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Boiler Efficiency Calculation Inquiry

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FAIZANYOUSUF

FAIZANYOUSUF

Mechanical
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Jan 23, 2016
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Hello experts and professionals,

I want to confirm one thing regarding boiler efficiency calculation method.

My feed water temperature at dearator tank is 95C after which it goes to air preheater before going finally into boiler. The water temperature after air pre heater is 110 C. So while calculation boiler efficiency via direct method, I have to take value of feed water at 95 C or 110 C?

My vote goes for 95 C since it is the heat given to water for heating. The rest temperature is raised via exhaust gases emission which should not be included as it is heat extracted from something which is not produced separately for water.

Please confirm.

Also, if anybody like to help me confirming/clarifying following,

To get natural gas fuel in KJ/KG from BTU/SCF, I am doing following steps

1- Multiplying BTU/SCF with KJ/cubic meter.
2-Dividing this figure by natural gas density (kg/cubic meter) to get finally KJ/KG

A much thanks in advance.
 
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Here is your problem with your OP. An air pre heater heats up combustion air before the combustion air is captured the burners. What warms up your feed water is the economizer, so research terminology associated with boiler operation before getting involved in determining the efficiency cycles of boiler operation. Study thermodynamics on boiler cycles and you'll get your answers.
 
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Yeah right its economizer instead of air pre heater but it does not mean I am not aware of boiler operations terminologies.
 
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To get to kJ/kg from Btu/scf,(use FG lower heating value LHV)
Btu/scf x 379.5 = Btu/lb mole
Btu/lb mole x 1.055 = kJ/lbmole
kJ/lbmole x 2.204 = kJ/kgmole
kJ/kgmole / FG mol wt = kJ/kg

If you are looking for overall eff, use 95degC, the feed temp to the BFW preheater / economiser.

 
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The boiler cannot count any "running start" it gets from preheating water, air, or fuel. The boiler manufacturer typically states that his boiler will produce X #/hr of steam at Y pressure, based upon conditions like feedwater temp, air temp, at a given blowdown rate, etc

Think of the boiler as the engine in a car. When an economizer is added, that doesn't change the boiler - it's like adding a another higher gear to the transmission in a car. The car will get improved fuel economy, and will go faster, but that modification had nothing to do with the engine. The engine itself remained unchanged.
 
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