Document 3Q5nBjK7g3o011q7q8J4kLxYE

342 CHAPTER 31 1962 Guide And Data Boole Table 2 .... Load Calculations, Beef Holding Cooler Sire (Feel)] 72 XI09X 15 Cooler Capacity: 520 carton** AtnaMd CM) brie 10 F deg in 4B hr Assumed Air Cintdatiom 4QJ00Q ctm Loading Them loaded at random Average Corcon Weigh*: 560 lb Assumed Air lo Col: 34 F, 96% Hi Aturned Fan Horsepower: 10 hp Het Gain--Rood lood 1. Transmission, infiltration, personnel, and equipment beat 2. Product heat (average, first 24 hr): a. 520 X 560 - 0.75 X 10 X 0.03 b. 520 X 3.5 X 0-03 X 1070 3. Total heat gain (room load), Btuh (Items 1 + 2a 4- 2b) Seeable Heat 128,500 65,000 -59,000 134,500 4. Air circulation, lb/hr dry air 40,000 X 0.08 X 60 = 192,000 5. Heat removed per Ibdry air, Btu (Total heat Item 3)/(ltem4) = 1.023 6. Air-vapor enthalpy, Btu/lb dry air; a. Air to coil, 34 F 96% rh b. Btu removed, temp drop 2.9 F deg c. Air from coil, 31.1 F 100% rh 7. Coil air-vapor heat removal, Btuh (Item 4) (Item 6b) 8. Room vapor condensed to fog (Item 7) - (Item 3) 9. Water (ice) removed by coil 520 X 3.5 X 0-03 X 144 10. Total heat removal (coil load), Btuh (Items 3+8+9) SeesSde Heat __ -- 8.167 .700 7.467 134,500 -- -- 134,500 toads, Btuh latent Heat 3,000 59,000 62,000 loads, Stall Ictod Heat __ -- 4.227 .323 3.904 62,000 -- 8,000 70,000 Total Heat 131,500 65,000 196,500 Total Heat __ -- 12.394 1.027 11.371 196,500 --8,000 204,500 avenge rate for the first 24 hours (3 percent) should be used for design. Under peak chilling and holding room conditions, water trapped and condensed out by the coils imposes a further hidden or latent load on the evaporators, This occurs in the form of heat extracted to freeze condensed water to ice, or of heat removed to chill the returning wanned and strengthened spray solution. Ln the absence of a more complex evaluation, this load may be considered roughly equal to the latent heat of fusion (144 Btu/lb) of the water removed. Based on the above data, cooler loads may be calculated as illustrated in Tables 1 and 2. Transmission, infiltration, personnel, and equipment loads are estimated by standard methods. Note that sensible losses are less in the holding room than in the chilling room because of the lesser fan horse power required. The complete calculation is made to illustrate the heat removal process associated with the chilling-drying of the carcass; and in particular to illustrate that the sensible heat ratio of the heat transfer taking place in the coil cannot be used to measure the amount of water removed from the space when fog is involved, in practice, the designer soon learns to make selection on the basis of total heat load alone without performing the complete calculation, but he should under stand the role of fog in heat transfer under conditions of high vapor load. Evaporator Selection Evaporator selection requires the specification of coil ca pacity, refrigerant temperature, and air-vapor circulation rate. This is a procedure of approximation only, because of the inaccuracies of load determination on the one hand and of predicting sustained field performance of coils on the other. Furthermore, complete freedom of specification rarely exists; for example, air-vapor circulation rate for a given coil may be . limited to avoid spr&y solution carry-over, or to avoid exces sive fan horsepower. Sprayed and dry coil systems can do equally well with re spect to shrink, provided compressor capacity is adequate and the evaporators are correct for the system selected. Evaporator requirements vary widely with the type of system. Table 3 gives comparative evaporator data on typical suc cessful ftooded-coil installations in the chilling cooler. The chilling capacity has been expressed as the product of the coil U-vaJue, the total ooil surface area, and temperature difference between refrigerant and room air. The coil U-value (overall heat transfer coefficient) and air flows shown describe sustained field performance under actual chilling conditions and loads, and are not to be con fused with clean-coil test ratings. The U-value varies greatly with the character of the ooil and its operation. It is influ enced by such variables as the ratio of extended-to-prime surface, which may, for example, range from 7-to-l to 21-to-l in standard dry coils; coil depth, which ranges typically from 8 to 12 rows in sprayed coils and from 4 to 6 rows in dry coils; fin spacing, which may be 3 or 4 per inch in typical dry coils; condition of the surface, which may be continuously defrosted or coated generally with frost; and air flow, which may vary from 250 to 750 cfm per sq ft coil face area. Greater temperature differences (TD) than those shown are sometimes used, but a higher TD is valid only at high room temperatures. The lower TD (10 F) shown for dry coils is desirable to limit frosting. Many dry coil evaporators have higher ratios of extended-to-prime surface and higher air flows per square foot face area than shown. As stated previously, sprayed and dry coil systems, well designed, can do equally well with respect to shrink. The choice between them therefore lies in evaluation of evaporator cost and capacity, operating attention required, effect of brine corrosiveness on cost and maintenance of space and equipment, cost and rate of loss of the brine, and cost and complexity of brine-handling equipment on the one band and defrosting equipment on the other. The difficulties involved in obtaining accurate shrink figures and Fresh Fish Table 3 .... Typical Evaporator Installations, Beef Chitting Cooler Six (Fw*)> 72X )0SXI5 Cooler Capacity: 520 rowtai Oeep-Bauod CIA To 60 f in 20 hr Oetrga Load: I/J73J300 Btvh Col Operation: Flooded Loading Than: 4 hr oaidavta Arerage Carcass Weights 560 lb Attorned Air to Col: 33 F 100% rfc Seats* Heat Batio: 53% 343 Coil Description: pufspacing--fins/inch Co# depth--number of pipe rows Ratio, extended to prime surface Coil face ana, sq ft . Coil surface area, sq ft total Bare pipe 8-row 16 400 Finned 6/inch 8-row 14 16 2,400 Finned 3/inch 4-row 8 16 1,100 Finned 4/inch 6-row 15 14 1,650 grins Flaw {Surface Spray): Flow, gpm flow, gpm/sq ft cou face a pump horsepower 40 2.0 2.5 0.3 0.5 Fan Description, Air Flow: Type of fan Fhyw through coil, cfm Flow, cfm/sq ft coil face a Fan horsepower Vnil Rating* (Total Heat) : U-value, Buih/(sq ft) F deg TD for capacity rating,* F Chilling capacity, Btuh Temp, drop, air through ooil, F Equipment for 6SO Carcasses: Number of unite required Total bp, fans and pumps Coil surface per carcass, sq ft Air flow per carcass, cfm Centrifugal 9,000 550 3 15 90,000 12 40 Centrifugal 12,000 750 5 3.5 15 125,000 50 40 200 Centrifugal 3,000 200 1 2.2 10 24,000 3.7 45 45 Propeller 5,000 350 1.5 2.3 10 38,000 3.7 42 90 250 * TD ia temperature difiereoee between nfrisemnt ead etr. _ ,,. . , ... . . .__ . * atown ere retimeted koto pofcwnnace at setae) eyeteae. Dry eoO r*Uo ere et verse frost eoadiboo. with air Sow reduced by frost Detraction. Wh3% actual tbey are not to be interpreted as aoeepted standards. Otba tsataOatioss empkijine both more asd teas eqmrrom* are in anrrnif il operation. oa carcasses chilled to a specified degree cause wide differences of opinion as to the coil capacity required for good chilling. While the data shown describe actual successful installations, other installations also considered to be successful may differ from thm HOG CHILLING AND TEMPERING The internal temperature of hog carcasses entering the chill coolers from the killing floor varies from 100 to 160F. The specific heat is 0.50 to 0.57. The dressed weight varies from 90 to 450 lb approximately, the average being near 180 lb. Present accepted practice requires that dressed hogs be chilled and tempered to an internal temperature.of 37 to 39 F on &n overnight This limits the chilling and tempering time to 16 to 22 hr. " The cooler and refrigeration equipment must be designed so that the hogs are chilled thoroughly with no frosen parts *t the time the carcasses are moved to the cutting floor. Car cass crowding, reducing exposure to circulated chilled air, and excessively high peak temperatures are detrimental to proper chilling. The following design details of a hog chill cooler will pro ride: 1- Sufficiently quick chilling to retard bacterial development "5 prevent deterioration. cooler shrink from 1.2 to 1.5 percent. A lower shrink "*ud be obtained with additional evaporator surface, however, B^ger research and data available are not conclusive as the benefits. A shrink below 1.2 percent could require that all fresh cuts be rechilled before shipping toprevent off condition due. to excessive moisture in the product. This extra handling could result in added cost that would offset the shrink savings. 3. Firm carcasses suitable for efficient cutting that are dry and bright without frosen surface or internal frost. A description of the design of hog coolers is necessary for both new units as well as remodeled coolers since there exists at the present time many brine spray decks that should be equipped with modern refrigeration equipment. Most brine spray deck refrigerated coolers have rails.that are unusable because of back-lash or carry-over of-brine. The brine laden air results in excessive maintenance due to the corrosion and rusting of steel rails and beams. Design of New Hog Coolers The dressing rate of hogs and the planned hours of operation set the capacity of the hog coolers. However, onaoneahiftbasis it is economically sound and basically desirable to provide cooler hanging capacity for 10 hours dressing in order to (1) properly handle the chilling of large sows that require more than 24-hour exposure in the chill room, (2) handle increased dressing volumes when market conditions warrant overtime operation, and (3) have some flexibility in the unloading and loading of the cooler during normal operations. On a two shift bams extra cooler capacity for overtime operation is not necessary. The rail height should be 9 ft to provide adequate clear ance between the floor and the largest dressed hog. This rail bright is ^Isn the requirement of the M.I.D. and roost