The EPA has come to our facility and has required us to raise 60 of our exhaust stacks to a given height.

I am wondering with initial data how i can get final CFM.

So for example. We have a stack with a current height of 41.92'. Height required is 66.58' which is an increase of 24.66'. The stack system has been designed such that there is 36' of stack height that starts at a furnace exit (which is decoupled) and goes up to rooftop height and ties inline with an exhaust fan which in turn has additional stack height inline above it. We'll be replacing the 5.92' of stack above the fan with a 30.58' stack which will get us to our goal of 66.58'. I don't want to overdraft nor under draft the furnace with the stack height modification.

If I'm not mistaken, as long as final CFM doesn't change or at least stays within a tolerance of initial CFM the additional stack height will not over-draft nor under-draft the furnace. enter image description here

Again all that is changing is stack height.

Inner duct diameter is the same at 1.33FT. Temperature is 112 F. Initial SCFM is 2684 before modification. I believe the static pressure for the specific duct is .4inwc per 100ft.

Fan details: design CFM - 3570. Design Static Pressure - 2.5inwc. Fan RPM 1518 and designed max RPM is 2026.

I need help finding a formula or something to get final CFM.

  • 3
    $\begingroup$ Alex - please don't repeatedly post the exact same question under different accounts. I noticed that this account is unregistered whereas your other account is registered. If you would prefer this question closed as a duplicate of the other, I can switch that out. I chose to keep this question open as Chris put some effort into editing the question. $\endgroup$
    – user16
    Aug 12, 2015 at 16:36

1 Answer 1


The resistance against the fan is influenced by the total resistance of the pipe down wind of of the fan (in additional to the resistance on the inlet side). The total resistance is calculated from the skin friction, diameter and length with equivalent lengths added for bends/turns, joints and the type of plenum used on the outlet. When you add length to this pipe you will increase the resistance on the fan and depending on the existing fan operating point (on the fan curve) it may reduce the CFM output.


1) Hire a mechanical/ventilation engineer to ensure that those fans do not stall and destroy other equipment in your factory by not properly handling exhaust.

2) Gather all the information you have available about the fans and the pressure on them, then hire a mechanical/ventilation engineer to do the work based on that data.

3) Put the pipe on there and see what happens. You could increase the pipe size or reduce the skin friction so that it would be equivalent to the friction that the fan is experiencing pre-retrofit. You may be able to change the pitch of the fan blades so that its operating curve will be able to meet the new requirements. Removing bends within the system will also do wonders for any system.

Essentially, I believe your best option is to bring in an expert that deals with this day in and day out. Its not one stack at a small operation you're dealing with, you have 60. What is the cost of hiring a Qualified & Licensed Engineer to do this work vs the down time incurred by shutting down one of the furnaces due to a calculation error.


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