Load estimation in Air Conditioning

To design any air-conditioning unit, estimation of heating or cooling load is very important. It helps us in design different devices most importantly the humidifier (in case of winter) or de-humidifier (in case of summer).

1. Load Estimation Presented By : Parth Prajapati Mechanical Engineering Department

2. Introduction Heating and cooling load calculations are carried out to estimate the required capacity of heating and cooling systems. Information regarding indoor and outdoor conditions , specifications of the building, specifications of the conditioned space, occupancy, equipment used should be known. Comfort applications : on basis of thermal comfort Industrial or commercial applications : on basis of particular processes being performed or products being stored.

3. Heating Load Heating Load Calculations are carried out to estimate the amount heat energy that should be added to the building to arrive at required heating capacities. During winter, peak heating load occurs before sunrise and after sunset. Internal heat sources are beneficial as they assist in heat addition process. Normally, to simplify the problem, heat load calculations are carried out assuming steady state conditions i.e. no solar radiation and steady outdoor conditions and neglecting internal sources This is simple but it leads to over-estimation of the heating capacity For accurate results, each and every factor should be taken into account which makes the calculation complex

4. Cooling Load For cooling load calculations, one has to consider unsteady outdoor conditions (as the peak load occurs during the daytime) In addition, all internal sources and solar radiation will have their effect on the average room temperature and neglecting them would lead to under-estimation of the required system. Thus cooling load are more complicated as it involves unsteady equations with unsteady boundary conditions.

5. Outside Temperature For any building there exists a balance point at which the solar radiation ( Qsolar ) and internal heat generation rate ( Qint ) exactly balance the heat losses from the building. where UA is the product of overall heat transfer coefficient and heat transfer area of the building, T in is the required indoor temperature and T out is the outdoor temperature.

6. Outside Temperature If the outdoor temperature is greater than the balanced outdoor temperature given by the above equation, i.e., when T(out) > T( out,bal ), then there is a need for cooling the building. On the other hand, when the outdoor temperature is less than the balanced outdoor temperature, i.e., when T(out) < T( out,bal ), then there is a need for heating the building. When the outdoor temperature exactly equals the balanced outdoor temperature, i.e., when T(out) = T( out,bal ), then there is no need for either cooling or heating the building

7. Outside Temperature For residential buildings (with fewer internal heat sources), the balanced outdoor temperature may vary from 10 to 18 o C. A cooling system is required when the outdoor temperature exceeds 18 o C. If the building is well insulated (small UA) and/or internal loads are high, then the balanced outdoor temperature will reduce leading to extended cooling season and shortened heating season. If there are no internal heat sources and if the solar radiation is negligible, then from the heat balance equation, Tout,bal = Tin. Th is implies that if the outside temperature exceeds the required inside temperature (say, 25 o C for comfort) then there is a need for cooling otherwise there is a need for heating.

8. Methods of Estimation Generally, heating and cooling load calculations involve a stepwise procedure, taking into account all the building energy flows. Variety of methods ranging from simple rules-of-thumb (based on assumptions) to complex Transfer Function Methods are used in practice to arrive at the building loads. Thumb rules are useful in preliminary estimation of the equipment size and cost. The main conceptual drawback of rules-of-thumb methods is the presumption that the building design will not make any difference. Thus the rules for a badly designed building are typically the same as for a good design.

9. Required Cooling capacities for various applications based on rules-of-thumb

10. Methods of Estimation More accurate load estimation methods involve a combination of analytical methods and empirical results obtained from actual data, For example the use of Cooling Load Temperature Difference (CLTD) for estimating heat gain and the use of Solar Heat Gain Factor (SHGF) for estimating heat transfer through arrangement of windows in a building. Widely used by air conditioning engineers. Accurate results and estimations can be carried out manually in a relatively short time. More accurate methods that require the use of computers have been developed for estimating cooling loads, e.g. Transfer Function Method (TFM). These methods are expensive and time consuming they are generally used for estimating cooling loads of large commercial or institutional buildings.

11. Cooling Load Calculations Design cooling load takes into account all the loads experienced by a building under specific set of assumed conditions. Assumptions: Outside conditions are selected from a long term statistical database. Design data for outside conditions for various locations of the world have been collected and are available various handbooks. The load on the building due to solar radiation is estimated for clear sky conditions. The building occupancy is assumed to be at full design capacity. All building equipment and appliances are considered to be operating at rated capacity

12. Cooling Load Calculations External Load: It consists of heat transferred through the building envelope (walls, roof, floor, windows, doors etc.) Internal Load: H eat generated by occupants, equipment, and lights. 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.

13. Sensible Load Heat flowing into the building by conduction through exterior walls, floors, ceilings, doors and windows Heat received from solar radiation Heat given off by lights, motors, machinery, cooking operations, industrial processes etc. Heat liberated by occupants Heat carried out by outside air which leaks in through the cracks in doors, windows and through frequent openings

14. Latent Load Heat gain due to moisture in the outside air entering by infiltration Heat gain due to condensation of moisture from occupants The heat gain due to condensation of moisture from any process such as cooking foods etc. Heat gain due to moisture passing directly into the conditioned space through permeable walls or partitions from the outside region where the water vapour pressure is higher

15. Heat transfer through conduction

16. Heat transfer through conduction

17. Overall Heat Transfer Coefficient

18. Sensible heat gain through outside walls and roofs Temperature of wall rises with rise in outside air temperature and the heat is stored in wall which has a considerable storage capacity. Thus the heat transferred to the room is reduced. The stored heat in wall is given off to the room when the outside air temperature falls. Shaded area above the actual load shows the heat stored and below the actual load shows the heat released by the walls and other structures

19. Sensible heat gain through outside walls and roofs A convenient method of taking into account this lagging effect is to use an equivalent temperature differential.

20. Sol Air Temperature A hypothetical temperature used to calculate the heat received by the outside surface of a building wall by the effect of convection and radiation.

21. Heat gain due to infiltration Heat transfer due to infiltration consists of both sensible as well as latent components. The sensible heat transfer rate due to infiltration is given by: The latent heat transfer rate due to infiltration is given by: The infiltration rate is obtained by using either the air change method or the crack method. Air changes per hour , or  air change rate , is a measure of the air volume added to or removed from a space (normally a room or house) divided by the volume of the space. If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced.

22. The infiltration rate by the crack method is given by: where A is the effective leakage area of the cracks, C is a flow coefficient which depends on the type of the crack, ΔP is the difference between outside and inside pressure and n is an exponent whose value depends on the nature of the flow in the crack. The pressure difference Δ P arises due to pressure difference due to the wind ( Δ P wind ) , pressure difference due to the stack effect ( Δ P stack ) and pressure difference due to building pressurization ( Δ P bld ) Heat gain due to infiltration

23. heat transfer depends mainly on the population and activity level of the occupants. Heat Gain from occupants

24. Heat Gain through power equipments For other equipment such as computers, printers etc, the load is in the form of sensible heat transfer and is estimated based on the rated power consumption.

25. If the cooling coil has a some by-pass factor , then some amount of ventilation air directly enters the conditioned space, which becomes a part of the building cooling load. The sensible and latent heat transfer rates due to the by-passed ventilation air can be calculated by replacing V with (V x BPF) In addition to this, sensible and latent heat transfer to the building also occurs due to heat transfer and air leakage in the supply ducts. Miscellaneous External Load

26. Overall Balance