WATER TUBE BOILER

It is a horizontal, externally fired, natural draught, stationary, water tube boiler. It is generally a high capacity boiler and can produce steam at a pressure of 4Mpa at a rate of 40000 kg/hr.

CONSTRUCTION FEATURES

Consists of a water drum which is connected to two headers at the front and back ends, the headers are connected by a large number of water tubes which are inclined upwards from down take header to uptake header. Further, a mud drum, in which heavier sediments of water settle down and are blown off from the blow off pipe, is also provided. The combustion chamber is located below the boiler drum. A chain grate with stoker is provided from burning coal. Hot flue gasses produced after combustion of fuel are made to pass over the water tubes in several passes with the help of baffles. The draught is regulated using dampers provided. Finally, chimney is provided to permit exit of waste flue gasses.

A super heater is also provided to super heat dry and saturated steam. It consists of U tubes and is placed in the path of flue gasses.

Several mountings which ensure safe and efficient operation of boiler are also provided. These include water level indicator, Pressure Gauge, Dead weight safety valve etc.

WORKING

Water is filled in boiler shell through the feed valve up to 2/3 rd of shell. Water flows down through the down take header via descending water tubes and rises upwards through the uptake header and ascending tubes. Inclined tubes help in setting up water circulation currents. The hot flue gasses coming from combustion chamber are made to pass over the tubes, under side of the drum and super heater tubes. Baffle plates ensure longer contact time of flue gasses with the tubes. When water in tubes is heated, it moves upwards through the ascending tubes and cold water takes its place through the descending tubes. In this way convection currents are set up.

The hottest gases being raised from the grate come in with that portion of water tube which is located on the highest side near the upper header. Water begins to evaporate and mixture of water and steam thus formed reaches into the boiler drum. Steam is collected in space above water. This steam is then lead to Super heater tubes for superheating. Superheated steam thus produced flows out through the stop valve.