Excess air provides safe operation above stoichiometric conditions. A burner is typically set up with 15% to 20% excess air in higher firing ranges. Higher excess air levels result in fuel being used to heat the air instead of transferring it to usable energy, increasing stack losses and significantly decreasing efficiency. Boilers with lower excess air throughout the operating range have higher efficiencies.
In theory, to have the most efficient combustion in any combustion process, the quantity of fuel and air would be in a perfect ratio to provide perfect combustion with no unused fuel or air. This type of theoretical perfect combustion is called stoichiometric combustion. In practice, however, for safety and maintenance needs, additional air beyond the theoretical "perfect ratio" needs to be added to the combustion process - this is referred to as "excess air".
This is because, under normal circumstances, the exhaust temperature of the boiler cannot completely condense the water in the flue gas, and the difference between the low calorific value and the high calorific value is mainly in the part of the latent heat of vaporization, so the low heat is used. The value is calculated to reflect the true efficiency of the boiler. However, there will be some special circumstances. For example, if the boiler is a condensing boiler, the calculation of the condensed water portion should be calculated using the high calorific value.
In a boiler, energy from the fuel is transferred to liquid water in order to create steam. Once the water is heated to boiling point, it is vaporized and turned into saturated steam. When saturated steam is heated above boiling point, dry steam is created and all traces of moisture are erased. This is called superheated steam.
A fire tube steam boiler is a boiler where the combustion gases from the burner are channeled through tubes that are surrounded by the fluid to be heated. The boiler body is the pressure vessel and contains the fluid. In most cases, this fluid is water that will circulate for heating purposes or become steam for use in processing.
Each set of tubes through which the combustion gas passes, before making a turn, is considered a "step". Consequently, a three-step boiler will have three sets of pipes with the outlet located at the rear of the boiler.
