Secondary and Primary Centrifugal fan Calculation

[thesnipe]

Hi,

Please see the attached drawing.

My question seems complex in nature. I suspect a fan should have been replaced with a larger version upon modifications to the process system at my place of employment.
( the modifications are drawn in green on my drawing)

The EXHAUST system is as follows:

Primary Exhaust Booster Fan = 4500 cfm (Circled in black)
Secondary Exhaust booster fan(s) = 300 cfm (circled in red)


I'm curious about how to go about calculating the pressure requirement of the new secondary exhaust fan.

The tanks that were added onto the circuit require an exhaust rate of at least 90 cfm.

Because I was not involved in the original design process, I do not have the original calculations handy.

How do I do these calculations? ( i know technically how to do them but I don't know what assumptions to make here) Which ducting lengths and fittings should I use etc etc.?

Any and all help is greatly appreciated!

Kind Regards,
The_Snipe

Snipe

 

[Drazen]

I fear you expect from eng-tips more than intention of the site is.

tips give ideas, help in process, but it seems you need complete engineering guidance in your first steps, which requires guy next to you.

in my years of experience i belive one of the invaluable things is to have such guidance in first one or more years. without that, no matter how bright you can be, you mess opportunity to get a grip of what is called "good engineering practice", which truly cannot be learned the other was.

in general, drawing you provided gives layout of ducting, and knowing exact lenghts you should be able to use smacna tables, while correction for friction might be necessary as you did not mention what are you exhausting.

plenum between primary and secondary needs some kind of check, the best would be to measure actual static pressure at full operation, and to include it in your calcs.

these are to be taken as tips only, as there is really much more to be checked, almost certainly.

 

[thesnipe]

Drazen,

I appreciate your insight and help. I should have been more clear in my explanation of the issue.

I'm dealing with a PVC process exhaust (tank exhaust) system. I know where the fittings (as well as what they are)and I know all of the required lengths of ducting.

I'm unsure, however, of whether or not to consider the system as a whole when specifying a fan for this branch or to just consider the pressure drop/losses in that particular branch line.

Hope this helps.....


Thanks,
Kevin

Snipe

 

[willard3]

Drazen is right, you are going to have know about stuff that is not going to come from an engineering tips forum.

Parallel/series systems are pretty basic stuff if you are doing air systems.

 

[ChasBean1]

Your main 4,500 cfm fan I’d guess modulates speed to maintain pressure at PDT-(illegible). Say this is minus (-)0.5” based on your diagram. At the point where you tie in your new exhaust, go with that value (-0.5”) on the outlet plus duct losses on the inlet.

You should discuss with the owner and take an opportunity to redesign this system. All of the ‘sub’-fans should instead become flow controlled terminal devices and the 4,500 cfm fan could be modified to a more robust fan if required…