Air Exchange Rule of Thumb 3

[Rich134]

Is there a "Rule of Thumb" for the number of air exchanges for a light manufacturing industry? There are no chemicals in the indoor environment.

I have gone through Industrial Ventilation: A Manual of Recommended Practice by ACGIH without finding any guidelines that apply to my situation. TIA

Rich

 

[CHD01]

American Society of Heating Refrigeration and Air Conditioning Engineers has this information, MARKS Mechanical Engineering Handbook also has a chapter on this I believe. I'm sure there are other references.

Do NOT use the following as absolute limits to follow, but for an idea I have seen the following criteria used with the actual value selected based on type of room:

5 to 50 cfm fresh air per person occupancy

0.05 to 4.0 ft3/min per ft2 of floor area

Air Changes (I have seen 1 to 50 air changes/hr; but I do not mean to limit you to that range)

I would pick the maximum recommended considering all the above and the room type.

There is also the issue of whether you need a clean or steril room which requires a minimum differencial pressure between clean and ordinary room environments. See a previous thread on this on Room Static Pressure Measurement.



The more you learn, the less you are certain of.

 

[Yeldud]

You don't say whether your light manufacturing space is only ventilated or ventilated and air conditioned. Also the term air changes per hour frequently is referring to air changes per hour of outdoor air (ASHRAE 62 uses this), but also is used in reference to number of air changes or air turnovers in the space without regard to fraction of return air vs outdoor air. Ranges quoted by CHD01 are valid but represent extreme ends of the range... for typical light manufacturing spaces without unusually large internal cooling loads or exhaust requirements, you could probably narrow the ranges to outdoor air value of 10-20 cfm per person to meet the code ventilation requirements, 0.75 to 1.25 cfm per square foot of floor space for air conditioning (mechanical cooling), or 8 to 15 air changes per hour of outdoor air for a ventilated only space. Hope this clarifies things a little.

 

[CHD01]

Yeldud: Would you agree with a normal 20% of total recirculated air for a fresh air requirement; the max would be 100% oc course and the min 0% (if there was alot of infiltration I guess) - but what would you place a normal design range at for % fresh air?

Thanks


The more you learn, the less you are certain of.

 

[lilliput1]

If ventilated only, use multiple supply and exhaust fan systems that can be sequenced on/off by a space thermostat except the minimum at 0.05 CFM/SF should always be on. Provide unit heater that can handle this ventilation load + transmission load in winter. At peak summer, rule of thumb is 2 CFM/SF. Locate fans to sweep the area with supply at one end & exhaust at the far diagonal corner, etc.

 

[lilliput1]

Also supply low to blow at the occupied area & exhaust high since heat rises. Points of concentrated heat should be exhausted directly overhead so the heat does not get to the occupied spaces.

 

[Bittooria]

No there is no rule of thumb for air exchanges. Ideally in HVAC, the ventilation rate shall be determined by the number of persons as recommended by ASHRAE 62.
The air exchange method is not an energy efficient method and is recommended for hazardous areas for dilution of indoor contaminants. People tend to use 1 or 1.5 air changes for non-hazard applications and 6 to 12 air changes for hazardous application when it is defined by some codes.

 

[quark]

Yeldud is right. We say ACPH with respect to the total flow rate because this will give exact picture of the cleanliness we maintain (particularly in Pharma Plants). The minimum ACPH for a typical class 10000 room should not be less than 20 as per USP, but above 30 are generally maintained.

20% fresh air is a good number. Some vaccine manufacturing facilities and cytotoxic formulation facilities, however, use once through systems (due to dearth of validatable data).

Fresh air requirement per person is generally 16 cfm based on dilution of CO2(by respiration) to safe levels. A good deal of information and calculations are provided in Air Conditioning book by Shan K Wang. Right now I don't have the book with me, but I will try to get the calculation.

Regards,

 

[lilliput1]

ASHRAE recommends 20 CFM per person if there is no smoking in the room.

 

[VSSriram]

Air Changes per hour can sometimes be misleading. If the height of the building is more (say 7 or 8 metres), then air changes per hour can give significantly high values resulting in higher energy costs as well. Probably, air changes per hour could be applied based on "net occupiable space" or a standard 3 to 3.5 metre height ?