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What is adequate ventilation? 


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  • 2 years later...
On 4/2/2018 at 2:27 AM, Silk City Distillers said:

One of the points the consulting engineers bring up is:  How can we determine there will be no pooling of the vapors that would exceed 25% of the LFL?

Ask your engineers to calculate the amount of ethanol required to be vaporized to be able create pooling of vapor at 25% LFL with no ventilation in a 13,000 square foot space. 


Someone check my math...

Let's just say, for giggles, we are talking about 2 feet of pooled vapor in the 13,000 square foot space, 26,000 total cubic feet at 3.3% (the LFL of ethanol).

Let's say 736 cubic meters to keep things simple.  At 3.3% ethanol, we're talking about 24.3 cubic meters of ethanol vapor in that bottom 2 foot space (736 total)

Stay with me here, that's 24,300 liters of ethanol vapor.

I might be wrong but a mole of ethanol is 22.3 liters volume.

24,300 liters / 22.3 moles = 1089.69 moles of ethanol

Ethanol is 46.07 grams per mole.

That's about 50.2kg of ethanol, or about 64 liters, a little under 17 gallons of pure ethanol vaporized to fill a 2 foot tall, 13,000 square foot space (no walls) to hit the lower flammable limit.

Now, you'll have a 125 gallon still.  Playing with numbers, you would need to vaporize all of the alcohol in a 125 gallons of a 13.6% wash to hit the lower flammable limit.  

This would probably take something like 4 hours of blowing vapor out of the still, with no air exchanges.  I suspect this would be noticed.  Or at least I hope it would.

So, to hit 25% LEL, in that 2 foot tall space, we'd be talking about blowing vapor for a full hour.

However, fill both stills with 30% alcohol, and blow vapor, and you'll clearly see that it is possible to hit the 25%LEL in something closer to 15 minutes.  Garbage truck backs into your SUV in the parking lot, you run outside flustered and lose track of time, because you are having a shitty day, you have a water issue and the condensers lose water flow.  You'd still need to blow vapor for about an hour to hit the actual LFL.

Point being, under normal operation it's highly unlikely to liberate enough vapor to ever hit the LEL of ethanol, it's only under a catastrophic situation that this would happen (read up on your area classifications, as this is a really important point).

Also, looking at it from a storage perspective.  Assuming only 1 air exchange per day, you would need to lose approximately a full 10 gallon barrel of whiskey at 120 proof to hit the LEL in the same scenario.  Think about this, you would have to evaporate nearly a full 10 gallon barrel of whiskey, every single day, to get to the LFL, that's losing more than 3500 gallons of whiskey a year.  Again, highly unlikely.

All that said, when you spill ethanol, realize that none of the math above is relevant anymore.

I have been asked by a neighbouring distillery for some numbers he can use to counter a ridiculous request from our local authority regarding build-up of ethanol fumes in his bond store.

I haven't read all this thread recently, it is late here in Australia and I need to get to bed.

My conclusion is similar to Silk's conclusion, except I don't believe alcohol vapour will pool in a hot distillery because of dilution with air by thermal air currents.( probably discussed earlier on this thread.)


My calculations and response to my neighbour


The lower explosive or flammable limit for ethanol air mix is 3.3% at 25 deg C. This number found on several internet sites.

Your floor area is 360 square metres times height of 4.5m giving an air volume of about 1,600 cubic metres

3.3% of 1,600 is 52.8, say 53 cubic metres or 53,000 litres of ethanol

The whisky in the barrels is about 64% ethanol

So about 82,000 litres of whisky would need to evaporate and all of it remain sealed in the shed before it could potentially catch fire and probably explode.

You say it is at capacity with 100,000 litres in the bond shed.


Maybe I am misunderstanding the 3.3%. IS THAT 3.3 LITRES OF ETHANOL LIQUID EVAPORATED INTO 100 LITRES OF AIR ??

(please feel free to check my maths, I am wondering why people get so worried about the vapours. Spilled liquid is much more dangerous)



Ethanol fumes are often quoted as being 1.5 times heavier than air so it will sink and follow drains and low areas and possibly find an ignition source, then flash back to its source.

1.5 is Vapour density, it is ethanol vapour with no air present.

But ethanol vapour very easily mixes with air and becomes diluted.

The air/ethanol mix is only very slightly heavier than air and is dispersed and diluted by even slight air turbulence.


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I did miss a step. I studied this subject at University nearly 50 years ago, but as expected some details don't come back spontaneously.

I felt there must be something missing and that is why I posted for help.



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It is quite a calculation to convert the lower flammability limit figure of 3.3% into kg or litres of ethanol.

At only 3.3 % we can regard the 1600 m3 as pure air to make the calculation easier.  At 25 C and atmospheric pressure the density of air is 1.18 kg/m3 and the 1600 m3 will contain 1888 kg of air.

The 3.3% is in terms of molar percent (same as volume % at this temperature and pressure) so we need to convert the mass of air to moles.  The molecular mass (molecular weight in the US) of air is 29 so the 1600 m3 contains 65 kgmols of air.  The 3.3% would be 2.15 kgmols of ethanol - or 98.7 kg of ethanol.

I don't have a full chemical simulator, but a rough eyeball interpolation tells me that if the room were sealed, the barrels open and the spirit at 64 % abv the 3.3 % level would be achieved if the spirit was above 18 C (64.4 F).  Of course the room is not sealed and the barrels are sealed so this is a worst case situation. 

I agree that the ethanol vapour would mix quickly with the air, but it is very difficult to say how quickly so it is safest to assume that the ethanol vapours could sink into drains etc.  I know of men being suffocated by CO2 where it was denser than air because it was very cold and the CO2 sank into a pit - even though it would eventually all be mixed.

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Keep in mind though, the rate of vaporization of stored/barreled ethanol.

Typically, distilleries see barrel evaporation at 10% volume per year.  Based on this, we're talking about 0.03% evaporation per day.

If we look at this in the context of a single day evaporation - 98.7kg of ethanol (total kg necessary to be vaporized) / 0.03% (average vaporization a day) = 329,000 kg of ethanol stored in average conditions (roughly 1500-2000 53 gallon barrels).

Not even sure it's physically possible to store this many barrels in 1600 cubic meters.  In addition, as little as 2 complete air changes *per day* would mean LFL would not be hit.

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