Two stage compressors

[Tycho]

I could say I have a slight idea what it is, but I'd be giving you a "white lie"

I''ll give you a background story here: http://www.refrigeration-engineer.co...6515#post56515 to give you an idea where I'm coming from...

All I have to say is that with the intermediate pressure, I had some idea, but with "two stage" I've heard it mentioned, and thats about it... I get the feeling it has to do with both screw and piston compressors...

as I said in my previous thread, I have never been hands on, or anywhere near one, but I would like to know more, one engineer to another...

I would very very much appreciate your reply

[US Iceman]

Hi Tycho,

as what I say here might lead someone unquallified to try something that will leave them with a compressor in bits and pieces

That's good advice for all of us. All too often an experienced person may leave out some minor detail in an attempt to explain something and unfortunately, someone reading the reply may assume that is all there is to the story.

Trying to give advice is sometimes like fortune telling. You have to try to look out and anticipate what might happen.

Intermediate pressure is a term commonly used on two-stage systems, or compound compressors.

In a two stage compression system, the intercooler pressure is the intermediate pressure. This term goes back to the days before screw compressors.

A recip. compressor has limits on maximum pressure ratio and discharge temperature. So in order to keep the pressure ratio under these limits it was necassary to split the pressure ratios across two compressors (lo-stage & hi-stage. We sometimes refer to the lo-stage compressors as boosters or booster compressors).

In some applications with no cooling load at the intercooler pressure the pressure ratios were evenly split. This way the pressure ratio was the same for both boosters and hi-stage compressors.

On other applications you may have a cooling load higher or lower than the intercooler pressure, so the actual intermediate pressure could be adjusted up or down during design.

You would see different pressure ratios for the boosters and hi-stage compressors, but it does not matter as long as the maximum values are not exceeded.

With screw compressors, I would tend to use the term side-port pressure instead of intermediate pressure. The side-port pressure just happens to occur where the port is placed on the compressor and is usually closer to suciton than discharge.

With all of this different terminology that sounds similar I like to only use intermediate pressure for two-stage compression systems or compound compressors.

The system could be also be a two-stage system with screw compressors and the side ports are also being used. That could get really confusing.

Does that make sense?

[NH3LVR]

Tycho;
Was just about to reply, when US Iceman beat me to it. His answers are as usual, correct and complete.
Seeing as you referred to "Two Stage Compressors" though I might mention that this is used occasionally for a single compressor set up to work two stage.
There are at least two ways this is done.
The simplest is to pipe the discharge of some of the cylinders into a manifold that is cooled by a TX valve, then into the remaining cylinders. This is called internally compounded or interstage liquid injection.
The other is to use a Intercooler between the stages where the the discharge gas from the first stage is piped into a vessel partially full of liquid refrigerant, allowing the discharge gas to be cooled to the intermediate temperature.
In either case you end up with one compressor body and a two stage system.
It can have some disadvantages in control of the intermediate pressure, but is common in smaller systems.
Selecting an appropriate intermediate temperature/pressure can be involved depending on the loads you want run off the intermediate. For instance a cooler or an ice machine.

[winfred.dela]

All too often an experienced person may leave out some minor detail in an attempt to explain something and unfortunately, someone reading the reply may assume that is all there is to the story.

Trying to give advice is sometimes like fortune telling. You have to try to look out and anticipate what might happen.

Hi US Iceman,

Sometimes Experience persons have to leave some minor (sometimes not so minor) detail:

- to be able to explain in the shortest possible way.

- to make the other person research or study more regarding the particular question.
Thus, making him work & sweat a little.

- to be able to keep the discussion a little more interesting.

Women always does this THING to men, especially the third one . . .


Regards & Merry Christmas,
Winfred

. . .

[karelsdr]

Both guys have done a great job explaining it. But here is my take on an explanation. On a recip. The low side suction is the lowest temp. you are trying to evaporate at. The discharge of the low side is the same pressure as the low side of the high side. This is normally the press./liquid of the liquid you are supplying your evaporators with .Then the discharge of the high side is your condensing pressure.On a screw the suction is your evap. pressure and the discharge is your condensing. The side load is the suction of your supply liquid being subcooled.

[US Iceman]

- to make the other person research or study more regarding the particular question.
Thus, making him work & sweat a little.

Hi Winfred,

Sorry for not replying sooner. I must have missed this thread after the holidays.

Your comment above is the way I was forced to learn from the old timers. They would give just enough information to get you into trouble. That forced me to think about the problem, instead of just asking for the answers.

After a while I learned it was better to think about the problem first, then start working on the problem.