automotive ac compressor

[airtight]

Hi all,
I have recently purchased an atomotive style a/c compressor for an emergency glycol chiller on a small boat.

It was sold as a 3kw output however on measuring we have only 1.6 kw. The engineers say it is working finem

My question is that in the spec it says operating speed 800 rpm - 8000 rpm. Are these type of compressors self regulating once they get beyond a certain speed?

Our setup is running at 2200 rpm and I am just wondering if i increased it to its max speed (over quadruple) if the output would rise.

Its a bit of a longshot that anyone can help me, im really only in to traditional reciprocating compressors that would have a pro rata output for their speed.

Thanks in advance. Neville. Scotland

 

[MintJulep]

It's still a positive displacement compressor. Capacity is a function of speed.

 

[NickE]

1) most engines couple to the AC with a slight gear up. Thus the AC is spinning ~1.1-1.5 times the speed of the crank.

2) more than likely the compressor you have is a reciprocating compressor. most are, unless its a scroll design. (although you probably have anywhere from 5-9 cylinders, and they reciprocate parallel to the input shaft.)

3) The design speed of the compressor is likely somewhere around the top gear 55mph cruise RPM of the vehicles it was intended for.



Nick
I love materials science!

 

[BrianPetersen]

The power demand on the compressor (and cooling output) will probably also depend on the evaporator and condenser temperatures and pressures, and on the amount of flow of condensed refrigerant that the orifice lets through.

 

[Warpspeed]

These compressors are especially designed to work over a very wide range of condensing and evaporating pressures, as well as a very wide variations in driven speed. For this reason, they have some particular design features such as deliberately restrictive intake valving, and a deliberately large cylinder head clearance volume.

The restrictive intake valving along with relatively large piston displacement allows useful cooling capacity at only 800 rpm, but deliberately restricts the mass flow and hence discharge pressure at 8,000 rpm.

The result is useful cooling at engine idle speed, without mechanically overloading anything at flat out maximum engine rpm. Increasing the drive rpm will increase the cooling capacity, but almost certainly not by as much as you may be expecting. Once the intake valves go sonic, that is the end of any flow increase with speed.

Likewise the large internal clearance volume limits the mass flow as the differential pressure rises. This is to limit compressor drive torque and prevent belt slippage.

The whole thing has been designed for safe reliable operation in a vehicle (nothing bursts, breaks, or slips) and reasonably useful cooling capacity over a very wide envelope of operating parameters.

You could try speeding it up, but I would expect only a minor improvement in doing so.