Cooling towers1

Cooling TowersCooling Towers

Topics CoveredTopics Covered

Types of cooling tower Types of cooling tower Performance evaluationPerformance evaluation Efficient system operationEfficient system operation Flow control strategiesFlow control strategies Energy saving opportunitiesEnergy saving opportunities Assessment of cooling towersAssessment of cooling towers

What is the importance/necessity of a What is the importance/necessity of a cooling towercooling tower

Reject heat into the atmosphere Reject heat into the atmosphere

Represent inexpensive and Represent inexpensive and dependable means of removal low dependable means of removal low grade heatgrade heat

Cooling tower typesCooling tower types

Natural DraftNatural Draft

Mechanical draftMechanical draft

Natural Draft Cooling towersNatural Draft Cooling towers

Use very large concrete chimneys to Use very large concrete chimneys to introduce air through the mediaintroduce air through the media

Used for water circulation rates Used for water circulation rates above 45,000 cum/h (Normally for above 45,000 cum/h (Normally for power plants, where condenser water power plants, where condenser water requirements are high)requirements are high)

Mechanical Draft Cooling towersMechanical Draft Cooling towers

Utilize large fans to force or suck air Utilize large fans to force or suck air through circulated waterthrough circulated water

Water falls downwards over fill Water falls downwards over fill surfaces, which help increase of surfaces, which help increase of contact time between water and air contact time between water and air (increases heat transfer)(increases heat transfer)

Types of Mechanical Draft Cooling towersTypes of Mechanical Draft Cooling towers

Counter flow induced draftCounter flow induced draft

Counter flow forced draftCounter flow forced draft

Cross flow induced draftCross flow induced draft

Cross flow induced draft towersCross flow induced draft towers

Hot water enters at the top and Hot water enters at the top and passes over the fill.passes over the fill.

Air is introduced at the side, either Air is introduced at the side, either on one side (single flow tower) OR on one side (single flow tower) OR opposite sides (double flow tower)opposite sides (double flow tower)

Counter flow induced draftCounter flow induced draft

Hot water enters at the top, while the Hot water enters at the top, while the air is introduced at the bottom and air is introduced at the bottom and exits at the topexits at the top

Here both forced and induced draft Here both forced and induced draft fans are usedfans are used

What does a Cooling tower consist ofWhat does a Cooling tower consist of

Frame and casingFrame and casing FillFill Cold water basinCold water basin Drift eliminatorsDrift eliminators Air inletAir inlet LouversLouvers Nozzles and FansNozzles and Fans

Frame and casingFrame and casing

Many towers have structured frames Many towers have structured frames that support the exterior enclosures that support the exterior enclosures (casings), motors, fans and other (casings), motors, fans and other components.components.

With some smaller designs such as With some smaller designs such as some glass fibre units, the casing some glass fibre units, the casing may essentially be the frame.may essentially be the frame.

FillFill Plastic or wood. Fills can be splash or film type.Plastic or wood. Fills can be splash or film type.

In splash type, water falls over successive layers In splash type, water falls over successive layers of horizontal splash bars, continuously breaking of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill into smaller droplets, while also wetting the fill surface. Plastic splash fill promotes better heat surface. Plastic splash fill promotes better heat transfer than wood splash filltransfer than wood splash fill

Film fill consists of thin, closely spaced plastic Film fill consists of thin, closely spaced plastic surfaces over which the water spreads forming a surfaces over which the water spreads forming a thin film in contact with the air.thin film in contact with the air.

Film type—MORE EFFICIENT and provides same Film type—MORE EFFICIENT and provides same heat transfer in a smaller volume than the splash heat transfer in a smaller volume than the splash fillfill

Cold water basinCold water basin Located at the bottom of the tower, Located at the bottom of the tower,

receives the cooled water that flows down receives the cooled water that flows down through the tower and fill.through the tower and fill.

The basin usually has a sump or low point The basin usually has a sump or low point for cold water discharge connectionfor cold water discharge connection

Drift EliminatorsDrift EliminatorsThese capture water droplets ENTRAPPED These capture water droplets ENTRAPPED in the AIR STREAM that otherwise would in the AIR STREAM that otherwise would be lost to the atmospherebe lost to the atmosphere

Air inletAir inlet This is the point of entry for the air entering a This is the point of entry for the air entering a

tower.tower.

The inlet may be at the ENTIRE side or CROSS The inlet may be at the ENTIRE side or CROSS FLOW DESIGN or be located low on the side or FLOW DESIGN or be located low on the side or the bottom of counter flow designsthe bottom of counter flow designs

LouversLouvers Equalize air flow into the fill and retain the Equalize air flow into the fill and retain the

water within the towerwater within the tower

NozzlesNozzles Provide water sprays to WET the FILLProvide water sprays to WET the FILL

FansFans Both AXIAL (propeller type) and Both AXIAL (propeller type) and

CENTRIFUGAL fans are usedCENTRIFUGAL fans are used

Axial fans are used in INDUCED draft towersAxial fans are used in INDUCED draft towers

Both AXIAL and CENTRIFUGAL fans are Both AXIAL and CENTRIFUGAL fans are found in forced draft towers.found in forced draft towers.

Cooling tower materialCooling tower material Wood--- frame, casing, louvers, fill, and cold Wood--- frame, casing, louvers, fill, and cold

water basin (or concrete)water basin (or concrete)

Galvanised steel, various grades of stainless Galvanised steel, various grades of stainless steel, glass fibre and concrete, aluminium steel, glass fibre and concrete, aluminium and various types of plastics for some and various types of plastics for some componentscomponents

Large towers are made of CONCRETELarge towers are made of CONCRETE Plastics are widely used for FILL, including Plastics are widely used for FILL, including

PVC, polypropylene and other polymersPVC, polypropylene and other polymers Plastics also find wide use in nozzle materialsPlastics also find wide use in nozzle materials

Cooling tower PerformanceCooling tower Performance

Cooling tower performanceCooling tower performance

Range : (water in – water out temperature)Range : (water in – water out temperature)

Approach : (water out – WBT)Approach : (water out – WBT)

Effectiveness : Range/(Range + Approach)Effectiveness : Range/(Range + Approach)

Cooling capacity: Heat rejected in kcal/h or Cooling capacity: Heat rejected in kcal/h or TRTR = m x Cp x temp. difference= m x Cp x temp. difference

Evaporation loss:Evaporation loss:

= (Circulation rate x temp. difference)/675= (Circulation rate x temp. difference)/675

Cycles of concentration (C.O.C) is the ratio of Cycles of concentration (C.O.C) is the ratio of dissolved solids in circulating water to the dissolved solids in circulating water to the dissolved solids in make up waterdissolved solids in make up water

Blow down: depends on C.O.C and evaporation Blow down: depends on C.O.C and evaporation losses = Evap. Losses/(C.O.C. -1)losses = Evap. Losses/(C.O.C. -1)

Factors affecting Cooling tower performanceFactors affecting Cooling tower performanceCapacity utilizationCapacity utilization Amount of water circulatedAmount of water circulated

RangeRange Determined by the process it is servingDetermined by the process it is serving Determined by heat load and water circulation rateDetermined by heat load and water circulation rate Thus Range: f (Heat load & water circulation rate) Thus Range: f (Heat load & water circulation rate)

Wet Bulb temperature: design range is specified at Wet Bulb temperature: design range is specified at certain WBTcertain WBT

The closer the approach to the WBT, the more The closer the approach to the WBT, the more expensive the cooling tower due to increased size.expensive the cooling tower due to increased size.

Factors affecting Cooling tower performanceFactors affecting Cooling tower performance

Wet Bulb TemperatureWet Bulb Temperature

WBT of air entering the cooling tower WBT of air entering the cooling tower determines operating temperature levels determines operating temperature levels throughout the plant, process or system.throughout the plant, process or system.

Recirculation raises the effective WBT of the Recirculation raises the effective WBT of the air entering the tower with corresponding air entering the tower with corresponding increase in cold water temperature.increase in cold water temperature.

Approach and FlowApproach and Flow

Approach is dependent on WBT of air Approach is dependent on WBT of air entering the cooling tower.entering the cooling tower.

Water circulation rate is directly Water circulation rate is directly proportional to the heat loadproportional to the heat load

Factors affecting Cooling tower performanceFactors affecting Cooling tower performance

Range, Flow and Heat LoadRange, Flow and Heat Load

Range is a direct function of the quantity of Range is a direct function of the quantity of water circulated and the heat load.water circulated and the heat load.

Increasing the range as a result of added heat Increasing the range as a result of added heat requires an increase in tower size. requires an increase in tower size.

If the hot water temp is constant and the If the hot water temp is constant and the range is specified with a lower cold water range is specified with a lower cold water temp, then the tower size required for such temp, then the tower size required for such applications would increase considerably.applications would increase considerably.

Approach and Wet Bulb TemperatureApproach and Wet Bulb Temperature

Design WBT is determined by the Design WBT is determined by the geographical location.geographical location.

Usually the WBT selected should not Usually the WBT selected should not exceed 5% of the time in that area.exceed 5% of the time in that area.

Higher WBT, smaller the tower required to Higher WBT, smaller the tower required to give a specified approach to the wet bulb give a specified approach to the wet bulb at a constant range and flow rateat a constant range and flow rate

Factors affecting Cooling tower performanceFactors affecting Cooling tower performance

Fill Media EffectsFill Media Effects

Function: Heat exchange between air and water is Function: Heat exchange between air and water is influenced by surface area of heat exchange, time of influenced by surface area of heat exchange, time of heat exchange and turbulence in water effecting heat exchange and turbulence in water effecting thoroughness of intermixing.thoroughness of intermixing.

Due to fewer requirements of air and pumping head, Due to fewer requirements of air and pumping head, there is a tremendous saving in power with the there is a tremendous saving in power with the intervention of film fill.intervention of film fill.

Recently, low clog film fills with higher flute sizes Recently, low clog film fills with higher flute sizes have been developed to handle high turbid waters. have been developed to handle high turbid waters. (sea water)(sea water)

Cooling Water TreatmentCooling Water Treatment For controlling suspended solids, algae For controlling suspended solids, algae

growth, etc.growth, etc.

Improving treatment methods, increases Improving treatment methods, increases C.O.C, thereby reduces make up water C.O.C, thereby reduces make up water requirements.requirements.

For large Cooling towers (especially power For large Cooling towers (especially power plants), water treatment is the key area for plants), water treatment is the key area for energy conservation.energy conservation.

Efficient System Operation of a Cooling towerEfficient System Operation of a Cooling tower

Drift lossDrift loss

Should be less than 0.02% of the Should be less than 0.02% of the circulation ratecirculation rate

With technological development, With technological development, incorporation of efficient designs of drift incorporation of efficient designs of drift eliminators enables to specify to as low as eliminators enables to specify to as low as 0.003 to 0.001%.0.003 to 0.001%.

Cooling Tower fansCooling Tower fans

PurposePurpose: To move a specified quantity of air : To move a specified quantity of air through the system, overcoming the system through the system, overcoming the system resistance, which is defined as the pressure resistance, which is defined as the pressure loss.loss.

Work done by the fanWork done by the fan: air flow x pressure loss : air flow x pressure loss

Efficient System Operation of a Cooling towerEfficient System Operation of a Cooling tower

Fan efficiencyFan efficiency::• Dependent on profile of the blade like Dependent on profile of the blade like

tip clearance, obstacles to air flow and tip clearance, obstacles to air flow and inlet shape etc.inlet shape etc.

• Metallic fans doesn’t have the ideal Metallic fans doesn’t have the ideal aerodynamic profile characteristicsaerodynamic profile characteristics

• Hence FRP fans finds use for such Hence FRP fans finds use for such application, where power consumption application, where power consumption is as low as 80 to 85% compared to is as low as 80 to 85% compared to metallic ones.metallic ones.

Fan efficiency contd..Fan efficiency contd..• Due to light weight, FRP blades require Due to light weight, FRP blades require

low starting torque, resulting in low kW low starting torque, resulting in low kW of the motorsof the motors

• Light weight increases the life of the Light weight increases the life of the gear box, motor and bearings and gear box, motor and bearings and allows for easy handling and allows for easy handling and maintenancemaintenance

Typical measurements and ObservationsTypical measurements and Observations

Inlet and outlet water temp.Inlet and outlet water temp.

DBT and WBTDBT and WBT

Heat loads of processHeat loads of process

TDS of cooling towerTDS of cooling tower

BlowdownBlowdown

Make up water requirementsMake up water requirements

C.O.C at site conditionsC.O.C at site conditions

Performance Assessment of Cooling towersPerformance Assessment of Cooling towers

Replace splash bars with self Replace splash bars with self extinguishing PVC cellular film fillextinguishing PVC cellular film fill

Install new nozzles to obtain a more Install new nozzles to obtain a more uniform water patternuniform water pattern

Optimise blow down rate as per COC Optimise blow down rate as per COC limitlimit

Energy saving opportunitiesEnergy saving opportunities

Installing FRP blades in place of metallic Installing FRP blades in place of metallic bladesblades

Incorporation of thermostatic controls for Incorporation of thermostatic controls for fan operationfan operation

Consider COC improvement measures for Consider COC improvement measures for water savingswater savings

Evaluate the efficiency of CT pumps on a Evaluate the efficiency of CT pumps on a periodic basisperiodic basis

Energy saving opportunities contd..Energy saving opportunities contd..