How A Freezer Works?

[karimihassan]

THANKS FOR PAYING ATTENTION TO MY QUESTION AND ANSWERING IT.

I WOULD LIKE TO GET SOME INFORMATION ABOUT THE PRINCIPLE OF OPERATION OF BELLOW ZERO (eg:-30 degree celsius)FREEZER UNITS.

FOR EXAMPLE ,HOW THE GAS PRESSURE IS CONTROLLED IN THE EVAPORATOR TO GET THIS TEMPRETURE.
IS IT POSSIBLE TO USE NORMAL EXPANSION VALVES AND COMPRESSORS FOR THIS PURPOSE .

[Samarjit Sen]

Hello Karimihassan,

For any low temperature application, the basic system remains the same. The changes are as follows.
1. The refrigeration Compressor has to be selected for the required Te and the refrigerant to be used.
2. Depending on the Room Temperature to be maintained, you have to select the fin spacing of the Evaporator Coils. For below -30oC Room Temperature your coil fin should have a spacing of 3-4 fpi.
3. There has to be a defrost system. There are three types of defrost. One is the Hot Gas Defrost, the second is the Electrical Defrost and the third is the Air Defrost which is meant for say 0oC Room Temperatures.
4. Depending on the Te, you will have to select the Expansion Valves. If you go into any of the TEV manufacturers technical Datas, you will find different charges for different applications.

There might be something that I have missed, which I am sure some one will fill it up. If you desire the complete design of a system, I can help you, but you have to give the basis datas as to the size of the room, the insulation, the required temperature, the product to be stored, the pull down time etc.

[TXiceman]

Depending on the size of the unit, I generally do not like to use DX at the lower temperatures. You start to experience oil return problems and proper feeding of the circuits. If you are talking about ammonia, do not use DX below -10 dF ET.

I all but the smallest plants, you need to be looking at a two stage system for efficient operation.

Ken

[US Iceman]

Low temeprature refrigeration systems are not much different than high temeprature refrigeration systems.

In a high temperature refrigeration system operating at say 1.6°C evaporating temperature, the operation does not basically differ from a system operating at say -40°C.

The compressor maintains the lower evaporating pressure thus producing colder temperatures.

[Samarjit Sen]

For a temperature of say -40oC Te, I had used a single stage compressor with DTC. But I feel it is better to use a two stage compressor for such application. The refrigeration is R 22.