The Main Elements of a Boiler

The first boilers were of a cylinder shape. The external surface of a cylinder at the same time was the heating surface. The construction of the boilers was improved in two ways: 1. the internal heating surface of the drum boilers of the burning pipe and smoke pipe was increased and 2. the external heating surface of water pipe boilers was increased.

In the drum of the burning pipe boilers there are one or two pipes of a large diameter – burning pipes in front part of which furnaces are installed. In the smoke pipe boilers flue gas flows in many pipes of small (50…130 mm) diameter. The increase of heating surface of these boilers increased their output. Favourable conditions are formed for convection heat exchange, if smoke quickly flows in the pipes of a small diameter. So nowadays these boilers are installed, if it is necessary to employ the heat of low temperature smoke (e.g. smoke of industrial furnaces). Sometimes, (e.g. in the locomotives) joint boilers made of a short burning pipe and a smoke pipe are used.

Boilers of 4 water pipes designed later than the boilers of the flue gas pipes appeared more promising. These boilers consist of many pipes of a small diameter filled with water. The external surface is heated by smoke. At first pipes were joined to the special chambers, the latters – to the drum. Later inconvenient pipe chambers were substituted by drums. Almost vertical banks of the pipes were joined to the drums. Such boilers were called boilers of the vertical water pipes.

Further development of the boilers was related with the method of creation of the dust shape solid fuel combustion. Walls of the furnace so that high temperature and liquid slag would not affect them were covered protecting the pipes-shields in which boiler water as circulating. It proved that shield pipes not only protect well the walls of the boiler, but are very effective heating surfaces getting radiant heat straight from the burning torch. So the surface of the shield pipes was increased and became the main element of the water evaporation. Only one drum in the modern boiler of the water pipes is installed.

Single-pass boilers without a drum were designed making an attempt to create cheaper, of simpler construction ones. In these boilers water moving is forced because it flows due to the pumps. Nowadays boilers of the shield water pipes and single-pass ones are used.

The improvement of the evaporation surfaces were made not only improving the construction of the boiler. Already in the 19th century heaters of the water-economizers, steam superheaters and air heaters were used. Principle scheme of the shield tube boiler settled in 1937…1938 practically speaking is unchanged till nowadays. In the boiler smoke at first rises up, then flows horizontally and goes down, so its trajectory resembles letter pi. Such scheme of the smoke moving is characteristic to the most modern boilers.

Every steam boiler is characterized by three main parameters: output, pressure and temperature. The output of the boiler is the amount of the steam produced per time unit and measured in kg/s (or t/h).

Steam boilers are divided into low pressure boilers which produce steam of 0,9 or 1,4 MPa pressure, medium pressure – 2,4 or 4,0 MPa, high pressure – 10 or 14 MPa and more than critical – 25,5 MPa pressure steam.

Steam boilers are small – up to 1,8 kg/s (6,5 t/h), medium – up to 2,8…20 kg/s (10…75 t/h) and of very large output – more than 140 kg/s (500 t/h).

According to purpose steam boilers are distributed into industrial and power. Industrial ones are of low and medium output producing steam of low parameters. Steam is overheated a little or not overheated at all in these boilers. The produced steam is employed for industrial or everyday needs. In the power boilers 4 MPa and larger pressure steam is produced and it is always overheated. If 4,0 MPa pressure exists in the boiler, the temperature of the overheated steam is 440° Celsius. In the higher pressure one – it is up to 540…560° Celsius. Such boilers are used in the thermal power stations and their largest output is 1100 kg/s (3950 t/h).

According to the character of the water moving boilers are divided into three groups: of natural circulation, frequent forced circulation and single-pass.