Internal suction on building fabric wall due to exhaust fans
Internal suction on building fabric wall due to exhaust fans
(OP)
I have a situation where the one side of a fabric building wall is seeing suction when the exhaust fans are running. The wall seeing the suction is opposite of the exhaust fans. There is a hybrid system being used in that the supply is natural and the exhaust is mechanical. I feel like the reason why the suction is occuring is because the supply louvers are positioned parallel to 45degrees to the prevailing winds. This would cause a reduction in the CFM drawn through the free area based on 88*Cv*A*U. Cv would drop from .5 to .25. However, does this equation state still apply when you have a mechanical exhaust pulling through the air?
Here is what I have
The exhaust fans are 11,242 CFM * 2 = 22,484 CFM.
The intake louvers have a free area (A) = 7.44 sq ft * 2 = 14.88 sq ft. w/ 2500fpm/ fan
Volume of building = 524275 ft^3
Capacity = 300 people
Static pressure = .00
Based on 3 air changes - 26,213 CFM are required
Based on 25CFM per person for a gym like enviroment - 7500 CFM
If i use the 22484 cfm achived through the mechanical exhaust system and divide it by the 2500 fpm achieved through the louver intake system I would need 9sq ft of free area required. Which I have 1.6x's more. This would not cause suction on the intake wall though right?
Or do I look at it as creating a balanced system through CFM. The max achievable CFM through the louver can be 22484 with a windspeed of approximately 65mph and a min of 523 CFM with virtually no wind. As suggested above I don't know if this theroy applies when I have a mechanical exhaust system pulling through air???
I believe the exhaust fans needs to be variable speed in order to limit the CFM. Another engineer suggested a larger free area natural intake. Either or, I think that they both accomplish the same results?
Here is what I have
The exhaust fans are 11,242 CFM * 2 = 22,484 CFM.
The intake louvers have a free area (A) = 7.44 sq ft * 2 = 14.88 sq ft. w/ 2500fpm/ fan
Volume of building = 524275 ft^3
Capacity = 300 people
Static pressure = .00
Based on 3 air changes - 26,213 CFM are required
Based on 25CFM per person for a gym like enviroment - 7500 CFM
If i use the 22484 cfm achived through the mechanical exhaust system and divide it by the 2500 fpm achieved through the louver intake system I would need 9sq ft of free area required. Which I have 1.6x's more. This would not cause suction on the intake wall though right?
Or do I look at it as creating a balanced system through CFM. The max achievable CFM through the louver can be 22484 with a windspeed of approximately 65mph and a min of 523 CFM with virtually no wind. As suggested above I don't know if this theroy applies when I have a mechanical exhaust system pulling through air???
I believe the exhaust fans needs to be variable speed in order to limit the CFM. Another engineer suggested a larger free area natural intake. Either or, I think that they both accomplish the same results?





RE: Internal suction on building fabric wall due to exhaust fans
RE: Internal suction on building fabric wall due to exhaust fans
RE: Internal suction on building fabric wall due to exhaust fans
RE: Internal suction on building fabric wall due to exhaust fans
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RE: Internal suction on building fabric wall due to exhaust fans
The fan has an equation that relates flow and pressure drop.
If you have one in and one out the flow must be the same.
Two equations, two unknowns, solve.
RE: Internal suction on building fabric wall due to exhaust fans
RE: Internal suction on building fabric wall due to exhaust fans
MintJulep, if the natural ventilation has a mechanical exhaust does the airflow from the natural ventilation still have to come from wind? I might not be seeing this correctly but you can't create an airflow without the wind needed for natural intake, correct? Especially since there is no chimney effect. The system is sits at the same height on opposite walls.
The pressure drop is .075lbs per cubic foot.
RE: Internal suction on building fabric wall due to exhaust fans
RE: Internal suction on building fabric wall due to exhaust fans
RE: Internal suction on building fabric wall due to exhaust fans
If you have an exhaust fan you don't have a natural ventilation system.
Your description of the system and your problem is sufficiently poor that we are all making guesses about what the problem might actually be. Answers are based more on those guesses than any information you have provided so far.