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YORK Process Systems - Notes and News
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CO Cascade System
Applied To Low Temperature Process System |
| Cascade systems
are used for very low temperature applications. They cascade heat rejection
from the low temperature refrigerant to the high temperature refrigerant
through a cascade condenser. At normal condensing temperatures, low temperature
refrigerants usually have high coincidental (saturated) pressures ... the
temperature level required to reject the entire refrigeration system heat to
ambient. |
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Typical low
temperature refrigerants have a very low specific volume. This means a much
smaller compressor can be installed than when refrigerants such as R-22 or
ammonia are used at very low temperatures.
Issues related to ODP (Ozone Depletion Potential) and GWP (Global Warming
Potential) have eliminated some traditional halocarbon refrigerants such as
R503 and R13 and made the remaining choice, R23, 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. |
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| CO2
is very low in cost with excellent thermodynamic properties at low temperature,
including a low specific volume that allows for much smaller compression
hardware. Most important, it is environmentally benign, allowing you to
eliminate the costly expansion tank associated with traditional cascade
refrigerants. |
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YORK Process
Systems recently started up a -49°F CO2 cascade
system at a petrochemical plant in Mobile, Alabama. The system was commissioned
smoothly and successfully, and now provides the plant its desired capacity and
temperature. The plant chose the refrigerant on the basis of smaller low stage
compressor costs, the environmental compatibility of CO2,
and the availability and low replacement cost compared to traditional
halocarbon refrigerants.
The user is a multifaceted petrochemical complex producing many chemical
intermediates used in adhesives, solvents, cleaning agents and a number of
consumer products. The unit has been running very 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 US for the
petrochemical market. The unit is a 150-ton CO2/R22
package. It uses two Frick screw compressors at 250HP and 400HP cooling
d-Limonene in a flooded evaporator to -49°F. The high stage R22 compressor was
selected for future conversion to R507.
Both the CO2 and high stage compressor are
factory standard rotary screw compressors. 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 since they are positive displacement,
high flow compressors that can easily adjust to variations in pressure rise and
capacity. |
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