Boiler Flue Gas Economizer Calculations

[EnerEn]

Can any help with how to select Boiler Flue Gas Economizer, and how to calculate gas out temperature and water in to feed tank?

Thanks.

 

[davefitz]

Most people use the simple method of Q=UA*LMTD, where
Q= heat transferred from gas to water,
U= overall heat transfer coefficient, inclcuding resistance to heat transfer caused by fouling and tube metal conductivity( extended surfaces need to include a fin efficiency)
A= total surface area of the heat exchanger

A simple starting point is to assume a gas velocity thru the bank of 50-60 fps , and the UA = 1.5 *WgCp,g ( this is equivalent to an NTU = UA/WCp = 1.5 ). The number of tubes operating in parallel to yield a tubeside water velocity of 10-15 fps.

A more accurate method is the e-NTU method, but I think you might not be ready for that yet.

 

[Feric]

Hi there:

On the gas side, it would be good to know the flue gas composition.

There is a possibility that you can deal with the enthalpy values directly for the amount of heat that gets exchanged.

Q = (h gas in - h gas out) * gas mass flow rate [kW]

Q = (h water out - h water in) * water mass flow rate [kW]

Then you figure out the required surface for such amount of heat transfer, as indicated above.

Thanks,

Gordan Feric, PE
Engineering Software

http://members.aol.com/engware

 

[greenche]

The ESCOA manual has a calculator included for sizing economizers.

It can be downloaded from this page:

http://www.fintubetech.com/escoa/