Boilers and Furnaces Flashcards
Difference between boilers and furnaces
Boiler - heat water for applications (hot water or space)
Furnaces - heat used directly
2nd law of thermodynamics
Heat is the absence of external assistance can only flow from a hotter to a colder body.
Heating Value
HHV
Amount of heat per unit of fuel when completely burned and combustion products cooled to room temperature.
Assumes the water in the gas is completely condensed, recovering latent heat.
Carnot Cycle
Carnot efficiency
Heat > Work
maximum = 1-Tc/Th
Efficiency of boiler
useful energy delivered/energy available in the fuel
Load factor
Actual energy input/ energy input if operated at max output.
Efficiency is greater at higher load factors.
3 sources of heat loss in boilers
Radiation, blowdown, flue gas
What are radiation losses in boilers
surface losses, constant with load so low load represents greater % loss
What are blowdown losses in boilers
As the water is evaporated, salts and impurities must be blown down to remove.
Also prevents fouling
What affects the flue gas losses in boilers
Combustion conditions, primarily the A/F ratio. Increases with temperature and excess air.
How to minimise boiler losses (4 points)
1.) Controlling A/F ratio
2.) Using heat to preheat water
3.) Recovering flash steam from blowdown
4.) insulating pipes and boiler
What are condensing flue boilers? Which type is used for process heating?
Improves efficiency by extracting latent heat from vapour. Important for NG as there is high water in products.
Wet
Some characteristics of a domestic condensing boiler
two HX zones, condensate cant be in contact with flame.
Fans
Exhaust gas must be cooled below the dew point.
Why is the dew point relevant in condensing boilers?
What changes it?
Exhaust gas must be cooled below dew point for condensation>
PP, decreases with excess air , meaning lower T required.
Impact of humidity on boilers?
Combustion air contains water which will be condensed, improving efficiency.
