Exhaust Question

[Phunkphlex]

We are planning on putting a 2-ton wall-mount a/c unit on a small building that has about 2000 cubic feet of interior space. The unit puts out about 800 cfm of air, with up to 25% (200 cfm) of the intake being outside air.

Question is, what would be required for the exhaust air? Would a simple wall-mounted louver do the trick? Would a barometric damper be needed? And if so, how would you install it (no exhaust ductwork is planned). Or would you need an exhaust fan?

Thanks for your help!

[Gib's Son]

What type of space is it? 25% osa is getting up there. I would not worry about exhausting 800 cfm let alone 200. But to answer your question, exhaust the cfm of OSA you are providing.

[bob_scheel]

We are planning on putting a 2-ton wall-mount a/c unit on a small building that has about 2000 cubic feet of interior space. The unit puts out about 800 cfm of air, with up to 25% (200 cfm) of the intake being outside air.

Question is, what would be required for the exhaust air? Would a simple wall-mounted louver do the trick? Would a barometric damper be needed? And if so, how would you install it (no exhaust ductwork is planned). Or would you need an exhaust fan?

Thanks for your help!

The answer to your question depends on what the occupancy of the space is. Requirements vary depending on how many people and what activities are going on. Specific uses like medical and industrial have their own requirements for ventilation.

[amickracing]

200 CFM isn't very much extra air and unless there's a balance job done, it might even be a bit less than the expected 200 cfm, good odds are there will be enough leaks in the wall/ceiling penetrations you'll be losing that much at least anyway.

You obviously don't want to go to extremes, but a bit of positive pressure is usually a good thing to keep infiltration away and somewhat controllable.

[Phunkphlex]

The space is used to do work on torpedoes. The a/c will only be operated a small fraction of the year (approximately 1 month of every year). When the a/c is operating, typically only two people will occupy the building. Also, it's safe to assume the building is tight construction, so I believe exfiltration/infiltration will be kept to a minimum.

With the building having tight construction, I was assuming that exhaust would be needed. But is 200 cfm for a 2000 cf space small enough where it would not over-pressurize the building?

Also, if exhaust is required, we would need to protect the opening with a louver, since we get hurricanes over here. Would a louver itself be enough in this case (where the occupants would just close the slats when they leave)? Or would a separate damper or exhaust fan be needed?

[acwizard]

Since you are working on torpedoes, is it safe to assume this is for the Navy. If so then the Navy would have specific requirements. What are all of the design parameters. Adding a fan for exhaust would insure that there is outside air entering the space.

[Gib's Son]

With only 2 persons that would be 40 CFM. HOWEVER, you need allot more information from them if they are working on torpedoes. Is the space considered a hazardous location? Ask for the design criteria, they must have it developed. Ask is there is a requirement for ventilation and OSA.

In terms of a louver, just get a Miami Dade Hurricane rated louver and you should be good. Size for 6-700 fpm.

[Phunkphlex]

The space is considered a Class I Division 2 environment, meaning flammable vapors are present, but are kept under storage during normal conditions. The building is already equipped with a manual localized exhaust contraption that the occupants use whenever any flammable products are used. In other words, we're just supplying comfort cooling. However, considering this a hazardous environment, any electrical equipment used inside the space is required to be explosion-proof, hence the reluctance to use what could be an expensive exhaust fan.

I extracted this excerpt from ASHRAE 62: "Naturally ventilated spaces shall be permanently open to and within 25 ft of operable wall or roof openings to the outdoors, the openable area of which is a minimum of 4% of the net occupiable floor area. Where openings are covered with louvers or otherwise obstructed, openable area shall be based on the free unobstructed area through the opening."

If I'm understanding this correctly, with a 225 sf floor area, the exhaust opening needs to be approximately 9 sf (225 x 0.04). The louver I'm looking at has a 59% free area, which puts the louver at 48" x 48". That seems pretty large for the area we're working with... am I misinterpreting something here?

As far as the OA intake goes, the a/c unit contains an on/off motorized intake damper that allows up to 25% of OA introduction, which is why I used 200 cfm of OA. As far as I can tell, the damper is non-adjustable, although I could be wrong about that.

[bob_scheel]

The space is used to do work on torpedoes. The a/c will only be operated a small fraction of the year (approximately 1 month of every year). When the a/c is operating, typically only two people will occupy the building. Also, it's safe to assume the building is tight construction, so I believe exfiltration/infiltration will be kept to a minimum.

With the building having tight construction, I was assuming that exhaust would be needed. But is 200 cfm for a 2000 cf space small enough where it would not over-pressurize the building?

Also, if exhaust is required, we would need to protect the opening with a louver, since we get hurricanes over here. Would a louver itself be enough in this case (where the occupants would just close the slats when they leave)? Or would a separate damper or exhaust fan be needed?

I am going to assume that there are explosive materials present within this space. (when a torpedo is present.) Also high energy batteries or fuel cells. You need an expert to design any ventilation within this space who is familiar with all of the NFPA and Life Safety requirements. This means a licensed engineer and architect should sign off on the plans. As I said before the requirements vary depending on the use. (which includes an analysis of the materials being used within the space.)

[acwizard]

Torpedoes are not cheap, nor is a man's life. This room needs to be designed correctly. The louver size would be correct, but there is a problem using a louver and that is lack of control with outside building pressures. The a/c needs to meet the class1 spec , along with the damper motors. Do not forget about static discharge or kaboom.