Pressure drop in refrigerant lines 1

[CamLoch]

I have heard of a general rule of thumb for R134a refrigerant that 1ºF of cooling is lost for 2psig pressure drop on high side. If the pressure drops on the discharge side, the SCT decreases and thus the we would be closer to the SCT for the discharge pressure. Is this not an advantage to performance? The only reason I can think of for the loss of cooling is the pressure loss means more work for the compressor which then increases the THR for the condenser.

Am I out to lunch on this?

 

[CH5OH]

the amount of cooling would be expressed in kJ
the temperature being a result of the differential between added heat and retracted heat.
what do you mean by the high side
what is SCT and THR?

 

[CamLoch]

I am trying to determine the factors which contribute to the cooling loss rule of thumb. The high side is from the compressor outlet to the TXV. SCT is saturated condensing temperature and THR is Total Heat Rejection of the condenser which typically is the heat removed by the evaporator and the heat of compression from the compressor.

 

[CH5OH]

the pipe losses on the high side would be critical for:
liquid (condensor to TXV):if Pressure condensor < pressure drop pipe + pressure evaporator
vapour (compressor to condensor):if Pressure discharge < pressure drop pipe + pressure condensor
the biggest cooling loss comes from the pipe between evaporator and compressor, there you could have a rule of thumb: every degree of temperature increase is so much increase in suction pressure

 

[MintJulep]

Yes, you're out to lunch.

Condenser performance is determined by the amount of heat that must be rejected and the condenser air temperature.

If there is pressure drop in the line between the compressor and the condenser the system compensates for this by increasing the discharge pressure. Increased discharge pressure = decreased performance.

If there is a pressure drop in the line between the condenser and the expansion valve the system again compensates by increasing the discharge pressure, but in this case the pressure in the condenser is also increased, increasing the SCT.

 

[CamLoch]

Thank you for your candour MintJulep!!

When the system compensates for the pressure loss in the discharge line, does that mean the compressor is working harder and so adds work heat to the system which is where the performance loss is derived?

 

[imok2]

The pressure drop in the discharge line increases the compressor power required per unit of refrigeration effect and it also decreases the amount of sub-cooling that occurs in the condenser. The reduction in sub cooling decreases the refrigerant flow rate through the metering device and the systems capacity. In addition, the pressure drop generated across the discharge line is added to the saturation pressure of the condenser in order to determine the discharge pressure of the compressor. Consequently, as the pressure drop across the discharge line increases because the discharge line is too small the discharge pressure also increases. This response increases the compression ratio, heat of compression, and saturation temperature of the condenser reducing the efficiency of the system. The solution: Increase the discharge line to a larger pipe diameter for the application.