Steam boilers are mechanical pieces of equipment designed to convert water in liquid form to steam through the combustion of a fuel source like natural gas. There are many different types of boilers but most are characterized by pressure and heat exchanger type.
Low pressure boilers operate below 15 PSI, high pressure boilers operate above 150 PSI and medium pressure boilers operate in between 15 and 150 PSI. The different types of heat exchangers describe the location of the fuel and the water. Water tube boilers have water in tubes, with the hot combustion gases around the tubes, while fire tube boilers have combustion gases flowing through tubes that are submerged in water.
It is important for the engineer to understand the three different systems that comprise a boiler system, the (1) Feed-water System, (2) Combustion System and (3) Steam System.
(1) The feed-water system describes the incoming fluid water to the boiler. It consists of a feed-water pump, water softeners to remove minerals that can damage boilers and de-aerators to remove oxygen. Feed-water is provided by a mixture of the water supply and condensate return. The important part of the feed water system is to be able to determine the entering enthalpy of the feed water, depending on the pressure and temperature of the incoming water. As previously discussed, water in the sub-cooled region has enthalpy values that are a function of temperature.
(2) The combustion system describes the fuel portion of the boiler. The combustion system consists of oxygen supply, typically provided by a fan or air is naturally induced, an ignition and the fuel supply and piping. It is important to be able to determine the total heat supplied by the fuel. Total heat is shown as Q, which is a function of the mass flow rate of the fuel, the higher heating value (HHV) of the fuel and the boiler efficiency. The HHV can be found in the Mechanical Engineering Reference Manual. Boiler efficiencies are a function of the losses in the system, like
(3) The steam system is the output portion of the boiler. It consists of the outgoing steam piping to the steam consuming pieces of equipment, which in the HVAC and Refrigeration field are steam heating coils for air distribution and for water distribution. The output of the boiler is either saturated steam or a super-heated steam and the values for this steam output can be determined from the saturated steam tables or the super-heated steam tables.
Efficiency of a boiler is found by dividing the output energy by the input energy. The output is found by determining the change in enthalpy between the feed-water and the super-heated steam. The input is determined by the mass flow rate of the fuel and the higher heating value of the fuel.
This efficiency often referred to as the fuel-to-steam efficiency and is a true measure of the boiler input to output efficiency. There are other efficiencies that are out of the realm of this book, like the thermal efficiency and combustion efficiency. These items are more representative of the Thermal and Fluids topic and this page focuses more on the HVAC and Refrigeration topic.