wind vs building air exchange rate
wind vs building air exchange rate
(OP)
I'm looking for a ball park figure on the relationship between wind speed and a building's natural air exchange rate(i.e, no mechanical ventilation). A couple sources seem to indicate it is linear, although less than linear as wind speed increases, while others indicate it varies with wind pressure (1/2 of vel. squared).
One of my software packages calculates it (toxic vapor cloud dispersion), using the linear method, but provides no documentation as to where the relationship comes from.
Thanks,
One of my software packages calculates it (toxic vapor cloud dispersion), using the linear method, but provides no documentation as to where the relationship comes from.
Thanks,
Stephen Haines, PE, CFEI
Haines Fire & Risk Consulting
www.hainesfire-risk.com





RE: wind vs building air exchange rate
I would say the velocity-squared relation (wind pressure) seems more logical.
But the actual amount of air exchange for a building due to natural effects (wind, draft)... there are so many factors that are of influence I don't think there is a good way of calculating this.
RE: wind vs building air exchange rate
Delta T
Building height
Age of construction, window types, etc
Type of construction
This list is not exhaustive.
RE: wind vs building air exchange rate
Wind pressure on the building is P=½ x ρ x Vw^2 where
P = wind pressure
ρ = air density = 1.2kg/m3
Vw = wind velocity
Pressure is +ve upwind, neutral inside, and -ve downwind
Pressure loss through the building = A x K x V^2 where
A = Total area of openings
K = pressure loss coefficient
V = velocity through openings
Total infiltration = VA
Also need to do the above for both upwind and downwind facades, and what about facades not perpendicular to wind?
The difficulty is determining A and K for which you really need to pressure test the building to establish a pressure vs leakage rate factor or know the maximum leakage rate the building will be designed for (eg 10m3/hour/m2 @ 50Pa)
I just assume the following:
0.25 ACH if no windows or doors
0.5 ACH if windows and no doors
1.0 ACH if there are doors
or run a CFD model
In reality you are totally dependant on how well the building is constructed which is a total guess (eg did the guy have a big night on the booze the night before?)and rules of thumb work as well as any other
RE: wind vs building air exchange rate
Stephen Haines, PE, CFEI
Haines Fire & Risk Consulting
www.hainesfire-risk.com
RE: wind vs building air exchange rate
TTFN
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RE: wind vs building air exchange rate
Stephen Haines, PE, CFEI
Haines Fire & Risk Consulting
www.hainesfire-risk.com
RE: wind vs building air exchange rate
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: wind vs building air exchange rate
RE: wind vs building air exchange rate
Just curious but why are you trying to find out ACHs without mechanical ventilation?
RE: wind vs building air exchange rate
Note the Infiltration model section lower down. The wind factor is clearly a non-linear function.
Being that you've run a blower door, you should have good data to run through this model.