Copeland scroll low temp txv question

[anthonyac1]

So for some reason I “think” I read somewhere with the Copeland scroll compressors in a low temp applications you won’t need a pressure limiting powerhead on the txv. The scroll compressor can handle the higher load. I know on reciprocating compressors they can over amp due to a higher suction pressure during a high load such as after a defrost.Does anyone know if this is true? In a low temp -20 degree coil temp you can use a regular powerhead without the pressure limiting. This particular condensing unit is a ffap-015z-cfv-160.

Is this correct? Or maybe I am wrong.

[trippintl0]

Yes it is true, there is no issue with overamping as the ZS09KAE in the unit you have is rated for low and medium temp.

I think the main risk in your scenario is the fact that the condenser coil is sized for the normal BTU load. After a defrost, suction pressure will run abnormally high for a time and this might cause the head pressure to rise to an unacceptable level. Definitely make sure your evaporator fans delay after defrost.

The particular condensing unit you listed is compact, and only has a single condenser fan. If ambient conditions will exceed 95f then i would use the pressure limiting powerhead for that reason alone. Not that it wouldnt work, but once the condenser gets a little dirty and it's hot, you may get nuisance HP trips. The Emerson units use a Dixell type pressure switch will manually lock out after a couple of HP trips, I believe.

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[pecmsg]

Install a pressure limiting just to be on the safe side

[icemeister]

It's true. As trippintl0 said, the ZS low temp scrolls can handle the amp load over a wide range of suction temps. It's true for the ZF series as well.

It's also true that the condenser size may be a limitation because as the compressor capacity increase, so does the total heat of rejection (THR) load on the condenser. To get a rough idea of what this would mean, consider for example a case where you have a low temp air-cooled condensing unit. By checking the manufacturer's specs you find that at -20°F evap temp and 90°F ambient, the design condensing temp is 105°F. This would represent a 15°F TD.

So hypothetically, if the suction is allowed to increase enough to double the compressor capacity, you would expect the THR to roughly double too. The resulting TD would than be 30°F. For R404A this would mean your discharge pressure would go from about 255 psig to roughly 305 psig or so during a pulldown or after a defrost. Not a big deal there. But if your condensing unit's condenser isn't sized so generously and its normal TD is already 30°F, then doubling the TD to 60°F for pulldown would surely trip the high pressure control.

Another point to think about here, which actually would work as a self-limiting factor, is the TXV capacity. In my hypothetical above, I used a doubling of the compressor capacity as an example. According to the performance charts the compressor would definitely have the capability of doubling capacity given the right conditions, but the TXV's flow capacity is limited primarily by its fixed port size. So if you think about it, if your TXV is sized for 1 ton at -20°F SST, it's going to be seriously undersized if the system is suddenly requiring 2 tons. The result would be the TXV will throttle the suction naturally.

My reasoning for not using the MOP type TXV is essentially because it limits evaporator flow to limit the compressor suction pressure. A CPR valve limits compressor suction pressure directly while allowing the TXV to control superheat normally, which therefore allows for full evaporator distribution...ie, not starving the evap. It also provides better compressor cooling because of lower suction gas temperatures during pulldown.

A compressor with enough motor power to avoid over-amping at high suction conditions doesn't need a CPR or an MOP TXV. The end benefit is a faster pulldown.

I was going to post some performance data, but the Copeland OPI page isn't working for me today.

[bob1420]

icemeister I have been in trade 30+ years and every post of yours I bow down to you,you are a true master of our trade thank you

[pecmsg]

icemeister I have been in trade 30+ years and every post of yours I bow down to you,you are a true master of our trade thank you

X several #'s

[icy78]

icemeister I have been in trade 30+ years and every post of yours I bow down to you,you are a true master of our trade thank you

Yepper!
X2 on that. ^^^

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