- #1
MacLaddy
Gold Member
- 291
- 11
- 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
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