Compressor Limts / Envelope

[kfjoe]

hi all

I am trying to understand what causes the compressor limits / envelope , or to be more precise the maximum evaporating temperature SST given by the manufacturer.
In that is it the temperature of the return gas that would give insufficent cooling to the suction cooled windings or is it the pressure relevant to the SST that would overload the compressor due to a high suction pressure ?

If anyone could help me it would be great;y appreciated

Thanks

[mad fridgie]

Limits,
Understanding the compressor first.
If it is a fixed speed, the it can move a certain volume. How dense the inlet refrigerant you are moving and how far you are moving it determines how much power you require(suction to discharge) "Like driving a car up a hill."
Then you need to know the efficiency of the compressor, both mechanically and electrically (for semi and hermetic)
This gives you your upper limit.
When you have low(er) suction pressure, (you have less refrigerant mass) you increase your compression ratio, which increase the discharge temperature (which if it gets to hot can cause problems with oil and some parts of the machinery), If in some cases the motor inside the compressor is gas cooled (often known as suction cooled compressor) then a certain amount of mass of refrigerant is required to remove the heat from the motor. The refrigerant internally becomes greatly superheated.
These combinations determine the lower limits.
You also have limits on minimum compression ratios, valves can chatter if ratios are to low
Plus the manufactures are sometimes a little conservative especially around maximum SST.
Hope this is enough

[Gary]

Limits,
Understanding the compressor first.
If it is a fixed speed, the it can move a certain volume. How dense the inlet refrigerant you are moving and how far you are moving it determines how much power you require(suction to discharge) "Like driving a car up a hill."
Then you need to know the efficiency of the compressor, both mechanically and electrically (for semi and hermetic)
This gives you your upper limit.
When you have low(er) suction pressure, (you have less refrigerant mass) you increase your compression ratio, which increase the discharge temperature (which if it gets to hot can cause problems with oil and some parts of the machinery), If in some cases the motor inside the compressor is gas cooled (often known as suction cooled compressor) then a certain amount of mass of refrigerant is required to remove the heat from the motor. The refrigerant internally becomes greatly superheated.
These combinations determine the lower limits.
You also have limits on minimum compression ratios, valves can chatter if ratios are to low
Plus the manufactures are sometimes a little conservative especially around maximum SST.
Hope this is enough

Exactly so. I especially like the car-up-the-hill analogy.

Can this motor drive the car up the hill without overloading?

That depends:

a. How heavy is the car (mass refrigerant flow)?

and

b. How steep is the hill (compression ratio)?

[mad fridgie]

Exactly so. I especially like the car-up-the-hill analogy.

Can this motor drive the car up the hill without overloading?

That depends:

a. How heavy is the car (mass refrigerant flow)?

and

b. How steep is the hill (compression ratio)?

Cheers Gary, I think that explains it very well

[kfjoe]

Thanks Guys
that helps me
great analogy Gary!

So I guess I am trying to say if i can keep the suction pressure down with a CPR that will stop the compressor overloading , but i think the return gas could be above the maximun SST given as the evaporatoe pressure temp could be high . ?

[mad fridgie]

Thanks Guys
that helps me
great analogy Gary!

So I guess I am trying to say if i can keep the suction pressure down with a CPR that will stop the compressor overloading , but i think the return gas could be above the maximun SST given as the evaporatoe pressure temp could be high . ?

May be your are confusing SST (saturated suction pressure, or in other words a temperature and pressure at which both liquid and vapour exist with no further changes happening like in a closed refrigerant bottle) and superheat. You have a maximum SST (maximum weight of the car) and controllerd by the CPR and maximum superheat (the difference between the SST and the actual temperature) if the superheat is to high, then discharge temp can be to high.
The CPR should not be used as a main control but for short term protection (after defrost, or short bursts of high load)
How the superheat is affected by the CPR is dependent on the system. (this is where you will get different points of view) A CPR causes a pressure drop thus in therory increase superheat, but A CPR resticts the flow out of the evaporator thus allowing the evap to have more liquid in than normal, possibly causing a slight wet suction, so when this wet suction goes through the CPR at little bit of phase change can happen and no excessive superheat occurs (a bit hard to write in a few words)

[Gary]

Thanks Guys
that helps me
great analogy Gary!

So I guess I am trying to say if i can keep the suction pressure down with a CPR that will stop the compressor overloading , but i think the return gas could be above the maximun SST given as the evaporatoe pressure temp could be high . ?

This is an area where mad fridgie and I have a technical disagreement.

I say that the CPR, (installed near the compressor inlet) will do the trick no matter what the evap pressure/temp does.

Try it out and let us know if the discharge temp rises.

[Joey.zhang]

Limits,

When you have low(er) suction pressure, (you have less refrigerant mass) you increase your compression ratio, which increase the discharge temperature (which if it gets to hot can cause problems with oil and some parts of the machinery), If in some cases the motor inside the compressor is gas cooled (often known as suction cooled compressor) then a certain amount of mass of refrigerant is required to remove the heat from the motor. The refrigerant internally becomes greatly superheated.

The internal compression ratio is constant due to constant internal volume ratio for Piston/Srew/Scroll compressors. Evap Temp(ET), Cond Temp (CT) and presssure drop of suction line and discharge line will also affect the compressor operation.

ET -pressure drop of suction line , decide SST, which plus SH will decide the compressor inlet condition, including mass flow.

and then, internal pressure of compressors can be gotten due to fixed compressor ( constant internal volume ratio), discharge pressure, is NOT equal to the internal pressure of compressors, and affect by the pressure drop and actual condensing pressure which is up to the heat exchanger performance.

So DST maybe increase , or also Not.

I agree that low mass flow will affect the compressor's low limit.

[mad fridgie]

The internal compression ratio is constant due to constant internal volume ratio for Piston/Srew/Scroll compressors. Evap Temp(ET), Cond Temp (CT) and presssure drop of suction line and discharge line will also affect the compressor operation.

ET -pressure drop of suction line , decide SST, which plus SH will decide the compressor inlet condition, including mass flow.

and then, internal pressure of compressors can be gotten due to fixed compressor ( constant internal volume ratio), discharge pressure, is NOT equal to the internal pressure of compressors, and affect by the pressure drop and actual condensing pressure which is up to the heat exchanger performance.

So DST maybe increase , or also Not.

I agree that low mass flow will affect the compressor's low limit.

Thanks Joey, I think what Joey is saying, is that if you had you discharge pipe (outlet) is open (no restrictions) then changing your inlet pressure/mass flow will not change the internal compression ratio, because we have a fixed swept volume. How ever in practice the discharge is not open it is restricted (condensor & TEV) then changes to the inlet to the compressor change compression ratios, because the outlet remains more constant in comparission.

[Joey.zhang]

Thanks Joey, I think what Joey is saying, is that if you had you discharge pipe (outlet) is open (no restrictions) then changing your inlet pressure/mass flow will not change the internal compression ratio, because we have a fixed swept volume. How ever in practice the discharge is not open it is restricted (condensor & TEV) then changes to the inlet to the compressor change compression ratios, because the outlet remains more constant in comparission.

Thank Mad for the explanation. Yes, you are correct. Compression ratio will change in pratice as SST and SH varies. Compression ratio may either incease or decrease if the SST is lower, because compressor will over or off compress in pratice due to effect of the actual condensing compressor.

[mad fridgie]

kfjoe, confused yet?
A very good question, very hard to explain in a few words

[sterl]

Piston compressors are NOT fixed Vi machines providing the valves are pressure activated....If they seated and unseated by mechanical means the compressor makes a Vi; which may be near or far from the system's operating requirements.

High suction pressures make high inlet side velocities and high top of piston pressures. The high velocities largely effect suction filters and similar but can lead to oil lift....The high pressures lead to both high motor loads and high stresses on pistons, rods, crank and so on.

[mad fridgie]

Hi Iso, I aplogise if I insulted you, but you did change the working conditions. This does make me a little suspicious?
DesA clearly states my (our) reasons for being cautious.
Like many on here, we are willing to assist if we can (for the good of the industry), but we are not here to give away our Intelectual Property!
Pay the cash , then not a problem!
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

[Gary]

Where there is giving without taking there is no incentive.

Where there is taking without giving there is no justice.

Where there is give and take in both directions there is balance and everyone benefits.

What goes around should come around.

[isobutane]

Where there is giving without taking there is no incentive.

Where there is taking without giving there is no justice.

Where there is give and take in both directions there is balance and everyone benefits.

What goes around should come around.

Agree!
That makes the forum more reasonable

[desA]

^ A sage in our midst... indeed. Excellent.

[2cool4u]

few....!
and relax!