# Thread: Air Change in airconditioning systems - definition and calculation

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## Air Change in airconditioning systems - definition and calculation

Hi, Need some advice as I am in a "disputed" teritory. I have asked my service provider to confirm if the Air Change for baby's nursery in a healthcare facility is within the desired requirement (specs say "The ventilation for the newborn nursery shall: i) have a minimum ventilation rate of twelve air change per hour which is provided by mechanical supply and exhaust air systems;") The vendor measures the total supply air rate through the supply air diffusers in CFH of the room and devides by room volume to arrive at Air Change. That is ((CFM per diffuser X Number of diffuser)X60mins/room volume) I disputed this method as to me it only indicated the "movement" of air due to recirculation and does not tell me the ventilation required. In my opinion the method of calculation should be based on how much fresh air (Outside Air) is injected into the AHU such that 12 times the room volume will be injected in the room in one hour by way of diluting the supply air. That is (12XRoom volume)/60mins to give fresh air intake rate in CFM measured at the fresh air intake grill/duct leading to the AHU. Hope someone will be able to correct me if I am wrong.

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That is (12XRoom volume)/60mins to give fresh air intake rate in CFM measured at the fresh air intake grill/duct leading to the AHU.
12 times the room volume is merely the total amount of air per hour to be moved. Required fresh air cfm is typically much less. I cite 2003 International Mechanical Code section 403.3 called Ventilation rate, also using table 403.3. Example: A hospitals' medical procedure room has an estimated maximum occupant load of 20 persons per 1,000 square feet and minimum cfm of outdoor air per person is 15. In the same building, but in the operating room the cfm of fresh outdoor air per person is double that, while the number of occupants remains limited to 20 per 1,000 square feet of floor space. This amount of fresh air is infused into the total airflow, yet is typically NOT the total airflow of ventilation. Or so I believe...

12 air changes per hour is much greater than the required cfm per person of fresh air to be included within said 12 air changes per hour.

3. 12 ACH of fresh air is a ridiculously high number, even taking into account that a nursery is a poop factory.

4. I found this interesting HHE (Health Hazard Evaluation) report:
http://www.cdc.gov/niosh/hhe/reports...-0187-2544.pdf

ASHRAE Ventilation Evaluation Criteria
Newborn nursery
Outside air 5 ACH
total air changes/hour 12

and in many other hospital ventilation specifications documents online, both outside (fresh air) ACH and total ACH are specified. Therefore I think you needed to specify both outside ACH and total ACH. If you only specified a "ventilation rate", it's reasonable to equate it to a requirement for total ACH. It's not the fault of the contractor if you didn't specify the outside ACH.

I hope that helps.

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12 ach is approaching the required outside air ventilation rate of an operating suite at 15. for your suite ot utilize 12 changes of outside air would likely require specifically designed equipment ot handle the loads, over a system that does 12 air changes per hour. also, since its medical use, there are specific filtration requirements to meet modern codes, including two types of filtration in most cases at the air handler, sometimes three.

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Originally Posted by flange
12 ach is approaching the required outside air ventilation rate of an operating suite at 15. for your suite ot utilize 12 changes of outside air would likely require specifically designed equipment ot handle the loads, over a system that does 12 air changes per hour. also, since its medical use, there are specific filtration requirements to meet modern codes, including two types of filtration in most cases at the air handler, sometimes three.
Hi Revolver,pmeunier & flange,
Thanks a ton for your valuable feedback. I really appreciate the quick response.

There are a couple of things I'll need to highlight. The specification only says "ventilation rate of 12 ACH" it does not indicate anything to say about a minimum outside air change requirement as stated in ASHRAE Ventilation Evaluation Criteria for Newborn nursery (Minimum Outside air of 5 ACH out of the total air changes/hour 12). I beleive the CDC environmental infection control guidelines also stipulates the same. The other thing is in my country the ACH requirement is stipulated that way ("ventilation rate of 12 ACH") in the Private Healthcare Act. The Act does not mention anything about the outside air injected being 5 ACH out of the 12 ACH. Hence my interpretation that the word "ventilation" would mean the injection of outside air. If nothing is mentioned about the minimum requirement, we could shut off or reduce the fresh air intake amount (only in theory that is) to save energy. But that would compromise clinical requirements. Hence the confusion.

Feedback appreciated.....

7. 12 ACH of HEPA filtered air (which is what I would call ventilation) for a newborn nursery makes sense to me, to capture dust, allergens and airborne pathogens. However I can't imagine why you'd need that to all be outside air unless you have them packed at 1 baby/cubic foot. Outside air requirements are based on occupancy and sources of pollutants in the space. Unnecessary outside ACH is a waste of money, energy, and is anti-green, and inflates already astronomical hospital bills.

8. Because the law is poorly worded, perhaps you do need 12 outside ACH just to avoid the cost of silly lawsuits, and the risk of jury awards, regardless of scientific basis. I'm not a lawyer.

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ach is just that. you circulate and filter the air in the space twelve times in 60 minutes. that means you change the air every five minures. simple math until you figure ventilation requirements and pressurization schemes, then it gets a little more complex. local healthcare codes dictate ventilation rates.

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Originally Posted by pmeunier
12 ACH of HEPA filtered air (which is what I would call ventilation) ...
Technically, general air changes have no stipulations regarding filtration. If the room happens to be a negative pressure room, the general air changes are calculated by the exhaust leaving the room. Obviously there are filtration requirements, but they do not always play a role in air changes.
In most cases, both general air changes and outside air changes are specified. In the negative pressure room mentioned above general air changes would be calculated via exhaust, but the outside air changes would be calculated via a percentage of outside air in the supply air being supplied to the space. Thankfully, for someone in my business, most 'critical' areas are 100% outside air, thus making these calculations easy. However, from time to time, I come across a situation where, as mentioned previously, outside air is reduced to save energy.

Without looking it up I would guess a nursery would be designed positive to the adjacent sterile corridor. With that assumption in mind, one would use the supply air to calculate air changes. I would also venture a guess that in high temperature load conditions it will take enough conditioned air to maintain temperature set points that general air changes will be met by default. Point is, 12 air changes is not excessive.

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Hi tnhikr44, flange ,pmeunier,revolver
My heartfelt thanks for the invaluable information. It has been an enlightening discussion. I guess I will be having a tough time resolving or convincing this issue as the guidelines are not clear. Will try to get hold of the ASHRAE guidelines to throw the book ( figuratively that is! ) at anyone when required

Once again, thanks a million

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An air change per hour is a measure of how many times the air within a defined space is replaced.The actual amount of air changed in a well-mixed ventilation scenario will be 63.2% after one hour and 1ACH.
You can use following formula to calculate it-
N=60Q/Vol

Where:
N=Number of air changes per hour
Q=Volumetric flow rate of air
Vol=Space volume

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