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CO2 for cooling

CO2 for cooling

RE: CO2 for cooling

CO2 for closed cycle compression refrigeration has great thermal properties and is particularly applicable at temperatures from -25 F. to maybe -65 Deg. F.   Below that temperature, the triple point is encountered.  In large central systems, it has been applied as the low temperature end of a cascade system, often with R-717 as the high temperature side of a 2-temperature plant.  There have probably been 40 such systems constructed in the last 5-years.

CO2 at higher evaporating temps involves some pretty high pressures; (45 deg. F. and Satn is around 600 Psia) and direct to ambient condensing results in a transcritical system: rejected heat is all sensible heat, none of the advantages of Phase Change available.  This last condition leads to low COP's on systems condensing over 88 deg.F.; and the equipment for 88 deg. F. would have to withstand about 1300 Psig to achieve "normal" conformity to codes, etc.

Sanyo has designed a suitable compound compression circuit for lower capacities and transcriticalapplications; it inherently supports a gas displacement overcirculation arrangement.  Other players include Linde, Nestle, and Star Refrigeration in the UK.  

By comparison with R-717, major driving forces for design with CO2 include: Smaller Vessels, compressors and piping; easier tranportation for oil and liquid in an overfeed arrangement; and lack of toxicity should a leak or discharge develop. There is an inclination with "compact" CO2 circuits to simply leave out a number of the valves that would nominally be on an industrial system, simply because the easiest way to manage charge during servicing is to dump it and renew it.

RE: CO2 for cooling

(OP)
Thanks for your insight!...specifically were looking at the IT application.  This sounds like a viable option for a high power density application and avoids the mixing of electrical and CHW inside the cabinets:

http://www.troxaitcs.com/aitcs/news/news/0206_co2_mcc.php

The chiller system appears to be designed around standard air conditioning parameters.  The primary refrigeration system: R134a chiller supplying 43degF CHW absorbs about 10degF worth of heat from the secondary refrigerant-CO2.  The secondary CO2 refrigerant system is self contained except for the interconnecting piping.  One drawback as you mention is the high operating pressures ie. 50bar or 735psi.  I suspect this turns the project into a hybrid type of HVAC piping project with the additon of process piping (hi pressure welded SS tubing) not to mention additional costs.

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