space pressure and infiltration

[norske35]

if a conditioned space has positive pressure how much of door infiltration is applicable (1 story building with automatic door and no vestibule)
Initially looking at supply/exhaust
Process exhaust and Mua
Bathroom exhaust
Ventilation
space was slightly negitive . so i can introduce more mua through Rtu to make positive ( say 1000 cfm)

when i look at infiltration from an automatic door using ashrae eq Q= Ca*A*Rp the infiltration is about 2000 cfm
can i neglect the 2000 cfm since space is positive ?

 

[DRWeig]

Not if you have a receptionist 10 feet straight in front of the door. In the winter, you'll hear nothing but misery about cold feet. In your case, that may be the nearest sales clerk or the folks behind the checkout stand.

An air curtain can mitigate a lot of what will sneak in. A vestibule or revolving door even better.

In my area, the folks in the small markets just have to dress warmly and wear double socks in the winter.

Here's another good paper for you to supplement the ones I posted in your other thread:

http://www.sciencedirect.com/science/article/pii/S1359431106002274

If you're determined not to do any reading, just consider this: If you can maintain even a (very high) 0.10" WC positive pressure in your building, how much pressure will be exerted on the area of your door by a 20 mph breeze? I'll leave that calculation to you.

Best to you,

Goober Dave

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[EnergyProfessional]

you should calculate (ASHRAE fundamentals chapter 16 equation 24,25 ) the pressure due to wind and dT and then add them up. you likely come up with a few in. wg (not sure though). Unless you are willing to create 10" of positive pressure, you won't keep the cold air out.

don't quote me on the numbers, I'm too lazy to actually calculate it now.

 

[norske35]

ok
i see ashrae formula Ps=CpPv and Pv=rhoUh^2/2gc then Uh=Umet(delta met/Hmet)^alpha met(H/delta)^(a)
i'm having finding cofficients to plug in

i found another formula in(MODERN HVAC) Pw=0.000482Vw^2
Pw = VELOCITY PRESSURE HD in inwg
Vw= wind speed mph
for 20 mph = 0.1928 inwg
so, if space is at .1 in wg there will be infiltration with door openings

i looked at other posts on pressurization and found alot of opinions . someone said they they add extra supply at .5 cfm/sf to pressurize to 0.04in wg
i am working on hvac plan for 5000 sf (1) story market with two automatic doors
i calculated my heat and cooling loads and thought the infiltration rate from automatic door was too high . the heat load is getting to 60 btu/sf with refrigeration effect.
some other books use air change factors based on building tightness . that may be the result of study. i don't know how reliable it is

 

[tys90]

In my area, a 5000 SF area would require a vestibule. Even if it's not required, they are very helpful in reducing infiltration if used correctly.

 

[DRWeig]

Yes, tys90. Revolving doors are even better and much more fun!

Best to you,

Goober Dave

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[cry22]

see ASHRAE 90.1 5.4.3.4 for Vestibule requirements, depends on your climate zone, building size, etc.