"Heat rejected by compressor"

[PhilippF]

Hi guys!

As I am still learning, I more often come across this term but still dont get what it means. Would anyone be so kind and explain it briefly?

Thanks in advance!

Regards,

Philipp

[Xeon th MG Pony]

When a hermetic compressor runs it converts electrical energy to mechanical, a fundimental law of thermal dynamics says nothing can be 100% efficient, thus we have losses, some electrical energy is converted to heat, plus some mechanical energy is converted into heat.

So with a running compressor we must remove this heat added by the 2 losses of energy, as the hermetic is surounded in cool refrigerant vapour we naturaly dump the heat there and discharge it along with all the latent heat absorbed from the evap.

The term Total Heat Rejection refers to A: Rejecting the heat taken from evap & B: The heat taken to cool the compressor its self. On average the heat added by the compressor is between 25 to 35% depending on the type.

Technicaly we have three heat gains, one is the evaporator, two is the Suction Line, and finaly Three is the Compressor, when we add all three togeather we get the Total Heat of Rejection, and that is the size of the condencer you will need, in order to get good efficiency it is concidered good practice to then over size it by 15 - 25% to help give reserve capacity during hot pull down or un-usualy hot days.

[Xeon th MG Pony]

So in short the term of heat rejected by the compressor means How much heat the compressor can move at a set of points for example it can move 6,000BTU/h @ 35C(95f) condensing @ 7.6C (45.7F) Evaporating. You use this to match a compressor to your aplication.

Things you need to select a compressor:
The evap temp
Amount of heat to be removed at evap temp
Refrigerant type
Condencing temp

The compressor must fit into the opperating window of the design.

Total heat of rejection is that + 35% and is used in sizing the condenser.

[DetroitAC]

The only way I know of that a compressor can reject heat is by shell cooling, in the case of a hermetic, air flowing over the steel shell. There are semi-hermetic that have water cooling for the motor (like Copeland Copelametic), and some that even have water cooled heads (Bitzer).

Plenty of ways in which it adds heat, that's the only way I know of that it rejects heat.

[Xeon th MG Pony]

The only way I know of that a compressor can reject heat is by shell cooling, in the case of a hermetic, air flowing over the steel shell. There are semi-hermetic that have water cooling for the motor (like Copeland Copelametic), and some that even have water cooled heads (Bitzer).

Plenty of ways in which it adds heat, that's the only way I know of that it rejects heat.

Hermetic shell compressors are considerd vapour cooled, the motor winding and compressor head or in an atmosphere of cold refrigerant gas, where is the heat going to go? The heat you feel on a hermetic shell is mainly from the oil running down the sides, it helps but is not the bulk of the cooling.

If the compressor has an oil cooling loop they will partialy condence the refrigerant and send it down that loop to cool the oil then back to the main condencer, water too can be used with the oil cooling circuit.

Depending on the design of a semi hermetic it will be suction gas cooled or air cool, and of cours water cooled.

[DetroitAC]

Xeon, I think I'm interpreting the word "rejected" very differently than you are. Most of what you are talking about I would call heat "added" by the compressor, and heat "rejected" by the system.

The question was about the term "heat rejected by compressor", now way back in the dark ages when I was taking thermo classes, we used the word reject to mean "get rid of it to the ambient". We would also get points taken off for using C,K and F temperatures all in the same formula just giving you a hard time...

[PhilippF]

Thanks, Xeon! But I still dont get the term "rejected by compressor". Thats what is confusing me.

[Xeon th MG Pony]

Xeon, I think I'm interpreting the word "rejected" very differently than you are. Most of what you are talking about I would call heat "added" by the compressor, and heat "rejected" by the system.

The question was about the term "heat rejected by compressor", now way back in the dark ages when I was taking thermo classes, we used the word reject to mean "get rid of it to the ambient". We would also get points taken off for using C,K and F temperatures all in the same formula just giving you a hard time...

Ya here we are metric and for some assinine reason the book is in imperial, I didn't have time to do all the conversions lol

Normaly I only give Metric with imperial in brackets.

And it seems we interpret "rejected" here differently indeed, when the term is used it it is not binded to ambient, it is binded to the opperating mechanism whether it be refrigerant, water, air exctera

Heat rejected by compressor is "Some" of the manufacturors way of saying "This how much heat the compressor will move in this situation X Condencing @ Y evaporating" unles they specify other wise.

[Xeon th MG Pony]

Thanks, Xeon! But I still dont get the term "rejected by compressor". Thats what is confusing me.

Refer to above post, last peragraph.

[DetroitAC]

Heat rejected by compressor is the manufacturors way of saying "This how much heat the compressor will move in this situation X Condencing @ Y evaporating" unles they specify other wise.

Really? I happen to be employed by a compressor manufacturer, and we would call what you have described the "Capacity" of the compressor, and what is meant by that is the cooling capacity in the conditions referenced.

Please post a link to a company that uses the term in that way, as certainly not all companies or people use the same terminology.

Maybe we can clear this up for PhillippF yet! PhillippF, can you describe the context in which you're seeing this term?

[Xeon th MG Pony]

Really? I happen to be employed by a compressor manufacturer, and we would call what you have described the "Capacity" of the compressor, and what is meant by that is the cooling capacity in the conditions referenced.

Please post a link to a company that uses the term in that way, as certainly not all companies or people use the same terminology.

Maybe we can clear this up for PhillippF yet! PhillippF, can you describe the context in which you're seeing this term?

Here in Canada They seem to call it both, I've often heard both terms used.

If I recall I've seen it used by Copland lititure befor, Cant recall where, I do recall it was from data on an old series recip.

I too would call it capacity, but I have seen that term used, and as we see here it is confusing to see it used in such a way, or some times they give a percentage and would refer to the compressor cooling its self

Capacity then heat rejected by the motor to the refrigerant, over time I have seen it used in such ways.

[jinu117]

Rejected in here is compressor dumping heat to air.... bit interesting. (from compressor at least).

[PhilippF]

Perhaps I have a little problem with my English here. I understand "rejected" in opposite to "accepted". So I thought what kind of heat the compressor could "reject" instead of "accepting" heat. I think I got all this wrong.

If I am understandy Xeon right: Cooling capacity + Heat rejected by compressor = Power to dissipate in condensor

The worse isentropic efficiency of a compressor, the more heat is "rejected"?

[Xeon th MG Pony]

Perhaps I have a little problem with my English here. I understand "rejected" in opposite to "accepted". So I thought what kind of heat the compressor could "reject" instead of "accepting" heat. I think I got all this wrong.

If I am understandy Xeon right: Cooling capacity + Heat rejected by compressor = Power to dissipate in condensor

The worse isentropic efficiency of a compressor, the more heat is "rejected"?

That works.