Refrigeration

Abstract

Cooling is a fundamental operation in food processing and preservation. Removal of heat could involve either the transfer of heat from one fluid to another or from a solid to a fluid, or it could be accomplished by adiabatic vaporization from liquid water. Knowledge of the principles of heat transfer is an essential prerequisite to the understanding of the design and operation of refrigeration systems.

Problems

• 10.1. An ammonia refrigeration unit is used to cool milk from 30 °Cto1 °C (86 °F–33.8 °F) by direct expansion of refrigerant in the jacket of a shell and tube heat exchanger. The heat exchanger has a total outside heat transfer surface area of 14.58 m² (157 ft²). To prevent freezing, the temperature of the refrigerant in the heat exchanger jacket is maintained at −1 °C (31.44 °F).

  • (a) If the average overall heat transfer coefficient in the heat exchanger is 1136 W/m² ≅ K (200 BTU/h ≅ ft² ≅ °F) based on the outside area, calculate the rate at which milk with a specific heat of 3893 J/kg K(0.93 BTU/lb ≅ °F) can be processed in this unit.

  • (b) Determine the tons of refrigeration required for the refrigeration system.

  • (c) The high-pressure side of the refrigeration system is at 1.72 MPa (250 psia). Calculate the horsepower of the compressor required for the refrigeration system assuming a volumetric efficiency of 60%.

• 10.2. A single-stage compressor in a Freon 12 refrigeration system has a volumetric efficiency of 90% at a high side pressure of 150 psia (1.03 MPa) and a low side pressure of 50 psia (0.34 MPa). Calculate the volumetric efficiency of this unit if it is operated at the same high side pressure but the low side pressure is dropped to 10 psia (68.9 kPa). Assume R12 is an ideal gas.

• 10.3. Calculate the tons of refrigeration for a unit that will be installed in a cooler maintained at 0 °C (32 °F) given the following information on its construction and operation:

• The cooler is inside a building:

• Dimensions: 4 × 4 × 3.5 m (13.1 × 13.3 × 11.5 ft)

• Wall and ceiling construction:

• 3.175 mm (1/8 in.) thick polyvinyl chloride sheet inside (k of PVC = 0.173 W/m ≅ K or 0.1 BTU/h ≅ ft ≅ °F)

• 15.24 cm (6 in.) fiberglass insulation

• 5.08 cm (2 in.) corkboard

• 3.17 mm (1/2 in.) PVC outside

• Floor construction:

• 3.175 mm (1/8 in.) thick floor tile (k = 0.36 W/(m ≅ k) or 0.208 BTU/(h ≅ ft ≅ °F)

• 10.16 cm (4 in.) concrete slab

• 20.32 cm (8 in.) air space

• Concrete surface facing the ground at a constant temperature of 15°C (59°F)

• Door:

• 1 m wide × 2.43 m high (3.28 × 8 ft)

• Design for door openings that average four per hour at 1 min per opening

• Air infiltration rate:

• 1 m³/h (35.3 ft³/h) at atmospheric pressure and ambient temperature

• Ambient conditions:

• 32 °C (89.6 °F)

• Product cooling load:

• Design for a capability to cool 900 kg of product (C_P = 0.76 BTU/lb ≅ °F or 3181 J/kg ≅ K) from 32 °C to 0 °C (89.6 °F to 32 °F) in 5 h. The freezing point of the product is −1.5 °C (29.3 °F).

• 10.4. The “stack effect” due to a difference in temperature between the inside and outside of a cooling room is often cited as the major reason for air infiltration. In this context, ΔP is positive at the lowest section of a cooler and is negative at the highest section, with a zone, called the neutral zone, at approximately the center of the room where the ΔP is zero . If the area of the openings at the lowest sections where ΔP is positive equals the area of the openings in the highest sections where ΔP is negative, air will enter at the top and escape at the openings in the bottom at the same volumetric rate of flow (assuming no pressure change inside the room). If the room allows air leakage at the rate of 2% of the room volume per minute at a ΔP of 0.5 in. wg (124 Pa), determine the rate of air infiltration that can be expected in a room that is 2 m (6.56 ft) high to the neutral zone if the interior of the room is at −20 °C (−4 °F) and ambient temperature is 30 °C (86 °F). The rate of gas flow through the cracks is proportional to the square root of ΔP. Assume air is an ideal gas. ΔP due to a column of air of height h at different temperatures = g(ρ₁ − ρ₂)h, where ρ₁ and ρ₂ are the densities of the columns of air .

• 10.5. For a 1-ton refrigeration unit (80% volumetric efficiency) using refrigerant 12 at a high side pressure of 150 psia (1.03 MPa) and a low side pressure of 45 psia (0.31 Mpa), operating at an ambient temperature of 30 °C (86 °F), determine the effect of the following on refrigeration capacity and on HP/(ton)_r. Assume the same compressor displacement in each case.

  • (a) Reducing the evaporator temperature to −30 °C (−22 °F). High side pressure remains at 150 psia (150 MPa).

  • (b) Increasing ambient temperature to 35 °C (95 °F). (Low side pressure remains at 45 psia [0.31 Mpa]). The high side pressure is to change such that ΔT between the hot refrigerant gas and ambient air remains the same as in the original set of conditions.

  • (c) Air in the line such that the vapor phase of refrigerant always contain 10% air and 90% refrigerant by volume. Assume condensation temperature of hot refrigerant gas and temperature of cold refrigerant gas are the same as in the original set of conditions (partial pressure of refrigerant gas at the low and high side pressures are the same as in the original set of conditions, 45 and 150 psia or 0.31 and 1.03 MPa). Use R = 1.987 BTU/(lbmole ≅ °R) or 8318 J/(kg ≅ K). The specific heat ratio C_P/C_v for air is 1.4.

  • (d) Oil trapped in the vapor return line such that ΔP across the constriction is 10 psi (68.9 kPa). Assume evaporator temp. = 0 °C.

• 10.6. Chopped onions are frozen in a continuous belt freezer at using −50 °C air at high velocity. When onions with a moisture content of 86% are loaded on the belt with a thickness of 2 cm., it took 20 min for the temperature to drop from 10 °C to −20 °C. Onion juice is added and mixed with the chopped onions in a ratio 0.10 parts juice to 0.90 parts of the chopped onions. The freezing point of the chopped onions is −0.5 °C. The juice contains 1.5% solids (all soluble). The juice has a freezing point of −0.16 °C. Assuming that the rate of heat transfer is the same, calculate the time required to freeze the onions with the added juice.

• 10.7. A MAP pouch with a volume of 2 l empty is filled with chicken thigh meat at a temperature of 5 °C. The pouch is positioned inside a vacuum packaging machine inside a processing room where the room temperature is the same as that of the chicken meat. A vacuum of 25 in. Hg is drawn, and the pouch is back flushed with a gas mixture consisting of 30% CO₂ and 70% N_2. By volume. If the vacuum drawn was 25 in. Hg, and the ratio of void volume to meat volume in the pouch is 1:2 by volume, calculate the resulting gas composition in the pouch after the pouch is sealed. Atmospheric pressure is 101 kPa, and air originally inside the pouch is 21% oxygen and 70% nitrogen by volume .