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CO2 system and HP expansion valve operation
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I am new to this forum and find it and the r744.com website well organized and immensely informative. Thank-you for all the knowledge out there and for asnwering my questions. I look forward to future threads and bein a new member.
A refrigeration system operating at 30degC ambient would be in the trans critical cycle. A high pressure expansion valve feeding the cooled discharge gas from the gas cooler into the intermediate vessel and a gas bypass valve are necessary for correct distribution of CO2 liquid to the evaporators. How does the high pressure expansion valve behave when the outdoor ambient is in the 0 to 10 degree range? Does it control by subcooling and does the gas cooler act as a conventional condenser in this situation?
If a plate heat exhanger were connected to a water loop using 4 to 5 degreeC chilled water as the condenser, wouldn't this be operating in the subcritical range?
If so then the CO2 vapor would condense to a liquid in this heat exchanger, could the liquid then be sent to a conventional liquid receiver to be used by the evaporators?
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RE: CO2 system and HP expansion valve operation
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When running in subcritical mode, CO2 is no different than other refrigerants. What you typically get in a booster system, is a 2 stage expansion. The high pressure valve is controlled by subcooling in this case. This algorithm is build in Danfoss EKC 326A controller. You can find more information on this system in Danfoss CO2 application handbook in the knowledge section.
If you can guarantee that your system always operates in the subcritical mode, you can avoid having a high pressure valve. However that would require a constant supply of very cold water, which is in most cases not practical.
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RE: RE: CO2 system and HP expansion valve operation
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Mr. Anatolii Mikhailov posted on 2009-11-08 09:24:05: When running in subcritical mode, CO2 is no different than other refrigerants. What you typically get in a booster system, is a 2 stage expansion.&nbs ... Jump to Post Thank-you for the explanation. When a transcritical system is operating in the subcritical mode you say the high pressure expansion valve operates by subcooling. Do you know what the control temp (how many degrees subcooling is typical?
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RE: CO2 system and HP expansion valve operation
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Hi
can any body explain to me in simple terms how the hp valve works. i have asked this to many people who i've met on sites and all have just said 'its software driven' or 'programed to run for maximum efficency'. i (think) know the control device looks at the high pressure reading and gas cooler outlet temp but what relationship are we looking for to get maximum efficency?
thanks Rowan
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RE: CO2 system and HP expansion valve operation
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When in transcritical state pressure and temperature of CO2 are decoupled: that is for any given temperature (defined by the ambient and gascooler) one could have a range of pressures. At one of them COP is maximal (defined by CO2 properties). Software knows this optimums and adjust the valve accordingly, that is why there are 2 input signals - both pressure and temperature. We have a good description with diagrams in our (Danfoss) application handbook, that you can download in the knowledge section or from our web.
Regarding the previous question about the subcooling, I don't have any specific recommendations, 2-5 K should do.
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