(also once-through boiler), a steam boiler in which water is completely evaporated during a single direct pass through an evaporative heating surface.
In a flow-through boiler a circulation pump feeds water to an economizer. The water then passes to the coils or rising pipes, which constitute the evaporative surface and are located in a combustion chamber. In the output part of the coils any remaining moisture is evaporated, and the superheating of the steam begins. In this conversion zone the steam content of water reaches 90–95 percent by volume and scum forms rapidly if the feed water is not sufficiently pure. For this reason, coils in the conversion zone must be protected from overheating and are therefore partly relocated from the combustion chamber to gas conduits, where there are fewer thermal stresses. After passing through the conversion zone, steam is completely superheated in radiant or convection superheaters.
Flow-through boilers do not have drums or down pipes, which significantly reduces the quantity of metal required per unit and lowers the cost of the boiler. One disadvantage of flow-through boilers is that any salts entering the boiler with the feed water are either deposited on the walls of the coils in the conversion zone or enter the steam turbines with the steam, where they are deposited on rotor blades and lower the efficiency of the turbine. For this reason, the quality of feed water used in flow-through boilers must satisfy stringent requirements. Another disadvantage is the increased energy expenditure required to drive the circulation pump.
Flow-through boilers are primarily installed in condensation electric power plants, where boilers are fed with saltless water. Their use in central steam power plants results in the added expenses needed to chemically clean supplementary water supplies. Flow-through boilers are most efficient at supercritical pressures, that is, pressures above 22 meganewtons/m2 (MN/m2), at which other boilers cannot be used.
In the USSR, flow-through boilers were built by the Bureau for Construction of Flow-Through Boilers, under the direction of L. K. Ramzin. The first experimental flow-through boiler (the Ramzin boiler) was put into operation in 1932; it had horizontally arranged coils, a vaporative capacity of 3.6 tons per hour, and a steam pressure of 14.1 MN/m2. The first industrial flow-through boiler was developed in 1933; it had a vaporative capacity of 200 tons per hour and the same pressure as its predecessor. (The parameters of modern Soviet flow-through boilers are discussed in BOILER UNIT.)
The Ramzin boiler is used outside the USSR, as are the Benson boiler, which has vertical rising pipes, and the Sultzer boiler, in which the evaporative surface consists of vertically arranged coils for the upward and downward flow of water.