CO2 Refrigeration Systems
The use of carbon dioxide as a refrigerant has seen increasing interest in recent years. As a result, there is also a lack of familiarity with this refrigerant among the general industry.
The use of CO2 in conventional refrigeration systems presents several interesting properties that must be addressed. The pressure/temperature relationship of this refrigerant is one of the primary concerns.
For an example of the pressure/temperature relationship of CO2 as vapor pressure curves see IP units or SI units (PDF files).
The pressures observed with the use of CO2 are much higher
than those normally found in ammonia refrigeration systems. Due to the
higher pressures found with the use of carbon dioxide it becomes
necessary to implement certain principles to limit the pressure
increases at higher temperatures. Some of these design
guidelines are used to allow standard refrigeration components to be
utilized in CO2 refrigeration systems.
A cascade refrigeration system is one method to provide this capability. In this process a separate refrigeration system uses a different refrigerant to condense the CO2. The CO2 is maintained at relatively low pressures by the low temperatures created by the separate refrigeration system. With this type of system configuration, standard refrigeration components are used in the CO2 refrigeration system.
It is common to find ammonia (NH3 or R-717) being used as the higher temperature refrigerant to condense the CO2 used in the lower temperature refrigeration system. Below is an example of a typical CO2/NH3 cascade system in a basic configuration.
It is also important to be aware of other concerns in the event of power outages, or intermittent operation of CO2 systems. If the ammonia system is not able to provide cooling of the CO2, the resulting pressure increase in the CO2 system may cause relief valves to lift. This can release CO2 to the atmosphere and result in loss of the operating refrigerant charge. In order to maintain the pressure of the CO2 system below the maximum allowable design pressure of the CO2 system other design requirements may become necessary.
According to the safety and environmental regulations in the USA refrigeration systems having a threshold quantity of ammonia greater than 10,000 pounds have specific criteria to meet for compliance. The use of CO2 for these low temperature applications provide one means of limiting the total ammonia inventory.
Please Contact Us if you would like to learn more about the use of CO2 as a refrigerant. Read more about industrial refrigeration systems and specific properties of commonly used refrigerants.
