Looking for heating coil

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Hi Freaks,

Has anyone come across a 8.5 inches circumference by 8 inches height spiral heating coil? I am trying an app. for which I need to heat this coil for about 2 minutes with a battery to about 60-70C.

I will keep looking googling it as well.

Thanks.

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Not enough information - Voltage, current, power expectations...

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That is a really odd size & shape. I have seen line voltage heaters that are on a ceramic form about 3" diameter and maybe 8" long and the wire is in the form of a long spring that lays in spiral groves on the outside of the form.

However, as sbennett points out, there is not enough information. 60-70C depends on too many things. Does it have to heat something or is this purely radiant? The temperature is really the point at which the incoming power balances the outgoing power, and outgoing power depends on temperature. Without the heat loss at desired temperature, there is no way to help you!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Ok this is to heat coconut oil in a bottle. The dimensions given are those of the bottle. Oil is in the solid form and I want to melt it.

Thinking out loud here if I use H = m x c x deltaT to calculate the heat I will need, then I get:

H = 0.5 (kg) * 2100 * 35 = 36,750 Joules/2 minutes which is 18,375 joules/minute

Now the energy in two 12V , 2200 mAh batteries is:

(12 * 2200 * 3600) / 1000 = 95,040 Joules

So the battery will drain in about 5 minutes.

So my options are a)slow down my heating rate to increase my battery life or b)use a higher mAh or Ah battery or c) use the mains outlet.

I prefer using a battery for this.

Does this calculation sound ok?

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I do not know what kind of wattage you will need to raise the volume of oil to final temp in two minutes, but there are large power resistors with metal heat sink mounts built in that could me mounted to a metal plate in series or parallel to make a heater. Ohms law would tell you the wattage for given set of resistors and power input. I think you need to get the wattage from a formula to figure it out.

For information you might visit http://www.chromalox.com/

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1 Joule is 1 Watt per second, so 95040J per minute is 1584J per second, so I guess you need a heating coil of 1600W, not taking in account of other heat losses.

1600W is not too exotic, most household deep frying pans and coffee makers are similarly rated; a $15 frying pan is likely a good parts donor :)

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On a battery???

And, of course, the back of the napkin, first cut calculations assume an isothermal fluid bath, and uniform heat distribution. Neither of these are true, and both lead to significantly underestimating the heat required, unless your system is well insulated, and has an agitator.

JC

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To a degree, lowering the power output to extend the warming time is an energy looser. The issue is that your battery contains a fixed amount of ENERGY. If you lower the power into the heater, it takes longer to heat, and you loose more heat (through radiation, conduction, and convection).

If you have to do it with battery power, then your "solution" is a bigger battery (that holds more Joules of energy).

By the way, first order approximation of battery energy storage is Vbattery * AmpHr. You would use "average" battery voltage. The result is in Watt-hours. As, JayJay points out, a Joule is a Watt-second, so, if you really want Joules, you would multiply by 60*60. However, for comparison purposes, Watt-Hr is adequate.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Just curious.. What is the end use? How critical is the temperature you need to reach? Does it have to be 12 VDC or could you use 120VAC ?

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For a little practical "grounding", here, lets try an example.

Suppose a 12V car battery. Nominal 12V, lets say 80Ahr, That translates to 960Watt-hr.

Lets suppose that you actually can do this in 5 minutes and that it takes 400W to maintain the 60C-70C temperature. This means that 400W is going out (through losses) just balancing the 400W coming in. That comes out to be 400W * (5 min/60 minperhour) or 33Watt-hr.

Then, with the 12V car battery, you could do this 960/33 = 29 times.

Now, if you have one of those small 12V, 7Amp-hr batteries which holds 84Watt-hr, you could do it twice with a bit left over.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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The end use is for heating oil. The oil frequently gets frozen in winter and I need it as a liquid to get it out of the bottle. Of course I have other simpler options like immersing it in hot water, using antifreeze (possibly?) etc. but I want to try an electrical option.

The temperature is not critical, the end result is more important ie. liquifying the oil in < 3 minutes.

@Jim: I agree with the principle and the calculations.

I wanted to try it with a battery (to make it more convenient and so I could carry it around) but it does look like I may have to go for the mains to power this thing.

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There are those 12v auto beverage heaters which run off the cigarette lighter plug. They can heat 12 to 16 oz of water.

http://www.thefind.com/appliance...

You can probably find these in Pep Boys or other stores.

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You can try a Peltier junction type heater. I built a custom cooling system for an internal chamber to test the breakdown voltage of air at various temperatures and drove them pretty hard with a 13.8 volt DC power supply. I drove them so hard that I immersed the "hot" side of the junction in an ice bath:0) It was for this hot meteorologist I was dating at the time:) What we won't do for a beautiful women!

Here is a groovy kit. There are so many junctions available at various voltages and current ratings, but then you will have to get creative with your custom built heating apparatus!! I sandwiched mine between heavy copper plates and sealed the edges with high temp silicone. I used tons of heat sink compound on the copper plates and then just submerged the leads in the ice bath...FYI, they do wear out over time!! Gotta love that Frenchman:)

Just some guy

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Thanks, John and alwelch. I had seen that beverage heater earlier on amazon. I will certainly consider these options. I will have to modify the peltier trick to make it cylindrical so I can heat the bottle. Another idea I though of was to take one of those bed warming mattresses and use the coil from that.

So John, I assume you used a DC power supply powered from mains?

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Yes sir, primary was 120 volts AC!!

You might peruse tank heaters used in aquariums also...might find some good ideas there!!

John

Just some guy

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One more...you may consider heat trace tape. This tape is rated in watts per foot and comes in a 12 volt variety for RV applications. It can be cut to length and then wrapped around the container and then insulation can be added on the container.

Here is a supplier that sells this tape in the 12 volt variety!

John

Just some guy

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I can't back this up with figures, but I'm confident you will not be able to liquefy a gallon of solidified oil in 3 minutes using this kind of external heat source. The best you will achieve in that time is a slightly smaller bottle-shaped solid mass floating within a skin of liquid that convects the heat away. You'd have a lot better results with an immersion heater, or just a low-powered ambient heater that stops it getting cold enough to freeze in the first place. A watt or so in a polystyrene foam cooler box would probably be enough.

Remember, when you're calculating the heat energy required, that phase changes have a "latent heat" that needs to be considered along with the thermal mass.

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npat_avr wrote:
Hi Freaks,

Has anyone come across a 8.5 inches circumference by 8 inches height spiral heating coil? I am trying an app. for which I need to heat this coil for about 2 minutes with a battery to about 60-70C.

I will keep looking googling it as well.

Thanks.

In answer to your question; I've seen heating elements which you are able to bend into any shape you desire - they are supplied straight in varying lengths.

Below is a link to Farnell for a 240v AC heater as an example, but I'm sure that if you spoke to the supplier of similar products you would be able to find one for a suitable voltage - I've seen a number of 12v/24v/48v immersion heaters on eBay, so they certainly exist in low voltage versions.

http://uk.farnell.com/_/2440-1-5...

-Tim

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Quote:
I can't back this up with figures, but I'm confident you will not be able to liquefy a gallon of solidified oil in 3 minutes using this kind of external heat source.

I agree...I would need to measure the caloric energy required for the transition before I could consider a solution. A simple experiment to calculate the minimum energy requirement would not be too hard to set-up.

Just some guy

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Beware of burning the oil too. There are a lot of topics on biodiesel forums where they are heating oil (sometimes from solid,, but usually just liquid to 55'C) and burning it due to high power immersion heater reaching over temperatures due to low flow rates.

-Tim

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Thanks for the pointers, Tim.

That link says it is out of stock but I will keep checking.

@peret:
That makes sense, I did not take the latent heat into account. I am going to try ripping out the coil used in those heated mattresses and use it to try and heat the oil. Will take longer to liquefy it but I don't think it will burn it. (based on the fact that when it heats the bed it is warm but not burning hot)

Of course I will use some jacket to prevent direct contact of the coil with the bottle.