CLASS NOTES FOR
Cascade refrigeration creates low
temperatures in the range of -700C to
-1000C. For the same purpose, multi-
stage vapor compression refrigeration
systems are in use. Cascade refrigeration
system has numerous advantages over
multi-stage vapor compression systems.
Cascade Refrigeration Systems
Need of Cascade Refrigeration
Many industrial and medical applications require very low-temperatures. Single-stage compression refrigeration systems cannot achieve such low temperatures. It is because of limitations of an individual refrigerant. In such situations, a cascade system is a better alternative. Multi-stage refrigeration systems are possible. These have inherent disadvantages compared to Cascade refrigeration systems.
Practical Applications of Cascade Refrigeration
To store blood at around -700C in the hospital to increase the life of the blood
To cool light hydrocarbons gases
At minus 70 degree Celsius, Pfizer’s ultra-cold COVID-19 vaccine requires storage. Novel technology is the base of the vaccine. It uses synthetic mRNA to activate the immune system against the virus. It needs a temperature of minus 70 degrees Celsius (-94 F) or below.
Ultra low temperature freezer required for sea food and meat
In a cascade refrigeration system, use two or more vapor-compression cycles. These use different refrigerants. Two stage cascade refrigeration systems create temperature around -750C. In this, there is low temperature and high temperature refrigeration systems (Fig.1). Three stage cascade refrigeration is to produce temperatures up to -1000C. In this, use low, intermediate and high temperature systems (Fig.2). Use different refrigerants to avoid vacuum operation in the low temperature evaporator. Remove heat from the space cooled using a low temperature (LTE) evaporator. It transfers heat to high temperature evaporator in the cascade heat exchanger. The high temperature system transfers heat to the atmospheric air or water in a conventional condenser. Use of different refrigerants improves overall COP.
Fig. 2 stage Cascade Line diagram & on p-h Chart
LTE low temperature evaporator
HTE high temperature evaporator
LTC Low temperature condenser
HTC high temperature condenser
1234 low temperature refrigeration cycle
5678 high temperature refrigeration cycle
1 and 5 are inlet of compressors
Fig. 3 stage cascade line diagram & on p-h Chart
IT intermediate temperature
Advantages of Cascade Refrigeration System over multi-compression refrigeration system
Energy input is less.
Easiness in operation because of different independent refrigeration systems.
COP is more
Avoid vacuum operation at the low temperature evaporator. Vacuum operation increases inward leakage of air. Vacuum operation increases volume handled by the compressor.
Refrigerants used in Cascade Refrigeration
These should have the following properties.
Less leaking tendency
High latent heat
Low specific volume to reduce the size of compressor and power requirements
High thermal conductivity
Lower normal boiling temperature
Lower freezing temperature
High critical temperature & pressure
No Global warming potential
No Ozone depletion potential
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