Small scale flue gas/waste heat heat exchanger utilizing low quality heat

lassej

Hey all,

I am currently researching the possibilities of utilizing waste heat from a catalytic process to heat a small hot water coil in an adsorption dehumidifier for a utilities company.

Currently I'm wondering whether there is any available small scale component that I can fit on the chimney and let the natural draft of the flue gas release its heat to a copper tube wound around the inner circumference of the chimney? Or does anyone have similar experience with using low quality heat where I cannot interfere too much with the pressure in the system?

My main problem is the constraints on the catalytic process, where I have found that most heat exchangers will cause too large of a pressure drop and affect the process.

Currently I have a flue gas of :

• Around 134 deg C
• Heat capacity of 1.013 kJ/kg*K
• Flow of around 425 kg/hour.
• Relatively clean/ non corrosive gas: (0,05% methane, 17% oxygen, 3,5% CO2, 3% H2O and the rest +70% being nitrogen)
• Goes out of a chimney, thereby at atm. pressure.

The water loop has constraints of:

• 80 deg C inlet (max 100) and 40 deg C outlet
• Flow: 0,13 liters/s and max pressure 10 bar (g)

It is a relatively small installation, whereby my thinking is that most industrial heat exchangers are too beefy and expensive. Thus, I'm looking for a more small scale technology that would be economically viable.

Thanks in advance!

-LJ

hot_rod

what type of device is connected to that flue? You need to be careful not to pull too much heat and cause problems in the flue, condensation, corrosion etc

Sounds like this may need to be a custom built HX. If so tube outside not inside the flue.

Kaos

A larger diameter drain water heat recovery pipe might work. Just check that nothing the flue gas will corrode the copper pipe.

These are meant to work with a fluid film over the wall pipe for heat transfer, so not sure how much you can get out of it with air. Some baffle/turbulator (twisted flat plate, similar to the one in the flue of a gas water heater) in the middle might help without adding too much restriction.

PS. Having just written the above, why not get a gas fired water heater, remove the burner bits and use the flue as your heat exchanger. You get the heat right into the water heater without any extra bits.

lassej

https://forum.heatinghelp.com/discussion/comment/1818630#Comment_1818630

I was kind of hoping that I could just have the end of the cooled down flue be released to the atmosphere, perhaps drill a drain hole after having exchanged heat, so that condensate could run out. But I see that a custom built HX may not be economically viable, as I'm essentially just trying to save some electricity by compensating for an electric heating coil with my set up.

lassej

Thanks the replies!

I had an idea and was wondering if anyone has inputs to the feasibility.

What about the possibility of using a water jacket, like on a wood fired stove?

Something like this one;
https://hitze.pl/en/product/heat-exchanger-wcpw01w-fi180/

lassej

https://forum.heatinghelp.com/discussion/comment/1818632#Comment_1818632

Interesting about the gas fired water heater! So what you are saying is essentially to just shoot my hot flue gas into the water? I'm afraid that would cause some pressure serious pressure drops though, having to push the gas into a body of water, or am I way off here?

fentonc

@lassej - I think the idea is to just remove the burner from the water heater and connect your existing flue to the bottom of the water heater flue (where the burner was). The flue gases heat up the water in the tank on their way through the water heater flue, but no gases come into contact with the water.