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Posted By Jason McQueen
Whats the best way of calculating the number of air changes that a room has?
I guess its a case of working out the volume of the room and then dividing this by the air speed per hour but whats the best way of determining the air speed for a standard, non forced/mechanically vented room? Is it a case of standing there with an anemometer?
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Posted By Tony abc jprhdnMurphy
Leakage, Infiltration, Size of room, extract system? All info you need
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Posted By Bossa nova
Jason,
If you haven't got forced ventillation you will have nothing to measure. Are you checking for compliance or because there is a problem?
Crossover
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Posted By Jason McQueen
Its as a result of a DSEAR assessment. We need to ensure that one of the rooms in which we stored flammable liquids has six changes of atmosphere per hour. The room is vented via natural draught ventilation but nothing else.
We therefore need to calculate if this is sufficient to generate the six changes and if not, look at forced ventilation.
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Posted By Bossa nova
A little quick to hit the send button earlier. If you have a flow to measure, assuming this is continuous for the purposes you state! The you measure the velocity multiply this by the open inlet area and this gives you your volumetric flow, divide the room volume by this and hey presto.
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Posted By Guderian
Buried somewhere deep in the text of one of the DSEAR guidance books, there is some guidance on the size of ventilation louvres etc. From memory if the area is 1.5% of the wall size, then ventilation is likely to be ok.
If you consult BS60079-10, there is a formula in there for calculating air flow rates and air changes. Sorry can't send you a copy, its a copy right document.
Also consider the existing/proposed layout, check that there are no stagnant areas.
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Posted By Guderian
Slight correction to my earlier post. The percentage area of ventilation vents is given in the following publication:
The Storage of Flammable Liquids in Containers ISBN 071761479 (1998 Ed) Para 61
Recommends 5 x air changes per hour
The area of the vents should be between 1-3% of the total roof and wall area.
The building/vents should not be obstructed from external winds etc.
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Posted By Bossa nova
A little more involved than a straight 1-3% I would suggest you getting a copy of HSG51, a nominal investment considering the task you are undertaking.
Blowover
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Posted By Guderian
The info in my last post is taken from HSG 51. Your advice is not too specific, to help the original poster of the query they had.
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Posted By Paul Leadbetter
Jason
Have you considered trying to measure the air change rate?
Paul
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Posted By Guderian
In practice it is quite difficult to measure natural ventilation rates - the air flow velocities are usually too small to get accurate measurements. Anemometers/turbine flow meters requires probably in excess of 0.5-1m/s wind speed to give anything like an accurate reading. In which case if you have these speeds, then the ventilation is probably ok.
Not to mention other inaccuracies caused by draughts around doors/windows etc.
Its not so much the amount of air flow (although still important), as to where the air flow goes. High flow rates can sometimes mask leaks/build up of vapours. Hence my earlier comment about giving consideration to areas where stagnant air/vapours might gather.
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Posted By Paul Leadbetter
It is out of print now but the method in MDHS 73 was pretty straightforward.
Paul
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