5 replies to this topic

[PRkumar]

Dear all,

             How can we design a optimize centrifugal  compressor. if anyone have appreciable documents pls give me.

 

 

Thanks & Regards

pranav

[ankur2061]

Pranav,

 

What is the definition of "optimized" compressor in your mind?

 

Basically a compressor design from process point of view is to determine the gas power of the compressor in terms of kW or hp. The inputs to determine the kW or hp of a compressor are pretty straightforward:

 

a. Inlet Volumetric Flow Rate (at defined standard conditions) or Inlet Mass Flow Rate

b. Suction Pressure

c. Discharge Pressure

d. Suction Temperature

e. Gas Molar Composition (this will provide the molecular weight, specific heat ratio)

f. Inlet Compressibility Factor at suction

g. Outlet Compressibility factor at discharge

 

The compressor calculations done based on the above data will provide you the discharge temperature, the polytropic or isentropic head, the gas power and if you consider some mechanical losses then the shaft power for the compressor. You can also calculate the number of stages based on poytropic head per stage and rotational speed of the compressor.

 

To conclude, the so-called "optimum" design of your compressor is the one which fulfills your requirement of flow rate, pressure ratio, head  and discharge temperature.

 

Regards,

Ankur.

[PRkumar]

Dear ankur sir,

                       i generally asked for optimize design of compressor(like optimum design of any equipment).if you have anu appreciable documents pls foeward me or suggest me any site. thanks for reply.

 

Regards

pranav

[Bobby Strain]

You get the most effective design by giving a duty spec to compressor vendors along with you driver power cost. They will do their best to meet your needs. Unless you are just doing an exercise.

 

Bobby

[nike.256]

Mr Ankur,

what do you mean calculate the number of stage based on polytropic head per stage?

does it mean if we propose to use 3 stage, so we need 3 compressor map ( say 200 to 400 psig, 400 to 800 psig, and 800 to 1500 psig) and then condition between stage will be optimized?

or predict compressor map of total operating, (say 200 psig to 1000 psi), so from the compressor map, we adjust how much stage will be used?

[ankur2061]

nike.256,

 

You are confusing the meaning of stage for a centrifugal compressor. Following is how I define a stage

 

A stage is defined as an impeller-diffuser pair. A series of stages will comprise a 'section' or 'process stage' between which there is no intercooling or loss or gain in flow (i.e. no sidestreams, bypassing or injection) & whose rotational speeds are in fixed ratio to each other.

 

 

For each stage as defined the polytropic head cannot exceed a limit based on the molecular weight of the gas. As an example if your molecular weight is 16 the polytropic head per stage will be limited to approximately 12,000 ft-lbf / lbm. If your molcular weight is as high as 50, the polytropic head per stage will be limited to approximately 8,000 ft-lbf / lbm.

 

When you calculate the total polytropic head for the compressor and then divide it by the polytropic head per stage you will get the number of stages.

 

Now how did I arrive at the values of polytropic head per stage based on the molecular weight. I will quote from a book on compressors written by an expert on compressors regarding this:

 

"To determine the number of stages (using the impeller and diffuser defined as the stage), assume 10,000 ft-lbf/lbm of head per stage. This value can be used if the molecular weight is in the range of 28 to 30. For other molecular weights, this initial value must be modified. As a rule of thumb, lower the head per stage by 100 ft-lbf/lbm for each unit increase in molecular weight. Conversely, raise the allowable head per stage 200 ft-lbf/lbm for a unit decrease in molecular weight. The rule of thumb gives the best results for a molecular weight range of 2 through 70. Because this sizing procedure is being used only to establish the rough size of the compressor, the upper range may be extended with some loss in accuracy." - Page 196, Compressors: Selection and Sizing, 3rd Edition, by Royce N. Brown

 

When you divide the total polytropic head by the polytropic head per stage you may not get an integer and instead get a fraction. Round-off the fraction to the next integer. For example if you get a fraction X.2 then round it down to integer X and if it is greater than X.2 then round it up to X+1.

 

Having obtained the number of stages re-calculate the polytropic head per stage by dividing the total polytropic head with the number of stages.

 

Hope I have been able to clarify what it means by centrifugal compressor stage. Please note that the definition of stage which I have provided is strictly limited to centrifugal compressors.

 

Regards,

Ankur.