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rate at which heat would have to be removed to convert 1,000 kg of water at 0°C into ice at the same temperature in a 24-hour period. The latent heat of fusion of water is 79.68 kcal/kg. Hence, a ton of refrigeration is equal to a rate of heat removal of 3,320 kcal/h. Enthalpy. Enthalpy, a measure of the heat quantity in a product, is a relative quantity. For most refrigerants, the enthalpy of the saturated liquid at 0°C is by convention set at 200 kcal/kg. For frozen foods, the enthalpy at — 40°C is by convention set at 0 kcal/kg. Refrigeration effect. The net refrigeration effect for a system under given operating conditions can be calculated from the pressure-enthalpy diagram as

RE = iig — Hf where RE is the refrigerating effect (kcal/kg), Hg is the enthalpy of vapor refrigerant leaving the evaporator, and H{ is the enthalpy of the refrigerant entering the evaporator. In the example of the mechanical refrigeration cycle in Figure 4, RE is given by H2 — Hv Quantity of refrigerant circulated per ton of capacity. This quantity is self-defining.

Heat of compression. This is the enthalpy difference in the refrigerant between compressor discharge and inlet. In Figure 4, it is represented by H3 — H2. It can be expressed as kcal/kg.

Work of compression. This is the product of refrigerant mass flow and heat of compression. It can be expressed as kilocalories per minute. Standard conversions allow it to be expressed in other units where necessary. Condenser heat load. This is determined by subtracting the enthalpy of the saturated liquid leaving the condenser from the enthalpy of the superheated vapor entering the condenser: H3 — H^.

Coefficient of performance. This is the ratio of output to input. The theoretical value is given by the refrigeration effect divided by the heat of compression. This, however, ignores friction losses in the compressor, which cause the heat input from the compressor (ie, the work of compression) to be less than the energy required to operate the compressor. The practical coefficient of performance is often only 60% of the theoretical value.

The choice of refrigerant is made on the basis of several criteria. These include (10):

1. The latent heat of evaporation. A high latent heat is preferred.

2. The pressure that would be experienced in the condenser at the condensing temperature. Excessively high pressures are to be avoided.

3. The freezing temperature of the refrigerant must be below the temperature of the evaporator.

4. The critical temperature of the refrigerant must be sufficiently high to give an effective working diagram for the temperatures experienced in the high-pressure side.

5. The refrigerant should ideally be nontoxic, noncor-rosive, and chemically stable.

6. It should be easy to detect leaks in the system.

7. For industrial applications, where large quantities of refrigerant are used, low-cost refrigerants are preferred.

Clearly, not all these criteria can be satisfied by any one refrigerant. Ammonia has a high latent heat of evaporation, and leaks are easily detected. However, it is toxic and corrosive. On balance, the ease of leak detection renders it safe for use despite its toxicity. The fluorocarbons are also commonly used in refrigeration units. They have a wider range of applicable temperatures than ammonia, but environmental concerns are associated with their use.

Where lower temperatures are required, it is possible to use multistage refrigeration, where essentially two or more refrigeration cycles are coupled—the low stage of one is the high stage of the next. An alternative approach is to have two-stage compression (Figure 6).

To freeze a product, it must in some way be coupled to the refrigeration effect. Freezing equipment uses a variety of solutions. Where mechanical refrigeration is used, the cooling source is the evaporator. It is possible to put the product in direct contact with the evaporator, as is the case in some designs of plate freezers, where the plates themselves are machined internally as the evaporator. It is more common, however, to use some form of secondary refrigerant, which is cooled by passing through the heat exchanger of the evaporator, and then to contact this fluid, directly or indirectly, with the object. The secondary cooling medium may be a variety of fluids or air. The ability of this fluid to transport heat has an influence on the freezer design. Fluids based on aqueous solutions have a high volumetric heat capacity, so the volume of fluid required to transport a given quantity of heat is not too high. Air has a much smaller volumetric heat capacity (given the much lower density), so the volume of air required to transport the same quantity of heat is much greater. Freezers that use air as the transfer fluid need powerful fans to blow large volumes of air past the product to be frozen if heat

Figure 6. Two-stage refrigeration, schematic.

removal is to be rapid. A variety of freezer designs exist (11). In plate freezers, the product is in contact with metal plates, cooled either directly (ie, the plates are themselves the evaporator coil) or indirectly, by circulating a secondary refrigerant (usually an aqueous solution such as glycol solution or brine of sufficiently low freezing point) through the coolant channels. This gives effective cooling of the product if the geometry of the product results in good thermal contact with the plates, but poorer cooling occurs if air spaces interrupt the thermal contact. An alternative cooling method is direct immersion of the product in the secondary cooling fluid. Using aqueous secondary refrigerants with high volumetric heat capacity, immersion results in excellent cooling rates, but the potential for contamination of the product exists. Using air as refrigerant, the potential for contamination is much less, but the volumetric heat capacity is much lower, so the heat transfer capabilities depend on the volume of cold air that can be brought into close contact with the product in unit time. A wide variety of freezer designs exist that use air as the heat transfer fluid. Static freezers use natural air circulation, and forced air (blast) freezers use a high-velocity cold air stream that passes by the product. Many mechanisms have been evolved for ensuring product throughput, and batch and continuous processes exist. In either case, air temperature and air velocity are critical to the rate of heat removal.

The common characteristic of the mechanical refrigeration systems so far discussed is their ability to produce cooling at the throw of a switch. This characteristic is also found in thermoelectric cooling systems (12). Thermocouples are devices that produce an electrical voltage as a result of temperature difference. Thermoelectric coolers are the reverse concept—devices that produce a temperature difference on passage of an electric current. If the higher temperature node is at ambient temperature, the lower temperature node is a heat sink. Using modern semiconductor materials, effective thermoelectric cooling devices have been manufactured that can be used to provide refrigeration. These are used primarily in special situations. They do not have the high capacities of mechanical refrigeration systems.

Mechanical or thermoelectric refrigeration is not the sole means of providing a heat sink. Just as ice has traditionally been used as a cooling source, cryogenic media may be used as a cooling source. As with ice, these are not turnkey systems to produce cooling at the throw of a switch. Rather, these are systems where cooling capacity is stored in a convenient form. Special cooling processes are used to produce the cryogenic medium. In food freezing, common cryogenic coolants are liquid nitrogen or solid carbon dioxide. These are produced centrally and distributed and stored in special insulated containers before use. The properties of the cryogens are summarized in Table 3. These cryogens can be used directly, bringing them into contact with the product to be frozen. The vaporization of the cryogen is an important component of the heat sink capacity, but the heat required to raise the temperature of the cryogen is also an important source of cooling capacity. Because the three primary drivers in rates of heat transfer are the volumetric contact rate between coolant and product, the volumetric heat capacity of the coolant, and the

Table 3. Thermal Properties of Cryogenic Refrigerants

Property

Nitrogen

Carbon Dioxide

Boiling point (°C)

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