GL's non siphoning heater tube
Written by: Graham Laming
Reprinted with permission from www.graham-laming.com
Heating any flammable liquid can be hazardous, so I've tried to find a simple method of reducing (not eliminating - you still need to use common sense and vigilance) the risk of accidental ignition.
I have purposefully not relied on float switches, level sensors or other means to control the heater, because these can fail or be fooled, for example in winter, a blob of frozen biodiesel can hold a float switch in a false position - I don't trust any 'float' system when there's the possibility of the liquid freezing and leaving a false residue of material.
This idea, instead, relies on keeping the heating element always covered in liquid, regardless of valve positions, tank contents, temperature etc., so that air cannot get to the hot surface of the element.
A pair of thermostats in the liquid ensure that the liquid is never allowed to reach its auto-ignition temperature.
The heater consists of a horizontal tube, with an inlet and outlet on its top surface, so that the tube can never accidentally empty.
Looking end-on, the inlet and outlet pipes are at the TOP of the HORIZONTAL tube, with tangential feed...
Here we see a diagram showing the tube end-on, partly filled with biodiesel. You'll see that you can't empty the tube via the ports.
There's a small airspace shown here above the liquid.
As soon as you empty the tube enough to create that airspace, the two ports are joined by that airspace, which prevents liquid from being sucked or drained out of the tube. This is the key reason why the element will always remain covered in liquid.
Inside the tube is the heater and thermostats.
The thermostat assembly contains 2 thermostats - an adjustable version covering 30 to 70C and a fixed, non-auto-resetting version, which operates at 90C or above.
Imagine you lose pump flow, and the tube has stagnant biodiesel in it, with 3kW of heating. The thermostat is right next to the element, and will respond quickly to a temperature rise above the adjustable set temperature and above the 90C cutoff.
We want to ensure we switch off the power to the heater WELL before we reach the auto-ignition temperature of the liquid. This is the temperature at which the liquid will combust of its own accord, without spark or other added ignition source when in contact with air. (not the same as flashpoint).
* For biodiesel this is ?? Degrees C (data needed - NOT flash-point)
* For methanol this is 450 Degrees C (850 F) [Reference]
* For glycerol this is 400C (750 F)
You can get heater elements and thermostats from here.
Extra Info: See this link on how to re-range this type of thermostat.
Fill the thermowell with silicone grease, then fit the thermostat, to ensure efficient and rapid heat transfer to the thermostat.
The heater tube is fabricated from mild steel sheet and seam welded.
The heater tube is to be mounted HIGH UP in the system, something like this ...
If you use a venturi in your system , place the venturi in the heater tube's OUTLET line, between the heater and the top of the tank.
The reason for having the heater tube high up is two-fold...
* Less liquid above heater, reduces draining when pump switches off
* Less chance of it being filled with glycerol
Why would it empty less if high up?
It's to do with the fluid momentum you build up in the tubing and heater tube. A top mounted heater tube will lose very little fluid if you open the bottom of the pipework, whereas a bottom mounted heater tube will loose more.
Look at these 2 skiers, who can explain the reasons more clearly...
At each finish line is a hungry polar bear who likes chewing on skiers. (We can't see him, he's white and it's been snowing, but he's there! Hunters look for the glint of his eyes...)
The 1st skier won't build up much momentum when he starts off, and probably won't even be able to leave the trap area, just a few metres from the start line. Lucky for him!
The 2nd skier, however, will have built up a LOT of momentum by the time he reaches the trap area and will find it very difficult NOT to leave the trap.
I can imagine him doing a spectactular somersault as he goes over the finish line, , landing in the jaws of the waiting polar bear, while skier 1 will probably still be struggling to leave the trap just a few metres from the start line.
The high-up trap location is best in this case too!
Positioning the trap region IS affected by height
Also, as mentioned, you'll have less likelihood of the tube filling with glycerol if it is mounted high up. Approx 25% with typical brew, max, for a top mounted, 100% glycerol fillup guaranteed for a bottom mounted tube.
Heater on the inlet or outlet of the pump?
Your pump will be less likely to cavitate if you mount the heater tube in the outlet of the pump - most centrifugal pumps prefer working into a load, rather than sucking thru a restriction.
Cavitation is a boiling action, so with the heater at the pump's inlet you are doing 2 things to promote cavitation ...
1. Heating the meth up to the highest temp in the system
2. Causing the pressure to the inlet of the pump to drop.
Also the pump will run cooler if you have the heater on the pump's outlet side.
Try it and you'll see what I mean about the emptying - I know it seems 'intuitively' the same either way, but it isn't the same practically.
High up is best.
Here's a short video about it...
Tim Palmer's version of the non-siphoning heater
Tim sent me some photos of his design, using standard iron fittings. Remember that the tube must be kept horizontal so that it doesn't drain.
A nice solution, using standard fittings, which Tim got from Ace Hardware in the States, for around $25.00 total.
Many thanks, Tim!
Here's an exploding water heater, just for entertainment. Remember folks, don't block your vent and make sure your thermostat is working properly!...