Exhaust fan sizing

[mayoj]

I've been given a project to replace 2 existing exhaust fans. There are 4 total in two systems which are similar but operate separately. Each system has a duty fan and a stand-by. A few years ago the other two fans were replaced with smaller capacity fans. The original fans were 48,000 cfm and the new fans are 36,000 cfm. They are captureing exhaust gases from a system of furnaces which burn natural gas. When I questioned how the new fans were sized I was told they took air flow measurements one day with 10 furnaces running and that's the number they used. I am not covinced this is the worse case scenario. I know I have 10 furnaces which could be running at the same time. Each furnace can put out 3,600,000 BTU/hr. The actual BTU's varies by product being run in them. Natural gas is roughly 1000 BTU/cu.ft and it requires 10 parts air to 1 part natural gas for combustion. When I work this out, it gives me approximately 2,400,000 cfm of combustion products I need to exhaust which makes no sense as my fans could not physically do that.

as you can see, I'm not a ventilation guy and I could use some assistance on this. I basically need to know how to calculate the volume flow for products of combustion from my furnaces.

 

[cdxx139]

Have you asked or looked into the boiler catalog or asked the boiler manufacturer rep what the exhaust flow rate of each boiler is?



knowledge is power

 

[mayoj]

They are annealing furnaces from the early 80's and manufacturer gone out of business.

I think I've figured it out. First I was multiplying by 60 not dividing to get btu's per minute... I also found a formula in the Combustion Handbook for flue gas flow in cfs = [BTU/hr input * ( flue gas temp + 460)] / 177500000. Numbers seem to make sense now.

 

[cdxx139]

Thanks for sharing, I never saw that formula. Very helpful.

The 460 is the temp converison, b ut what is the 177,500,000 number a constant of?

knowledge is power

 

[mayoj]

No idea. Can not find anything stating what the constant is of.