Introduction to Clothing Comfort
Details about clothing comfort science, factors affecting clothing comfort, ways to measure clothing comfort...
Clothing Comfort A Combination of Objective and Subjective Evaluation
Presented to: Prof. Dr. Lubos Hes
By: Muhammad Mushtaq Ahmed Mangat Technical University Liberec April 12, 2010
Clothing Comfort: A Combination of Objective and Subjective Evaluations Introduction Clothing is one of the fundamental needs of the human being. It serves various and diverse purposes. Clothing selection is based on the needs and desires of the people. It may be to satisfy some aesthetic needs or to fulfill any particular demand of human being. People’s selection of clothing depends upon their perception and feeling about the clothing. In some cases it is recommended to wear certain clothing and selection is not possible, for example dress of a firefighter, military uniform, etc. However, it is very common that there is a dynamic and fundamental changes in the preferences of people with the change in the context; season, climate, age, type of activity, etc. It is highly linked with the core requirement why a person is wearing any particular clothing. Moreover, clothing requirements are rather different depending upon the type of activities of any person. However, comfort is a basic and introductory prerequisite of the people in all situations and is considered a threshold in selecting the clothing.
Comfort is difficult to explain since it is a complex and interdependent combination of physical, psychological and sensorial perceptions and highly depends of subjective evaluation of the individuals. It is not possible that comfort level of every one sitting in an office could be same, even temperature, air velocity and other parameters, which they are experience are same and comparable, even then comfort sensation is quite diverse. However if more than 80% people feel comfort, then it can be said that such environment provides a comfort (ISO). Same is the case with clothing comfort.
Literature provides a number of mathematical models to predict comfort but still final decision is made based on subjective findings by using clothing in real world. There is a continuos research to produce clothing which should be able to provide higher level of comfort. It is not possible to make clothing suitable for every situation. Industry is producing different designs, colors, patterns to provided better look for wearers and at the same time functional clothing to fulfill certain demands. For example, water proof jackets, fire fighting suits, etc. This work is an effort to describe different faces and facts of clothing comfort. The core objective is to provide a summary of clothing comfort, which is science as well as an art. It is evident from fierce competition in clothing market, that expectations of consumers have increased many folds
and in diverse directions. Clothing comfort parameters, all three; physiological, psychological and sensorial are now market tools. More over people are also considering the ecology. They prefer supplier which provides information about their contribution to in making this planet a better place to live. Beside that current time is an era of competition based on performance. Protected and restricted access of suppliers is a diminishing idea. Import transaction barrier are being removed, additionally IT has provided maximum information under finger tip and in most of the areas of the world. Clothing suppliers are having a strong contact with their costumers and end users. Companies to have pay attention to the voice of customers and they live on their feed back rather than relying on government policies to provide any protection from competitors. Emergence and Development of Clothing Comfort Science Hollies et al. (1979) has discussed the emergence of clothing comfort science and quoted various examples which indicate how people have turned their faces towards clothing comfort. Extension of thermal adaption approach by ASHARE, work of Rees to add surface characteristics, research carried out by Hock et al., Bogaty & Mehrtens are few examples, which provide enough information to workout that 1940-1950 period can be considered as birth period of clothing comfort science. During this time, clothing comfort was taken as a separate discipline labs were set, courses were taught, books were written and many thing more, however it is hard to fix the exact time. Still, debate is continue about the birth of clothing comfort science. It is evident from the survey of literature, which provide a serious discussion difference between heat-balance approach and heat adaptive approach. People have different point of view about the initiating point.
Temperature and humidity are the main contributor in the overall comfort. This dependency becomes more significant when we are the ends of the scale. As a matter of fact, some people have more ability to adapt the changed environment. People response with different magnitude on any change in temperature or humidity, which are fundamentals of comfort. Harmony among all factors concerning with the human being is fundamental requirement for comfort. It may be physical, psychological, sensorial, environmental, economic and many more. Literature provides evidence that there are three broad categories of comfort; psychological, sensorial and physiological. It is more complex and tiresome to conclude which one is preferred by people. Nonetheless, it is true that context plays an important role in preferences. For a military commander, while taking salute, it is most important that how clothing looks. He wants to deliver a non verbal message that he owns authority. At this stage psychological comfort is much significant and physiological comfort has
second position. By and large people can live under wide range of temperature and humidity, which are major contributor in thermophysiological comfort. Contrary to this there are people who reach in discomfort situation even there is a slight unwanted change in environment.
Literature provides a number of studies conducted in lats couple of years to note down the preferences of the customers. Kaplani and Okur (2008) have quoted some of them, like, study of Fujiwara et al. (1994) to assess consumer perception of apparel quality, work of Heller and Schiff (1991) to determine comfort, fit, style, color and quality. Survey of Silverman (1999) and Wong and Li (2002) about the stated comfort, garment fit, easy care and durability. Results of the effort of Wu and Delong (2006), which was focused to find out the significance of design, fashion, comfort, fit and guaranteed quality, survey of Zhang et al. (2002) which concluded that fit, comfort, style, color and workmanship as the most important casual wear attributes, Richards and Horridge (1984) survey of a group of American home economists, study about consumer clothing quality evaluation by Swinker and Hines (2006), investigation through two survey studies by Hong and Guolian (2004). It is not an exhaustive list. There are many more studies reported in literature and it is presumed that in future reliance of clothing manufactures will increase on customer feed back and survival will depend upon the best match between preferences of the customers and technical parameters of the clothing.
Among all mentioned surveys, work of Kaplani and Okur (2008) is most latest one. It is imperative to quote here their findings so that one may get an idea what people are looking for. They have summarized their findings in the following table. This table tells that nearly one-third focus of customers is on cool and warn feeling and lowest consideration is given to laundry instructions. It is obvious from the table that most of factors are related to heat and moisture balance. This is the area which will define the role of objective and subjective evaluation standards and will play most significant role in the marketing and companies will be judged based on their performance in this area.
All above discussion is an effort to describe the significance of clothing comfort which has been a point for discussion in last few decades. It is also important to note that US has played a major role in this area and they developed most functional clothing for military personnels. Despite that, it is not possible to define exact time when clothing comfort science became part of syllabus.
Clothing Comfort: A Descrete and Ubiquitous Concept We start our discussion keeping in mind that comfort is a property or characteristic of clothing which is not so easy to quantify due to various complexities attached with it. For example, thermal comfort a major part of clothing comfort and is ruled by the heat transfer laws. In case of thermal clothing comfort it cannot be judged purely based on heat transfer equations. Because the presence of human being on one end is not so simple as heat transfer phenomenon. Human being has its own dynamics, which are quite diverse and cannot act in a uniform way. People will react in a different way and intensity of reaction will also be different. This is the main reason that we consider clothing comfort is a subjective feeling and it is quite difficult to define all factors and their contribution level which can affect this feeling. Nevertheless, up to certain extend we can predict the response of human being by conducting different tests, particularly, related to heat and moisture transfer, surface friction, bending rigidity, etc. Thereby we can divide factors which can influence comfort into three main categories: 1. Factors related to wearer (metabolism of person, age, experiences, level of health, mental and ecomoc position, types of activities) 2. Clothing structure and chemical nature of fibers (fiber and yarn types, fabric structure, mecahnical and thermal properties of fabric, clothing design, fitting)
3. External Conditions (moisture, ambient and radiant temperature, wind speed)
Clothing Comfort concept is much easy to understad from the following statement, which is quite comprehensive: It [comfort] depends on many factors such as the temperature of the environment, the relative humidity, the wind velocity, the metabolism of the wearer and, of course, the characteristics of the clothing materials, e.g. materials’ thermal comfort properties, which display their abilities to transport heat and moisture from the human body’s surface into the environment. The measuring values that reflect this ability are clothing’s thermal resistance or thermal insulation, and water vapour resistance. Many other factors such as colour, fashion, a person’s physical and psychological state also influence the feeling of comfort (Mecheels, 1998 as cited by Celcar; Meinander and Gersˇ, 2008).
It can be conclude that one of the major findings relating human comfort to the atmospheric environment is the recognition that comfort depends upon more than one atmospheric variable. Thus, the sensation of heat or cold depends upon variables other than measured atmospheric temperature. This observation has resulted in the development of biometeorological indices that attempt to predict the various human responses to the sensations of warmth and cold and to further assess the physiological strain imposed by the combined atmospheric variables ( Yan and John, 1995)
There are a number of definitions of clothing comfort coined by researchers and scholars. Here we are giving few definitions to understand the idea of clothing comfort. “Comfort is defined as the absence of perceived pain- and discomfort as taught by Prof. Lubos Hes. Mine et al. (2009) explain “Clothing comfort is a state of satisfaction indicating physiological, psychological and physical balance among the person”. Above both definitions are quite interrelated and intriguing the same idea but with a subtle difference. Clothing Comfort perception put forward by Celcar; Meinander and Gers (2008) gives more elaborated view, it states “Clothing wear comfort is a state of mind influenced by a range of factors and is the result of a balanced process of heat exchange between the human body, the clothing system and the environment.” ASHRAE defines thermal comfort as an expression of mind that expresses satisfaction with the environment.
Above mentioned four definitions are well defined and have certain framework, which is mainly consist of thermal and moisture balance. There is a lack in above definitions that having a required and acceptable level of thermal and moisture balance between human body and the environment does not provide the manifest to define clothing comfort in a comprehensive way. There are many factors which are missing in these definitions. The most important is psychological and sensorial comfort, influence of emotional and economical level, variation due to previous experiences, etc.
Diagram 01, well explains the response of different factors which contribute in overall comfort.
Diagram 01: Human Signal System Source: http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio %20102%20lectures/sensory%20systems/sensory.htm
Y. (2001) proposes the following model which explains the clothing comfort process.
Figure 02: Clothing Comfort Sensation Process (Y. (2001)
In nut shell, comfort is an out come of a network of different physiological, psychological and sensorial feelings, which are based on the information provided by different parts of body, like, eyes, nose, hands, tactile senors, heat observers, brain. In addition it is the state when human is at its lowest stimulation. It can be said it is a state when brain feels a minimum stress and strain. This level is different for different persons and even for a same person varies a lot with the variation in the context. For example, a bed sheet is much comfortable when ambient temperature is between 20-25 Centigrade and it will give uncomfort if the ambient temperature is much low or high. Human body reacts when there is a change in the environment. Intensity of reaction is well defined mathematically. Diagram 03 is a pictorial explanation to understand the reaction of human body on change in its temperature.
Figure 03 Reaction of Human Body on Temperature Variation Source:http://media.johnwiley.com.au/product_data/excerpt/53/04716896/0471689653.pdf.
Building Blocks of Clothing Comfort Relying on information available in literature, it can be deduced that clothing comfort is considered as function of aesthetic, sensorial and thermophysiological comfort. Raj and Sreenivasan (2009) along with many scholars are of the same view. They also classify the clothing comfort into three main categories; aesthetic, thermophysiological, and tactile. All three categories have quite diverse in nature and serve different purposes and perform different functions.
The most significant factor is thermophysiological comfort and it defined as the process which explains the changes occur in human body due to change in temperature. It is a known fact that human body is a thermal engine and produces heat and it has a strong link with environment temperature. Furthermore, there is a constant change in the environment, thereby, body has to react accordingly and take necessary actions to keep itself intact from the vraition of ambient temperature. Human body takes different steps to have a balance with external environment. During this process there will be heat transfer phenomenon. In cold weather, body will lose its heat and to stop it we use clotheing and in hot summer, we wear light weight and porous clothes so that our body heat may be reduce and it hleps in heat loss through convection, radiation, conduction and
evaopoartion. All such efforts are focused to satisfy soft needs of wearer and to develop a thermal balance. Aesthetic comfort is highly dependent on designing, coloring, sewing, packing, embellishment, etc. It needs a separate discussion. In this report our main purpose is to discuss the two building blocks; thermophysiological and tactile (sensorial) comfort. However, we will provide a brief of psychological comfort.
Psychological Comfort Although discussion about the psychological comfort is out of the scope for this work, it is found imperative to list down the factors which has a significant correlation with the psychological comfort. Prof. Lubos Hes provides the following list of factors which have a great affect psychological comfort: 4. Economical aspects: Resources, technology of food and objects manufacture, skills, political system, 5. Historical aspects: Inclination to products made of natural materials, to products simulating nature, to products of natural smell. Tradition in lifestyle and fashion. 6. Cultural aspects: religion, habits (in Arabic countries women are fully covered) 7. Social aspects: age, qualification, social class, rank or position in this class 8. Individual and group aspects: the effect of fashion, style, colors and luster, trends, personal preferences
It is presumed that intensity of factors varies a lot depending upon the context, which people are experiencing. There may be many more factors which are missing from the list and has become significant with the passage of time.
Sensorial Comfort: Easy to Estimate Sensorial comfort is relatively a young subject which has attracted much attention of people. Currently it is the topic which is highly focused (Y. 2001). Most of the clothing as a whole or partly touches the human skin. Under garment are fully in touch with human body, however jackets, coats or sweater may have partially contact with human body. Beside that, their hand feel is also important since our hands are regularly in touche with the clothing. Sensorial comfort depends upon
mechanical and chemical properties of fabric. It is not possible to discuss sensorial comfort without discussing the thermal comfort. Anyhow, we have tries to discuss both topics separately.
Figure 04: Skin Details Source: http://i.imgur.com/wGB4Jl.jpg
Importance of sensorial comfort can be judged by the work J.Y. Hu Lubos Hes Y. Li, K.W. Yeung and B.G. Yao. They successfully developed, Fabric Touch Tester (FTT), which is used to integrate evaluation of thermal–mechanical sensory properties of polymeric materials. Inventors of FTT are fully convinced that clothing which is directly in touch with the skin have effect on overall comfort. FTT measures, records and analyses thermal and mechanical sensory stimulations and propagations occurred while clothing becomes in touch with the human skin. FTT can be used to predict many mechanical properties of clothing, like smoothness, softness, prickliness, warmth and dampness. Instead of the highest precision level of FTT, even then there is a need of subjective evaluation of the clothing.
People generally estimate clothing comfort level just by touching with hands and have to rely on their touch sense and they take decision based on their very raw observation. Generally following factors of clothing can be judged by human fingers:
1. Softness 2. Smoothness 3. Bending rigidity 4. Elasticity 5. Warm and cool feeling, 6. Thickness of the fabric 7. Shear rigidity 8. Hand fullness 9. Wrinkle behavior
In current era there is a rising trend to do shopping online, and it is becoming difficult to touch the clothing before buying. For this reason there is a dire need to have some mechanical evaluations, enough to predict different characteristics of the fabric. For such information there is a need to have a uniform and acceptable method. There is a successful effort by different institute. One of the most significant example is of Hohenstein Institute in Bönnigheim, which was established 1946 and has gained international reputation. This institute provides labels, which are attached with the clothing.
Figure 05: Clothing Comfort Labels
Another example is of Kawabata system which is also used to have better idea about the comfort. There are five Kawabata Evaluation System (KES) instruments. These instruments measure 4 modulus and 16 parameters, like, fabric bending, shearing, tensile and compressibility along with surface smoothness and friction and many more.
Figure 06: Friction Measuring Instrument by KES Japan
Other than above instruments, Dr. Lubos developed ALAMBETA and PRMATEST to measure thermal properties and moisture permeability of fabrics. These testing machine can measure heat and moisture behavior of different fabrics. Results can be used to predict the clothing comfort level. There are many universities and manufacturing firms using these two equipments. List of instruments and laboratories is not short. All efforts is an evidence that there is an increasing trend to have mechanical evaluation of fabric before sending it to market. These evaluations serves many purposes. The foremost is a base to develop new products. Today it is used to market the products, particularly; it is much useful in case of online clothing marketing. Prof. Lubos explains the German method of objective evaluation of complex sensorial comfort of worn garments, which is based on large experimental investigation. They have developed following equation based on a large experiments:
Where: Imt = index of water vapor transmission Ik = index of water vapor transmission IB = index of wetting Io = Surface index nk = Number of contact points S= Bending rigidity
Numerical values of constants: !1 = -2,537
!5 = 1,71.10-3
!2 = 1,88.10-2
!6 = 3,86.10-2
!3 = 2,29.10-3
" = 0,36
!4 = 2,09.10-2 As per calculation total sensation 1 is equal to .35 sensorial and .65 thermophysiological comfort. Equation for Thermophysiological Comfort
imt= Water vapor transmission Fmt= Dynamic water vapor absorption
Kd= Liquid moisture buffering coefficient Bt= Temperature buffering coefficient Kf= Moisture permeability
Numerical values of constants: ! = -5,640
! = -4,512
! = -0,375
! = -4,532
! = -1,587
Total comfort: TK
= 0,35 . TK + 0,65 . TK H
Lubos further explains that sensorial effect is a combination of of mechanical, thermal, acoustic and visual perceptions of comfort. The index range is 1-6, where 1 means the best and 6 the worst.
Y. (2001) has listed 26 sensory desciptions after a thorough survey. Some pof them are listed here: 1.Snug
2.Loose 3.Stiff 4.Light weight 5.Staticky 6.Non-absorbent 7.Sticky 8.Heavy 9.Cold 10.Damp 11.Clammy 12.Clingy 13.Picky 14.Rough 15.Scartchy 16.Hot 17.Soft 18.Warm 19.Prickly 20.Itchy 21.Chill 22.Sultry 23.Tickling 24.Raggy 25.…. 26.….
Skin Sensibility and Its Measurement
Skin is highly sensitive to pressure and change in temperature along with humidity level on its surface. Clothing is an extension of the human body its first layer has a direct touch with the skin. At the same time micro climate is produced, where on one side there is a skin and on other hand there is a clothing layers. Micro climate is quite dynamic in nature as compare to the macro climate. Any change in micro climate is sensitized by the skin and skin sends signal to brain which develop
a local and overall comfort due to clothing. There are many test available to test the changes in skin due to changes in micro climatic conditions. For example, Hohenstein Institute has developed many tests to evaluate cling index, number of contact points and surface index, stiffness and sorption index. A group Yehia El Mogahzy, leader; Faissal Abdel-Hady, Roy Broughton, Gisela Buschle-Diller, Ramsis Farag, Asaad Mohamed, David Pasco, B. Lewis Slaten (Auburn), Bhupender S. Gupta (NC State) Radhakrishnaiah Parachuru (Georgia Tech) are developing a Design-Oriented Fabric Comfort Model. The blueprint of the model shows (Figure 07) that they have taken both micro and macro climate into account. Their aim is to develop "virtual intimacy measurement of fabric comfort using structural, fuzzy-logic and psychological modeling to replace the traditional intimacy of touch.”
Figure 07: Design-Oriented Comfort Evaluation Model by Yehia El Mogahzy and Associates
Sensorial sensation can be divided into two main categories; neurophysiological and thermophysiological comfort. Neurophysiological parameters are mainly concerned with the sensation (stimulation) occurred when our skin, which is composed of number of nerves becomes in contact with the surface of fabric. Skin, which is highly sensitive can observe the level of surface smoothness, crispness, roughness, stiffness. Even our skin can make observation regarding warm and cool effect of the fabric. Whereas, thermophysiological comfort parameters are mainly related with the heat and mass transfer phenomenon, which covers, change in skin temperature, level of moisture at the skin surface and pressure of air flow.
After carrying many tests, Hohenstein Institute has put forward results which reveals that thermophysiological characteristics account for about 66% and skin sensibility contributes nearly one-third in the overall perception of clothing comfort.
Steven (1906-1973) proposed a power law. This law governs the relationship between magnitude of the stimulus and its impact perceived:
Where: I = Magnitude of the physical stimulus !(I) = Magnitude of the sensation a = Exponent depending upon the type of stimulation k = Proportionally constant related to the type of stimulation and the units used.
Where: I = Magnitude of the physical stimulus !(I) = Magnitude of the sensation a = Exponent depending upon the type of stimulation k = Proportionally constant related to the type of stimulation and the units used.
Table 02: Sensation Index
Deformation Angle Fabric rigidity plays an important role in its overall comfort. Human body is not straight. It has lot of bends, furthermore, during performing different tasks, different parts of body have to move. This movement is mainly consist of bending. Bending of fabric depends upon its thickness and length.
y = F l3 cos2" sin" / 3 E I Where: Y= Deformation F= Force I= Inertia moment
Where Inertia moment is:
I = # d4 / 64
Above equation depicts that fiber deformation is directly proportional to third power of its length and indirectly to fourth power of its diameter. Due to this fact micro fibers bend easily and provide large area of contact between skin and fabric surface. Thereby, heat transfer area increase and we feel fabric surface cool. Overall fabric made by microfibers provides cool feeling.
Bending righty also depends upon their packing coefficient. Prof. Nescar has developed this model. Following equation explains the relationship: B=E.I Where: I = b.h3/12 Drape Angle and Its Theory Drape angle is another parameter of fabric in clothing comfort. Niwa and Seto have developed following equation to estimate the drape behavior of the fabric. regression equation given below,
DA = DA=C + C (B/W)0,33 + C (G/W) 0,33 0
There are many ways to measure it. Here we explain a very simple method as taught by Dr. Lubos.
Figure 08: Drape Angle Measuring Method
All above discussion provides compelling evidence that sensorial comfort is a one of the major areas of concern in overall clothing comfort. It is quite complex and improper evaluation can lead to some wrong decision. For example, as described by Hipler and Elsner (2006) “The skin sensorial comfort is negatively affected by hydrophobic, smooth [flat] surfaces that easily cling to sweatwetted skin, or which tend to make textiles stiffer. As guidelines for the improvement of the thermophysiological or skin sensorial wear comfort, it is recommended to use hydrophilic treatments in a suitable concentration and spun yarns instead of filaments.”
Thermophysiological comfort is a fundamentally important element of outdoor garments since they are intended to be worn in various weather conditions, often while the wearer engages in strenuous activities; the anticipated environmental conditions and the clothing system are accepted to be the other key elements to consider during design and manufacture. Ruckman et al (1998)
Papkov (1981) views the thermal imbalance as the core reason of discomfort. Papkov prefers to discuss discomfort instead of comfort. Since explanation of discomfort can easily be discussed and simple to understand. It may be due the fact the human being sense is more prone to discomfort as compare to comfort. They easily and quickly identify the discomfort as compare to comfort human body is a thermal engine, which produces heat to run its different functions of the body. Our food is main source of energy. Nevertheless there are many other ways to produce heat, like, running can increase the body metabolism, which increaes the enternal and skin temperature. There is a direct and indirect interaction of skin and environemnt, both surfacrs have different temperatur. This temperature gradinet provides the basis of heat transfer. Common heat transfer laws govern the heat flow from body to environment or vise versa. If there is a heat flow from body to environment due to cold weather or from environmnet to body in a hot atmosphere. In both cases there is a chance that temperature of human skin will fall over and under the temperature which creates discomfort for the human body.
Barker (2002) points out two main distinct features of thermophysiological comfort. One is normal wearing and the other is transient wearing conditions. In both cases there is a need of thermophysiological comfort which mainly depends upon the thermal balance between body and the environment. Clothing is an extended part of body and sometimes it is stick to the body. For example, ski player wear suits which are quite stick to body. Whereas, people prefer to wear suits which are loose fitted so there is a gap between the body and the clothing. In both cases thermal regulation will be different and role of clothing thermal properties cannot be ignored.
To avoid any uncomfort there is a need to create an equlibrium between the internal and external temperature. For example, in cold weather, there are two ways to keep the balance; either more heat production (by doing exercise) to keep body warm or covering the body with such materials which could stop heat flow through radiation and conduction. Same is the case in hot environment. In this case, one has to keep body covered from heat or exposouser of body to atmosphere so that there is a sweat evapration, which ultimately reduces the body temperature.
In both cases there is need of such clothing which could provide comfort to human body. The propoerties of clothing which deals with comfort are quite complex and there are numerous factors, which can affect clothing comfort. Some scholars have discussed this phenomenon under the
heading of thermal comfort. Satsumoto et al (2009) investigated the comfort of underwear by using different materails and their combination and concluded that mositure absorbency has a significant role in provision of comfort. Thermal comfort depends upon the imbalance in the heat. For better control of temperature to keep it at low level it is necessary that there should be a evaporation of sweat. Evaporation is a endothermic process and keeps temp low which provides a comfort to body. The premsie of this whole phenomenon is based on the mositure absorbency and evaporation capabilities of the fibers. This study reinforced the idea that thermal comfort of human body is based on the heat tranfer but it is quite complex as compared to the process where there is no moisture.
Heat and Moisture Transfer from Skin
Human body tends to be at steady state with environment by keeping a thermal balance. Extra heat produced by metabolic process or due to any activity is transferred to environment primarily through convection, radiation and small portion through conduction. If these mediums become insufficient then evaporation is the last method. Working of thermal engine of human body depends upon many factors, which are mainly related to the physiology of human body and its working conditions. Intensity of thermal generation will be different for a child and an old man. Furthermore, human body will produce less quantity of heat while it is in rest position as compared to when a person is running. Additionally, there are many more factors, like, type and quantity of food, health conditions, age, etc.
In heat transfer phenomena two terms are most common; heat and temperature. These two terms are quite distinct in nature but sometimes used as synonymous of each other. Temperature is a relative term to feel cold or hot. It is average KE possessed by the substance. Whereas, heat is an energy in transit, ready to transfer from hot body to a cool body. Temperature gradient is the main driving force in heat transfer. For temperature we use thermometer and measure in Celsius and Kelvin scales. Contrary to this heat is measured in Joule and Calorie and symbol is Q. Heat transfer process is denoted by q. We can concluded that heat is an energy in transit ready to move from one substance to other. Its direction is determined by the temperature difference. It always move from hot to cool place. For this reason a negative sign is put in heat transfer equation.
Heat production is a regular function of the human body. It is much required to keep the whole body in function. Production of heat depends upon many factors, like level of activities, age, food taken, etc. There is need to dissipate the extra heat produced by the body. If this extra heat is not removed from the body, it will create uncomfort for the human. There are two main ways to dissipate this extra heat; through skin and through respiration system. In respiration we take moisture and heat from lungs. The discussion about this process is out of the scope of this article. The second method in which heat transfer takes place through skin is the area where we need to discuss in detail.
Figure 09: Body Temperature Source: Barite (2008)
Our body has its own thermal regulation system. 1Keith C. Heidorn, PhD provides a in-depth information about it. This report reveals that there are two sets of heat sensors present in the human body. First indicates the outflow of heat from human body. It lies close to the surface of skin and it is concentrated in the finger tips, nose and bends of elbow. The second works in case when body temperature is low as compare to environment and body gains heat. It is present deeper in the skin and is concentrated in the chest, upper lip, chin, nose and forehead. The core function of both sensors is to send signals to brain, which take action to counter the affect of heat transfer from body to environment or vise versa.
Thermal regulator of human body is called hypothalamus. It is a gland which is present at the base of brain, just above the pituitary. It is set very close to 98.60 F (370 C) and keeps monitoring of the body temperature through blood temperature. Blood circulation is the main source of thermal distribution throughout the body. After sensing any change in blood temperature, hypothalamus 1
reacts accordingly and initiate physiological response to increase or decrease the temperature of the body. This all efforts is to have body temperature very close to the set temperature 98.60 F (370 C), at this temperature human body has the best comfort level. Any drastic change in blood temperature, low or high will cause uncomfort.
Heat Transfer Through Evaporation There is a strong need to understand the whole mechanism of heat transfer in the presence of moisture. Ruckman et al (1998) describe that “ Theories and mechanisms have been established on the basis of Fick’s Law and it is now widely believed that both heat transfer and moisture transfer”. We can estimate the amount of heat produced by human body. Literature provides a number of way to estimate the heat produced by the human body. Papkov (1981) has discussed it in length, we have derived following information from Papkov (1981).
Human body needs different amount of energy to maintain its activities. It ranges from 7,560 J/day to more than 21,000 J/day. It is equal to 420 J/h per meter square of the human body. This variation depends upon the working of the human body. It is very simple to understand that if one person is in a rest position it just needs heat to maintain the functions of the body, like respiration, etc., but in case when a person is running and doing sever exercise it will burn more calories and will need more calories to maintain the working of human body. However, on average person needs 840-1260 J/m2//h.
There are two ways to remove heat from the body. One is heat transfer through convection, radiation and two is energy consumed in making evaporation of sweat evaporation (mass transfer). In this case sensible heat is used in latent heat. Both processes are happening at the same time and this continue till there is a temperature gradient becomes zero or negligible or not enough to work as main source of heat and mass transfer.
In case when human body is directly in touch with environment, main heat transfer takes place through convection and radiation. Heat transfer through conduction is quite less since direct contact of human body with any other substance is quite low. It may be through feet if these are directly in touch with the floor.
When human body is covered by clothing, which is most of the cases happens. Notwithstanding, there are certain parts of the body which are not less often covered but generally major part of the human body is covered for most of the time. Body is producing heating and transferring it to environment. If this transfer is stopped by the clothing shield, humidity will tend to increase and discomfort is the ultimate result. Since most of our fabrics have fine capillaries to transfer this moisture from skin to environment and this evaporation process continues. This evaporation follows all laws related to heat and mass transfer.
There is a need of 2,436 J to evaporate 1 gm of water. This heat is drawn from the skin to evaporate water from the skin surface. According to calculation there is a need to remove 0.0021 J/m per square centimeter of the body. It is about 10-6 g of water per cm 2.min. It is a known fact that evaporation is directly proportional to the difference in partial vapor pressure between two systems. As it is obvious from the whole process that there is accumulation of moisture on the skin due to clothing shield and ultimately there will be less difference of moisture percentage on skin and in environment. Result is low evaporation. In this situation, there are two possibilities; first use of human body to work as latent heat and the second is movement of air to supplement the evaporation process. In clothing comfort removal of moisture is extremely important since more moisture will create uncomfort in shape of heat, sticking, cling, irritation, etc.
In real world there is a diverse situation about the RH and air velocity. However, for calculation purpose 4-7 m/minute is taken as standard. Under the standard humidity around the surroundings and standard air velocity the rate of evaporation from a surface is in the range of 10-3--10-4 g of water per cm 2.min. By comparing the required amount of heat to be removed from body, which is 10-6 g of water per cm 2.min, depicts that outer surface of cloth cannot work as the limiting factor.
Above discussion provides a valid argument that outer layer of clothing cannot affect the evaporation process. It is the humidity level which is main reason of slow evaporation, since drop in humidity is the main driving force. A fabric which can transfer moisture from inner side, means from the skin surface can contribute in evaporation process. For better transport of moisture from skin to outer layer needs a contact angle less than 900. Hydrophobic material cannot form this angle, it is alway near to right angle, which affects its ability to transport the moisture. It shows that in case of hydrophobic material force is required to transport from one side to other, more explicitly speaking from skin to environment. It can be calculated by using the following equation.
Where: h= Height of water ascent through the capillary $ = Surface tension r = Radius of the capillary g= acceleration of the free fall
Figure 10: Wetting Angle
It can be observed by putting a drop of the cotton fabric which is free from any sort of finishes. Water drop will spread easily and the contact angle will be quite less than 90 degree. It may be in the range of 0-30 degree. In case the surface is hydrophobic it will be near to 90 degree. This angle is a function of interface behavior among three phases; liquid, solid and the ambient atmosphere. Polypropylene, polyethylene, polyester, teflon and many more plastic material have low surface energy. Surface energy which is also called wet-ability is measured in dynes/cm. PP and PE have less than 30 dynes/cm, which is quite insufficient fir wetting.
Due to low surface energy and consequently higher angle of contact inhibits synthetic fibers to transport moisture from inner side to environment through capillary actions. Considering this discussion it can be concluded that if the inner surface of the fabric is made of such fabric then there is less chances that moisture will shift from skin to environment. The accumulation of moisture on skin creates discomfort.
Table 3: Wetting Angle Wetting Angle (0)
Polyester and Nylon 6
Here there is a model which explains the moisture removal process. It is enough to understand that material which have highest wetting angle is not capable to transport moisture from skin and not suitable foe better comfort.
Figure 11: Heat and Moisture Flow Papkov (1981) sums up the discussion and concludes that comfort depends upon the moisture transfer from skin to the environment. This transfer depends upon the wetting power of the fibers used, porosity (inter-fibre capillary) of the yarns, determination of fabric bulkiness and capacity of reversible deformation of fabric. This can be achieved through a comprehensive consideration of all factors. It is possible to achieve the required target by using different combination of fibers, testing different fabric formations, application of various surface treating chemicals and making a suitable design which should moisture transfer. One should not ignore various disadvantages of using synthetic fibers, one is production of static charges, which can create uncomfort of human being.
Factors affecting Thermal Balance There are nine possible factors which can contribute in achieving thermal balance. These factors belong to five main areas:
10. Body metabolic activity (Physiological function and behavioral response or level and type of activities) 11. Mass transfer (sweat evaporation and respiration system) 12. Heat transfer (through conduction, convection and radiation) 13. Ambient and radiant temperature 14. Level of insulation (clothing)
In this part of the article we will focus on physiological working and heat and mass transfer. Other areas will be discussed later.
What we eat, it is converted into energy which is required for growth, tissue regeneration, operation of body and physical activities. Our metabolic process has 20% efficiency, or we can say that our body requires 20% of the energy produced through metabolic process all rest is converted into heat and major part of the heat is rejected by body. To remove this extra heat there is a need to have a direct contact of the body with environment so that it should give energy to environment through radiation, conduction and convection. If environment temperature is already high then heat transfer
will not occur. In such case body temperature will keep on increasing and body will take help from sweating process to convert the sensible heat into latent heat, which will convert moisture into vapors. It is a known fact the evaporation reduces the temperature and ultimately skin will be cooled down and thermal balance will be achieved. Barker (2002) explains Woo and Barker model which is used to heat and moisture vapor transfer, which occurs simultaneously. This model is written as:
Where: Mn= Net metabolic heat which must be dissipated Q= Heat lost through the clothing H= Heat lost through thermal transfer E= Heat lost through evaporation (mass transfer)
Woo and Barker (1988) calculate a comfort range with heat generated. The model is based on the first criterion of clothing comfort which holds that the net metabolic heat generated (Mn) must be dissipated through garments worn (Q).
Moisture Transfer and Clothing Comfort The ability of a clothing material to transport moisture from sweat skin is crucial to perceived wear comfort. A modified gravimetric absorbency testing system (GATS) is used at NCSU to measure the moisture accumulation associated with the wicking of liquid moisture from sweating skin. The GATS procedure measures demand wet-ability. The test indicates the lateral wicking ability of the fabric, or the ability of the material to take up liquid in a direction perpendicular to the fabric surface.
The GATS apparatus was modified to incorporate a special test cell and cover to assess absorption behavior in the presence of evaporation. In this arrangement, liquid is drawn from a fluid reservoir by the capillary action of the fabric. The hydrostatic pressure of the fluid delivery system is adjusted by controlling the position of the sample platform. Liquid is delivered to the test material placed on a porous plate. Numerous pins, distributed over the area of the test surface uniformly restrain the test fabric. The amount (grams) of liquid siphoned from the reservoir is recorded as a function of time. These data are used to calculate absorption capacities and rates, and the percentage of
moisture evaporated by the fabric. Applications of this device are discussed by Barker and Choi (2001).
Thermal Equation of Human Heat is an energy in transient. In case of human body it will be produced depending body metabolic system and level of activities. According to laws of thermodynamics, equilibrium is the ultimate end when two hot bodies or systems are put into contact. It will happen here and human body will strive to have balance with the environment, it may be atmosphere or micro climate created due to the clothing shield.
Following equation describes this process:
Where: M = Metabolic rate of human body E = Heat loss through evaporation and aspiration (wet heat) R = Heat loss or gain through radiation C = Heat loss or gain through conduction S = Heat stored or consumed by body
Figure 12: Body Heat Balance Source:http://media.johnwiley.com.au/product_data/excerpt/53/04716896/0471689653.pdf.
Human body is quite flexibly and has the highest adaptability. It acclimatize easily with environment. Physiological conditions of human plays a significant role. If it is not in good condition (overall health of the person), then there is a strong need to take measures to keep thermal balance of human body. It may be change in ambient and radiant temperature, less or more clothing, change in activities, etc.
There are certain differences in the final calculation of human body temperature. This variation is with in a narrow limit. In addition, human body shows different temperature at different parts. Furthermore, inner body temperature is different from the skin temperature. Nevertheless on the whole 37.0 Centigrade is most acceptable and considered normal human body temperature of a health person. There is a big variation in skin temperature. It may go down up to 40C and maximum it can attain 40 0C. In both cases body feels discomfort. Generally, at 270C ambient temperature body creates a thermal balance with the environment. In case of decrease in ambient temperature, body starts to lose its heat and cold feeling signal is transmitted to brain, which takes necessary action. It may be shivering of body. However, if the ambient temperature increases then body starts to take temperature and again a discomfort is sensed and body starts sweating to lose its heat.
Considering this discussion it can be said that thermal imbalance is one of the reasons of discomfort.
Figure 13: Radiation and Hunan Body
Figure 14: Reaction of Human Body and Temperature Variation
Figure 15: Association Between Temperature and Body Heat Loss Fairy (1994)
Figure 16: Metabolic and Activities
Table 04: Metabolic Rate and Activities
Clothing Insulation and its Measurement In case of imbalance between body temperature and ambient temperature, we take help of clothing to attain balance which is highly required for thermal comfort. The main function of clothing is to provide insulation between the body and environment to trap the heat inside so that there should be no decrease in skin temperature. ISO 7730 explains the details of thermal clothing and provides useful information how insulation works to provide thermal insulation for the protection of human body from cold environment.
Clothing insulation properties depends upon type of fiber, yarn specification, fabric formation process and designing of clothing. All is under the thermodynamics principles and laws. For example, clothing which can trap air better insulator since air has lowest thermal conductivity.
Traditionally clothing insulation is measured in a unit which is called clo. It is equal to 0.155m2. 0C/W. This value is almost zero for a naked person and for a business two piece suit and accessories it is nearly .05 clo. There is a list of different clothing and their insulation values given in Table 5 Table 05: Clothing Insulation
Figure 17: Temperature and Clothing Insulation Source:http://media.johnwiley.com.au/product_data/excerpt/53/04716896/0471689653.pdf.
Figure 18: Clothing Insulation Values Source:http://media.johnwiley.com.au/product_data/excerpt/53/04716896/0471689653.pdf.
Figure 19: Temperature Variation and Effect on Human
Cellar et al.2008) take total heat transfer from the human body as the total of dry heat and wet heat. They consider that heat loss through convection, conduction and radiation is dry heat loss, whereas evaporation is wet heat loss. They have used a sweating manikin to calculate dry and wet heat loss.
Heat Transfer through Conduction, Convection and Radiation
Figure 20: Heat Flow Through Conduction
Fourier Law is explains the heat transfer through conduction. It is written in the following form:
It is important to note that heat transfer is not a scaler quantity, rather it is a vector quantity. It is better to indicate direction with q. Normally x is used to indicate the one dimensional flow, however for circular flow r put as subscript with q.
Convection Newton law of cooling describes heat flow through convection. In this case heat is carried a moving particle. It may be gases or liquid. Making water hot in a pan is best example to explain.
Following equations is used to describe the heat flow through convection:
q=h As %T
Where: q = heat flow from surface of the substance (W) h = heat transfer coefficient (depends upon the geometry of the substance) (Wm2 K-1) As = Surface area in touch with environment (M2) %T=TS-T! Temperature Difference (K)
Free Convection Free convection is a common method in heat transfer process from human body. In forces convection there is force which is used to keep in touch the medium by force. For example, fan is used to take away moist from human body. In case of free convection, fluid or gas motion is due to the buoyancy, which takes away the warmer fluid and cooler fluid becomes closer to substance which has higher temperature. If we use any external force then convective rate will increase.
Free convection is well explained through Nusselt number. It is a ratio and dimensionless number and indicates the conductive rate:
Q/A = Thermal flux (W/m2) L =Length of the body k = Thermal conductivity of the fluid or gas &T = Temperature difference between the body and the surrounding fluid
We can have Nusselt number by using the Grashof number and the Prandtl number.
The Grashof number is given by2
Where: " = Thermal expansion coefficient L =Length of the body V= Velocity &T = Temperature difference between the body and the surrounding fluid
The Prandtl number indicates a ratio of the kinematic viscosity to the thermal diffusivity. For example Pr number of air is nearly 0.7. It also depends upon temperature.
It is to note that the product, Gr*Pr is the Rayleigh number. Ra, is used to find the Nusselt number. However, relationship between Nu and Ra is based on empirical experiments.
Value of Ra is about 1010 indicates transition region and turbulent region. Where
Nevertheless I is used in vertical plates or cylinders alike. It can be applied to do calculation of human body.
Human skin temperature is nearly 34°C. After having clothes it may be changed increased or decreased. For calculation, we consider that ambient temperature difference is &T=10°C (10K). We take an adult man body height L=2 m. By taking Ra=8x109, Nu is nearly 200. It means that conduction will be very low. As it is known that Nu is the ratio of convective to conductive thermal transfer rates.
Since k=0.026 W/m/K for air The convective thermal flux rate is
Heat Flow through Radiation
Radiation is the third way to transfer heat. In this case electromagnetic waves produced due to temperature difference and heat is transfer from a hot body to a cooler body. In this case no medium or matter is required to carry the heat. Heat transfer due to radiation is well described by the following equation: Emissive power of a surface:
E=$'Ts 4 (W/m2)
Where: E= Emissive Power of surface of the of a substance $= Stefan Boltzmann constant ' = emissivity of the substance. It is 1 for a black body) Ts= Absolute temperature of the surface (K)
Third option of heat transfer is radiation and it can be calculated by Stefan-Boltzmann equation, which is as under:
T0 = Ambient absolute temperature, ( = Emissivity $ = The Stefan-Boltzmann constant
It has been worked out that skin works like a black body. Emissivity of skin and clothing is taken nearly one. Its impact is less when &T is small compared to the ambient temperature, T0.
Wien's Law This equation is used to predict temperature of any body by calculating the wavelength.
"max= Peak wavelength T = Absolute temperature of the blackbody b= Constant of proportionality, Wien’s displacement constant:
Figure 21: Wavelength and Light Example: Wavelength of light emitted by human body:
Comparison of convective and radiative rates
We can measure ratio of convective to radiative transfer. It can be observed that its dependence on temperature difference is not significant:
It is obvious from the above ratio that &T of 10°C is not making a big difference convection and radiation. It shows that radiative heat radiative heat loss is double to the convective rate. However forced convection can increase the convective heat transfer.
Literature provides many more results derived by researchers. There is a wide range of results and all are based on some assumptions which ultimately lead to some differences in the outcome. We can conclude all discussion that heat transfer from human body is primarily thorough convection and then radiation. Very low percentage thorough conduction. In case heat transfer process fails to create balance between human body temperature and ambient temperature, human body takes help of perspiration and then mass transfer phenomenon takes place. There is a big analogy between heat transfer and mass transfer.
Clothing Heat Transfer Coefficient Clothing Heat Transfer Coefficient explains the all parameters which can influence the convection heat transfer of clothing. It depends upon conditions of boundary layers, surface geometry and nature of fluid.
Thermal Parameters of Fabric One of the main functions of our clothing to work as thermal insulator. These parameters determine the behavior of clothing under different circumstances. In the following lines we will discuss the tests generally carried out to predict the behavior of clothing under different climatic conditions. The main source all here under information is based on the lectures by Dr. Lubos Hes.
Thermal conductivity, which is normally denoted by k is a property of any matter to pass the heat. It is mainly related with the chemical structure of the material and in case of gases, it is also a function of pressure. However, in case of solid, it mainly depends upon the chemistry of the mass. There are many ways to measure it. Its significance in clothing comfort cannot be ignored. Clothing are made of different fibers. It may be natural or man-made. Different fibers have different thermal conductivity. As said earlier that thermal conductivity as an indicator of the response of material, when it will be exposed to any situation where there is a temperature difference between fiber and the environment. It is quite easy to understand from the pan, used to fry an egg. Temperature of the handle is quite low as compare to base of the pan. It is mainly due to the thermal conductivity of material used for handle and for the base. In the following there is a table showing thermal conductivity of different materials: Table 06: Thermal Conductivity of Different Substances
Thermal conductivity equation ( in some equations k is used instead of )):
Where: Q= amount of conducted heat F = area through which the heat is conducted * = time of heat conducting %T = drop of temperature $ = fabric thickness.
Thermal diffusively or thermal diffusion is a property of any substance which indicates the ability of the substance to adjust its temperature according to the environment. Higher thermal diffusivity means that substance will adjust its temperature quickly. In more simple word, it is property of the substance that depicts the rate at which heat diffuses through a substance. Furthermore, it is a combine effect of thermal conductivity and its specific heat. Both are opposite. Higher thermal conductivity will increase the body's thermal diffusivity whereas, high specific heat reduce the thermal diffusivity. Its equation is as under and unit is m+/s:
Where: k : Thermal conductivity W/(m,K)) - : Density (kg/m.) cp :Specific Heat Capacity (J/(kg,K))
In case of fabric it is the ability to heat flow through the fabric and how quickly fabric adjusts its temperature according to the surroundings.
Table 07: Thermal Diffusivity of Some Common Materials
Source: J.P. Holman, Heat Transfer, as cited on Wikipedia
Thermal absorption is related to the cool-warm feeling of the surface of the fabric. This parameter is related to the surface of the fabric. Cotton has higher value than synthetic fibers. Due to this reason cotton is most suitable for summer as compared to synthetic fibers. Frydrych et al.2002) have compared properties of different fibers and concluded that this parapets is mainly related to the fabric surface so it can be altered by applying different finishings. Its equation is as under:
Where: ) = Thermal conductivity - = Fabric density c = The specific heat of the fabric
Frydrych et al.2002) have also found a correlation between fabric surface and the thermal absorption. Smooth surface will have higher thermal absorption and gives cooler effect as compared to a rough surface. Considering this observation, it can be deduced that twill fabric will have low thermal absorption as compared to plain fabric. Also course yarn will have warm than fine yarn.
Thermal Resistance and Resistivity
Thermal resistance is a property of material which is directly proportional with thickness and inversely proportional with thermal conductivity. Its equation is as under:
Where: $ = Thickness ) = Thermal conductivity
It is reported by Frydrych et al.2002) that fabric with smooth surface has low resistance, whereas, fabric with structured surface, like, twill will have higher resistance. This is the reason that when we touch a smooth surface, we find it cool as compared to a rough surface. It is mainly due to the thermal resistance difference. It is also important to note that certain finishes can alter the thermal resistance by creating a change on the surface of the fabric. Peaching of the fabric is another convincing example. After peaching fabric surface gives a cool feel.
Resistivity is a different concept, it relates to the object property and associated with the resistivity and thickness of the object. It is denoted as:
R= 1/k Where: R= Thermal resistance per unit area of the piece of material (m+K/W), l = represents the thickness of the material (m), and k = represents the conductivity of the material (W/mK)
Maximum and Stationary Heat Flow Density Ratio Heat flows from hotter to a cooler body. It is denoted by heat flux, thermal flux, heat flux density or heat flow rate. It is a flow of energy per unit of area, which is meter in SI units. It is expressed as W.m- 2 and denoted a subscript q. It is vector quantity since it has both direction and magnitude. Its equation is a sunder:
Where: Q = Amount of heat F = Area through which the heat is conducted * = Time of flow.
In textile ratio of maximum and density heat flow ratio is a property which indicates certain characteristics of the fabric. Frydrych et al.2002) have put forward that this ratio of cotton fabric is much higher than the Tencel fabric. Maximum heat flow density indicates that when a cold fabric is touched with skin a cool feeling will be observed. It is the reason that when we wear cotton in summer, it gives a cool feeling, which is mainly due to the high ratio of maximum to stationary heat flow of cotton. It is a function of thermal insulation and thermal absorption. Moreover, plain fabrics have higher ratio as compared to rough surface.
Air and Vapor Permeability Air and water vapor permeability is feature of fabric which is used to predict its contribution in overall clothing comfort. Air permeability is amount of air passed through fabric in a certain period. It is highly concerned with the flow of air from environment to body or vise versa. More air passing means for heat and moisture exchange through convection. It depends upon the structure of the fabric, yarn compactness, porosity of the fabric, cross-section and its shapes. In addition air and moisture permeability have a great influence on thermal properties. This property is also under influence of the type of weave and chemical application. Since both can alter the surface structure. Equation of air permeability:
Where: V= Capacity of the flowing medium F= Area through which the medium is flowing * = Time of flow %p= Drop in pressure of the medium.
Thermal Equilibrium and Non-Physiological Factors Other than physiological parameters, environment is the second most important factor. It relates to air temperature, mean radiant temperature, air velocity, humidity. It may be an indoor or outdoor environment. It is a well known fact that the temperature gradient is responsible for heat exchange under natural conditions. Whereas, vapor pressure, which is measured as relative humidity is crucial for moisture transfer under normal conditions. In case of indoor environment, we mange all above four factors by using different methods. It may be heaters, air conditioners, fans, curtains, paints on the wall, humidifiers or dehumidifiers. However in case of open environment, we prefer to go where we can have environment suitable for our body. It may be sunlight in winter or shade of tree in hot summer. In addition, we adjust our clothing to have a thermal balance between body and environment. It is done by having more or taking off clothing according to the requirement. In the following lines we will discuss all above mentioned factors in detail.
Air and Mean Radian Temperature Temperature is a measure of average kinetic energy of any substance in any shape; solid, liquid, gas or elementary plasma. It is linked with the movement of speed particle and their mass. High speed of particles will have higher temperature. Mass of the particles have a direct affect of temperature. Same mass if moves faster will have more K.E. and ultimately its average K.E. will increase which can be measured as temperature of the whole particles. Kinetic Energy is core concept of mechanics and has a lot of applications. It is a product of mass and square of velocity. It is also referred as thermal energy of the whole system and generally known as heat. There are certain laws which govern heat transfer phenomenon. In SI temperature unit is Kelvin (K), which is equal to -273.15 Centigrade. In thermodynamics K is most commonly used. In clothing comfort air temperature is
one of the main elements which is hard to ignore when we are talking about the comfort. It is the parameters which leads the flow of heat and mass fro human body. Radiant temperature which is not so popular as air temperature but it is equally important for clothing comfort. It has not known to ordinary people only it is used in labs where clothing comfort is measured. As it is obvious from the name it is related to radiation. Every substance emit radiation and level of radiation depends upon its temperature along with its emissivity power. In any room, there are many things, like walls, curtains, furniture. They take energy from same environment but their radian temperature is much different from each other depending upon their thermal conductivity and specific heat. Thereby their radiation is also different which is much associated with the temperature (Planck’s law and constant). The mean radiant temperature of the things sitting in a room have overall effect on air temperature. For example, In winter, walls will become cold if they are not properly insulated. Thus they will radiate less and absorb more heat and ultimately will effect the over all temperature. It is generally denoted as Mean Radiant Temperature (MRT). MRT also depends upon the net exchange of energy (absorbency and reflection). It is proportional to the difference of temperature of both items multiplied by their emissivity.
Where: T = Surface temperature / = Surface exposure angle (relative to occupant) in degrees
MRT is an indicator of a uniform temperature of the surrounding of any object. For this reason it is measured at all possible angles so that we may have a uniform radiant temperature. That is the reason that it is not measured by a normal thermometer, rather a globe meter is used for this purpose. It is important to note that human body has very high emissivity and in measuring MRT, human body is considered as black body. For measuring purposes, 1 is taken as emissivity of balk body and then emissivity of any object is measured a ratio to black body, which is 1 at that temperature.
Emissivity of human body is .97, which is highest that any other known object available on earth. It is quite near to 1, which is a hypothetical value. Any minor change in environment, human body response in a great manner and ultimately there is a change in thermophysiological comfort, which is mainly associated with the thermal balance. Consequently, there is a great impact on physiological equivalent temperature (PET) and predicted mean values (PMV). These both factors are major contributor in overall comfort of human being. Globe meter, which is used to measure MRT is normal dry bulb thermometer in a case of 150 mm diameter matte-black copper sphere. Its absorptivity is equal to human skin. Globe meter measures MRT and it depends upon the heat transfer through convection and radiation. However, there is a possibility to adjust the effect of convection by increasing the size of the dry bulb. Consequently, we can have globe temperature (Tg) and air temperature (Ta). By increasing the air velocity, there will be small gap between air and globe temperature, or MRT temperature. Relative Humidity Relative Humidity (RH) is an indicator of presence of water vapor presence in the air. Simply it is the ratio of water vapor present in the air and the maximum amount of water vapor which air can hold at that temperature or to attain saturated state. It can be expressed as:
0 = Relative Humidity ew = Water vapor in the air mixture
e*w = Saturated water vapor pressure
There are many meters to measure RH, the most common is a meter fitted with dry and wet bulb. The difference is used to calculate the RH, which is given on chart. Application of RH measurement is quite significant. It is better than the absolute humidity which just indicate the amount of vapors present in air. It is also a known fact that there is a constant change in moisture with the change in temperature. In clothing comfort measurement, RH is preferred on absolute humidity measurement.
Human body is thermal engine and produces heat on a regular basis. A part of the heat is used to run its different functions and rest of the heat is shifted to atmosphere. Convection and radiation are the main two ways to transfer heat. In case when human body produces extra ordinary heat due to any physical activities, consumption of spicy food or extra ordinary increase in air and MRT , human body produces sweat. This sweat is evaporated and resultantly a cooling effect is generated and body achieves a thermal balance. For evaporation, the driving force is air velocity and RH.
In case when RH is quite high, the mass transfer (transformation of water into vapors) becomes very slow. However, when there is a dry environment, water evaporates quickly. It is generally observed that people feel less comfort in humid atmosphere as compare to highly humid atmosphere. Due to this human body feels any change in RH very strongly and sends signals of comfort or discomfort to brain. It is also important to note that presence of water on human skin increases its skin temperature, since water takes a lot of heat generated by the human body and holds it to increase its temperature. This way we find a regular storage of heat on the skin. In such case the comfort is only possible by removing the moist from the skin by using air velocity or by changing RH in the environment. In such cases we use fan or air conditioning machine to reduce the skin temperature which provides ultimately comfort.
Bending Rigidity B= E.I I= b.h3 By increasing height bending rigidity increase three times Power (W)= Amp* Volts (I*V) Air velocity (#)1/Moisture resistance Absolute water vapor permeability= Pa m2. W-1
Air Velocity and Comfort
Air circulation or movement increases heat transfer due to convection. Nevertheless, air movement also serves many other purposes, like, removal of odor or any other unpleasant smell. It also takes in fresh air which has normally higher percentage of oxygen and desired fragrance. If there is no air circulation, then there are bright chances that unpleasant smell will increase and uncomfort will be observed. In addition our skin also needs oxygen.
Table 08: Air Velocity and Reaction
Heat/Mass Transfer and Role of Textile Özdi et al.2007) have studied impact of different properties of yarn on thermal properties and have linked it with clothing comfort. They conclude that combed and fine yarns have lower thermal conductivity higher vapor permeability. Fabric made from such yarn give warmer feeling. Such fabrics also have lower thermal absorptivity. Literature provides many more studies which explain the correlation between thermal properties of fabric and warm and cool feeling.
Thermal of fabric governs the flow of heat from human body to environment and vise versa. A man in desert where the temperature is at its peak will prefer clothing which should not allow heat to cross the clothing layers from outside to inner side. Since outer temperature is quite high as compared to body temperature. In such case best clothing are which have lowest thermal conductivity as well as lower thermal absorptivity and higher resistance. Such clothing will give
cool effect. However, in a case where temperature is freezing, a person needs such clothing which should not allow to pass his or her body heat from inner side to environment to keep the body warm. In such cases we also need a clothing with lower thermal conductivity, lower thermal absorptivity and higher resistance. Such clothing will give a warm feeling. All above discussion, supports the idea that clothing with low thermal conductivity and lower thermal absorptivity and higher resistance are most suitable. Cotton, which is most common in use and highest percentage of consumption has the lowest values of thermal conductivity and thermal absorptivity.
The second factor which plays an important role in keeping our body warm is the capability to trap air. We prefer wool in winter because it keeps our bodies warm in winter. Wool has almost same thermal conductivity as cotton but it gives higher warm feeling. It is mainly due to the wool fiber structure which is highly capable to capture air. It is a known fact that air has the lowest thermal conductivity and protect human body from cool. In case the wool is fine and have a dense weaving it also gives cool effect.
Polypropylene is a synthetic fiber which possess the lowest thermal conductivity almost near to cotton and wool. It is much appreciated in making socks, legging, vests etc to give a warm feeling in winter. One can find clothing made of PP in market, even denim trousers, sweat shirts are also available.
Above discussion leads to the fact that if we want to keep ourselves warm, we need a material with low conductivity and capability to trap air. For this purpose fleece made of PP is the best product. Even fleece made of polyester also serves this purpose. In addition PP and polyester does not lose their thermal conductivity behavior when they are wet. Whereas, cotton shows a higher thermal conductivity when it gains moisture. This is the main reason that people have developed fleece made of polyester and PP inside so that it keep its thermal conductivity intact even there is a moisture. PP is more effective in this case since its moisture absorbency is near to zero and it receives no impact of moisture and shows low thermal conductivity.
Multi Fiber Fabric in Clothing Comfort As it is obvious from Tables (1,2,3) that different fibers have different properties. It is not possible to construct any fabric able to work in every situation due the huge variation and diversity in their fundamentals characteristics. However, it is quite possible to blend different fibers to get most
optimum results. Blending of two different fibers to make yarn is quite common. Another way is to use two or more than two different yarns in fabric formation. Along with that adhesion of two or more than two lyres of fabrics is another way to have better results. Different yarns on opposite sides is another solution. Moreover, filling of fiber between two layers of fabrics also serves certain demands. Filling of staple fiber in quilts is a common example.
Wu et al.2009) and Wu and Fan (2008) have conducted studies to evaluate impact of blending of different fibers and concluded that it is quite possible to achieve better results by combining more than one fibers. Wu et al.2009) tested 10 different shirts made of different fibers and found that there is a great variation in the results which is primarily due to fiber properties. They did subjective evaluation by following proper protocol in a controlled chambers. It was observed that natural fibers are more damper and more thermal than regenerated fibers. Wu et al.2009) have proposes that clothing made by assembling of different fabrics exhibits much better thermal and moisture balance and boost overall clothing comfort perception. Wu and Fan (2008) put different layers to develop a unique fabric and found that hygroscopic pads plays a significant role in controlling moisture management. This is mainly due to contrast thermal and moisture control properties of different fibers.
Modification of Fibers and Thermal Parameters Although chemical structure of fibers is the core factor responsible for heat transfer in clothing comfort process. Largely, our focus is to have blend of different fibers. Schacher et al.2000) have studied the difference between traditional polyester and microfibers. They report that microfibers exhibits low thermal conductivity while keeping all other parameters same. Lower conductivity provides a warm feeling which in some cases highly desired. This study is an evidence that even having same chemical structure, change in the physical structure helps improve the moisture and thermal management. Tzanov et al.1999) conducted tests of different fabric after treating them with various chemicals. They conclude that surface modification affects thermal properties of fabrics.
Fabric Properties and Comfort There are number of properties of fabric which plays a significant role in overall comfort. Some are listed below:
15. Bending rigidity
16. Flexural rigidity 17. Thickness 18. Grams per meter square (density) 19. Surface smoothens 20. Compressibility 21. Stretchability 22. Shear rigidity 23. Bulkiness 24. Surface friction 25. Warm and cool effect 26. Air permeability 27. Moisture permeability 28. Absorbency
Clothing Moisture, Thermal and Air Transfer at Wetting State Water has higher thermal conductivity as compare to most of natural and man-made fibers. Moreover, when fibers gains moisture, consequently their thermal conductivity also increased and thermal resistance drastically decreased. Hes (2008b) has conducted an in-depth studies and conclude that moisture replaces air from the fabric pores. It is a known fact that air has quite less thermal conductivity as compared to water. Moreover water is absorbed by certain fibers and this also increase the thermal conductivity.
From the presented results follows, that with increasing fabrics humidity their thermal resistance can even significantly decrease. This is caused by substituting of the air in pores by water with higher thermal conductivity. The ALAMBETA instrument proved to be suitable for the investigation of thermal properties of fabrics in wet state. It was found, that some fabrics with certain structure and composition keeps higher level of thermal resistance even in wet state. Hes (2008b) also identified that air and moisture permeability decreases. Nevertheless it was found that in some fabrics change is not so significant and it is compensated with other phenomenon. Effect of moisture on thermal, moisture and air permeability properties needs a great attention of the fabric designers since it can alter the overall clothing comfort level.
Objective Measurement of Clothing Comfort: An Effort for Precision
The significant of objective measurement is becoming popular even there are many questions of its accuracy and adaptation are attached with this concept. There are a number of testing machines have been made to have better results. NCSU have developed a sweating manikin to measure effect of moisture on the thermal balance. Such system was also devised by Meinander. Kuklane et al. (2004) have reviewed different manikins based on their sizes and functions. This review provides quite useful information and reveals the struggle to have better results from testing before conducting subjective evaluation.
Barker (2002) raises point of complexity of human body and mind, while we are applying scientifically based approaches for objective measurement of fabric or its next shape clothing. It should be given a due weight. Because comfort is not a function of some mechanical properties of fabric tested in isolation. There is a strong need to intervene the human perception process, which is quite influenced by many factors associated with people and have a significant variation from person to person. Even same person would have different reaction if there is a change in context. Human will use his or her personal feelings, preferences, experiences, state of mind, level of emotion, education, training as filters before reaching on any conclusion.
Study of Kuklane et al. (2004) points out performance variation problem associated with manikins which are widely used in objective measurements. There is a drastic but acceptable variation in the structure of male and female manikins. It shows that in real life difference between the bodies of male and female is also visible even in manikins. Such observations support the point that reliance on objective measurement is not enough to declare any product best fit for human being. This study recommends that there is need of utmost care while interpreting the results by using different manikins of different sizes and of different fitness along with of male and female.
There are thermal manikins which are equipped with a system to produce moisture from different parts. Barker (2002) reports that manikin made at NCSU can produce moisture from 189 different
points (human body has more than 2.6 million sweat glands). Thermal manikins are used to measure moisture and heat loss while having different type of clothing.
There are a number of tests under practice to test the clothing comfort. All these tests are carried out to measure some mechanical and electrical changes. Nevertheless all such tests are being adopted after doing many simulations. A number of tests by using some subjects were found having a significant correlation. Thereby, we can rely on these tests to a certain length. There is a long list of such instruments. However, we will list down here some most common instruments being used to predict clothing comfort.
Kawabata Evaluation System (KES) KES can measure the following parameters of fabric: 29. Warm cool feeling 30. Smoothness of surface 31. Bending rigidity 32. Shear rigidity
KES work on different principles. For example, it was investigated by Kawabata that there is significant correlation between transient heat flux and the warm or cool effect. Base on this principle, KES measure the warm or cool effect of the fabric. Alambeta
Dr. Lubos Hes has patent of this testing machine. It measure the thermal conductivity coefficient, heat transfer resistance, height of the fabric, heat flux peak, thermal absorptivity. In this machine fabric is packed between the two plates which have 10 degree difference of temperature. During a short period contact heat flow takes place and all parameters are measured.
Permetest Dr. Lubos Hes has its patent and the main function of this machine is to measure vapor permeability of fabrics.
Figure 22: Fabric Touch Tester J.Y. Hu, Lubos Hes Y. Li, K.W. Yeung and B.G. Yao have developed “Fabric Touch Tester (FTT)”, which is used to integrate evaluation of thermal–mechanical sensory properties of polymeric materials. This instrument measures, records and analyses thermal and mechanical sensory stimulations and propagations occurred while clothing becomes in touch with the human skin. FTT can be used to predict many mechanical properties of clothing, like smoothness, softness, prickliness, warmth and dampness. In stead of the highest precision level of FTT, even then there is a need of subjective evaluation of the clothing.
Figure 23: Fabric Touch Tester
Hohenstein Hohenstein Institute working since 1946 and provides services to measure clothing physiology. This institute is using different methods of clothing physiology to develop different products. For example, items of clothing, as well as bedding and sleeping bags, shoes, socks etc. (for information:www.hohenstein.de)
Figure 24: Working at Hohenstein Institute
Sensorial Comfort Testing at NCSU NSCU has developed a testing machine which tests the sensorial comfort. It measures moisture transport and vapor buffering capacity. There is a dynamic sweating plate in this machine, which is used to assess the impact of microclimatic changes in fabric due to pulsed heat and moisture changes. Finally changes in micro climate and dynamics of fabric are processed and based on the results sensorial comfort can be predicted.
Figure25: Dynamic Sweating Machine at NCSU
Moisture and Wicking Test NCSU has developed a Gravimetric Absorbency Testing System (GATS) to measure the moisture presence and its wicking from the sweating skin. This testing machine works on the principle of demand wet-ability. From this machine we can measure the ability of fabric to transport moisture from skin to environment.
Figure 26: Gravimetric Absorbency Testing System at NCSU
Figure 27: Sweating Manikin used at NCSU
Figure 28: Walking manikin used by XIaoming Qian and Jintu Fan
Nondestructive Testing of Clothing Hes (2008a) has provided details of the some instruments which are capable to carry tests without destruction of the clothing. These tests are possible to conduct after making clothing.
Subjective Measurement and its Limitation As discussed in previous pages that there are a number of machines available to measure different thermal and mechanical properties of different fabrics. Nevertheless these tests are all incredible. There is one short coming associated with these tests that they cannot predict the overall comfort. They can be used to predict certain properties. For example, wicking properties, surface smoothness, etc. Even though there are many factors which are missing, particularly, fitting and cutting of clothing. If there is designing or fitting problem, this will supersede all mechanical and thermal comfort.
The second important point is that the human being which is final user of the product itself has immeasurable variation. One temperature may suits to a young person but may not to an old person. Keeping all difficulties in view subjective testing by using human being as a subject is the most common method applied to measure clothing comfort. If we find a strong correlation between two then it is considered as the success of the tests.
Subjective measurement is not free from errors. Rather it has more chances of errors. However, in spite of many drawbacks, it is the final way to test the clothing. The main issue is the creation of the environment which has a great analogy with the real world. We can maintain temperature, humidity, air flow in a closed chamber. In real world there is a huge dynamical changes in all three parameters. There is no way to accommodate such changes. Clothing may be highly comfortable at a certain temperature and humidity, but sudden changes may make it uncomfortable. The second issue is the selection of the subjects. Every human being is quite influenced by the age, experiences, metabolic rates, mental and physical health, education, training, etc.
There are many places where subjective evaluation is being carried out. They have tried their level best to have more accurate results by developing highly controlled chambers and doing lot of care
in selecting of people. This evaluation is mainly used to correct the clothing and fabric and this leads to develop new fabrics and clothing.
NCSU has developed a well equipped labs to evaluate the fabric and clothing through subjective evaluation. For this purpose they use different protocols. In some cases five-period test sequence is used (activity-rest). There is no final and uniformly acceptable protocol, rather it depends upon the type of clothing and institute itself.
Table 09: Overall Comfort Rating
Model for subjective evaluation provided by Jason Wickwire and associates
Literature provides many reports which are big critics of subjective evaluation. Cent and Dearb (2001) consider that chamber developed to measure subjective perception are not a successful experiment, rather such chambers fail to provide an experimental realism. They prefer field studies, which of course are not free from errors, but their error is much less than the chamber evaluation. The main difference is absence and deletion of contactual factors, which are mainly associated with the human. The emotion, competing sense, adapting attitude, adjustment of posture and clothing are few factors which cannot be taken in chambers. Subjects selected in chambers lack in such matters and they just try to perform as a machine without any sort of soft issue of the evaluation.
Models for Clothing Comfort Prediction
Wong and Li (ND) have developed a hydride model. In my personal view this looks one of the best model which have taken maximum factors. Writers have used objective and subjective testing, both to develop this model. They developed 33 different fabrics and carried out their tests. For subjective testing they involved 38 adults and followed the standard procedures and protocol. They used three different models (HM-LM, HM-NN and HM-FL). In this model writers measures eight factors (objective measurement, five mechanical and three related to transport of mass and heat). They took three sensory factors (thermal-wet comfort, tactile comfort and pressure comfort). Researchers found a high correlation between objective and subjective predictions and summed the whole work in the following words, “Based on the study in this paper, it can be concluded that fabric physical properties can be used to predict overall clothing comfort with hybrid predictive models….
Figure 29: Hybrid Modeby Wong and Li (ND) Raj and Sreenivasan (2009) Raj and Sreenivasan (2009) have successfully developed index to predict the clothing comfort.
Sampling scheme (Raj and Sreenivasan, 2009)
Raj and Sreenivasan (2009) present their findings which are quite invaluable. This outcome can be used to predict the overall clothing comfort behavior. Summary of their result is as under.
Fabric made of yarn which has been produced by using fine and longer fiber exhibits better extensibility and higher tensile energy and such fabric need less bending force and have lower compressional energy. Moreover, such type of fabric has lower bending recovery. Researcher also point out that increase in twist can increase bending rigidity, extensibility, compressional energy, and residual strain. On the other hand, it will decrease the shear rigidity, hysteresis, and tensile resilience. Considering this observation it can be inferred that twist level can be used to predict certain quality parameters of the fabric. Research further explains that finer yarn is source of better smoothness and softness, which plays an important role in overall comfort.
Raj and Sreenivasan (2009) have also measured impact of different weaving types and found a strong correlation between weaving design and mechanical properties. They have also studied affect of vapor permeability, fabric thickness, fabric surface friction on comfort. Considering all this, they have presented a Total Wear Comfort Index, which can be used to predict overall clothing comfort.
All above discussion provides a convincing arguments in the favor development of some models able to predict overall clothing comfort. This effort is still on and in our view these model can serve purpose to a certain extent. These models can be used to develop new fabrics. The matter of fact is that subjective evaluation along with the objective testing of the product is the most reliable method to predict the overall comfort level, which could have more than 80% accuracy and acceptance level length.
Overall Comfort: A Desired Outcome
All above discussion is to explain the clothing comfort phenomenon and factors affecting it. As concluded by Barker (2002), objective and subjective measurements, both are incredible. They are capable to server certain purposes. Although non of these is fully able to predict the overall comfort. Testing machine can tell thermal conductivity of any fabric, friction of the fabric surface and many things more. Human can give his or her observation about the skin sensation, warm or cool effect, etc. but not the overall comfort. We cannot ignore the role of cutting and designing, fitting. Human body has lot of shapes and there is a big variation among women, men, young people, kids and even people living in different parts of the world.
Table 10: Wear Trail Protocol at NCSU
Baker (2002) provides list of different sensations. It is obvious from the radar chart that perception about different factors are is quite different from each other.
Figure 30: Human Sensation Source: Baker (2002)
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