Constituents of Steam Power Plant and Layout | Electrical Engineering

The important parts and auxiliaries of steam power plants are discussed in brief as follows:- 1. Steam Generating Equipment 2. Condensers 3. Evaporators  4. Feed Water Heaters  5. Prime Movers  6. Spray Ponds and Cooling Towers  7. Control Room 8. Switchyard

1. Steam Generating Equipment:

(a) Boilers:

Boilers or steam generators convert water into steam and form one of the major equipments in a steam power plant. Boilers used in steam power plants are of two types namely fire tube boilers and water tube boilers. In fire tube boilers the tubes containing hot gases of combustion inside are surrounded with water while in water tube boilers the water is inside the tubes and hot gases outside the tubes. Fire tube boilers are compact in size, have low initial cost and have the ability to raise rapidly large quantities of steam per unit area of fire grate but have the following drawbacks.

As water and steam, both are in the same shell, higher pressure of steam are not possible, the maximum pressure which can be had is about 17.5 kg/cm² and with a capacity of 15,000 kg of steam per hour. For higher pressures or higher rates of evaporation, the shell and fire tube boilers become extremely heavy and unwieldy. In the event of a sudden and major tube failure, steam explosions may be caused in the furnace due to rush of high pressure water into the hot combustion chamber which may generate large quantities of steam in the furnace.

Fire tube boilers use is, therefore, limited to low cost, small size and low pressure (to about 10 kg/cm²) plants.

For central steam power plants of large capacity water tube boilers are used. Water tube boilers essentially consist of drums and tubes. The tubes are always external to drum. In comparison to fire tube boilers the drums in such boilers do not contain any tubular heating surface, so they can be built in smaller diameters and consequently they will with­stand high pressure.

The water tube boilers have got follow­ing advantages over the fire tube boilers:

1. High evaporation capacity due to availability of large heating surface.

2. Better heat transfer to the mass of water and better efficiency of plant owing to rapid and uniform cir­culation of water in tubes.

3. High working pressure due to small size of drum.

4. Quick raising of steam owing to large ratio of heat­ing surface to water volume.

5. Safety in operation.

6. Less space occupied.

7. Better overall control.

8. Easy removal of scale from inside the tubes.

The design of boiler depends upon several service fac­tors such as weight, height, portability, safety, and bulk, char­acter of operating labour, life, efficiency and cost. The boilers may be straight or bent tube of diameter ranging from 25 mm to 100 mm; longitudinal or cross drum, horizontal, vertical or inclined tube, forced or natural circulation, single or multi-drum, sectional or box header, cross or parallel baffles, marine or stationary.

Header boilers, using straight tubes have been superseded by curved-tube drum type. Natural circulation (by density difference) of water is employed with pressure up to 175 kg/cm². Pumps giving controlled circulation are often preferred on design deliver­ing more than 340,000 kg of steam per hour at pressures above 100 kg/cm².

Research and development during last 60 years has led to large steam generating capacity boilers having improved design and construction. Today boilers are more efficient, compact and lighter in weight in comparison to old time boilers and can burn low grade fuels. Supercritical pres­sure boilers with pressures as high as 350 atm tempera­ture 600°C and capacity 1,000,000 kg/hour are available nowadays.

The selection of the size and type of boiler depends upon:

(i) The output required in terms of amount of steam per hour, operating temperature and pressure,

(ii) Avail­ability of fuel and water, and

(iii) The probable load fac­tor.

The other factors which influence the choice of a boiler are availability, initial cost, maintenance costs, labour costs, fuel costs and the space requirement.

Water tube boilers are used where large amounts of steam are to be produced at high temperature and pressure and weight and space considerations are important. To meet a required demand, the choice between two boilers will be based on economic considerations i.e., total annual cost (fixed cost plus running cost). The noteworthy point is that the total cost of the fuel used by the boiler in its lifetime may be 3 to 4 times the initial investment.

(b) Boiler Furnaces:

A boiler furnace is a chamber in which fuel is burnt to liberate the heat energy. In addition, it pro­vides support and enclosures for the combustion equipment i.e., burners. The boiler furnace walls are made of refractory materials such as fire clay, silica, kaolin etc. Such materials have the property of resisting change of shape, weight or physical properties at high temperatures.

The construction of boiler furnace varies from plain refractory walls to com­pletely water cooled walls, depending upon characteristics of fuel used and ash produced, firing methods, nature of load demand, combustion space required, excess air used, operat­ing temperature, initial and operating costs.

The plain refractory walls are suitable for small plants where the furnace temperature may not be high. The ar­rangement may consist of a single section of homogeneous refractory or insulation may be placed in between the refractory and the casing.

For larger plants, where the furnace temperature is quite high, refractory walls are made hollow and air is circulated through hollow space to keep the temperature of the furnace walls low. This type of arrangement is no more preferred.

Partially water cooled walls are similar to the plain refractory type but with a portion of the surface covered by water tubes. A proper balance can be made between the water cooled section and the refractory section to give best results. This type is used both for stoker fired and pulverised fuel fired boilers.

The recent development is to use water walls. Water walls are built of tubes of diameter ranging from 25 mm to 100 mm variously spaced with or without fins or studs and bare or with different thickness of moldable refrac­tory on the inner face. Heat transfer rates run from 0.5 × 10⁶ to 1.4 × 10⁶ kilocalories per hour per cubic metre of surface. To meet these requirements of heat transmission, circulation on the water side must be adequate, obtained by convection or by pumps. This type is suitable for pul­verised fuel fired boilers and high steaming rates can be maintained.

(c) Super Heaters and Re-Heaters:

A super heater is a de­vice which removes the last traces of moisture from the saturated steam leaving the boiler tubes and also increases its temperature above the saturation temperature. For this purpose, the heat of combustion gases from the furnace is utilised.

Super heaters consist of groups of tubes made of steel (carbon steel for steam temperature up to 950°F, carbon- molybdenum steel for steam temperature of 1,050°F and stainless steel for steam temperature of 1,200°F) with an outside diameter ranging from 25 mm to 64 mm. Tube bundle location and arrangement, with counter current, and/or parallel flow is dictated by type of firing, required steam temperature, and steam-temperature characteristic. The super heater tubes are heated by the heat of combus­tion gases during their passage from the furnace to the chim­ney.

Super heaters are classified as radiant, convection or the combination of both.

Radiant super heater is located in the furnace between the furnace water walls and absorbs heat from the burning fuel through radiation. It has two main disadvantages firstly, owing to high furnace temperature; it may get overheated and, therefore, requires a careful design. Secondly it gives drooping characteristics i.e., the temperature of superheat falls with the increase in steam output, because with the increase in steam output, the furnace temperature rises at a much less rapid rate than the steam output and the radi­ant heat transfer being a function of furnace temperature increases slowly with steam flow or the steam temperature falls.

Convection super heater is located well back in the boiler tube bank, receives its heat entirely from fuel gases through convection. It gives rising characteristics i.e., the tempera­ture of superheat increases with the increase in steam output because with the increase in steam output both gas flow over the super heater tubes and steam flow with in the tubes in­crease which causes increase in the rate of heat transfer and mean temperature difference. Convection super heaters are more commonly used.

Desired control of characteristic is obtained by:

(i) Proportioning and locating surfaces in series,

(ii) Using internal dampers on boiler gas side,

(iii) Attemperating by water or

(iv) Supplementary burners.

Heat transfer rates of 30 to 35 kilo-calories per hour per square metre per degree temperature difference are representative.

The steam is superheated to the highest economical tem­perature not only to increase the efficiency but also to have following advantages:

(i) Reduction in requirement of steam quantity for a given output of energy owing to its high internal energy reduces the turbine size.

(ii) Superheated steam being dry, turbine blades remain dry so the mechanical resistance to the flow of steam over them is small resulting in high efficiency.

(iii) No corrosion and pitting at the turbine blades occur owing to dryness of steam.

The function of the re-heaters is to re-superheat the partly expanded steam from the turbine. This is done so that the steam remains dry as far as possible through the last stage of the turbine. Modern plants have re-heaters as well as super ­heaters in the same gas passage of the boiler. They can also be of combination type using both radiant and convective heating.

(d) Economiser and Air Pre-Heaters:

When the combustion gases leave the boiler after giving most of their heat to water tubes, super heater tubes and re-heater tubes, they still possess lot of heat which if not recovered by means of some devices. Economiser and air preheater are such devices which recover the heat from the flue gases on their way to chimney and raise the temperature of feed water and air supplied for combustion respectively.

Economiser raises boiler efficiency (by 10-12%), causes saving in fuel consumption (5-15%) and reduces tempera­ture stresses in boiler joints because of higher temperature of feed water, but involves extra cost of installation, mainte­nance and regular cleaning and additional requirement of space.

High cost of fuel consumption, high load factor and high pressure and temperature conditions, all justify the use of economiser. For pressure of 70 kg/cm² or more the econo­miser becomes a necessity. Economiser tubes are made of steel either smooth or covered with fins to increase the heat transfer surface area. The tubes can be arranged in parallel continuous loops welded to and running between a pair of water headers, or in return bend design with horizontal tubes connected at their ends by welded or gasketed return bends outside the gas path.

The feed water flows through the tubes and the flue gases outside the tubes across them. The heat transfer from flue gases to feed water is by convection. The feed water should be sufficiently pure not to cause forming of scales and cause internal corrosion, and under boiler pressure.

The tempera­ture of feed water entering the economiser should be high enough so that moisture from the flue gases does not con­dense on the economiser tubes, which may absorb SO₂ and CO₂ from the flue gases and form acid to corrode the tubes. The temperature of the feed water entering the economiser is usually kept above 84°C. In a modern economiser, the tem­perature of feed water is raised from about 247°C to 276°C.

The economiser chamber should be leak proof against air infiltration; otherwise boiler draught may be adversely affected.

Air preheaters are employed to recover the heat from the flue gases leaving the economiser and heat the in­coming air required for combustion. This raises the tem­perature of the furnace gases, improves combustion rates and efficiency, and lowers the stack temperature, thus improving the overall efficiency of the boiler. It has been found that a drop of 20-22°C in the fuel gas temperature increases the boiler efficiency by about 1%.

The use of air preheater is more economical with pulverised fuel boilers because the temperature of flue gases going out is suffi­ciently large and high air temperature (250-350°C) is al­ways desirable for better combustion. The use of water cooled furnaces and a feed water heater has also resulted in the increasing use of air preheaters.

An air preheater should have high thermal efficiency, reliability of operation, less maintenance charges, should occupy small space, should be reasonable in initial cost and should be accessible. In order to avoid corrosion of the air preheaters, the flue gases should not be cooled below the dew point. This can be achieved by passing some air around the heaters or by recirculating air from the heater outlet to the forced draught fan inlet.

The air pre-heaters are basically of two types viz., recu­perative (plate type or tubular type) and regenerative types. Recuperative type air preheaters are continuous in action while the regenerative type are discontinuous in action and operates on cycle. In recuperative type of heaters, the two fluids are separated by heat transfer surface, one fluid flowing constantly on one side and the other fluid on the other side of the surface.

In the recuperative type of heaters, the rate of heat transfer is low, space occupied is large and cleaning of surface is diffi­cult. The plate type recuperative heater consists of rectangu­lar flat plates spaced from 12.5 mm to 25 mm apart, leaving alternate air and gas passages. In tubular type recuperative heaters, there is a counter flow, the flue gases pass through the tubes and air flows across the tubes or vice versa, but the first method is preferred owing to convenience in cleaning of tubes. Horizontal baffle plates are used so that maximum heat transfer takes place. The di­ameter of steel tubes is about 40-75 mm.

A regenerative heater consists of a rotor made up of corrugated elements. The rotor is placed in a drum which has been divided into two compartments, air and gas compartments. To avoid leakage from one compartment to the other seals are provided. The rotor rotates at a very slow speed of 3-4 rpm. As the rotor rotates, it alternately passes through flue gases and air zones.

The rotor elements are heated by the flue gases in their zone and transfer this heat to air when they are in air zone. The regenerative heaters are compact, have less weight, occupy less space and provide enormous amount of heat transfer surface and are, therefore, widely used in steam plants.

To get very high temperatures (320 to 420°C), two stage preheaters are used with the economiser installed in between because it is economically inexpedient or even impossible to preheat air to a high temperature in a single stage air pre-heater.

2. Condensers:

Steam, after expansion through the prime mover, goes through the condenser which condenses the exhaust steam and also removes air and other non-condensable gases from steam while passing through them. The recovery of exhaust steam in the condenser reduces the make-up feed water that must be added to the system, from 100% when exhausted to atmosphere, to about 1-5% and thereby reduces considerably the capacity of water treatment plant.

The ex­haust pressure may be lowered from the standard atmos­pheric pressure to about 25 mm of Hg absolute and thereby permitting expansion of steam, in the prime mover, to a very low pressure and increasing plant efficiency. Maintenance of high vacuum in the condenser is essential for efficient opera­tion.

Any leakage of air into the condenser destroys the vacuum and causes:

(i) An increase in the condenser pressure which limits the useful heat drop in the prime mover, and

(ii) A lowering of the partial pressure of the steam and of the satu­ration temperature along with it.

This means that the latent heat increases and therefore, more cooling water are required. Also, the under cooling of the condensate is likely to be more severe. This will result in lower efficiency. As it is not possible to eliminate air leakage completely, a vacuum pump is necessary to remove the air leaking into the condenser.

Condensers are of two types namely jet or contact con­densers and surface condensers. The essential difference between a jet condenser and a surface condenser is that in the former, the exhaust steam mixes with the cooling water and the temperature of the condensate and the cooling water is the same when leaving the condenser; and the condensate cannot be recovered for use as feed water to the boiler; heat transfer is by direct conduction; in the latter i.e., in surface condenser, the exhaust steam and cooling water do not mix with each other, the water being circulated through a nest of tubes, the heat transfer being by convection.

The tempera­ture of the condensate may be higher than the temperature of the cooling water at outlet and the condensate is recovered as feed water to the boiler. Both the cooling water and the condensate are separately withdrawn. Advantages of jet con­densers are low initial cost, low requirements of floor area and cooling water and low maintenance charges. However, its disadvantages are that the condensate is wasted and high power is required for pumping water.

The use of jet condens­ers is, therefore, limited to small industrial applications (1,000 kW) where high vacuum is not required (50 mm to 125 mm Hg abs). Modern power plants mostly use surface condensers from which condensate can be used as feed water requiring less pumping power and high vacuum is created at the turbine exhaust though they are costlier in initial cost, require large floor area and have higher maintenance charges.

The jet condensers may be further classified as:

(i) Parallel flow type jet condensers, and

(ii) Counter flow type jet condensers.

In parallel flow type jet condensers, the steam and cooling water enter at the top of the condenser and flow downwards in parallel. The coldest water is thus in contact with the hottest steam and, therefore, efficiency is low. In counter flow type jet condensers, the steam flows upwards through the condenser, meeting the cooling water flowing downwards from the top. The air is removed at the top and the condensate and water separately, at the bottom.

In this type, since the hottest steam is in contact with the hottest cooling water, it is thermodynamically the most efficient, because heat transfer approximates towards reversibility. Also, the cooling of air is most effective and it will reduce the capacity of the air suction pump.

The counter flow type jet condensers are of two designs namely, low level and high level counter flow type jet condensers. In low level jet condenser the supply of cold cooling water is drawn into the condenser shell due to the vacuum head created in the shell by the air pump. The water is sprayed downward in the shell into the up flowing steam.

The condensed steam and cooling water flowing downward is discharged into the hot well. The high level jet condenser is also called the barometric jet condenser. In case the bottom of the condenser is not less than, say 10.5 metres above the level of water in collecting tank (hot well), condensate extraction pump is not required and the condenser is self-discharging. But, a pump is required to inject the cooling water into the condenser shell, from the cooling pond.

In ejector condenser, the cooling water enters the condenser at the top under a head of 4.5-6 metres and flows downward through a number of coaxial guide cones in a tube. As the water rushes across the gap between the central parts of nozzles (cones), it drags in the exhaust steam and air. The steam gets condensed in contact with cooling water and the air is carried forward with the water. The condenser thus acts as an air pump as well as a condenser.

The usual construction of the surface condenser is that there is a cast iron or steel shell fitted with a tube plate at each end. A large number of tubes extend between these end plates to form the cooling surface. Surface condensers can be classified depending upon whether the water flows through the tubes or steam flows through the tubes.

The usual flow pattern is that the cooling water flows through the tubes and the exhausted steam is circulated around the tubes as the outside of the tubes is not contaminated by the clean steam. The steam enters the condenser through an opening in the top of the shell. The steam after being condensed leaves the condenser through a hole at the bottom of the shell. The condensers may be single pass or two pass.

In single pass condenser, the cooling water flows in one direction only through all the tubes and in the two-pass type the cooling water flows in one direction through part of the tubes and returns through the remaining of the tubes. Surface condensers are also classified as parallel flow, counter flow or cross flow depending upon the direction of flow of the condensate relative to the tubes.

They can be further classi­fied as down flow type, central flow type and inverted flow type. In the down flow type, the steam enters at the top of the condenser and flows downwards over the tubes through which cooling water flows as the extraction pump is at the bottom. The cooling water flows in one direction through the lower half of the tube nest and return in the reverse direction through the upper half of the tube nest.

The air associated with the steam is also extracted from the bottom of the condenser where the temperature is lowest, so that the work of the air pump is reduced. In order to keep the velocity of steam across the tubes, approximately uni­form, the cross section of the shell of the condenser is gradually reduced in width towards the bottom. Also the tubes are generally placed close together in the lower part.

In central flow type, the suction pipe of the air pump is located at the centre of the tube nest. The condensate then leaves at the bottom where the conden­sate extraction pump is placed. In this type, the steam comes into close contact with the whole periphery of the tubes. In the inverted type, the air suction pump is at the top. The steam flows upwards and then condensate re­turns to the bottom of the condenser by flowing near the outer surface. The condensate pump is at the bottom of the condenser.

The condenser is attached to the turbine by:

(i) Direct bolting to turbine exhaust flange (up to 20 MW).

(ii) Direct bolting to turbine exhaust flange with spring supports under the condenser.

(iii) Solid support of condenser with expansion joint between the turbine and the condenser.

3. Evaporators:

Evaporators are employed for supplying pure water as make-up feed water in steam power plants. In an evaporator raw water is evaporated by using extracted steam and the vapours so produced may be condensed to give a supply of distilled or pure feed water. These vapours can be condensed in feed water heaters by the feed water or in separate evaporator condensers using feed water as the cool­ing medium. There are two main types of evaporator’s viz., film or flash type and submerged type.

In the former one (film or flash type evaporator), there are tubes or coils through which the steam is passed. Raw water is sprayed by means of nozzles on the surface of these tubes and some of the raw water will be converted into vapours. These vapours are collected from the evaporator and are condensed to give pure and distilled water for boilers. In the submerged type evaporator, the tubes through which the steam is passed are submerged in raw water.

The vapours rising from the raw water are collected and condensed to provide a supply of pure make-up feed water. Because of continuous operation of raw water, concentration of impurities goes on increasing, so periodic blowing down of raw water is essential. Scales formed on the surface of the tubes will retard the heat trans­fer rate and so its removal is also necessary. This is removed by draining the raw water from the shell and then spraying the tubes with cold water while the tubes are kept hot by flow of steam through them. The scale is cracked off and is washed away by the spray.

4. Feed Water Heaters:

These heaters are used to heat the feed water by means of bled steam before it is supplied to the boiler.

Necessity of heating feed water before feeding it back to the boiler arises due to the following reasons:

a. Overall plant efficiency is improved.

b. Thermal stresses due to cold water entering the drum of boiler are avoided.

c. There is an increase in the quantity of steam pro­duced by the boiler.

d. The dissolved oxygen and carbon dioxide which would otherwise cause boiler corrosion are removed in the feed water heaters.

e. Some other impurities carried by steam and conden­sate, due to corrosion in boiler and condenser, are precipitated outside the boiler.

Feed water heaters are of two types viz.:

i. Contact or open heaters and

ii. Surface or closed heaters.

i. Open or Contact Heaters:

Open or contact heaters are usually constructed to re­move non-condensable gases from water and steam along with raising the feed water temperature. Such heaters are also called the deaerator. The amount of gas dissolved in water depends upon its temperature. This decreases sharply with the increasing temperatures and falls to almost zero at the boiling point. Such feed water heaters are used in small power plants.

ii. Closed or Surface Heaters:

Closed or surface heater consists of closed shell in which there are tubes or coils through which either steam or water is circulated. Usually, the water is circulated through the tubes and the steam and water may flow either in the same direction or in opposite directions. Such heaters may be either vertical or horizontal type. In such heaters, the feed water can never be heated to the temperature of steam. For maintaining a high overall heat transfer for the heater, the water velocity should be high but pumping costs limit the velocity to about 1-2.5 m/s.

5. Prime Movers:

The prime mover converts the stored energy in steam into rotational mechanical energy. Steam prime movers are either reciprocating engines or turbines. Nowadays steam engines have become obsolete due to their reciprocating motion and the steam turbines are usually employed as prime movers.

Steam turbines give high speed (standard speeds are 3,000 rpm and 1,500 rpm), maximum size (1,000 MW), minimum floor space, bulk and weight, maximum efficiencies in large sizes, suitability for highest steam pressures and steam temperatures. All large units (above 1 MW) are steam turbines.

6. Spray Ponds and Cooling Towers:

In the modern fossil-fuel steam power plants, about 10-15% of the heat input is rejected to the atmosphere through boiler chimneys, while 48-52% of the heat input is rejected to cooling water system through the steam condensers. The quantity of cooling water needed for condensing the steam in the condenser is, therefore, quite large. Roughly each kg of steam needs 100 kg of cooling water for the condenser.

A typical 2,000 MW power plant requires about 300 × 10⁶ litres of cooling water per hour. Such large requirement of cooling water can be conveniently met if the plant is located near a natural or artificial source of water such as river, sea, lake or canal. In such a case, water is pumped from the source of water to an open channel and taken to the circulating water pumps through strainers.

The circulating water is then pumped through the condenser tubes. For such arrangements, a settling tank and syphon system is used before the circulating water pump at the inlet. The circulating water takes up the heat of the exhaust steam and itself becomes hot. This hot water coming out from the condensers is discharged at a suitable location down the source of water.

In case natural or artificial source of water is not available as a source of cooling water for condensers, the warm water coming out of the condenser has to be cooled and reused. In this case, water is first obtained from a tube well or some other source and stored in a tank. From the tank it is pumped into the condenser, usually by a centrifugal pump. The water there absorbs the latent heat from the exhaust steam and gets warm, the warm water is then cooled by means of spray ponds or cooling towers. Small plants use spray ponds and medium and large plants use cooling towers.

(a) Spray Pond:

Spray pond consists of a water tank in which hot water is distributed by pipes throughout and is sprayed in air through nozzles at a suitable pressure. The water is cooled by both convection and evaporation. The sprayed water comes in contact with the atmospheric air and is cooled. Mostly the water is cooled by evaporation as the heat for evaporation is withdrawn from the water itself with the result that it is cooled.

A small amount of cooling is due to conduction and radiation also. Since the cooling is mainly affected by evaporation it is, therefore, necessary to ex­pose as large surface as possible. In the design of spray ponds the most important part is nozzle and it is neces­sary that it should produce spray, not sheet of water. The spacing between nozzles should be around 2.5-3.0 metres and about 1.2-2.5 metres above the surface of water.

Further the nozzles should be adjustable for load and climatic conditions. The drawbacks of cooling ponds are that con­siderable quantity of water may be carried away in sus­pension in air when its velocity is high and loss due to evaporation and the large ground area required especially in cases of large sized power plants.

(b) Cooling Tower:

A cooling tower is a wooden or metallic rectangular structure inside of which is packed with baffling devices. The hot water is led to the tower top and falls down through the tower and is broken into small parti­cles while passing over the baffling devices. Air enters the tower from the bottom and flows upward. The air vaporises a small percentage of water, thereby cooling the remaining water. The air gets heated and leaves the tower at the top.

The cooled water falls down into a tank below the tower from where it is pumped to the condenser and cycle is re­peated. The splitting of water into small droplets, the draught provided by the tower and the large evaporating surface help to cool water very quickly-practically during the time while it is descending.

Although eliminators are provided at the top of the tower to prevent escape of water particles with air but even then there is a loss of water to the extent of around 5 per cent and this loss has to be made up by water drawn from well or any other source. Air can be circulated in cooling towers through natural draught, me­chanical draught or mechanical natural draught.

In natural draught, air movement is induced by a large chimney and the difference of densities between the air inside and outside the chimney. In mechanical draught, air is moved by fans, either of the induced type that pull the air through the tower, or the forced type that push the air through.

Combination of mechanical and natural draught incorporates both a chimney and fans. The natural draught towers are being gradually replaced by mechanical draught type be­cause the latter requires less pumping head, less space and less wind age loss. Induced draught tower is considered to be better than the forced draught tower because the latter type requires more power and involves costlier maintenance of fans.

The induced draught tower occupies less space as the fan drives are placed at the top of the tower. Moreo­ver, since air is drawn from all the sides of the tower, the cooling effect is distributed across the entire cross section of the tower. Also, since the fans handle warm air, they are non-freezing. Again, as the air leaves the tower at a high speed, this type is non-circulating.

7. Control Room:

The control room, in case of remote con­trol, houses all the necessary measuring instruments for each panel of alternator and feeder, synchronizing gear, protective gear, automatic voltage regulator, communication arrange­ment etc. A separate battery room and a motor-generator set or a rectifier is also installed for supplying to make and trip circuit of switchgear. In case of outdoor switchgear normally compressed air is used for operation.

8. Switchyard:

The switchyard houses transformers, circuit breakers and switches for connecting and disconnecting the transformers and circuit breakers. It also has lightning arrestors for the protection of the power station against lightning strokes.

The supply to the bus-bars from alternators is taken through transformers and circuit breakers of suitable ratings.

The power station over and above this requires work­shop, store, labour canteen, library and also residential ac­commodation for the operating staff.