Heat Rejected from boiler into boiler room

[BronYrAur]

I am trying to determine the heat lost from a boiler into the boiler room. I want to make sure I have proper ventilation in the summertime.

I have gone back-and-forth with my vendor and received a calculation sheet, but most of it doesn't make a whole lot of sense. See attached. The rep told me that the important value is the difference between the "Combustion Efficiency" and the "Fuel to Output Efficiency". So in my example of a Burnham 3P-200-G boiler, the difference is 82.629% - 80.217% at the pressure, ambient temperature listed, etc., which is 2.412%.

So, does that mean that 2.412% of my input BTUH (which is 8,346,131 BTUH) is rejected into the boiler room? So approximately 200,000 BTUH needs to be accounted for as I ventilate the room, correct?

Thanks for your help.

 

[PaulV7427]

Hi BronYrAur,

Unfortunately with your scenario there are still a lot of unknowns but I will do my best to ensure that all my assumptions are fully stated.

After looking through the PDF that you attached, I perceive the combustion efficiency as the overal efficiency of the combustion reaction inside the boiler. Therefore, with all the fuel that is supplied to the chemical reaction, only 82.629% of the energy is output inside the boiler. From there, due to miscellaneous losses (sound, overall design inefficiency, etc.) not all of that 82.629% energy is directly utilized to heating the medium. Therefore, the Fuel-to-output efficiency is slightly less (80.217%). Now, to determine the energy being lost from the boiler into the room I will need to assume that your flue gas from the boiler is not being let out into the boiler room. With that assumption, your resultant energy los of 2.412% is correct. This is a maximum heat loss percentage due to the fact that not all of the miscellaneous losses result in energy being put out into the boiler room.

However, if my earlier assumption of the flue gas not being let out into the boiler room is incorrect, and for some reason you do have the flue gas being ventilated inside the boiler room, then your energy output to the room is no longer just the miscellaneous losses but also the energy present in the flue gas. The total energy is then 100% - 80.217% = 19.783%.

The main reason for the difference between the two efficiencies is the misellaneous losses. If the design were perfect and we assumed that all energy that was created by the combustion reaction was absorbed by the medium, then the remaining ~18.4% of energy is captured by the combustion by-products (aka the flue gas).

Another important note is that these percentages only apply at the stated environmental conditions in the PDF that the vendor supplied. If the temperature, elevation, etc. changes from the stated values in the document, your percentages will vary.

In summary your 2.412% is correct and can be used as a worst case scenario for energy needing to be ventilated from the boiler room.

 

[macmet]

I think Paul put together a pretty good summary. But, just to add to it...I'm used to using a standard heat loss rate as a standard percentage of fuel input for solid fuel boilers.

But the Steam book is a better resource and it has this note in section 23.,

"Heat losses, corresponding to these surface temperatures (135F), range from 90 to 130 btu/hr/ft2...

I thought that they had it stated somewhere earlier in the book as well but I wasn't able to find it in my quick search.

Did you happen to ask your rep what "standby radiation losses" are? I'd be curious what they mean by that.

 

[MisterDonut]

BronYrAur,

200,000 BTU/hr appears correct. Also make sure you are satasying applicable mechanical and fuel gas codes. Example: International Mechanical Code requires 1 CFM per 2400 BTU/HR fuel input for ventilation/combustion air/dilution air.