HVAC - A Beginner's Guide

HVAC HVAC is an acronym for Heating, Ventilation and Air-Conditioning. An HVAC system maintains air purity, air temperature and relative humidity (RH). HVAC systems are implemented in office, manufacturing units or homes. The basic purpose of having an HVAC system at homes and theatres will be only for comfort purpose, whereas in a pharmaceutical or textile industry all parameters need to be monitored and maintained. The various equipment, which are used to implement HVAC, is DX (Direct Expansion) units, AHU (Air Handling units), VAV (Variable Air Volume) units, Chillers and Heaters. The basic model that can be used to explain the HVAC cycle is the room airconditioner, which is familiar to all. This uses the same principle, which a centralised chiller of say 300 tonnes of air conditioning capacity would use. The various components of the room airconditioner is: 1. Evaporator ( the coil you see in the front). 2. Compressor ( the one that makes the most noise!) 3. Condensor ( the one that blows the hot air on the other side of the room) 4. Expansion: (May not be visible on first sight.) The following cycle will explain how the above components work as a part of the whole cycle. R Reeffrriiggeerraattiioonn C Cyyccllee:: The refrigeration cycle in a cooling system is as shown in the diagram below.

The evaporator has a cooling coil filled with the refrigerant in liquid form. The refrigerant is chosen based on it’s property to change state ( evaporate) at room temperature, i.e. conversion from a liquid form to a vapour form takes place at room temperature ( say 30 degreesC). In technical terms, it’s boiling point is very low.(Similar to any perfume/after shave lotion). Remember the fundamental fact: Evaporation causes cooling! So, in order to cause cooling, we ‘evaporate’ the refrigerant by causing it to get in contact with air at room temperature in a heat exchanger ( evaporator).Heat exchange happens between air in the warm area (or the area to be cooled) and the liquid refrigerant. The refrigerant goes to the Vapour State as it has absorbed heat and the room is cooled. We need to bring back the refrigerant to it’s liquid state, so that we can continue the cooling cycle above. For that we need the refrigerant to reject the heat it has absorbed from the conditioned space. But the problem is that it is already at ambient temperature and so there is no temperature difference for heat transfer to take place. So,

this

vapour

is

compressed

(in

the

compressor) to increase its pressure ( and therefore it’s temperature, i.e high pressure, high temperature) so that it is now ready to reject heat. In the condenser stage the heat absorbed by the refrigerant is rejected, by passing the hot refrigerant gas through a heat exchanger ( Condensor) where ambient air is used to cool the refrigerant from it’s hot vapour stage to liquid stage.( Low temperature, high pressure). The next stage is expansion where the refrigerant in the liquid+gaseous stage ( from the condensor) is transformed into the liquid stage ( low pressure, low temperature). This cycle is continued . The above example is a Vapour Compression Cycle, which is most commonly used. Another method of providing cooling is the Vapour Absorption Cycle, which is slowly gaining acceptance in certain applications. This uses, steam as one of the components to produce cooling! ( Thermax are one of the people offering this system where steam is required for process and cooling is required for comfort applications). That is another story, another time!

There are 2 classification of air conditioning systems, namely the DX system and Centralised System. D Diirreecctt E Exxppaannssiioonn ((D DX X)) U Unniittss:: These units are called as DX units because the refrigerant directly cools the warm area ( through a heat exchanger called, you are right,

Evaporator + Expansion units

Condensor + Compressor

the evaporator). Window AC’s ,Split AC’ s, Packaged Units(some of them) are examples of DX units. In a split AC the evaporator with the expansion valve is present inside the room, whereas the compressor & condensor components are present outside the room. In case of Split AC, normally there can be no provision for fresh air to enter the room. But in window AC there is a provision for letting in fresh air. The amount of fresh air required by a person is 15 cfm (cubic feet /minute).

Normal range of window A/C’s (above picture) as you are aware is 0.5TR, 0.75 TR, 1 TR, 1.5 TR, 2 TR. ( TR?-see glossary). These are, as the name suggests, wall mounted.Split units: 1 TR, 1.5 TR, 2 TR. ( non-ductable), upto 7.5 TR ( ductable). These are normally ceiling mounted.Packaged units are slightly bigger in size ( in capacity of cooling) and in the range of 5 TR, 7.5 TR, 10 TR, 15 TR. One difference in

packaged units is that the evaporator with expansion valve and the compressor is in one unit and the condensor unit is outside. These are normally floor mounted. Package Unit :

R Roollee ooff B BM MS S iinn D DX X uunniittss C Coonnttrrooll & &M Moonniittoorriinngg:: ♦ Time-Schedule based On/Off of DX units is possible. ♦ Common fault,blower status and heater status can also be monitored in case of Precision AC’s C Ceennttrraalliisseedd S Syysstteem m:: This is for larger capacities of cooling requirements. It uses a. A Chiller to produce the cooling centrally b. Ahu’s ( Air Handling Units) to provide cooling locally, by using the cooling energy produced at the Chiller. The Chiller and it’s components are usually referred to as the ‘High’ side and the Ahu’s and the components are referred to as ‘Low’ side of the HVAC contract. C Chhiilllleerr:: Each chiller is controlled by a microprocessor-based control panel. Each chiller has its own evaporator, compressors, condenser and expansion components. (In other words an expanded room air-conditioner, expanded multifold times.) The number of compressors in a Chiller is usually 2 but it may also vary with the manufacturer & size of chiller. The water used in a chiller system is DM (demineralized) water. In a chiller the refrigerant cools water (called Chilled Water), by using the same DX principle as that of a room air conditioner/split or packaged unit. The only difference is that the capacity of the compressor is much more. This Chilled water is then pumped into the Heat Exchanger of the AHU ‘s (called ‘Cooling coil’).

This Chilled water which is at a temperature of 5.5 degree centigrade to 6 degree centigrade then cools the air from the conditioned space. Hence in a chiller system the refrigerant does not directly cool the warm area.

The above diagram shows 3 chillers. The primary pumps are used to pump the water into the chillers. Depending upon the amount of cooling required the compressors in each chiller is switched on or off. The secondary pumps pump the cool water coming out of the chillers into the cooling coils of the various AHU’s present in the building. There are basically 2 types chilled water distribution. 1) Constant flow and 2)Variable flow ( above drawing). In case of constant flow a constant amount of chilled water is sent from the chillers into the cooling coils of the AHU. (Just remove the secondary pumps in the above sketch and you have a constant flow system). In case of variable flow system,

apart from the primary pumps which circulate constant flow through the chiller, secondary pumps are present which vary amount of water flowing into the cooling coils of the AHU’s. The refrigerant in the chiller is either air-cooled or water-cooled. In case of water-cooled chillers the cooling tower is used to cool the water which cools the refrigerant. The cooling tower has fans. There is make-up water available to make up for any loss of water. Water Cooled Chiller:

Air Cooled Chiller:

Water Cooled Chiller :

R Roollee ooff B BM MS S iinn C Chhiilllleerrss C Coonnttrrooll & &M Moonniittoorriinngg:: ♦ The cooling demand required to be produced by the chiller will depend on how much of chilled water the chiller is producing is being used by the Ahu’s. A measure of the load condition is the difference between the supply and return header temperatures. By measuring this difference, the load can be determined and energy saving means adopted, as will be illustrated below: ♦ If there is only one chiller, the microprocessor of the chiller will receive inputs of the supply and return header temperatures, ( apart from safety interlocks like water flow, refrigerant pressures etc) and based on the difference, a decision on how many compressors can be switched on will be determined. ♦ Example: Let us take the case of 2 chillers, each with it’s own microprocessor panel. In case of a 50% load ( meaning requirement of one chiller only), each chiller will operate with one compressor each ( assuming two compressors per chiller). This is because the microprocessor of chiller 1 will not know that there is another microprocessor panel of chiller 2 next to it. ( Assuming no BMS or any other controls from Chiller manufacturer). Each chiller will switch off one compressor, as both the chillers have received same supply and return header temperature input. Please Note: Whether BMS is there or not, the chiller manufacturer will use temperature sensors in supply and return line of the chiller as inputs to the microprocessor. Panel. Coming back to our above example:This creates a problem because, we have a situation where two chillers are operating on 50% load conditions. Most chillers’ efficiency drop drastically if they are operated below 80% load conditions. So, we can use the BMS to switch off one Chiller and to make the other chiller run to cater to 50% load, which in this case would be operating both compressors of that particular chiller. Also, one chilled water pump can also be shut down.The same example can be extended to more number of chillers. Please note: The decision to run the compressors of the chiller is taken by the micrprocessor panel of Chiller and not by BMS. BMS will only decide whether one or more chillers need to run.The operating of the compressors of the chiller, is referred to in HVAC industry, as ‘loading’ and ‘unloading’ of the chiller. ♦ While switching off the chiller, The chiller is first switched off, then primary pumps and then the butterfly valve is closed. When again switching on the chiller the reverse sequence is followed.

A Aiirr H Haannddlliinngg U Unniitt:: The filter in the AHU filters the dust particles in the air from the return air duct and fresh air supply. This air then passes across a cooling coil inside which cold water is running (from the chiller, through the pumps). Heat exchange occurs between the air and the chilled water in the cooling coil. The cooled air is then circulated into the supply air duct by means of a fan. Using ducts this cooled air is distributed to the various areas where this AHU is catering to.

Return Air

Supply Fan

Fresh Air

Supply Air

Filter R Roollee ooff B BM MS S iinn A AH HU UC Coonnttrrooll & &M Moonniittoorriinngg:: ♦ The filter status is monitored to find out if any clogging has occurred. A differential pressure switch connected across the filter is used to give a digital output when the filter is clogged. ♦ A differential pressure switch is used to monitor the functioning of the fan ♦ The AHU can also be operated on time schedule basis. For e.g.. The AHU can be switched on from 9.00 am to 9.30 p.m. ♦ The return air temperature of

each AHU is used to monitor the opening and closing of the valve

connected at the outlet of the chilled water line (cooling coils) . If the return air temperature is high then the valve is opened wide . If the return air temperature is low, then the valve is closed allowing only little water flow.

AHU :

V Vaarriiaabbllee A Aiirr V Voolluum mee U Unniittss:: VAV is used to control the amount of cool air flowing into each room or zone. The air enters each room via a damper system. Each zone will have varying temperature and pressure requirements. This requirement is met by varying the closing and opening of the damper which in turn controls the amount of cool air flowing into the zone.

A VAV unit :

Comparing the AHU, VAV combination with the water system we have at home and offices, the AHU is similar to the overhead water tank. The VAV’s are similar to the taps in each of the rooms. The water available in the overhead tank can be used in the rooms as per each room’s requirement by variably opening and closing the taps. R Roollee ooff B BM MS S iinn V VA AV VC Coonnttrrooll & &M Moonniittoorriinngg:: ♦ ♦ A temperature sensor is placed in the supply air duct from the AHU. This temperature is monitored while using VAV. ♦ ♦ A pressure sensor is installed at 2/3 rd distance of the supply air duct serving the various zones. If the pressure is too high a Variable frequency drive is used to lower the speed of the AHU fan. R Reedduucciinngg R RH H ooff aa rroooom m: In pharma and textile industries a specific RH has to be maintained. While monitoring RH using an RH sensor there may be situations where we need to reduce the RH. To reduce RH we increase the cooling. Increased cooling leads to dehumidification. Due to dehumidification the water content in the air reduces. Thus the RH of the room is reduced. After this process if the temperature of the room has to be increased a little then we can switch on the heater. G Glloossssaarryy:: Humidity: It is the measure of moisture content in a substance. Air is never 100 % dry. Air always exists in the form of a mixture of dry air and water vapour. Relative Humidity: The ratio of the actual moisture content of air at a given condition to the moisture condition that would have been present at saturated conditions for the same temperature. Dew point temperature: It is the temperature at which air can no longer hold the moisture and when condensation commences. Boiling point: We can alter the boiling points of liquids by changing the pressure. When pressure increases the boiling point increases. In other words the temperature of the liquid increases when we increase the pressure. Anamolous Expansion of water: Between 0 deg centigrade and 4 deg centigrade water actually contracts. Hence care should be taken that the water flowing in chiller never reaches this temperature.

Heat Exchange : Exchange of heat happens from a higher temperature area to a lower temperature area. Btu/hr : The abbreviation for British thermal units. The amount of heat required to raise one pound of water to one degree Fahrenheit per hour, a common measure of heat transfer rate. Tonnage(TR) : The unit of measure used in air conditioning to describe the cooling capacity of a system. One ton of cooling is based on the amount of heat needed to melt one ton (2000 lbs.) of ice in a 24 hour period. One ton of cooling is equal to 12,000 Btu/hr. Fresh Air : Outdoor air introduced into HVAC system.

**************************************