Many people regard carbon dioxide as the most promising refrigerant for several application areas. It's a natural substance with excellent heat transfer properties and when used a refrigerant generally provides opportunities to enhance system energy efficiency and considerably lower overall environmental impact.
However, using CO2 as a refrigerant brings some new technical challenges because both its critical point and triple point fall within the envelope of practical use. In addition, some thermodynamic and transport properties (e.g., saturation pressure level and liquid viscosity) are extreme compared to traditionally used refrigerants (NH3, HCFCs and HFCs). Table 1 compares some properties of select refrigerants relevant to process plants and refrigerant pumps. The most significant differences between CO2 (R744) and the other refrigerants are its low critical temperature, high saturation pressure, very low kinematic viscosity and low surface tension.
In the last 10 years, many technical challenges related to CO2 have been successfully overcome for components such as compressors, valves and heat exchangers. Now, commercially competitive versions of these components designed and optimized for CO2 are available on the market.
Refrigerant pumps have been offered for 60+ years. Most were typically designed for general chemical industry service and then modified to operate with the substances used as refrigerants (e.g., NH3 and R22). Due to increasing demands for low leakage levels, most refrigerant pumps today rely on canned motors or magnetic couplings. Commercially competitive refrigerant pumps designed and optimized for CO2 have been on the market for less than one year (Figure 1).
The specific challenges for refrigerant pumps operating with CO2 are:
High system pressure. The saturation pressure of CO2 is much higher than that of the traditionally used refrigerants. At 40°F (4°C) the saturation pressure is 550 psig (38 bar g). Today 750 psig (52 bar g) seems to satisfy most demands -- but in the near future some types of systems will need a maximum system pressure of 950 psig (65 bar g). In addition, standstill requirements for many systems demand a maximum system pressure up to 1,750 psig (120 bar g).
Subcooling from liquid column. A column above the pump inlet changes the liquid refrigerant condition from saturated to subcooled. This ensures that internal losses in the pump's inlet section don't generate vapor that can reduce pump performance. For most traditional refrigerants each foot of liquid column provides considerable subcooling; at nominal operating conditions the net positive suction head required (NPSHR) typically is limited to 3–5 ft (1–1.6 m). For CO2, a 3–5-ft liquid column produces practically no subcooling due to the different relation between saturation pressure and temperature. Therefore, reducing inlet losses and establishing a high degree of hydraulic robustness to vapor bubbles become very important design elements for a CO2 pump.