Emergency Generator Ventilation 1

[kahnman]

I'm new to this forum, and I can't find the code reference I'm looking for. A little background: We have a generator room that is using unconditioned outside air for cooling and combustion air, powered in using fans. There are three (3) generators in the room. I have a good idea of the airflow required, and selected my fans appropriately. However, I know there needs to be a discharge damper on the outlet of the fans. Through using projects with similar scope around the office, I saw that the requirement for the motorized discharge damper is "spring open/power close" as opposed to most dampers on the market. I received a submittal that shows "spring close/power open" which is opposite of what I specified. I seem to remember seeing the "spring open/power close" requirement in NFPA something or another. I haven't been able to find it in NFPA 110, 90, 92, or 70. Any ideas where I could find this control damper requirement, or am I just making this thing up?

 

[tys90]

Even if it isn't in writing somewhere, it sounds like a good idea. You wouldn't want your discharge damper closed while the generator is running.

 

[HVACMech]

Either way they will need to be interlocked with the generators, so I am not sure if spring return open or close would matter in that situation.

 

[urgross]

If the MOD is on normal power that makes sense. An end switch on the damper would provide an easy interlock. If the generator does not turn on, you have a larger problem than freeze-up potential.

 

[LittleInch]

Might come from a requirement to close the dampers if gas is detected on a petrochem plant?

In your case as this is inlet air for an emergency gen set, when, presumably, power has been lost before you fire up the emergency gen sets, the dampers need to be fail open, not fail closed. I doubt this is regulated as each site and location could be different.

Can't see a big deal about reversing the action.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kahnman]

Thanks everybody. I think I'll approve it and hope the AHJ signs off on it. Much appreciated. If the AHJ doesn't, is it an easy fix? Seems like an actuator switch-out if I understand it correctly

 

[HVACMech]

Yes it would be just an operator switch out. The AHJ is typically more concerned about the interlock. They want to make sure the generator doesn't try to run when there is not fresh air available.

 

[berkshire]

kahnman (Mechanical)
Most generator rooms I have worked on have had louvered dampers that have been airflow open, gravity close.
B.E.

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

 

[lukaiENG]

I have always specified generator room dampers to be spring open, power closed. This is so they will fail to the operational position.
I can't imagine there are many clients who would be happy if their generators failed due to a fault on the low voltage control circuit which energizes the dampers.

On another note, it is a little unusual to have air inlet fans. I guess if you're in a basement or away from accessible exterior walls this might make some sense. The normal design strategy is to have as few active/powered components in the system in order to minimize the risk of failure.

Finally, you want to pay attention to the operation sequence at start up. If your generator and radiator fan start up too quickly you risk damaging the dampers.

 

[kahnman]

Thank you for the advice.

 

[ME27272727]

I've seen a situation flawed control sequence put a pause between generator startup and opening the dampers. I couldn't believe it, but the static was strong enough that it caved in a portion of the wall!

 

[lukaiENG]

a lot of generator installations have a re-circulation damper on the discharge side of the radiator - this can serve 2 purposes:
1. prevent sucking in the damper/walls
2. provide some heat to the space when it is very low ambient conditions.

These dampers are normally set to normally open. when the generator has started and the inlet air dampers are open, these dampers will modulate down to maintain a space minimum temperature (closing if it is exceeded)

 

[QualityTime]

Let’s make this simple to understand why you need a spring open, power closed damper:

[ol 1]
[li]When there is loss of power the generator will start up but it will take time for it to come up to speed AND there is a timer before generator power is transferred to the MCC via the robonic transfer switch. Therefore there will be no power to operate the combustion air fans or the exhaust dampers.[/li]
[li]Meanwhile, combustion air is needed for the generator and during this time the room also kind of gets smoky and hot.[/li]
[li]That is why when there is a power loss the outside air dampers need to spring open to allow combustion air to come in immediately by gravity without the use of power to open a motorized damper[/li]
[/ol]

I don’t think there is a code for this

 

[urgross]

It's common sense. If you are in a climate where freeze-up is a hazard, you fail open. If you have no power to actuate a damper that needs to be open, fail open.

 

[FreddyNurk]

Quite hard to do in this case, but most of the generator installations I'd worked on had the power for radiator and ventilation fans coming off the generator rather than through a central MCC. In the same manner, it'd be possible to do power open for generator dampers so that they only open when the set is running. Of course, if your sets are larger than LV, its a different argument entirely.

Admittedly my applications never had dampers fitted, and it was one generator per room, so making sure that the fans are powered by the correct generator is much easier. Its a different argument in a generator hall with 3 sets, in that case you'd want it the way its specified.

As an aside, are there any competing requirements for dampers and fire suppression, or does that not come up much as no one fits fire suppression (in my case it was never done either)?

I don't have any code references unfortunately, I don't think the codes you referenced were applicable to my location either.