Internal suction on building fabric wall due to exhaust fans 3

[stillfan]

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?

 

[jmk407]

The intake velocities appear to be high. I typically keep them at 500 FPM to limit moisture entrainment. What is the pressure drop through the louvers when the fan is running full speed?

 

[miningman]

Despite what you state the static pressure within the building is not zero. I suspect it is close to negative 0.4 inches WG, which will be close to the pressure drop you measure across the louvers.

 

[stillfan]

To be quite honest with you. I don't know what the pressure drop is. How would I determine this?

 

[willard3]

You would first hire an Engineer familiar with ventilation and you would then read forum policies which start as follows:

>>>These fora should not be used to bypass your own in-depth research on the issues that affect you, nor is it intended to be >>>a substitute for appropriate professional assistance within your field or geographical region.

 

[MintJulep]

The louvers have an equation that relates flow and pressure drop.

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.

 

[KiwiMace]

Most of the vendors produce a graph of the airspeed versus pressure drop for the different louvre types. Make sure you are determining the free area of the louvre correctly. The W x H is the area, the free area will be around 60% of this depending on the mullions and blades, which the vendor catalog will tell you.

 

[stillfan]

Willard3, I love how you assume that I have not contacted another engineer. Is that not another area of ethics, that you should not assume! You must know everything in detail before you accuse someone of being unethical. I have in fact contacted another engineer who actually designed the system. In the meantime, I am doing some research on the possible causes and have come up with the following questions based on my research. I don't believe there is a law against that.

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.

 

[miningman]

Stillfan , you really need to listen to Willard's advice. Not once did he mention ethics.. you brought up that subject. Then take a look at Willards profile... 567 posts in the HVAC section alone. I suspect he is quite knowledgeable on HVAC matters. You are 100% wrong when you suggest that the pressure drop is 0.075 lbs per cubic foot. That value is the density of air at sea level at 70 degrees F. The fact you dont know how to measure the existing pressure differential, as opposed to doing calculations which at best will provide an theoretical estimate of the pressure differential doesnt inspire confidence in your abilities either. Plus in my opinion, you have some rather strange beliefs on the matter of natural ventilation

 

[jmk407]

This isn't really a natural ventilation issue. When the fan is on, most of the air will come from the path that creates the smallest pressure drop. The pressure drop through the louver will be provided as a function of CFM and in units of inches wg.

 

[MintJulep]

if the natural ventilation has a mechanical exhaust does the airflow from the natural ventilation still have to come from wind

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.