# Turn spacing for helical heating coil?

I'm planning to develop a helical heating coil wound around a ceramic rod. The purpose of this coil is to radiate heat to its nearby surroundings.

I want to know if there should be a spacing between each turn in the coil? If yes does the heat radiated depend on the spacing? If yes what should be the spacing inorder to attain maximum intensity of heat radiation, keeping all other parameters constant?

I'm guessing, the lesser the spacing more are the turns in the coil hence more heat radiation, thus the spacing should be as little as possible (but not zero) inorder for maximum heat radiation. Am I right?

EDIT: I'm interested in a high intensity heat to generate a high temperature in a small volume. Basically I just want to heat the surface just below the coil. The distance between the coil and the surface is not more than two millimetres.

• What is a helical heating coil? Apr 18 '20 at 9:51
• @JakobHalskov a wire made of nichrome that is helically wounded across the length of a ceramic rod. Apr 18 '20 at 14:57
• @Somanna So...just a coil? Apr 18 '20 at 17:18

## 3 Answers

While I would be quick to jump on the "just simulate it" bandwagon for tasks like this, I'll take a shot

1. Reflections off the rods, assuming the wire of the coil radiates fairly evenly about its circumference, having a wider gap between the coils means there is a larger proportion of paths from that circumference that can either freely escape, or reflect off the rod and miss another heating coil, so the heat is spread over a wider angle on average, meaning the wire should be slightly cooler.

2. Resistance, The smaller the spacing, the longer the coil is in total, you cannot trip the circuit its on, so the resistance has to be high enough to not do this, equally to get the maximum wattage out of a given supply voltage you want to be as close to the ideal resistance as possible, e.g. for a 240V, 10A circuit, P = IIR, so 2400W / 10 / 10 = 24 ohms, matching the resistance of your wire to reach this resistance would be ideal

3. Thermal change in resistance, the larger the surface area the same heat energy is spread across, the lower the temperature on that surface, so the more turns in your coil, the lower the temperature in the wire, and the lower the temperature change from ambient, the less increase in resistance when at operating tempeature,

Based on all this, I would work to find your ideal resistance by knowing your supply voltage and current limits, find a wire thickness and number of turns that meets that at expected hot and cold temperatures, that fills as much of that surface as possible, increasing the spacing only if you need a wider angle for the heat to be spread over.

Define "heat radiation" for your purpose.

The total power radiated depends on the voltage and current. The current depends on the coil resistance : increasing resistance reduces the current and total power.

Reducing turns spacing will increase the coil temperature and thus the radiated heat intensity, but the increased temperature will (with most coil materials) increase the coil resistance and thus reduce the overall power.

So, the answer depends if you are interested in a high intensity heat to generate a high temperature in a small volume, or maximising heating at a lower temperature in a larger volume. A good pottery kiln is an inefficient way to heat a large room.

I cannot tell which you want from the question.

• I'm interested in a high intensity heat to generate a high temperature in a small volume. Basically I just want to heat the surface just below the coil. The distance between the coil and the surface is not more than two millimetres. Apr 18 '20 at 14:56

You will want to maintain appropriate watt density (diameter-> surface area and wattage per unit length) on the resistance wire so that you get reasonable life at your operating temperature. The quality of the contact with the ceramic rod will affect how much the wire overheats. If the ceramic rod has grooves in it, it will be much better. Of course you have to leave enough room to ensure the turns don't touch. Sometimes you need the element helically coiled and then that coil coiled about the surface.

I really suggest you start with a known-good commercial product and modify from there. Or, better yet, just buy a commercial heater if that makes sense. You have not mentioned temperature range or wattage, but there is a very wide range of commercial heaters that are available with guaranteed specifications. Exposed elements are not all that common in most, for good reasons, but necessary in a few types for severe cost constraints or extreme temperature reasons.

• Reasonable life? Life of what? The whole coil setup? Apr 18 '20 at 15:58
• Life of the heater wire, which determines the life of the heater. Apr 18 '20 at 16:50