1、24.1CHAPTER 24REFRIGERATED-FACILITY LOADSTransmission Load 24.1Product Load. 24.3Internal Load. 24.3Infiltration Air Load 24.4Equipment Related Load. 24.6Safety Factor . 24.7Load Diversity. 24.8OTAL refrigeration load includes (1) transmission load, whichTis heat transferred into the refrigerated sp
2、ace through its sur-face; (2) product load, which is heat removed from and produced byproducts brought into and kept in the refrigerated space; (3) inter-nal load, which is heat produced by internal sources (e.g., lights,electric motors, and people working in the space); (4) infiltrationair load, wh
3、ich is heat gain associated with air entering the refrig-erated space; and (5) equipment-related load.The first four segments of load constitute the net heat load forwhich a refrigeration system is to be provided; the fifth segmentconsists of all heat gains created by the refrigerating equipment.Thu
4、s, net heat load plus equipment heat load is the total refrigera-tion load for which a compressor must be selected.This chapter contains load calculating procedures and data forthe first four segments and load determination recommendations forthe fifth segment. Information needed for refrigeration o
5、f specificfoods can be found in Chapters 28 and 30 to 42.TRANSMISSION LOADSensible heat gain through walls, floor, and ceiling is calculatedat steady state as q = UA t (1)whereq = heat gain, Btu/hA = outside area of section, ft2t = difference between outside air temperature and air temperature of th
6、e refrigerated space, FThe overall coefficient of heat transfer U of the wall, floor, orceiling can be calculated by the following equation:(2)whereU = overall heat transfer coefficient, Btu/hft2Fx = wall thickness, in.k = thermal conductivity of wall material, Btuin/hft2Fhi= inside surface conducta
7、nce, Btu/hft2Fho= outside surface conductance, Btu/hft2FA value of 1.6 Btu/hft2F for hiand hois frequently used forstill air. If the outer surface is exposed to 15 mph wind, hoisincreased to 6 Btu/hft2F.With thick walls and low conductivity, the resistance x/k makes Uso small that 1/hiand 1/hohave l
8、ittle effect and can be omitted fromthe calculation. Walls are usually made of more than one material;therefore, the value x/k represents the composite resistance of thematerials. The U-factor for a wall with flat parallel surfaces of mate-rials 1, 2, and 3 is given by the following equation:(3)The
9、thermal conductivities of several cold storage insulations arelisted in Table 1. These values increase with age because of factorsdiscussed in Chapter 25 of the 2009 ASHRAE HandbookFunda-mentals. Chapter 27 of that volume includes more complete tableslisting the thermal properties of various buildin
10、g and insulationmaterials.Table 2 lists minimum insulation thicknesses of expanded poly-isocyanurate board recommended by the refrigeration industry.These thicknesses may need to be increased to offset heat gaincaused by building components such as wood and metal studs, websin concrete masonry, and
11、metal ties that bridge across the insulationand reduce the thermal resistance of the wall or roof. Chapter 27 ofthe 2009 ASHRAE HandbookFundamentals describes how to cal-culate heat gain through walls and roofs with thermal bridges. Metalsurfaces of prefabricated or insulated panels have a negligibl
12、e effecton thermal performance and need not be considered in calculatingthe U-factor.In most cases, the temperature difference t can be adjusted tocompensate for solar effect on heat load. Values in Table 3 applyover a 24 h period and are added to the ambient temperature whencalculating wall heat ga
13、in.The preparation of this chapter is assigned to TC 10.8, Refrigeration LoadCalculations.U11 hi xk 1 ho+-=Table 1 Thermal Conductivity of Cold Storage InsulationInsulationThermal Conductivityak, Btu in/hft2FPolyurethane board (R-11 expanded) 0.16 to 0.18Polyisocyanurate, cellular (R-141b expanded)
14、0.19Polystyrene, extruded (R-142b) 0.24Polystyrene, expanded (R-142b) 0.26Corkboardb0.30Foam glassc0.31aValues are for a mean temperature of 75F, and insulation is aged 180 days.bSeldom-used insulation. Data are only for reference.cVirtually no effects from aging.Table 2 Minimum Insulation Thickness
15、StorageTemperature, FExpanded Polyisocyanurate ThicknessNorthern U.S., in. Southern U.S., in.50 to 60 2 240 to 50 2 225 to 40 2 315 to 25 3 30 to 15 3 415 to 0 4 440 to 15 5 5U1x1k1 x2k2 x3k3+-=24.2 2010 ASHRAE HandbookRefrigerationLatent heat gain from moisture transmission through walls,floors, an
16、d ceilings of modern refrigerated facilities is negligible.Data in Chapter 27 of the 2009 ASHRAE HandbookFundamentalsmay be used to calculate this load if moisture permeable materialsare used.Chapter 14 of the 2009 ASHRAE HandbookFundamentalsgives outdoor design temperatures for major cities; values
17、 for 0.4%should be used.Additional information on thermal insulation may be found inChapters 25 and 26 of the 2009 ASHRAE HandbookFundamentals.Chapter 18 of that volume discusses load calculation procedures ingreater detail.Heat Gain from Cooler FloorsHeat gain through cooler concrete slab floors is
18、 predicted usingprocedures developed by Chuangchid and Krarti (2000), who devel-oped a simplified correlation of the total slab heat gain for coolersbased on analytical results reported earlier by Chuangchid andKrarti (1999). Parameters in the solution include slab size and ther-mal resistance, insu
19、lation thermal resistance, soil thermal conduc-tivity, water table depth and temperature, and indoor and outdoorair temperatures. The design procedure accommodates four slabinsulation configurations: no insulation, uniform horizontal insu-lation, partial-horizontal-perimeter insulation, and partial-
20、vertical-perimeter insulation. The slab size characteristic is expressed as theratio of slab area A to exposed slab perimeter P. The result is an esti-mate of the annual mean and amplitude cooler floor heat gain that,when combined, gives the instantaneous floor heat at a specifictime of year. The ti
21、me-variation of the ground-coupled heat gainq(t) for cooler slabs isq(t) = qm qacos (t ) (4)whereqm= annual mean slab floor heat gain, Btu/hqa= amplitude of annual variation slab floor heat gain, Btu/h = phase lag between cooler floor heat gain and outdoor air temperature variation, dayst = time, da
22、ys = constant for annual angular frequency, 0.0172 radians/day These quantities are functions of input parameters such as buildingdimensions, soil properties, and insulation thermal resistance. Notethat the time of origin (t = 0) in Equation (4) corresponds to the timewhen the soil surface temperatu
23、re is minimum (typically January 15in most U.S. locations). Details of the calculation method are givenby Chuangchid and Krarti (2000).For the conditions in Table 4, results for cooler slab floor heatgain are computed based on the method. As shown in Figure 1, heatgain through the cooler floor signi
24、ficantly varies and may even benegative at certain times of the year. The influence of partial-horizontal and partial-vertical-perimeter insulation on cooler floorheat gain for Table 4 conditions can also be seen in Figure 1.Figure 2 shows the results for maximum heat transfer per unit areaqmax/A by
25、 calculation where typical cold and warm climate temper-atures are applied. These outdoor temperatures are given in Table 5.qmax/A is the sum of the mean and amplitude of heat gain per unitarea and is, therefore, the maximum heat transfer rate that occurs atsome time during the year. The influence o
26、f floor slab width andlength is captured by the A/P parameter. Widths of both 30 and 100 ftTable 3 Allowance for Sun EffectTypical Surface TypesEastWall, FSouth Wall, FWestWall, FFlatRoof, FDark-colored surfacesSlate roofing 8 5 8 20Tar roofingBlack paintMedium-colored surfacesUnpainted wood 6 4 6 1
27、5BrickRed tileDark cementRed, gray, or green paintLight-colored surfacesWhite stone 4 2 4 9Light colored cementWhite paintNote: Add to the normal temperature difference for heat leakage calculations to com-pensate for sun effect. Do not use for air-conditioning design.Table 4 Example Input Data Requ
28、ired to Estimate Cooler Floor Heat GainRequired Information Example ValuesSoil thermal conductivity ks10.47 Btuin/hft2FSoil thermal diffusivity s7.66 106ft2/sTotal floor area A 32 ft 50 ft = 1600 ft2Exposed perimeter P 164 ft)Slab thickness y 4 in.Slab thermal resistance Rf3.33 ft2hF/BtuInsulation t
29、hermal resistance Ri(partial uniform, partial, vertical)20 ft2hF/BtuPartial insulation length c 3 ft)Cooler inside temperature Tr35F)Annual mean Tm40FAnnual amplitude Ta60FWater table depth b Ignored (if depth is greater than 7 ft, water table effect can be ignored)Table 5 Typical Annual and Annual
30、Amplitude Outdoor Temperatures for Warm and Cold ClimatesTemperatureTypical ColdClimate, FTypical WarmClimate, FAnnual mean Tm40 70Annual amplitude Ta60 30Fig. 1 Variation of Cooler Floor Heat Gain over One Year forConditions in Table 1Fig. 1 Variation of Cooler Floor Heat Gain over One Year for Con
31、ditions in Table 4Refrigerated-Facility Loads 24.3were used to generate Figure 2. The fact that the qmax/A curves inFigure 2 coalesce for different slab areas while still having the sameA/P ratio and the same indoor, annual mean outdoor, and annualamplitude outdoor temperatures suggests that slab ar
32、ea does notaffect the cooler floor slab heat gain rate per unit area with floor areasgreater than about 1075 ft2.PRODUCT LOADThe primary refrigeration loads from products brought into andkept in the refrigerated space are (1) heat that must be removed tobring products to storage temperature and (2)
33、heat generated byproducts (mainly fruits and vegetables) in storage. The quantity ofheat to be removed can be calculated as follows:1. Heat removed to cool from initial temperature to some lowertemperature above freezing:Q1= mc1(t1 t2)(5)2. Heat removed to cool from initial temperature to freezing p
34、ointof product:Q2= mc1(t1 tf)(6)3. Heat removed to freeze product:Q3= mhif(7)4. Heat removed to cool from freezing point to final temperaturebelow freezing point:Q4= mc2(tf t3)(8)whereQ1, Q2, Q3, Q4= heat removed, Btum = weight of product, lbc1= specific heat of product above freezing, Btu/lbFt1= in
35、itial temperature of product above freezing, Ft2= lower temperature of product above freezing, Ftf= freezing temperature of product, Fhif= latent heat of fusion of product, Btu/lbc2= specific heat of product below freezing, Btu/lbFt3= final temperature of product below freezing, FThe refrigeration c
36、apacity required for products brought intostorage is determined from the time allotted for heat removal andassumes that product is properly exposed to remove the heat in thattime. The calculation isq = (9)whereq = average cooling load, Btu/hn = allotted time, hEquation (9) only applies to uniform en
37、try of product into stor-age. The refrigeration load created by nonuniform loading of awarm product may be much greater over a short period. See Chapter28 for information on calculating the cooling load of warm product.Specific heats above and below freezing for many products aregiven in Table 3 of
38、Chapter 19. A products latent heat of fusion maybe estimated by multiplying the water content of the product (ex-pressed as a decimal) by the latent heat of fusion of water, which is144 Btu/lb. Most foods freeze between 26 and 31F. When the exactfreezing temperature is not known, assume that it is 2
39、8F.Example 1. 200 lb of lean beef is to be cooled from 65 to 40F, then frozenand cooled to 0F. The moisture content is 69.5%, so the latent heat isestimated as 144 0.695 = 100 Btu/lb. Estimate the cooling load.Solution:Specific heat of beef before freezing is listed in Table 3, Chapter 19,as 0.84 Bt
40、u/lbF; after freezing, 0.51 Btu/lbF.To cool from 65 to 40F in a chilled room:2000.84(65 40) = 4200 BtuTo cool from 40F to freezing point in freezer:2000.84(40 28) = 2016 BtuTo freeze: 200100 = 20,000 BtuTo cool from freezing to storage temperature:2000.51(28 0) = 2856 BtuTotal: 4200 + 2016 + 20,000
41、+ 2856 = 29,072 Btu(Example 3 in Chapter 19 shows an alternative calculation method.)Fresh fruits and vegetables respire and release heat during storage.This respiration heat varies by product and its temperature; the colderthe product, the less heat of respiration. Table 9 in Chapter 19 givesheat o
42、f respiration rates for various products.Calculations in Example 1 do not cover heat gained from productcontainers brought into the refrigerated space. When pallets, boxes,or other packing materials are a significant portion of the total massintroduced, this heat load should be calculated.Equations
43、(5) to (9) are used to calculate the total heat gain. Anymoisture removed appears as latent heat gain. The amount of mois-ture involved is usually provided by the end user as a percentage ofproduct mass; so, with such information, the latent heat componentof the total heat gain may be determined. Su
44、btracting the latent heatcomponent from the total heat gain determines the sensible heatcomponent.INTERNAL LOADElectrical Equipment. All electrical energy dissipated in therefrigerated space (from lights, motors, heaters, and other equip-ment) must be included in the internal heat load. Heat equival
45、ents ofelectric motors are listed in Table 6.Forklifts. Forklifts in some facilities can be a large and variablecontributor to the load. Although many forklifts may be in a spaceat one time, they do not all operate at the same energy level. Forexample, the energy used by a forklift while it is eleva
46、ting or low-ering forks is different than when it is moving.Processing Equipment. Grinding, mixing, or cooking equip-ment may be in refrigerated areas of food processing plants. Otherheat sources include equipment for packaging, glue melting, orshrink wrapping. Another possible load is the makeup ai
47、r for equip-ment that exhausts air from a refrigerated space.Fig. 2 Variation of Qmax/A with A/P Using Conditions fromTables 1 and 2Fig. 2 Variation of qmax/A with A/P Using Conditions from Tables 4 and 5Q2Q3Q4+n-24.4 2010 ASHRAE HandbookRefrigerationPeople. People add to the heat load, in amounts d
48、epending onfactors such as room temperature, type of work being done, type ofclothing worn, and size of the person. Heat load from a person qpmay be estimated asqp= 1295 11.5t (10)where t is the temperature of the refrigerated space in F. Table 7shows the average load from people in a refrigerated s
49、pace as cal-culated from Equation (10).When people first enter a storage they bring in additional surfaceheat. Thus, when many people enter and leave every few minutes,the load is greater than that listed in Table 7 and must be adjusted.A conservative adjustment is to multiply the values calculated inEquation (10) by 1.25.Latent Load. The
