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How to make your own heat exchanger
I have made three types of heat exchanger so far, each using a slightly different design. The first was a simple water over oil double tube heat exchanger with a 15 mm central pipe held within a 28 mm pipe.
The second was inspired by the picture I was sent by Stephan Helbig from Germany who made a similar heat exchanger made with a coil of micro-bore wrapped around the central copper pipe.
The third version has two stages so the fuel is warmed before it goes to the normal fuel filter (avoiding the need for the extra Peugeot filter I fitted to our VW Passat). The heater we sell is a three stage unit with an electrical boost which comes on only when the glow plugs are activated or when the wwater temperature is below a set limit.
The external coil looked easiy to make, but in fact it was quite tricky to make a neat job! It is essential to fill the 8 mm micro-bore pipe with fine dry sand or salt before attempting to bend it around the core, otherwise the pipe simply buckles. I used salt, which I had first dried in the oven and then ground to an almost liquid dust with a Moulinex electric soup blender. I then poured the salt dust into the pipe using a small diameter plastic funnel taped to the end of the pipe. (Beware. Fine salt dust stings your nose if sniffed). I then closed the ends of the pipe with wooden pegs to prevent spillage of the salt. It is possible to get quite a tight fitting spiral to the micro-bore pipe if you twist the pipe as you form the coil. It also helps to use a block of wood with a grove in it to hold the pipe as you can easily distort the soft pipe if you exert pressure using your naked fingers. Instead of using salt it is possible to fill the pipe with water or oil and clamp the ends.
Ready to fill the micro bore with dry salt before coiling
When the coil is formed, the bungs can be removed, and the salt shaken out of the pipe whilst rotating it slowly, to enable the salt dust to fall out from the lower end. It will be necessary to tap the sides of the coil with a wooden spoon to release the salt. If it does not clear completely then you will have to soak it in hot water to dissolve out the salt. It may take a while but it does all come free in the end. I have not tried using sand, because I imagine it may be more difficult to remove the sand. Salt seems to work well so long as it is very dry, and you take great care to fill the tube without any air gaps inside. Keep tapping the sides of the tube as you fill it to shake the salt all the way down to the bottom.
The rest of the heat exchanger is made from standard Yorkshire plumbing fittings. Notice how the nozzles for receiving the rubber pipes are extended using half of a straight connector so the two solder rings make a convenient trap for the jubilee clip to grip the rubber pipe between. It may be better to braise the joints rather than use solder as the water can get very hot and melt the joints if the engine boils, however I can hear some people saying that the radiator is all made of soldered joints anyway!
I also attach a wire strap around the coil to keep it tightly secured to the 28 mm copper pipe, and then I run plenty of solder around the coil to weld it to the copper pipe as much as possible. The whole heat exchanger is then wrapped within a thick pipe insulation tube.
When using the double flow heat exchanger, the supply fuel goes around the coil and the return oil goes through the middle pipe.
Two stage heat exchanger ready for soldering the coil to the main water pipe
Detail of one end showing method of forming outlet and inlet ports.
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John Nicholson September 2005
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