CHAPTER 28 NONRESIDENTIAL COOLING AND HEATING LOAD CALCULATIONS COOLING LOAD PRINCIPLES ............................................ 28.1 Example Cooling Load Calculation ...................................... 28.33 Space Cooling Load Calculation Techniques ......................... 28.2 CLTD/SCL/CLF CALCULATION Initial Design Considerations ................................................. 28.4 PROCEDURE ................................................................... 28.39 Heat Gain Calculation Concepts ............................................ 28.5 Synthesis of Heat Gain and Cooling Load Heat Sources in Conditioned Spaces ...................................... 28.7 Conversion Procedures ..................................................... 28.40 Infiltration and Ventilation Heat Gain ................................. 28.11 Heat Sources Within Conditioned Space ............................ 28.51 HEATING LOAD PRINCIPLES ........................................... 28.16 Example Cooling Load Calculation Using TRANSFER FUNCTION METHOD CLTD/CLF Method ........................................................... 28.54 CALCULATION PROCEDURE ........................................ 28.17 TETD/TA CALCULATION PROCEDURE ........................... 28.56 Basic Cooling Load Analysis ................................................ 28.17 Cooling Load by Time Averaging ......................................... 28.58 Heat Gain by Conduction Through Example Cooling Load Calculation Exterior Walls and Roofs .................................................. 28.17 Using TETD/TA ................................................................. 28.59 HIS chapter presents three methods of calculating air-condi- components are often not in phase with each other, each must be Ttioning cooling load for sizing cooling equipment and a general analyzed to establish the resultant maximum cooling load for a procedure for calculating heating load, for nonresidential applica- building or zone. A zoned system (a system of conditioning equip- tions. In addition, the fundamental principles for calculating heating ment serving several independent areas, each with its own temper- loads are presented as a counterpart to cooling load calculation. For ature control) need recognize no greater total cooling load capacity residential applications, consult Chapter 27. For information on than the largest hourly summary of simultaneous zone loads cooling and/or heating equipment energy use, consult Chapter 30. throughout a design day; however, it must handle the peak cooling The heat balance approach is a fundamental concept in calculat- load for each zone at its individual peak hour. At certain times of the ing cooling loads. While generally cumbersome for widespread or day during the heating or intermediate seasons, some zones may routine use, this underlying concept is the basis for each of the three require heating while others require cooling. simplified procedures outlined for varying purposes. Calculation accuracy. The concept of determining the cooling The cooling calculation procedure most closely approximating load for a given building must be kept in perspective. A proper cool- the heat balance concept is the transfer function method (TFM), first ing load calculation gives values adequate for proper performance. introduced in the 1972 ASHRAE Handbook of Fundamentals. This Variation in the heat transmission coefficients of typical building computer-based procedure takes place in two steps, first establish- materials and composite assemblies, the differing motivations and ing the heat gain from all sources and then determining the conver- skills of those who physically construct the building, and the man- sion of such heat gain into cooling load. Developed as an hour-by- ner in which the building is actually operated are some of the vari- hour calculation procedure oriented to simulate annual energy use, ables that make a numerically precise calculation impossible. While its normalizing characteristics make it particularly appropriate for the designer uses reasonable procedures to account for these factors, that application. the calculation can never be more than a good estimate of the actual A simplified version of the TFM, which can be used with certain cooling load. types of buildings for which application data are available, was Heat flow rates. In air-conditioning design, four related heat presented in the 1977 ASHRAE Handbook of Fundamentals. This flow rates, each of which varies with time, must be differentiated: one-step procedure uses cooling load temperature differences (1) space heat gain, (2) space cooling load, (3) space heat extraction (CLTD), solar cooling load factors (SCL), and internal cooling load rate, and (4) cooling coil load. factors (CLF), to calculate cooling loads as an approximation of the Space heat gain. This instantaneous rate of heat gain is the rate TFM. Where applicable, this method may be suitable for hand cal- at which heat enters into and/or is generated within a space at a culation use. given instant. Heat gain is classified by (1) the mode in which it An alternative simplification of the heat balance technique uses enters the space and (2) whether it is a sensible or latent gain. total equivalent temperature differential values and a system of Mode of entry. The modes of heat gain may be as (1) solar radi- time-averaging (TETD/TA) to calculate cooling loads. Also a com- ation through transparent surfaces; (2) heat conduction through exte- puter-based, two-step procedure (heat gain, then cooling load), first rior walls and roofs; (3) heat conduction through interior partitions, introduced in the 1967 ASHRAE Handbook of Fundamentals, this ceilings, and floors; (4) heat generated within the space by occu- method gives valid broad-range results to experienced users. pants, lights, and appliances; (5) energy transfer as a result of venti- lation and infiltration of outdoor air; or (6) miscellaneous heat gains. COOLING LOAD PRINCIPLES Sensible or latent heat. Sensible heat gain is directly added to the conditioned space by conduction, convection, and/or radiation. The variables affecting cooling load calculations are numerous, Latent heat gain occurs when moisture is added to the space (e.g., often difficult to define precisely, and always intricately inter- from vapor emitted by occupants and equipment). To maintain a con- related. Many cooling load components vary in magnitude over a stant humidity ratio, water vapor must condense on cooling appara- wide range during a 24-h period. Since these cyclic changes in load tus at a rate equal to its rate of addition into the space. The amount of energy required to offset the latent heat gain essentially equals the The preparation of this chapter is assigned to TC 4.1, Load Calculation product of the rate of condensation and the latent heat of condensa- Data and Procedures. tion. In selecting cooling apparatus, it is necessary to distinguish 28.2 1997 ASHRAE Fundamentals Handbook SPACE COOLING LOAD CALCULATION TECHNIQUES Heat Balance Fundamentals The estimation of cooling load for a space involves calculating a surface-by-surface conductive, convective, and radiative heat bal- ance for each room surface and a convective heat balance for the room air. Sometimes called “the exact solution,” these principles form the foundation for all other methods described in this chapter. To calculate space cooling load directly by heat balance proce- dures requires a laborious solution of energy balance equations Fig. 1 Origin of Difference Between Magnitude of involving the space air, surrounding walls and windows, infiltration Instantaneous Heat Gain and Instantaneous Cooling Load and ventilation air, and internal energy sources. To demonstrate the calculation principle, consider a sample room enclosed by four walls, a ceiling, and a floor, with infiltration air, ventilation air, and between sensible and latent heat gain. Every cooling apparatus has a normal internal energy sources. The calculations that govern energy maximum sensible heat removal capacity and a maximum latent heat exchange at each inside surface at a given time are: removal capacity for particular operating conditions. Space cooling load. This is the rate at which heat must be m () () removed from the space to maintain a constant space air tempera- qi, θ = hci ta, θ – ti, θ + ∑ gij tj, θ – ti, θ Ai ture. The sum of all space instantaneous heat gains at any given time j = 1, ji≠ (1) does not necessarily (or even frequently) equal the cooling load for ,,,,, the space at that same time. +RSi, θ +RLi, θ +REi, θ for i = 123456 Radiant heat gain. Space heat gain by radiation is not immedi- where ately converted into cooling load. Radiant energy must first be absorbed by the surfaces that enclose the space (walls, floor, and m = number of surfaces in room (6 in this case) θ ceiling) and the objects in the space (furniture, etc.). As soon as qi,θ = rate of heat conducted into surface i at inside surface at time these surfaces and objects become warmer than the space air, some Ai = area of surface i of their heat is transferred to the air in the space by convection. hci = convective heat transfer coefficient at interior surface i The composite heat storage capacity of these surfaces and objects gij = radiation heat transfer factor between interior surface i and interior surface j determines the rate at which their respective surface temperatures θ ta,θ = inside air temperature at time increase for a given radiant input, and