Exhaust Only Air Changes 1

[edtpro]

Our company does environmental testing in medical settings for a variety of chemicals. A debate has come up I need some guidance on.

If I have a room that has either ONLY supply or ONLY exhaust, can air changes per hour be calculated using the standard N=60Q/Vol? I have an building manager who is arguing that ACH can only be calculated if there is BOTH a supply AND an exhaust/return in the room

So, if I have a room that has a volume of 500 CuFt, Supply of 100 cfm and exhaust of 0 cfm, the ACH = 12. Is that right? Also, if the room had exhaust of 50 cfm, the ACH would still equal 12? And, if the room had exhaust ONLY of 100 cfm, the ACH would still be 12?

Any basic help would be greatly appreciated as we are trying to develop a spreadsheet to use.

 

[IRstuff]

Well, I would think that if you're actually getting 50 or 100 cfm, then you have both ends of the ACH. Otherwise, you'd either kill people with too high pressure or a vacuum in the room.

If there's air being moved in or out of the room, then there's sufficient openings to let air out of or suck air into the room.

TTFN

FAQ731-376

 

[mauricestoker]

The ACH should be measured based on either exhuast or supply, whichever predominates. If under negative pressure, ACH is from exhuast. If on positive pressure, ACH is on supply. Otherwise, you have more than the required ACH.

 

[edtpro]

Thank you for your replies

 

[willard3]

This is a case of the blind leading the blind........

 

[mauricestoker]

What part can't you see? This is very basic, simple calculations.

 

[Ziggypump]

I think what your manager is trying to say that an air change will only occur with a supply and return/exhaust in any way it occurs. You can have a toilet room with only exhaust but still have air changes, because the supply air comes from the adjacent space. Otherwise you would have a vacuum for a room. Air changes come from transmission of air throughout the building. If the space can get or relieve air to an adjacent space, then you will have an air change.

So in essence your manager is correct, but his explanation doesn't look at the entire picture. The air required for the calculation can come from other places, not just that one room. Looking at the building pressurization is helpful as well (and can help determine if the front doors will be pushed out by the air, or vacuum seal the building).

 

[edtpro]

Ziggypump -

Not exactly. What he is trying to say is that in order to correctly calculate air changes for a room, the room must have BOTH a supply and an exhaust (or return) IN THAT ROOM.

This is nonsensical to me because there are plenty of rooms out there (toilet rooms in hospitals, for instance) that are designed with just an exhaust. The reason that room is not a vacuum is that it is drawing offset air from the adjacent corridor, room, etc.

The issue really came about when we were measuring a decontamination room and found that while it had an active supply, the exhaust was not pulling anything. It also measured positive pressure under the door to .0034" wc. Therefore, the room failed on our report and we stated that the room showed 17 ACH from the supply when it was supposed to have 10 ACH supported by the exhaust. THAT opened a long discussion about how to properly calculate ACH that I asked about yesterday. I finally felt our method was vindicated by a few comments in this forum and through a call to ASHRAE.

Hope this helps to clarify. And, if anyone has some more helpful info, please let me know. Great site with good stuff.

 

[edtpro]

BTW, I found mauricestoker's comment to be the most helpful here. Thanks, Maurice!! You confirmed what we already knew.

 

[KiwiMace]

Air will always take the easiest path. In your case study, you say the air is not exiting through the exhaust, but through the other room openings.

If there is supposed to be a fan running in the exhaust, you have a chance to make the room negative, but it is the balancing of the supply inlet that will reduce your air changes.

Adding a preferential exit path can only increase the ACH without also choking the inlet down.

 

[mauricestoker]

I think if you looked at the typical layout for a toxic compounding USP797 pharmacy, or at BSL-3/ABSL-3 design layounts, you would see very quickly that a room does not have to have both supply and exhaust devices to attain air changes. I finished a USP797 design about two years ago (before the current criteria, but matching exactly with current criteria, and beyond) that illustates the point. For toxic compounding, the product needs to be protected, so it is isolated on a BSC. The personnel need to be protected, so the compounding must be done in a negative environment. A vestibule is included under relative positive pressure between the workroom and the entry vestibule, and entry vestibule muust be negative relative to surroundings. Relative postive in the middle is required for product protection. The ACH must meet ISO requirements, so exhaust drives ACH for the work room and the entry. The middle vestibule must be positive, so ACH is counted from the supply side. Easiest way to make this work was to have only supply in the vestibule, exhaust in the entry, and supply and exhaust in the workroom to set differntial flows at TAB setting of dP and damper setting for relative differential pressure (majority of exhaust through BSC). The ACH varies between supply and exhaust depending on relative differential pressure requirements, but obviously does not need a supply and exhaust device in each area.

If you have excess supply air (cutting back from 17 to 9 ACH) I would recomend looking into sending it to perimeter area outside of containment. That won't help make air changes, but it does help with maintaining dP. A magnahelic on the wall helps.