One-Stage & Two-Stage Refrigeration Cycles

[jamesk]

Hello ~Everybody,,

Can someone please tell me the difference between a one-stage and two-stage refrigeration cycle?

I have come across a diagram here: http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1413&context=iracc, pg.220 but still can't grasp the basic difference. Is it simply the fact that between the first and second compressors there is a cooling line in the two-stage cycle and not the other? Or does a two-stage cycle even need 'two' compressors at all?

I am quite confused because there are soo many different types of one-stage and two-stage systems, a broader view would be very helpful, Thank you.

[Rob White]

.

One stage is where the compression cycle (the compressor) is done in one go.

Two stage is when the compression cycle is done in two goes.

Don't confuse the fact that one stage can use more than one compressor.

The reason why we have two stage compression is mainly to do with the pumping
efficiency of the compressor.

Most reciprocating (piston) comps only have a pump ratio of about 1 to 8, that means
it can lift pressure by about 8 times the input. 1 bar in will be pumped out at 8 bar.

Scroll, screw and rotor comps vary between (1 to 10) to (1 to 12).

If you have a low temperature freezer system working at -30 (evap temp) the pressure
could be as low as 1 or 2 bar so to pump that up to the 20 or so bar needed to condense
you have to do it with a two stage system. One compressor pumping into another (or one set
of pistons on the same compressor pumping into others).

It is referred to as a compound system (google compound refrigeration systems).

Two stage is not limited to just pressure (temperature) difference but its a good start point.

The two stage system can then be used for 2 temperature applications and many, many
other things that are for other days to talk about :D

Regards

Rob

.

[jamesk]

Thanks Rob, I read a few times, but still "confused the fact that one stage can use more than one compressor."

I read it as two stages requires two compressors (one after the other), so how can a one stage have more than 'One' compressor and remain 'One' stage? Unless the one stage has parallel compressors (Not one after the other)?
I really did not mean to use 1 so much.
Thanks for your help

[Rob White]

.

One stage uses one or more comps in parallel whereas
two stage has one comp (or piston) pumping into another, in series.

Regards

Rob

.

[Rob White]

.

Imagine a system that requires 40kw of cooling you could do
it with 1 X 40kw comp or 4 X 10kw comps (or combinations in-between).

That is a single stage system because there is only 2 pressures one low and one high.

Now imagine a low temperature system.
One comp pumps the pressure up to an intermediate pressure and the second comp
pumps that intermediate pressure to a high pressure. The complex thing is it can be
done in one comp using different pistons but don't worry about that yet, just think
of separate comps.

Rob

.

[jamesk]

Excellent, thank you. it is crystal clear to me now.

James

[haris]

--> Rob,
Superbly explained, Thank you!
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-==-Imagine a system that requires 40kw of cooling you could do
it with 1 X 40kw comp or 4 X 10kw comps (or combinations in-between).-==-