Pipe Heater Equation(s) 2

[4301JGS]

It has been a while since I have dealt with heat transfer, but I have the following issue. I need to heat up water going to some equipment that needs a min temp. The supply temperature is somewhat variable and with the onset of cold weather, we are going below the temperature we need.

One option thrown out is to route our supply pipe next to a duct of hot flue gas and recover the heat from that. The design would be to route the pipe close to the duct and then insulate the pipe and duct together. Essentially the water pipe would have all kinds of heat available since it is a large duct.

Basically, can someone point me in the right direction for calculating this. Would this just be a straight conduction problem since the hot service would be a constant temperature?

 

[MintJulep]

I would not advise that you pursue this without some greater degree of consideration.

Many of the things in flue gas are the components of acids. When flue gas condenses, you make acid. This acid will corrode your duct and your pipe.

Flue gas flow is typically buoyancy driven. If you take enough heat out of a flue gas stream you can stop the flow.

 

[chicopee]

As MintJulep points condensation of carbonic acid could take place, that's why flue gas temperature should not be lowered than 450 deg.F or thereabout. If your flue gas temp was a lot higher, let's say 900 deg F, then I would say go for it, however I would explore the idea of putting the cold water pipe with fins in the exhaust duct and in essence you would have an economizer.

 

[4301JGS]

Thanks chicopee and mintjulep for your answers. I think I have found a better way, using a direct steam injection into the water. I appreciate your input!