I am making a set of 5 metal heating trays for food or liquid, and need to warm them with some evenness from about -10F to about 150F. The heat needs to conduct up from the bottom of the trays and radiate down from the tray above it. This I was planning to run them off of 120V wall current if that works. How can I go about determining how to heat my trays? it should be noted this will all be occurring in a medium level vacuum, where air conduction is not great.
Thanks!
Two panels of nichrome wire wound similar to a toaster. One above and one below.
How quickly do you need to heat the food? You may have to do some experimentation to get the correct power dissipation so as to heat but not burn.
Start with a short length to see how much power will make it glow. This will give you power rating per metre at that voltage so can then work out how long to make the element for 120v.
Weedpharma
Very slowly, actually, over a period of many hours. Does making it longer increase heat? Also, do I need to be worrying about diameter or anything? Will it handle being plugged straight into the wall outlet through a relay? I had simply planned to hook it up to a relay board to turn power on/off for temperature control (unless someone tells me that won't work XD )
Every resistor is a heating element. It converts electrical energy to heat with 100% efficiency. The heating power is used for 2 things:
So your power needs are dependent on this. You can make practical tests by submerging a immergion heater in a tray with liquid (with known thermal mass and known power) and recording the temperature rise. This will give a very good estimate of the systems behavior.
^that would indeed be a fun experiment. For the time being, I was hoping to get an idea of how this stuff behaved before beginning experimentation. There are all different gauges and lengths, and I have no idea which one I should get. Presumably a larger gauge would decrease resistance, and therefore increase heat. Then surely a larger gauge would handle more power, right? anyone have any ballpark guess ideas which gauge wire, or how much would be needed? (6 sheets including the top, each about 1x2feet)
There's actually an online calculator available. Check here for details But in your case, it'll probably get hotter than you expect (due to the reduced air pressure.)
rytcd:
Very slowly, actually, over a period of many hours. Does making it longer increase heat? Also, do I need to be worrying about diameter or anything? Will it handle being plugged straight into the wall outlet through a relay? I had simply planned to hook it up to a relay board to turn power on/off for temperature control (unless someone tells me that won't work XD )
Please give more information on the project. Why very slowly over many hours?
This tells us that you don't want high power as it will cook and burn the food. What temperature are you wanting? In what volume of space?
Give as much information as you can so we don't have to guess.
Weedpharma
I would use a grill element from an electric cooker, you can get them as replacement parts off eBay for quote cheap.
I like the idea of buying a heating element or [u]power resistors[/u] rather than making your own from nichrome wire. (That resistor is just a random example of what they look like... You may want a different power rating and you'll need to calculate the resistance value.)
If you make your own heater from nichrome wire, make sure to electrically isolate/insulate it from from the trays! You'd like good thermal conductivity, but no electrical connection.
The big unknown is the wattage, and you will have to experiment. A typical food warming tray is probably 300-1000 Watts. ...25W won't be enough and 2000W will blow a 15 Amp circuit breaker.
A toaster or hair dryer is closer to 1500W, and that's about the maximum you can get from a typical 15 Amp circuit. (Power = Voltage X Current.) With 5 trays, the total amount of power available could be an issue. (That's why catering companies use "canned heat" or propane.)
Typically, the power is cycled on & off to maintain the temperature the same way your furnace or air conditioner cycles on & off.
With more power, you can get hotter and you can get hotter quicker.
With a given amount of energy, you'll get a given amount of temperature rise.... If the environment is 10 degrees hotter, your tray can get 10 degrees hotter.
A thicker-heaver tray will heat more evenly than a thin tray, but it will take longer to heat up.
Power can be calculated as Voltage Squared/Resistance. So for example, at 120VAC a 100 Ohm resistor will dissipate 144 Watts. The power dissipated in parallel resistors adds-up just like two 100W light bulbs make 200W.
The equivalent circuit will be something like this.
Thermal power is equvalent to current
Thermal resistance is equivalent to resistance
temperature is equivalent to voltage
thermal mass is equivalent to capacitance
VS1 = element temperature
R1 terma resistance from element to tray
R2 thermal resistance from tray to ambient
C1 thermal mass of tray
DVDdoug:
...... and 2000W will blow a 15 Amp circuit breaker.
A toaster or hair dryer is closer to 1500W, and that's about the maximum you can get from a typical 15 Amp circuit. (Power = Voltage X Current.) With 5 trays, the total amount of power available could be an issue.
Perhaps you need to get your outlets rewired? UK Power sockets are rated at 13A, and 3kW is no problem!! http://www.argos.co.uk/static/Product/partNumber/4151797.htm
http://www.argos.co.uk/static/Product/partNumber/1463347.htm
Regards,
Graham
Since you are working in a vacuum, resistive type heaters will not work (unless in direct contact) as they require air to transfer the heat.
Perhaps consider infrared heaters which do not require air.
Bathroom style heat lamps are plentiful and cheap and control could be via standard light dimmer.
Standard dimmer has a range of approx. 400va. so perhaps 2 lamps (150w each) per dimmer.
Resistive heaters do radiate with IR, you see them glow red don't you?
It is all part of the black body radiation curve.
Anyway the OP said medium level vacume which means there is air present.
ghlawrence2000:
Perhaps you need to get your outlets rewired? UK Power sockets are rated at 13A, and 3kW is no problem!!
Graham
15A is standard in the U.S. I didn't know 13A is standard in the UK.
Sometimes, you'll find 20A circuits and we have 240V circuits for electric ranges, clothes dryers, air conditioners, etc. My dryer circuit is 240V/30A. These are wired to special outlets with different connectors... We don't have 240V outlets in living rooms or bedrooms, etc. There are generally no "extra" unused 240V outlets unless you have a gas dryer or gas range and your house is also wired for electric appliances.
Grumpy_Mike:
Resistive heaters do radiate with IR, you see them glow red don't you?
Infrared (IR) is invisible radiant energy
bluejets:
Infrared (IR) is invisible radiant energy
A little knowledge is a dangerous thing, I would be quiet if I was you......
I used to work in a hospital as medical physics technician, and we had a sleep monitoring system which among other things used infra red lamps and cameras. I was amazed to find that the incredibly expensive infra red lamps, were nothing more than a 240V50W dichroic halogen bulbs behind a thin bit of metal..... in the absolute darkness, there was a dim deep red glow, presumably from the heat of the lamp..... but I tell you what, 2 of those 'shining' at the bed from different angles, was like daylight to the video cameras!
Infrared radiation is emitted by any object that has a temperature (ie radiates heat)
So would you like to retract your last statement?
Merry Christmas everyone,
Graham
"IR is invisible radiant energy" is a true statement. The energy cannot be seen unless converted to visible spectrum via some other system, in your case, a camera.
Weedpharma
bluejets:
Infrared (IR) is invisible radiant energy
Wow twenty years of teaching physics at a University an I never knew that ---- not.
Anything with a non zero temperature radiates black body radiation, the fact that some of that radiation is in the visible part of the spectrum is indicative of the actual temperature. IR is only part of the energy it is radiating. It works in a vacuum, that is why we feel the sun as being hot. It has traveled most of the way to the sun in a vacuum.
See this link:-
back to the project.
you can buy nichorome and make a grid. how much energy is dependant on the heat thransfer of the medium, the condutivity of said raided temp of the container and the mass of the project to be heated.
several hours ? no problem. if funds are not an issue, buy a large roll of wire and run it out then back, then put it into your chamber with a known volume of water.
monitor temperature change over time. if you get the wire to a suitable temperate then note the power, temp and such. you can then make more loops. the charts will give you a guide of power needed.
if you want to try different wires, you can buy small loops of different size.
I too like the idea of using elements from a toaster oven. there are different types, glass quartz and stainless steel sheaths.
since you mentioned cold, do you have a source of cold ? or do you have to generate that ? a bucket of ice water will deliver close to 32° constantly. that could be in contact with the pan.
one thing not mentioned is evaporation. the evaporation over time will be a marked cooling effect an will need heat to compensate.
I would think the simplest way to do this is to use those rod quartz halogen bulbs they use in shoplights.
Just buy a small halogen shop light, and take the glass cover off.
You can also buy IR heat lamps.
This seems a lot simpler than trying to calculate and then cut-and-try nichrome wire heaters.