recovering heat from hot air stream

[waterworks178]

i have an 3 cylinder air cooled diesel engine, with a vertical hot air duct, 650 cm2, @ 80 degrees C max , i want to pre heat vegetable oil fuel from this air stream, what kind of heat exchanger would suit this purpose ?

naturally the air flow cannot be interupted much, or the engine will overheat.

 

[TD2K]

An external jacket doesn't affect the air flow at all except for the reduction in its temperature. The downside is that it takes a long jacket to get much heat transfer area.

At 80C, I'm sort of doubtful how much heat you'll be able to transfer. Have you done any preliminary heat balances around the system?

 

[MortenA]

Its not a great deal of heat (and in the US it very unlikely it will be economical). I think e.g. a spiral heater would be suited. But you have to consider that a HX is a kind of blockage of the exhast and most engines does not react positively to this unless prepared for it. Also smoke is quite agressive - sespecially when cooled and the material for the HX may have to pretty high grade. (read your post alittle closer and saw that you are aware of the blockage issue). A spiral heater could be a candidate.

 

[waterworks178]

The plus side is the fuel consumption to the engine is only 1 liter an hour, which i want to get near to 80 oC using the hot air stream.

 

[btrueblood]

For 80C, it might be safer to use the coolant, rather than exhaust heat. If you are dead-set on using exhaust heat, consider a system which recirculates some of the heated fuel to the tank, in order to maintain the injection temperature at a stable value. Not sure what temperature veg. oil will start coking, but it wouldn't be much fun to have to replace the heat exchanger every year because it fouls from overheating.