1st and 2nd Stage Delivery temps equal? 1

[NeilSF]

Am I right in thinking that in a two stage recip compressor, each cylinder should have the same compression ratio, do an equal amount of work, and that the delivery temperature from each cylinder should be the same?

I have just been speaking with a tech who's said that this is not the case.

Any insight would be welcomed.

Best regards,

NF

 

[pmover]

what about inlet temps for each state?
in theory, yes; provided the gas composition does not change or differ between cylinders and inlet temps are the same. however, each cylinder likely has differences that cause different outlet temps - i.e. increased flow resistance/valve losses. true, for minimizing work, the compression ratio for each stage of compression should be equal as should the inlet temps. for 2nd stage, if inlet temp is higher than 1st stage inlet temp, then outlet temp will be higher.

trust this helps!
-pmover

 

[zdas04]

If they are ever the same, it is a coincidence. Typically, fin-fan coolers are designed for 15-20F approach to ambient. If the inlet gas is 70F, then the interstage outlet temp will be 85-90F. If for some unfathomable reason the clearance and cylinder sizes resulted in exactly the same compression ratios, then second stage outlet temp would be higher than first stage.

Typically the mix of cylinder sizes and clearance available results in one stage taking more ratios than the other. This is a desirable condition. With real-world machines I regularly see second-stage temps very close to first stage, but this happens because the first stage is doing somewhat more ratios.

As long as all the temps are below design max, then there is no reason to expect them to be the same.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[25362]

To obtain equal adiabatic hp of compression in both stages, one needs to have equality in: "suction (p.V)", [κ]=Cp/Cv, and compression ratios, beside equal stage efficiencies (mechanical and otherwise). If not equal, changes in these constraints would have to compensate, thereby cancelling out the effects of their variations.

While analysing the various factors that may affect these variables, one concludes that only by chance one could get absolutely equal amounts of work for both stages.

I believe past threads dealt with these subjects when dealing with real-life compressors.

 

[Montemayor]

Neil:

Dave Simpson has answered your question correctly, as usual. There would have to be a strange and lucky coincidence if the compression ratios in a 2-stage reciprocating compressor were identical. I would bet that this has never happened – within a margin of 10% between them.

But you are also correct in thinking that there is a goal to achieve the same compression ratio and amount of work between them. This means that there is a definite interest in obtaining the same compression ratio in both cylinders – or in all cylinders, if that is the case. Here, bear in mind that most people would visualize only one cylinder per stage; this may not be so. There have been cases where we have used two cylinders for one stage. The reason Dave is correct goes back to the case of reality as compared with theoretical, “virtual compression” as found in your thermodynamics text book. All reciprocating manufacturers, to my knowledge, don’t necessarily have cast iron cylinders on their shelves. But they have a very well-guarded and secured wooden form cache of potential cylinders to be cast on demand or purchase order. All cast cylinders have a corresponding wooden form made and shaped by skilled craftsmen. Most of the people who made some of the more classic and successful wooden forms are no longer alive – and it has become almost impossible to find and hire some of these “old-timers”. I have personally toured the storage rooms for these wooden forms at Ingersoll-Rand and Norwalk when these companies were successfully operating in the 1960-1970’s. It was a sight to see. This is valuable Hi-tech as important as it can get. It takes a lot of skill and know-how to carve, shape, and assemble one of these forms and they represent a lot of technology investment for the manufacturer.

Manufacturers will almost always select the most applicable cylinder forms in their collection in order to cast the “correct cylinders” for your 2- stager. You can imagine the odds that you would have to beat to come up with identical compression ratios. But the equal compression ratio still is a goal – albeit un-achievable. The reason is not so much to produce equal amount of work between them and to generate identical discharge temperatures as it is to achieve as balanced a machine as possible in order to reduce wear and tear. This is particularly important in balanced-opposed models.

So, your technician is also correct although he didn’t give you the reason as to why it is so. I have operated up to 5-stage reciprocating compressors and my records reveal that only one machine – a Clark Bros. model – ever came within the 10% margin. It was a 2-stager, balanced-opposed and ran like a Swiss watch.


Art Montemayor
Spring, TX

 

[crjones]

You did not say whether the gas composition contained water. But if it does, as the compressed gas from the first stage passes through the intercooler you would form a condensate and change the mass flow into the second stage.

 

[25362]

To crjones, this can also happen, for example, with mixtures of hydrocarbons, due to lube oil absorption (dilution of the lube) or, as you mentioned, a consequence of intercooling.