Heat Exchanger Design

[djdemzy]

Good Evening,

I am currently desiging a heat exchanger to heat Air from 10 oC to 110oC and cool flue gas from 1259.4 oC to 725 oC.

As both fluids are gases, i would like to know if it is advicable to use a shell and tube heat exchanger?

I read online that the convertional gas-to-gas heat exchanger is vertical shell and tube heat exchanger


Thank You Very Much
Kind Regards


Paul

 

[speco]

djdemzy,

A shell and tube design could be used for this. However, it really depends on the operating pressures. If both sides are operating at essentially atmospheric pressure, then your optimal design may be more rectangular.

Gas to air heat exchangers are generally fabricated using either tubes and tubesheets, or are plate type. Either one may work in your case, and it could be done with a simple cross-flow exchanger since the temperatures do not cross. However, your hot gas is VERY hot, and you will need to pay special attention to your materials of construction. Another problem area is differential expansion and leakage.

Regards,

Speco

 

[djdemzy]

Thank You Very Much Speco. You have really helped.

 

[speco]

djdemzy,

One thing I forgot to add. Since both air and flue gas are poor heat transfer media, you need as much pressure drop as you can get in order to get a decent heat transfer coefficient.

Good luck!

Speco

 

[gr2vessels]

speco,
If you are heating the combustion air, you'd be better off using a hot radiator (finned exchanger installed in the air duct) to heat the air. That's the cheapest way to heat air and the control is very easy. In any case, the finned heater is far better option than expensive TEMA exchangers.
Cheers,
gr2vessels

 

[MJCronin]

Nope.....

Rotary regenerative heat exchangers (slowly rotating plate type) are the best and most economical solution for this gas to gas HX.... a Ljungström ! !

http://www.engineersedge.com/heat_transfer/regenerative_heat_exchanger.htm

http://www.thermopedia.com/content/1087/

http://en.wikipedia.org/wiki/Air_preheater

Ask any coal fired power plant ! !

 

[djack77494]

Too big an issue for generalizations. It will depend on quantities (i.e. Ljungstrom might work for a "large" system but would be impractical for smaller systems), pressures and acceptable pressure losses, and other factors. In some situations S&T, plate, and Ljungstrom exchangers might all be "the right answer".

 

[rmw]

And... if you can stand 8% leakage right out of the box brand new degrading to up to 25% over a period of years, go for the Ljungstrom.

Now that is a 100+ year old technology that can't seem to die. Wow, think if Stanley Steamers had been as resiliant.

rmw

 

[gr2vessels]

Hey Paul,
Tell us, do you design a power station?

 

[rmw]

Temperature range on the flue gas doesn't sound like any power station I ever heard of. Same with the air side I guess. Sounds like a lab project, maybe even homework.

rmw

 

[gr2vessels]

Once again, the facilitator has failed to properly select the posting candidate;- at least should consider purging the list from people who are not considerate or plainly just using the forum for their own benefit without contributing at all to the community.