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340 CHAPTER 31 1962 Guide And Data Boole \\ 22 cul^ MeursfuniiacE tEmskur**a*tcut Ew* it. /I HOC* VAPOH MCSSU T ii 11 i i i n:ACTUAL CAftCAU WOCHT WASHED AMO 16 TJ -T1d. 2HMCWKM LESS 0>rr SMMUO . o 4 12 16 20 24 X032 34 40 H 4 32 M 40 04 tt ft TIME P*M STMT Of CHLL' HOWS Bg. 3----- Typical Beef Carcass Chilling-Holding Shrinkage Curves air raises the temperature of the air immediately surrounding the carcass. The rate of transfer is increased by more rapid circulation of air against the carcass, and by lower air tem perature, but this is limited by the necessity of avoidingsurface-freezing. The exposed fat of the skinned and shrouded beef carcass absorbs and carries into the cooler a large amount of wash and shroud water. This continuously vaporizes and enters the surrounding space, thereby cooling the surface. The vapor pressure of the water solution in the carcass surface is governed by the temperature and dryness of the carcass'surface, and the opposing-pressure of the surrounding atmos pheric vapor is measured by its temperature and relative humidity. The spread between the two pressures is very large at the beginning of the chill, when the carcass is warm and wet. Evaporation proceeds rapidly and in proportion to the spread until the carcass is thoroughly chilled, causing loss of wash and shroud water and some actual shrinkage from dry carcass weight. Estimated differences in vapor pressure between surface water (at average surface temperature) and atmospheric vapor during a typical chilling-holding cycle, and the corre sponding shrinkage curve for an average carcass, are shown in Fig. 3. Note the tremendous vapor pressure differences causing evaporation during the early part of the chill cycle, when the carcass is warm. It is reasonable to question whether the evaporative loss can be reduced by beginning the chill with room temperature high, then lowering it slowly so as to mini mize at all times the pressure difference between carcass sur face water and room vapor. This however, slows the chill and prolongs the period of rapid evaporation. The weight of evi dence is in favor of the quick ehill. Evaporation from the warm carcass in cool air is to a con siderable degree independent of room relative humidity, because the warmth of the carcass surface generates much higher vapor pressure than that of the cooler vapor surround ing the carcass, even when the room air is saturated. If the space surrounding a warm carcass is saturated, evaporation continues nevertheless, with the formation of a fog. This can be observed at the beginning of any chill. The existence of the fog at no time increases atmospheric vapor pressure above that at saturation. The formation of fog in a saturated atmosphere involves a two-stage heat transfer. The worm carcass supplies the heat required to vaporize its surface water, and thereby lower carcass temperature. The freed vapor then (in condensing to fog) transfers its heat of vaporization to the cool air, thu* raising air temperature. The net result is a sensible transfer of heat from the carcass to the air, and a physical movement of water out of the carcass. Evaporation from the well-chilled carcass with surface temperature at or near room temperature is another matter. The spread between surface and room vapor pressures is narrowed, and approaches zero when the room air is near saturation. Evaporation pioceeds slowly, without the for mation of a general fog. Under these conditions, since atmos pheric vapor pressure is proportional to relative humidity, an increase in relative humidity has a pronounced effect io reducing evaporation. It is a misconception to suppose that evaporation invariably ceases when the room is saturated. This is true only if the carcass is chilled through to room temperature and no lw^t transfer is taking place. The room air-vapor mixture is heated by transmission and infiltration, and by heat-producing equipment, as well as by the carcasses. Water vapor enters the space by infiltration, from personnel, and from the carcasses. The resulting mixture (air, vapor, and fog) is cooled sensibly during its passage through the refrigeration coils, typically by 1 to 3 F deg. To the extent that the temperature of the vapor is lowered below the dewpoint, water is condensed out on the coils and removed from the space. Fog is trapped physically on the coil. The ultimate disposition of the water condensed or trapped out on the coils depends on the temperature of the coil sur face and the method of coil operation. In continuous defrost (sprayed coil) operation, condensed and trapped water arepicked up by the solution sprayed over the coil, resulting in its dilution. In non-frosting dry coil operation, condensed water falls to the evaporator pan and drains to sewer. Water frozen on the coil is lost to the sewer if removed by hot gas or coil spray defrost. Periodic room air defrost, however, vaporizes part of the ice and returns it to room atmosphere, while losing the remainder to sewer. The weight changes that take place, in. good current prac tice, may be summarized asfollowsfor a beef carcass of average weight: In tbb Chuxixo CoolS3 Initial dry Weight Wadi water pick-up Initial wet weight Shroud water pick-up* Drip Joss, not vaporized . Weight at start of chill* Weight loss in chill cooler Weight at end of 20-hour chill* Net loss through chill, wet baas*1 Net loss through chill, dry baatf In tbb Boldtwo Cootn Initial weight (shroud removed) Weight loss during 48-hour hold Weight loss per day Final weight after 48-hour bold Lb 550 8 558 6 4 560 13.5 546.5 11.5 3.5 PzaCXKT 2.0 0.7 546.5 3.5 1.7 543 0.3 * Exclusive of 44b weight of the dry rhroud doth. * Ctron weighed ia alter washing, but before shrouding. * Carcass weighed in before washing and shrouding. It should be noted again that chilling of the beef carcass is not completed in the chill cooler, but continues in the holding cooler at a reduced rate. A carcass well-chilled when it Miters the holding cooler will show minimum holding shrink, and one poorly chilled will show high holding shrink. A word of caution on shrink values is in order. If these are Meat- ond Fresh Fish 341 Table I .... Load Calculations, Beef Chilling Cooler Sir* (fed) 72X108 X13 Coelm Capodty: 520 careens* Assumed QaS Roto: 40 F in 20 heart Assumed Air CtrcufofMftr I35JDQ0 efm Heal Cain--Rooo Load j yjnwmfwitm infiltration. oeihcnneL and eouipment heat 2 Product heat (average, first 4 hr): a. 520 X 560 X0.75 X 40 X 0.08 b. 520 X 13.5 X 0.08 X 1070 3. Total heat g*in (room load), Btuh (Items l + 2a + 2b) Heel Removal Ca4 lead 4. Air circulation, dry air Ib/hr 135,000 X 0-08 X 60 " 650,000 * Heatremoved per lbdry air, Btu (Total heatItem3)/(Ttem4) = 1.525 6. Air-vapor enthalpy, Btu/Ib dry air: a. Air to coil, 33 F 100% rh b. Btu removed, temp drop 3.7 F deg e. Air from coil, 29.3 F 100% rh 7. Cod air-vapor heat removal, Btuh (Item 4) (Item 6b) 8. Room vapor condensed to fog (Item 7) -- (Item 3) 9. Water Cce) removed by coil 520 X 13.5 X 0.08 X 144 .10. Total heat removal (coil load), Btuh (Items 3+8+9) Loading Tim*; 4 hr maximum Avtng* Cerceu Weight: 560 lb Assumed Air to Coft 33 F 100% Ht Amrn**** Fan Horsepower. 45 hp Sensible Heai 288,000 700,000 -620,000 368,000 Sensible Heat -- -- loads, Btuh loteat Heat 3,000 620,000 623,000 Loads, Stub latent Heat _ -- Total Heat 291,000 700,000 991,000 Total Heat _ -- 7-927 0.880 7.047 572,000 204,000 572,000 4.242 0.645 3.597 419,000 -204,000 82,000 501,000 12.169 1.525 10.644 991,000 -- 82,000 1,073,000 to have any significance, they must be derived with great care. Actual product loss must be determined by first weigh ing the dry carcass prior to washing, then out of the cooler with the shroud removed. In-motion weights are not suffi ciently precise; carcasses must be weighed at rest. Scales must be accurate, and the same scale if possible used before and after chilling. If the shrinkage is to have any comparison value it must be measured on carcasses chilled to the same tem perature, since the chilling occurs largely by evaporative wright loss. Design Conditions and Refrigeration Load It is sound practice to base equipment selection on condi tions at peak load, when product loss is greatest. Room losses, equipment heat, and carcass heat add up to a total load that varies greatly throughout the chill, not only in magnitude but also in proportion of sensible to total heat (sensible heat ratio). As the chill progresses, the vapor load decreases and the sensi. ble load becomes more predominant. ' Under peak chilling load, more water is vaporized from the. warm carcass than can exist as vapor at room temperature. The.excess condenses into fog--enough to warm the airvapor-fog mixture to the sensible heat ratio of the heat re1320vai process. The heat removal process of the coil therefore under-estimates the actual rate of water removal by the amount of vapor condensed to fog, as shown in Table 1. Under later chilling room loads, and all holding room loads, fog does not form generally, although it may form locally and re-vaporize. Sensible heat ratios of air-vapor heat gain aad air-vapor heat removal are then equal, as shown in Table 2. It has been customary in beef chilling rooms, particularly with sprayed coil systems, to provide evaporator capacity aufficient to hold room temperature under load approximately to shown in Fig. 2. This results in a rise in room temperature to 35 to 40 F, with gradual reduction thereafter to 32 to 34 F. Many recent installations, however, provide greater capacity. particularly dry oil systems which thereby avoid excessive coil frosting. In batch-loaded coolers, room temperature may be as low as 25 F under peak load, provided it is raised to 30 F as the chill progresses, without surface-freezing of the beef. The shrink improvement effected by. such lower tem peratures, however, tends to be less than expected (in beef chilling) because of the relatively small part played by sensible transfer of heat. It is standard practice in the holding room to provide evaporator capacity to hold the room temperature at 32 to 34 F at all times. Holding room coils sized at peak load, low air-vapor circulation rate, and a coil temperature IQ F below room temperature tend to maintain the 90 to 95 percent rela tive humidity that avoids excessive shrink while preventing surface sliming. From the average temperature curve of Fig. 2 and the shrinkage curve of Fig. 3 certain generalizations, useful in calculating carcass shilling load, may be made-. In the chilliog cooler, the average carcass temperature is reduced approximately 40 F (from 85'F average first-hour temperature to 45 F) in 20 hr. Simultaneously, about 13.5 lb of water is vaporized per carcase (of which only 3.5 lb ia actual shrinkage). The loss of sensible heat and of water occur at about the same rate; averaging 8.6,4.5, 3.5, and 3 percent of the total per how during the five successive four-hour periods represented. For a chilling cooler batch-loaded over a four-hour period, the maximum rate of 8 percent should be used in design. This tallies with the old rule-of-thumb that peak load is approximately 1.5 times average load. In the holding cooler, the average carcass temperature is reduced approximately 10 F deg (from 45 to 35 F) in 48 hours. Simultaneously, about 3.5 lb of water will be vaporized per carcass (all actual shrinkage). Here also, the loss of sensible heat and of water occur at about the same rate; aver aging 5, 4,3, 2, 2,2, 2,1,-1,1,1, and 1 percent of the total per hour during the twelve successive four-hour periods repre sented. Because the holding cooler is loaded at random, the