Generally almost all the refrigeration systems used commercially and domestically have similar principle of operation. They conduct refrigeration by vapor compression cycle. However in some cases it is more effective to use an advanced configuration. One of these refrigeration setup is called Cascade Refrigeration System.
Cascade Refrigeration System uses multiple refrigeration cycles coupled with each other via heat exchanger to improve the refrigeration effect. It uses multiple refrigerants depending on the stages. It can achieve reduced temperature and better efficiency compared to conventional refrigeration systems.
The refrigerants that are used in each cycle have different boiling points, freezing points and critical pressures. This mainly depends on the lowest temperature that has to be achieved and refrigeration effect required at evaporation coil.
There are multiple cycles in cascade refrigeration system. A two stage cascade system has high temperature cycle and low temperature cycle. The selection of refrigerant is also based upon cycle in which the refrigerant is flowing.
Conventional Refrigeration Cycle
It would be a good idea to revise the simple refrigeration system operating on vapor compression cycle. It will revise the basic cycle used in any stage of cascade refrigeration system. Further it will make it easy for you to compare multiple stage cascade system with traditional refrigeration system.
The refrigerant is allowed to compress adiabatically. This means no absorption or rejection of heat. The low temperature and low pressure refrigerant is converted to high temperature and high pressure state.
The high temperature and high pressure refrigerant rejects heat during its movement through condenser. This is done by heat exchanger that rejects the heat to the air or water surrounding the refrigerant coil. The process is isothermal and isobaric i.e. no change in temperature and no change in pressure, respectively.
The high pressure refrigerant with reduced temperature is allowed to pass through expansion valve. Here it is expanded to low pressure state resulting in excess drop of temperature. The process is isenthalpic i.e. with no enthalpy change. The refrigerant is in the coolest state after this.
The cool refrigerant almost 75 percent liquid and 25 percent gas moves through the evaporator. Evaporator is actually a heat exchanger with refrigerant within its coils. As the refrigerant passes through evaporator it absorbs heat from the surrounding. The amount of heat absorbed is called the refrigeration effect. Theoretically, it happens with no change in temperature and pressure.
How a Cascade Refrigeration System Works?
Cascade refrigeration system is the combination of multiple refrigeration cycles. This allows a configuration of refrigeration cycles in such a way so as to provide a higher refrigeration effect.
It is used for purposes where the temperature required is very low as compared to conventional refrigeration system. Even if the desired temperature is achieved by the conventional refrigeration setup the overall efficiency will be very low.
Further the pressure ratios required in conventional refrigeration system would be too high. Pressure ratio has inverse relation with volumetric efficiency. This means that in ordinary refrigeration system to achieve close effect to cascade refrigeration system they will have very low volumetric efficiency. This low volumetric efficiency is definitely not desirable.
Cascade refrigeration system uses two different refrigerants in both cycle. This depends on the requirement of temperature and the refrigeration effect required. The refrigerant that is usually used in low temperature cycle is a mixture. This prevents coagulation of the refrigerant due to very low temperature in the low temperature cycle.
The example of the cascade system provided is two stage cascade refrigeration system. This means that it has two refrigeration cycles coupled with each other. What is a stage? How more than two stages operate? This is covered below in the separate heading. Let us now discuss the most common two stage refrigeration system.
High Temperature Cycle
The high temperature cycle in the above cascade system is the one with higher evaporator temperature. The common refrigerant used in high temperature cycle is ammonia.
Low Temperature Cycle
The low temperature cycle is the one that has lowest evaporator temperature. In fact this is the cycle that will provide the desired low temperature in the evaporator. The lowest temperature in this cycle defines the refrigeration effect of the respective cascade system.
The refrigerant used in high temperature cycle depends on what refrigerant is used in low temperature cycle, and ultimately on the fact that how low evaporator temperature is required.
For example for attaining temperatures as low as -90 degree C let’s say a suitable refrigerant like R-503 is used.
In this case depending upon the refrigerant in the second stage and suitable temperatures required it would be better to use R-500 in high temperature cycle.
Operation of Cascade Refrigeration System
First consider the high temperature cycle. The compressor in this cycle is called high temperature compressor. The refrigerant in this cycle is compressed by the compressor and sent to condenser.
The movement of refrigerant through condenser coil rejects heat energy to the external environment. The high pressure and low temperature refrigerant is expanded to cool to evaporator temperature.
In low temperature cycle the compressor is called high temperature compressor. After refrigerant is pressurized by compressor. It is at inlet condenser temperature.
The refrigerant passes through the condenser coils that are actually coupled with the evaporator of high temperature cycle via heat exchanger.
The evaporator of high temperature cycle is coupled with the condenser of low temperature cycle. This coupling is via a heat exchanger. The role of the heat exchanger is to reject the heat of the refrigerant from condenser of low temperature cycle to evaporator of high temperature cycle.
The common or intermediate temperature of the condenser and evaporator at heat exchanger is also called coupling temperature.
Hence by the use of two or more refrigeration cycle coupled with each other refrigeration effect is increased. This is indicated by lower evaporator temperature compared to conventional refrigeration setup.
In general coefficient of performance of cascade refrigeration system increase with increase in evaporator temperature of low temperature cycle and decreases with increase in evaporator temperature of high temperature cycle.
What are stages in Cascade Refrigeration System?
Stage are referred to the number of refrigeration cycles coupled with each other in cascade refrigeration system. Each stage has different refrigerant in it. The goal of adding another stage is to get even lower temperature.
For example we can achieve temperatures of the range of -120 degree C in the above cascade refrigeration system. In that case we may add another stage with a refrigerant that has lowest boiling point compared to other two stages.
A refrigerant like R-15 would be suitable to be used in the third and lowest temperature stage if R-500 is used in first stage and R-503 in the second stage.
Note: As we go from higher temperature stage to lower temperature stage, the boiling points of the refrigerants used in each stage reduce respectively.
Now-a-days ammonia is used a refrigerant in high temperature cycle and carbon dioxide is used as a refrigerant in low temperature cycle
Advantages of Cascade Refrigeration over Conventional Refrigeration
Low Operating Cost and Energy Efficient
Cascade Refrigeration System can give low temperature operation with high energy efficiency. One of the major reasons is lower compressor work required for the refrigeration operation equivalent to conventional refrigeration system. In general power consumption is reduced to about 10 percent.
Low Temperature Operation
Excitingly low temperatures can be achieved by using cascade refrigeration (as low as -120 degree C). In case two stage cascade system cannot provide the required refrigeration, another stage may be added with lower boiling point refrigerant to achieve lower temperatures.
The overall maintenance of cascade refrigeration system is easier compared to equivalent conventional refrigeration system.
Some of the reasons of convenient maintenance is low pressure ratio and lower lubrication oil temperatures during the operation. Compared to conventional system, cascade system require less compression ratios in the different stages. This results in lower lubrication oil temperature resulting in better operation.
Higher COP and Lower Compressor Work
Cascade systems provide higher COP compared to equivalent conventional systems. This is due to the fact that they provide more refrigeration effect with lesser compressor work input.
Higher Volumetric Efficiency
The pressure ratio in refrigeration cycle is inversely proportional to volumetric efficiency. Cascade refrigeration systems use low pressure ratios compared to conventional refrigeration systems. This implies that cascade refrigeration system have higher volumetric efficiency, which is desirable.
Applications of Cascade Refrigeration System
Cascade Refrigeration is used for medical storage purposes i.e. storage of blood, vaccines, bone banks etc. Biological fluids require very low storage temperatures to sustain the constituents within. After which they are delivered to the required locations where they are instantly used.
It also finds it use in deep freeze food storage facilities where the food must sustain its quality for very long duration of time. These facilities require chilling temperatures to maintain the quality of food.
CRS is utilized for storage of elite quality adhesives and tools.
Liquefaction of gases require extremely low temperatures. These temperatures can be provided by cascade refrigeration system.
For some applications requiring very low temperatures and high refrigerating effect conventional refrigeration system are not sufficient. Thereby one of the configuration of refrigeration systems called cascade refrigeration system is used. They allow coupling of multiple refrigeration cycles to provide greater refrigeration efficiently, economically and abundantly.