Compression Ratios

[Burrah Boy]

Hi, I am reading a book on two stage low temperature refrigeration (-70 deg C). They say that compression ratio`s shouldnt exceed 7-9 for a recip compressor (I presume this has to do with excessive current draw on start up, but unloaders should cater for this), but if using a screw, compression ratio`s can be 15 or higher. If dont want to embarrass myself by giving my opinion, so can someone give me their understanding of why a screw can handle such a higher ratio than a recip, even though both have means of capacity control.
Thanks

[US Iceman]

The limits on compression ratios stated by compressor manufacturers are typically based on refrigerants used for the specific application. If the compressor is adequate for both ***** & ammonia, the compression ratio limit is dictated by the refrigerant.

The general rule-of-thumb is:

Recip.'s
Ammonia: 8:1
*****: higher than 8:1 (but still dependent on refrigerant used)

Screw's
Ammonia: 18:1 (perhaps a little higher depending on compressor)
*****: Probably higher than ammonia.

The big concern is temeprature of the discharge gas. On recip compressor the discharge temperatures are very high with ammonia, so the manufacturers limit the compression ratio.

On screws you have oil injection for cooling and sealing of the rotors. The injection oil is cooled by external oil coolers or injection of liquid refrigerant (depending on circumstances or preferences).

Capacity control has nothing to do with it.

[sterl]

All of US Iceman says is correct; and its safe to add that lubricants play a big role in determining the character of the operating envelope for each style of machine.

Most recips pass suction gas over the oil in crankcase, and thus past a collection of moving metal parts that are both tossing some of the oil around and warming the refrig gas. Oil at 250 means parts are going to be warm which means gas is going to get heated before it is pulled into a compression cylinder; thus recips on ammonia will warm suction gas by 30 or so degrees. Consider that Suction strainer on a recip may pull frost; but frost on the outside of crankcase indicates big trouble...

Most screw arrnagements sump oil on the discharge side; hottest oil tends to be at same termperature as discharge gas...and is cooled by 50 or 60 degrees F before being injected to compression space as US indicates. Screws employed as boosters on 2-stage plants often are frosted well into the compressor body.

Adding temperature to the suction gas both increases the discharge gas temperature and makes the machine operate at lower volumetric efficiency. In addition: Conventional mineral oils become questionable lubricants where local temperatures (bearing surfaces and similar) exceed 300; and break down pretty quickly should "bulk" temperatures exceed 265 or so.

[US Iceman]

Screws employed as boosters on 2-stage plants often are frosted well into the compressor body.

Just want to add to the above...

Frosted booster compressor bodies do not mean the compressor has liquid in it! This question comes up a lot...

[Burrah Boy]

Thanks guys, seems very interesting, low temp refrigeration is a field I would like to eventually get into as it seems a rather specialised field. Reading about it has really interested me.
Thanks!

[US Iceman]

Just remember all of the same principles apply. It does not matter whether you are considering low temperature systems or high temperature systems. The low temperature stuff though you have to be careful with.