Air handling unit smoke dampers

[remp]

I have a new air handler and exhast fan serving 3 floor of production area. The air handelr is located on the top floor and serves each floor via a masonary shaft. The consultant wants smoke dampers on each floor take off so that on fire mode the air handler shuts down and the smoke dampers close. I think this is fine.

However he also wants smoke dampers on the discharge side of the air handler and on the intake of the fan to do the same. I dont know why these are required. Only 20ft away from the air handler and fan the floor smoke dampers exist and will close anyway. There is no take off between the air handlier and the first set of floor smoke damper. Is this a waste, can I delete them?

 

[friartuck]

Hi Billyq

The SD's are there for a number of reasons.

a. Building Regs requires it (depends where you are)
b. To prevent smoke spread from one floor to another.
c. To prevent smoke (from an external fire) from entering the building.
d. To prevent a fire in the AHU from blowing smoke into the building.

Smoke dampers are as they say...smoke rated (and fire rated)...however smoke appears immediately a fire sterts and it is generally 'cool' due to it being diluted. (Cool can mean anything up to about a 100C. In the early stages of a fire, the smoke would not normally be hot enough to set off fire dampers (thermally operated)and so people would be exposed to cool smoke which would create problems.

You can also use the smoke dampers in clever ways, for instance, if the fire is on the third floor, you could blow air into the other floors (and not the third) to keep them free of smoke (depending on your fire codes).

Alternatively you could use your extract in a similar way but this time shut off all vent to smoke free floors and extract only from the fire floor.

If the whole system is deactivated during a fire, and the smoke dampers were not fitted, the smoke could get into the duct at one level and leak into the floor above (or indeed go downwards if the space below had extract vent running)

Ciao

Friar Tuck of Sherwood

 

[remp]

Friar Tuck

We have smoke dampers to each floor available and these will isolate each floor in the event of fire/smoke activation...no problems with this. My question was why do we need them on the outlet of the AHU when 20ft downstreem we have them again where it serves the first floor?

Billyq

 

[friartuck]

Is the ductwork between the ahu and the smoke dampers fire rated? I would guess not. If a fire brike out in the ahu and got into the duct, the duct might melt or its integrity may fail. If however you use fire rated duct or a brick duct, then I would agree with your question.....

Friar Tuck of Sherwood

 

[remp]

Friartuck,

The duct between the AHU and the smoke damper on the wall of the plantroom is not fire rated. And is doesnt have to be fire rated because the AHU is located in the plantroom and the plantroom is its own fire compartment. The AHU is not an essential service and if it burns down along with the entire plantroom during a fire, thats ok, the other smoke/fire compartments are protected.

Asking for a smoke damper on the discharge side of the AHU is like treating the AHU as a seperated fire/smoke compartment, absolute madness and I dont believe this is a code reuqirement.

Thats my 2 & 6 pence worth anyway.

 

[friartuck]

Billq

common sense should tell anyone that if the room is a fire compartment and it has segregation at the compartment walls, then that should be enough. Why the 'consultant' should want to provide dampers at every opportunity is a nonsense and a waste of time and money. Sometimes if you simply say 'no' they will back down. It takes some guts but it does work. The other option is to ask them which code requires this particular fire dampering arrangement.

Good luck

Friar Tuck of Sherwood

 

[SAK9]

Billyq,

I have seen this kind of requirement in some 'Grandfather' specs which has been passed down the genarations.I would say fire damper rather than smoke damper is a must on every floor take off simply because fire can spread across floors through a ventilation duct.The need for a smoke damper could be an insurance requirement as they want to cover for all sorts of imaginable risks.Is the AHU located in an explosive atmosphere such as near exhaust from solvent rooms etc? I have seen this in some pharma projects.As Friartuck suggested,if the fire starts in the AHU,it could travel down the duct and spread to areas where the duct is exposed.But I would not buy this argument as the air handler is of the same material as the duct connected to it and in case of fire in the AHU both the duct and AHU would burn down in about the same time.

Insurance companies have crazy requirements such as putting a fire sprinkler on top a cooling tower!What you should do is ask the consultant for an explanation why it is required.If he can not convince you,then he needs to get convinced!

 

[remp]

I have decided to purchase NFPA 92A & 92B over the net. This should tell me what is required. Thanks for all your inputs, to this thread.
In the mean time we are progressing on site with what the consultant requires, no more time for arguing, but NFPA standards will be good for future reference.

 

[chicopee]

You have not stated what type of production area that you have on each floor. Also, what are some of the other occupancies on each floor?

 

[ChasBean1]

Billy,

Smoke dampers are not required on each floor. Only fire separations are required between floors. Each takeoff should have fire rated dampering (typically 1½ hours per damper) with fusible linkage designed to melt at a certain temperature that shuts fire dampers at duct takeoffs to separate the building into boxes much like a navy ship shuts hatches during general quarters to prevent sinking upon a hit.

The source of smoke in supply air will be your fan motor and belt or reingestion from an outside source. For this reason, and because filters can ignite, the smoke isolation damper is at a single location at the discharge of the AHU, after the filter bank; not at each floor. These dampers shut upon activation of a supply duct smoke detector alarm. Smoke dampers are ONLY needed on each floor if an engineered smoke control system separates a building into two or more smoke zones (generally only in hospitals) to allow smoke-free areas of refuge on a single floor.

I agree with you in that smoke dampers on each floor is not a bad idea; but ALL smoke dampers would need to close if supply air smoke is sensed at the central AHU (at the detector installed in accordance with NFPA-90A), but this end is more simply met by a single AHU discharge smoke/isolation damper.

Additionally, the firebox ratings (e.g., fire dampering at shafts and between floors) generally come into effect in high-rise buildings (>70 ft in US). Fire dampering requirements may be more stringent in non-sprinklered buildings.

-CB

 

[lilliput1]

For AHU(s) greater than 2000 CFM but not more than 15,000 CFM, the 2003 International Code (paragraph 606.2.3)requires a smoke detector in the return upstream of the filters but the 2003 NFPA 90A Paragraph 6.4.2.1 (1) requires the smoke detector upstream of the filters, in the supply duct.
For AHU(s) greater than 15,000 CFM, the 2003 NFPA requires also a smoke damper at the discharge of the AHU downstream of the filter but upstream of any supply air branch. The 2003 International Code do not require the smoke damper at discharge.
Both however require, for AHU(s) larger than 15,000 CFM, smoke detectors at each floor.
So the safest way is to meet the requirements of both ifou units 2000 CFM to 15000 CFM and for units greater than 15000 CFM.