Hemp me dehumidify
April 25, 2012 11:34 AM   Subscribe

How can I quickly reduce relative humidity in a small enclosed environment within a fume hood without raising air temperature?

We have a small chamber (30x30x30cm) inside a fume hood. The chamber is not gas tight, it just serves to protect the apparatus inside from the fast moving air outside (i.e. the fume hood environment). We do this because we're drying a film on glass inside the chamber and we want it to dry in still air so that it drys as evenly as possible.

Our problem is relative humidity, which we need to keep at about 25% to make good film. For safety reasons the lab has to be a single pass air system, that means that the air conditioner takes whatever air mother nature gives us does its best to heat or cool it and pumps that into the lab, then the fume hoods suck that out and dump it. It doesn't get recirculated because it contains the bad stuff from the fume hood. Thus the air handler only gets one pass at conditioning and so we have poor control of humidity within the lab. What we want is a way to aggressively remove moisture from the air within that small chamber.

Here's what we're already doing or thinking about:

Today we heat the air inside the chamber which reduces the RH, but the increased temperature also interferes with the film formation.

Typical desiccants like silica gel just don't work fast enough.

We are looking at pumping dry compressed air or dry nitrogen into the chamber.

What I would really like is a vigorous chemical reaction that strips water out of the air. We're a chemistry R&D company so we should have no problem acquiring the materials.
posted by Long Way To Go to Science & Nature (9 answers total) 1 user marked this as a favorite
You've probably already thought of this, but is it not possible to get some powerful dehumidifiers for the whole lab? Either sitting around the room or attached to the air conditioner so it dries incoming air? That seems like it'd be much simpler than rapidly drying the air in the chamber.
posted by randomnity at 11:42 AM on April 25, 2012

Since you need to have ~25% humidity, a vacuum chamber is out of the question.

How long does it take the dry the film?
posted by Slackermagee at 11:43 AM on April 25, 2012

**meant to say I guess a vacuum chamber is out of the question
posted by Slackermagee at 11:44 AM on April 25, 2012

Do you have moderate money to spend? Desiccant based commercial dehumidifiers can knock humidity out of a room REALLY fast. You could also use a smaller one and direct the output toward the hood. Some will do this without raising temperature. That's probably your best bet.
posted by pjaust at 11:44 AM on April 25, 2012

That chamber is pretty small. I'd think pumping air/nitrogen would be your best bet. If you need to up the humidity to a certain level, you could introduce a small volume of water inside the chamber.
posted by beepbeepboopboop at 12:02 PM on April 25, 2012

There are commercially made constant humidity chambers (google for a whole slew of them), but I assume you've already rejected this idea. So what about modifying your little box to include an airlock? You could pass stuff in and out without wholesale exchange of the air inside.

To maintain a constant humidity inside the box, you can use concentrated solutions of certain salts. The CRC Handbook has tables of desired humidity and the corresponding solution to use. This is slow, so you'd want to set up your chamber in advance and let it equilibrate, then use the airlock to avoid changing the atmosphere as much as possible. Of course, you'd also need a platform for your sample to rest on, to keep it out of the solution.
posted by Quietgal at 12:20 PM on April 25, 2012

Forgot to mention I was thinking about a sort of glove-box arrangement, so you could manipulate your sample as required. So why not get a ready-made glove box? (Several suppliers offer various sizes and models; this just came up first on Google). They have an airlock already built in, and you can get one that will fit in a fume hood. Load it up with dessicant or constant-humidity solution as needed, and let it get good and dry before you introduce your sample.
posted by Quietgal at 4:06 PM on April 25, 2012

In the summer I dehumidify my swamp-cooled house by lighting a candle. Would a small tea light raise the temperature?
posted by TooFewShoes at 6:32 PM on April 25, 2012

I would try lowering the overall room humidity with some standard dehumidifiers, as randomnity suggested, but with the lab's air turning over so quickly they might just get overwhelmed. Still, it wouldn't be hard to try. You could also get one (or a portable air conditioner) and point it into the fume hood to get some dry air in that area. Maybe even duct it into the hood's intake for maximum effect.

If that doesn't work, I think you're probably down to pumping dry compressed air into the drying chamber. You should be able to regulate it and baffle it such that it doesn't make too much wind, all you really need after all is a little positive pressure, not a gale of dessicated air. That's good because it means that you won't be blowing through air cylinders too rapidly. There might be commercial solutions for this but if anyone in your lab is handy enough I bet it wouldn't be too hard to rig something up on a DIY basis, as long as you are OK with a little trial and error at first.

Good luck.
posted by Scientist at 9:42 PM on April 25, 2012

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