How can I recover waste heat from hot air exaust

Add a hot water coil to the exhaust stack, pump and piping to your tank. As far as turning pump on you could either have it turn on based on exhaust stack temp or water temp at the discharge side of the coil.

Yeah, I'd want to put some of that heat back in my wallet, too.

Water would probably be the most efficient way to extract heat from the exhaust stream, as BRL suggests. Cramming that much air through a truck radiator would probably need a more powerful fan to keep the required airflow going, though.

Just keep in mind that if that exhaust temp is 350F or more it could easily boil water to steam. You want a bulletproof way to make sure the pump is circulating water through your heat exchanger any time the exhaust temp is over maybe 180F (or something) for safety margin sake.

P.S. You might need to deal with some fume condensation from all the water in that large a gas flame. Mostly acidic water from the gas fire and whatever fumes your process might be giving off.

What's the cfm of the exhaust fan? What kind of particulates does the forge throw out? I'd be worried about that clogging something as fine as a car radiator or HVAC water coil. Maybe duct it into the bottom of a NG boiler, something with large flue passages and just treat the exhaust as a flame.

Do you know anyone that could work with stainless? Maybe fab up a heat exchanger with a bunch of 1.5" tubes about 3' long (just like gasifiers use) with water surrounding them. Just make sure the area of all the tubes equals the area of your 12" existing and you shouldn't effect airflow. If I have time I'll draw up something to give you the idea. Brian

find a stump 11" or so diameter. get 3/4"? coppertubing & wrap around the stump however many times. remove 11" coil from stump & install on exhaust flue, it should fit tight on 12" flue. install 1 pressure releaSE VALVE AT EACH END OF COIL. PUMP FROM TANK INTO COIL & BACK.

I would go with a cast iron radiator!

what fuels the forge?

I like the tube idea. I just removed a steel tube boiler from my house that would have been heavy @ 250lbs + water but nice easy flue passages to brush once a year, and about 80% efficient depending on the air flow. Just cut a larger hole into the firing chamber.

I don't think you're going to get enough heat transfer from an exterior coil. I think your best chance for maximum heat transfer would be something like the heat exchanger of a pool heater or as you suggested a radiator would pull the heat through.

Rather than a truck radiator, how about a big unit heater (Modine, etc.) as an exchanger. It's designed for large air flows and, perhaps just as important, easy water flow through its larger tubing. You can find these as used industrial surplus sometimes. They also are designed to get all the air out of the water lines, too.

water has a higher heat capacity than air, much better 3400 times better for the same volume.

Conduction is the best heat transfer mechanism. Wrapping a copper coil around the hood or pipe would be the best way to scrub some heat energy out.

But watch the flue gas temperature above the coil or hx. You need to stay above dew point. Condensing in the flue stack will cause dripping and rapid deterioration of the pipe.

hr

You'd have to flatten one side of the pipe to get any meaningfull transfer of heat, then insulate it to keep room air convection from robbing you of your gained heat.. Even then you will only be capturing the energy that comes in contact with the pipe while missing a majority of the area in the pipe, which is over 3x the circumference. If you've got a 12" pipe then your circumference is less than 37" but your area is over 113". The copper could work, but for every inch of exhaust pipe you'd need over 3' of copper tubing, getting expensive in a hurry @ $3-5 per foot.

I like the Modine heater idea. Made to hang, already plumbed for water, and pretty cheap if you get lucky. Problem however is the units use a mechanical fan to transfer heat past the coils/fins, which would have to be removed in this situation. What kind of reasonable constriction could be asked of draft fan on the forge?

if u lower the temp below 212*f u get condensation

Keep CO (carbon monoxide) in mind if you pull byproducts of combustion from the exhaust hood. It's a silent killer. CO detectors are cheap insurance.

hr