How can adjacent air be accounted for in fresh air exchanges for HVAC systems?

[MacLaddy]

TL;DR Summary

Investigating methods to calculate fresh air intake from adjacent spaces

Good morning folks. I have a question about fresh air in an HVAC system. The explanation I provide are a bit tedious, but the question is eventually asked towards the bottom.

I am evaluating a negative air pressure room for a small medical laboratory. Using ASHRAE 170, I know this room needs to have a minimum total of 6 air changes per hour (ACH), with 2 of those ACH coming from fresh outside air. The room air should also be exhausted directly to the outdoors and not returned to the system.

Using a calibrated air hood, we measured the following.

Supply: 165 CFM
Exhaust: -190 CFM
No Return.
Room Volume: 400 CF

Knowing this, I can easily calculate total air changes per hour.

$ACH = \frac{190 (ft^3/min) * 60 (min/hr)}{400 ft^3} = 28.5$

This easily meets the criteria for Total ACH. However, the fresh ACH is more difficult. We are going to attempt the Outside Air Percentage (OA) method.

$OA = \frac{MAT - RAT}{OAT - RAT} \times 100$

Where:

RAT = Return Air Temperature.

MAT = Mixed Air Temperature.

OAT = Outside Air Temperature.

Now my question. If we aren’t meeting requirements via the OA% method, is there a way to figure out how much fresh air is coming from the adjacent area? From the numbers, we can see there is a deficit of 165-190 = -25 CFM. This is approximately 3.75 ACH.

When is fresh air not considered fresh anymore? Is there a method to account for adjacent air at all, or is it typically ignored and considered just a positive or negative relationship?

Thanks,
Mac

 

[anorlunda]

Maybe @russ_watters can help.

 

[russ_watters]

This easily meets the criteria for Total ACH. However, the fresh ACH is more difficult. We are going to attempt the Outside Air Percentage (OA) method.

$OA = \frac{MAT - RAT}{OAT - RAT} \times 100$

You're saying you don't have a direct reading of the outside air fraction at the air handling unit? That's a little surprising, but ok...

Now my question. If we aren’t meeting requirements via the OA% method, is there a way to figure out how much fresh air is coming from the adjacent area? From the numbers, we can see there is a deficit of 165-190 = -25 CFM. This is approximately 3.75 ACH.

When is fresh air not considered fresh anymore? Is there a method to account for adjacent air at all, or is it typically ignored and considered just a positive or negative relationship?

That's the million dollar question with ventilation. ASHRAE and its interpretations are not always completely clear, but the basic logic is that air is "fresh" if it can do what it's supposed to do and hasn't already been used for that purpose. In other words, if you are supplying fresh air (from outside) to one room at a code required rate, you can't then transfer it to another room and still call it "fresh", even if there was nobody in that room to contaminate it. But if you need the air for dilution of chemical contaminants in a lab, for example, and there's no lab in the adjacent corridor, then all of the air transferred from that corridor is usable for dilution of the chemicals in that lab.

AHRAE 170-2008 (the latest I have...), sect 7.1.1b says:
"The air change rates specified are for supply in positive pressure rooms and for exhaust in negative pressure rooms."

In other words, your lab is negative because it is protecting the adjacent rooms from what's going on inside, so the air transferred from those adjacent rooms is "fresh" for that purpose (otherwise, those rooms would be negative too). Applying this broadly to your problem, I'd say you are taking the analysis further than you need to: your room is 100% exhausted so for the purpose of your question all of that 28.5 ACH is OA/fresh/ventilation. Period, analysis finished.

 

[MacLaddy]

You're saying you don't have a direct reading of the outside air fraction at the air handling unit? That's a little surprising, but ok...

No. It's an older system where multiple small heat pumps (2 to 5 Ton) are spread through the clinic and all supplied from one DOAS.

That's the million dollar question with ventilation. ASHRAE and its interpretations are not always completely clear, but the basic logic is that air is "fresh" if it can do what it's supposed to do and hasn't already been used for that purpose. In other words, if you are supplying fresh air (from outside) to one room at a code required rate, you can't then transfer it to another room and still call it "fresh", even if there was nobody in that room to contaminate it. But if you need the air for dilution of chemical contaminants in a lab, for example, and there's no lab in the adjacent corridor, then all of the air transferred from that corridor is usable for dilution of the chemicals in that lab.

AHRAE 170-2008 (the latest I have...), sect 7.1.1b says:
"The air change rates specified are for supply in positive pressure rooms and for exhaust in negative pressure rooms."

In other words, your lab is negative because it is protecting the adjacent rooms from what's going on inside, so the air transferred from those adjacent rooms is "fresh" for that purpose (otherwise, those rooms would be negative too). Applying this broadly to your problem, I'd say you are taking the analysis further than you need to: your room is 100% exhausted so for the purpose of your question all of that 28.5 ACH is OA/fresh/ventilation. Period, analysis finished.

That's a great way to view it and one that I logically agree with. One caveat I worry about is the ASHRAE Standard specifically says the lab requires 2 "Outside ACH." But yeah, for the adjoining space to stay positive it would require fresh air. Analysis Finished.

Thanks. That was very helpful.
Mac

 

[Lnewqban]

... It's an older system where multiple small heat pumps (2 to 5 Ton) are spread through the clinic and all supplied from one DOAS.

For an existing system like that one, the minimum required volume of air that the dedicated outdoor unit supplies to all air handlers should have been calculated based on the ventilation rate procedure of ASHRAE 62.1.
That means that certain amount of outside air is fed to the whole building based on a theoretical need of multi-zones, but in no way each of those zones receives the volume of fresh air that "belongs" to it.

If the room you are evaluating is exhausted at 28 changes per hour, I would say that it is receiving the equivalent of 3 to 8 changes per hour of fresh air.
Practical ventilation is never less than 10% of the supplied air volume for dealing with sensible and latent loads of commercial buildings, for medical facilities is higher than that.

Copied from pages 8 and 14 of ASHRAE standard 170-2017, under Space ventilation-Hospital spaces and in reference to Table 7.1:
"7.1.a.6.i.i The minimum outdoor air change rate listed in this standard shall be interpreted as the $V_{oz}$ (zone outdoor airflow) for purposes of this calculation."