Health Care - Negative Isolation Rooms. 1

[FBW]

Im having trouble trying to get some legit answers on if i should be using the supply or the exhaust to calculate ACH for negative isolation rooms. One would think that this would be obvious, as i was told that ACH is always calculated from the larger of the 2. However, i just read an ASHRAE document that states "12 ACH on supply air as minimum". And also the original commissioning documents are based on supply air.

My co-workers think I'm wasting my time and over thinking this. Am I?

Thanks

 

[IRstuff]

You can't exhaust more than you supply, so how could the air changes be based on the exhaust?

TTFN
faq731-376

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[urgross]

Yes, you can exhaust more than you supply, which is how the AII rooms maintain 0.01 to 0.05 IWC relative negative pressure. For relative negative space, room ACH is counted by the exhaust; for relative positive, or protected environment, ACH is measured from the supply. Same holds true for AAALAC, BMBL and USP. See ASHRAE 170, para. 7.1.a.3.

 

[IRstuff]

The extra is sucked into the room through openings, not from the inlet, which ought not count as an air change.

TTFN
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[317069]

Question 1:If we have a fully isolated room (no infiltration at all), let us say we supply 1000cfm by a make up air unit and exhaust 1500 cfm by a dedicated exhaust fan, everything is fully isolated, what the room pressure would be after running for 5 hours, 10 hours, three days for example, would be constant at a certain value or it would get lower and lower until the room's walls collapse.?

Question 2: why don't we consider the air coming to a space by infiltration as a supply air for pressure calculation purpose.?

ASHRAE follows fluid mechanic, not fluid mechanic follows ASHRAE

 

[FBW]

Sorry if i wasn't clear enough, I'm talking about a patient isolation room in a health care facility. That means it's not isolated in the sense that its 100% sealed and the walls will implode, as a human being has to remain alive inside, and people will be entering and exiting often. And we don't calculate pressure in the room from supply and exhaust flow, you commission the flow controllers to reach the pressure you want.

And to my understanding, air pulled in through the openings is considered an air change, as in its not the same air that was in the room before, which could potentially be full of nasty stuff. Its not an "outdoor air change", which, yes, would be based on fresh air from the inlet. Outdoor air changes and air changes per hour have different requirements, the former being a portion of the latter. I'm not trying to argue the terminology here, i just want to know why I'm getting contradictory information, and i want to make sure i'm doing things correctly.

For example, here is a document from 2014 that would appear to support my original position that ACH for AII rooms is based on exhaust flow, as stated by urgross.
[link

http://grummanbutkus.com/assets/upl...HRAE_Seminar_-_Isolation_Room_HVAC_Design.pdf

]Link[/url]

Which was fine until i found this document that states on page 15 "12 air changes per hour on SUPPLY air as a minimum".
[link

https://www.google.com/url?sa=t&rct...t0cC_3jur3UE5XA&bvm=bv.96783405,d.cGU&cad=rja

]Link[/url]

Also, as i mentioned, the original commissioning docs are based on supply air, which is adding to the confusion.

The reason this is important is, I have I have to know which of the flows (supply or exhaust) needs to remain constant to meet ACH requirements, and which one i can manipulate to achieve the pressure i want.

 

[urgross]

Are you following ASHRAE 170? ASHRAE 170 is incorporated in whole as part of the 2014 FGI. The first link is not a standard or criteria followed by the joint commission. I'm certain the joint commission will follow ASHRAE 170 and FGI.

 

[317069]

I know it is not fully isolated room, but I put my question in general to discuss both opinions.
if you consider infiltration or door opening then your exhaust would be equal to your supply, then you don't need to use any document since you have 1000 cfm exhaust and (800cfm from supply duct +200 cfm from infiltration) are running in your room.

 

[FBW]

I see your point, 317069, and in that regard i can now understand why it is the larger of the 2 MEASURED flows (inlet / outlet) will always define the actual ACH. Which answers my question. The real flows are equal, as you point out. But i'm only actually accurately measuring one of them at any given time, since i'm not measuring infiltration via my flow controllers. Right?

urgross, im going to get myself a copy of the documents you mentioned, so i don't have to rely on third hand information.

Awesome, thanks all.

 

[marcoh]

Most isolation rooms I have seen are based on 12ACH supply air, exhaust is oversized by about 20%, and the the exhaust is balanced at commissioning to achieve the required pressure differential.

 

[SAK9]

Air change rate is to be based on supply air only as exhaust rate can vary depending on how airtight your room is.Moreover the pressure differential control output goes to the exhaust valve and therefore exhaust flow rate can vary if one of the door seal comes loose.