Flue gas water

[omph]

That's a good question. I think the reason we don't see it, is because when steam is very hot, like it will be as it leaves the flame ((hundred of degrees), it is purely in the form of water vapour, which is an invisible gas. When we see steam coming out of a kettle, I don't think that is pure water vapour. Instead, it is mixture of water vapour and water droplets, and we are seeing the water droplets.

Interesting points. What stumped me is when I read that condensing boilers can produce up to 2L of water per hour. That is a lot of water and is what provoked my question. Gas boilers must use a lot of gas. I always thought the flame on a hob and in a boiler's chamber would be roughly the same.

 

[JonathanM]

Interesting points. What stumped me is when I read that condensing boilers can produce up to 2L of water per hour. That is a lot of water and is what provoked my question. Gas boilers must use a lot of gas. I always thought the flame on a hob and in a boiler's chamber would be roughly the same.

The flame on the hob and the flame in the combustion chamber will be the same temperature. The flame in the boiler will be about ten times bigger. Like the hob, the boiler will also produce invisible water vapour. It is only when it is deliberately cooled down later that it condenses back into water.

If you go back to the equation:

CH4 + 2O2 → CO2 + 2H2O

The molecular weight of methane is 16, and the molecular weight of water is 18. So, every 16g of methane we burn, produces 36g of water.

If a typical boiler is about 90% efficient, and burns at a rate which produces say 10KWh of heat per hour (in cold weather), it will burn about 11KWh worth of gas in that hour. 11KWh worth of gas is about 1 cubic metre of gas. The density of natural gas in about 0.7 Kg per cubic metre. So in that hour we are burning about 0.7Kg, or 700g of gas.

700/16x36 = 1575g

This means that the 700g of gas burnt in an hour would produce about 1.6Kg of water, which is 1.6L of water. So, the 2L figure sounds about right.

 

[omph]

The flame on the hob and the flame in the combustion chamber will be the same temperature. The flame in the boiler will be about ten times bigger. Like the hob, the boiler will also produce invisible water vapour. It is only when it is deliberately cooled down later that it condenses back into water.

If you go back to the equation:

CH4 + 2O2 → CO2 + 2H2O

The molecular weight of methane is 16, and the molecular weight of water is 18. So, every 16g of methane we burn, produces 36g of water.

If a typical boiler is about 90% efficient, and burns at a rate which produces say 10KWh of heat per hour (in cold weather), it will burn about 11KWh worth of gas in that hour. 11KWh worth of gas is about 1 cubic metre of gas. The density of natural gas in about 0.7 Kg per cubic metre. So in that hour we are burning about 0.7Kg, or 700g of gas.

700/16x36 = 1575g

This means that the 700g of gas burnt in an hour would produce about 1.6Kg of water, which is 1.6L of water. So, the 2L figure sounds about right.

Excellent post, this is what I was looking for. Do you know how 2 molecules of O2 are acquired ? How is the fuel/air ratio calculated ?

 

[JonathanM]

Excellent post, this is what I was looking for. Do you know how 2 molecules of O2 are acquired ? How is the fuel/air ratio calculated ?

I'm not very good with the engineering side of things. So I will tell you the simple bits I know!

On modern boilers, the combustion chamber is sealed from the dwelling. The air/oxygen is brought into the boiler, from outside the building, through the flue. What happens after that is a bit hazy for me. The air is mixed with the gas in some way, and passed through the burner. I don't know exactly how.

I've read that "excess air" is usually used, to minimise things like soot and unwanted combustion products, like carbon monoxide. So it sounds like the mixture will be made a bit lean on purpose. As far as I can remember, that reduces efficiency a little, but I can't remember why.

EDIT: I think I might be able to show you how the fuel air/ratio is calculated, if you would like. I've been reading that it is about 10:1 air to fuel. So that must supply the 2 molecules of oxygen for every methane plus the required "excess air", which I've read is typically 10% to 20%,

 

[flameport]

I always thought the flame on a hob and in a boiler's chamber would be roughly the same.

Typical gas hob burner on full power = 3kW
Typical combination boiler 30kW or even more.

 

[Madrab]

Also, to explain the mechanics. When NG burns, the heat from the burner travels down the Heat exchanger (HEX), transferring that heat into the CH system water as it goes, as it reaches the bottom of the HEX it has cooled significantly, from ~1000+ deg down to ~70odd deg. If the return temperature of the CH system water is balanced correctly then that water reaches the boiler ideally at <55degC.

This water temp is the ideal temp to allow the flue gases to release the last of it's heat and as the dew point of the flue gas is around 55deg then as the gases transfer the last of that heat into the CH water, it reaches the dew point and the water vapour condenses out of the flue gases and turns into water - there's the condensate (water).

edited for typo

 

[Harry Bloomfield]

On modern boilers, the combustion chamber is sealed from the dwelling. The air/oxygen is brought into the boiler, from outside the building, through the flue. What happens after that is a bit hazy for me. The air is mixed with the gas in some way, and passed through the burner. I don't know exactly how.

Not an expert either, but....

Air is drawn into the boiler by a variable speed fan, speed monitored and driven by PWM. The fan, being functional, is part of the boiler's essential startup checks. The fan's speed is carefully matched to the burner's gas flow, to ensure precise combustion. The flue is concentric, air drawn in, and combustion gases expelled concentrically.

 

[lloyda]

If the return temperature of the CH system water is balanced correctly then that water reaches the boiler ideally at >55degC.

Presumably a typo, and you meant less than 55 degC?

 

[Madrab]

Presumably a typo, and you meant less than 55 degC?

yup should have been <55Deg

 

[John D v2.0]

dew point of the flue gas is around 55deg then as the gases transfer the last of that heat into the CH water, it reaches the dew point and the water vapour condenses out of the flue gases and turns into water - there's the condensate (water).

Just to add to that, the dew point is when the water starts to condense out. At that point the humidity would be 100% by definition. as the temperature drops the capacity of the air drops still further and water continues to condense out (all the way down to room temperature and beyond)

Not an expert either, but....

Air is drawn into the boiler by a variable speed fan, speed monitored and driven by PWM. The fan, being functional, is part of the boiler's essential startup checks. The fan's speed is carefully matched to the burner's gas flow, to ensure precise combustion. The flue is concentric, air drawn in, and combustion gases expelled concentrically.

I wouldn't say the burner has a gas flow itself, actually the gas is transferred into the air stream by the movement of air from the fans speed. Then the premixed combination enters the burner and then into the combustion chamber.

 

[JonathanM]

I wouldn't say the burner has a gas flow itself, actually the gas is transferred into the air stream by the movement of air from the fans speed. Then the premixed combination enters the burner and then into the combustion chamber.

Like a carburettor?

 

[omph]

Just to add to that, the dew point is when the water starts to condense out. At that point the humidity would be 100% by definition. as the temperature drops the capacity of the air drops still further and water continues to condense out (all the way down to room temperature and beyond)


I wouldn't say the burner has a gas flow itself, actually the gas is transferred into the air stream by the movement of air from the fans speed. Then the premixed combination enters the burner and then into the combustion chamber.

Thanks. Where does the condensation happen ? So the heat exchanger is where the combustion transfers the heat to water, but where does the returning CH flow and the flue gases meet ? and from what I've observed condensation can take at least a few seconds, and thats a long time when talking about the movement of gas.
Also, does the latent heat extraction happen with hot water too ? or is it limited to CH ?

 

[John D v2.0]

Like a carburettor?

I'm too young to know how one of those works, but if you Google something along the lines of
premix Venturi zero governor
You shouldn't be too far away from the answers

 

[JonathanM]

I'm too young to know how one of those works

 

[John D v2.0]

Thanks. Where does the condensation happen ? So the heat exchanger is where the combustion transfers the heat to water, but where does the returning CH flow and the flue gases meet ? and from what I've observed condensation can take quite some time, and thats a long time when talking about the movement of gas.
Also, does the latent heat extraction happen with hot water too ? or is it limited to CH ?

The useful condensation happens anywhere there is a surface cooler than the flue products. So it could be the heat exchanger all the way through to the terminal of the flue.
Anywhere beyond that and the heat will be lost outside into the great outdoors.
It's not something that happens once and it's done, the same flue gas loses more moisture as it cools further and further.

Condensation isn't something quick or slow, it just happens as soon as the conditions are met. Look how quick it happens when you breathe on a cold window pane.

It happens just the same with heating and hot water, if the heat exchanger is cold enough.