ASHRAE FUNDAMENTALS IP CH 18-2013 Nonresidential Cooling and Heating Load Calculations.pdf

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1、18.1CHAPTER 18NONRESIDENTIAL COOLING AND HEATING LOAD CALCULATIONSCooling Load Calculation Principles 18.1Internal Heat Gains . 18.3Infiltration and Moisture Migration Heat Gains . 18.12Fenestration Heat Gain 18.14Heat Balance Method 18.14Radiant Time Series (RTS) Method 18.20Heating Load Calculatio

2、ns 18.28System Heating and Cooling Load Effects. 18.32Example Cooling and Heating Load Calculations 18.35Previous Cooling Load Calculation Methods 18.49Building Example Drawings 18.52EATING and cooling load calculations are the primary designHbasis for most heating and air-conditioning systems and c

3、om-ponents. These calculations affect the size of piping, ductwork, dif-fusers, air handlers, boilers, chillers, coils, compressors, fans, andevery other component of systems that condition indoor environ-ments. Cooling and heating load calculations can significantly affectfirst cost of building con

4、struction, comfort and productivity of occu-pants, and operating cost and energy consumption.Simply put, heating and cooling loads are the rates of energyinput (heating) or removal (cooling) required to maintain an indoorenvironment at a desired temperature and humidity condition. Heat-ing and air c

5、onditioning systems are designed, sized, and controlledto accomplish that energy transfer. The amount of heating or coolingrequired at any particular time varies widely, depending on external(e.g., outdoor temperature) and internal (e.g., number of peopleoccupying a space) factors.Peak design heatin

6、g and cooling load calculations, which are thischapters focus, seek to determine the maximum rate of heating andcooling energy transfer needed at any point in time. Similar princi-ples, but with different assumptions, data, and application, can beused to estimate building energy consumption, as desc

7、ribed in Chap-ter 19.This chapter discusses common elements of cooling load calcu-lation (e.g., internal heat gain, ventilation and infiltration, moisturemigration, fenestration heat gain) and two methods of heating andcooling load estimation: heat balance (HB) and radiant time series(RTS).COOLING L

8、OAD CALCULATION PRINCIPLESCooling loads result from many conduction, convection, and radi-ation heat transfer processes through the building envelope and frominternal sources and system components. Building components orcontents that may affect cooling loads include the following:External: Walls, ro

9、ofs, windows, skylights, doors, partitions, ceil-ings, and floorsInternal: Lights, people, appliances, and equipmentInfiltration: Air leakage and moisture migrationSystem: Outdoor air, duct leakage and heat gain, reheat, fan andpump energy, and energy recoveryTERMINOLOGYThe variables affecting cooli

10、ng load calculations are numerous,often difficult to define precisely, and always intricately interrelated.Many cooling load components vary widely in magnitude, and pos-sibly direction, during a 24 h period. Because these cyclic changes inload components often are not in phase with each other, each

11、 compo-nent must be analyzed to establish the maximum cooling load for abuilding or zone. A zoned system (i.e., one serving several indepen-dent areas, each with its own temperature control) needs to provide nogreater total cooling load capacity than the largest hourly sum ofsimultaneous zone loads

12、throughout a design day; however, it musthandle the peak cooling load for each zone at its individual peak hour.At some times of day during heating or intermediate seasons, somezones may require heating while others require cooling. The zonesventilation, humidification, or dehumidification needs mus

13、t also beconsidered.Heat Flow RatesIn air-conditioning design, the following four related heat flowrates, each of which varies with time, must be differentiated.Space Heat Gain. This instantaneous rate of heat gain is the rateat which heat enters into and/or is generated within a space. Heat gainis

14、classified by its mode of entry into the space and whether it is sen-sible or latent. Entry modes include (1) solar radiation through trans-parent surfaces; (2) heat conduction through exterior walls and roofs;(3) heat conduction through ceilings, floors, and interior partitions;(4) heat generated i

15、n the space by occupants, lights, and appliances;(5) energy transfer through direct-with-space ventilation and infiltra-tion of outdoor air; and (6) miscellaneous heat gains. Sensible heat isadded directly to the conditioned space by conduction, convection,and/or radiation. Latent heat gain occurs w

16、hen moisture is added tothe space (e.g., from vapor emitted by occupants and equipment). Tomaintain a constant humidity ratio, water vapor must condense on thecooling apparatus and be removed at the same rate it is added to thespace. The amount of energy required to offset latent heat gain essen-tia

17、lly equals the product of the condensation rate and latent heat ofcondensation. In selecting cooling equipment, distinguish betweensensible and latent heat gain: every cooling apparatus has differentmaximum removal capacities for sensible versus latent heat for par-ticular operating conditions. In e

18、xtremely dry climates, humidifica-tion may be required, rather than dehumidification, to maintainthermal comfort.Radiant Heat Gain. Radiant energy must first be absorbed by sur-faces that enclose the space (walls, floor, and ceiling) and objects inthe space (furniture, etc.). When these surfaces and

19、 objects becomewarmer than the surrounding air, some of their heat transfers to the airby convection. The composite heat storage capacity of these surfacesand objects determines the rate at which their respective surfacetemperatures increase for a given radiant input, and thus governs therelationshi

20、p between the radiant portion of heat gain and its corre-sponding part of the space cooling load (Figure 1). The thermal stor-age effect is critical in differentiating between instantaneous heatgain for a given space and its cooling load at that moment. PredictingThe preparation of this chapter is a

21、ssigned to TC 4.1, Load Calculation Dataand Procedures.18.2 2013 ASHRAE HandbookFundamentalsthe nature and magnitude of this phenomenon to estimate a realisticcooling load for a particular set of circumstances has long been ofinterest to design engineers; the Bibliography lists some early workon the

22、 subject.Space Cooling Load. This is the rate at which sensible and latentheat must be removed from the space to maintain a constant spaceair temperature and humidity. The sum of all space instantaneousheat gains at any given time does not necessarily (or even fre-quently) equal the cooling load for

23、 the space at that same time.Space Heat Extraction Rate. The rates at which sensible andlatent heat are removed from the conditioned space equal the spacecooling load only if the room air temperature and humidity are con-stant. Along with the intermittent operation of cooling equipment,control syste

24、ms usually allow a minor cyclic variation or swing inroom temperature; humidity is often allowed to float, but it can becontrolled. Therefore, proper simulation of the control system givesa more realistic value of energy removal over a fixed period thanusing values of the space cooling load. However

25、, this is primarilyimportant for estimating energy use over time; it is not needed tocalculate design peak cooling load for equipment selection.Cooling Coil Load. The rate at which energy is removed at acooling coil serving one or more conditioned spaces equals the sumof instantaneous space cooling

26、loads (or space heat extraction rate,if it is assumed that space temperature and humidity vary) for allspaces served by the coil, plus any system loads. System loadsinclude fan heat gain, duct heat gain, and outdoor air heat and mois-ture brought into the cooling equipment to satisfy the ventilation

27、 airrequirement.Time Delay EffectEnergy absorbed by walls, floor, furniture, etc., contributes tospace cooling load only after a time lag. Some of this energy is stillpresent and reradiating even after the heat sources have beenswitched off or removed, as shown in Figure 2.There is always significan

28、t delay between the time a heat sourceis activated, and the point when reradiated energy equals that beinginstantaneously stored. This time lag must be considered when cal-culating cooling load, because the load required for the space can bemuch lower than the instantaneous heat gain being generated

29、, andthe spaces peak load may be significantly affected.Accounting for the time delay effect is the major challenge incooling load calculations. Several methods, including the two pre-sented in this chapter, have been developed to take the time delayeffect into consideration.COOLING LOAD CALCULATION

30、 METHODSThis chapter presents two load calculation methods that varysignificantly from previous methods. The technology involved,however (the principle of calculating a heat balance for a givenspace) is not new. The first of the two methods is the heat balance(HB) method; the second is radiant time

31、series (RTS), which isa simplification of the HB procedure. Both methods are explainedin their respective sections.Cooling load calculation of an actual, multiple-room buildingrequires a complex computer program implementing the principlesof either method.Cooling Load Calculations in PracticeLoad ca

32、lculations should accurately describe the building. Allload calculation inputs should be as accurate as reasonable, withoutusing safety factors. Introducing compounding safety factors atmultiple levels in the load calculation results in an unrealistic andoversized load.Variation in heat transmission

33、 coefficients of typical buildingmaterials and composite assemblies, differing motivations andskills of those who construct the building, unknown infiltrationrates, and the manner in which the building is actually operated aresome of the variables that make precise calculation impossible.Even if the

34、 designer uses reasonable procedures to account for thesefactors, the calculation can never be more than a good estimate ofthe actual load. Frequently, a cooling load must be calculated beforeevery parameter in the conditioned space can be properly or com-pletely defined. An example is a cooling loa

35、d estimate for a newbuilding with many floors of unleased spaces for which detailedpartition requirements, furnishings, lighting, and layout cannot bepredefined. Potential tenant modifications once the building is occu-pied also must be considered. Load estimating requires proper engi-neering judgme

36、nt that includes a thorough understanding of heatbalance fundamentals.Perimeter spaces exposed to high solar heat gain often need cool-ing during sunlit portions of traditional heating months, as do com-pletely interior spaces with significant internal heat gain. Thesespaces can also have significan

37、t heating loads during nonsunlithours or after periods of nonoccupancy, when adjacent spaces havecooled below interior design temperatures. The heating loadsinvolved can be estimated conventionally to offset or to compensatefor them and prevent overheating, but they have no direct relation-ship to t

38、he spaces design heating loads.Correct design and sizing of air-conditioning systems requiremore than calculation of the cooling load in the space to be condi-tioned. The type of air-conditioning system, ventilation rate, reheat,fan energy, fan location, duct heat loss and gain, duct leakage, heatex

39、traction lighting systems, type of return air system, and any sen-sible or latent heat recovery all affect system load and componentsizing. Adequate system design and component sizing require thatsystem performance be analyzed as a series of psychrometric pro-cesses.System design could be driven by

40、either sensible or latent load,and both need to be checked. In a sensible-load-driven space (themost common case), the cooling supply air has surplus capacity todehumidify, but this is usually permissible. For a space driven byFig. 1 Origin of Difference Between Magnitude of Instantaneous Heat Gain

41、and Instantaneous Cooling LoadFig. 2 Thermal Storage Effect in Cooling Load from LightsNonresidential Cooling and Heating Load Calculations 18.3latent load (e.g., an auditorium), supply airflow based on sensibleload is likely not to have enough dehumidifying capability, so sub-cooling and reheating

42、or some other dehumidification process isneeded.This chapter is primarily concerned with a given space or zone ina building. When estimating loads for a group of spaces (e.g., for anair-handling system that serves multiple zones), the assembledzones must be analyzed to consider (1) the simultaneous

43、effects tak-ing place; (2) any diversification of heat gains for occupants, light-ing, or other internal load sources; (3) ventilation; and/or (4) anyother unique circumstances. With large buildings that involve morethan a single HVAC system, simultaneous loads and any additionaldiversity also must

44、be considered when designing the central equip-ment that serves the systems. Methods presented in this chapter areexpressed as hourly load summaries, reflecting 24 h input schedulesand profiles of the individual load variables. Specific systems andapplications may require different profiles.DATA ASS

45、EMBLYCalculating space cooling loads requires detailed building designinformation and weather data at design conditions. Generally, thefollowing information should be compiled.Building Characteristics. Building materials, component size,external surface colors, and shape are usually determined fromb

46、uilding plans and specifications.Configuration. Determine building location, orientation, andexternal shading from building plans and specifications. Shadingfrom adjacent buildings can be determined from a site plan or byvisiting the proposed site, but its probable permanence should becarefully eval

47、uated before it is included in the calculation. The pos-sibility of abnormally high ground-reflected solar radiation (e.g.,from adjacent water, sand, or parking lots) or solar load from adja-cent reflective buildings should not be overlooked.Outdoor Design Conditions. Obtain appropriate weather data

48、,and select outdoor design conditions. Chapter 14 provides informa-tion for many weather stations; note, however, that these designdry-bulb and mean coincident wet-bulb temperatures may varyconsiderably from data traditionally used in various areas. Usejudgment to ensure that results are consistent

49、with expectations.Also, consider prevailing wind velocity and the relationship of aproject site to the selected weather station.Recent research projects have greatly expanded the amount ofavailable weather data (e.g., ASHRAE 2012). In addition to the con-ventional dry bulb with mean coincident wet bulb, data are nowavailable for wet bulb and dew point with mean coincident dry bulb.Peak space load generally coincides with peak solar or peak drybulb, but peak system load often occurs at peak wet-bulb tempera-ture. The relationship between space and system loads is discussedfurther in