Twin Lobe Gas Blowers Outlet temperature

[Sam1390]

Hi ,
I am working on a project which has a Twin Lobe GAs Blowers. I've been asked to calculate the discharge temperature at lower flow rate. Vendor has already specified the temperature rise at max flow. I assume the capacity won't remarkably affect the temperature as it's variable speed.
and the temperature rise is mainly proportional to compressor efficiency and compression ratio.

Looking forward to hearing from you.

Cheers,

 

[pierreick]

Hi,
Consider this link:

https://www.eng-tips.com/viewthread.cfm?qid=243308

Why don't you ask vendor?
Pierre

 

[Sam1390]

Thanks Pierre,
I’d seen that and already know how to calculate the temp. But I want to know if the gas flow rate change how I can justify that it has no impact on discharge temperature.
The formula is only proportional to pressure ratio and irrelevant to volume.

 

[pierreick]

Hi,
less RPM, less flow rate, less Pressure discharge thus lower temperature. this technology is a positive displacement blower.
Apply the isentropic calculation to the new pressure discharge.

https://tmcfluidsystems.com/blog/lobe-blowers-working-principle.html

Pierre

 

[LittleInch]

Sam1390,

The key issue is what does a reduced flow do to the pressure on the outlet?

If this stays the same then lower speed / lower flow would seem to give you a similar temperature differential, but not clear if there is a difference in efficiency at this lower flow / speed or greater slip at the lower speeds.

The key is that if anything else changes other than speed / flowrate, then this can impact the temperature rise.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

The lower the flow rate, the less adiabatic the process will be, in reality. But, theoretically, neglecting heat loss to the environment, there should be no difference. Roots blowers pump a certain volume per cavity and the compression actually occurs when this volume gets exposed to the higher downstream pressure.

 

[Sam1390]

Thanks everybody for detailed responses.
I’ve nearly got my answer.
Just wondering if there is any formula that shows the speed change that leads to flow rate change will not affect the temperature rise.

 

[LittleInch]

That's the performance curve of the blower which is measured, not calculated.

Don't think what you're looking for exists.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi,
another paper:

https://www.ppipumps.com/performance-and-fault-diagnosis-of-the-twin-lobe-roots-blower.html

note:
Get a copy of the reference underneath:
Compressor selection and sizing (3rd edition) by Royce N.Brown
extract from internet:

https://books.google.co.th/books?id...N.+Brown&cd=1&redir_esc=y#v=onepage&q&f=false

Pierre

 

[georgeverghese]

If the lower flow requires turndown in speed to below the lower limit of the VSD, then the pressure recycle loop will be active and discharge temp will most likely rise. Am assuming recycle gas downstream of the recycle valve will be warmer than fresh feed at compressor suction.

 

[Sam1390]

Thanks everyone for the detailed responses. I got what I was looking for.