Chapter 23

Steam Boilers

Boiler System Performance

There are several factors that we would consider as part of the overall performance of the unit. Not all performance factors are relevant in all applications, so it is important to understand how the boiler will be used, and which performance factors are key to the application.


This is simply the amount of energy that can be handled by the boiler. Unfortunately, there are several different units of measure used by the industry, often determined by the type of product. Some common terms are:


There are several ways to determine steam boiler efficiency. Different methods calculate different factors that eventually lead to boiler efficiency. In calculating boiler efficiency, a ratio varying from the expected ratio indicates the need to modify the operation process or get in touch with the steam boiler manufacturer for their superior knowledge and assistance.

Combustion Efficiency

Combustion efficiency is an indication of the burner’s ability to burn fuel and the ability of the boiler to absorb the heat generated. The amount of unburned fuel and excess air in the exhaust are used to assess a burner’s combustion efficiency. Combustion efficiency is not the same for all fuels; generally, gaseous and liquid fuels burn more efficiently than do solid fuels.

Thermal Efficiency

Thermal efficiency assesses the efficacy of the heat exchanger that enables the transferring of the heat energy to the water or steam. With setting up of soot or water scaling, poor condition of the heat exchanging surface within the boiler results in affecting the thermal efficiency negatively, which, in turn, lowers the overall efficiency of the steam boiler.

Fuel-to-Steam Efficiency

Fuel-to-steam efficiency or fuel-to-water efficiency (also referred to as “boiler efficiency” determines the overall efficiency of the boiler. Fuel-to-steam efficiency is equal to combustion efficiency less the percent of heat losses through radiation and convection.

Startup Time

The time it takes to bring a boiler on line, and up to the operating steam pressure will vary with the boiler type. Some boilers are designed specifically for quick startup, and can go from cold to operating steam pressure in minutes

Steam Quality

Steam quality is a measure of the moisture in the steam. Moisture can be disruptive to the application, and users generally want a very low moisture content. This becomes more difficult in lower operating pressures simply because a pound of steam requires a much larger space

Furnace Size

In most boilers, a relatively large furnace is used to contain the fire. The size of the furnace can be important, up to a point. If the furnace is too small, it can be difficult to obtain good combustion. Low NOx emissions and oil firing generally require larger furnace sizes.

Total Contained Energy

One of the primary considerations in the dynamic efficiency is the amount of energy required to bring the boiler up to steam pressure. This is important because applications that have a lot of “off” time can lose a considerable amount of energy during the off cycle. As a broad general statement, boilers with a lot of heating surface tend to have good steady state efficiency, but due to their larger size, they have more water, steel and refractory to heat up, and a higher heat loss in the off cycle.

Click on the following topics for more information on steam boliers.