Perfect Engine revs for fridge compressor?

[allatsea99]

What revs would you run a 3 cyl - D1-30 Volvo to bring a eutectic refrigeration system down with standard belt driven compressor? I've been subjected to such different advice here in all my commercial years in chartering but no one has given me technical justification for determining the best rev range. Any ideas from the informed amongst the forum please? Would save unspoken disagreement from my very capable wife aboard! Thanks.

[Sandero]

Great question.... I always assumed that when the pulley size was chosen it was meant to provide max cooling at higher than idle and less than typical cruising RPM. But I have no idea about this and would like to know.

The compressor is a load and so I suppose running the engine on the hook to cool the refer would be considered loading the engine which is supposed to be better for the diesel. In that sense the alternator is a load as well (more so when it's kicking out lots of amps).

You could run some tests... with a thermometer... in the box... run for say 1 hr at various revs with same starting fridge temp and compare the results.... or the time it take to reach a designated temp at various RPMs. Probably 3 trials would be enough.

[shakey doug]

What type of compressor do you have ie. Reciprocating or rotary? Make? Refrigerant? TX valve size?

[allatsea99]

Hi Shakey Doug... your asked for some details...
Compressor is a Unicla UP-90 -10 cylinder swashplate design, running on R134M refrigerant, - not sure on the TX... imprinted in the Danfoss casting is 158 3002. This help? Thanks.

[sfnz]

Tecumseh/ blissfield state maximum rpm of compressor for low temp as 850rpm, alas the pulley sizes to achieve this with a high speed diesel aren't that belt friendly, have used a 2" engine pulley in the past but it's viscious on belts 2 1/2" seems to work well which with a swash plate comp will give you roughly a 2 to 1 ratio, basically the slower the better, 2 reasons for this, design of comp is for auto A/C which is designed with high back pressure which equates to high flow and high oil return and comp subcooling, drop the temp required and oil/ lubricant return as well as subcooling suffer,as an aside, it is usual practice to add oil (100ml for a fridge, 150 ml for a fridge and freezer) above the recommended . 2nd reason is to keep refrigeration system low side out of the negatives pressure wise to negate any likelihood of moisture laden air into the system via the shaft seal .and in extreme running in a vacuum it is possible for the compressor to dump most of its oil into the system, usually the evaporator...
so
!/ try keep the comp rpm as slow as you can
2/ regular drier changes
3/ add oil