Negative pressure formula

[HAVENOIDEA]

Is there any formula with which I can determine the cfm needed to eliminate the negative pressure inside a room, if I know the volume and negative pressure? We have fans blowing in and out, we need more blowing in, but without knowing the capacity of each fan, it is quite difficult for me to come up with anything. Appreciate it in advance.

 

[Latexman]

Instead of adding more blowing in, can you put a damper on one blowing out and adjust the pressure of the room down. It may be less expensive. Just a thought.

Good luck,
Latexman

 

[HAVENOIDEA]

that is a good thought, thank you. first i just need to find the differences between cfm in and cfm out. But I know that I need to measure the cfm, there is no way around it. Thanks again for that idea, I will definitely use it once i find out the cfm difference.

 

[handleman]

Why would you need to measure CFM? If the goal is to equalize inside/outside pressure, all you need to do is adjust the dampers suggested by Latexman until the measured pressure is the same.

 

[HAVENOIDEA]

we want to make sure everything makes sense numerically as well, and that sufficient air is being circulated. thanks though.

 

[bvanhiel]

Pressure and room volume are insufficient to fully define this problem.

Think of it like this: you have a pond with a stream going in and a stream going out. If your goal is to keep the pond at the same level, it doesn't matter how wide or deep the pond is. The only thing that matters is the flow into and out of the pond.

You need to know how big the various fans are to solve this problem.

-b

 

[HAVENOIDEA]

I am currently measuring those capacities now. Thanks for the input.

 

[swearingen]

bvanhiel, I'm sorry but I have to disagree. You can easily make sure that the same amount is going in that is going out by keeping tabs on the level of the pond. You could do that by checking the pressure head under the surface which is similar to monitoring the pressure in the room and comparing it to the outside. The dampers latexman speaks of are analogous to a downstream weir that moves up and down to maintain the level in the pond. If your pressure meter goes up, you raise your weir and vice versa. The pond maintains its level and the flow in and flow out are kept the same as a consequence.

The fan cfms are not needed unless you want to assure a certain number of air changes per hour.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[handleman]

I believe bvanheil was referring to the original question - can cfm be determined from pressure and room volume. That answer is certainly "No." I don't think anyone would argue with the fact that you can control the room pressure using a damper, but in any case the size of the room is entirely irrelevant to either question. That was the point of the pond analogy.

 

[zekeman]

Since you have negative static, you have excess exhaust fan.Remove or damper one or more exhaust fans as suggested to balance the room static to zero.
Now simply measure each of the supply fan cfms at this condition.
The total cfm is obviosuly the sum of all the measured supply cfms.

 

[XELR8]

Hello Folks,

If all your worried about is the air pressure I suggest installing a recirculation duct from your high pressure output back into the room and control the air flow with a make up damper. Or you could recirculate the air into the inlet stream to the room and increase that pressure also.

Good Luck!