: A question for those of you in the frozen/freezing/thawing north..

: I've been building a set of display freezers - double glazed
: acrylic box with stainless plates for Ice sculpture exhibition.
: They're 400mm cubes, with a small fan circulating air. The
: stainless plate is 3mm thick with the refrigeration evaporator
: tubing soldered to the underside.

: The problem I'm having is that the 'moist' air is pulled down
: around the edges of the stainless plate, and recirculated up the
: inside of the inner perspex box - as it is pulled down around
: the edges of the plate, the moisture turns to ice crystals and
: gradually blocks the ventilation channel. That in turn causes
: the sculpture to gradually melt from the top down and refreeze
: as a small mound of 'frost' in the bottom of the unit.

: I've been working on a defrost system, but we don't have a lot of
: 'spare' refrigeration capacity, and the defrost I have at the
: moment takes longer than I'd like to clear the channel.

OK, I'm not sure why you put the cooling coils under the ice sculpture. Seems like you would get a much better cooling job with the coils mounted near the top of the box. Natural convection currents would bring warm air up, where it would hit the coil, cool, and drop down again. Having a fan would accelerate the process, but it would not always be needed. But, since the coils are on the bottom, you need to get cool, DRY air to the top.

Can you put in a dessicant to absorb any moisture? If the air is dry there won't be significant humidity to form your ice crystals.

How big is the channel you need to clear? A 15 watt night light would stay warm enough to prevent ice forming. You might even get a smaller one. Stuff it inside the channel and no ice is going to form on it, or within a few mm of it, so the channel will stay open. If you don't want the light visible, wrap the bulb with a piece of aluminum foil, which will transmit the heat but block the light.

: So, the question: Is there something I can treat the edge of the
: stainless plate with to prevent ice forming there? We're talking
: about -15C with air moving over it.

Try painting it with sodium chloride (table salt) calcium chloride or potassium chloride. Make a concentrated solution, brush on a coat to only the areas you want to keep ice free. If you want to build up a good ammount of the stuff, let it dry, brush on a second coat, and repeat for a third coat. Potassium chloride is available at any supermarket as a salt substitute. Look in the spice aisle. You only need a teaspoon of the stuff to try it.

: We spend 99.9% of the year well above freezing here, so this is
: foreign territory for me - for those of you who spend half the
: year building kayaks because the outside world is white and
: solid it's probably an easy question!

Actually, it is not the kind of thing one runs into around Chicago, where the weather gets cold in the winter. Most of the time we try to keep those cold things outside, and if ice crystals form, then so be it. the only ice crystals we care about melting are the ones on the sidewalks and roadways. When it gets really cold then the salt used on the roads is not very effective--so they mix it with calcium chloride.

Minus 15 C works out to 5 degrees F. Not sure if you know that the Fahrenheit scale is based on a solution of equal weights of salt and ice. The brine is a stable zero on the Fahrenheit scale. Pure water would the freeze at 32, etc. Applying a gram of salt to the edge of your plate would be enough to keep a gram of water in a liquid state (no ice crystals) down to a temperature a bit lower than where you are operating. If you want to paint the edge of the shelf with water and drop on some rock salt, or heavy Kosher salt, such as is used for pickling, then the salt crystals might dissolve a bit in that fresh layer of water, and stick to the shelf, looking very much like ice themselves.

The salt absorbs water, and would act slightly as a dessicant, besides preventing ice crystals forming on its surface. If your ice sculptures have a wedding theme, a box of rice would be a neat prop, and the rice, too, will absorb some moisture from the air.

Hope this helps.

PGJ