29 replies to this topic

[Brijesh Patel]

Hi

 

I have one problem regarding inter stage pressure in multi stage  reciprocating compressor.

I have four stage reciprocating compressor with inter cooler. I also have inlet and discharge pressure. here volume of each cylinder are known with clearance. Then what is procedure to find out inter stage pressure

 

Regards

Brijesh

 

[Art Montemayor]

This is NOT a computer simulation topic.  This is nothing more than a simple application of calculating the theoretical interstage pressures in a multi-stage reciprocating compressor.  Any good compressor textbook covers this subject and the method is amply covered in our Forum Search Machine or in Google.

 

If you have a reciprocating compressor problem, you should research the subject matter as indicated, do the calculations and if needed, check with our expert members by submitting your calculations.

[breizh]

http://checalc.com/calc/compress.html

 

Consider this resource to support , good to find a textbook to understand the way the calculation is performed.

 

Good luck

 

Breizh

[Brijesh Patel]

Thanks Art & Breizh

 

Actually I already referred book of reciprocating compressor. I find out inter stage pressure based on formula P_i=(P₄/P₁)^(1/n)

For Example :

P4=650 psia

P1=14.5 psia

n( No of stages)= 4

P_i= 2.5875

 

Above calculation is correct for ideal case. Here P_i will remain same for all stages.

 

But in actual case pressure ratio (P_i) at each stage will be different. I am trying to find out actual pressure ratio require for each stage to reach up to 650 Psia.

I have volume of compressor cylinder at each stage as below

V₁=2042.82 in³

V₂=779.31 in³

V₃=298.64 in³

V₄=113.09 in³

In short In this case I have fixed volume of compressor cylinder at each stage, and I need to adjust pressure ratio at each stage based on inlet and discharge pressure. I have required inlet capacity is 900 icfm

 

I think it may be iterative process. But I have no idea how to adjust pressure ratio at each stage

[breizh]

https://books.google...epage&q&f=false

 

Hi ,

Let you try to get a copy of the book mentioned above.

 

Good luck

 

Breizh

[Brijesh Patel]

Thanks Breizh,

 

I already referred this book, they have given calculation same as I mentioned above and suggested that in actual case it may be different due to volumetric efficiency. I tried with maximum possible way for calculation of correct pressure ratio at each stage but I am not able to breakthrough.

 

can I get your Email Id .I want to share one Document of performance of compressor. so you can guide me in good way.

 

Regards,

Brijesh

[breizh]

Hi ,

Better to share your calculations with the community for the benefit of all  members .

 

Breizh

[Brijesh Patel]

Hi Breizh,

 

I have attached data sheet of compressor data. I am calculating for four stage reciprocating compressor whose all input data are  given in attached file on the top, and result is also given in this sheet.

 

I am trying to calculate pressure ratio( Compression ratio) for this data. but it is not matching with this sheet.

you can observe in this sheet pressure ratio( Compression ratio) is different at each stage. and I am trying to get value given in this sheet by hand calculation.

 

Biggest confusion is how to adjust pressure ratio at each stage

 

Regards,

Brijesh

Attached Files

•  Compressor Data sheet.png   134.63KB   28 downloads

[Art Montemayor]

Brijesh Patel:

 

Please tell us what, specifically, you are trying to do.  This is important to know in order to best answer your query.

 

It is obvious you have no experience in process designing, specifying, purchasing, installing, or operating a multi-stage compressor.  That is O.K., most people don't.  Some of us on this panel do have that experience and can share their knowledge with you.  But you must be candid and open about your query in order to help you.

 

Go to the following past thread on this topic:
http://www.cheresour...age-compressor/

 

I don't know if you are trying to "simulate" a reciprocating compressor on a computer program or not.  I don't know if you are dealing with an actual machine that you may be presently operating.  I don't know these things because you don't tell us.

 

If you are trying to predict the accurate, expected compression ratio in each stage of a multi-stage gas compressor you are wasting your time.  I hope you gather as much from the referenced thread.  The multiple-stage cylinders whose performance you are trying to predict ALREADY EXIST - either in a warehouse or in casting drawings, diagrams, or wooden molds for casting.  All compressor manufacturers have already decided on the size, specifications, and ratings of the cylinders they will mount on the frames that they also have already decided to produce.  They will fit your specifications to the existing cylinder specification that they already possess.  They are not going to go to the expense and risk of casting or forging a special cylinder for you - unless you want to pay for the exorbitant cost of doing so for one machine produced.  Those are the facts of life as they exist in the engineering world of reciprocating compressors - and most major and expensive equipment.

 

I am trying to get this thread back on a logical and realistic path so that all our members can share in helping and learning from it.  I hope you can help by furnishing us with all the details of your query.

[Bobby Strain]

Brijesh,

      No one would actually use manual calculations for this. Best that you focus on understanding the thermodynamics involved and the calculation required to find the capacity of a single compressor cylinder given pressure and temperature requirements along with machine parameters. Like those shown in your post. In practice, one would use trial and error to arrive at interstage pressures for a multi stage compressor. The more stages involved, the more complicated the calculations. Once you have a cylinder model, then you add a second stage and solve. Then add third stage and solve, beginning with the first two and assumed second stage discharge. Once solved, add the fourth stage. In each case, you begin with solving 2, then 3, then 4 stages. The magic is in an efficient model to vary unknown pressure. The solution is achieved when each cylinder has the same mass capacity, assuimng no interstage condensing. If there is condensing between stages, the model gets more complex.

 

Bobby

Edited by Bobby Strain, 21 May 2016 - 09:46 PM.

[Brijesh Patel]

Dear Art,

 

​You are absolutely right, I accept this fact that i am new in the field of  compressor. 

 

Previously shared sheet of compressor data is software generated sheet. Nowadays i making one excel sheet to calculate performance of multi-stage compressor( 2 stage, 3 stage or 4 stage) and trying to get all parameters same as available in sheet. Shared sheet is for 4 stage compressor. For this compressor i have volume of each stage with clearance, All input data like Inlet temperature, Inlet pressure, atmospheric temperature, required flow, Relative humidity, Outlet pressure are available. Also i have number of stages.

 

For this calculation i require correct pressure ratio at each stage. But i am not getting it same as available in software generated sheet.

 

In other way i can say that i am generating my own Excel sheet which will calculate require horse power for multi stage compressor as per input data. for this calculation i am considering this sheet as reference and i am trying to get all output parameters ( mainly discharge pressure at each stage and Horse power) as same as in sheet.

 

At this stage i am able to find out Horse power based on equation available from books. but for that correct pressure ratio at each stage is require. and in this sheet pressure ratio at each stage is different. 

 

so my question is about how to calculate pressure ratio at each stage. Because it is obvious that available software is calculating this pressure ratio based on some equation. there may be possibility of iteration. If it is iterative process then i can do programming in any other programming language. But for that logic behind calculation is require.

 

Brijesh

[Brijesh Patel]

Bobby,

 

You are absolutely right. I am trying to do the same thing as you explained.  But due to less experience in this field, I am struggling with that.

 

Can you explain this trial and error method - or can you give me any reference material for this type of calculation?

 

This would help me in trying to built Excel sheet to calculate the performance of multi stage compressors.

 

Regards,

Brijesh

[breizh]

hi Brijesh ,

If you own the equations , you can build a model and use Solver from Excel to calculate the intermediate stages .

 

http://www.freestudy...amics/t2201.pdf

 

Good luck

 

Breizh

Edited by breizh, 22 May 2016 - 03:14 AM.

[Bobby Strain]

Brijesh ,

    You might succeed only by using VBA for the calculations. Or C# in a windows form, using Excel for input and output.  And it is a monumental task. You would better spend your time with some more productive effort since you are a student.

 

 

     Visit this site.  https://jensapardi.w.../compressor/.  The site owner will send you an Excel workbook. But I wouldn't recommend that you use it because every calculation is in the spreadsheet, not VBA.

 

     Visit this site. http://checalc.com/calc/compress.html. It doesn't answer you needs. But the calculations are done in javascript. Find them, unravel them, and use them as a guide.

 

     You can download a program here. https://www.arielcorp.com/.

 

     This should help you get started.  http://www.ijceronli...AD022395402.pdf

 

     You should chose a solution algorithm that matches the mass flow of all the cylinders. When you find the pressures where each stage mass flow is the same, then you can calculate power requirements, rod loads, etc. So, build your model for one cylinder, then connect them together in however many stages you have.

 

    I have developed my own calculation for 3-stages. Believe me, it was quite an effort. I started in 1982 with a program already written in Visual Basic. It had bugs, and it took quite an effort to find and fix them. I later ported the code to Excel to eliminate all the input/output screen reading/writing.

 

Good luck, but reconsider taking on this task.

 

Bobby

Edited by Bobby Strain, 22 May 2016 - 03:07 PM.

[Brijesh Patel]

Thank you very much Bobby for your valuable help,

 

Actually I have done with calculation using reference given by you in your first link. But there is no success.

 

Also I have generated one Excel sheet for calculation of pressure ratio using SOLVER as suggested by Breizh. In that case I am arrived near by to my expected answer but not exactly to correct answer.

 

So, Can you send your calculation of 3 stage compressor. So I can use it as my reference.

 

Thanks once again.

 

Brijesh

 

[Bobby Strain]

Sounds like you have made good progress. If you post the second solution maybe you will get some help.

Attached are results from my software. The pressures match closely. I don't know about the capacity. The calculation is based on dry air with no condensation between stages. This method is likely to be close to the one used to prepare your performance data. The software uses the following methodology.

1. Provide an estimate for the first stage mass flow. My software uses 80% of the piston displacement for this starting estimate.

2. Starting from the last stage discharge, vary the stage inlet pressure until the computed clearance matches the machine clearance.

3. Repeat this for each stage, finally calculating a pressure for the first stage inlet.

4. Calculate a new estimate for inlet flow, and repeat. Continue this iterative procedure until the calculated first stage inlet pressure matches the specified pressure.

5. Calculate other parameters and output to spreadsheet.

 

The method is simple, but there is lots of programming to be done. If you include condensation between stages, the calculations become more complex. Humid air is not too bad. But multicomponent systems with interstage condensation are best done with a process simulator. Or use the software from Ariel.

 

Bobby

Attached Files

•  Air_Compressor.pdf   13.21KB   112 downloads

Edited by Bobby Strain, 24 May 2016 - 12:58 PM.

[Bobby Strain]

For some reason the edit above didn't become visible. So see above.

 

Bobby

[Brijesh Patel]

Thank you very much Bobby..

 

In my calculation I am varying volumetric efficiency to get desired inlet flow for minimum horse power. Because I have fixed clearance for particular cylinder. Vol. Efficiency is depend upon clearance and pressure ratio only and I have fixed clearance so I am changing my pressure ratio in such a way that It will give minimum power for required inlet and outlet pressure.

 

but Value of standard flow (scfm)  at each stage is not remaining same at all stages

 

Is this a correct way???

 

Brijesh

Edited by Brijesh Patel, 25 May 2016 - 07:05 AM.

[Bobby Strain]

No. You should follow the steps that I outlined. You find interstage pressures such that the flow is the same in all stages and match the inlet pressure for the first stage.

 

Bobby

[Brijesh Patel]

Yes bobby I am following steps provided by you. but I am confuse in my case with Volumetric efficiency at Outer End and Frame End.

Here I have double acting cylinder so I have vol. Effi at both the end. So for calculation of power which efficiency is require?

 

In present calculation I am using average efficiency. Means average of Outer end and frame End volumetric efficiency.

 

Brijesh

[Bobby Strain]

Use average clearance and calculate volumetric efficiency on this basis.The file I posted does this. If you want better precision, then calculate each end and add the capacities for total flow. You will need to specify the clearance on one end and relate the other end to the specified value.

 

Bobby

Edited by Bobby Strain, 26 May 2016 - 10:06 AM.

[breizh]

Hi,

You may find some interest reading the document attached .

 

Hope this helps

 

Breizh

Edited by breizh, 26 May 2016 - 07:47 PM.

[Brijesh Patel]

Hiii Bobby

 

thanks for your great help..

Iam almost reach up to solution using solver in Excel. But still I am considering compressibility =1. I observed in your result. you are calculating compressibility at each stage. I also want to calculate it by using Nelson obert chart. I also referred excel sheet available in Cheresources but they are not using Nelson obert chart.

 

So which equation and procedure you are using to calculate compressibility?. and how I can calculate compressibility using Excel or any other programming tool?.   Please suggest me

 

Regards

Brijesh

[Bobby Strain]

You should not range too far from your first objective. So, simply input the property values for each stage and match the datasheet performance. Then you can expand your program to calculate properties. There are lots of correlations available. I use a component library and calculate the necessary properties based on gas composition.

You should look at Prode.

 

Good luck,

Bobby

Edited by Bobby Strain, 08 June 2016 - 03:26 PM.

[Brijesh Patel]

Heey Bobby,

 

I am near about to get answers as per data sheet. and write now I am getting answer in 2% range for four stage machine. But with similar procedure I am getting 12% variation from data sheet for same ICFM.

 

Means I am getting same value of pressure ratio at each stage and ICFM. But getting value of Horse Power to far from data sheet( 12% difference).

 I am using following equation for Calculation of horse power

 

HP={(144*Suction Pressure*Suction Capacity*(k/(k-1))*[{Pressure ratio ^ ((k-1)/k)}-1]}/(33000* Volumetric efficiency)

 

Suction Capacity= ICFM at inlet of particular stage