Mechanical Engineering: high pressure boilers

Showing posts with label high pressure boilers. Show all posts

Loeffler Boiler

This is also a modern high pressure water tube boiler using the forced circulation principle and named after Prof.Loeffler.

Salient features of Loeffler Boiler
The novel feature of the Loeffler Boiler is to evaporate water solely by means of superheated steam. The furnace heat is supplied only to economiser and superheater. In other words, steam is used as a heat absorbing medium.

The major difficulty experienced in La-Mont boiler is deposition of salt and sediment on the inner surfaces of water tubes. The deposition reduces the heat transfer, ultimately, the generating capacity. This difficulty was solved in Loeffler boiler by preventing the flow of water into the boiler tubes. Feed water is evaporated in the drum using part of the superheated steam coming out from the water-heater. Thus only the dry saturated steam passes through the tubes. Poor feed water can, therefore, be used without any difficulty in the boiler, which is great advantage of this boiler.

Working principle of Loeffler Boiler
The image shows the outline diagram of Loeffler Boiler.

Economiser
The feed water from the feed tank is supplied to the economiser by feed pump. In the economiser the feed water is made to flow through a number of tubes surrounding which the hot gases leaving the furnace pass over. There is a heat exchange from the hot gases to the feed water, which is preheated in the economiser.

Evaporated Drum
It is housed away from the furnace. It contains a mixture of steam and water. The feed water from the economiser tubes enters the evaporator drum into which is also passed two-thirds of the superheated steam generated by the boiler. The superheated steam gives its superheat to the water in the drum and evaporates it to saturated steam.

Mixing Nozzles
The nozzles distribute and mix the superheated steam throughout the water in the evaporator drum.

Steam circulating pump
A steam circulating pump forces this saturated steam from the evaporator drum to the radiant superheater through the tube of the furnace wall.

Radiant superheater
The radiant superheater is placed in the furnace. The hot gases in the furnace are used for superheating the saturated steam from the drum. The radiant superheater receives heat from the burning fuel through radiation process.

Convection superheater
Steam from the radiant superheater enters the convection superheater where it is finally heated to the desired temperature of 500’C. The convection superheater receives heat from the flue gases entirely by convective heat transfer. Both radiant and convection superheater are arranged in series in the path of the flue gases.

Steam outlet
About one-third of the superheated steam from the convection superheater passes to the steam turbine while the remaining two-thirds is passed on to evaporator drum to evaporated the feed water to saturated steam.

Capacity
Capacity of the Loeffler boiler is about 100 Tonnes/Hr of superheated steam generated at a pressure of 140 kgf/sq.cm and at a temperature of 500’C.

La Mont Boiler Working and Construction

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.

I will explain 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.

Modern High Pressure Boilers

A boiler which generates steam at a pressure of 85 kgf/sq.cm or above is termed as a “high pressure boiler”. The present tendency is towards the use of high pressure boilers in power plants. The modern high pressure boilers used for power generation have capacities of 40 to 1600 tonnes/hr of superheated steam with a pressure upto 210 kgf/sq.cm and a temperature of about 650’C. One of the largest modern steam power plants in the world is in japan with a steam capacity of 1600 Tonnes/hr. In India, the trombay power plant has a steam generating capacity of 550 tonnes/hr, Ramagundampower power plant with 320 tonnes/hr and bokaro plant with 160 tonnes/hr.

Water tube boilers are generally preferred for high pressure and high output whereas fire tube boilers for low pressure and low output.

Advantages of high pressure boilers:

Method of water circulation

Water circulation through the boiler may be either natural circulation due to density difference or by forced circulation. In high pressure boilers, water circulation is made with the help of a centrifugal pump which forces water through the boiler tubes. This is called “forced circulation of water”. The use of natural circulation is limited upto 120 kgf/sq.cm. Steam pressure and forced circulation upto 210 kgf/sq.cm. Forced circulation increases the rate of heat transfer and hence increases the steam generating capacity of boilers.

Size of drums

The high pressure boilers are characterized by the use of very small steam separating drums or by the complete absence of any drum.

Type and arrangement of tubes

The heat of combustion is utilized more efficiently by the use of small diameter and light weight tubes in large numbers. To avoid large resistance to the flow of water , the high pressure boilers have a parallel set of arrangement of tubes.

Compactness

The boiler components can be arranged horizontally, giving greater accessibility and operational convenience as high head required for natural circulation is eliminated by using forced circulation. The space required is hence less and arrangement is compact.

Foundation cost

Due to the light weight tubes and small size drums required and the arrangement being compact, the cost of foundation is reduced.

Efficiency

The efficiency of the power plant is increased upto 40%, by using high pressure superheated steam. Also steam can be raised quickly after the boiler is fired.

Cost of electricity

Since efficiency of the plant is increased by using high pressure boilers, the cost of electricity production is reduced.

Overheating

All the parts are uniformly heated; therefore the danger of overheating is reduced. Also thermal stress probelm is avoided.

Scale formation

The tendency of scale formation is eliminated due to the high velocity of water through the boiler tubes.

Types of high pressure boilers

In the upcoming posts, I will write about the types of high pressure boilers in breif. If you have any comments please let me know.