When does Boiler room ventilation to cool room become too excessive based on air changes per hour?

[BronYrAur]

At what point does ventilation of a boiler room become excessive based on air changes per hour? I have several rooms that I am considering, so I will just give one of the more extreme examples here.

I have a requirement to maintain a boiler room at no more than 10 degrees above outdoor ambient conditions. I have been using the sensible heat formula of Q=(CFM)*1.08*Delta-T to calculate the required CFM at a 10 deg Delta-T. In this one room, I have estimated the boiler heat loss to be 42,000 BTUH, which give me a calculate CFM of approx 3,900 CFM.


My room is very small (only 3,500 cubic feet). When I compare the calculated ventilation rate to the room volume, I get 1.11 air changes every minute, or 67 air changes per hour. That's insane!!! With an air flow rate like that, won't the heat be immediately swept off the boiler and not actually heat up the room very much?

I think the basic sensible heat equation "breaks down" (for lack of a better term) in such a small space. Although it may be true that the air sweeping across the boiler is receiving a 10 degree temp rise, it would seem to me that the room itself will be much closer to ambient since so much air is being ventilated.

Is there a better way to perform this calculation?

 

[berkshire]

Is there any Way you can install floor and roof vents on this room and allow natural convection to do your ventilation for you?
Trying to stay within 10 degrees of ambient is a tall order. You are going to end up with a room like a wind tunnel.
B.E.

You are judged not by what you know, but by what you can do.

 

[BronYrAur]

My question pertains to many rooms, but in this one example, NO. I must use fans and side walls to bring air in and out. My exact concern is that the room will be a wind tunnel.

 

[IRstuff]

You could possibly apply a wind chill factor to account for the cooling effect of the air flow. Nonetheless, your heat load is 12kW, so it's nontrivial and would definitely get hot without some air exchanges.

Alternately, you could enclose the boiler in its own air space. We used to have diffusion furnaces dumping about 60 kW into a 6000 cfm room, but the heat from the furnaces were always dumped directly into their own air streams, so little of the heat was actually getting into the inhabited space.

TTFN
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[walkes]

Is this a process boiler application or heating boiler? If the latter does the load drop when the out door air temperature is warmer? Can you use a reduced load for the jacket losses from the boiler?

Can you insulate the boiler jacket more to reduce losses?

 

[BronYrAur]

The boilers are by and large heating, but they are steam. I figured the jacket losses will be similar year round since the steam pressure will not change even though the load will.

 

[chicopee]

The one aspect that you have not thought or advanced in this discussion is the fact that the burner is to draw out a certain amount of CFM from the boiler room, therefore, you should recalculate with the knowledge that part of the make up air into the boiler room will be evacuated by the burner.