Pressure differential across air conditioned rooms

[Rabindra]

Hi,

What relation or relations are known to give the best results in calculating the air flows when some pressure differential is to be maintained between the room or corridor for clean room applications of air conditioning.

The room which is to be at lower pressure with respect to the corridor should be supplied less air from AHU than the corridor, which should obviously have more air. I am requesting to please let me know the best way to do it.

Rab

[The Viking]

Normally, the clean room would be completely sealed and served by it's own AHU / extract. That way you can balance the supply/extract air.

[Tesla]

I've seen one simmilar to the one Viking has described, The one I saw had ( I think for memory ) a supply air fan for the corradore with a VSD and a differential pressure sensor accross the two spaces.

[frank]

Hi,

What relation or relations are known to give the best results in calculating the air flows when some pressure differential is to be maintained between the room or corridor for clean room applications of air conditioning.

The room which is to be at lower pressure with respect to the corridor should be supplied less air from AHU than the corridor, which should obviously have more air. I am requesting to please let me know the best way to do it.

Rab

Using that combination of pressure differential will result in contaminants entering the Clean Room.

It is normal to ensure that the Clean Room is at a greater pressure than adjacent areas by at least 10% to prevent the ingress of "uncontrolled air".

[Ravi]

You should never play with the supply quantity as you have to, always, supply the required quantity in all conditions(by no. of air changes method, sensible heat method, total heat method or any suitable method). What you should control is the return air quantity.

Mark all the supply flow rates of individual rooms on a layout. Then plot leakage from all the rooms on the layout drawing. Add all the leakages into the room and deduct all the leakages out of the room from supply flowrate and this gives you the return air flowrate.

For calculating the leakage across the rooms (unintentioal openings) use Q = 2910xAx(dP^1/2), where Q is in cfm, A is leak area in sq.ft and dP is pressure differential in inches WC. Mostly, the leak path is through door gaps. 5mm in the bottom, 2mm from the sides and 1mm from the top can give you good estimates. Add individual supply flowrates and return flowrates of the areas covered by one AHU. If the aggregate return is more than aggregate supply(i.e air leaking in), you need to exhaust air from the return duct at a rate equalent to return flowrate - supply flowrate. If supply is more than return then you have to bring in fresh air.

[Rabindra]

Hi
Thanks a lot. Can you please give me the reference to the relation Q = 2910xAx(dP^1/2). Is it from ASHRAE
Thanks again
Rab


You should never play with the supply quantity as you have to, always, supply the required quantity in all conditions(by no. of air changes method, sensible heat method, total heat method or any suitable method). What you should control is the return air quantity.

Mark all the supply flow rates of individual rooms on a layout. Then plot leakage from all the rooms on the layout drawing. Add all the leakages into the room and deduct all the leakages out of the room from supply flowrate and this gives you the return air flowrate.

For calculating the leakage across the rooms (unintentioal openings) use Q = 2910xAx(dP^1/2), where Q is in cfm, A is leak area in sq.ft and dP is pressure differential in inches WC. Mostly, the leak path is through door gaps. 5mm in the bottom, 2mm from the sides and 1mm from the top can give you good estimates. Add individual supply flowrates and return flowrates of the areas covered by one AHU. If the aggregate return is more than aggregate supply(i.e air leaking in), you need to exhaust air from the return duct at a rate equalent to return flowrate - supply flowrate. If supply is more than return then you have to bring in fresh air.

[Ravi]

Yes, but you have to search the older HOFs. The new books give you the equation Q = C*(dP^n), where you have to get the value of C1 and n by blower door test, emperically. Once you establish the building leakage curve (by curve fitting actual data), the leak rate at any pressure can be extrapolated.

[Rabindra]

Thanks Ravi,

The earlier relation Q = 2910xAx(dp^1/2) seems easier to use. Since it is probably an emperical relation I think the units of A and dp should be consistent o the relation.

PLEASE LET ME KNOW THE UNITS (A IN FT^2 OR M^2 AND DP in what unit)

Thanking you once again and waiting for your reply.

Rab


Yes, but you have to search the older HOFs. The new books give you the equation Q = C*(dP^n), where you have to get the value of C1 and n by blower door test, emperically. Once you establish the building leakage curve (by curve fitting actual data), the leak rate at any pressure can be extrapolated.

[Ravi]

The last paragraph in my first post described the units. This is not an empirical relation and it can be derived. The flow velocity for a given static pressure difference is expressed as V = 4005x(dp^1/2). 2910 is nothing but a correction, interms of flow coefficient, to velocity. 2910/4005 = 0.73, which means you are treating the leak area like a square edged orifice. (Though you can establish the leak rates and do a fair design for area pressurisation, this is inferior to blower door test. However, I have been using this for 10 years in pharma industry)

The idea is, the static pressure differential across the rooms is establishing a flow across an opening and the dynamic pressure due to the flow should equal the static pressure differential.

[Rabindra]

Thanks

Rab