The Renewable Energy Refrigeration Facility (RERF) at USQ uses a vapour-compression
refrigeration cycle operating with R134a as the working fluid to extract heat from a cold room at the
rate of 4 kW. An illustrative diagram of the cycle is shown in Figure 3. The temperature of the air in
the cold room is maintained at 4°C, and to enable the use of a moderate-sized evaporator, the
temperature difference between the cold air and the refrigerant should be 10°C. In a conventional
cycle, the condensation of the refrigerant would have to occur at 45°C to enable effective heat
exchange with the outside air which is at a temperature of 30°C. However, the advantage of the
RERF cycle lies in the fact that the refrigerant condenses at a relatively low temperature of 25°C in a
heat exchanger. In the RERF cycle the heat from the cold room is delivered via a counter-flow heat
exchanger into a heat pump arrangement driven by solar-thermal energy. The target operating
condition requires 6°C of superheating on the inlet to the compressor as a margin for safety as we
don't want any possibility of liquid entering the compressor. Sub-cooling of 3°C is specified for
R134a at the end of the condensation process in the heat exchanger. The pipework connecting the
components and the heat exchangers have been well sized so that we don't have to consider pressure
changes in these devices when analysing the system.
to solar thermal
Figure 3. Illustration of the RERF cycle.
(a) Determine the values of pressure and specific enthalpy at the entry and exit of the compressor, and
at the entry and exit of the expansion device.
(b) Sketch a pressure-enthalpy diagram showing the cycle, with key points labelled, relative to the
(c) What refrigerant mass flow rate (expressed in kg/s) is required?
(d) Calculate the compressor power (kW), on the assumption that the compression process is
(e) Determine the COP of the system.
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Renewable energy Refrigeration Facility (RERF) at USQ using Vapor compression refrigeration cycle.
The working fluid in the refrigerator is R134a
The rate of heat extraction from the cold rook (cooling load) = 4KW
The temperature of the air (evaporator) in the cold room = 4 degree Celsius.
The temperature difference between the cold air and the refrigerant should be 10 degree Celsius.
The temperature of the refrigerant = - 6 degree Celsius.
Condenser fluid temperature =45 degree Celsius.
Environmental temperature (Surroundings to condenser) = 30 degree Celsius.
Superheating before entry to compressor is 6 degree Celsius...