calculate heat transfer for condensing economizer

Please can anyone help me to calculate heat transfer for condensing economizer for steam boilers.
Fuel is natural gas.
I can calculate heat transfer if outlet gas temperature is min. 100oC. But I can't do it when outlet gas temperature is lower than 100oC. In that case begin aditional proces of heat exchaning and with condensating vapor of water as product of burning.

Construction of economizer is: cold water (15oC) goes in steel pipes with square steel rib . Outlet gas goes around.

Please excuse for my english

dragniko@mail.com.mk

 

[TD2K]

I'm assuming you can't calculate the heat transfer from the water side of the economizer as that would be easiest (water flow rate and inlet/outlet temperatures would give you the duty). I don't think your 100C figure is correct BTW, you aren't talking about 100% water vapor, there are a lot of other components in the stack gas, water is just one.

You could always calculate the heat transfer from the condensing water vapor. From your fuel flow, composition and excess O2/air, determine the stack gas composition. Use the stack temperature out of the economizer along with steam tables to determine the partial pressure of water. If the calculated partial pressure is less than this, you are above the water dewpoint. If it is below, then some of the water will condense. Calculate how much condenses (basically, how much water vapor along with the other components gives you a saturated stream, water wise) and determine the amount of heat the condensed water contributes to the total Q. The other components that don't condense contribute sensible heat based on their flowrate, effective Cp and dT across the economizer.

Sorry, this is pretty short I know.

If you have Pro/II or Hysys, stick the stack gas composition in that and let it handle the details.

 

Thak TD2K veru much for your answer.
I will try to input heat of condensing in your way.

p.s. 100oC means 100 degrees celsius, I work in SI sistem of measures.
You are wright when you think that the question is for the gas side, I miss to precise.

dragniko@mail.com.mk