I have been working with OA evaporation lately and thinking of designs to improve on application methods. 

My ruminations ion my diary evolved into discussion on my forum:
http://www.honeybeeworld.com/forum/viewtopic.php?f=10&t=289&p=1529#p1529
and I am repeating one post here because I need some input, particularly if there are any who are current in their chemistry. 
 Fifty years ago, I could have calculated the volume of gas given off by one gram of oxalic acid dihydrate in a jiffy, and probably found the heat of fusion and the heat of vapourisation of the dihydrate, too.  These days, I'm a bit rusty.

I'm betting we have some chemistry whizzes reading who could offer assistance.
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Without going into the ultra compact design, which involves engineering and expense to prototype, there are a number of possibilities for a hivetop applicator.

A blower could be as simple as a smoker bellows, but I prefer a muffin fan. Alternately, a pulse from an air tank and small compressor could be used.

An electric iron has good temperature regulation and heat inertia, but requires 600 to 1000 W of 110 V power, depending on the model selected. The draw is intermittent, but that draw means either a big inverter and some batteries in addition to a vehicle with an alternator -- or a big generator if more than a couple were to be run at once.

The iron would be mounted inverted under a centre hole in a deep lid. The OA is dropped through the hole and then the air stream is applied through the same hole or nearby. The iron surface set on "rayon" should be very stable in temperature and have enough heat in its mass to sublimate the OA with minimal or zero decomposition, unlike some current commercial models which have no regulation and depend on airflow for modulation of the temperature.

My chemistry is rusty, so I have not calculated the actual volume of gas generated by 1 gram of oxalic acid dihydrate, but the interior volume of a hive body is only 1.5 cu ft before the frames and bees are considered and the frames are 0.8 cu ft., leaving about 0.7 cu ft of airspace per box. That is not much air. Current power applicators inject far more.

As for a compact, battery model, electric drills, electric snippers, etc have established systems for charging, and standard spare and replacement batteries available for affordable cost, so that is a start. The evaporator design depends IMO on a number of factors. At present, I do not know the heat of fusion or the heat of evaporation of the OA component. Water properties are well-known. These numbers determine the heat input required to achieve the evaporation and the feasibility of using battery power. Both the heat of fusion and the heat of evaporation for the measured quantity of the dihydrate must be supplied, along with enough energy to overcome any heat leakage or losses over a short time in order to achieve satisfactory and complete sublimation. The sublimated OA must then be distributed quickly. 

My understanding is that the vapour phase is transitory since the cooling air will precipitate it very quickly. If achieving a very fine particle is all this elaborate process accomplishes, then I wonder why we need to heat the OA at all. Maybe just grinding it very fine and blowing it into the hive would accomplish the same end result?

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