Superheat on compressors

[2sac]

Okay, so 1* would be vapor.

Theoretically.

Yes. Theoretically 1* of SH is enough.

I've taken a couple Copeland classes thru RSES and I've always heard 20-30* at the compressor is ideal. I can also tell you that I've seen extended runs that were poorly insulated with 40-60*SH and the compressors ran just fine.

Ideal evap SH should be 4-8 on low temp and 8-12 on med temp.

All the txv adjusting in the world won't bring down compressor SH if the suction line is poorly insulated

[hvacskills]

Copeland says that they are concerned about the comp SH,not the evap SH. So, having a higher evap SH is the less of two evils. Protect that comp.

as usual you are correct ... but you can do both, protect the compressor and also ensure system performance .... IE, heat exchanger, suction line accumulator .... ICE is the expert here

this link clearly states it is preferred NOT to adjust the TXV to obtain the 20 degree minimum

https://www.achrnews.com/articles/96...ding-superheat

HOWEVER, as has been pointed out previously some manufacturers DO specify to adjust the TXV to obtain the 20 degree minimum, my point is you do not starve the evaporator or impair system performance to achieve this ... just look over the system and evaluate how it is performing, maybe the system will operate fine running with a higher evaporator outlet superheat, maybe the evaporator outlet superheat needs to be lowered and the 20 degree minimum total superheat needs to be addressed by installing a heat exchanger or an accumulator ? we are all supposed to be competent HVACR people we should be able to evaluate a system and figure that out .....

for example, the recent post about the True ice cream dipping freezer .... it was designed for a capillary tube, my guess it was probably running with a 10 - 15 degree superheat at the compressor, that is fine, a capillary tube is not going to spiral out of control and slug or flood the compressor under low load conditions because you can adjust the thermostat and charge to prevent that from happening, but then it was converted to a TXV, so what do you do, starve the evaporator coil that is supposed to be almost flooded to obtain the 20 degrees or install a heat exchanger ?

what about the thread about the freezing evaporator coil, OP adjusted the charge / TXV to obtain 40 degrees at the compressor and was left with a 2.7 degree evaporator temperature, medium temperature ?

what about the 70 year old Pepsi cooler that ran a high superheat with a cap tube and the compressor got warm and seized ?

last I knew nobody ever said there was an easy or simple way to do this type of work if you want it done right

[hvacskills]

Copeland says that they are concerned about the comp SH,not the evap SH. So, having a higher evap SH is the less of two evils. Protect that comp.

you, and most of the other guys on this website have far more experience and are far more knowledgeable in refrigeration than myself, so I am not or am not claiming to be an expert ( except maybe for medium temperature cap tube units ), but I think I understand this part

[Olivero]

Interesting point you bring up.

If I were to choose myself, I would rather ensure the comp is getting the right temp gas back to it so it remains cool and doesen't get slugged over the effeciency of the evap.

If it were a different unit that gets a much higher load and has a much harder time keeping up, I would and I have spent much longer on getting the SH dialed in but this one seems to be happy. It's an R22 medium temp system so with a SH at the evap of 10-15* and the comp at 20-25* I'd say it's pretty good.

Line's are all well insulated, luckily 80% of the run is easily visible and accessible so I can see it.

[BBeerme]

By Copeland saying they want no less than 20*F SH at the compressor, they are simply building in a huge safety margin. Nothing more, and nothing less.

The reality is that you will see a wide variation between applications and temp ranges. For example, the older Manitowoc ice machines were dialed in at 1-2*F evap SH right before harvest. Don't know if they still do that.

Interesting point you bring up.

If I were to choose myself, I would rather ensure the comp is getting the right temp gas back to it so it remains cool and doesen't get slugged over the effeciency of the evap.

If it were a different unit that gets a much higher load and has a much harder time keeping up, I would and I have spent much longer on getting the SH dialed in but this one seems to be happy. It's an R22 medium temp system so with a SH at the evap of 10-15* and the comp at 20-25* I'd say it's pretty good.

Line's are all well insulated, luckily 80% of the run is easily visible and accessible so I can see it.

[Olivero]

By Copeland saying they want no less than 20*F SH at the compressor, they are simply building in a huge safety margin. Nothing more, and nothing less.

The reality is that you will see a wide variation between applications and temp ranges. For example, the older Manitowoc ice machines were dialed in at 1-2*F evap SH right before harvest. Don't know if they still do that.

Right, which makes sense. Kind of a way of idiot proofing it by giving you such a large margin of operation.

I wasn't aware that the droplets were present or even presenting a problem, so I'm glad I now know that.

Your signature made me laugh, a co worker and I actually did solder a relay onto a scotsman control board by soldering wires to the board so they were sticking out and then soldered the relay onto those wires, worked for months.

[hvacskills]

By Copeland saying they want no less than 20*F SH at the compressor, they are simply building in a huge safety margin. Nothing more, and nothing less.

The reality is that you will see a wide variation between applications and temp ranges. For example, the older Manitowoc ice machines were dialed in at 1-2*F evap SH right before harvest. Don't know if they still do that.

that is why it is nice to understand ' why ', allows you to adapt to different situations

walk in cooler I service, older circular evaporator coil with a built in heat exchanger, liquid line running about 4 feet inside the suction line ....

in the summer with a heavy load the unit runs fine with steady evaporator outlet superheat of 10 - 12 degrees

last winter with a light load the walk in cooler would reach temperature in around 4 minutes, not long enough for the TXV to stabilize, TXV was hunting all over the place and the evaporator outlet superheat was running low at times, figured this must be the low load they are talking about and how the TXV cant maintain superheat, because of the heat exchanger I still had superheat at my compressor so I just bumped up my box temperature a few degrees as an extra cushion so the load wasnt ' as low ', figured who needs ice cold beer in the winter time anyhow

understanding ' why ' allowed me to adapt to that situation

[Olivero]

that is why it is nice to understand ' why ', allows you to adapt to different situations

walk in cooler I service, older circular evaporator coil with a built in heat exchanger, liquid line running about 4 feet inside the suction line ....

in the summer with a heavy load the unit runs fine with steady evaporator outlet superheat of 10 - 12 degrees

last winter with a light load the walk in cooler would reach temperature in around 4 minutes, not long enough for the TXV to stabilize, TXV was hunting all over the place and the evaporator outlet superheat was running low at times, figured this must be the low load they are talking about and how the TXV cant maintain superheat, because of the heat exchanger I still had superheat at my compressor so I just bumped up my box temperature a few degrees as an extra cushion so the load wasnt ' as low ', figured who needs ice cold beer in the winter time anyhow

understanding ' why ' allowed me to adapt to that situation

Exactly, that's why I wanted to get a better understanding of it.

[BBeerme]

Right, which makes sense. Kind of a way of idiot proofing it by giving you such a large margin of operation.

I wasn't aware that the droplets were present or even presenting a problem, so I'm glad I now know that.

Your signature made me laugh, a co worker and I actually did solder a relay onto a scotsman control board by soldering wires to the board so they were sticking out and then soldered the relay onto those wires, worked for months.

[2sac]

By Copeland saying they want no less than 20*F SH at the compressor, they are simply building in a huge safety margin. Nothing more, and nothing less.

The reality is that you will see a wide variation between applications and temp ranges. For example, the older Manitowoc ice machines were dialed in at 1-2*F evap SH right before harvest. Don't know if they still do that.

It is my understanding that ice machine compressors have beefier internal parts than low or med temp pumps

[BBeerme]

If memory serves, ice machine compressors are medium temp.

For what it is worth, compressors can handle those minute droplets of refrigerant entrained in the oil. Just think about a hot gas defrost, when the evap is actually being used as a condenser. As long as the oil in the compressor is plenty warm, then you're good to go.

It is my understanding that ice machine compressors have beefier internal parts than low or med temp pumps

[On The Path]

Where and when has Dick Wirz stated at 5*SH liquid refrigerant is present? I personally own a few tomes of his, and do not remember anything of the sort?

[Core_d]

Ive seen oil sight glasses churning and boiling at 8 or better SH.


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