air leakage velocity calculation

[saliba11]

I have simple question which I couldn't figure out.

Let say, there is 50 Pa pressure difference continuously between a closed room and outside which is in atmospheric condition. The room pressure is bigger than the outside. When opening the door (Area of door is 3 m2) of room to outside, what will be the velocity of the escaping air from room to the outside? Room air is also can be taken as atmospheric condition, except pressure which is 50 Pa bigger than the outside (1 atm) pressure.

could you kindly help me about this problem.

Thanks in advance

 

[lownox]

So you have a continuous source of air that is pressurizing the room to 50 Pa (.2"WC) above atmospheric pressure. Velocity pressure for standard air at .2"WC is about 570 fpm. So... somewhere in that range.

Also, assuming your door is probably big enough that whatever is feeding the room will not be able to sustain the 50 Pa static pressure difference with the door open... the velocity will taper off quickly.

Something like that IMO.

 

[saliba11]

Thank you lownox for the reply. the door is closing immediately. What happens in that case. what is the procedure for calculating this velocity.

kind regards

 

[lownox]

Not entirely sure I understand what you are doing, but here is the basics.

Room is pressurized to 50Pa, door opens... pressure drops while air is rushing out. For standard air density...

V = 4005*Sqrt(vp)
V=velocity in fpm
vp=velcity pressure in "WC

Seems that the maximum velocity you would see would be 1791 fpm or so. The 570 from my first post was incorrect.

 

[IRstuff]

The equation is similar to that for aerodynamic drag:

http://en.wikipedia.org/wiki/Drag_coefficient

Set area = 1 and solve for v

TTFN

FAQ731-376

 

[burnt2x]

Hi,
Flow through intentional openings are proportional to the differential pressure happening across the opening and the flow restrictions on the said opening! So I guess the formula lownox presented did not consider flow restrictions! Here it is:

Q = K x 4005 x area of opening x square-root of DP

Q/A = velocity, in FPM

Q then becomes:

Q = K x 4005 x square-root of DP

K = is the flow coefficient through the opening, and if you assume there is no restriction, K=1. There being no perfect world, it is best to use flow coefficients,0.73 perhaps.
DP = differential pressure in Water Column = 50 pa,which is equal to 0.20 inches WC

So, you have V = 1307 CFM with 0.73 flow coefficient.

 

[teoman877]

Regarding that question, let me explain the real case;

With my colleague saliba11, we are trying to design an airlock with dimensions 2,5*3*3(h) m for Refuge buildings. This room under +50 Pa according to the outside (1 atm) in normal case, which is pressurized by HVAC system.In case of any kind of emergency (toxic gas release at outside), the workers will enter to the Refuge by passing the airlock. The door of airlock (dimension is 0,9*2,15) will be stay open approx. 3 sec. for one person inlet.At that period the room pressure will decrease because of escaping of air to outside.Without closing outside door,inner door will not be opened.In order to keep the released toxic gas outside of the building,we need to continue to pressure the airlock by other system which is compressed air bottles. This closed comprresed air bottles will work also in case of HVAC shutdown at an emergency situation. Max. 200 person will use one refuge. Emergency case scenario time is 3 hours.

For this case,we need to size the bottles by describing pressure and volume.

The company is to state the air escaping velocity is 0,3 m/s for +50 Pa difference according to the outside (1 atm)when door is opened. But, according to the above written solutions, that defined escaping velocity (0,3 m/s) is too low.

UCould you please help me for that under real case scenario?

Thanks a lot.

 

[IRstuff]

Perhaps you're not reading your specification correctly. What is the actual text of the specification regarding air speed?

TTFN

FAQ731-376

 

[teoman877]

Hi,

I have checked again the actual text, but, it is written that the escaping velocity is 0,3 m/s as mentioned above.What is your suggestion? They may give wrong information.

Thanks a lot..

 

[IRstuff]

Can you post the exact text?

TTFN

FAQ731-376

 

[MintJulep]

With a big door, the pressure will drop to atmospheric almost immediately. After that, air will flow out the door at the same rate that your HVAC system puts air into the air lock.

So, for the transient: P1V1 = P2V2

For the duration: CFMin * 3/60 or in metric l/s in * 3

 

[teoman877]

IRStuff, Please find the excat text;

For the sizing of the compressed air bottle banks the airflow should be determined as follows: For the air flow from the outside door when open, a face velocity of 0.3 m/s for the duration of 5 seconds shall be taken. Also to allow one person per entry, dependant upon the number of personnel allocated to use each Temporary Refuge.

Thanks a lot..

 

[MintJulep]

Given the requirement as-written, I would simply do:

0.3 m/s x 3m^2 x 5 s = 4.5 m^3

However, please consider faq403-1308

 

[IRstuff]

That's why we asked for the actual text. It does not say that the velocity IS 0.3m/s. It essentially says to ASSUME that velocity for the purposes of doing the gas capacity calculation, as done by MJ, above.

Additionally, the paragraph indicates that you also need to multiply by 200, the number of people allocated to your building.

TTFN

FAQ731-376