The device above is part of what is called a “bubble pump” from a coffee maker. It is an aluminum tube with resistors wrapped around it. The resistors get current and heat up the fluid in the tube. In the coffee maker there is a one-way valve on one end of the tube. The water in the tube heats up and gets to the boiling point. When it boils it pushes out in both directions from the tube. The one way valve activates and then all of the boiling water is pushed out by its own pressure to the coffee. That is how 99% of home drip coffee makers work. Neat, isn’t it?
You can dig one of these out of almost any coffee maker you find in a thrift shop or on the side of the road. It will be in the base of the unit, under where the pot rests. There may be a lighted switch somewhere in the middle. Keep that – it may be useful when you build your enclosure.
By removing the one way valve and putting it in-line with a heat tolerant pump, this resistive heating tube can be exactly what you need in order to keep your mesh temps from dropping.In this photo you can see that the liquid tube is below, and the resistor is poking out of the upper section. The other component in this particular model is a thermostat to turn it off when it reaches a certain temp.
Now the unit above as pictured is VERY DANGEROUS. It needs to be shielded from both fingers and water. You need to have good strong hose clamps on all of the (high temp) tubing, and you should be certain to never run the heater when the pump isn’t on. I’m not yet sure what kind of enclosure I’ll put around mine. It doesn’t need a fan per se, but it should provide a bit of slash resistance and a switch and an indicator light would be nice.
Obviously this isn’t a ferarri of a RIMS tube, but it does work pretty well. I was using it in my basement to warm up some water for bottle washing. In 30 minutes it went from 45 to 115 with around 4.5 gallons of water and no insulation. Ambient temp = 45. This could be used to slowly bring the hlt up to temp, and also to give the mash a little extra boost. Unless you had ten of these I doubt you would be able to get much of a boil going though!