Low liquid viscosity. CO2's liquid viscosity is lower than that of NH3, HCFC and HFC. For media-lubricated bearings, viscosity is a very important parameter for bearing lifetime.
Low surface tension. CO2's surface tension also is lower than that of NH3, HCFC and HFC. Vapor generated will appear as numerous very small bubbles -- typically creating a foamy mixture instead of a separated liquid and vapor flow.
Low refrigerant temperature. During evacuation, service and charging processes low temperatures occur -- the pressure can drop below the triple point pressure of 75.4 psia (5.2 bar), leading to formation of dry ice. At atmospheric pressure, dry ice sublimates at a temperature of -108°F (-78°C) or below. Temperatures in refrigerant pump environments can reach -150°F (-100°C).
Operation "close" to critical point or triple point. The difference between liquid and vapor density generally is lower for CO2 than for all other refrigerants. As the critical point is approached, the density difference goes toward zero. Therefore, density-difference-driven liquid/vapor separation isn't as effective for CO2 as for other refrigerants. As the condition comes closer to the triple point, dry ice may form inside the pump, potentially damaging canned-motor centrifugal pumps.
Figure 4. Carbon dioxide serves as the only refrigerant in
These challenges call for a pump design with few seals and small dimensions. Hydraulic robustness to vapor in the inlet must be high. Vapor bubbles will form there frequently because liquid subcooling will be practically zero and the vapor and liquid don't separate well. Therefore, cavitation likely will occur often -- but because CO2's surface tension and latent heat of vaporization both are low, it shouldn't have nearly the same damaging effect as it does in water and NH3. The pump should feature materials with cavitation-resistant properties and surfaces, and should handle temperatures down to -150°F (-100°C).
In addition, the pump should satisfy general design requirements such as compactness, light weight, high energy efficiency, variable speed capacity control, easy integration, minimal maintenance needs and easy servicing.
CO2 can be used as a refrigerant in three fundamental ways:
1. Indirect system. CO2 serves as a secondary volatile refrigerant with circulation established by a mechanical pump. The main drivers for this option are to considerably reduce system energy consumption, significantly decrease primary refrigerant charge when that refrigerant has a high global warming potential (GWP) or is flammable or toxic, and increase system energy efficiency. Studies show that indirect systems with CO2 have the same first cost as traditional systems.
Figure 5. Pump circulates refrigerant throughout pipe network