Water circulation in a boiler

The boiler operates effectively only when the tubes of its heating surfaces are intensively cooled by water, steam or their mixture. So a coolant must circulate at a certain velocity in the pipes. In economizers or air heaters water and steam moving is forced due to a pump or the difference of the steam pressure in the drum and superheater. In the evaporating surfaces of the boiler water circulation can be natural and forced.

The simplest scheme of the natural water circulation in the closed circuit of the boiler consists of these elements: drum, lower header of tubes and connecting them two rows of pipes. Circuit is filled up with water. Its altitude is a half of the drum. Tubes are intensively heated, pipes are outside the boiler masonry and are not warmed. The bubbles of the steam formed in the heated tube mix with the water and rise up into the boiler drum. Here separated steam flows out of the drum and additional water is supplied instead. Water comes down by unheated tubes; these pipes are called downcomers, heated ones-upcomers or evaporation ones.

In the unheated pipes water density is larger than density of the water and steam mixture in heated ones. Driving force of the circulation (Pa) takes place due to density difference in the connected pipes.

Steam boilers of natural circulation

The most popular are steam industrial boilers DKVR (reconstructed two drums boiler of vertical water pipes) made in Russia. The output of these boilers is 1,04, 1,67, 2,71 and 4,17 kg/s, steam pressure – 1,4 MPa. In most cases saturated steam is produced in these boilers, but, if it is necessary, a part of the water pipes can be taken out and instead of them a steam superheater is installed. Boilers are heated by gas or mazut.

The same construction scheme is characteristic to all DKVR boilers: they are two drums boilers of natural circulation with a shielded furnace. Boiling pipes layed out in a corridor order are fixed to the drums installed along. Shield pipes of furnace and boiling ones are joined to the long upper drum. Diameters of the pipes and drums, the distances between their centers are the same, so it is easier to produce them. Side walls of the furnace are entirely shielded. Front and rear shields are installed in the boiler DKVR. Ceiling of the furnace chamber consists of the bent ends welded to the upper drum of upper shield pipes. Lower ends of the shield pipes are fixed to two headers installed along side walls of the boiler. In order to form circulation circuit a front part of the header is fixed to the upper drum by unheated downcomer pipe. Other part of the header is joined to the lower one by unheated short pipes. The bottom of the boiler leans on the steel frame, so warmed it can get longer upwards. The furnace is installed under the projected out part of the upper drum. In front wall of this boiler burner for gas and mazut is installed.

Combustion products get into the final combustion chamber through orifice of the furnace rear wall where gas burns down. Smoke gets into the bank f convectional boiling pipes taking the roundabout way of the brick partition. It makes two runs due to a cast-iron partition. Flue gas leaves through a rear wall orifice of the boiler. Water circulates in the pipes because the pipes installed nearby furnace are heated more intensively than those at the end of the boiler heated by taking off smoke.

If it is necessary to overheat steam, superheater is installed at the beginning of the boiling pipes bank.

Settlings from the external surface of the boiling pipes bank are removed by the steam blown through stationary blasting equipment or by compressed air supplied through the openings of the side walls.

Boilers DKVR entirely or partly assembled are transported by railway to the mounting place.

Boilers KE (Russia) with stoker – fired furnace are produced to burn solid fuel. Their construction differs a little from DKVR ones, output – from 0,7 to 7 kg/s. Boilers DE (Russia) predestinated to burn gas and mazut are manufactured of the same output range. Water heaters-economizers and air heaters are installed in the exterior of the boiler as separate blocks.

In the industrial boilers large convectional evaporating equipment is installed which is not used in modern power ones. In such boilers water boils and vapours getting heat from burning torch by radiation in the shield pipes of the furnace. Such power boiler consists of all the elements characteristic to the modern water pipes boiler: shield pipes, superheater, air and water heaters and so on. A boiler consists of one drum, it is heated by different sorts of coal dust or crumby peat. Heating by peat in front of the boiler two bruising mills are installed. All walls of the furnace are shielded by pipes. In the funnel shaped bottom part of the furnace slag gets colder and falls out into the slag chamber. In the upper part pipes of the rare shield make up a bank of four rarefied rows of pipes-festoon, behind it a convectional steam superheater of two grades is installed. In the upper part of the boiler emergency exploiding valves are arranged necessary according to the rules of the boiler operation burning gas, mazut and fuel dust. In the convectional shaft economizer and air heater of two grades are installed.