I have my methanol still's heating element wired to only come on when the pump is on first. I'm pretty sure the switch for the element is just powered by the pump switch and both the pump and the element share the same neutral.
Now I haven't worked with 240V much, and never an application where I have tried to power a 240V component and a 120V component together. But that is my plan. The element needs to be converted to 240V to speed my heating while keeping the pump on 120V and keeping the safety aspect in place.
Here's what I THINK I need to do, correct me if you see a fault here.
1) One of the 120V leads of my new 240V plug will replace the 120V lead from current plug in the control box.
2) The pump and the element will no longer share the neutral: the pump will return to the neutral, keeping it on 120V total, while the element will return to the second 120V lead of the 240V plug (the -120V, essentially), putting it at 240V.
The internal wiring of the control stays the same: 120V passes through the single switch for the pump to power the element's switch. The element's return path is at -120V, so when the switch is thrown to apply 120V, its running at 240V.
My concern is that I am putting the element at -120V potential, but there is no return path, so no current flows. But if the element shorted and grounded somehow, I would have current flowing, wouldn't I? Hmmm...should the element's switch be replaced with a single-throw, double-pole (I think that's right name) switch that cuts both power leads when its off? Wiring the switch to cut into both the power and the "return" line.
You have got it pretty much right. You only need to control (switch) one leg of the 240v element not both, this is standard practice. However, you should have a separate 2 pole switch for servicing it as there will always be one leg on the heater alive, or if your unit is plugged in the plug could also be used as the service switch.
You may not even need a neutral, your pump and PID may be rated for 240v as well as 120v, just check the nameplates.
Now for my professional opinion,
Having the heater tied in with the pump switch offers ZERO safety of any kind to the circuit. When your pump stalls or doesn't start etc. you still have the same safety issue, pumps always fail, its just a matter of when... Use a float or flow switch and you have an actual safety circuit that you can leave unattended.
Here is a drawing I did awhile back for Drewry here who also needed to run his pump on 120 and heaters at 240 as well as some other things for a push pull setup, its far more involved then what you propose but it will give you an idea of how things should be wired. HERE
Here is an affordable flow switch that would add a level of safety to your setup http://www.dwyer-inst.com/Prod...ddle/SeriesV10/Specs. Murphy also has free instructions on his website on how to build a reliable float switch.
Simple schematic for a pump and heater control with a high limit
Sensor for the biodiesel/glycerin layer
Well, I wouldn't say ZERO. It does prevent me from turning on the in-line element and rapidly boiling methanol in the line and then burning the element out.
But there is no protection from pump failure currently, correct. I also don't run it if I'm not within earshot; part of the reason I am trying to speed it up. But if I have a spot available, the flow switch would be good.
The water circ. pump motor burned out last weekend. I heard its tone change, walked up to it and watched the current climb slowly (its on a 12V power supply) and then I shut it off when it tried to pull 15V on a 12V pump and limited out my power supply.
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