Subcooling, refrigeration

Linear regression results show that b is approximately 0.5 for each experimental condition and that a increases with the rate of crystal growth due to environmental conditions. The refrigerant subcooling, aT, was seen to directly effect the value of a, whereas, since the bulk subcooling changed with time, there was no clear correlation between a and aT),. Further work will be necessary to delineate these relationships for both batch and semi-batch conditions. 

Some assumptions must be made regarding the condenser coil performance, and this may have a AT of 14 K between the entering air and condensing refrigerant and subcooling the liquid 5 K, with suction gas entering the compressor with 6 K superheat. 

Calculation of the power requirements for the three refrigerants requires the flowrate to be calculated for a duty of 3 MW. This can be calculated from the enthalpy difference across the evaporator (H2 — H1). The enthalpy difference across the evaporator is assumed to be the difference between the saturated vapor enthalpy at the evaporator pressure and the saturated liquid enthalpy at the condenser pressure. This assumes no subcooling of the refrigerant. 

The tubes in the condenser required for subcooling steal heat-transfer surface area required for condensation. In effect, the condenser shrinks. This makes it more difficult to liquefy the refrigerant vapor. The vapor is then forced to condense at a higher temperature and pressure. Of course, this raises the compressor discharge pressure. And, as we have seen in the pressure section, this increase in compressor discharge pressure invariably reduces the compressor s capacity and may also increase the horsepower needed to drive the compressor. 

An increase in the elevation that the condenser effluent must flow up into the receiver will reduce the pressure of the liquid refrigerant, in the same way as the 10-psig piping friction losses. This loss in pressure, due to increased elevation, will also require an increase in the surface area of the condenser sacrificed for subcooling. 

The effect of subcooling in the film will next be considered. As indicated previously, the sensible heat transfer associated with the subcooling of the liquid film is often small compared to the latent heat release and has been neglected in the above analysis. However, for working fluids with low values of latent heat, such as most refrigerants, a correction to the above analysis to account for subcooling may sometimes be needed. 

A vapor-compression refrigeration system is conventional except that a countercurrent exchanger is installed to subcool the liquid from the condenser by heat exchange with the v stream from the evaporator. The minimum temperature difference for heat transfer is 10(°F). Amm is the refrigerant, evaporating at 22(°F) and condensing at 80(°F). The heat load on the evapo is 2,000(Btu)(s) . If the compressor efficiency is 75 percent, what is the power requirement ... 

FIG. 11 -75 Refrigeration system with a heat exchanger to subcool the liquid from the condenser. 

The bypassed vapor heats up the liquid there, thereby causing the pressure to rise. WTien the bypass is closed, the pressure falls. Sufficient heat transfer surface is provided to subcool the condensate, (f) Vapor bypass between the condenser and the accumulator, with the condenser near ground level for the ease of maintenance When the pressure in the tower falls, the bypass valve opens, and the subcooled liquid in the drum heats up and is forced by its vapor pressure back into the condenser. Because of the smaller surface now exposed to the vapor, the rate of condensation is decreased and consequently the tower pressure increases to the preset value. With normal subcooling, obtained with some excess surface, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control, (g) Cascade control The same system as case (a), but with addition of a TC (or composition controller) that resets the reflux flow rate, (h) Reflux rate on a differential temperature controller. Ensures constant internal reflux rate even when the performance of the condenser fluctuates, (i) Reflux is provided by a separate partial condenser on TC. It may be mounted on top of the column as shown or inside the column or installed with its own accumulator and reflux pump in the usual way. The overhead product is handled by an alter condenser which can be operated with refrigerant if required to handle low boiling components. 

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