Boiler Efficiency is a term which establishes a relationship between energy supplied to the boiler and energy output received from the boiler. It is usually expressed in percentage. As a general rule, “Boiler efficiency (%) = heat exported by the fluid (water, steam) / heat provided by the fuel x 100.”1
Types of Boiler Efficiency
The efficiency of a boiler may be classified into following three major types:
- Combustion Efficiency
- Thermal Efficiency
- Fuel-to-Steam Efficiency
Amongst all the three above mentioned boiler efficiencies, the fuel-to-steam efficiency is considered to give the most accurate representation of boiler efficiency on the whole. This is due to the fact that fuel-to-steam efficiency takes into account, the radiation and convection losses while performing efficiency calculations. Typically, it is the job of the boiler manufacturer to define boiler efficiency so that any type of economic analysis could be done properly.
Combustion efficiency generally gives an idea about the fuel burning capability of a burner. This type of efficiency is determined by the quantity of fuel which is left unburned in the boiler along with the surplus exhaust air. For getting high boiler efficiency, their burners should be well designed to provide low quantities of unburned fuel and excess air.
Combustion efficiency tends to vary with the types of fuel sources. In general, gaseous and liquid fuels result in very small amount of unburned fuel as well as 15% surplus air levels; hence they offer highly efficient burning as compared to solid substances. “By operating at only 15% excess air, less heat from the combustion process is being used to heat excess air, which increases the available heat for the load.”2
This type of boiler efficiency is only used to assess the performance of heat exchanger units used in boilers. It basically determines the efficacy via which a heat exchanger would convey heat generated by burning process to the fluid in the boiler. While doing so, it does not take into account the radiation and convection losses occurring in the boiler sections. Hence, thermal efficiency is not considered valuable for economic analysis since it doesn’t reflect correct fuel consumption of a boiler system.
Fuel-to-steam efficiency is helpful in determining the overall efficiency of a boiler since it takes into consideration both the thermal efficiency i.e. heat exchanger effectiveness and the radiation and convection losses. This is the type of boiler efficiency which is ought to be used for making all types of economic assessments.
Methods of Determination
The two major methods employed to find out the fuel-to-steam efficiency of a boiler are explained below:
This method of efficiency determination largely depends upon the input-output ratio determination of the boiler. In this method, the output of the boiler derived in BTUs is divided by the boiler input supplied in BTUs and then the resulting number is multiplied by 100.
Heat Loss Method
It is also referred to as heat balance efficiency measurement method. This method of efficiency determination takes into account all kinds of heat losses occurring inside the boiler. The true boiler efficiency is calculated by summing up the percentage of all stack, radiation and convection losses and then finally deducting the resultant sum from 100 percent. This entire calculation will provide actual fuel-to-steam boiler efficiency.
Types of losses
Two major types of losses which take place inside a boiler system are mentioned below:
“The stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the boiler and reflects the energy that did not transfer from the fuel to the steam or hot water.”3
In other words, it gives an indication about the quantity of heat energy lost due to dry exhaust gases and moisture loss. It is found valuable in determining the true efficiency of a boiler. A lower value of the stack temperature is always preferred for gaining well efficient heat exchanger performance and greater fuel-to-steam boiler efficiency.
Radiation and Convection Losses
Radiation losses are defined as the losses which occur due to radiation i.e. emission of heat energy out of the boiler whereas convection losses are the losses happening due to the air circulating around the boiler. Nearly all kinds of boilers experience these two significant losses. “Radiation and convection losses, expressed in Btu/hr, are essentially constant throughout the firing range of a particular boiler, but vary between different boiler types, sizes, and operating pressures.”4
Boiler Design Criteria
To achieve high efficiency out of a boiler system, it must be designed in such a way that it meets all the required design criteria which mainly includes:
- Required number of boiler passes
- Suitability of boiler and burner
- Availability of fuel control within the boiler system
- Adequate heating surface of the boiler
- Well designed pressure vessel