Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Replacing the Original Heater

In my day job, we frequently work with cartridge heaters, both for hot-melt glue application, and 3D printers. So, being somewhat familiar with their ins and outs, I decided to try cartridge heaters to replace the missing B31 heater unit. Mind you, this was long before the recent solutions had been worked out, namely improvising the 35MM Chinese band heater, or simply winding a new heater with nichrome wire, as pioneered by Junkologist (Mike). In my case, even with those approaches, I would have to come up with a way to hold the heater in place, since my engine didn't even have the internal expanding clamp.

Given all that, I went ahead with designing a cartridge heater solution. It turns out that cartridge heaters are not all that well-suited to this application. The only length that will fit is a 1" long heater, and the largest diameter in this length is 3/8 inches, which is rated at only 150 watts. If I used two of them (300 watts), this would be somewhat less than the 350 watts of the original heater. If I used three of the 150 watt heaters, I would be well above the original rating at 450 watts. I was a bit leery of exceeding the nameplate wattage, even if performance might suffer as a result. As it turned out, I wish I had went with the three heaters.

Here is one the heaters that I purchased. It's a Tutco #CH15766, rated at 150 watts, 120 volts:

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Due to the cramped quarters beneath the boiler, I would need to bend the heater's leads at a sharp right angle. To be able to see what I was doing, I decided to remove the factory-installed hi-temp sleeving. Here's what the heater looks like with the sleeving removed, and the heater wires bent:

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I used some PTFE heat-shrink tubing to re-insulate the heater wires where the bends were made. I then dressed the leads to length, and crimped some ring terminals to the wires. Here is the cartridge heater pair, ready for installation:

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For best performance and life, cartridge heaters should be installed in a close-fitting hole, preferably in a aluminum block which can conduct the heat efficiently to the target medium (in this case, the boiler sheath, and the water within.) I decided to make an aluminum plug to fit the I.D. of the opening in the bottom of the boiler. It would be split into two parts, each with a reamed hole for a heater cartridge. A set screw between the plug halves would force the two halves firmly against the sides of the opening.

Here is a what the plug looks like after turning to size on the lathe:

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A rear-mounted parting tool was used to cut off the plug:

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Reaming the drilled holes to size for the heaters:

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A set screw hole was tapped using a 5/16-18 taper tap:

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I clamped the plug to the bed of my cutoff saw to split it down the middle:

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Here is the finished plug assembly and heaters, ready for installation:

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(Stay tuned for more...)