Return gas temperture calculation

[jaybee]

How is the return gas temperture determined on compressor? Will it be the suction pipe temp 6" from service valve +75 ? just want to know how thetemp is determined? Any replies are thanked in advance.,

[jpsmith1cm]

I think it is a simple line temp measurement. At least that is all that I do.

Somebody wiser will surely chime in and answer with the facts.

[jaybee]

I think it is a simple line temp measurement. At least that is all that I do.

Somebody wiser will surely chime in and answer with the facts.

I thought that it will be higher than temp measured at the line because it will pick up heat before entering the suction chamber from the motor& houseing. Is this corerect????

[H/P Man]

The return gas temperature at the compressor is based on super heat. Each compressor manufacture requires a minimum amount of super heat at the compressor. Copelands requirement is 20 degrees Superheat at the compressor. you can find this information on the compressor manufacturs websites.

[thefreezerguy]

The return gas temperature at the compressor is based on super heat. Each compressor manufacture requires a minimum amount of super heat at the compressor. Copelands requirement is 20 degrees Superheat at the compressor. you can find this information on the compressor manufacturs websites.

I have always known it to be superheat. I believe Copeland's spec is 65 degrees of superheat maximum at the compressor on suction cooled machines. However I work in markets primarily on racks, A/C may be different.

[jaybee]

The return gas temperature at the compressor is based on super heat. Each compressor manufacture requires a minimum amount of super heat at the compressor. Copelands requirement is 20 hdegrees Superheat at the compressor. you can find this information on the compressor manufacturs websites.

So the superheat is the return gas temp? the additonal heat picked up by the motor is not a factor? so how can i determine what temp my suction gas temp should be on a low tep r-22 system running -40 degrees to keep discharge temps to safe operating levels? I have seen comp @ 20 degree sh but yet high discharge temps at the comp.

[jpsmith1cm]

So the superheat is the return gas temp? the additonal heat picked up by the motor is not a factor? so how can i determine what temp my suction gas temp should be on a low tep r-22 system running -40 degrees to keep discharge temps to safe operating levels? I have seen comp @ 20 degree sh but yet high discharge temps at the comp.

WOW!

-40 degrees on R-22? Have you considered upgrading to a 2 stage pump?

at -40 degrees suction temp, I'd be looking for -20 MAX.

Do you have demand cooling on those compressors? Are they working correctly?

What is the head pressure? Can you lower it?

As far as motor heat, I'm thinking it is already factored in. The mfr knows how much heat their motor will produce.

[thefreezerguy]

With that kind of compression ratio demand cooling might not even work. The head would have to be LOW.
BTW, are any copeland compressors rated to run -40 r22???

[thefreezerguy]

Are we talking about Copeland?

Is it a Carlyle or Copeland compound? What application? Conventional or rack? Why is the suction temp so low????

I have never seen R22 used down to -40 before!:anyone:

[craig1]

WOW!

As far as motor heat, I'm thinking it is already factored in. The mfr knows how much heat their motor will produce.

I would think It has to be factored in by the manufacturer because it is different for each compressor. a hermetic would add more heat to the suction gas than a Discus (refrigerant cooled), which would add more heat than a K body (air cooled)

[jaybee]

WOW!

-40 degrees on R-22? Have you considered upgrading to a 2 stage pump?

at -40 degrees suction temp, I'd be looking for -20 MAX.

Do you have demand cooling on those compressors? Are they working correctly?

What is the head pressure? Can you lower it?

As far as motor heat, I'm thinking it is already factored in. The mfr knows how much heat their motor will produce.

sorry type o i mean -20 degree suction approx10# suction a rack system

[thefreezerguy]

sorry type o i mean -20 degree suction approx10# suction a rack system

I don't like to see anything under 20 superheat at the suction header. Especially if there isn't a double header. Then I use 65 maximum superheat at the compressor as a guide for proper compressor inlet temps. Though, I try to stay away from the max high, especially on low temp 22.
With low temp r22, run the suction as high as possible and the head as low as practical. The compressors can use all the help they can get!

Good Luck

[jpsmith1cm]

Run 'em cold.

Nice frosty suction valves. I've seen compressors that you have to knock the ice off with a hammer to get at the suction valve last for YEARS!

Cool compressors are happy compressors.

Is this a Hussmann rack? If so, look closely at the demand cooling liquid feed. It probably isn't piped right. Repipe it correctly and your compressors will be happier. Liquid wants to come off of the bottom of the line, not the top.

Also, run a minimum of 20% receiver on those racks for the same reason.

[John Markl]

Run 'em cold.

Nice frosty suction valves. I've seen compressors that you have to knock the ice off with a hammer to get at the suction valve last for YEARS!

Cool compressors are happy compressors.
Is this a Hussmann rack? If so, look closely at the demand cooling liquid feed. It probably isn't piped right. Repipe it correctly and your compressors will be happier. Liquid wants to come off of the bottom of the line, not the top.

Also, run a minimum of 20% receiver on those racks for the same reason.

You sound just like me !! We use the Sporlan mechanical injector though. (Y-1037). No electronics, and they modulate, by having the bulb clamped on the hot gas line of the compressor.

[jpsmith1cm]

You sound just like me !! We use the Sporlan mechanical injector though. (Y-1037). No electronics, and they modulate, by having the bulb clamped on the hot gas line of the compressor.

I used to see them on Carlyle compressors, especially the compound cooling ones.

Haven't seen one in a few years, though.

[John Markl]

I used to see them on Carlyle compressors, especially the compound cooling ones.

Haven't seen one in a few years, though.

Yep. They were for Carlyles. But the Copeland Demand Cooling units were a pain in the rump, plus the Sporlans modulate. So we put them on both low temp racks in that store I posted pics of. They're great :)

FWIW, I don't like the Copeland Sentronic oil safeties either. Johnson Controls for me ;)

[Phase Loss]

Y-1037's work nice and simply. I am not a fan of Copeland's Demand cooling or there Sentronic's for that matter.

I believe Copeland requires a MINIMUM of 20* Compressor superheat and allow you to go higher in superheat until you are out of the safe discharge temperature range OR no higher than 65* return gas temperature. What ever limit you reach first is the stopping point.

Anything out of those ranges would require a de-superheating valve.

But parameters of that range would be specialty refrigeration for weather chambers and lab equipment and other systems of that nature.

[stanbyyourword]

I believe heat added by compressor motor only becomes a factor in "total heat rejected" calculations for condensers,

[jaybee]

Y-1037's work nice and simply. I am not a fan of Copeland's Demand cooling or there Sentronic's for that matter.

I believe Copeland requires a MINIMUM of 20* Compressor superheat and allow you to go higher in superheat until you are out of the safe discharge temperature range OR no higher than 65* return gas temperature. What ever limit you reach first is the stopping point.

Anything out of those ranges would require a de-superheating valve.

But parameters of that range would be specialty refrigeration for weather chambers and lab equipment and other systems of that nature.
Oh , someone stated earlier that your sh is your return temp, which is it??
and getting back to my original ? how is return gas calculated in the field?

[icemeister]

Oh , someone stated earlier that your sh is your return temp, which is it??
and getting back to my original ? how is return gas calculated in the field?

Compressor suction superheat (SH) is not the same as the return gas temperature (RGT). The RGT is used to calculate the SH and they are both measure at the same point (6" before the suction service valve) but those are essentially the only connections between the two values.

Also, 65ºF RGT is commonly misunderstood and often misused in the field. It's not a target suction temperature for proper operation, rather it is simply a standard rating point which was selected many years ago to simplify compressor performance ratings.

Here's a good read from Copeland which will help explain it:
http://www.hvacrinfo.com/cope_ae_bulletins/TAE1260.PDF

[Phase Loss]

Oh , someone stated earlier that your sh is your return temp, which is it??
and getting back to my original ? how is return gas calculated in the field?


Nope compressor superheat is not the same as return gas temperatures. There is no SET number on maximum compressor superheat. Copeland recommends no less than 20* compressor superheat and you can go higher as long as you can maintain safe discharge temperatures.

The method of checking return gas temperatures only requires the use of your thermometer mounted to the suction line no closer than 6" away from the compressor.

Personally I do not use return gas temperatures as a single diagnostic tool. I always combine it with the SST so I can read compressor superheat. The highest SST I typically work with would only be +38SST and If you target 20* minimum superheat, that puts you at 58* return gas temperatures. And that still keeps you away from 65*. The lowest SST I usually work on is -35*SST and keeping the same 20* minimum target compressor superheat in mind, this would put you in the -15* return gas temperature.

The closer you can get your return gas temperature to your SST temperature (superheat) the suction gas will be more dense and the compressor mas flow rates will increase because it is pumping more refrigerant per stoke and staying cool at the same time. Pair that with running maximum suction and minimum head to lower compression ratios and you get an efficient compressor that will last a life time.

[rayr]

Listen to Icemeister, he is a great tech and besides that he got 21 on the Cruel Cwiz!!!! Good job Ice

[thefreezerguy]

Compressor suction superheat (SH) is not the same as the return gas temperature (RGT). The RGT is used to calculate the SH and they are both measure at the same point (6" before the suction service valve) but those are essentially the only connections between the two values.

Also, 65ºF RGT is commonly misunderstood and often misused in the field. It's not a target suction temperature for proper operation, rather it is simply a standard rating point which was selected many years ago to simplify compressor performance ratings.

Here's a good read from Copeland which will help explain it:
http://www.hvacrinfo.com/cope_ae_bulletins/TAE1260.PDF

Thanks for the link. I had remembered it wrong as a maximum superheat value. Knowing it is a RGT rating makes it somewhat irrelevant. It all depends on the SST.
I am going to continue as always. Insure proper superheat at the compressor and "keep 'em cool" like JP said.

[jaybee]

Nope compressor superheat is not the same as return gas temperatures. There is no SET number on maximum compressor superheat. Copeland recommends no less than 20* compressor superheat and you can go higher as long as you can maintain safe discharge temperatures.

The method of checking return gas temperatures only requires the use of your thermometer mounted to the suction line no closer than 6" away from the compressor.

Personally I do not use return gas temperatures as a single diagnostic tool. I always combine it with the SST so I can read compressor superheat. The highest SST I typically work with would only be +38SST and If you target 20* minimum superheat, that puts you at 58* return gas temperatures. And that still keeps you away from 65*. The lowest SST I usually work on is -35*SST and keeping the same 20* minimum target compressor superheat in mind, this would put you in the -15* return gas temperature.

The closer you can get your return gas temperature to your SST temperature (superheat) the suction gas will be more dense and the compressor mas flow rates will increase because it is pumping more refrigerant per stoke and staying cool at the same time. Pair that with running maximum suction and minimum head to lower compression ratios and you get an efficient compressor that will last a life time.

so u are saying return gas temp=sh+ sst ?

[stanbyyourword]

return gas temp is the max sensible physical temperature they want to see returning to that compressor period, regardless of superheat or sst , ;) stan

[Phase Loss]

so u are saying return gas temp=sh+ sst ?

not even close.

your return gas temperature is nothing more than a true pipe temperature.

[stanbyyourword]

I beat ya!! sorry!! ;)

[klove]

so u are saying return gas temp=sh+ sst ?

Yes, but you're approaching it backwards. You can't figure superheat until you already know return gas temp. Suction superheat = return gas (actual suction) temperature as measured with your thermometer minus saturated suction temperature.

Out of curiosity, why the uproar about return gas temperature?

[jpsmith1cm]

your return gas temperature is nothing more than a true pipe temperature.

There is your answer.

I think that this got over complicated with compressor superheats and other minutia.

Return gas temp is return gas temp. Measure the temperature of the line and write it down.

SST (Saturated Suction Temperature) is SST. Measure it and write it down.


Compressor superheat is compressor superheat. Using the above figures, compute it.

This ain't rocket science although sometimes we like to pretend that it is.