5 replies to this topic

[jase123]

Hi, I am new to this forum and have a quick question.

 

Normally for centrifugal compressors, compression ratios are selected to roughly balance to equal horsepower between stages.

 

What about for reciprocating compressors? Say a 3-stage recip compressor, does the above apply as well?

 

 

 

Thank you, any valuable insight into this is deeply appreciated.

[PingPong]

In general: compression ratio is set by the maximum allowable discharge temperature for a particular service or medium.

 

With recip compressors there is possibly the additional limitation of maximum force on the piston shaft, which is determined by pressure difference, not pressure ratio.

[Art Montemayor]

 I have responded to this question a lot of times in the past and I will quote from a seminar I gave on this subject in the past:

 

In understanding compressors, it is more important to consider the compressor ratio than the discharge pressure. 

 

The compression ratio per stage, R, is based on absolute pressures and the number of stages required.  To minimize power usage, one should initially select an equal compression ratio in each stage.  The number of compression stages is arrived at by dividing the total work done equally between all the stages of compression.  The compression ratio is defined as follows: 

 

R = (Pd/Ps)^1/n

where,

Pd = discharge pressure, absolute

Ps = suction pressure, absolute

n = number of compression stages

 

The ultimate compression ratios supplied by a compressor manufacturer will only approximate the calculated, ideal, equal ratios because the manufacturer is limited by the existing, pre-designed cylinder sizes available to a compressor package.

 

It is desirable to limit discharge temperatures to below 121 – 135 °C (250 – 275 °F) to ensure adequate packing life and to avoid cylinder lube oil degradation.  At temperatures above 149 °C (300 °F), eventual lube oil degradation is likely, and if oxygen is present, ignition is possible.  This specific hazard has occurred in Air Separation plants in the past and is well documented.  Under no circumstances should the discharge temperature be allowed to exceed 177 °C (350 °F).

 

Employing a “non-lube” design doesn’t remove the temperature limitation on the cylinder.  Polymers and plastics employed as piston and rider rings are also subject to the same temperature limit.

 

The discharge temperature can be lowered by cooling the suction gas or reducing the compression ratios per stage by adding more stages of compression.  Depending on the gas, a compression ratio between 2 and 3.75 is the usual practice.

 

The compression ratio per stage is limited for any reciprocating compressor by the need to limit the discharge temperature.  As the ratio per stage increases, the discharge temperature increases.

 

Normal compression ratio limits per stage vary from slightly over 1.0 for booster service to roughly 4.0 for special high ratio machines.  Staging compression is efficient because it saves compression power.  For example, by installing a second stage, one may cool the gas discharged from the first stage – which is the suction to the second stage.  The actual volume of gas to be compressed is then lower than it would have been if the gas had continued to be compressed in a single stage compressor without interstage cooling.  The energy saved by performing the compression in two stages.  This savings must be balanced by the cost for increased piping and equipment, energy lost due to pressure drop in interstage piping, and increased maintenance costs.  Note that staging is, in reality, an approach to Isothermal Compression - which is the most efficient method of compression.

 

[jase123]

Thank you Mr Art ,

 

My next question on this is what is the other impacts on compressor if theres highly unequal compression ratios between stages other than power consumption efficiency?

 

 

 

 

 

 

[Art Montemayor]

Other impacts to a compressor operating with distinctively different compression ratios in its multiple stages depends on the mechanical design and type of compressor used.  There are many configurations possible for a multi-stage reciprocating compressor:

• In-tandem design (horizontal and vertical);
• Horizontal, balanced-opposed (horizontal);
• Radial design;
• "L" design (horizontal and vertical)

What I have commented on is on the thermodynamic design of the machines; with excessively different compression ratios given to each stage the problems encountered now fall in the mechanical design and limitations of each type of design.  One outstanding issue would be the creation of unbalanced forces on the machine and the development of turning moments - creating a mechanical balance and vibration problem for the machine that would lead directly to accelerated wear and excessive vibration with its hazards and maintenance problems.

 

If the machine(s) in question were designed and fabricated by a recognized, experienced manufacturer, your question is only theoretical.  Such a design and machine would never make it out through the front door.  However, if the owner takes an existing machine and modifies/uses it for a totally different compression application then that owner has taken on the total liability and responsibility for the operational results.  For example, I wouldn't do it without the original designer/manufacturer's blessings.  I have done a lot of reciprocating compressor modifications in the past - even going so far as to design and machine a 3rd stage for a discharge service of 1,200 psig with an additional suction side stream in a developing country (Peru).  The machine was an in-tandem design and it turned out exactly as calculated and predicted.  I used a scrapped, huge steel roller mill from a sugar refinery as the cylinder material and had the cylinder machined out of it.  I mention this only as an example of what can be done; I wouldn't do it any more.  At the time, I was young, daring, with scarce financial resources and no time to wait in order to produce more product for a growing market and thereby forestall competition.

 

[jase123]

Thank you Mr Art for clearing this up.