Boiler Efficiency

[firefastball]

Hi there,

Can anyone help to clarify the boiler efficiency calculation particularly on the output part?

Control Volume is only on the boiler
Example: Assuming that there is no blowdown,

Mass balance:
Mass of feedwater = mass of steam
Energy Balance:
Mass of feedwater X Feedwater enthalpy = Mass of steam X Steam enthalpy

So the output on boiler efficiency equation would be:
Mass of steam X Steam enthalpy - Mass of FW X FW enthalpy
Replacing mass of FW to mass of steam would give us
Mass of steam (Steam enthalpy - FW enthalpy)

Now usually have blowdown to remove impurities
Looking at the same control volume with blowdown would give the following equations:

Mass balance:
Mass of FW = mass of steam + mass of blowdown
Energy Balance:
Mass of FW X FW enthalpy = mass of steam X steam enthalpy + mass of blowdown X blowdown enthalpy

Then the output on the boiler efficiency would be:
Mass of blowdown X blowdown enthalpy + mass of steam X Steam enthalpy - Mass of FW X FW enthalpy
Replacing of Mass of FW would give
Mass of BD X BD enthalpy + mass of steam X Steam enthalpy - mass of steam X FW enthalpy - mass of BD X FW enthalpy
or
Mass of Steam ( Steam enthalpy - FW enthlapy) + Mass of Blowdown (Steam enthalpy - FW enthalpy)

Am I doing it right?
Sorry for the long equations. Hope you guys can understand what I'm trying to ask.

 

[davefitz]

I would suggest you read the power test code ASME PTC 4.1 to get a better idea of how boiler efficiency is calculated, or read the book "Steam" by B&W , in particulate the chapter on combustion and boiler efficiency.

"Whom the gods would destroy, they first make mad "

 

[gruntguru]

Your energy balance equations should include heat input eg

Mass of feedwater X Feedwater enthalpy + Heat Input = Mass of steam X Steam enthalpy

je suis charlie

 

[EmmanuelTop]

See also

http://www.arb.ca.gov/cc/capandtrade/allowanceallocation/boiler_efficiency_calc.pdf

http://www.mae.ncsu.edu/eckerlin/courses/mae406/chapter6.pdf

http://www.energyefficiencyasia.org/energyequipment/assessment_boiler_indirectmethod.html

Dejan IVANOVIC
Process Engineer, MSChE