Hot Water & Steam Boiler Systems
Two general categories of boilers are hot water and steam boilers. Most smaller commercial buildings use hot water boilers where water is heated to appropriate distribution temperatures (typically 140 - 180°F). These systems are often "closed" with virtually no fresh water makeup. Hot water boilers are often preferred because they normally do not need an operator or special water chemistry, and they run at higher fuel conversion efficiencies than steam boilers.
Steam boilers are found in many different configurations, but all serve one purpose: to contain water and transform it into steam by the application of heat. The two basic boiler designs for buildings are fire-tube and water-tube. In fire tube boilers, hot combustion gases pass through tubes submerged in water.
In water-tube boilers, the water is contained in tubes located inside a furnace and hot flue gases pass over the tubes, heating the water, and then exit out the stack.
While both steam boiler designs offer comparable efficiencies, they are not interchangeable. This is primarily due to structural considerations. Because of their greater structural integrity, water-tube boilers are specified in all situations where operating pressures of 250 pounds per square inch gauge or greater are required. Individual fire-tube boilers are used in most commercial applications and are usually preferred in low-pressure applications between 3,500 (100 boiler hp) and 35,000 ( 1,000 boiler hp) pounds per hour of steam. In sizes below 3,500 Ib/hr and above 35,000 Ib/hr, water-tube units are often preferred. Whatever the design, the primary function of a boiler is the efficient transfer of heat from hot combustion gases to water or steam. To accomplish this task, three variables must be properly controlled: combustion efficiency, heat transfer and steam distribution effectiveness.