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     CO2 as Refrigerant 
        R744 System 

Environmental impact

ODP CO2 is a very attractive refrigerant with zero ODP and a GWP of 1. It is a naturally occurring substance and abundant in the atmosphere.

 

Pressure and temperature

CO2 is a high-pressure refrigerant where high operating pressures are required for efficient operation. During a standstill, the ambient temperature can reach and exceed the critical temperature and the pressure can exceed the critical pressure. Hence systems are typically designed to withstand pressures up to 90 bar, or sometimes even equipped with a small standstill condensing unit to keep pressures low.

At the same time, CO2 has a low compression pressure ratio (20 to 50% less than HFCs and ammonia), which improves volumetric efficiency. With evaporation temperatures in the range of -55 ºC to 0 ºC, the volumetric performance of CO2 is for example four to twelve times better than that of ammonia, which allows compressors with smaller swept volumes to be used.

The triple point and critical point of CO2 are very close to the working range. The critical point may be reached during normal system operation. During system service, the triple point may be reached, as indicated by the formation of dry ice when liquid containing parts of the systems are exposed to atmospheric pressure. Special procedures are necessary to prevent the formation of dry ice during service venting.

 

Material interaction

CO2 does not react with common metals or with Teflon®, PEEK, or neoprene components. However, it diffuses into elastomers and can cause swelling with butyl rubber (IIR), nitrile rubber (NBR), and ethylene-propylene materials (EPDM).

The density of liquid CO2 is about 1.5 times that of ammonia, resulting in a higher mass charge in evaporators, such as large plate chillers in large industrial systems. Higher density means higher oil circulation as well, which in turn requires effective oil separators for industrial systems.

 

Cost efficiency

CO2 is a by-product in several industries, so the price of power consumption on CO2 is low. However, CO2 systems tend to be more expensive than traditional systems due to higher pressures (in trans critical systems) or increased complexity (in both transcritical and subcritical systems). The complexity of systems seems to be decreasing with the entrance of Booster systems and as the number of CO2 installations has increased, history has shown that the cost approaches the cost of the reference systems using HFCs.

Secondary, large CO2 systems, especially in industrial refrigeration, may be less expensive to build than their glycol counterparts and thus offer lower initial and life-cycle costs.

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