Choice of temperature range, capacity, cycle, boundary conditions and component efficiencies
As an example we have chosen a 500 kW refrigerator, which cools a load from 60 to 40 K. For this temperature range there is no real alternative to a Brayton cycle withcompression at ambient temperature, counter-current heatexchange and the production of refrigeration by work extracting expansion. Fig. 2 shows the chosen process,boundary conditions and component efficiencies, which have been used for the initial process calculations.
The refrigerant is compressed in a three-stage turbo-compressor with inter- and after-coolers, followed by abooster compressor C4, which is driven by turbine T2. The coldbox contains two cryogenic heat exchangers HX1 and HX2. The turbine T1 is introduced to cover the losses of exchanger HX1, whereas turbine T2 produces the refrigeration. The power of T1 is about five to ten times smaller than the power of T2. Therefore no direct power recovery is foreseen. The flow diagram shows a generator brake, but this power recovery is not considered in the overall energy balance, because it is only about 1-2 % of the main compressor input power.
The suction pressure of the main compressor has a large influence on the compactness of the plant, i.e. the volume of the heat exchangers and the diameter of the pipes,as well as on the efficient operation at part load. For this example a pressure of 0.6 MPa has been chosen.
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