99.99% Efficient Boilers 3

[ChrisConley]

It seems like boiler manufacturers are making more and more claims towards incredible thermal efficiency. I've seen a half a dozen website claiming efficiencies over 97%. If you read further there is a list of conditions that is nearly impossible to achieve for this effciency to occur (return water under 100 degF, altitude?)

I guess my question/tip is. What is the maximum theoretical combustion efficiency of a boiler? And does anyone have any comments on a practical maximum effciency? I've heard from a reputable manufacturer that 94% is the highest efficiency to be expected from a natural gas boiler.

Any comments?

 

[imok2]

Those efficiencies are possible with furnices but only because they have double heat exchangers the squeeze out the heat of condensation, so if there is some way to take advantage of this process,say with the stack gas and bring the same process to bare then I don't know how they can get that much efficiency. Others may have the answer, good post!

 

[walkes]

In most boilers the heat from the combustion of natural gas is broken down into the following components 10.2% latent heat and 89.8% sensible.
Of the 89.8% sensible it is further broken down to 4% flue gas, 4% jacket loss and the remaining ~82% is used to heat the fluid. This occurs with a net stack temperature (the temperature of the flue gas above the primary air temperature)in the area of 350°F and a CO2 output of 10% ( the CO2 produced is in relation to the amount of excess air during combustion in this case 15-20%).

Now by doing some creative things the efficiency can be increased. By adjusting the fuel-air mixture the efficiency can be increased. By reducing the net stack temperature the efficiency is increased (achieved by forcing the combustion process into the condensing range and increasing the amount of heat transferred to the fluid). By insulating the boiler better the jacket loss is reduced.

The 10.2% latent heat is regained by condensing in the boiler and the heat is transferred to the fluid the loss is less than 2%. The cooler flue gas radiates less heat the loss is now less than 1%. Well insulated boilers can reduce heat loss to less than 1%. Now you have ~96% of the combustion energy transferred to the fluid.

The 96% is likely only achievable when doing domestic water heating from 40°F to 140° staying in the condensing range as much as possible.

There are other factors that will affect the dewpoint of the flue gas like altitude of the system, and the exact combustion process (excess air etc).

Hope that helps somewhat.

 

[Accystan]

Wilkes:

That's a really good post; an excellent explanation/overview of the combustion process within boilers. Very well broken down into understandable chunks.

Thank-you.

Just out of interest, are the percentages you quote hard and fast numbers or do they vary appreciably under the influence of external factors (gas composition, combustion air temp/humidity, altitude etc.)?

 

[walkes]

The percentages are not hard and fast, they will vary with gas composition, combustion air temperature/humidity altitude etc.

combustion air intake temperature from 0°F to 100°F will vary the combustion efficiency +/- 1.5% compared to 70°F combustion air.

I attended a condensing boiler technology seminar put on by Viessmann Boilers in Waterloo, Ontario, Canada. They explained the combustion process very well, I guess some of it stuck with me. Another resource for combustion analysis would be Bacharach (they produce flue gas analyzers and other combustion analysis equipment but I don't have any contact information)

 

[NorgenLou]

People sometimes overlook that if the equipment is in an area that needs heat anyway (many residential units), the efficiency issue becomes much less important since many of the "losses" aren't really losses.