Heat Recovery @ 1250 F

[AO1958]

Hello there,

hope somebody could help me since I am dealing with an issue not faced to the forum up to now, - according to a research I have made a search in past posts -

Indeed the topic is quite crossing different forums. I hope to be correct posting it here.

I have to design an heat exchanger where air, derived from a combustion process is entering @1250F.
The aim is to recover heat, therefore air entering is the higher temperature.

I am just beginning, therefore I'd like to start with basic elements:

I'd like to ask you following pieces of information:
- have you ever faced to similar tech issue ?
- which kind of heat exchangers are utilized in these cases ? tube with external fins ?
- which materials are utilized in these conditions ? I would suppose that recovering air from combustion would imply fouling and corrosion issues, too.


Many thanks

 

[metengr]

What do you intend to do with the heat removed from flue gas, make process steam or other?

 

[IRstuff]

Where exactly did you search? There are plenty of heat recovery questions asked in the HAVC/R and Heat Transfer forums.

TTFN

FAQ731-376

 

[Compositepro]

Google regenerative and recuperative heat recovery.

 

[AO1958]

Many thanks to all for your answers.

To Mr. Metenger.
Yes,the aim is to warm process fluid.

To Mr.IrStuff
You are right concerning the existing topics.
Nevertheless I was not able to find any dealing with these conditions (air with so high temperature).
What I'd like to know at first is
- which materials has to be employed in this case
- which type of heat exchanger is commonly utilized in these cases ?

Many thanks

 

[davefitz]

At what pressure is the hot air?

If it is close to ambient pressure, then there are hundreds of examples of HRSG's heat recovery steam generators located downstream of a gas turbine's exhaust, typically 1150 F. The heat exchanger is composed of finned tubes, containing superheated steam.

If the hot air is pressurized , a similar concept can be used. For example in the 1960's the concept was called a "supercharged boiler". The overall size of the boiler can be greatly reduced with pressurized air, as the density/ convective heat transfer coeficient/ gas radiative emissivity increases with the air pressure.

One means of improving the efficiency of a standard combined cycle plant is to reduce the use of excess air in the combustion chamber. This in turn increases the adiabatic flame temperature above current metallurgical limits of tehe gas turbine blades. However, if one places a supercharged boiler between the pressurized combustion chamber and the first row of turbine blades, the supercharged boiler can be used to reduce the 3700 F adiabatic flame temp down to a reasonable 2600 F prior to entering the first row of blades. The supercharged boiler section can be mostly superheater / reheater surface, for fianl steam temp of 1300 F ( current research objective). The lack of corrosive combustion deposits avoids isseus with liquid phase coal ash corrosion, as occurs with an advanced coal fired unit.

 

[AO1958]

Many thanks Mr. Davefitz,

air is @ atmospheric pressure.
Therefore if I have well understood I have to google "heat recovery steam generators", haven't I ?

I would anyhow expect that this kind of heat exchangers are made by steel. Please, could you confirm this supposition ?

Many thanks

 

[rmw]

Also google "Thermal oil heaters" as the construction of a process fluid heater will be different than the typical HRSG construction which is for making steam and superheated steam from water.

Also check "fired heaters" which heat petroleum products in the refining process.

rmw

 

[plasgears]

Flue gas heat has been recovered by using Ljungstrom type units. It rotates a drum exposing successive sectors to heat, and the heated sectors are then exposed to a cool flow that recovers the heat in insulated ducts.

A fixed heat exchanger with counterflow elements would be simpler. Basically, it would have cool liquid flow pipes running across the flue and running down insulated tubes to the user.

3M published a heat recovery booklet some time ago, which covered a lot of ground. 3M's expert gave a presentation at a plant eng convention, and I strongly encouraged him to publish, which he did.