Volume of air through a hole - how to calc.

[Harry Bloomfield]

How do you calculate the volume of air passing through a hole, given the hole size and the pressure please?

Lets say 0.3mm diameter, with a pressure of 30PSI.

[GB]

Google comes up with *many* references. This one seems useful:

[Onetap]

Volume of what? Air, water, treacle, bricks?

[Clive George]

Um, you've quoted the answer to your question.

[Onetap]

I don't recall the formulae off-hand, but a couple of brain cells eventually connected and I remembered that it's covered by these;

You should find all the details in there or other sites dealing with orifice meters. They're the commonest main flow measuring device in building services (heating, chilled water, some air) so there's lots of literature.

There is a large pressure loss immediately downstream due to local turbulence; some of the fluid static pressure is converted into velocity. The pressure increases downstream (10D) as the turbulence dies away.

[Onetap]

Bummer. Senior moment there, apologies offered to all inconvenienced

[Harry Bloomfield]

What I'm trying to work out is very roughly, supposing I had a compressor which store 4cf of air at 100PSI, followed by a pressure reducer dropping it down to 30PSI which then fed a hole which was 0.3mm diameter, with atmosphere at the other side - how long would it last, before the 100 PSI in the tank fell to 30PSI.

I found this, but I'm confused as to how to feed the information in -

[ARW]

:-). And when I said to the site manager today that the cooker extracter hoods had been fitted too low he asked me "which room is this in?".

There is no inconvenience, just the reassurance that it is not just me who can sometimes miss the blinding obvious and that we are all human. I am not telling you what I did wrong.

[newshound]

Well to a reasonable approximation the mass flow rate will be about 0.6 x Area of hole x square root of (density of air x pressure difference). You need to use consistent units. I'm about to go out, but will try to plug some numbers into MathCAD some time tomorrow.

[Harry Bloomfield]

Thanks, absolutely no rush.

[John Rumm]

I remember one engineer coming up to me with spec document clutched in hand... "It says that to select this function you set the command word to 1... It that a 1 in hex or a 1 in binary?"

[Matty F]

I'd do some experiments with different size holes. I don't trust those engineers and their formulae! Some engineer got the formula wrong with the Manapouri power tunnel and made it 10% too small. There was more friction than they allowed for. They had to dig a new tunnel in rock for 20 kilometres at a cost of $220 million.

[Onetap]

Charts for calibrated orifices here;

Lots of other information on the theory on that site, or Google for micro o rifice plate.

They're surprisingly accurate because they're used so much and have been te sted and refined by experimenters over many decades.

I mostly used charts to estimate pump sizes and you never heard how accurat e the estimate was, unless something went wrong. Everyone tended to add in random factors of 10% since it's easier to reduce flow than it is to upgrad e pipes &?or pumps.

I only once calculated the resistance of an existing installation (with CIB SE charts), that I had measured and my 'estimate' was within 1% of the meas ured resistance; the orifice plate was only accurate to +/- 5%. A commissi oning technician had falsified test data.

ade it 10% too small. There was more friction than they allowed for. They h ad to dig a new tunnel in rock for 20 kilometres at a cost of $220 million.

Rubbish in, rubbish out.

[newshound]

Hmmm. My first calc gives about 9 hours. That seems a bit large to me (but I can't spot any obvious fault).

I tried that calculator link first, but it did not work at all for me.

[Harry Bloomfield]

9 hours is much longer than I expected, but if it is in hours that is good enough for my purpose - I thinking to carry out some experiments with Babington type burners.

[Harry Bloomfield]

Harry Bloomfield brought next idea :

Sorry, ommited to say - thanks very much.

[Dave Baker]

Discharge coefficient of a non radiused hole that small in normal thickness sheet metal will be somewhat higher than the 0.6 of a conventional thin plate orifice - call it 0.7.

Flow rate at a pressure difference of 30 psi absolute is therefore about 0.7 litres/min.

Tank capacity at approx 28 litres per cubic foot is 112 litres. At 70 psi over regulator pressure that equates to about 70 / 14.7 x 112 = 533 litres at atmospheric.

Discharge time = 533 / 0.7 = 762 minutes = 12.7 hours.

[Harry Bloomfield]

Dave Baker formulated on Friday :

Not that far from Newshaounds result of 9 hours...