Compressor loading, cooling, and range of operation.

I thought I would bring up the subject of compressor motor load, and cooling based on the evaporator temp. (The way I see it)

Or basically how they classify compressors based on high temp, low temp, and medium temp operation. And why they do so. And how you can use that information when applying that compressor to a job it isn’t listed for.

The program I used to make the graphs I attached to this post is graphmatica. Do a google search for it and you will find it. I added the temps and labels with a paint program.

The base point is motor loading based on evaporator temperature. (With the condenser temp as reference)

The first line on the graph.(white)
Y=2(((1+ln(1/X))X)-X)

The base equation for energy/force from/to a compression/expansion system is...

(mean effective pressure) = (initial pressure abs)*(1+hyp.log(no of expansions)/(number of expansions) -(back pressure abs)

That is drawn from the “Audels engineers and mechanics guide 1” copyright 1921. (Which I have in hand)

To reduce that equation. The initial pressure is the reference point (1)

So that reduces the formula to

MEP=(1+hyplog(no of exp)/(no of exp)-(BP)

hyp.log=(log natural) so..

MPE=(1+ln(no of exp)/(no of exp)-(BP)

To graph it on a 1 by 1 Graph as 1 equal to the condensing pressure. The number of expansions equals 1/X. That makes it.

MEP=(1+ln(1/X)/(1/X)-(BP)

Reduces to
MPE=((1+ln(1/X)X) -(BP)

Back pressure on the cylinder is the intake pressure. (pressure in the compressor case) which is X

MEP=((1+ln(1/X)X)-X

The two on the front of the equation used in the graph is to scale it to make it fit in the 1 by 1 graph nicely.

It will give you a relative load graph for any compressor/ based on evaporator pressure with a fixed condensing pressure. X=1 when the evap pressure and condenser pressure are the same. (no load). X=0 when the evap pressure reaches pure vacuum and there is no vapor to compress (no load)
Refrigerant temperature will affect the density and push the peak a little bit one way or the other. but it is ignored to make the equation simpler.

Since the vapor density roughly follows absolute pressure (ideal gas law) (ignoring temp induced shifts). Vapor density at the intake equals X so the efficiency of the compressor based on evap temp is roughly is...

X/(above equation)

That is the second line in the graph (green)
The higher you get the evaporating temp (closer to condensing) the lower the energy used to move every BTU of heat. Or basically the smaller the evap/condense temp split the higher the EER and COP.

Both lines are affected by volumetric efficiency which will push the figures a little. But with the scale of this graph it is to little of a change to worry about.

All in all, the graph is just something to get a general idea across so don’t nit pick it to death. (please!!)

Second graph is a close-up of the normal operating range people deal with.

Now onto the zones of operation.

The peak load is around X=0.37

If we reference the condensing point (X=1) as 120F that will give you an idea of where the peak point lands.

Peak is about 55F for R12 or R22 based on the vapor density chart.(Gas temp equal to evaporating temp)

Or about 60F boiling point pressure based on pressure alone (ideal gas law) ignoring temperature induced changes.(Assuming gas temp to be 120F.)

So it will be somewhere in between.

Boiling point temperatures on the graph are based on density listings from a pressure temperature chart. Not on the ideal gas law with a fixed 120F gas temp.

The peak point will always roughly be the same percentage of the condensing temp. So if the condensing point goes up, then the point of the peak load evaporating temp will go up by the same percentage. (In addition to being a larger peak load.)

If we have a condensing of 90F then....
Peak is about 30F to 40F for R12 and R22.

IF you go to a higher evap temp than that then motor load goes down. If you go to a lower evap temp than that then motor load goes down.

The cooling effect of the refrigerant coming in goes down as the vapor density goes down. So, at the bottom end of the graph the cooling effect of the returning vapor becomes really low for the displacement..

That will give you an idea why they categorize compressors as high temp, med temp, and low temp. Which part of the graph they operate in. That is limited by the maximum motor horsepower displacement and cooling.

(per embraco specs)
Low range is -31F to 14F (-35C to -10C)
Med range is -4F to 40F (-20 to 10C)
High range is 23F to 59F (-5 to 15C)

The higher you go in the range, the higher the motor load will be, and the higher the vapor density, and cooling effect of the vapor will be.

There is two losses in an electric motor, the no load losses that occur as a result of magnetizing losses and copper losses. There is the load losses that result from copper losses and rotor slippage. The no load losses are constant, while the load losses go up with the square of the load. The load losses limit the max horsepower of the motor. You can reduce the no load losses by making the motor larger for it’s relative max horse power. Which reduces magnetic density in the windings, and magnetic losses. But that makes the motor heavier and more costly.

Small low temp compressors have a relatively large displacement for the motor size. (Small motor for the displacement) If you try to bring up the condensing temp (move it closer to the peak) the small motor will overload and will burn out. But the motor has a low no load loss for it’s rated horsepower to deal with the low cooling of the low density vapor.

Large low temp compressors like bolted hermetic units have outside cooling fins which air is blown across to cool the motor. They don’t have to worry about low cooling from the return vapor. The motor, just like the small units, is undersized for the displacement. If you try to go toward the load peak, the motor will overload.


On medium temp units the motor has a larger horsepower for the displacement. But to make the motor smaller they push down the size to the minimum with pushes up no load losses. The motor makes a higher minimum heat load for the vapor to carry away, so they rely on vapor density to improve cooling, which is why they will overheat if operated at low temp. They can make it where it will operate at low temps, but it requires a moderate bit of overbuilding by keeping the motor relatively large and keeping down no load losses. That way, the low vapor density can cool it at reduced load.

With high temp units. They have the largest motor of any of them. The designer usually relies heavily on the cooling effect of the refrigerant coming in to help cool that motor. They can because of the high vapor density that that is present. They can design the compressor with enough cooling effect to tolerate operating at medium and low temp if they want to but it makes for a larger, bulkier and more expensive compressor with a large motor for it‘s rated horsepower.

Cross listed compressors (multi duty) can operate over a wider range.

Low/Med units and Med/High temp units have the larger motor to run in the higher load range that can deal with the low cooling from the vapor because they designed the motor with low no load losses. That causes the heat load to drop down enough with vapor flow to avoid overheating.

Low/Med/High units have the largest motor that is designed to handle the highest loads, but it has real low no load losses for it’s rated horsepower so it can handle the low vapor density that is present at low temp operation.

AC compressors are in the high temp range and usually rely heavily on the cooling effect of the suction gas to keep the motor cool. They have a small highly compact motor that has real high no load losses, so it will quickly fry if you run it with low vapor flow across the compressor at low load.

If you try to run a high temp compressor at medium temps the motor may overheat from lack of cooling if it is fighting a high condensing pressure. You can help it operate at medium temperatures by keeping down the condensing pressure/temperature way below max rated specifications.

A compressor that is designed for low temp operation with a higher pressure refrigerant can be used for low, medium, or high temp operation with a low pressure refrigerant. (R12 or R134a in a R22 or R502 low temp compressor.) The compressor has a larger motor in relation to it’s displacement to pump the higher pressure refrigerant with a higher pressure shift. So it can operate with a low pressure refrigerant in the medium and high temp ranges without overloading the motor. And it will operate in the low temp range just fine because of the reduced load in that range. The motor will have a lower vapor density for cooling at that range, but it will also have a much reduces load at the same condensing temp. So motor heating or overloading will not be a problem across any range. At the medium and high temp range the vapor density will be above what it would see with a high pressure refrigerant at low temps at the same load, so it will be running nice and cool.
Low and medium temp compressors can be operated on the upper side of the peak without overloading. (Above high temp range). 60F to 90F evap temp with a 90F condensing temp.

Medium temp compressors can be used for high temp operation when you keep the condensing temp way below maximum specifications.

Medium temp high pressure compressors can be used with a low pressure refrigerant in the high temp range.

High temp high pressure suction cooled compressors can usually be used with a low pressure refrigerant in the same application but the range of operation before motor overheating occurs will be smaller. It won’t be able to simultaneously handle as high of condensing temp, and as low of evaporating temp. One or the other limit by it’s self is fine, but it you push both limits at the same time, something may fry.

High temp high pressure discharge cooled compressors should love running with a low pressure refrigerant because of the lower heat of compression. The gas flowing around the windings will be a lot lower temp than you would see with a high pressure refrigerant.(lower heat of compression) so it should be running nice and cool.

Anyone see any obvious flaws in my logic?
Anyone find any of this info useful?
Or am I just wasting my time? :D

Yuo need to get a life mate!

Very interesting. Impressive! I didnt know of the relationship between the physical size/power and the zero load/full load losses. Electrical engineering is a very interesting subject.

Quote:

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Originally Posted by Refrigerologist

Yuo need to get a life mate!

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Whats wrong with investigating deep into a subject? If no one ever did it we would probably still be in the stoneage!

Nice one SteinarN!It is an interesting post.Clearly passionate about the trade,which i good to see.Nobody knows everything in this trade,so every experience and piece of information shared is valued.Thanks Air Duster.Some useful info,some common knowledge and some to look into....

As compressor manufacturer, I will go into your subject and come back with my opinion in the next days. Thanks for your time so far.

Very interesting post - thanks.

Most consideration are too general to be applied to real cases. For example, a low temperature compressor can be used for high temperature, provided superheat is kept low enough to boil it off.
Also: the rotor can be modified on the fly to accomodate a wider or narrower rotating disk and cool the motor at the expense of discharge temperature.
So, I do not agree with you 100%.

Also please note that low, medium or high temperature is not a feature of Embraco, but it is decided by european norm EN12900.

Seems like Air Duster isn't active in here any more.

I wish he was.....

thanks air duster

is this same for r407c