Circulating Water System

7/30/2019 Circulating Water System

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24 December 2012 1http://studygalaxy.com/


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Bulk requirement of water is used in thermal plantsfor the purpose of cooling the steam in condensers.

The requirement of water for this purpose is of theorder of 1.5-to2.0 cusecs/MW of installation.

Where sufficient water is available once throughsystem is used.

Where water supply is not consistent, closed loopcooling system with cooling tower is used.


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A circulating water pump house

Intake channel

Trash rack

A chlorination plant

Traveling water screen

Connecting pipe line to condenser

Outlet channel

A cooling tower


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We shall discuss the open loop system first.


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A circulating water pump house

It houses the main CW pumps. Vertical

submergible pump with operating pressure

around 1 Kg/ cm2.

The pump house resides on the fore bay.

The fore bay is flooded through intake system.


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It is RCC open trench from where Raw water is taken

through canal/reservoir. The intake level is normally 10-15 ft. above from flood

level of the water source and 12 m in depth.

Intake

Trash Rack

Trash rack to avoid entering wood, tree branches, animal,plastic, floating object.

provides uniform flow/suction to the CW pump

Traveling Water Screen

It catches small pieces of coal, sand, gravel, wood,plastic, herbs, leaves which can go into the impeller andmay choke/damage the pump.


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Chemical dosing done for disabling micro organismdevelopment inside the tube.

Condenser is fitted with 4 way valve to enablereverse flow through condenser when required.

Hot water coming out from condenser is allowed topass through long outlet channel to get cool down

before meeting the main stream of water. Performance of condenser mainly depends on Inlettemp. to the condenser, Condenser tube fouling, Airingress in condenser etc.


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Open Loop system:

Water is abundant

Reduction in the APC..

Condenser

River Flow

Steam from Turbine

Pump

Hotwater

Cold Water


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CW scheme

Reservoir/ River Canal Intake

Trash rack

TWS

CW pumps

Condenser

Hot Pond

CT pumps

Cooling tower


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HotPond:

Hot Water from the condenser discharged to hot pondand provide source for the CT pump.

CT Pump

It takes suction from the Hot pond and discharged

the water to the riser tubes of Cooling towers


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Cooling Tower - Introduction

A cooling tower is an air and water management

device, which consists of fan stacks, fans, drift

eliminators, fill and water/air distribution systems. It uses principle of evaporation of water in

unsaturated air.


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Cooling Tower - Introduction

Cooling tower is the most important equipment for

closed cycle water recirculation system.

The efficiency of cooling tower has direct effect on

condenser vacuum and in turn, the heat rate of thestation.

1oC rise in cold water temperature in a 200MW unit

corresponds to 5mm vacuum loss leading to 7.5Kcal/kwh loss in heat rate and in 500MW, 5.74

Kcal/kwh loss in heat rate..


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Wet type

Dry type

Wet type cooling towers are two type

1.natural draft 2.mechanical draft

Mechanical draft may be devided in to two type

1.forced draft

2.induced draft Both of these can be counter flow type or cross flow

type.


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Cooling Towerin NTPC Ltd.There are 75 cooling towers in operation in NTPC.

Induced Draught Cooling Towers

Cross flow type 24 Counter flow splash bar fill type 21

Counter flow film fill type 20

Natural Draught Cooling Towers

Natural draught type. 08


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It produces air flow through utilization of stack orchimney effect.pressure diff. causing air flow isgiven by p (pr head developed)=(p outer-p

inner).H.g The fill is located in the lower portion of shell with

upper 85 to 90% of shell empty which is used tocreate chimney effect.

They are more suitable in the area of high relativehumidity and low temp.

They may be of cross flow or counter flow.


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Forced draft

Advantage:-fan is subjected to less severecondition

Disadvantage:-recirculation more Induced draft

Advantage:-min recirculation

Disadvantage:-fan is subjected to warm humidcondition.


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Counter flow :-fill is arranged over entire towerplan.fill is typically film type.

Advantage:-more thermal efficiency, smallesttower,lower capital cost, lower pumping head

Disad:-increased pressure drop requires morehorse power of fan.

Cross flow:-fill is arranged at the outer perimeter.

Advantage:-large air inlet area hence pressure dropis less.

Disad:-high pumping head due to more height of fill.

MECHANICAL DRAFT C.T.


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T f C li T


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Types of Cooling Towers


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24 December 2012 22A BIRDs EYE VIEW OF NDCTs OF DADRI-Coal (4 X 210 MW)


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1.Rectangular type:-each cell indentical, more long

area required, low capital cost,for plant upto

250mw.

2.Round type:-less recirculation, more capitalcost,for plant more than 250mw.


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Parameters are used to develop the tower design :

Water flow rate Cooling range

Design Heat load

Design Wet-bulb temperature.

Recirculation and interference

DBT and relative humidity


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Condenser and c.t. are designed on the basis of

MCR load point.


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The impact the design wet-bulb temperature has on the

size and power requirements of a cooling tower is criticalto optimizing the cooling tower economics.

In the majority of applications, the design duty of anevaporative cooling tower is based upon anacceptable/required cold water return temperature

If investment and operational costs were not a concern,the ideal design wet-bulb temperature would be equal to,or higher than, the highest local wet-bulb temperaturerecorded.

In this way, the returned water temperature would neverbe higher than the acceptable/required cold watertemperature.

Design W.B.T.=DESIGN AMBIENT D.B.T.-RECIRCULATION ALLOWANCE.


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The component most likely to provide improvementin tower performance is the fill packing.

The earliest and most common designs utilized

splash type fills Film type counter flow designs using relatively low

cost PVC materials.

The new film type designs provide energy savings

both in fan power and pump head through the highsurface areas per cubic feet of fill.


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Splash type consisting of splashBars used for crossflow and

Counterflow.

Film type consisting of thin film of

Sheets used mostly for counterflow


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In splash fill, the hot water strikes the bar and

breaks up into many smaller drops.


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Film fill consists of flat or formed sheets to provide

a surface upon which water and air come in

contact for heat exchange.


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A cooling tower drift eliminator is a low pressure,

momentum filter. Components are arranged to force the air leaving the fill

section to make a series of directional changes.

Water droplets, which cannot negotiate these turns,impinge on the surface of the eliminator, from which they

are collected and drained back into the wet side of thetower.

The designer's goal is to provide the maximum driftelimination at reasonable cost and minimum pressureloss.

The design of drift eliminators has undergonetremendous improvement in the last decade.

New eliminator configurations accomplish thisimprovement while actually reducing eliminator pressure

losses


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Drift Eliminators

Cellular eliminators are typically constructed of PVC

sheets vacuum formed into very precise, compound

shapes, with an integral honeycomb strength. The

compound shape allows significant improvements indrift eliminations and the use of cellular structure

appreciably reduces the pressure losses through the

eliminator when compared to either the wood lath or

wave form eliminators.

The net free are of well-designed, modern cellular

eliminators is in excess of 95%.


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Air-Water Distribution System Design1) Water Distribution:

Assured performance is a function of nozzle design, nozzle installationpattern, nozzle distance, and the structural cleanliness of the spraychamber.

To provide the primary function of precise water distribution, thenozzle must be designed with other considerations in mind:

The location of counter flow nozzles and the potential for poor qualitycirculating water demands that the nozzle system be designed tominimize fouling.

The nozzle must be capable of providing uniform distribution over awide range of flows, without significant loss in nozzle performance.

The nozzle must be capable of efficient operation while consuming aminimum of expensive pump energy.


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Air-Water Distribution System Design2)Air Distribution: Three variables control the distribution of air to

the fill in a counter flow configuration.

The first is the air inlet geometry.

Pressure Ratio : The pressure ratio reflects the ratio ofresistance to available entering air energy. The higher the ratio,the better entering air will be spread out before entering the fill.The lower the pressure ratio, the less uniform, and less stablethe distribution of air flow becomes. The degradation of air flowuniformity is readily apparent, particularly at the inlet.

(Pressure Ratio = Static Pressure / Velocity Pressure at Air Inlet)

The third is the fan coverage over the eliminators


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Each fan has only one design point which isestablished by a specific air flow, total pressure,air density, and fan speed.

The factors that must be known when replacing

a fan on an existing installation are:

1. Fan diameter.

2. Installed motor horsepower.

3. Gear reduction ratio of gear reducer.4. Shaft size or gear reducer model.

5. Some estimate of elevation above sea level ofinstallation.


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Cooling Tower Performance


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Cooling Tower Performance

Cooling Tower Effectiveness=ActualCooling/Maximum Cooling Possible =Range/(Range+Approach)

Cooling Tower Capacity = mass flow rate X specificHeat x Temperature Difference

Evaporation Loss in cub mtr/Hr =0.00085x1.8xcirculation rate x Temperature Diff.

Cycle of Concentration = Dissolve solid in circulatingwater/Dissolve solid in make up water

Blow Down =1-1.5 percent of total water flow

To replenish the losses make up water 2-2.5% ofwater flow is added


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Intake Channel Level (Normal)---------- 202 mtrs

C.T.PUMP

Nos 2 for each 210 MW unit

Discharge 4160 L/sec or 15000 m cube per hour

Head 17.855 mwc

Speed 405 rpm

Make KIRLOSKAR BROTHERS LTD.(U5)


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Type Induction Motor

Speed 421 rpm

KW 930

Current 108.5 amps

Volts 6600 volts, 3 Ph. 50 C/S

Make BHEL


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Make Gammon(U4)/ Paharpur(U5)

Number 1 for each unit of 210 MW

No. of cells/tower 16

Rate of flow/unit 27000 m cube per hour per tower

Cooling Range 10 degree C

Max operating inlet water temperature 50 degree C


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Fan Type Induced Draft

No of fans/Tower 16

No of blades/fan 9

No of row of fan 2(8 in each row)

Dia. 7315 mm

Reduction gear Ratio 10:1 L.X.

Fan Speed 151 rpm


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Make N.G.E.F

Volts 415 V

Current 130 amps

K.W. 75

Speed 1475 rpm


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Circulating Water System