Gentlemen,

Thank you for all the replies, and I'm happy to say that I agree with all of the suggestions offered and have already implemented my own version of most of them.

For the heating element safety interlock, I've used a slave switch that will only apply power to the element once the motor is up and running - motor stops, element stops.

It has a further refinement in that the element is run through a drop out relay. If the power is interrupted or the motor stops for any reason, the element is cut off and won't reconnect until a reset button is pushed.

David, you asked what sort of things can be done to comply with ATEX rules. The simple answer is to replace every component with Ex certified units. Heating elements for example are available in Ex 'd' which means that they hook up to their electrical supply via a junction box that will contain any flame/explosion.

Needless to say, such exotic components are expensive and have fairly stringent servicing requirements.

The other way is to think laterally and start replacing components with intrinsically safe versions. For example, industry makes extensive use of hot water for process heating and it wouldn't be very difficult for me to put in a water/oil heat exchanger in one of the recirculation pipes - added advantage being that I can then offer a solar collector and storage tank as part of the deal. Sounds a bit ridiculous, but I can still come in under the cost of a dread Fuelmeister.

In truth, I'm still thinking through the various permutations possible, and I'm sure the eventual bright side is that it will improve my designs.

BTW, I'm new to the whole ATEX thing myself so appreciate any advice. Grimwood mentioned the evaluation of hazardous areas, but I can't see how any biodiesel plant would not be rated at the very least as a Zone 2, with any vents into the processor tank as a Zone 0. The only variability seems to be in the area that the hazardous area occupies.