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RWolff 09-08-2013 01:21 AM

Heat thermostat for special purpose needed
 
I need to construct a small insulated metal box, say a 3 to 4 foot cube for drying out ceramic sculptures prior to being placed in the kiln.

I would like the box to be heated to anywhere from say 150 to no more than 190 degrees F, it can get no hotter than 200 because water in the clay would turn to steam and explode the pieces, ruining them.
I've run experiments using the kitchen oven on "warm" which provides 150-160 degrees and that temperature is fine, warmer still is even better, but the kitchen stove is not meant for this kind of use, and during the winter with the windows shut it wouldn't be good with an unvented gas stove, but it was an experiment.

I have not decided on a heat source yet, it could be one of those metal oil filled radiator type heaters, a series of 100 watt light bulbs, heat lamps, a forced air blower or any number of options, but whichever is selected it will be electric not gas, and I'll need an external surface mount thermostat with a probe or sensor to control it based on the internal box temperature.

Does anyone have any suggestions on what type of thermostat would serve for this use to work in the temperature range I need, and function with NC contacts to open them by 190 degrees F ?

The heat source would be 120v and I don't think it would take any more than 1200 or 1500 watts, quite likely less to generate that temperature in so small an insulated box, the cycle I plan to run the heat in the box would be around 10 hours.
My kiln could do this task but this would add a lot of extra unnecessary wear and tear on the contact relays, electronic board and elements.

I know I've read a few blogs of people who built similar boxes but out of wood for drying bows, fishing rods and other things using a couple or so light bulbs as a heat source and getting in the 120 to 130 degree range, so it would not seem to need a large heat source.

HVAC1000 09-08-2013 01:26 AM

Quote:

Originally Posted by RWolff
I need to construct a small insulated metal box, say a 3 to 4 foot cube for drying out ceramic sculptures prior to being placed in the kiln.

I would like the box to be heated to anywhere from say 150 to no more than 190 degrees F, it can get no hotter than 200 because water in the clay would turn to steam and explode the pieces, ruining them.
I've run experiments using the kitchen oven on "warm" which provides 150-160 degrees and that temperature is fine, warmer still is even better, but the kitchen stove is not meant for this kind of use, and during the winter with the windows shut it wouldn't be good with an unvented gas stove, but it was an experiment.

I have not decided on a heat source yet, it could be one of those metal oil filled radiator type heaters, a series of 100 watt light bulbs, heat lamps, a forced air blower or any number of options, but whichever is selected it will be electric not gas, and I'll need an external surface mount thermostat with a probe or sensor to control it based on the internal box temperature.

Does anyone have any suggestions on what type of thermostat would serve for this use to work in the temperature range I need, and function with NC contacts to open them by 190 degrees F ?

The heat source would be 120v and I don't think it would take any more than 1200 or 1500 watts, quite likely less to generate that temperature in so small an insulated box.

A good option may be one of those old helix style main limit/ fan controls. It has a set of NO and NC contacts with adjustable points

RWolff 09-08-2013 01:35 AM

Quote:

Originally Posted by HVAC1000 (Post 1239361)
A good option may be one of those old helix style main limit/ fan controls. It has a set of NO and NC contacts with adjustable points

I checked McMaster Carr, it looks like from your description something like this might work:

Line Volt Heat/Cool Thermostat with Thermowell


120-240 VAC, SPDT Switch, Built in Sensor


VAC120 to 240

Amps 10

Temperature Range, F +100 to +240

Switch Type
SPDT

Sensor
Built In


http://images2.mcmaster.com/Contents...g?ver=29336088

I bookmarked it to check into and we'll see what else others might suggest too.
Tho this one above is rated for 10 amps, if I wound up using a 1500 watt heater this won't make it. I noticed another one that handles 15 amps but it's electronic, but I'll look at other models in that category.

yuri 09-08-2013 09:41 AM

get a Honeywell L4064 fan and limit control from americanhvacparts.com or elsewhere. comes in 5/8/11 inch lengths. probably more sensitive and faster acting than the well type and cheaper. use the limit control terminal for your cut off. rated for 8 amps on the limit.google L4064 and there is a pdf file from Hwell

you would use the limit control side of it to turn the heater on/off and ignore the fan side

DO NOT buy a used one as it will be inaccurate.

http://www.ebay.com/sch/i.html?_nkw=l4064

Technow 09-08-2013 09:59 AM

Quote:

Originally Posted by RWolff (Post 1239363)
I checked McMaster Carr, it looks like from your description something like this might work:

Line Volt Heat/Cool Thermostat with Thermowell


120-240 VAC, SPDT Switch, Built in Sensor


VAC120 to 240

Amps 10

Temperature Range, F +100 to +240

Switch Type SPDT

Sensor Built In


http://images2.mcmaster.com/Contents...g?ver=29336088

I bookmarked it to check into and we'll see what else others might suggest too.
Tho this one above is rated for 10 amps, if I wound up using a 1500 watt heater this won't make it. I noticed another one that handles 15 amps but it's electronic, but I'll look at other models in that category.

You can always use a relay to control the heater separately from the control.

RWolff 09-08-2013 11:02 AM

Quote:

Originally Posted by yuri (Post 1239417)
get a Honeywell L4064 fan and limit control

DO NOT buy a used one as it will be inaccurate.

Thanks for that, yes that looks like it's workable, or if not it at least sends me in the right direction!
As someone else said, I could add a relay for higher capacity, though I'd rather keep things simple with one unit if possible.

It will depend on what I select for the heat source, I mean I might find it only takes 4-6, 100 watt lightbulbs or heat lamps to get the temperature
and that might take some experimenting to see what it takes to get the temperature I want, and then go from there.

I'll be working out the details and probably start on building it later this fall.

I would never buy used, good advice though! Funny thing is on Ebay I found a dealer selling new ones, more than 10 available, and it says to save on shipping they are shipped without the cover- to use the old cover! LOL I'm sure that little sheet metal cover is REAL heavy!

biggles 09-08-2013 11:19 AM

couple of 250W flood lights on the lid closing down on a metal lined box:wink: I have seen 250W floods in commercial retail spaces banging 120F off the glass lense showing products...during no cooling calls for customers.heard back that 250W flood produces 500 BTU of heat during cooling load calculations..on jobs :whistling2: enclosing it would help you in reaching a controlled temp there...TC-1 (in HVAC LINGO) as shown by other...with a capillary bulb/or probe rod with a desired setting range...then will cycle the bulbs to maintain box temps check this site for bulb styles http://www.cureuv.com/low-intensity-...amp-par38.html

yuri 09-08-2013 11:44 AM

I really don't trust Z Bay as it is not brain surgery to stick old used parts in new boxes or clean stuff up. americanhvacparts.com seems legit and good.

biggles 09-08-2013 06:38 PM

ever consider using a round/rectangular coolers if it is made to hold iced beers and soda same will happen holding heat generated from a light

SeniorSitizen 09-08-2013 09:33 PM

A 1500 watt electric heater will heat a 1800 cu. ft. room in my house with the outdoor ambient around 0F. I'm seeing something like a 100 W bulb or less to maintain the desired temperature in your insulated box of 3-4 cubic ft.

RWolff 09-11-2013 02:09 AM

Quote:

Originally Posted by biggles (Post 1239559)
ever consider using a round/rectangular coolers if it is made to hold iced beers and soda same will happen holding heat generated from a light


Well, the box needs to have at least a couple of open/wire metal racks in it for the stuff to rest on so air circulates around each piece evenly, and there could be several hundred pounds of sculptures in this too.


Quote:

A 1500 watt electric heater will heat a 1800 cu. ft. room in my house with the outdoor ambient around 0F. I'm seeing something like a 100 W bulb or less to maintain the desired temperature in your insulated box of 3-4 cubic ft.

Yeah I wouldn't think it should take a whole lot to do this, small space, can be insulated to any degree and I could only go by what I've seen people do for similar applications to dry fishing rods and bows or raw lumber, using a few lightbulbs, but it seemed most mentioned temperatures around 120 degrees, but of course I wouldn't know how insulated their box is if it even is, or how many bulbs and wattages, but to be on the safe side I figured I would wire and control it for up to a space heater's 1200-1500 watts, though I doubt it would need anywhere near that I'd rather not have to make this over.

I also will probably have some amount of air circulation in and out to remove the moist air, so that will increase the amount of heat required to make up for that loss, it might only need a small computer fan to keep the air positively moving.
Some of this will be somewhat experimental to see how it works and what it needs, the cheapest heat source to experiment with would be wiring up 4 sockets for 4 light bulbs and trying some 60 watt bulbs or 75 and see where it goes. Nice thing about the bulbs is it would be easy to vary up or down simply by changing any of the bulbs to get the temperature I want.
I am aware the incandescent bulbs are on the way out, but I have a few cases on hand of 75 and 100 watt bulbs and they will still be available for some time to come yet, and I can always use floods or infrared bulbs too.

beenthere 09-11-2013 06:29 AM

Probably don't need a fan. Natural convection should allow plenty of air to enter and rise out of the box, with a few vent holes on the bottom and top.

How quick you need the ceramic to heat up, will have more to do with how much heat you need. then the box size. A 50 watt bulb will heat an insulated 4 cu ft box to over 150F.

yuri 09-11-2013 09:58 AM

and if U want to get real fancy U can put a dimmer switch/rheostat in series with your bulbs to fine tune their output.:thumbsup:

SeniorSitizen 09-11-2013 11:02 AM

Being this may still be in the experimental stage and depending if appearance is a concern, a defunct upright or even a chest freezer is metal and well insulated with racks in uprights for certain and possibly some in chest type. The upright may naturally convect air better for your situation.

Adjustable ( outdoor grill vents come to mind ) lower vent holes of some type for innie air and possibly the same for upper outie air.


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