Cooling the flue on small furnace, any help apprec!

[adriangillitt]

I have already posted on here about the design of the flue for a small gas fired test furnace. The ID is 50mm and i have good insulation around the SS310. I would like the flue gas temperatures to come down to around 180C max. The volume of the furnace is 400 mm3 and temperatures inside are following ISO hydrocarbon curve so around 1100C.

I dont know if it is possible as we are on a tight budget, but could i tap in 3 water nozzles in the SS310 flue and spray a mist of thin jet of water into the flue? Would this cause material cracking/corrosion problems?

I can reallt afford to get an intercoller fitted as we are already over budget on the project?

Thank you in advance Adrian

 

[unclesyd]

We use water injection on a much larger scale to cool hot gas streams.
Your system is quite small water maybe a problem, though you could use steam. With steam you loose the latent heat but you don't have the problems you have with a water stream and you have better control.
For a system this small its going to take a very small and efficient atomizer or nebulizers to get the mist into the stream. If you try water use a supply of very pure water, we use condensate in one application and deionized water in
another. On a small system (6") we used a nozzle similar to an oil mist system nozzle.
Another problem using water is that you are going to eventually get thermal fatigue on the vent stack pipe, less so with steam.

 

[Montemayor]

adriangillitt:

I did a lot of this very thing some years ago. I used boiler flue gas at 500 - 600 oF, with 14 % CO2 for CO2 recovery by routing the flue line into a water scrubber. I don't know the size of your flue, but you can easily calculate the amount of water required to cool it. The problem might be that you don't want to cool it down to approximately 100 - 120 oF as it exits the top of the scrubber. If you don't, then this is not for you. If you do, then all you have to decide is how long do you want the small scrubber tower to last. You can build it out of carbon steel (& accept corrosion rates), stainless steel, or use porcelainized clay pipe. I used the latter with great success in a 1.0 meter diameter size in 2 sites in South America. I believe they are still operating after more than 30 years. I used ceramic raschig rings in one and chunks of coke in another as the packing media. A goose-neck overflow line was used to effect a water seal on the outlet. The scrubbers operated under approximately 10" of W.C. pressure. The outlet flue gas was cooled & subsequently taken to a soda ash scrubber and an Amine absorber.

We had a lot of corrosion in the carbon steel models and these lasted about 5 years before we had to replace them.

Hope this experience helps.

Art Montemayor
Spring, TX

 

[Cooky]

You could use a flue dilution system.

This consists of a duct mounted fan, which draws ambient air (say 20 deg.C) and mixes it with the flue gases (at 1100 deg.C), the effect of mixing the ambient air with the hot flue gasses will lower the exiting gas temperature.

You'll need to calculate the air volume to achieve a mixed gas temperature of 180 deg.C

This method is used successfully in commercial boiler installations.

Cooky.

 

[tkdhwjd]

Instead of using energy to cool down your flue stream, why don't you think of a way to capture to preheat/vaprize other streams in your process? If the energy savings are minor, then speding capital resources may not be a great idea, but a simple DCF calcs should be adequate how much you can invest.

 

[adriangillitt]

Thank you all for your help, but the furnace is very small and will be used to test the structural capability of FRP structures under both fire and load. Basically the furnace follows the ISO or Hydrocarbon curve with a FRP panel bolted to the open plate section on the front of furnace, whilst being put under load from a 50Kn screw jack.
THe problem is the furnace is going to operate in a LAB fume cupboard and the cupboard is only rated to a certain temperature and therefore i need to cool down the flue gas to not exeed this limit. Otherwise i would just pipe it to the atmosphere and not worry.

I want to cool it using a three stage water nozzle setup. So have three water nozzles laid out down the flue pipe the flow or spray rate increases as you go down the flue, this is to stop the metal flue from experiencing such a thermal shock from the cooling water, hence stop thermal fatigue or at least reduce it. And to stop any heavy corrosion etc. I am going to use Stainless Steel grade 310 for the flue which is rated at 1200-1300C so it should cope i hope.

Any further tips would be greatly appreciated.

Adrian

 

[GenB]

Exit the furnace horizontally, then vertically and tee downwards, spray the water from top to bottom,you will get 140-180F at the flue outlet, you can reuse the hot water for a process, make yourself a nice water heater, at 99% eff.
ER