higher SST
higher SST
(OP)
Forgive me for my ingnorance, I have what may be a elementary question regarding DX cooling system.
Q.What would be the highest acceptable saturated suction Temprature(SST)in R-22 DX ? how to achive higher SST in the system ?
Thanks in advance for any input.
Q.What would be the highest acceptable saturated suction Temprature(SST)in R-22 DX ? how to achive higher SST in the system ?
Thanks in advance for any input.





RE: higher SST
RE: higher SST
On an oil injected screw compressor, you need to watch the inlet pressure and the compressor Vi, (volume index). Too high of a suction pressure and the compressor can achive rather high internal pressures before the rotos open to the discharge port.
Inshort, you need to explain a bit more of your concernor purpose and the type system you are applying.
TXiceman
www.rae-corp.com
RE: higher SST
TXiceman: the reason for my concern is to increase the compressor effficincy, The higher the suction gas superheat, the lower the gas density and therefore, the lower the compressor mass flow rate.
most Manufacturers set SST from 20-50*C , is there any problem in the system to select the 50*C SST.?
and how this can be done?
Thanks
RE: higher SST
most Manufacturers set SST from 20-50*F
RE: higher SST
High suction temp will not increase compressor efficiency. Yes the mas flow is down, but so will be the refrigeration effect (NRE). When you do the mass flow calculation for an evaporator, you will calculate a certian mas of refrigerant to provide "X" BTU/HR of cooling capacity. If you cross exchange this vapot with the liqyuid to increase superheat, you will subcool the liquid and lower the enthalphy to the evaporator. The gas temp will increase and the increase in volume is off set by the increased NRE. If you add heat to the suction w/o cross exchanging, you will decrease compressor performance.
By cross exchanging in a "liquid to Suction" heat exchanger, you have the same BTU/LB increas and decrease on both sides of the system, so the net change in system performance is for most part zero. On some refrigerants, such as propane (R-290) you may see a slight increase in compressor capacity. It all is controlled by the specific vapor of the refrigerant.
If you lok at the Copeland compressor ratings, they are rated at 65 dF suction gas temperature, irreguardless of the evaporator temperature. This in my opinion is not correct. Refigeration systems using DX (thermal expansion valve) control, run with a superheat of 10 to 15 dF as normal. Copeland is assuming that the superheat is all "useful" or that you get a cooling benifit to the evaporator to get the gas to 65 dF. Few if any systems run this way.
And most manufacturers limit the suction temperature to about 70 dF as a maximum. Some lower pressure refrigerants will allow you to go higher.
Ken
TXiceman
www.rae-corp.com