Suction Pressure and the CCPR

[liptow]

Hello Refrig Fans,

R134 small HP compressors from Copeland and others base capacity numbers on 65f return gas temperature. Does that mean that the compressor can handle suction pressures up to 63 psi (ie: saturation)? Or do I assume a 20f superheat and say the max suction pressure should be 40 psi? Basically, I want to know where to set the CCPR. Evap temps range from 20-40f in these rooms. System usually have HG defrost/modulation.

Some engineers here even set the CCPR as low as 35 psi.

Thanks friends.

[nike123]

What is compressor model#?

[liptow]

Hello,
Thanks for responding friend.
The Copeland compressor is 1.5 HP/R-134/
kala-016e-tac-001.

I have product lit and access to Copeland website
but do not find info to address max. suction pressure.

[nike123]

Hello,
Thanks for responding friend.
The Copeland compressor is 1.5 HP/R-134/
kala-016e-tac-001.

I have product lit and access to Copeland website
but do not find info to address max. suction pressure.

I also cannot find any data for R134a application. But for R12 is stated that is for medium and high temperature application.
Since R134a is replacement for R12 in high temperature application I think that you don't need CCPR at all.
Monitor your discharge temperature and amperage draw for various working conditions and base your decision according to that.

I also could be very wrong!:D

[Goober]

Regardless of the model number of compressor, why not just note FLA and set CCPR to that when comp running? Never overload compressor then.

[desA]

I can't locate a performance plot on the Copeland site, for that compressor. As you seem to be based in the US, at 60Hz, I'd estimate that your upper limit to Te,sat would be around 13'C. At 50Hz, Te,sat ~ 15'C seems to be mentioned in the typical compressor envelope information.

You would require pressure gauges on both sides of the CPR to set up correctly.

You may also want to consider a suction gas return temp limit, which would probably be safe at around 25'C max for a hermetic compressor.

[liptow]

Thanks for the input people.

I still find it interesting that compressor manufacturers list the max return gas temp at 65F/18C and yet mention nothing about pressure.

[desA]

Thanks for the input people.

I still find it interesting that compressor manufacturers list the max return gas temp at 65F/18C and yet mention nothing about pressure.

Not for all compressors, mind you.

For instance, for Copeland scrolls typically in ac applications, there is an envelope bound at Tc,suc of 25'C & also an SH bound of 10K.

You may want to chat to the manufacturer directly about the recommended limits for your compressor. Please do let us know what you find, as this would be of interest.

[3bets]

The 65 f return gas temperature is with repsect to the maximum return gas temperature that Copeland allows. If the refrigerant comes back higher than 65 f, then the compressor will over heat and go off on the internal.. Use of a cpr valve is recommended if during periods of operation the load is too high such that it will overload the compressor. Typically this would happen on a single compressor system when coming out of defrost. Every compressor manufacturer lists the allowable evaporating operating range for any compressor. For example a Tecumseh "low temp" compressor's application range is from -40 f to +10f. If the compressor would see evaporating temperatures higher than +10 f after defrost termination, and if that conditon was sustsained for a period of time, the coompressor would likely overload since the load is too high and the motor in the compressor isn't big enough to handle it.

The setting of the CPR valve would be the same as the highest allowable evaporating temperature (+10 F), converted to pressure for a given refrigerant. For r-404a +10 f is 43.4 psig. So, we would set the cpr valve to start throttling at just under 43 psig. This would keep the compressor on line and pulling as much vapor from the evaporator after defrost as possible, without the compressor overloading.

hope this helps,

3 bets

[desA]

The 65 f return gas temperature is with repsect to the maximum return gas temperature that Copeland allows. If the refrigerant comes back higher than 65 f, then the compressor will over heat and go off on the internal.. Use of a cpr valve is recommended if during periods of operation the load is too high such that it will overload the compressor. Typically this would happen on a single compressor system when coming out of defrost. Every compressor manufacturer lists the allowable evaporating operating range for any compressor. For example a Tecumseh "low temp" compressor's application range is from -40 f to +10f. If the compressor would see evaporating temperatures higher than +10 f after defrost termination, and if that conditon was sustsained for a period of time, the coompressor would likely overload since the load is too high and the motor in the compressor isn't big enough to handle it.

The setting of the CPR valve would be the same as the highest allowable evaporating temperature (+10 F), converted to pressure for a given refrigerant. For r-404a +10 f is 43.4 psig. So, we would set the cpr valve to start throttling at just under 43 psig. This would keep the compressor on line and pulling as much vapor from the evaporator after defrost as possible, without the compressor overloading.

hope this helps,

3 bets

Aren't you referring to the high temp thermal cut-out? This would be set to considerably higher temp than 65'F (18.3'C).

[liptow]

Gentlemen,
Upon discussion with Applications Engineer from Copeland, The 65F return gas temp takes into account about 20F assumed superheat so the CCPR would be set around 40 psi. Now if one had only about 2F superheat at evaporator outlet and a short insulated suction run to the compressor then the CCPR could possibly be set higher but to what benefit? If defrost is required this probably is not recommended, but I'm not sure. Does any one know what happens to stable superheat settings at the evap outlet when a hot-gas defrost evap goes into full defrost? I realize the hot compressor gas cools significantly and drops in pressure (ie: head to suction). Is superheat at evap outlet essentially ZERO in these situations? Or does the use of the MOP TXC allow the superheat to climb for long defrost periods? Sometimes I wish a were a refrigerant molecule getting circulated in a close system. It would be a better then any amusement park ride for sure.
Thanks again, Joe Liptow, P.E.

[liptow]

Sorry,
That's MOP TXV (max operating pressure thermal expansion valve) with hot gas side port downstream of nozzle.

[RonaldoDaLima]

very good.......needss a lot of study