Cen58933_ch04

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Unformatted text preview: in several hours by cold plates. Very high freezing rates can be obtained by immersing the packaged birds into a low-temperature brine. The freezing time of birds in 29°C brine can be as low as 20 min, depending on the size of the bird (Fig. 4–45). Also, immersion freezing produces a very appealing light appearance, and the high rates of heat transfer make continuous line operation feasible. It also has lower initial and maintenance costs than forced air, but leaks into the packages through some small holes or cracks remain a concern. The convection heat transfer coefficient is 17 W/m2 · °C for air at 29°C and 2.5 m/s whereas it is 170 W/m2 · °C for sodium chloride brine at 18°C and a velocity of 0.02 m/s. Sometimes liquid nitrogen is used to crust freeze the poultry products to 73°C. The freezing is then completed with air in a holding room at 23°C. Properly packaged poultry products can be stored frozen for up to about a year at temperatures of 18°C or lower. The storage life drops considerably at higher (but still below-freezing) temperatures. Significant changes Giblets Inside surface 13 mm depth Under skin 6 5 4 3 2 1 0 – 84 –73 –62 –51 –40 –29 –18 Air temperature, degrees Celsius –7 Note: Freezing time is the time required for temperature to fall from 0 to –4°C. The values are for 2.3 to 3.6 kg chickens with initial temperature of 0 to 2°C and with air velocity of 2.3 to 2.8 m/s. FIGURE 4–44 The variation of freezing time of poultry with air temperature (from van der Berg and Lentz, Ref. 11). cen58933_ch04.qxd 9/10/2002 9:13 AM Page 248 248 HEAT TRANSFER 5 0 Giblets Temperature, °C –5 FIGURE 4–45 The variation of temperature of the breast of 6.8-kg turkeys initially at 1°C with depth during immersion cooling at 29°C (from van der Berg and Lentz, Ref. 11). TABLE 4–7 Thermal properties of poultry Quantity Typical value Average density: Muscle 1070 kg/m3 Skin 1030 kg/m3 Specific heat: Above freezing 2.94 kJ/kg · °C Below freezing 1.55 kJ/kg · °C Freezing point 2.8°C Latent heat of fusion 247 kJ/kg Thermal conductivity: (in W/m · °C) Breast muscle 0.502 at 20°C 1.384 at 20°C 1.506 at 40°C Dark muscle 1.557 at 40°C –10 –15 Inside surface –20 38 mm depth 25 mm depth 13 mm depth 6.5 mm depth Under skin Skin surface –25 –30 –35 0 25 50 75 100 150 125 Time, min. 175 200 225 250 occur in flavor and juiciness when poultry is frozen for too long, and a stale rancid odor develops. Frozen poultry may become dehydrated and experience freezer burn, which may reduce the eye appeal of the product and cause toughening of the affected area. Dehydration and thus freezer burn can be controlled by humidification, lowering the storage temperature, and packaging the product in essentially impermeable film. The storage life can be extended by packing the poultry in an oxygen-free environment. The bacterial counts in precooked frozen products must be kept at safe levels since bacteria may not be destroyed completely during the reheating process at home. Frozen poultry can be thawed in ambient air, water, refrigerator, or oven without any significant difference in taste. Big birds like turkey should be thawed safely by holding it in a refrigerator at 2 to 4°C for two to four days, depending on the size of the bird. They can also be thawed by immersing them into cool water in a large container for 4 to 6 h, or holding them in a paper bag. Care must be exercised to keep the bird’s surface cool to minimize microbiological growth when thawing in air or water. EXAMPLE 4–5 Chilling of Beef Carcasses in a Meat Plant The chilling room of a meat plant is 18 m 20 m 5.5 m in size and has a capacity of 450 beef carcasses. The power consumed by the fans and the lights of the chilling room are 26 and 3 kW, respectively, and the room gains heat through its envelope at a rate of 13 kW. The average mass of beef carcasses is 285 kg. The carcasses enter the chilling room at 36°C after they are washed to facilitate evaporative cooling and are cooled to 15°C in 10 h. The water is expected to evaporate at a rate of 0.080 kg/s. The air enters the evaporator section of the refrigeration system at 0.7°C and leaves at 2°C. The air side of the evaporator is heavily finned, and the overall heat transfer coefficient of the evaporator based on the air side is 20 W/m2 · °C. Also, the average temperature difference between the air and the refrigerant in the evaporator is 5.5°C. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 249 249 CHAPTER 4 Determine (a) the refrigeration load of the chilling room, (b) the volume flow rate of air, and (c) the heat transfer surface area of the evaporator on the air side, assuming all the vapor and the fog in the air freezes in the evaporator. SOLUTION The chilling room of a meat plant with a capacity of 450 beef carcasses is considered. The cooling load, the airflow rate, and the heat transfer area of the evaporator are to be determined. Assumptions 1 Water evaporates at a rate...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.

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