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Refrigeration System of a Freeze Dryer

The job of the refrigeration cycle is to remove unwanted heat from one place and discharge it into another. The refrigeration system of a freeze dryer belongs to compression refrigeration, which is composed of four basic components:

  • Refrigeration compressor

  • Condenser

  • Evaporator

  • Expansion valve

They are connected in order by pipes to form a closed system. Refrigerant continuously circulates in the system, changes state, and exchanges heat with compressed air and cooling medium.

Refrigeration compressor draws low-pressure (low temperature) refrigerant from evaporator into the compressor cylinder. After the refrigerant vapor is compressed, its pressure and temperature increase at the same time. The refrigerant vapor with high-pressure and high-temperature is pressed to the condenser. In condenser, the higher temperature refrigerant vapor exchanges heat with the lower temperature cooling water or air. The refrigerant is condensed because its heat is taken away by water or air, and the refrigerant vapor becomes liquid.

This part of liquid is then sent to expansion valve, which is throttled to a low temperature and low pressure liquid and enters the evaporator. In evaporator, the low-temperature and low-pressure refrigerant liquid absorbs the heat of compressed air and vaporizes (commonly known as evaporation). After cooling, the compressed air condenses a large amount of liquid water. Refrigerant vapor in the evaporator is sucked away by the compressor. In this way, the refrigerant passes through four processes of compression, condensation, throttling, and evaporation in the system, thereby completing a cycle.

In refrigeration system of the freeze dryer, evaporator is a device that transports cold energy. In it, refrigerant absorbs the heat of compressed air to achieve the purpose of dehydration and drying. Compressor is the heart and plays the role of sucking, compressing, and transmitting refrigerant vapor. Condenser is a device that emits heat. It transfers the heat absorbed in evaporator to the cooling medium (such as water or air) along with the heat converted by input power of the compressor. The expansion valve/throttle valve plays a role of throttling and pressure reducing to the refrigerant. In addition, it can control and adjust the amount of refrigerant liquid flowing into evaporator, and divide the system into two parts: high-pressure side and low-pressure side.

There are many different kinds and variations of the refrigeration cycle components. For example, there are at least a half dozen different types of compressor, from the reciprocating, piston through a screw, scroll and centrifugal impeller design, but the function is the same in all cases - that of compressing the heat laden vapor into a high-temperature vapor.

The same can be said of the condenser and evaporator surfaces. They can be bare pipes, or they can be finned condensers and evaporators with electrically driven fans to pass the air through tem, or with a condenser pump to pump the water through a water-cooled condenser.

There are a number of different types of metering devices to regulate the liquid refrigerant into the evaporator, depending on size of equipment, refrigerant used, and its application.


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