9 replies to this topic

[rahulavyas]

What is the way to calculate "Cv" specific heat at constant volume, in determining "K" values.

I have bben using table 13-6 and 13-7 to find Cp, but no specific procedure is given to determine Cv.

 

Can some body provide guidance on this?

 

 

Note: I am fully aware of fact that K values can be determined by Aspen or other simulation softwares, but I would like to have it off hand in a spread sheet.

 

an excel sheet is attached for referance

Attached Files

•  Compressor Discharge Temp.xlsx   149.34KB   55 downloads

[breizh]

http://en.wikipedia....ulius_von_Mayer

 

This should help , Mayer's relation

 

Breizh

[Bobby Strain]

Looks like you have a spreadsheet that includes information from GPA. Note that they recommend that you not use their calculation method because it is not accurate. So, for a reasonable answer, your spreadsheet is going to include calculation of k with some kind of model. There are lots of correlations available. But none will be as accurate as EOS used by the commercial simulators. So, my advice: Don't waste your time reinventing the wheel (or k calculation).

 

Bobby

[rahulavyas]

Thnks for the inputs Bobby and breizh

[PaoloPemi]

when modeling a compressor you should consider that cp and cv are not constant over a range of values

(pin->pout) , better to consider enthalpy and entropy,

for a pure fluid you can solve directlly with the help of a chart/diagram with the properties of that fluid (useful from a didactical point of view),

for mixtures values can be calculated from ideal conditions + departure (with a EOS)

accuracy depending from the capability of EOS to model fluid properties,

nowadays these procedures are accurate within some % for most fluids.

For these calc's I prefer Excel, for thermodynamics you can link to a simulator

or use a process library, I have Prode Properties.

[Art Montemayor]

 

I agree with what Paolo has commented because many years of calculating compressor horsepower and discharge temperatures for a lot pure fluids (CO₂, nitrous oxide, N₂, O₂, H₂, CH₄, NH₃, Propane, Butane, Freons, etc, etc.) proved to me that the accuracy of using enthalpy charts and databases is as accurate (or more accurate, in some cases) than relying on the analytical calculation of k values and subsequent equations.  I have calculated the required horsepower for many reciprocating and centrifugal machines in my time and have never made a “boo-boo” in my predicted required HP or discharge temperatures.  And I have relied heavily on charts and databases when there were no digital computers or simulation programs.  I’ve also purchased and successfully installed and operated a lot of reciprocating compressors based on manual calculations using these methods.

 

And I am not the only one expounding the accuracy of thermodynamic databases and charts to do these types of calculations.  If you carefully read NGPA Engineering Databook editions of some years past, you will find repeated explicit instructions to do these type of calculations when dealing with compressors – for more accurate results than the analytical methods.  Although I am not privy to the algorithms used by Hysys or Aspen in doing these calculations, I am certain (as Bobby has commented) that they follow the same principles of using accurate thermodynamic databases or relationships.

 

For C_P and C_V values – as well as detailed enthalpy values, go to the NIST site (http://webbook.nist....hemistry/fluid/) and you can find all this valuable information for over 50 fluids.

 

[rahulavyas]

Certainly will do go with the base principle. thanks for the inputs

[eliealtawil]

I use Hysys to calculate fluid properties. For natural  gas, change of 10% in k factor is not going to affect your calculation that much, may be +/- 1 hp?? I deal only with US Iperial units, I hope the attached calcs for 2 methods to calculate k values helps

Attached Files

•  K-values.xls   24KB   30 downloads

[Art Montemayor]

A possible additional item of interest to this topic to our members - as well as to the Original Poster, Rahulavyas - is that the original submitted calculation by the OP has to do with comparing the GPSA method of estimating the calculated discharge temperature of a reciprocating compressor with that calculated for a CENTRIFUGAL compressor.  This is a total confused query that shows that the OP is not being careful of what he reads (or has the erroneous understanding that the discharge of both machines should be equal.

 

This has little to do with the accuracy of k values and more to do with understanding the difference on how each of the two types of compressors are calculated with respect to their discharge temperatures and their horsepower requirements.  One is an isentropic machine and the other is a polytropic machine.

[breizh]

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

 

Consider this great resource , you should find your answer .

 

Breizh