CO2 Refrigeration systems

Even though Hydrofluoro compounds (HFC) are widely used as refrigerants because of their environment friendly nature (no damage to ozone layer), CO2 is also a popular choice as refrigerant in CO2 refrigeration systems.

Some of the advantages of CO2 refrigeration system with CO2 as refrigerant are:

• widely available;
• high volumetric cooling capacity and heat transfer;
• no recovery or recycling required;
• non inflammable and non toxic;
• environment friendly;
• the compressors are compact in size

Some of the difficulties is using CO2 in CO2 refrigeration systems are: high working pressure and large pressure difference (3 to 5 times conventional refrigerants); low theoretical efficiency with normal refrigeration systems. Hence it requires advanced technology compressor and refrigeration system. The hoses need to be strong as well as the evaporators and gas coolers are used instead of condensers as there is no phase change of the refrigerant. They are widely used in vending machines.

One of the difficulties in CO2 refrigeration is the CO2 detection and level control.

Chemical sensor elements cannot reliably measure CO2 levels. Other alternate detectors include infrared sensors.

In a typical electronic CO2 level control system, when the sensor detects the CO2 refrigerant level is below a threshold value, it sends a signal to the control system. The control system actuates a valve that lets in CO2 from a storage tank.

The supply is stopped when the sensor detects that the CO2 level has reached the upper limit. Carbon dioxide systems have a higher likelihood of leakage than traditional refrigerant systems.

As there is less charge in a vapor compression system using carbon dioxide as the refrigerant, a leak has a greater influence on system performance.
Also a correct oil level in the compressors must be guaranteed in order to ensure proper lubrication.

Hence CO2 level control systems play an important role in operation of the CO2 refrigeration system.