Building a safe low-temperature heat box: heat sources

Question

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 optimized. 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-metallic 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.

Answer 1

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.

Answer 2

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.

Answer 3

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

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

this us a common requirement for incubators... a 100°F (99.5-99.8°) constant temp with intermittent door opening (swift reheating). Commercially avail incubators are $1000!,so many backyard farmers make DIY incubators out if well, anything they can get. My company is actually designing a DIY kit, which is why I'm a semi-pro with the concepts at least. #1) I would like to know the name of the temp controller you use/like. And now, on to my answer: The most effective solution is a dual heating option. One acute, one chronic if you will. Many inexpensive incubator controllers ($15 - $50) have two "relays" or power gates that will send power to H1 and H2/C1 (heat source 1, 2 or a cooler/heat remover such as a vent) Some are flush mount and hardwired and can fit into a small rectangle cut into the door (for external control and LCD temp monitoring). See inkbird stc-1000(?). Others are glorified power strip with 2 110v outlets mounded into the controllers housing, that allow you to simply plug in two heat sources or one and an exhaust fan (to pump out excess heat).

For my solution, I want the eggs to remain "at temp" should I open the door, and I want the chamber to reheat quickly after the door is closed again. So let's look at the acute (reheat) system first, as it's the easiest. The easiest way to move hot air is to blow a muffin fan over/thru a PTD heat source (or suck air from it, pending placement. A PTD heat source allows for finer tuning as the power can be regulated. As the unit starts to cool, a little juice turns the heater source on a little. It's a variable power input/Wattage output capable element, often made from ceramic with aluminum fins for heat dissipation. These are cheap on Amazon nowadays... $10-$20 for a 500W PTD heater brick with mounting tabs. With a little bent sheet metal, you can build a heater/fan unit or just use some pex tubing and make spacer bushings to create airspace for air to get into the fan and so the fan isn't too close to the heater. Look at incubator warehouse for their incustat system. They have their own model with controller and 1 or 2 heater/fan units pending the size of your chamber. Properly spaced, these will bring your chamber back to temp quickly. You could also have a metalshop bend a metal box heat shroud with one/two fan holes and simply set this above a 110v coil hot plate ($11 on myshire farms website Amazon store, or cheap from goodwill?). This allows you to have plug and play components that plug into an outlet that is wired to your controller, allows you to adjust the temp setting on the coil burner- lower in summer, higher if colder, or just right inside your home), and if any part fails, plug in a new one, Bing bang boom. Myshirefarms you tube has a Poppa builder of things Playlist that shows a large cooler being turned into a "gigantor" incubator... just under °100F per their needs.

Another cheap all in one blower/heater option can be scavenged from a harbor freight fake fireplace stove (or ebay, Craigslist, or FB marketplace). I bought one new for $40 at HF and these often have the advantage to include a temp dial, a light switch and the fan is already encased in a metal box and is a cylinder shaped blower fan, vs a radial muffin computer fan. The drawback is that if possible, you don't want to have to run a fan 24-7 for ever, so a chronic temperature solution, in conjuction with the forced air option dual set up is ideal. It would be great if the stire-bought heat mats went to 100°, but most are limited to °80 for reptiles or seedling mats. I like the idea of a custom mat, ni chrome wire with heat shrink or insulated with high temp silicone snaked and stitched into a sandwich of drawer liner silicone mats... a diy insole footballer video is available on YT. But that requires math to calculate the resistance and wire diameter needed per length, etc. You could also add some rice or beeds to more evenly distribute the heat (think electric oatmeal heat sacks folks put in the microwave) but probably not necessary in your application.
I am working on a winecooler at present and am thinking about threading premanufactured PTD cable or cord through the copper tubing that runs behind the back wall of my cooler, since the compressor has been removed and I have access to the tubing. This would give the box a low powered evenly dispersed and safe uniform heater. Using this to run the closed door heating (H2) and the above mentioned forced air options (H1), most any thermostatic controller could easily handle. Buy two, they are cheap and use one sensor on the rack and one sensor on the back wall and your waxing eloquently to say the least.

Answer 8

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.