18 replies to this topic

[tm1274]

I am trying to create a spreadsheet that would allow the user to easily calculate the number of air fills that can occur based upon the number of storage cylinders, their starting pressure and cubic footage of air. The storage cylinder would be a 4500psi, 444 cubic feet and 1.5 acf in each cylinder with a total of 4 cylinders. The cylinders being filled from would be 2216 psig at 45 cf. I would like to calculate the number of 2216 psi - 45 cf cylinders that can be filled and what the final pressure would be in the 4 - 4500 psi 444 cf after reaching the maximum number of fills. I would like to do the same with a different quantity of storage cylinders to give the user the ability to determine how many storage cylinders would give the user the most fills.

 

I am sure it is not as simple as 444 x 4 = 1776/45 = 39.47 but do not know how to calculate this correctly for accuracy. I also realize that there can only be a certain quantity of fills before the supply and the cylinder(s) being filled equalize so I like to calcuate what that pressure is as well.

 

Can anyone point me in the right direction of how to start the calculations required? Any advice or online examples that I can look at would be extremely appreciated.

Edited by tm1274, 22 October 2013 - 09:55 PM.

[thorium90]

You can use an equation of state. Calculate the density. Use a mass balance. An excel spreadsheet with goal seek would be helpful.

[tm1274]

Hi thorium90,

 

Thank you for your response.

 

I must tell you, this is totally foreign to me. So breaking down your suggestion, first I would need to select an equation of state, would this be the Ideal Gas Law PV = nRT, possibly keeping the temperature constant? I know that the Ideal Gas Law would not be 100% accurate but could this take the place of the equation of state and calcuating the density?

[Art Montemayor]

tm1274:

 

What you call the Ideal gas law, with a compressibility factor (Z), is an Equation of State (EOS).

[PingPong]

In general following applies: PV = nZRT

in which Z is compressibility factor.

For ideal gas Z = 1.0 but at 4500 psia Z for air is not equal to 1.0 but about 1.1

See table in: http://www.enggcyclo...y-factor-table/

or graph below.

 

Note that reference below graph is not entirely correct: that should read: Perry table 3-212 on page 3-162.

Instead of using an EOS you can easily make a simple formula for the data in the table to use in your spreadsheet.

 

[tm1274]

Hi Art and PingPong,

 

Thank you for responding to my need for assistance. So I would need to first calculate the Z compressibility factor of the specific gas, in this case air before I can perform the calculation required to create the spreadsheet? If I understand this, then the equation PV = nZRT now is for a real gas and no longer just an ideal gas equation? This would also allow me to calcuate the volume of gas at any pressure and temperature? Sorry for so many questions, I am just learning this and want to make sure I understand so I do not make any mistakes along the way.

[PingPong]

PV = nZRT is for every real gas.

For ideal gas with Z = 1.0 it reduces to ideal gas law PV = nRT

 

You can obtain Z using an EOS like Peng-Robinson, or from an applicable table or graph, whatever you prefer.

After that you can calculate specific volume or density of the gas.

[tm1274]

I am looking into actually calculating Z on another sheet within the spreadsheet so I have all the information and the understanding of how to calculate it rather than get the information from an online table. This will help me get a good overall understanding of the process it takes. I do see that it is pretty in depth though..

[Art Montemayor]

 

tm1274:

 

My initial comment was based on the fact that this is the Industrial Professionals Forum and it is populated by such professionals as thorium90 and PingPong.  I am sure that they, like me, are assuming that you also are a professional engineer.  I bring this up – as well as your knowledge regarding compressible fluids – because your query seems to be a very basic and simple problem and if there is any lack of understanding compressible fluids on your part, this could really evolve into a very long and difficult-to-understand thread.

 

Without a detailed sketch – such as I submit here as an Excel upload – it is difficult to understand exactly what you are describing.  I have sketched my interpretation of your description in order to make sure the thread is conveying the correct description.  Please correct the sketch and the operational description if it does not comply with what you are proposing.

 

You describe the storage cylinder(s) as 4,500 psi, 444 cubic feet and 1.5 acf.  I presume you mean to state that the maximum storage pressure in the cylinder is 4,500 psiG and that it has a total water capacity of 444 ft³, while containing 1.5 ACTUAL ft³ of air.  Please take note that you MUST always state the actual pressure units as either GAUGE or ABSOLUTE.  This is always required when dealing with compressible fluids.  Also note that the actual volume of air is not identified as to what pressure and temperature it is at.  You must identify these “actual” conditions in order to convert this to either mols or mass of air.

 

You are obviously starting out with the storage cylinders filled with air at their maximum pressure and some temperature, correct?  (You must identify that temperature)

You are using the stored air to fill smaller cylinders at a pressure of 2,216 psig and containing 45 ft³, correct?  If so, then you must state what are the conditions of the 45 ft³ – is this volume measure at “standard” or actual conditions, and what are those conditions?

Bear in mind that you are proposing what in industry is called a “cascade” type of gas transfer.  This transfer method is done by using the initial gas fill from one storage cylinder and “topping” the fill off by using higher-pressure storage cylinders when the pressure in the first storage cylinder falls below 2,216 psig.  The procedure is not as simple as you depict it.

 

Additionally, there will be an initial temperature increase inside the smaller cylinder being filled.  This is because of the compression taking place there.  There will also be some cooling taking place at the high pressure valve transferring the air.  This is due to the Joule-Thomson Effect.

 Cascade Gas Transfer.xlsx   11.62KB   44 downloads

 

[tm1274]

Hi Art,

 

I apologize for the lack of information given on my part. I do see the errors in my description as you have pointed out and will try to make sure this does not happen again. The fact that this is an Industrial Professionals forum is exactly why I am here, I am looking to professionals to help me understand how to do the calculations necessary to calculate a final value or values.. However, I am not a professional engineer and as such I should have been more clear in my statement that "this is totally foreign to me" and that I was only looking for assistance in creating a spreadsheet that would calculate the number of fills that can be obtained from a certain volume 444 cubic feet of stored supply air per cylinder, and 45 cubic feet of needed volume for the cylinder being filled based upon the manufacturers specification at 4500 psig, which I should have been more clear about as well as the 1.5 acf volume which is a function of 2750 cubic inches volume multiplied by 0.000578703704. I was certainly not trying to depict a simple procedure only trying to explain ithe procedure not knowing that you would need a more elaborate description so I apologize to everyone in this forum if I somehow mislead you.

 

I came here becuase I thought that the members in this forum could help me understand how to do something that I cannot do on my own due to the lack of knowledge and have already started taking the suggestions already given and started researching how to perform the calculations as I understand them. If I should not be in this forum asking such basic questions, I understand and ask that you please forgive my ignorance that this forum is for engineers only and I will not ask any other questions. If this is not the case and you are willing to assist me in my attempt to gain knowledge of the process required, then your example drawing in the spreadsheet, does in fact match what I am looking to accomplish with the exception of a total of 4 storage cylinders but this does not matter at this point. I did forget to mention the temperature as well and as you apparently are well aware, this could be anything, but 70 would be the standard temperature. The 45 ft³ is the value the manufacturer says the cylinder would hold at 2216 psig and 70 degrees. PingPong's statement of "Instead of using an EOS you can easily make a simple formula for the data in the table to use in your spreadsheet" is what I started out trying to do, but am quickly gathering that it may not be as easy to create a spreadsheet to accomplish this task as I thought.

 

Attached is the updated spreadsheet

Attached Files

•  Cascade Gas Transfer.xlsx   14.79KB   23 downloads

[thorium90]

During the course of my work, I made a simple spreadsheet to calculate how many nitrogen pallets I would need to charge up an accumulator at my workplace. I have made some slight modifications to your case and posted it here for you to use.

It uses the compressibility spreadsheet by Ankur. Pressing the button (macros must be enabled) uses goal seek to perform a mass balance.

Attached Files

•  N2 pallet usage calc.xls   51.5KB   56 downloads

Edited by thorium90, 23 October 2013 - 02:10 PM.

[tm1274]

Hi thorium90,

 

Than you for the example sheet. I am looking it over now..

[tm1274]

Hi thorium90,

 

I have spent a number  hours looking over the spreadsheet you sent me and I have a few questions. On the "Pallet usage" sheet, there are some items that I cannot find on any documentation I suppose due to the fact that Air is technically a mixed gas. Molecular Weight, M (28.013 g/g-mol), Crit. Temperature, Tc: (-146.956), Crit. Pressure, Pc: (33.9437 Bar). You mentioned that you had modified it for me and I do see that the volumes, initial and final pressures are based upon my original question. Were the Molecular Weight, Critical Temperature and Critical Pressure based upon Air as the gas? Are you aware of a chart that shows these properties for gases including air? Also, it appears that the only cells needing modified are on the "Pallet Usage" sheet, do I need to modify anything on the "Pallet Calc" sheet?

Edited by tm1274, 26 October 2013 - 09:23 PM.

[thorium90]

The spreadsheet is designed so that the only things to change are on the "Pallet Usage" sheet. Yes the spreadsheet was originally for nitrogen. Properties for other substances like air can be found here. You change cells B5 to B7 for a different substance. Apologies if it is not too user friendly as it has always been a personal use kind of spreadsheet...

 

http://www.engineeri...sure-d_161.html

 

http://www.engineeri...-air-d_679.html

Edited by thorium90, 26 October 2013 - 10:33 PM.

[tm1274]

Hi thorium90,

 

No need to apologize, your sheet is really simple to navigate and I don't think there is an easier way than what you have done to create a spreadsheet to accomplish this task.

 

I noticed that on the "Pallet Calc" sheet there appears to be a number of cells that appear to be the compressibility factor, B12, B46, F12, F46, and then there's the cells beside Z1, Z2, Z3 ad Z0. Can you please tell me which I would use as the "Z Factor" or compressibility factor?

Edited by tm1274, 26 October 2013 - 11:08 PM.

[thorium90]

In this case, Z0 is the factor.

 

The original spreadsheets can be found here.

http://www.cheresour...n-spreadsheets/

[tm1274]

Would Z be one of the others cells I mentioned at some other point (rather than Z0 for this case)?

Also, on the "Pallet usage" sheet based upon my application that you entered this page shows 303.9 barg as the final pressure of the N2 pallet. Would you suggest changing the Starting N2 pallet pressure from 310 barg to 303.9 barg to get the next final pressure and the putting that number in the starting N2 pallet pressure and so on until it cannot reach an acceptable final pressure to fill the accumulator and then adding the number of times that it reached an acceptable pressure to determine how many accumulators can be filled?

Edited by tm1274, 27 October 2013 - 09:28 AM.

[thorium90]

The reason i gave you the link to the other thread is so you can read about the eos being used and understand it. You didnt read it right...
You can easily modify the spreadsheet by changing the volume to be filled.

[tm1274]

Hi thorium90,

 

I did read all of the comments on the linked page but did not seeing any details about how the eos is being used. But I may just be missing something, I will continue to read about the different eos and their usage online to see how it may best used to accomplish my task.  I think you have already created a sheet that will work but I need to modify it slightly to get the results I would like to show. Thanks for your help.