51. How the induced draught is working?

In an induced draught system, a blower (induced draft fan) is placed near (or) at the base of the chimney. The fan sucks the flue gas from the furnace creating a partial vacuum inside the furnace. Thus atmospheric air is induced to flow through the furnace to aid the combustion of fuel. The flue gases drawn by the fan passes through chimney, to the atmosphere.

52. Why the balanced draught system is preferred than other system?

In the induced draught system, when the furnace is opened for firing, the cold air enters the furnace and dilate the combustion. In the forced draught system, when the furnace is opened for firing, the high pressure air will try to blow out suddenly and furnace may stop. Hence the furnace cannot be opened for firing (q) inspection in both, systems. Balanced draught, which is a combination of induced and forced draught, is used to overcome the above stated difficulties.

53. What is the difference between stocker firing and pulverised fuel firing?

The stocker firing method is used for firing solid coal where as pulverised firing method is used for firing pulverised coal.

54. What are the different types of stockers?

1. Over feed stockers

• Travelling grate stockers
• Spread stockers

2. Under feed stockers

• Single retort stocker
• Multi retort stocker

55. What is the use of pulveriser and name different types of pulverising mills?

The pulveriser is used to pulverise the coal in order to increase the surface exposure. Pulverised coal enables rapid combustion.

The different types of pulverising mills are

1. Ball mill
2. Hammer mill
3. Ball and race mill.

56. Name the two methods of pulverised fuel firing system?

1. Unit (or) direct system
2. Bin (or) central system.

57. What are advantages of unit (or) direct system of pulverised fuel firing?

1. The layout is simple and economical
2. It gives direct control of combustion
3. Coal transportation system is simple
4. Maintenance cost is less

58. How the ash handling system is classified?

1. Mechanical handling system
2. Hydraulic system
3. Pneumatic system
4. Steam jet system

59. Why ash handling system is’ needed?

• To remove the ashes from the furnace ash hopper
• To transport the ashes from furnace ash hopper to a storage
• To dispose the ashes from the storage

60. Name different types of dust collectors?

1. Mechanical Dust collector

• Gravitational separators
• Bag house dust collector

There are three types of bag house dust collector

• Open pressure type
• Closed pressure type
• Closed suction type

2. Cyclone. Separators

3. Elector Static Precipitator (ESP)

61. What is the main purpose of chimney?

The main purpose of chimney is to emit the flue gases at a considerable height to avoid nuisance to the surrounding people.

62. What are the different types of load act on the chimney?

There are two types of loads acting on the chimney namely.

• Its own weight which is considered to as a single vertical force acting through the centroid, and
• The Wind pressure, which is considered as the horizontal force acting on the centroid of vertical projected area.

63. Name different types of chimney?

1. Steel chimney
2. Site constructed chimney
3. Plastic chimney

64. Define forced draft and induced draft cooling towers

If the fan is located at the bottom of the tower and air is blown by the fan up through the descending water it is called as forced draft cooling towers.

If the fan is located at the top of the tower and air enters through the louvers located on the tower’s side and drawn up and discharge through the fan casing, it is called as induced draft.

65. What are the advantages of induced draft cooling tower over forced draft cooling tower?

i. The outlet water comes in contact with the driest air and warmest water comes in contact with most humid air.
ii. The re-circulation is seldom a problem.
iii. The first cost is low due to the reduction in pump power consumption.

66. What are the advantages of hyperbolic natural-draft cooling towers?

• Since no fans are used power cost and auxiliary equipments cost is reduced.
• Hyperbolic tower chimney creates its own draft even when there is no wind
• Ground fogging and warm air re-circulation are avoided in hyperbolic towers
• The structure is more or less self-supported.

67. What are the factors that affect the rate of evaporation of water in cooling towers?

• Amount of water surface area exposed
• The time of exposure
• The relative velocity of air passing over the droplets
• The RH of air
• The direction of airflow relative to water.

68. What is the working principle of Cooling Towers?

The hot water is sprayed from the top of the tower, while the air is made to flow from the bottom of the tower to the top. This air cools the hot water in the cooling tower. Air vaporises a small percentage of water, there by cooling the remaining water. The air absorbs the heat and leaves at the top of the tower and cooled water leaves at the bottom and recirculated to the condenser.

69. Name different types of cooling towers?

The cooling towers are classified as follows.

(a) According to the construction of material

1. Timber for small tower
2. Ferro concrete for large capacity stations.
3. Multi deck concrete towers for large steam stations
4. Metallic

(b) According to the nature of air draught

1. Atmospheric (or) Natural draught cooling system
2. Mechanical draught cooling tower.

70. How the atmospheric (or) natural draught cooling towers are classified?

In atmospheric (or) natural cooling towers, the natural air provides the required cooling without the use of fans.

This is classified into three types.

1. Natural draft spray filled towers
2. Natural draft packed type towers.’
3. Hyperbolic cooling towers

71. How mechanical draft cooling towers are classified?

Mechanical draft cooling tower is classified into three types

1. Forced draft tower.
2. Induced draft counter flow tower
3. Induced draft cross flow tower.

72. How the dry type cooling towers are classified?

The dry type cooling towers are classified into two types ‘as follows.

1. Indirect dry type (or) Heller cooling system
2. Direct dry type-cooling system

73. What are the methods to reduce the effects of particulates?

The effects of particulates can be reduced by the following methods.

• Coal cleaning
• Using improved electrostatic precipitator design
• Controlling the dust within allowable limit. This can be done by increasing the height of chimney thereby reducing the concentration

74. What are the equipments used for ash collection?

• Electrostatic precipitator
• Fly ash scrubbers
• Cinder catcher
• Cyclone dust collector

75. What is meant by ‘Desulphurisation’ and name the methods adopted for desulphurisation?

Desulphurisation of fuel is the process of reducing the sulphur content in the fuel. The following methods are adopted for desulphurisation.

1. Chemical treatment
2. Forth flotation
3. Magnetic separation