Compressor HP selection

[Blue warrior]

Hi,

When selecting a semi-hermetic compressor I noticed that sometimes you can get 2 compressors that can both do the correct duty and swept volume but the HP ratings are different.
Can anyone tell me if you have a compressor with a HP of 10 and one with a HP of 15 which one should be selected as both options give me the correct duty and swept volume?
Obviously the larger HP uses more electricity to operate so would automatically select the lower HP option.

Any thoughts?

Thanks

[chemi-cool]

Compressors are rated by evaporating temp. if you look at catalogs, you will see the difference.
When a compressor designed for high back pressure, it will have a lower pistons volume at a given speed.
In low back pressure, a larger pistons volume.

When you select a compressor, choose according to the back pressure you have designed the system for.
If you only replace a compressor but use different make, check the back pressure before.

[nike123]

As the suction pressure in compressor decrease, refrigerant density also decrease. Lower density mean less required work to compress refrigerant and therefore it can be done with less powerful engine.
Also, compressor is cooled by suction gasses. If suction gases are of less density, compressor have less mass flow and therefore worse cooling with suction gases. Electrical motor of smaller power have little smaller heat generation for same amount of generated work than bigger engine but not much significant.
Contrary, smaller power engine than original one could lead to stalling of compressor.

Compressor is defined by its swept volume (displacement), rotation speed and engine power.
Swept volume and rotation speed give us volume flow.

When you replacing compressor with other which is not same as original, you need to match same volume flow and similar motor power (or same area of usage: LBP, MBP, HBP, AC) as original one, and you will have perfect match.
Than you don't need to worry about refrigerating capacity stated in catalogues.

So, if original one is of swept volume of 20cc and rotation speed of 1440/min we have volume flow of 20x1440=28800cm^3/min or 1,728 m^3/h.
So in one catalogue it is stated in cc/rev and another one it is stated in m^3/h. Now you could compare these two compressors. What is left is to match their area of usage according to catalogues and your particular case.
Engine power does not necessary need to bi same. It is important that area of usage is matching.

Like in Dorin for example. In some cases you could use CC compressor and CS compressor for same application and in some only CC.
http://i44.tinypic.com/w71xfo.png

[monkey spanners]

Big motor=chiller/high temp, small motor=freezer/low temp, for the same displacement as a general rule.

Refer to manufacturers specifications for selection.