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higher SST 2
- Thread starter ASSADEQ
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It is important to understand how the various elements of a piping system contribute to the overall pressure loss when a refrigerant flows through it, and most important, how compressors are rated and at exactly what temperatures a manufacturer’s capacity rating applies. So when determining the highest SST you have to make sure that the pressure drop is minimal but at the same time you need to make sure that the velocity is maintained in order to get good oil return.
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- #3
What reason do you want to increase the SST on a unit? The highest actual sution temp a compressor can take is set by the manufacturer. He is concerned with the discharge temperature and the oil temperature. Inducing high suction temps or high superheat causes the discharge temp and oil temp to increase. Take a look at an R-22 P-H chart and follow the constant entropy lines at the suction.
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
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
- Thread starter
- #4
Imok2: thanks for your input
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
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
- Thread starter
- #5
ASSADEQ,
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
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
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