Single-pass steam boilers

Principle scheme of the steam boiler of frequent forced circulation differs from that of the natural circulation. A special pump in a circulation circuit behind the boiler drum is installed. It is possible to arrange evaporation surfaces freely, e.g. in the shape of the coil or similarly because circulation is forced. So in the boilers of forced circulation water and steam mixture can flow not only upward, but also downwards. Besides evaporation surfaces can be made of many pipes of small diameters, so the heat transfer by convection from smoke to the wall of pipes is more intensive.

In power engineering these boilers are not widespread, however boilers of frequent forced circulation are especially used to employ the heat of low temperature smoke such as flue gas removed from the industrial stoves or other equipment. Such boilers are called exhaust-heat ones. In this case evaporation surfaces are installed in narrow flue ducts sometimes rather far from the drum. Besides, utilizing the heat of low temperature smoke convection heat exchange is the most important.

There is no furnace in the boiler-smoke of 600…900° Celsius temperature gets into it from the industrial stove. Water heated in the economizer is supplied into drum, from that by circulation pump it is driven through evaporation coils. Water and steam mixture gets back from them into the drum where steam separates from the water. The water gets again into the circulation pump ant the separated steam gets into the superheater installed in the high temperature zone. In the boilers of this series steam is produced of 1,8…4.5 MPa pressure, output is up to 11 kg/s.

In the single-pass boilers water and steam flow only once through economizer, superheater and evaporation surface here water turns into vapour entirely. So in the single-pass boiler the equipment to separate steam from water, e.g. drum is not necessary. In the single-pass boiler water, flowing in the tubes, warms, evaporates and overheats. Large hydraulic resistance exists in the boiler due to long ay of the water in the pipes and its great velocity, so pumps use much power.

The construction of the single-pass boilers, as natural circulation ones, is of the letter pi shape. At first water is supplied into the economizer by a pump and out of it – into the lower header, at which shield pipes of evaporation are joined, installed in different ways along the furnace wall. Water boils in the lower part of these pipes and in the upper part the most of it is already evaporated. From the shield pipes water and steam get into the coils of intermediate zone, where it evaporates turning into dry saturated steam. At first steam is overheated in radiant, later – in convection part of the superheater.

In the intermediate zone at the end of turning into the vapour salts are separated from the water on the pipe surface, they make heat transfer worse. Shield pipes covered by salts can be shortly burnt. To avoid such phenomenon this part of the tubes is transferred to the comparatively low temperature zone. Boiler is washed up periodically to distinguish settled salts in the intermediate zone.

In the powerful thermal stations single-pass boilers nowadays are the main power ones. They are used mostly when steam pressure is more than 10 MPa. Producing steam of very high pressure (more than 18 MPa) only single-pass boilers are used because at such pressure densities of the water and steam differ only a little, so natural circulation is impossible. Single-pass boilers can be of unlimited output. E.g. a boiler of 1100 kg/s output supplying steam of 25 MPa pressure to a turbine of 1200 MW power is used.