Intake filter

[m2e]

Hi,

I am designing a ventilation system for an electrical room. The room is at the outside wall and a simple ventilation system is enough. The setup (from intake to exhaust) is like this:

1. Intake drainable louvre
2. Insect screen
3. Backdraft damper
4. Filter
5. Plenum
6. Transition and short straight duct run
7. Flex. duct connection
8. Inline centrifugal fan (BSQ)
9. Flex. duct connection
10. Short duct run
11. Louvered supply grille
12. Room
13. Motorized damper
14. Insect screen
15. Exhaust louvre

My question is:
a. Is the motorized damper needed? Or do I just need two backdraft dampers?
b. For the filter, I am going to need 30% filters, but will I need a pressure differential gauge to indicate the filter change? If so, what kind of pressure differential should it be?
c. For a simple setup like this, do I need a filter bank (zig-zag type), or a single pleated filter just perpendicular to the flow?
d. Is there any other problem you can see in this setup?

Thanks.

 

[ChasBean1]

I like the motorized damper on the non-fan louver. A BDD could rattle around in the wind and leak too much.

Potential issue: fan is sucking in OA directly, which is usually an uneven intake profile and can draw in snow/rain etc.

Suggest instead a very similar setup to what you have but use a large intake opening/plenum with motorized dampers and your 30% filters, set back from the intake as much as space allows. This should be low velocity <500 fpm and will help to promote a longer filter life. Turn the fan around and blow air out a smaller louver (make room ventilation draw through vs. push through).

A backdraft damper alone on the fan outlet, before the louver should be fine in lieu of a motorized damper.

Don't bother with filter DP gauges etc. unless it's a super critical application and your client has extra budget (not too common today).
-CB

 

[MintJulep]

I don't like the idea of draw-through. This makes the entire room negative pressure, and will suck in dirt under doors, through cracks, conduit penetrations, etc.

Motorized dampers seem like overkill to me. Yeah a back-draft damper might rattle when the wind blows - is that an issue in this application?

Filter arrangement is simply a matter of how long do you want to go between filter changes. Since the filters will probably never get changed anyway, may as well keep it simple.

 

[m2e]

It is indeed the reason why we wanted to keep the room positive pressurized; the area it is located is dusty. The very concern I have for filter indicator is that they need to tell the maintenance guys to "CHANGE ME"!

That leads me to two questions:
1. if the filter do get dirty and blocked, will the fan just stall and overheat? Isn't that going to be a problem?

2. Instead of using centrifugal fan, can I use wall mount propeller fan? The flow is about 400cfm, and obviously with the filter and louver, the initial SP is going to be around .3 and goes up to .55+ when the filter is clogged. I can't find a propeller fan that can take that kind of SP for such low flow. Do you have any suggestion? We wanted to use propeller fan to save the ductworks and labors.

 

[MintJulep]

will the fan just stall and overheat?

No, it just won't move any air.

 

[ChasBean1]

All this for 400 cfm???

This is usually a setup for a 12,000 cfm type elec. space.

Is there a building system anywhere nearby where you could tap a 400 cfm VAV box? Then forget the louvers, just undercut the doors...

 

[m2e]

yeah, it's just a 160 sq.ft. room... unfortunately, there's no vav box or anything like that, have to go through the walls.. That's why I'm thinking small propeller fans...

Do you think propeller fan will work here??

 

[ChasBean1]

Not if you want a filter, as Mint pointed out... the typical wall propeller fan won't move air with any pressure drop, even from a 30% filter.

Did you consider a mini split DX system?

 

[Compositepro]

"a simple ventilation system is enough"?!

Use an exhaust fan near the ceiling where the hot air is. A thermostat will save some energy.

 

[DrRTU]

This is an overkill for 400 CFM but if its a dirty industrial environment then it will work. Check the Greenheck line, they have a filtered supply wall housing for propeller fans.

http://www.greenheck.com/products/fans/sidewall.php

They offer a motorized damper and single point wiring. I use a motorized damper on the inlet and BDD if its on the discharge unless subject to wind or building pressure. Hartzell should also have a similar product.

 

[sailcat753]

I would definitely use a centrifugal fan if you are using merv 8 filters. I like the idea of filter pressure differential switch with a light outside the space to notify when filters get dirty. Did you run a load on the space to determine the 400 cfm? Q=1.08cfmdeltaT or Q=all load center heaters and transformers. transformer Q=1-3%of kva of transformer. solve for Q (Btuhs), then solve for cfm. 1-5F degree delta T. This is better than air change method. I would use a stormproof louver and check what the velocity that water penetration starts. Should be around 1200 fpm. I would use a motorized damper at the intake to close if and when the fan is not running to keep wind borne water from entering the ductwork. I would slope the first few feet of ductwork back to the intake louver. Would use a line voltage thermostat with a timer to override if someone is working in there for ventilation.

 

[RossABQ]

If this is an Electrical Equipment Room per the Code definitions, undercutting the doors or pressuring the room aren't good ideas. The walls and doors need to be 1-hr. rated, and generally these should be negative pressure to prevent pushing smoke out into the inhabited spaces. Not sure with the ventilation load of 400 cfm if this is really such a room, but check to see.

 

[DuctPipeMan]

Generally, I see unfiltered ventilation for electrical rooms with exhaust fan and intake louver (BDD on exhaust and motorized on intake). If dirty ventilation air is of concern then using a filtered supply fan and matching exhaust fan is a good way to go. The air flows can be matched, and the room pressure can remain neutral.

I do like using a T'Stat to avoid getting the room too cold.

Monitoring dP on the filters is something that is dependent on the end user. If there is a maintenance staff to use it, fine. If not, doing so only makes yourself feel better for doing the right thing.

 

[m2e]

Thanks Sailcat for your reminder of the heat calc. I redid it, and I think the reason my cfm is so low is because the delta-T is high. I've allowed the room to be at 86F while the outside is 68F. So, I think that should be ok.

Actually, do you really need filter for electrical room or a MCC room? Do you usually put in filter or no?

 

[ChrisConley]

I've allowed the room to be at 86F while the outside is 68F.

What about when the outside air temp is higher? Even in the great white north our outdoor gets around 90F in the summer, incidentally when transformer loads are typically the highest.

Typically, I get the maximum temp rating on the equipment and design the fan for a deltaT from summer design. If that is too much, a small mini-split is usually the way to go.