Flue gas water

[JonathanM]

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.

I think the flue gases are routed over the cold part of the heat exchanger by the fan, on their way to exiting through the flue. There's several different designs of heat exchanger these days. But the easiest one to visualise, I think, is a radial coil type heat exchanger. I found this video really helpful. Especially at the exact one minute point, where it shows the burner's placement within the combustion chamber, and how the flue gases flow over the coil. I've been told by a very knowledgeable poster that this is probably the world's most common heat exchanger, and is used in several major brands including Vaillant.

 

[MeldrewsMate]

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%,

Excess air is used to ensure thorough combustion of the methane. It's used because we want to reduce the partial combustion of methane (which produces carbon monoxide) to a minimum. Partial combustion occurs when the methane molecule doesn't happen to meet two molecules of oxygen in the air-gas mixture, so to reduce the chances of partial combustion we add extra (excess) air. Excess air causes inefficiency because it consumes some of the heat energy in itself being heated, thus reducing the flame temperature.
The perfect burner would need no excess air and would produce zero CO.

The reason for the 10:1 air to methane ratio is that air contains around 20% oxygen, so in the reaction equation given previously:
CH4 + 2O2 ----> CO2 + 2 H2O
for each methane molecule to have enough oxygen to fully combust we must supply it with 2 oxygen molecules, ie 4 atoms.
I think it was a geezer called Avogadro (drummed in by Frankie Marsden, my old chemistry teacher) who determined that every cubic metre of any gas at normal pressure and temperature contains the same number of ATOMS, so 1 cubic metre of methane needs 2 cubic metres of PURE oxygen to combust fully....as air (conveniently surrounding us) is only 20% oxygen then we need 5 times as much volume of air to get the oxygen required, and so 1 m3 of methane needs 2 m3 of oxygen, which means 2x5= 10 m3 of air.

MM

 

[JonathanM]

Excess air is used to ensure thorough combustion of the methane. It's used because we want to reduce the partial combustion of methane (which produces carbon monoxide) to a minimum. Partial combustion occurs when the methane molecule doesn't happen to meet two molecules of oxygen in the air-gas mixture, so to reduce the chances of partial combustion we add extra (excess) air. Excess air causes inefficiency because it consumes some of the heat energy in itself being heated, thus reducing the flame temperature.
The perfect burner would need no excess air and would produce zero CO.

Thank you, that's such a clear explanation.

The reason for the 10:1 air to methane ratio is that air contains around 20% oxygen, so in the reaction equation given previously:
CH4 + 2O2 ----> CO2 + 2 H2O
for each methane molecule to have enough oxygen to fully combust we must supply it with 2 oxygen molecules, ie 4 atoms.
I think it was a geezer called Avogadro (drummed in by Frankie Marsden, my old chemistry teacher) who determined that every cubic metre of any gas at normal pressure and temperature contains the same number of ATOMS, so 1 cubic metre of methane needs 2 cubic metres of PURE oxygen to combust fully....as air (conveniently surrounding us) is only 20% oxygen then we need 5 times as much volume of air to get the oxygen required, and so 1 m3 of methane needs 2 m3 of oxygen, which means 2x5= 10 m3 of air.

You've remembered your chemistry a lot better than me. I'd forgotten all about Avogadro's Law. It all makes sense now. Thanks!

 

[omph]

Great points. If the flue gases will cool where ever there is a surface cooler than the gas, what about flow return water ? The objective of the flow return is to have it low enough to maximise condensation, but what will that return water flow through ? The heat exchanger ? but isnt the HEX burning gas thereby reducing it's effect ?
I would have thought the flue gas would go through another block (like a plate heat exchanger) whereby the return temp would then cool the gases sufficiently.

 

[Madrab]

Great points. If the flue gases will cool where ever there is a surface cooler than the gas, what about flow return water ? The objective of the flow return is to have it low enough to maximise condensation, but what will that return water flow through ? The heat exchanger ? but isnt the HEX burning gas thereby reducing it's effect ?
I would have thought the flue gas would go through another block (like a plate heat exchanger) whereby the return temp would then cool the gases sufficiently.

You do get heat recovery systems (not cheap) that are an addition to the main HEX that fit around the primary flue on some boilers catching the last of the heat from the combustion process but if the main HEX is clean and working efficiently and the return system water temp (that enters the HEX at the bottom or the back, HEX design dependent) is balanced correctly, then that maximises the cooling and therefore condensing of the flue gases, and recovers the most amount of heat possible.

 

[MeldrewsMate]

I would have thought the flue gas would go through another block (like a plate heat exchanger) whereby the return temp would then cool the gases sufficiently.

In the days before condensing boilers the main heat exchangers used to be sized smaller so that flue gases would not condense; typical flue gas temperature of around 210 C - hex material was almost universally cast iron, and the acidic nature of condensate would lead to rapid corrosion and hex failure. Some of the first generation of steamers had secondary heat exchangers made of aluminium alloys or stainless steel, the purpose of which was to encourage condensing of the water content of the flue gases and recover the latent heat of evaporation (around 2000 kJ per kg if I remember correctly).
As hex materials and hex design improved a single larger hex is now utilised in most boilers, with exit flue gas temperatures typically 55 C. The flue gases condense more completely (and thus more latent heat is recovered) with lower flue gas temperatures; this is why boilers run more efficiently when return temperatures are lower, and why correct radiator balancing to get the return water as low as possible is so much more important than it used to be.

 

[omph]

In the days before condensing boilers the main heat exchangers used to be sized smaller so that flue gases would not condense; typical flue gas temperature of around 210 C - hex material was almost universally cast iron, and the acidic nature of condensate would lead to rapid corrosion and hex failure. Some of the first generation of steamers had secondary heat exchangers made of aluminium alloys or stainless steel, the purpose of which was to encourage condensing of the water content of the flue gases and recover the latent heat of evaporation (around 2000 kJ per kg if I remember correctly).
As hex materials and hex design improved a single larger hex is now utilised in most boilers, with exit flue gas temperatures typically 55 C. The flue gases condense more completely (and thus more latent heat is recovered) with lower flue gas temperatures; this is why boilers run more efficiently when return temperatures are lower, and why correct radiator balancing to get the return water as low as possible is so much more important than it used to be.

Do you know how they managed to fix the acidity eroding the HEX issue ?

 

[MeldrewsMate]

Do you know how they managed to fix the acidity eroding the HEX issue ?

With clever chemistry and better materials, do a search on t'interweb.

 

[JonathanM]

Do you know how they managed to fix the acidity eroding the HEX issue ?

By changing the heat exchanger material to one which does not corrode. The heat exchangers are now made from aluminium or stainless steel, instead of cast iron.

 

[omph]

By changing the heat exchanger material to one which does not corrode. The heat exchangers are now made from aluminium or stainless steel, instead of cast iron.

From what I understand, aluminium does corrode, although not sure if that is due to other casual factors or the acidity. The yearly servicing of the condensation trap would reveal black grit which apparently is HEX corrosion.

 

[Madrab]

You do get some aluminium oxide created by corrosion from the condensate in Alloy HEX's, as suggested that's where the white/grey particles on the HEX and condy trap comes from. A good reason why Alloy HEX's should be opened up and checked/cleaned/flushed regularly.

 

[vulcancontinental]

There is water content in the gas supply but kept within limits by the transporter.

The water vapour referred to in the OP is the result of a chemical reaction, a rearrangement of the atoms that are 'reorganized' as a result of the reaction created by the combustion process. Mix what goes in to the necessary proportions, methane and combustion air, introduce a source of ignition, result, a chemical reaction producing heat, the atoms are rearranged in the process to form molecules of CO2 and H2O.