Boiler Review

Boilers

Boilers, as the name implies boil water into steam usually for use in a steam turbine application. In the simplest of terms, boilers usually take the form of shell and tube style heat exchangers where feedwater enters the tubes of the boiler and a combustion fuel (methane, fuel oil, coal) is fired and run through the shell of the exchanger. A very simple but common configuration is as follows:

In the context of the PE exam, some of the main concepts you may be tested on are boiler efficiency, combustion/excess air required, and heat rate. Let’s look at each of these individually.

Boiler Efficiency

Boiler efficiency is defined as:

Or in other terms using the following schematic:

Where:
m ̇_steam= mass per unit time of steam flow
h2 = enthalpy of the steam leaving the boiler [btu/lbm]
h1 = enthalpy of the water entering the boiler [btu/lbm]
m ̇_fuel= mass per unit time of fuel flow
GCV = Gross calorific value [btu/lbm]

The only term here that isn’t straightforward is the Gross Calorific Value (GCV) of the fuel. The GCV, otherwise known as the higher heating value, the gross energy, or upper heating value, is defined as the heat produced by the complete combustion of a unit of fuel.

Higher heating value vs lower heating value

The equation given here for boiler efficiency uses the higher heating value (otherwise known and the Gross Calorific Value – GCV) in its calculation. However, the lower heating value (Net Calorific Value-NCV) is also a common fuel value as well. Knowing the difference between the two, and more importantly, how to convert one from the other may be valuable for the test.

Higher heating values are essentially the lower heating value of the fuel plus the heat of vaporization of the water in the fuel. The LHV is defined as:

Where:
HHV = higher heating value [btu/lbm]
Nh2o,p = moles of water in the products
hfg = latent heat for water at STP, 970.4 [btu/lbm]
Nfuel = moles of fuel burned
Mh2o = molecular mass of water [lb/mol]
Mfuel = molecular mass of fuel [lb/mol]

This would be used in a case where you have a known, balanced combustion equation.

Alternatively, higher and lower heating values can simply be referenced. Here’s a list of values for common fuels:

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