Boiler Heat Rejection Reduction at Various Boiler Loads

[John_187]

Hello, there is a project with an industrial boiler. The client is wanting to use the heat rejection of the boiler to help out the space heat load calculation (to require less unit heaters). The majority of the space heat load is from combustion air, so that airflow load would vary linearly with boiler load. Thus the combustion air load is only on when the boiler is on, so you can count the heat rejection.

The client says the boiler won't operate any higher than 75 % load, and that the reported heat rejection of the boiler into the space won't differ compared to 100 %. They claim that the surface temperature of the boiler will be the same, so the boiler heat rejection will be the same at 75 % load compared to 100 % load.

Is this assumption reasonable? Even if it is reasonable at 75 % load, it doesn't seem that it would be true at lower boiler loads (such as 50 %, 25 %).

What assumptions should be made here? That the boiler heat rejection would decrease linearly as boiler load decreases (starting at 75 % down to 0 % assuming their reported heat rejection at 75 % is correct).

Thanks for your input

 

[lukaiENG]

How are you going to recover the heat from the combustion air? The combustion products should be taken to atmosphere via a flue. Boilers normally have a pretty strict limitation on back pressure so doing some sort of heat recovery seems unlikely (and $$$) so the majority of the heat rejection is going to be lost to atmosphere. You will some radiant/convective gain off the surface of the flue and boiler themselves, but this will be small by comparison, particularly if the flue is double skin. Manufacturers can probably clarify this, any good boiler should be pretty well insulated.
You can probably get efficiency curves from the manufacturer which show part load performance, this should give you an idea of the expected heat loss under different conditions. Their assumption seems unlikely to me, yours about a linear relationship seems closer to the mark.
This seems like a lot of effort to eliminate some relatively cheap unit heaters.

 

[willard3]

If this is a modern condensing boiler, standby losses are very, very small. If you count on standby loss to heat the boiler room, you will have a cold boiler room

 

[chicopee]

What type of a boiler and which is the manufacturer. What is the exhaust gas temperature and the flow rate at 75% load. Remember reduction in exhaust gas temperature could create condensation problem in metal exhaust stack, therefore boiler manufacturers recommend a minimum exhaust gas temperature.