Page 236 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

For the simulation shown, pressure drops across piping and heat exchangers have not been considered.
When the circulation rate of R-152a is 65.3 kmol/h, the duty of the evaporator is 1 GJ/h. The compressor
is assumed to be 75% efficient and the loads on the equipment are as follows:

Compressor Power = 66.5 kW (at 75% efficiency)

Condenser Duty = 1.24 GJ/h

Evaporator Duty = 1.00 GJ/h
Compressor work per unit of cooling = (66.5)/(1,000,000/3600) = 0.2394

This value compares with 0.144 for the Carnot cycle. The main differences are due to the inefficiencies in
the compressor and the use of a throttling valve instead of a turbine.
The cost of refrigeration at 5°C = (66.5)(0.06) + (1.24)(0.354) = 3.99 + 0.44 = 4.43 $/h = 4.43 $/GJ

Using the results of Example 6.4, we can predict the cost of refrigeration at –20°C and –50°C as
The cost of refrigeration at –20°C = (4.43)(1.78) = $7.89/GJ
The cost of refrigeration at –50°C = (4.43)(2.96) = $13.11/GJ

For refrigeration systems operating at less than temperatures of approximately –60°C, the simple
refrigeration cycle shown in Figures 8.4 and E8.5 is no longer applicable. The main reason for this is that
there are no common refrigerants that can be liquified at 45°C under reasonable pressures (not
excessively high) and still give the desired low temperature in the condenser also at reasonable pressures
(not excessively low). For these low-temperature systems, some form of cascaded refrigeration system is
required. In such systems, two working fluids are used. The primary fluid provides cooling to the process