3 replies to this topic

[Genii]

Dear All,

I have used the following method for instrument air reciever sizing;

Basis:
The instrument Air receiver is sized to provide 15 minutes surge capacity at maximum usage rates. Volume is calculated based on normal operating pressure envelope of 8 barg to 6.5 barg.

Calculations:
V = RT (n1-n2) / (P1-P2)
where;
R = 10.7316 ft3. psi/(R.lbmol)
V = Actual vessel volume, ft3
n1 = lb-moles when the pressure is 100 psig
n2 = lb-moles when the pressure is 80 psig
T = Site temperature = 570 Rankine (110F)

Q = Total Air required, scfm
t = surge capacity 15 minutes
Standard Volume = 379 scf/lbmol

n1-n2 = Q . t /379 lb-moles



So vessel volume V, can be calculated from the above.

The methodology seems to be correct, but volume calculated from this method is very high for some reason or error as compared to rule of thumb 1 scfm = 1 gal. Please comment.

Early response will be highly appreciated.

Regards.

[jetul]

Hi,...

Let me give some thought on this,..As we had air buffer vessel for air hoisting purpose,..
The approach was used was same for 3 minute operation...

keep margine for friction losses which will take place while flowing air.

Use diynamic simulaltion for better results.

hope this helps....
thanks...

see attachments

Attached Files

•  air_reciver.zip   84.85KB   1063 downloads
•  air_reciver.zip   45.83KB   625 downloads
•  KAESER___Air_receiver_sizes.zip   25.37KB   667 downloads

[Genii]

Thanks jetul for your reply.

I would like to add that I hv already taken margin for pressure drop. IA (Instrument Air) system will operate from 7bar to 5.5 bar.

Secondly that KAESER calculator haven't taken the effect of temperature into account. whereas above formula incorporates that too.

I have checked this with dynamics and is confirming the formula results. I was wondering about the rule of thumb for IA reciever of 1 scfm = 1 gal. In my case, for IA requirement of 100scfm, vessel volumes comes to be 691ft3 @ 110F (115psig to 80psig) with surge time of 15 minutes.

Regards.

[DRS]

The apparent higher capacity of the receiver is due to the fact that you didn't consider the compressor capacity. During peak loads, compressor runs on full load, as usual, and receiver supplements the extra flowrate. You calculate based upon, say, Q1 instead of Q where Q1 is flowrate required from receiver. So, Q1 = Q-Q2, where Q2 is air compressor flowrate at full load.

PS: Check pressure unit conversion once again.