Hello all,
I hope it's not too late to jump into this topic.
Below are the sources of Anshul's formula:
http://www.tuthillva...atalog/3291.pdf
http://books.google.... vacuum&f=false
I can't comment on its applicability or accuracy because I have never used it, but I am quite sure to say that it is more accurate than the adiabatic compression one.
Going back to Anshul's calculation, I think the assumed volumetric efficiency of 0.8 is grossly excessive with such high P2/P1.
Volumetric efficiency decays steeply with increasing P2/P1; in Anshul's case it could be far less than 0.5, thus creating huge temp. increase.
As a rule of thumb (told to me by SIHI), the pressure ratio of single roots blower is between 1~10, with 5 being the usual max. design value.
Additionally, the pressure difference (P2-P1) of the blower should not exceed 20~60mbar.
Higher ratio can be selected at very low inlet pressure, where mass flow is very low.
Even higher ratio can be selected if roots blower is designed with internal cooling, which is much more expensive.
Above "rules" are meant to limit gas discharge temp. of each stage.
In my plant, two roots blowers are used to create 1mbarA final pressure against 35mbarA of backing pump inlet pressure.
Roughly, it is 10:1 in the first stage, and 3:1 in the second stage.
So, the P2/P1=100 mentioned by Anshul is simply impossible to attain with only one roots blower.
I anticipate that vendor will propose at least 3 stages of blowers.
Thanks.
xavio