CASCADE REFRIGERATION CLASS NOTES FOR MECHANICAL ENGINEERING

CASCADE REFRIGERATION

CLASS NOTES FOR

MECHANICAL ENGINEERING

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

  1. To store blood at around -700C in the hospital to increase the life of the blood

  2. To cool light hydrocarbons gases

  3. 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.

  4. Ultra low temperature freezer required for sea food and meat

Cascade Refrigeration

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

  1. Energy input is less.

  2. Easiness in operation because of different independent refrigeration systems.

  3. COP is more

  4. 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.

  1. Cheap

  2. Easily available

  3. Less leaking tendency

  4. High latent heat

  5. Low specific volume to reduce the size of compressor and power requirements

  6. High thermal conductivity

  7. Lower normal boiling temperature

  8. Lower freezing temperature

  9. High critical temperature & pressure

  10. No Global warming potential

  11. No Ozone depletion potential

  12. Non toxic

  13. Non corrosive

https://www.mesubjects.net/wp-admin/post.php?post=4198&action=edit        MCQ Vapor Comp Ref

https://www.mesubjects.net/wp-admin/post.php?post=4190&action=edit       MCQ Cascade REF