Building 12V Electric Pre-Heaters from Trash – Alternative Fuel Systems

November 27, 2008

Building 12V Electric Pre-Heaters from Trash – Alternative Fuel Systems
~James Dunn

Disclaimer:

Do NOT attempt to act upon or disseminate any portion of the following unless you read the full content of the Disclaimer linked herein: Click Here

Building 12V Electric Pre-Heaters from Trash – Alternative Fuel Systems

Safety Precautions (Partial List): The materials selected and procedures cited by the individual experimenter may contain asbestos, dangerous gases, explosive conditions, electric shock potentials, fire hazards, and other health hazards and hazards to equipment and structures not foreseen, …

Preface

Often, people experimenting with various ideas need a custom heat source for producing a desired effect. Standard heaters are often not commercially available in the configuration or voltages that are desireable for a particular setup. For instance, 12 VDC tubing heaters to pre-heat alternative fuels before combustion are expensive; when you can find them. Custom heaters can cost thousands of dollars for initial manufacturing setup. To reduce the initial research costs, custom heaters can be built from trash.

These heaters are especially useful for heating tubing, diesel fuel lines, etc. Again, assume worst case scenarios when engineering your heater.

Where to get your experimental parts

A heater can be easily built from old electric stove burners; even the ones no longer working. They cost about $30 a piece to purchase new, but you can go to any apartment complex and ask their maintenance supervisor to save you a couple of “stove eyes” or “oven burner elements” when they have to be replaced. Even the ones “burned out” can be made into useful heaters. Many maintenance people will do this happily if you tell them why you need the old burners. I find the oven elements are easier to work with.

If they have a stove they are throwing out, take the Oven Temperature Controller. The mechanical type has a bulb and capillary tube that is especially useful for controlling the temperature of tubing and is adjustable from 200F up to 550F.

Construction of heater element

Because the outside tube is isolated from the actual heating element, you can clamp these heaters directly to tubing or whatever else you are wanting to heat.

The elements are composed of a thin nichrome wire passing through a electrically insulative material,and clad with a stainless steel tube. The white electrically insulative material may be asbestos, though I do not know for sure. Do not let any of the dust get in your ears, eyes, nose, mouth, or other orifice and wash off any on your skin immediately.

As such, the outer body is in no way part of the electric circuit, it is only a durable surface through which heat is conducted. Unless of course through your experimentation you accidentally let the nichrome wire touch the stainless steel tube, then you have a potential shock and or equipment damage hazard.

The construction of these heating elements allows them to be bent “relatively” easily. Care should be taken to avoid kinking the tube when bending. Wrapping the tube around a pipe and pulling while bending helps. If it kinks, don’t use it. The kink may cause the electrified nichrome wire to touch the outer stainless steel casing and provide the potential for electric shock and/or equipment damage.

Working with heater elements

Check to see what voltage the heater operates at; most are 220 VAC in the United States.

When you hook up a working burner directly to 220 VAC, it will glow bright red. I’m guessing somewhere around 1000 Farenheit or hotter. If you need something that hot, then simply bend the element into the shape you need making large sweeping bends. Keep the electric terminal ends out into an area that will be no hotter than a few hundred degrees or your terminal ends will not survive long.

If you need a heater at a lower voltage and for safety you want to limit the maximum temperature achieved by the heater, then you can either simply hook heater elements in series with spade connectors to make them effectively longer (heat fuel line from the fuel tank all the way to the fuel injection pump).

If you want a 12 VDC heater that gets up to 250F when the vehicle is idling for instance, you can shorten the heating element and put spade connectors on them to make them operate at temperatures up to the normal operating temperatures for those heaters.

Special Note for those wanting to use this to pre-heat fuel lines. The temperature of a heater on a fuel line will vary significantly based upon the speed the vehicle is running. You might consider mounting the Oven Temperature Controller I mentioned earlier in a weather proof box near the battery to provide active temperature control of your fuel line.

No NOT use aluminum spade terminals; they will become loose and cause the related loose connection problems.

If you need lower temperatures or precise control over temperature, you can use a standard temperature controller. It will modulate the electric power to maintain a specific temperature range near the controllers temperature probe.

You can also use 12 VDC, 24 VDC, or 120 VAC instead of 220 VAC to run the heating elements at lower temperatures in conjunction with a temperature controller.

Or, if you do not want to purchase a temperature controller (>$100), then you can experimentally shorten or lengthen the heating element to get the steady state temperature you want to maintain in the system you are working on.

If you opt for a temperature controller, you can use a hotter heater. This will also provide a margin for operation accuracy in diverse operating situations (ambient temperature drift, differing flow rates, differing compounds, …). There are small Temperature Cutout buttons you can purchase from heating and supply vendors that can provide rough control at a low cost ($15) or if you choose to provided added safety margin as a cutout and/or alarm conditions.

Remember, when designing, remember to anticipate the worst case conditions. For an alternative fuel vehicle it might be: Heater on, car not running, very hot summer day in the direct sunlight, no wind, low humidity. Will the fuel in the fuel line that is being preheated get hot enough to cause problems? Heater on, car running down the road just after starting at 80 mph, freezing cold winter day, 25 mph head wind, relatively high humidity. Will the fuel in the fuel line that is being preheated get hot enough to run the vehicle as expected?

Realize, that in a perfectly insulated box, a heating element of any wattage will heat up and destroy itself. Running a current through an electric wire without any avenue for heat loss will melt the nichrome wire! Heaters rely upon ambient losses to protect themselves. If you are building a project and the heater is in a box without a temperature controller, put a damper on your experimental system to control the temperature.

You can lengthen or shorten the heating element by trial and error to get your desired heating power. Longer heater or lower voltage, lower temperature. If you need more heating capacity at lower temperatures, hook heating elements in parallel and make sure you have good contact with that which you are heating (tubing, heat exchanger, fins, …); a small air gap reduces heat conduction significantly.

Procedure for putting a spade connector on a shortened stove heating element

  • Put on a respirator suitable for working with asbestos, rubber gloves, and eye protection.
  • You simply cut the outer tubing with a tubing cutter.
    • Only cut the minimum necessary to sever the tubing.
    • Do not disturb the nichrome wire in the center.
  • Use a pair of side cutters (dikes) to cut through the nichrome wire and sever the tubing into two parts.
  • Mount the heater in a vise so the tube sticks straight up.
    • Use a hacksaw and cut a one inch slot in the center between the nichrome wire and the edge of the tubing; one inch along the length of the tubing from the end.
    • Fold the smaller flap of tubing back slightly so you can work.
    • Clean out the asbestos so that the spade connector crimp will have room in the tubing after it is crimped onto the nichrome wire.
  • Use a pair of needle nose pliars to pull the end of the nichrome wire out about 1/2 inch.
    • Take a bare zinc clad steel spade connector (no plastic insulation) and crimp it onto the nichrome wire.
  • Slide the neck of the spade connector gently back into the tube.
    • Position the spade connector so it wants to sit in position without having to hold it.
  • Mix up a small amount of High Temperature epoxy.
    • You can buy 500F 5-minute Epoxy from Wal-Mart for about $4
  • Coat the inside of the tube with epoxy.
  • Lay the spade connector within the epoxy so the spade connector des not touch the metal tube
    • axially so the nichrome wire lays straight
    • DO NOT put epoxy directly on the Nichrome wire.
      • this may cause a hot spot and shorten heater life
      • this may cause heating of the epoxy in excess of 500F
  • Coat all around the spade connector and fold back the tubing tab you had folded out earlier.
  • Let harden for about an hour.
  • Repeat this process for the other end.
  • Use an ohm meter to be sure the terminals and nichrome wire are isolated from the metal tubing before energizing.

Make sure the metal tubing near the epoxy does not exceed 400F or this will soften the 500 F epoxy and allow the spade connector to touch the tubing; causing a potential for short circuits, equipment damage, and electrocution. The temperature inside will be hotter than the outside.

Reverse Engineering to Specify Commercial Heaters

You can reverse engineer your heater to estimate the commercial heater specifications for your final product.

Resistance = Volts / Amps = Rref in heater formulas

Watts = Volts * Amps

Don’t forget to add a small margin for degradation over the life of the heater.

What to do with Custom Heaters

  • 12Vdc coffee cup warmer
  • 12 Vdc diesel fuel tank heater
  • 12 Vdc oil pan heater (cold climates)
  • 12 Vdc submersible cup warmer
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