Cooling Load Estimation of a Typical Class Room

COOLING LOAD ESTIMATION OF A TYPICAL CLASS ROOM

BY

UTAZI NDUBUISI DIVINE PG/M.ENGR./08/48726

A PROJECT PROPOSAL PRESENTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING UNIVERSITY OF NIGERIA, NSUKKA

SUPERVISOR  PROF.D.C.ONYEJEKWE

 NOVEMBER, 2009 1

ABSTRACT

Cooling load calculations are carried out to estimate the required capacity of cooling systems. The purpose of this project is to develop de velop a user-friendly program that can easily calculate spacecooling load of a typical class room taking some of the basic inputs like latitude, longitude time zone, building materials and other metrological data of the location.

This thesis uses the cooling load temperature difference (CLTD)/solar cooling load (SCL)/cooling load factor (CLF) (CLF) method. In the CLTD/SCL/CLF method, the CLTD will be used to calculate the sensible cooling load for the exterior wall and roofs. SCL represents the  product of the solar heat gain at that hour and the fraction of heat storage effect due to various types of room construction and floor coverings. CLF will be used to calculate internal sensible cooling loads. Beside, the project uses number nu mber of assumptions proposed by ASHRAE for its calculation of cooling load.

Finally, cooling load of an actual class room in University of Nigeria Nsukka will be computed using the developed program, and compared with manual version of CLTD/SCL/CLF Method.

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INTRODUCTION

Cooling load calculations are carried out to estimate the required capacity of heating and cooling systems, which can maintain the required conditions in the conditioned space. To estimate the required cooling capacity, one has to have information regarding the design indoor and outdoor  conditions, specifications of the building, and specifications of the conditioned space (such as the occupancy, activity level, various appliances and equipment used etc.) and any special requirements of the particular application. For comfort applications, the required indoor  conditions are fixed by the criterion of thermal comfort, while for industrial or commercial applications the required indoor conditions are fixed by the particular processes being performed or the products being stored. The design outdoor conditions are chosen based on design dry bulb and coincident wet bulb temperatures for peak summer or winter months for cooling and heating load calculations. For estimating cooling loads, one has to consider the unsteady state processes, as the peak  cooling load occurs during the day time and the outside conditions also vary v ary significantly throughout the day due to solar radiation. In addition, all internal sources add on to the cooling loads and neglecting them would lead to underestimation of the required cooling capacity and the  possibility of not being able to maintain the required indoor conditions. Thus cooling load calculations are inherently more complicated as it involves solving unsteady equations with unsteady boundary conditions and internal heat sources.

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The total building cooling load consists of heat transferred through the building envelope (walls, roof, floor, windows, doors etc.) and heat generated by occupants, equipment, and lights. The load due to heat transfer through the envelope is called as external load, while all other loads are called as internal loads. The percentage of external versus internal load varies with building type, site climate, and  building design. The total cooling load on any building consists of both sensible as well as latent load components. The sensible load affects dry bulb temperature, while the latent load affects the moisture content of the conditioned space. Buildings may be classified as externally loaded and internally loaded. In externally loaded  buildings the cooling load on the building is mainly due to heat transfer between the surroundings and the internal conditioned space. Since the surrounding conditions are highly variable in any given day, the cooling load of an externally loaded building varies widely. In internally loaded buildings the cooling load is mainly due to internal heat generating sources such as occupants or appliances or processes. In general the heat generation due to internal heat sources may remain fairly constant, and since the heat transfer from the variable surroundings is much less co mpared to the internal heat sources, the cooling load of an internally loaded building remains fairly constant. Obviously from energy efficiency and economics points of view, the system design strategy for an externally loaded building should be different from an internally loaded building. Hence, prior  knowledge of whether the building is externally loaded or internally loaded is essential for  effective system design.

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As mentioned before, the total cooling load on a building consists of external as well as internal loads. The external loads consist of heat transfer by conduction through the building walls, roof, floor, doors etc, heat transfer by radiation through fenestration such as windows and skylights. All these are sensible heat transfers. In addition to these the external load also consists of heat transfer due to infiltration, which consists of both sensible as well as latent components. The heat transfer due to ventilation ven tilation is not a load on the building but a load on the system. The various internal loads consist of sensible and latent heat transfer due to occupants, products,  processes and appliances, sensible heat transfer due to lighting and other equipment. Figure  below shows various components that constitute the co oling load on a building.

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OBJECTIVES Traditionally, load estimating for air conditioning systems is done either by manual calculation or judgmental estimation based on experience of the air conditioning practitioner. While manual calculation is laborious, estimate based on judgment is liable to error due to gigantic, complex and dynamic nature of present day architectural designs. Load estimating through computer  automation is likely to make a positive impact in the dynamic nature of air conditioning applications. The goals of this project are to:

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Develop a program that can easily calculate space-cooling load of a typical class room.

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Develop a graphic user interface interface (GUI) - To make the programs that will be developed user-friendly type.

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Calculation of cooling load of an actual building in University of Nigeria Nsukka with the developed program.

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Compare the results of the manual and an d computer-oriented versions of the CLTD/SCL/CLF Method.

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SCOPE The work to be carried out can be summarized as follows:

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Write algorithms of the problem.

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Draw the flowchart of the problem.

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Develop the program that can calculate cooling load of a class room with computer   programming language.

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Develop a graphic user interface(GUI) - To make the programs that will will be developed user-friendly type, GUI (graphic user interface) will be developed, So that any person with out knowing the detail of the program can run and have the cooling load of a class room.

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LITERATURE REVIEW

So much work has been done on this topic among the important ones, we mention: In 1937, ASHVE [1] Guide introduced a systematic method of cooling load calculation involving the division of various load components. In the ASHVE Guide, solar radiation factors were introduced and their influence on external walls and roofs was taken into consideration. co nsideration. Both the window crack and number-of-air-changes methods were u sed to calculate infiltration.

Mackey et al [2] first introduced the concept of sol-air temperature in 1944. In the same paper, they recommended a method of approximating the changes in inside surface temperature of walls and roofs due to periodic heat he at flow caused by solar radiation and outside ou tside temperature with a new decrement factor. In 1952, Mackey et al [3] analyzed the difference between the instantaneous cooling load and the heat gain owing to radiant heat incident on the surface of the building envelope.

In 1964, Palmatier [4] introduced the term thermal storage factor to indicate the ratio between the rate of instantaneous cooling load in the space and rate of heat gain. One year after, Carrier  Corporation published a design handbook in which the heat storage factor and equivalent temperature difference (ETD) were used to indicate the ratio of instantaneous cooling load and heat gain because of the heat storage effect of the building structure. This cooling load calculation method was widely used by many designers until the current ASHRAE methods were adopted.

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In 1967, ASHRAE [5] suggested a time-averaging (TA) method to allocate the radiant heat over  successive periods of 1 to 3 h or 6 to 8 h, depending on the construction of the building structure. Heat gains through walls and roofs are tabulated in total equivalent temperature differentials (TETDs). In the same year, Stephenson et al [6] recommended the thermal response factor, which includes the heat storage effect for the calculation of cooling load. The thermal response factor evaluates the system response on one side of the structure according to random temperature excitations on the other side of the structure. This concept had been developed and forms the basis of the weighting factor method (WFM) or transfer function method (TFM) in the 1970s.

In 1977, ASHRAE [5] introduced a single-step cooling load calculation procedure that uses the cooling load factor (CLF) and cooling load temperature difference (CLTD); these are produced from the simplified TFM.

Because of the wide adoption of personal computers, since 1980s, the uses of computer aided heating, ventilation air-condition and refrigeration (HVAC&R) was rapidly increased and various softwares have been developed that involved different assumptions. Among the recent ones are Energyplus, BLAST, HBfort, IBLAST, DOE-2 are worth mentioning. The above softwares differ from one another in the treatment of the heat gain in to cooling load.

Softwares like Energyplus, BLAST, IBLAST and HBfort use heat balance method for their  calculation of cooling load. Heat balance method implements the following basic assumptions: •

The air in the thermal zone is well mixed.

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Uniform surface temperatures.

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Uniform long wave (LW) and short wave (SW) irradiation.

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Diffuse radiating surfaces. One-dimensional heat conduction within the walls.

Within the frame work of the above mentioned assumption heat balance method treat the whole  problem by dividing the various heat gains g ains into outside face heat balance wall conduction  process, inside face heat balance and air heat balance. There are various methods for solving heat conduction in the wall. Energyplus software uses conduction transfer function for the transient analysis of wall conduction. Windows are described layers by layers as solid panes (glass, plastic film, etc) separated by gaps containing a gas fill (air, argon, krypton, etc) in Energyplus software. This program accounts for the temperature depend ence of the conductance of the glass fills. This method did not account for the number of times the windows are opened.

Hourly analysis program uses transfer function method for solving heat conduction across the wall. The Transfer Function Method is the culmination of work first published in 1967 by two scientists working for the Canadian National Research Council. The method is based on an idea known as the "Response Factor Principle". This principle states that for a specific room, the thermal response patterns (i.e., how the heat gain is converted to load over ov er a period of time) for  each specific type of heat gain will always be the same. The Response Factor Principle is in turn  based on three additional principles: •

The Principle of Superposition: The total room load is equal to the sum of loads calculated separately for each heat gain component.

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The Principle of Linearity: The magnitude of the thermal response to a heat gain varies va ries linearly with the size of the heat gain.

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The Principle of Invariability: Two heat gains of equal magnitude occurring at different times will produce the same thermal response in a room.

These principles allow the simplification of the Heat Balance Method analysis for a building.

In DOE-2, thermal loads are calculated by ap plying room weighting factors calculated in a  preprocessor, to hourly instantaneous heat gains from solar radiation, conduction, lights and  people / equipment. However, since the weighting factor method assumes time invariant room  properties, its accuracy is limited compared to the heat balance method, which allows timevarying properties. Some of the resultant limitations of the weighting factor method are: •

It assumes a constant value for inside air film cond uctance, which can over- or  o r  underestimate the rapidity with which heat stored in the thermal mass of a zone appears as a load.

In contrast, the heat balance method allows this conductance to vary with time depending on surface-to-air temperature difference, direction of heat flow and supply airflow rate. •

It assumes a constant distribution for solar radiation absorbed by inside surfaces.

In this thesis, CLTD/SCL/CLF method, which is a revised version of CLTD/CLF method, will  be used in developing user-friendly program that can calculate cooling load of a class room.

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METHODOLOGY For the purpose of this work, the cooling coo ling load temperature difference (CLTD)/solar cooling load (SCL)/cooling load factor (CLF) method is used. The CLTD/SCL/CLF method first calculates the sensible cooling load based on o n the transfer function method (TFM). The result is divided by the U value, shading coefficient, or sensible heat gain to generate the CLTD, SCL, or CLF. Thus, it provides a direct, one-step space cooling load calculation instead of a heat gain–cooling load conversion, a two-step calculation in TFM. In the CLTD/SCL/CLF method, the CLTD is used to calculate the sensible cooling load for the exterior wall and roofs, SCL factor represents the product of the solar heat gain at that hour and an d the fraction of heat storage effect due to various types of room construction and floor cov erings. CLF is used to calculate internal sensible cooling loads.

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ASSUMPTIONS Design cooling load takes into account all the loads experienced by a building under a specific set of assumed conditions. The assumptions behind design cooling load are as follows: •

Weather conditions are selected from a long-term statistical database. The conditions will not necessary represent any actual year, but are representative o f the location of  the building. ASHRAE has tabulated such data.

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The solar loads on the building are assumed to be those that would occur on a clear  day in the month chosen for the calculations.

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The building occupancy is assumed to be at full design capacity.

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The ventilation rates are based on maximum occupancy expected.

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All building equipment and appliances are considered to be operating at a reasonably representative capacity.

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Lights and appliances are assumed to be operating as expected for a typical day of  design occupancy.

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The latent heat gain is assumed to become cooling load instantly, whereas the sensible heat gain is partially delayed depending on the characteristics of the conditioned space.

REFERNCES

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1. Shan K. W., Handbook of Air conditioning and Refrigeration, 2nd edition, McGraw-Hill,

New York, 2001.

2. Mackey, C. O., and Wright, L. T., Periodic Heat Flow—Homogeneous Walls or Roofs,  ASHVE Transactions,

1944, pp. 293–312.

3. Mackey, C. O., and Gay, N. R., Cooling Load from Sunlit Glass, ASHVE  ASHVE Transactions,

1952, pp. 321–330. 4. Palmatier, E. P., Thermal Characteristics of Structures, ASHRAE Transactions, 1964, pp.

44–53. 5. Rudoy,W., and Robins, L. M., Pulldown Load Calculations and Thermal Storage du ring

Temperature Drift, ASHRAE Transactions, 1977, Part I, pp. 51–63. 6. Stephenson, D. G., and Mitalas, G. P., Cooling Load Calculations by Thermal Response

Factor Method, ASHRAE Transactions, 1967, Part III, pp. 1.1–1.7.

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