Maintain a minimum outward velocity in a room with 2 doors open? 1

[Speculiar]

I am trying to calculate how to maintain a 0.305m/sec(60 FPM) outward velocity in a room with 2 doors. The 2 doors in the room must be considered open simultaneously. So I guess my question is: Given the nature of this problem, what rule of thumb, model, or equation could I use to figure this out?

Room dimensions 29'x11'x8.5'
Door Dimensions 33.5"x77"


Thanks in advance!

 

[miningman]

( Area of Door 1 + Area of Door 2) x 60 = cfm of air that must be pumped into the room continously

 

[Drazen]

first question would be: what do you want to achieve?


within domian of ventilation and air distribution, specifying velocity is not so simple. what is certain is that there is no way you can have uniform flow through open door.

 

[Speculiar]

Miningman - Thank you for your input! You didn't reference anything about the room dimensions. Will the size of the room not effect the amount of cfm that must be pumped into the room?



Drazen - Thank you for your input! I should have said that a minimum of 0.305m/sec(60 FPM) outward velocity must be maintained. It does not need to be kept at that exact measurement.

 

[miningman]

Think about it in reverse. If a known volume of air is pumped into a room, or vessel, or any nominal closed system, and there is only one (or two) routes for that air to escape, how does the volume of the room affect the outflow.??

Drazen is correct in terms of uniformity of flow etc , but that is not what you asked

 

[Speculiar]

Miningman - Thank you again for your input! That is the same discussion my boss and I were having. We had doubts about the volume of the room not being an issue because of the specifications on different blower systems specifying a MAX sq. ft.

 

[miningman]

Ah well , now we are into considerations of turbulent flow vs laminar flow, possible statafication of air within the room causing possible temperature differentials at different elevations within the room, and if there is any kind of HVAC system servicing the room, there will definitely be some conflicts between the HVAC system and the fan / blower you use to provide the requitred flow thru your doors, but again, thats not what you asked for advice on.

 

[DRWeig]

All of the comments so far are very important factors.

One more that can be a problem: If your two doors are not on the same wall, there could possibly be a pressure difference between the two corridors that those doors open into. One door could be getting inflow while the other has more outflow than expected. This will be especially pronounced of one of the doors opens into the outdoors (bad) or both of them do (worse), even if they are on the same wall. Mother Nature can overcome whatever fan you might have blowing in there...

Best to you,

Goober Dave

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[DRWeig]

I hit submit instead of preview, sorry...

Interlock the doors so that only one can be opened at a time. Weatherstrip the heck out of any outside doors.

Best to you,

Goober Dave

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[Drazen]

in general, "shape" of flow through rectangular opening resembles shape of flame of hand matches. You have core zone where velocity remains the same at some distance.

hypothetically, core zone should cover whole door opening at zero distance from door opening, but that is not of use in practice. what can you do with air in one fixed, two-dimensional plane?

that is why i tried to understand what comes behind your request, to rephrase my question: for what purpose you need to maintain minimum air velocity at door?

 

[ChasBean1]

Just to add complexity to your seemingly easy question:

I think you need to review why you need 60 fpm across two open doors. The following are the points of concern:

- This means you would need 2,150 cfm more supply than exhaust.
- Assuming there is no exhaust, the room air exchange rate is 48 air changes per hour. A typical lab is 6-12 ACH (with exceptions for clean rooms and labs with hoods in smaller rooms, etc.).
- Assuming there is no exhaust, you have 6.7 cfm per square foot of supply air to the room. The typical value for space air conditioning is 1 cfm per square foot. Laboratories might see on the order of 2-3 cfm per square foot.
- If you have a hood in the room, it could have difficulty passing certification due to the amount of air being supplied, resulting in high drafts at the hood face.
- One of the two doors will never close, even with the closer set to its strongest closing force – unless there is a large alternate leakage pathway.

When you have a chance, please clarify why there is a design criterion of 60 fpm across two open doors.

 

[willard3]

You and your boss should hire an Engineer familiar with ventilation if this really makes any difference in the real world.

 

[Speculiar]

First off, Thank you all for your info. The reasoning behind the post is that one of our sales people is attempting to negotiate a deal with BP. (British Petroleum). We are a building manufacturer. We specialize in buildings that are designed to be in hazardous locations. For example, an area with flammable gas in the air. Well one of the BP requirements for any building in these types of areas is that they must have at least 60 fpm outward velocity from all openings with every possible opening open. So all doors, and anything else, open simultaneously.

He asked a bunch of us to try to figure out how much he would have to pump into the room to meet that requirement. Well we were all trying to figure it out, but nobody could agree on the procedure and what not. I was doing some research online and I happened across this forum.


Thanks Again!
Sal.