heat pump technical question

rwfegley

My Mitsubishi mini split service manual says:

In heating mode:

condensing pressure 309 psi
discharge temperature 139 F
condensing temperature 95 F


Question: if R410a is condensing at 95F, then the correct pressure is 309 psi. But if the refrigerant is discharging from the compressor at 139 F (which would correspond to 500+ psi) where is the pressure drop occurring between the compressor and the inside unit? There is no serious restriction in that line.

Maybe there is something I don't understand, or this is just a misprint in the manual.

Ron

beenthere

It doesn't discharge from the compressor at 500 PSIG. The hot gas from the compressor is super heated.

rwfegley

I have done some reading and studied the R410a phase diagram. I keep coming up with the compressor discharge pressure being 550 psi when the discharge vapor is 139 F. Since the line goes directly to the inside unit condenser through an oil separator, a 4-way valve, and a muffler (no serious restrictions), I can't understand why the pressure in the inside unit condenser wouldn't also be 550 psi.

Yet the specs from Mitsubishi say the condensing pressure and temperature is 309 psi and 97F.

I just don't understand!

Ron

rwfegley

I just studied the manual more, and I now see that the Discharge Temperature is determined by a thermistor located near the compressor output. It may not the temperature of the vapor??

beenthere

The compressor discharge gas is super heated, and will not be at saturated pressure.

rwfegley

Thanks for your help.

I finally "got it" that you can have a 139F vapor out of the compressor at a pressure of 309 psi. So I guess Mitsubishi is recommending that the superheat be 139 - 97 = 42 F.

Ron

beenthere

That sounds about right for discharge super heat.