Cascade System Eliminates Need for Expansion Tank
Environmentally benign, CO2 has a low specific volume that allows for much smaller compression hardware. Is this an option for your process? CO2 is an effective low temperature cascade refrigerant. Cascade systems are used for very low temperature applications by "cascading" the heat rejection from the low temperature refrigerant to the high temperature refrigerant through a cascade condenser.
The typical low temperature refrigerants have a very low specific volume, which means a much smaller compressor can be used compared to using refrigerants such as R-22 or ammonia at very low temperatures. However, these refrigerants normally have very high coincidental (saturated) pressures at normal condensing temperature; that is, the temperature level required to reject the entire refrigeration system heat to ambient. For this reason, the low temperature refrigerant must reject its heat to the high temperature refrigerant circuit through a cascade condenser.
Issues related to ozone depletion potential (ODP) and global warming potential (GWP) have eliminated some traditional halocarbon refrigerants such as R-503 and R-13 and made the remaining choice, R-23, prohibitively expensive. Hydrocarbons such as ethane and ethylene have the advantage of low specific volume and cost, but they are class A3 refrigerants with high flammability.
CO2 is low in cost with excellent thermodynamic properties at low temperature, including a low specific volume that allows for much smaller compression hardware. Most importantly, it is environmentally benign, which means the costly expansion tank associated with traditional cascade refrigerants can be eliminated.
A Cascade System at Work
Process Systems (PRS) in North America recently started up a -49oF (-45oC) CO2 cascade system at a petrochemical plant in Mobile, Ala. The system was commissioned successfully and now provides the plant its expected capacity and temperature. The plant chose the refrigerant on the basis of smaller, low-stage compressor costs, the environmental compatibility of CO2, and its availability and low replacement cost compared to traditional halocarbon refrigerants.
The end user runs a multifaceted petrochemical complex that produces many chemical intermediates used in adhesives, solvents, cleaning agents and a number of consumer products.
The unit has been running successfully for six months, and a second unit has recently been shipped to the same job site. This is the sixth unit of this type built by York Process Systems in the United States for the petrochemical market. The unit is a 150-ton CO2/R-22 package using two Frick screw compressors at 250 hp and 400 hp cooling d-Limonene in a flooded evaporator to -49oF. The high stage R-22 compressor was selected for future conversion to R-507. Both the CO2 and high stage compressor are factory standard rotary screw compressors as the CO2 requires no special materials as long as the refrigerant is dry and operating within the temperature parameters described. Screw compressors are uniquely adaptable to these refrigerant options because they are positive-displacement, high flow compressors that can adjust to variations in pressure rise and capacity.
Smaller compressors, high efficiency, elimination of an expansion tank and zero ODP make CO2 a preferred choice for low temperature refrigeration applications.
Ron Rose is vice president of York Process Systems, York, Pa. York Process Systems, a manufacturer of refrigeration and gas compression equipment at work in the petrochemical and gas processing industries. York was acquired by Johnson Controls in 2005. For more information from York Process Systems, call (717) 771-7890.