Experts in advanced refrigeration and air-conditioning

Cascade system

What is a cascade system?

A cascade system is a type of refrigeration system in which two systems are independent, and where one of the systems – the lower temperature system – uses a heat exchanger as a condenser, which is responsible for expelling the heat that accumulates in it. A more detailed explanation of how these systems work and how refrigerants are used for ultra-low or low-temperature applications is given below.

Cascade cooling system

A cascade cooling system is a system for ultra-low temperatures as low as -70 °C, with two dedicated cooling circuits. The first of these circuits is designated as the primary circuit, which is the high-temperature circuit; the second circuit is called the secondary or low-temperature circuit. As mentioned above, both circuits are independent of each other.

Within this two-circuit system, the primary circuit is responsible for cooling the secondary circuit, exactly the same as in the case of refrigerants. They are therefore particularly useful in a circuit where high reliability is required in terms of temperature.

It can be said that there are two variants of the basic vapour compression refrigeration cycle. The first is called the cascade cycle, where the vapour compression cycle can be used if the temperature difference between the condenser and evaporator is too high. A refrigeration system is used at this point, which absorbs the heat from the condenser during the evaporation process, as well as the heat from the compression of the refrigerant vapour as it passes through the compressor.

The first cycle thereby decreases the condensing temperature, creating a pressure drop later on. This is a more efficient method than heat extraction by convection processes. The reason is that, in a heat absorbing environment, a condensing device has a lower temperature than the air that is circulating through it.

The heat in the condenser is therefore absorbed by the evaporator in the second refrigeration system. Refrigerants generally vary according to the type of circuit, and the purpose of this variation is precisely to adjust the temperature and pressure parameters.

This is how a cascade cooling system works

In a cascade cooling system, two main stages can be distinguished. In the first stage (the high stage), the circuit takes over the cooling function. In this way, the refrigerant is discharged through the compressor at a high pressure and temperature and enters the condenser. This condenser is responsible for transferring the energy absorbed by the refrigerant from the environment, which will be released again as a liquid.

The next step must include refrigerant subcooling. The purpose of this operation is to gain capacity while ensuring that the liquid only travels in one direction. When the refrigerant comes into contact with the plate evaporator, the refrigerant in the low temperature circuit will be cooled. The evaporator – condenser cycle is thereby completed, and the high temperature refrigerant absorbs heat from the low pressure refrigerant.

A second stage can be distinguished after that. This second stage is often also referred to as the low stage, and takes place when the refrigerant is expelled from the compressor at high pressure and temperature. In this case, the condenser acts as a plate heat exchanger, and the system starts working at high pressures; it is then either cooled or converted from vapour to liquid by the high-stage evaporator. When the refrigerant is subcooled, it enters the evaporator, where it will be introduced at ultra-low temperatures.

Benefits of a cascade system

A cascade system offers specific advantages over other cooling methods. When a cascade system is used, the compression ratio of each system decreases. Volumetric efficiency is therefore increased in each stage and, by the same effect, the whole system as a whole becomes much more efficient. At the same time, it should be noted that a cascade cooling system also requires less compressor displacement.

Furthermore, the discharge temperature in the cascade system is lower than in a single-stage system; this effect is advantageous for lubrication and oil temperature. Finally, it should be noted that high-pressure refrigerants are also used in a cascade refrigeration system. This type of refrigerant causes the suction pressure to reach positive values. This means that this type of system does not need to work in a vacuum, even in cases where the saturated suction temperature is quite low.

Operating precautions in a cascade system

A number of precautions should be taken when using a cascade system. The first is to bear in mind that this is a system that has certain application limits, and that it is therefore important to monitor the lubrication condition of the system. It is also necessary to check that no liquid is being sucked into the system.

In the case of low-temperature systems, it must be ensured that the return gas temperature does not fall below -60°C. This is important in order not to damage the cast iron compressor. It is also advisable to monitor the oil temperature, so that it has the correct viscosity level; this will allow it to perform its function properly and prevent oil from flowing back. With respect to the oil, there must also be a high efficiency and well dimensioned oil separator. It should be noted that if it does not return, it is very likely that de-icing cycles will be necessary to increase suction pressure and mass flow. This is the way to decrease the viscosity of the oil, through increasing its temperature and causing it to return to the compressor.

In the case of the return temperature for the gas moving in the direction of the compressor, this must be high. In some cases, other compressor gases, including butane, may be tried for oil cooling. However, this operation should always be done by only applying a few grams of refrigerant gas.

As a last point related to the precautions to be taken in cascade cooling systems, we must mention the piping and its insulation. Due to them operating at very low temperatures, it is essential that the insulation of these components and all exposed parts ensure a temperature lower than the dew point. This is particularly important in areas where the temperature is below 0°C. This will prevent condensation and freezing of the moisture that is present in the air.

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