Boilers

Boilers Boiler is an apparatus to produce steam. Thermal energy released by combustion of fuel is used to make steam at the desired temperature and pressure. The steam produced is used for: (i) Producing mechanical work by expanding it in steam engine or steam turbine. (ii) Heating the residential and industrial buildings (iii) Performing certain processes in the sugar mills, chemical and textile industries.

A simple boiler In order to describe the principles of a steam boiler, consider a very simple case, where the boiler simply is a container, partially filled with water (Figure 1). Combustion of fuel produce heat, which is transferred to the container and makes the water evaporate. The vapor or steam can escape through a pipe that is connected to the container and be transported elsewhere.

Another pipe brings water (called “feedwater”) to the container to replace the water that has evaporated and escaped. Since the pressure level in the boiler should be kept constant (in order to have stable process values), the mass of the steam that escapes has to be equal to the mass of the water that is added. If steam leaves the boiler faster than water is added, the pressure in the boiler falls. If water is added faster than it is evaporated, the pressure rises. Figure 1: Simplified boiler drawing.

Boiler properties: (i) Safety. The boiler should be safe under operating conditions. (ii) Accessibility. The various parts of the boiler should be accessible for repair and maintenance. (iii) Capacity. Should be capable of supplying steam according to the requirements.

(iv) Efficiency. Should be able to absorb a maximum amount of heat produced due to burning of fuel in the furnace. (v) It should be simple in construction. (vi) Its initial cost and maintenance cost should be low. (vii) The boiler should have no joints exposed to flames. (viii) Should be capable of quick starting and loading.

CLASSIFICATION OF BOILERS According to what flows in the TUBE 1. Water tube. 2. Fire tube.

Fire – tube Boilers

Economizer

Water tube boilers

BABCOCK WILCOX BOILER

MERITS OF WATER TUBE BOILERS OVERFIRE TUBE BOILERS 1. Generation of steam is much quicker due to small ratio of water content to steam content. This also helps in reaching the steaming temperature in short time. 2. Its evaporative capacity is considerably larger and the steam pressure range is also high-200 bar. 3. Heating surfaces are more effective as the hot gases travel at right angles to the direction of water flow.

4. The combustion efficiency is higher because complete combustion of fuel is possible as the combustion space is much larger. 5. The thermal stresses in the boiler parts are less as different parts of the boiler remain at uniform temperature due to quick circulation of water. 6. The boiler can be easily transported and erected as its different parts can be separated.

7. Damage due to the bursting of water tube is less serious. Therefore, water tube boilers are sometimes called safety boilers. 8. All parts of the water tube boilers are easily accessible for cleaning, inspecting and repairing. 9. The water tube boiler's furnace area can be easily altered to meet the fuel requirements.

Demerits : 1. It is less suitable for impure and sedimentary water, as a small deposit of scale may cause the overheating and bursting of tube. Therefore, use of pure feed water is essential. 2. They require careful attention. The maintenance costs are higher. 3. Failure in feed water supply even for short period is liable to make the boiler over-heated.

advantages of fire tube boilers are as follows. (i) Low cost (ii) Fluctuations of steam demand can be met easily (iii) It is compact in size.

Water tube boilers are classified as 1. Horizontal straight tube boilers (a) Longitudinal drum (b) Cross-drum. 2. Bent tube boilers (a) Two pass (b) Three pass 3. Cyclone fired boilers

According to position of furnace. (i) Internally fired (ii) Externally fired In internally fired boilers the grate combustion chamber are enclosed within the boiler shell whereas in case of extremely fired boilers and furnace and grate are separated from the boiler shell. According to the position of principle axis. (i) Vertical (ii) Horizontal (iii) Inclined.

According to application. (i) Stationary (ii) Mobile, (Marine, Locomotive). According to the circulating water. (i) Natural circulation (ii) Forced circulation. According to steam pressure. (i) Low pressure (ii) Medium pressure (iii) Higher pressure.

We are gonna study………… Cochran BoilerCochran Boiler Lancashire BoilerLancashire Boiler Locomotive boilerLocomotive boiler Babcock Wilcox BoilerBabcock Wilcox Boiler

Fire – tube Boilers Cochran Boiler

Fire – tube Boilers

The outstanding features of this boiler are : ▫ It is very compact and requires minimum floor area ▫ Any type of fuel can be used with this boiler ▫ Well suited for small capacity requirements ▫ It gives about 70% thermal efficiency with coal firing and about 75% with oil firing

Fusible Plug. If the water level in the boiler falls below a predetermined level, the boiler shell and tubes will be overheated. Main parts ……

BOILER ACCESSORIES: Boiler can fail by explosion as a results of either faulty construction or improper operation.To minimize the danger of failure the following are added to the boiler: 1-Pressure gage for saturated steam pressure recording. 2-Relieve valve as a safety precaution to blow off at elevated pressure. 3-Water gage glass to indicate the level inside the steam drum. 4-Key valve to change water flow rate.

BOILER AUXILIARIES: 1-Superheater 2-Air preheater 3-Reheater 4-Economizer

Water Level Indicator. This indicates the water level in the boiler.

Pressure Gauge. This indicates the pressure of the steam in the boiler.

Steam Stop Valve. It regulates the flow of steam supply outside. The steam from the boiler first enters into an ant-priming pipe where most of the water particles associated with steam are removed

Safety Valve. The function of the safety valve is to prevent the increase of steam pressure in the holler above its design pressure.

Feed Check Valve. The high pressure feed water is supplied to the boiler through this valve. This valve opens towards the boiler only and feeds the water to the boiler.

Blow-off Cock. The water supplied to the boiler always contains impurities like mud, sand and, salt Due to heating, these are deposited at the bottom of the boiler, they have to be removed using blow off cock.

It is stationary fire tube, internally fired, horizontal, natural circulation boiler. This is a widely used boiler because of its good steaming quality and its ability to burn coal of inferior quality. These boilers have a cylindrical shell 2 m in diameters and its length varies from 8 m to 10 m. It has two large internal flue tubes having diameter between 80 cm to 100 cm in which the grate is situated. This boiler is set in brickwork forming external flue so that the external part of the shell forms part of the heating surface. Lancashire Boiler

Fire – tube Boilers Lancashire Boiler

Advantages The feed pipe projecting into the boiler is perforated to ensure uniform water distribution. Its heating surface area per unit volume at the boiler is considerably large. Its maintenance is easy. It is suitable where a large reserve of hot water is needed. This boiler due to the large reserve capacity can easily meet load fluctuations. Super-heater and economizer can be easily incorporated into the system, therefore; overall efficiency of the boiler can be considerably increased (80-85%).

LOCOMOTIVE BOILER Locomotive boiler is a horizontal fire tube type mobile boiler. The main requirement of this boiler is that it should produce steam at a very high rate. Therefore, this boiler requires a large amount of heating surface and large grate area to burn coal at a rapid rate. In order to provide the large heating surface area, a large number of fire tubes are setup and heat transfer rate is increased by creating strong draught by means of steam jet.

LOCOMOTIVE BOILER

Advantages 1. Large rate of steam generation per square metre of heating surface. To some extent this is due to the vibration caused by the motion. 2. It is free from brickwork, special foundation and chimney. This reduces the cost of installation. 3. It is very compact. The pressure of the steam is limited to about 20 bar.

Features 1. The evaporative capacity of this boilers is high compared with other boilers (20,000 to 40,000kg/hr). The operating pressure lies between 11.5 to 17.5 bar. 2. The draught loss is minimum compared with other boilers. 3. The defective tubes can be replaced easily. 4. The entire boiler rests over an iron structure, independent of brick work, so that the boiler may expand or contract freely. The brick walls which form the surroundings of the boiler are only to enclose the furnace and the hot gases.

Boiler performance 1) Evaporative capacity can be expressed in terms of: a) kg of steam/hr b) kg of hour/hr/m 2 of heating surface c) kg of steam/kg of fuel fired

Essentials of a Good Steam Boiler Followings are the important essentials of a good steam boiler: It should produce maximum quantity of steam with the minimum fuel consumption. It should be economical to install, and should require little attention during operation. It should rapidly meet the fluctuation of load. It should be capable of quick starting.

Essentials of a Good Steam Boiler It should be light in weight. It should occupy a small space. The joints should be few and accessible for inspection. The mud and other deposits should not collect on the heating plates. The refractory material should be reduced to a minimum. But it should be sufficient to secure easy ignition, and smokeless combustion of the fuel on reduced load.

Essentials of a Good Steam Boiler The tubes should not accumulate soot or water deposits, and should have a reasonable margin of strength to allow for wear of corrosion. The water and flue gas circuits should de designed to allow a maximum fluid velocity without incurring heavy frictional losses. It should comply with safety regulations as laid down in the boilers Act.

Selection of Steam Boilers The Selection of type and size of a steam boiler depends upon the following factors: The power required and the working pressure. The rate and which steam is to be generated. The geographical position of the power house. The fuel and water available. The type of fuel to be used. The probable permanency of the section. The probable load factor.

Boiler efficiency Ratio of heat actually utilized in the generator to the heat supplied by the fuel. Efficiency= m w (h-h f )/C Where C is the calorific value /