annaandherdad
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Hot Box
I'm slightly embarrassed to post this, in view of the sophistication of so much of the work on this forum, but it might be of interest to someone. In
this project as in many others my main purpose is to do something interesting and educational for my daughter and other kids, while practical and
economic considerations are secondary.
My photos were too big to upload to SM so I put them on my website. The link is:
http://bohr.physics.berkeley.edu/lab/lab.html
Some time ago I was working on a project of making silver mirrors, and I needed to evaporate some solution of silver nitrate in water and residual
nitric acid. For this I used a plastic kitchen container with NaOH as a desiccant. Unfortunately, my garage is rather cold, and it took more than a
month to dry out the silver nitrate completely. I figured there had to be a better way. I could have bought standard desiccation equipment (and a
vacuum pump), but I thought it would be more educational and fun to build something.
In view of the strong dependence of vapor pressure on temperature I decided to build a "hot box", a box with heating elements and a thermostat so I
could control the temperature.
I made the box of wood, with a layer of insulation on all inner surfaces (the insulation is pink in the photos). The apparatus was placed on an inner
floor, separated from the outer floor by the insulation layer, and supported by wooden pegs that poke through the insulation. Above the inner floor
is a set of ventilation and support ribs, between which are arranged a set of parallel resistor chains. I could have used light bulbs or something
like that for heating elements, but I wanted the heat to be as even as possible, without any hot spots that could produce a danger of overheating and
fire.
So I used chains of four 3.2K Ohm resistors in series, which dissipate altogether about 1W at 120 volts (standard US voltage). Each resistor is rated
1/2 W, but only dissipates 1/4 W in my setup, so they are within specs. When I started I didn't know how much power I would need to dissipate inside
the box to achieve a reasonable temperature, since I didn't know how effective the insulation would be. So I experimented, starting with just 1 Watt
and building up. I eventually installed 23 Watts (23 resistor chains, or about 100 resistors), which maxes out at about 60C inside the box when left
running for a long time. This is hot enough to evaporate things quickly, without running any danger of fire.
I installed a thermostat that allows me to set the temperature anywhere between garage temperature and 60C. If I had it to do again I'd set up
something electronic with a thermistor, because this thermostat has about a 5 degree range between turning on and cutting off. So the actual
temperature oscillates by this amount when it's controlled by the thermostat.
I've used the hot box with the top open and an evaporating dish in it; there's a picture of this in the photos. This is just using the heating
elements as a gentle source of heat. I've also used it completely closed under control of the thermostat. In those applications I put a fish tank
inside the hot box, which contains vapors (water and maybe acid) and prevents them from coming into contact with the workings of the hot box. I got
the fish tank from Carolina biological supply. I also built two pairs of small glass shelves (shown in the photos), which can hold trays of CaCl2 or
other desiccant.
The hot box works very well for rapid drying of things, for example, the silver nitrate project is finished in about a day. We've used it for quite a
few other crystallizing and drying projects, too.
We've used the hot box open for evaporation of quite a bit (36 liters) of seawater.
The hot box works well for melting honey that has crystallized. It's important that to keep the temperature below about 35C for melting honey.
It would be interesting to have a hot box that could be controlled in the range 100C to 200C. Things to do that require temperature control include
subliming oxalic acid (it makes beautiful, needle like crystals, but too much heat and it decomposes, producing CO), and making plaster of paris from
gypsum. That might be a future project.
Any other SF Bay chemists?
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elementcollector1
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For higher temps, how about nickel-chromium wire? Same stuff used in toasters.
Glad to see such an interesting project!
Elements Collected:52/87
Latest Acquired: Cl
Next in Line: Nd
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zed
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Ummm. For crude work, I like to use an large inverted cardboard box with a drop light under it. The bottom of the box can be propped up a little
bit to adjust internal heat. Great for drying sweaters on top of. Also great for chemical reactions that require moderate heat for a long period of
time. Though in that case, the reaction vessel would probably be kept inside of the box. A very convenient way to produce Nitro-propenyl-benzenes.
Mix up your aromatic aldehyde, some nitroethane, a little butylamine, and some ethanol.......Place in a foil wrapped mason jar to exclude light,
and stick it inside the box with the droplight. Prop up the bottom of the box, to adjust the temperature, until contents are held at a stable 100-120
F. Let your reaction proceed for a week. Great yields with minimal technological expense.
[Edited on 10-4-2014 by zed]
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Muffn Man
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I have seen smallish setups that use incandescent light bulbs for heat. Just keep them under a cookie sheet or put your liquids in a stainless steel
container (if the reaction/chemicals are compatible). Using a thermocouple with a Solid State Relay with an Arduino is very popular. You can also get
complete kits with a controller, solid state relay and thermocouple from ebay.
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annaandherdad
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Thanks for the tip about thermocouples and arduinos. I'll look for that.
Any other SF Bay chemists?
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Bert
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Mood: " I think we are all going to die. I think that love is an illusion. We are flawed, my darling".
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Use a PID for temperature control- It learns the response time of your system and anticipates.
You can get VERY stable temperatures.
Combined with an appropriately sized solid state relay, it will switch as much power as you care to use.
[Edited on 10-4-2014 by Bert]
Rapopart’s Rules for critical commentary:
1. Attempt to re-express your target’s position so clearly, vividly and fairly that your target says: “Thanks, I wish I’d thought of putting it
that way.”
2. List any points of agreement (especially if they are not matters of general or widespread agreement).
3. Mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism.
Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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Muffn Man
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PID is the easiest way to go. If you like learning and have a lot of time on your hands, though, an Arduino can open up a whole world of new and
exciting experiments.
As a note on Solid State Relays, make sure that the "Input" can take 3-5 volts DC (supplied by Ardino or PID controller) and that the "Output" voltage
(the controlled load, in this case would be your heat circuit) can handle 120volts Alternating Current (household current) or more. This one:
http://www.ebay.com/itm/NEW-Solid-State-Relay-SSR-40DA-40A-2...
for instance, is rated 24-380 Volts AC Output and 3-32 Volts DC for the Input.
Here is a hotplate build thread where the poster machined a large piece of aluminum to accept heater cartridges and a thermocouple combined with a PID
controller. Nice stuff.
http://www.sciencemadness.org/talk/viewthread.php?tid=23989#...
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aga
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Very nice project, and a great idea.
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