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HF concentration in a fume hood duct

2007

I need to provide our cleanroom contractor the potential HF concentration in the ductwork, if I have an open 50% HF bath in the hood.
It looks like the limiting transport factor here is the HF diffusion coefficient in water. May be I have to consider the water evaporation also.
Please help.

Iuliu Blaga
Microfab Engineer - Dublin, California, USA

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First of two simultaneous responses -- 2007

Whoa, there!

You're way into chemistry theory there. The diffusion coefficient of HF in water plays a role...as does the exact temperature, the rate of flow of air over the surface of the bath, the pattern of the flow of air over the surface of the bath...give me 30 minutes and I'll think of 3 - 4 other things.

I would do this: leave the HF in a place where it may be safely left open to the air, in a rectangular or cylindrical container. Have the temperature be the same as the temperature of the actual bath, in service. Leave it for, say, 72 hours. Determine the exact concentration of it, before and after, via acid base titration.

Now, by subtracting, you know the amount of HF that was released. Calculate the surface area of the actual bath, and the surface area of the test container. Divide the first number by the second one. Multiply the mass of HF lost by this ratio. Now, divide by three.

You now have an approximation of the mass of HF that will be lost in a 24 hour period. If you know the flow rate of air exhausted by the fume hood blower, a little arithmetic gets you to the approximate concentration of HF in the exhausted air.

It might also be possible to determine the air concentration in the air above the process bath with a Draeger tube, or similar device. Then, some calculation gets you to the number you need.

This leaves the main simplifying assumption that the rate of loss of HF from your little test vessel is going to be the same as from the process bath.

Perhaps somebody sees an easier way, but...it's a couple of acid-base titrations, and some math. HF is very nasty, and in the off chance that some accident happens down the road, everybody will sue everybody. If you have something like this documented, you'll be happy you do.

Dave Wichern
Consultant - The Bronx, New York

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Second of two simultaneous responses --

Hi Iuliu,

I don't understand your question ..... IF, IF, IF ... if you are worried about a future HF system, then please put a wide angle & easily removable spray jet in the ducting.

The ducting would lead, must lead, to a scrubber of some type. Materials? PVC. And slope the ducting by 2% so that you'd get proper 'drainage'.

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).
2007