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109 of the liquid prior to it entering the expansion device. By doing so, the capacity for refrigeration is increased. Figure 6.1 Diagram identifying one possible location for the subcooler. Figure 6.1 shows a suitable location for the subcooler in the system. It may be placed before or after the suction-line heat exchanger in both the freezer and the refrigerator cycles. In other words, the location of the subcooler is flexible, so long as the ultimate goal of subcooling the freezer liquid is achieved. In addition, the state of the refrigerant at the inlet or the outlet of the subcooler is not fixed. The state of the refrigerant on the refrigerator side Suction-line heat exchanger Subcooler Refrigerator evaporator Refrigerator & freezer condenser Capillary tube Freezer compressor Freezer evaporator Mullion coils Capillary tube Refrigerator compressor Mullion coils T 2 T 1 T 3 T 4
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110 may be saturated either at the entrance or the exit of this device. Hence, there exist a variety of combinations that are possible and may each produce a different effect on the performance of the overall system (the best location for the subcooler is explained in the next section). The construction of the subcooler may be similar to that of the suction-line heat exchanger (shown in Figure 5.4). The copper tubes that carry refrigerant from the freezer condenser and the refrigerator evaporator may be put in direct communication with other. The heat transfer rate depends, in part, on the surface area available for the fluids to come in contact. If need be, the subcooler can be made longer to increase the surface area, but this results in additional pressure drop. To prevent the refrigerator vapor from absorbing any heat from the environment, the tubes should also be well insulated. However, the transfer of energy from the freezer liquid to the refrigerator vapor by this method may require that both cycles be run simultaneously. To avoid this constrain, the freezer liquid-line may be connected to the refrigerator suction-line by a set of fins. When the freezer cycles on, the energy from the freezer liquid is transferred onto the fin, which “stores” this energy and transfers it to the refrigerator vapor when the refrigerator cycles on. The ability of the subcooler to transfer heat from the freezer to the refrigerator is described by its effectiveness. Since the product of mass and specific heat is larger for the refrigerant in the freezer cycle, the effectiveness of the subcooler is defined as 1 3 1 2 T T T T = ε Eqn. 6.1 where the subscripts correspond to the states in Figure 6.1. The effectiveness, then, is the parameter that affects the extent of subcooling that the freezer liquid experiences.
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111 6.2 Advantages and Disadvantages of Mechanical Subcooling In some ways, a subcooler behaves like a suction-line heat exchanger. The desired increase in capacity is achieved by means of subcooling the condenser liquid. Another advantage of subcooling is the removal of any residual liquid that leaves the evaporator, thus protecting the compressor.
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