why flue gas condensation takes place at 2nd pass in gas fired boiler, at 300 to 330 deg c of temp?

Answer 1

Can't think how water vapour would condense at that temperature (except at very high pressures which do not prevail). Is it possible that there is some oil or tar deposits in ducting which are smoking looking like condensation?

Answer 2

I'm not a boiler expert, but I don't see how this could be happening. If water is condensing out of the combustion gases at 300 to 330 C thats over 500 F. I don't have steam tables handy but it would require a pressure of over 100 lbs/in^2 to condense steam at that temperature.

Combustion products from burning natural gas should be less than 10% by volume water vapor it should require (if I'm thinking about this correctly) and even higher pressure to condense water out of the gas mixture than to condense steam.

The only thing I've ever seen that would simulate condensation in the fire tubes is if one of the tubes is ruptured and what you are seeing is water migrating from the boiler into the tubes. There is no way I can think of to build up enough pressure on the combustion side to condense water vapor at those temperatures.

Again I'm not a boiler expert but when this happened they shut the boiler down and had it repaired imediately.

Answer 3

I don't know the specifics, but generally, when a gas is first passed through a heat exchanger, its usually superheated steam.
The first heat exchange is to make the gas a saturated gas (has just enough energy to exist as a gas). We don't want it to be a two phase gas at this point because it requires a different setup to process (Ie: we treat a gas different than a gas/liquid system). For example, entrained fluid droplets can cause things like water hammer damage (ever turn on steam taps and hear a loud banging, that's water hammer and it needs lots of reinforcement or you can blow a pipe).
So, once all the excess energy is removed, we can pass the flue gas a second time and remove even more energy by condensing it.
Best of luck!

Answer 4

if water is condensing, it is because you have the right combination of pressure and temperature

at nearly 600 F, very high pressure is required for condensation

I think around 500 psig

if you have saturated steam, you should get some condensation