Hot water cylinders

Hot water cylinders are often used to make biodiesel processors. They come with several threaded ports and usually an immersion element. These cylinders are inverted, providing a dome for draining at the bottom, as well as correct placement of the immersion element in relation to the liquid.

Types of cylinder

• 

  Direct

• 

  Direct (economy 7)

• 

  Indirect

• 

  Primatic

Direct cylinders are the obvious choice, followed by indirect - the only disadvantage of indirect is that the coil occupies some of the cylinder's volume.

The comparatively rare "primatic" cylinder is unsuitable for use as a biodiesel processor, both because a much larger volume of the cylinder is wasted by the primatic apperatus, and also that not all of the liquid can be drained. If you are unsure if a cylinder is primatic, look into the side ports - if you can see a wall of copper just inside the port, then the cylinder is primatic. If the port forms a tube that bends away to one side, then the cylinder is indirect and you're looking into the end of the coil. Another quick check is to jiggle the cylinder - if there is a coil this can be felt wobbling around inside.

Hot water cylinder standard sizes

Standard Cylinders
British standard	External diameter of cylinder (excluding insulation)	External height over dome	Min storage capacity		Expected port fittings
			Direct	Indirect
	mm	mm	Litres	Litres	female, BSP
0	300	1600	98	96	1"
1	350	900	74	72	1"
2	400	900	98	96	1"
3	400	1050	116	114	1"
4	450	675	86	84	1"
5	450	750	98	95	1"
6	450	825	109	106	1"
7	450	900	120	117	1"
8	450	1050	144	140	1 1/4"
9	450	1200	166	162	1 1/4"
9e	450	1500	210	206	1 1/4"
10	500	1200	200	190	1 1/2"
11	500	1500	255	245	1 1/2"
12	600	1200	290	280	2"
13	600	1500	370	360	2"
14	600	1800	450	440	2"

Inherent risks and mitigation

Any processor with an immersion element directly heating the liquid presents a possible risk of fire or explosion. Incidents reported with the use of copper cylinders include:

• the element accidentally being turned on with the cylinder empty, but full of explosive vapour (NEVER put the element on a 24h timer as it may come on again the next day!)
• the element not being turned off when the cylinder is drained
• the liquid level dropping because of a leak, exposing the element
• the element not being completely covered by the liquid because the element is too long

The last of these is easily avoided; before being used as a processor, the heating element should be removed to determine its length. Unless the cylinder is particularly tall, long elements are not suitable, as the tip of the element may protrude above the surface of the oil and Methanol mix, igniting any Methanol vapour present. For this reason, long elements should be replaced with the folded 11" type.

The other three risks all relate to lack of liquid in the vessel. These can be protected against by using either a pressure switch or float switch to control when the immersion element is switched on.

Copper as a catalyst to oxidisation

Unfortunately, copper acts as a catalyst to oxidisation of biodiesel - even at the level of a few parts per million in the fuel. Fortunately, this only really applies to long term storage, and as the average homebrewer is likely to use their fuel within a couple of months of production, this does not cause any problems. It is, however, advisable to avoid storing your fuel in the copper cylinder if possible.