Insight about Air Changes per hour (ACH) in Mechanical Ventillation

[somashekar]

There is a requirement that in machanically provided ventillation, a certain number of ACH is maintained for factory environment (I guess it is 10). This comes directly from the cubic feet per minute (CFM) capacity of the Air handling unit (AHU) and is effected by the volume of the ventillated area.
Some questions:
1. Are the AHU designed to always draw in the fresh outside air OR certain designs allow circulation of the exhaust air for better effeciency of the cooling. In such a case is the ACH calculated really meaningful. In case it is partially circulated, how do we calculate the ACH.
2. To assess the volume of the area, do we consider the total height from floor to cealing or upto the false cealing from where air delivery and return paths are ducted.
3. If the ACH as said in the factory act is not met, could it be a OHS non-conformance / legal non-conformance.
4. If there are split a/c that are operating within the ventillated area, that circulate the air within the closed space, we do not add the CFM of these to the total CFM, in the ACH calculation. Am i right in this ?
5. How does this ACH have effect on the requirement of meeting the workzone air quality monitoring with regard to Time weighted avarage (TWA) limits of various VOC in air, stipulated in Factory act/ MSDS of the various solvents.
Can someone share there case sudy about these ....
(ACH = CFM of AHU x 60 / cubicfeet of space ventillated)

 

[Pancho]

Interesting questions. We sometimes get involved in helping our clients with vent calcs under our domes, and I have it a bit fresh from a recent case.

I don't think that the actual target for the required ventilation is the number of air exchanges per hour, at least not per se. What is necessary is to provide sufficient ventilation to ensure that concentrations of suspended particles in the volume of air inside the building are below the "Threshold Limit Values-Time Weighted Average" (TLV-TWA) or the standard imposed by the relevant authority. In the U.S., these values ​​are given by the American Conference of Industrial Hygienists (ACGIH) or the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE).

For example, if a building is occupied 8h per day and it contains a source of coal dust, the TLV-TWA recommended by the ACGIH is less than 2mg/m3 (ACGIH "Threshold Limit Values ​​for Chemical Substances in the Work Environment", 1997). The value increases if the periods of occupation are lower. If the volume of air in the building is about 18500 m3, the volume of suspended particles should be kept below 2mg/m3 X 18500m3 = 37,000 mg (or as appropriate depending on the volume occupied by the battery, and the applicable TLV). If you know the flow with which suspended particles are added at the source, then you can determine the amount of m3 of air to be vented. It maybe more or less than the typical rules of thumb.

In benign climates, natural ventilation may be all that's needed. Natural ventilation calculations are made with the method described in ASHRAE Fundamentals Handbook, Section 25.13.

For your questions:

1)
If you are heating or cooling, you may be able to recirculate the air provided that you filter it. How well your filters work dictate how many particles return with the recirculated air.

2)
I think you can probably figure only the occupied volume (under the ceilings), unless the ceilings are perforated or otherwise permeable.

3)
Yes. You are not providing a healthy workplace.

4)
Think of particles in and particles out to figure out any combo of fresh and recirculated/filtered air.

5)
Check out the references above.

Good luck!

Pancho

 

[Dr. L. Ramakrishnan]

Dear Somashekar,

In addition to the earlier response, I thought I would share with you some more details.

1. Are the AHU designed to always draw in the fresh outside air OR certain designs allow circulation of the exhaust air for better effeciency of the cooling. In such a case is the ACH calculated really meaningful. In case it is partially circulated, how do we calculate the ACH.

In most of the "Forced Ventilation" situations fresh air is drawn from the external environment and forced into the confined space. Hence it is normal to draw fresh air for the ventilation. In case an air conditioning unit is used for the Forced ventilation, there is a provision for recirculating a part of the exhaust air mixed with fresh air; this is just to save energy
2. To assess the volume of the area, do we consider the total height from floor to cealing or upto the false cealing from where air delivery and return paths are ducted.

Normally false ceiling is provided to reduce the space for maintaining ventilation requirements. If false ceiling is scientifically and technically sound, the space below the false ceiling is used for calculations
3. If the ACH as said in the factory act is not met, could it be a OHS non-conformance / legal non-conformance.
If the Factory Inspector has asked for a certain number of air changes in a Forced Ventilation situation, it has to be met; otherwise it is a non-compliance to legislation (in India -Factories Act / Factories Rules)
4. If there are split a/c that are operating within the ventillated area, that circulate the air within the closed space, we do not add the CFM of these to the total CFM, in the ACH calculation. Am i right in this ?

I am not sure if I have understood your question properly; if your question is about confined spaces (air conditioned rooms) within the space for forced ventilation, I would say it is similar to the situation with false ceiling. If the confined space (for example, a balance room in a shop floor) is indeed reasonably sealed from the outer space, you many not include it in the CFM calculation
5. How does this ACH have effect on the requirement of meeting the workzone air quality monitoring with regard to Time weighted avarage (TWA) limits of various VOC in air, stipulated in Factory act/ MSDS of the various solvents.

Unless there is a special requirement, it is always recommended to have a local exhaust to extract the emissions (like VOCs) so that they do not spread on the shop floor. In our factories in addition to the forced air ventilation, we have local exhausts to extract emissions from burners; these exhausts have two effects: one they extract the emissions from the burner and two they also extract the hot air, thereby reducing the heating of the shopfloor. Local exhausts also help to reduce emissions spreading on the shopfloor; they can be designed to ensure less than the TLV of emitted substance in the space around the operator. Normally for any new situation this local exhaust is designed to ensure that the air borne chemical concentration is less than half the TLV around the operator
Can someone share there case sudy about these ....

As a thumb rule, for a normal situation without any emissions of chemicals or heat, the recommended air change is about 6-8 per hour; in case of chemical emissions the number of air changes per hour will be anywhere from 12 to 18 depending on the quantum of emission. This is just a thumb rule. Always bear in mind that this is for FORCED VENTILATION situation. If you have natural ventilation with open windows and doors these recommendation are not appropriate. Of course local (at the source) ventilation is always recommended to reduce the spread of air borne chemicals in the shopfloor

If you are interested to know more on this subject I would recommend that your read the Chapter on Ventilation in Patty's Handbook of Industrial Hygiene.


With kind regards,

Ramakrishnan