A forced circulation boiler was first introduced by La-Mont in the year 1925 which is used in Europe and America. This is a modern high pressure boiler (water tube type steam boilers) working on forced circulation system.

Working principle of La Mont Boiler

The image shows the flow circuit of La Mont Boiler.

explained working of each and every part in La Mont boiler one by one.

Steam separator drum

The la Mont boiler consists of a steam separator drum which is placed wholly outside the boiler setting . The drum receives a mixture of steam and water from the evaporator tubes and feed water from the economizer. The steam is separated from water in the drum.

Circulating pump

The water from the drum is then drawn to the circulating (centrifugal) pump through the down-comer. The pump circulates water (“forced circulation”) equal to 8 to 10 times the weight of steam evaporated. This prevents the tubes from being overheated.

Distributing header

The circulating pump delivers the feed water to the distributing header with orifices at a pressure above the drum pressure.

Evaporator

The header distributes water through orifices into the evaporator tubes acting in parallel. Orifice in the header controls the flow of water to the evaporator tubes. Here part of the water is evaporated and a mixture of steam and water from these tubes enters the drum.

Convection superheater

The steam produced in the boiler is nearly saturated. This steam as such should not be used in the steam turbine. The presence of moisture in it will cause corrosion of turbine blades, etc. to raise the temperature of steam and thereby to increase the turbine efficiency, superheater is used.

The principle of convection superheater is similar to steam generating tubes of the boiler. The hot flue gases at high temperature sweep over convection superheated tubes and raise the temperature of steam. Convection superheater thus receives heat from the flue gases flowing from the combustion chamber, entirely by convective heat transfer. Such a superheater may be more conveniently located since it is not necessary for it to “see” the furnace.

Saturated steam from the top of the drum enters the convection superheater placed in the path of the flue gases and is superheated.

Steam outlet

Superheated steam from the superheater passes out to the steam turbine through the steam outlet.

Economizer

The quantity of superheated steam thus delivered to turbine is continuously made up in the form of feed water. Feed water supplied by the feed pump is heated in the economizer on its way to the steam separator drum.

The economizer is a device used to preheat the feed water using the hot gases leaving the boiler. Before the gases are let off to the atmosphere, they are made to flow in a definite passage in the economizer so that some of the heat in the hot gases, which otherwise gets wasted, can be used to preheat the feed water. The preheated water requires only a small amount of heat to be supplied in the boiler, resulting in some saving of the fuel burnt. This results in an increase in the boiler efficiency.

Air preheater

Since the heat of the exit gases cannot be fully extracted through the economizer, the air preheater is employed to recover some of the heat escaping in these gases. These exit gases preheat the air from the blower in the air preheater. The preheated air is supplied to the furnace for combustion.

Capacity

The capacity of la-mont boiler is about 50 Tonnes/hr of superheated steam at a pressure of 170 kgf/sq.cm. and at a temperature of 500’C.