# Building a safe low-temperature heat box : heat sources

I'm not sure if this is more of a physics or electronics question, but here goes:

I use a heatbox with a temperature in the range of 80-100F for warming wax (for sculpture modeling). My current heatbox is a peltier-based thermoelectric wine cooler with the poles switched on the peltier and an external thermostat controlling the power to the entire unit.

I have an old 7 cubic foot freezer and am debating on which heat source would be best. The issue is (1) i don't want any heating component within the freezer to be able to get above 180F or so and (2) i want effective heat transfer (therefore I'll either have to add a fan to recirculate air or use something that already integrates a fan).

Options:

1.) I know I could use a peltier (can't reach 180F with normal ambient room temperature, too big a differential) but I am thinking it's a rather advanced solution for a rather simple problem. Also I'm not sure if it can effectively bring the heat up at this larger volume. My current heat box (maybe 2 cubic feet) heats very slowly (24 hours for the wax to reach specified temp), I am not sure if this is in part because the fan is only on when the peltier is on and the air does not recirculate once the air temperature reaches the thermostats setpoint.

2.) A lightbulb has high surface temperature and the potential for breaking, though perhaps a very low watt halogen would be more appropriate. Many people use lightbulbs for this purpose but to me it sounds risky.

3.) A low-temperature heating pad type device would have to have enough surface area for a low surface temp and enough wattage to heat the box itself. Many have integrated thermostats which may be difficult to disable (may be deeply embedded) if they prevent the heat pad from operating in an environment above 80 or 90F (just need it to get to 100F max, so it's close). I could make my own out of nichrome wire embedded in silicone or ceramic and give it just the right current to keep the heat in range, but this seems more complicated than the peltier.

4.) I have a low watt hair dryer with an output temperature under 180 (on low) which has the advantage of acting as an air circulator. If located outside of the box with a metal inlet and outlet pipe its electronics are safe from wax getting on them and the air flow pattern can be optomized. I am concerned though that in the event of a thermostat problem it could overheat from recirculating hotter and hotter air, although it has protections built in for many such conditions (both a bi-metalic safety and a heat-fuse as well as a GFCI). It is not intrinsically limited in heat, is my concern.

5.) They sell recirculating tank heaters for automotive and aquarium purposes. One of these could be rigged up to copper tubing running throughout the heat box and generate a safe surface temperature and good heat exchange (fan would still be necessary though I assume). This would absolutely guarantee a low surface temperature to all heat components but has the potential for other safety issues I imagine? This seems like a complicated solution, though, and possibly more expensive.

Recommendations?

EDIT: Note I already have a controller I'm happy with and I don't need precise temperature control (+/- 3.0F has worked fine for me in the past), so I don't need heating solutions that inherently have tight control, I'm just seeking safety/simplicity recommendations, mainly.

• DO NOT use a on/off low speed switching rate thermostat top control a Peltier device. if the switching speed is slow compared to the thermal time constant then you will thermally-mechanically destroy it earlier than needs be. ie if the Peltier shifts in temperature significantly during a switching cycle change something. – Russell McMahon Nov 13 '12 at 23:39
• To limit maximum tempoerature safely you could add a large quantity of a phase change material that changes phase (melts, usually) at slightly above your max target temperature. Your wax is already a PCM - if you can adjust some wax so its temperature of melting is slightly higher it would do well. – Russell McMahon Nov 13 '12 at 23:41
• Do have a good look at my suggestion - just because nobody votes for it doesn't mean it's not a really good idea :-). It's hard to go past something which is self regulating , passive and relatively low cost and easy to implement. – Russell McMahon Nov 15 '12 at 11:30
• Hi Russel, I am having trouble understanding your idea in the nuts and bolts sense, a better description of how it would be implemented maybe would help. It sounds like it's not something I can 'set and forget' and leave unattended for a week then open it up and have warm wax ready to use, which is what I need and currently can do with my peltier heat box which has been on constantly for 2 years straight. – birchbark Nov 16 '12 at 17:33

It will be difficult to build something which is intrinsically limited by its heat output to a low temperature but still brings the contents up to temperature quickly.

Making some assumptions about the box:

• Wall thickness: 3 inches
• Surface area: 4460 square inches (random 7 cu ft freezer online, inset dimensions by half a wall thickness on each side)
• Insulation R-value: 1 (m^2 * K) / (W * in) [estimate based on Wikipedia numbers]
• Temperature difference: (100 - 72) = 28 deg F = 15.6 K
• Power to maintain temperature difference = $\frac{\text{surface area } * \text{ temperature difference}}{\text{wall thickness }*\text{ R-value}} = \frac{\left(4460 \text{ in}^2 \left(\frac{25.4 \text{ m}}{1000 \text{ in}}\right)^2\right) (15.6 \text{ K})}{ \left(3 \text{ in}\right) \left(1 \frac{\text{ m}^2\text{ K}}{\text{W in}}\right) } = 15 \text{ W}$

To guarantee that the temperature inside the box passively stays around 100 F, we would have to limit the heater's power to 15 W, which isn't much compared to the thermal mass of big hunks of wax. If the box is better insulated, then the power must be even lower. This also assumes that the temperature inside is completely uniform. If not, the power must be lower still to guarantee no hot spots.

I like the aquarium heater idea. It limits the possibility for overheating the wax both by the safety features of the commercial heater, and by the boiling point of water.

• The wax goes in cold, also, and I open the door often when working sometimes. On the other hand the wax, once warmed, holds its heat very well. – birchbark Nov 13 '12 at 23:26
• I am leaning towards the aquarium heater as well, if the complexity of setting it up is not too high and the durability not too low. It is doubly limited as you say, both by the thermostat inside it and the limits of water. This is why they use water-jacketing for commercial wax melting pots (which it would seem easy enough to engineer with just hte proper current running to a nichrome-ceramic jacket). – birchbark Nov 13 '12 at 23:31
• I recall having to make an environmental chamber to test an electronic box from -40 to +40C so I used two PC fans with Dry ice then replaced with a 25W solder iron. It worked perfectly and fast. so 15W sounds close. I think the electric car winter interior heater is a better bet if you can find one in your area. ( not florida) – Tony Stewart Sunnyskyguy EE75 Nov 14 '12 at 7:14
• I doubt 15W will do, keep in mind I already HAVE a heat box that's smaller. It's peltier based and it claims to consume 100 watts when running. In non-winter conditions it ran 4 out of every 10 minutes, therefore 40watts, at constant use, was necessary to maintain the wax temperature at 90ish. Now it is winter and the local temperature next to the heat box can get down to 60 degrees at night (turn house heat down). It runs quite frequently, but not constantly. So less than 100Watts, but close. And this one is 1.5 cubic feet, I'd say (though not as good insulation as the freezer). – birchbark Nov 16 '12 at 16:44
• If the lid is sealed and has good insulation 25W should be adequate and 100W will heat up faster and 750W heat up fastest but have higher risk of temperature gradients. – Tony Stewart Sunnyskyguy EE75 Nov 16 '12 at 17:10

You don't talk about budget, but Omega Instruments is just fantastic at this stuff, and very competitively priced. For less than $100 (well less, in fact), you can get a PID self tuning ramp-soak temp controller that uses fuzzy logic, and you won't have to worry about overshoot if you do it right. I suggest a heater in a tray of sand to even out your thermal mass, but the controller should be able to handle much of what you can throw at it. Pop a cheap thermocouple on your system, and you're good to go. Omega has some good selection guides that I've linked to below, and their customer service is great. Pick an appropriately powered heater, using the equations that others were nice enough to provide, close your loop with the thermocouple. Then, it's almost as simple as entering a self tune mode, keying in your ramp time and final temp, and hitting go. Of course, it's way overkill, and can do much more than you need it to do, but it really is inexpensive. Many of these controllers have serial outputs if you need that. Temp controllers Heaters UPDATE: I see I'm out of your budget from your post below, but Omega is still a nice source for reliable heaters at reasonable prices, like these Silicon Rubber heaters. You can start with these, controlled however you want to control them, and add a thermocouple, relay, and maybe a controller later. • I like their heaters but they're a bit pricey. As far as a controller I think a lot of people are misunderstanding, I am not interested in control advice, the issue is more in the most appropriate heating sources and or if there are any inputs on circulation perhaps. I have a thermostat/probe/controller I am happy with. – birchbark Nov 14 '12 at 1:41 • Sand is a great idea for a cheap, safe item with good thermal mass to coat a heating element with a high surface temperature! Downside is it has to be mounted horizontally, preferably at the bottom. And you don't want to get sand on your wax so I'd want to cover it with metal or silicone (I have lots of silicone handy because I make molds with it). – birchbark Nov 14 '12 at 1:44 revised Firstly, there are several design asssumptions that need to be validated and considered; 1. Heatup time , working time and frequency of reheating • This has to do with the type of wax, melting point, room temp, oven temp, heat source, heat energy/mass ratio, & insulation R factors. • How much time is wasted waiting for reheating, 50%? 30%? • Idealling you want temperature homogeneous to do coarse work at any depth. • Heating time will increase the lower you set the hot air tempearture. • Working time will decrease the cooler the room is but you may not want to work in a sweat shop. • Budget - low cost and simple to implement • Safety cutout a must in case of fan failure and regulator failure Recommendation • Use a 120Vac thermostat heater with a constant fan INSIDE your closed freezer set to 85'F or use a Honeywell thermostat rated for 120Vac with your hair dryer and wire up the heater but keep the internal fan circulating for homogenous air temp. • Add a couple large muffin fans to keep the air circulating to reduce heating time. • Add an additional safety thermal switch in case of overtemp to shut down • Get a light dimmer switch and a couple cheap 300W Halogen work lamps and use them as heat lamps WHILE WORKING to radiate heat to the wax from a distance of approx. 3 ft remembering the inverse square law where power doubles at half the distance. • added bonus of seeing better what you are working on and extended working time. Microwave may only work on special wax with a plastic powder additive that works. Beeswax has a special property of being transparent or lossless at microwave frequencies unless it is dirty. The same may be true for parafin wax, but I am not sure. If it did work, it would be quick and homogeneous temperatures, but more guesswork. A 1500W heater is overkill and may overshoot 85'F somewhat with low duty cycle but an additional internal ciculating fans will increase the rate of regulation and reduce overshoot but may wear out the thermostat sooner Oh well they are cheap. A professional oven uses proportional heater control. Another one • I'm not looking for solutions to my working process, I've been working with wax happily with my existing heat box for several years. I take advantage of the wax cooling down while working (I model quickly and rely on the cooling to add strength to the work so I can work with minimal armature). Also not really looking for ideal performance in terms of heat-up-time, etc, just OK performance. – birchbark Nov 16 '12 at 16:48 • Okay, trying to process your answer: by car heater I assume you mean a 12volt forced-air heater. How is that better than a blow dryer or AC ceramic heater? How does one mount it outside of the freezer so that wax cannot get into its internal components or onto its heating element; and why add the muffin fans if the forced air heater already contains a fan can't you just duct an inlet which receives cold air from the bottom of the freezer and an outlet which pushes hot into the top causing circulation? – birchbark Nov 16 '12 at 16:51 • The reason I considered the blow dryer was because it is easier to hook up an inlet and an outlet to because it's small size and circular form factor. It can be used with 3" diameter clothes dryer ducting or stove piping (I believe ABS/PVC wouldn't be as safe heat wise). A square form factor heater is more difficult to hook up ducting too as far as I can tell. Also you don't want the ducting to be too big because it will release heat. – birchbark Nov 16 '12 at 16:55 • Also, I already have a thermostat/controller/probe I like, so I would not use the integrated thermostat on any heat source I purchased (besides that they often don't go as high as I need and are not probing from the proper location). – birchbark Nov 16 '12 at 16:58 • I meant the 120Vac car heaters. The reason you dont use outside air is that it uses that air to regulate the heat flow and temperature. A hair dryer has no regulator. The reason for fans is in case the fan also shuts off or in case of fan failure. – Tony Stewart Sunnyskyguy EE75 Nov 16 '12 at 17:01 This is not really an answer but too long to be read comfortably in a comment: 1. What is your budget? 2. Also, have you considered indirect heating? In physics laboratories, if people want to keep a volume that burns easily (in the sense that it discolours) at a constant temperature, the go-to solution is a curie bath. Basically the idea is to have a container with the stuff to be kept warm "floating"/"hanging" in a tub of water which is kept at a constant temperature. You can find off-the-shelf solutions for keeping a water-bath warm. Look for immersion/circulation heaters. Depending on the consistency of your wax, you could even forgo the "container" for the wax and simply put your wax blocks in zip-lock bags (try to get all the air out) and let them float directly in the water. • 1.) Originally I wanted it to be free (the fridge was, and old hair dryers and pipe are, and I already have a thermostat that cna handle 1000 watts and does up to 109 degrees). I could spend up to$50 otherwise, so long as components are going to last. – birchbark Nov 13 '12 at 23:21
• 2.) I tried many solutions in the past, dry heat boxes are best. I go in and out of the heatbox constantly while working, water is messy and interferes with the adhesion of the wax, and if it gets in the wax at all accidentally in the process is dangerous when remelting it. I use lots of wax, and I use it quickly. I also like to be able to throw old sculptures (irregular shapes) in the heat box to soften them before breaking them down. Definitely no go on a water bath. – birchbark Nov 13 '12 at 23:25

Higher melting point phase change material buffer.

• Low tech

• Easy

• Cheapish

• Passive

• Non-electronic

solution

• that can actually be made to work superbly

Don't pass it by without a look at what's involved.

To limit maximum temperature safely you could add a large quantity of a phase change material that changes phase (melts, usually) at slightly above your max target temperature.

The mass can be arranged so that to melt it all with the energy available is impossible as the extra surface area provides more energy loss than is available at elevated temperatures. It only comes into action when the wax in the system has melted - usually its just an insulating layer. Simple radiating fins can be added to this buffer PCM that only provide a significant thermal path when their PCM buffer host starts to melt.

Your wax is already a PCM - if you can adjust some wax so its temperature of melting is slightly higher or if you can buy some wax that melts at a somewhat higher temperature than what you use now, it should do well.

If you want a material with much faster energy transfer rate look at the metallic PCMs. Gallium and similar. Search "Woods metal" (wrong temperature for you) and follow the thread from there. Large range of temperatures available.

Wikipedia - Phase Change Material
Coffee Joulies as a demonstration.

Related:

DO NOT use a on/off low speed switching rate thermostat to control a Peltier device. if the switching speed is slow compared to the thermal time constant then you will thermally-mechanically destroy it earlier than needs be. ie if the Peltier shifts in temperature significantly during a switching cycle you should alter your control method. I'd imagine that a switching rate of faster than say 10 Hz would begin to be safe - with higher safer.

• Looks like sculptors have a wide range of wax melting points from 65~110'C Bee, parafin, Microcrystalline etc. sculpt.com/catalog_98/Wax/Micro.htm#bees – Tony Stewart Sunnyskyguy EE75 Nov 14 '12 at 5:47
• Sorry, I don't understand the practical application of what you're suggestion. Are you saying to constantly insert new, pre-liquified wax att he right rate to keep up with the amount of wax and temperature of the wax that I am trying to warm up. This sounds absurd so I am 99% sure I am totally misunderstanding you. Requirements wise I need a system I can just open the door to, toss in an arbitrary amount of wax blocks, and then remove, fully warmed blocks at a predictable interval afterwards. And the thing has to be able to run safely 24/7 unattended/untouched. – birchbark Nov 16 '12 at 17:14
• I use microcrystaline with a melt temp supposedly of 180F but I find for it to liquify fully is more like 200. But my 'working' plasticity temperature is more in the range of 80-100F. – birchbark Nov 16 '12 at 17:15

Self-limiting heat cable (also called self-regulating heat cable or cord) fits all of these requirements. Self-limiting heat cable is basically two wires with a polymer between them whose resistance increases in proportion to heat. Basically, as the polymer expands from heat it becomes less conductive to the point where at a certain temperature of expansion no more current can travel across it.

Self-regulating heat cable is a type of "positive temperature coefficient" heater. It really is the only easily available PTC heater that self-regulates to the lower temperatures needed for a heat-box (150F models are common if I recall). There are other self-regulating heat sources that rely on a PTC effect, but they have reach much higher temperatures.

The other great advantage of this heat cable is that it can be installed in any configuration within any enclosure and at any required watt-density via doubling up or buying a higher watt-density to begin with. If a more rapid exchange of heat is desired you can simply add circulating fans.

Unfortunately it took years of various heat box designs before my research turned it up as a heat source. I'm not sure why no one was able to bring this to my attention as to my knowledge it is the only completely safe low-temperature self-limiting electrical heating possibility. Even using hot water or other liquids as an intermediary for heat transfer means the possibility of explosions (from pressurized water) or boiling-out of the water followed by dangerous over-heating and burn-out of the element.

Probably a bit late now but I'd try to get hold of an unwanted slow cooker, lots of people bought these and then left them unused at the back of a cupboard [U.S. = closet!]. Freecycle or similar may be able to provide. These have a long silicone-covered heating wire wrapped around the pot, generally glued on with more blobs of silicone. Slice it off carefully and re-silicone onto a sheet of metal at the bottom of your old freezer, connect your temperature controller, job done.