Air Changes per Hour vs. Fresh Air Volume 1

[MedicineEng]

Today I got into some discussion with a colleague regarding Air Changes and Fresh Air Volume and I would like to get a "referee" to solve this.
Imagine the following situation for the sake of argument:

Room Volume: 100 m3
ACH (Air Change per Hour): 5

My position is that the ACH figures tells me how many times in a hour I have to replace the air, so basically I would need to put in the room 500 m3 of fresh air
My colleague´s position is that ACH means that we will have to circulate the air through the HVAC system in an equivalent volume of 5 times the room´s volume (500m3) and that fresh air should be around 12 l/s per person which give much lower fresh air needs.

I see validity on my colleagues´approach and maybe I am being put off by the semantics, in which for be air changes per hour basically means that I have to replace air inside the room 5 times.

Which approach is the correct one?

Thanks a lot for your help.

 

[MintJulep]

You are right.

 

[MedicineEng]

MintJulep:

Thanks a lot for your feedback.

Nevertheless, and since meanwhile I also found some minimum fresh air tables that define a diferent approach for instance Part F UK building regulations 2010 calls for a 10l/s per person of fresh air.

Does thi smeans that if no Air changes are defined, we apply this value, if ACH is defined, then it will overule the minimum fresh air requirements, correct?

 

[DRWeig]

You can meet the higher ACH value and include the lower 10 l/s per person. It's a matter of mixing return and fresh air (and controlling same).


Best to you,

Goober Dave

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[sprinkler1000]

You have to check what are the code requirements.

ASHRAE clearly states "Air changes per hour of outdoor air" which is different from "Air changes per hour supplied to room". The latter refers to air movement.

 

[urgross]

You're friend is correct in my opinion. As an example, an OR room needs, under ASHRAE 170, to have 20 ACH and 4 ACH of outside air. Next point is that the ACH are measured ojn the supply side if space is relative positve differential pressure and off the exhuast/retrun if needed to be relative neg pressure.

 

[ProcessHVAC]

It goes down to code definition. For instance NFPA 820 defines ACH as per your definition (i.e. 100% O.A) but as mentioned in other standards, it could mean fan's CFM.

 

[MedicineEng]

Thanks everybody for yout answers.

I guess that we can call it a draw then.

The two approaches are correct, depending on the governing code.

 

[willard3]

You are arguing semantics and not physics.

 

[MedicineEng]

willard3:

In a sense you are right, but as someone pointed out above, different standards also have different understandings of the word.

Further to our discussion, today my colleague came with a portion of a HVAC tender that stated: " Air Balance Pressure 12 liters/sec per person" and he claims that this is related with our discussion of the air changes per hour. I have a feeling that it doesn´t but I also couldn´t explain why. Can anybody give a hint on what does this parameter relates with ACH?

Thanks a lot.

 

[MedicineEng]

I noticed now that I missed one line in my previous post:

The paper that my colleague showed me was written like this:

"
(...)
Area Air Requirements: 5 ACH
Air Balance Pressure: 12 lts/sec/person.
(...)
"

So that´s why he is claiming that the ACH figure is referring to recirculating air, while the air balance pressure is the fresh air.

Any thoughts on teh line of reasoning?

Thanks a lot

 

[dbill74]

ACH, by definition has nothing to do with the fresh air rate. ACH is defined as the number of times the air in a space is changed. For your 100m3 room, to get ACH=5, you would need to circulate 500m3/hr of air through the space (I don't know the conversion to l/s as I am in USA and work in CFM). What codes require in terms of l/s/person would equate to a percentage of the 500m3/hr. So you end up with 500m3/hr (ACH) of which X% is fresh air.

"Air Balance Pressure" has to do with the pressure in the space relative to another space, typically the outside and expressed as a pressure figure such as inches w.g. or as a percentage. As a percentage it would be calculated as the difference of fresh air intake and exhaust air volumes over fresh air intake: FA-EA/FA x 100%.

When designing HVAC systems, all these factors must be taken into consideration and each impacts the condition of the space differently.
ACH is your primary cooling/heating air removing or adding heat to the space as needed to maintain your thermostat's setpoint.
Fresh air rate maintains the CO2 levels in the space and prevents "stale air" in buildings.
Air balance is used to compensate for the fact that no building or space is ever truly 100% air tight. This is a positive value to keep unwanted moisture and other contaminates out of the building.