Questions answers on cooling tower and calculations

 

1-What is the cooling tower (CT)?









Cooling tower is a large & robust heat exchanger used to reduce water temperature. Here air & water are mixed to reduce water temperature.

2-What is the function of cooling towers (CTs)?

  • Cools the circulating cooling water by extracting heat from water
  • Provides sump or surge for cooling water
  • Provides sump or arrangement for chemical treatment of water

3-What action does happen in cooling towers for reducing water temperature?

In cooling towers, water temperature is reduced by evaporating small amount of water naturally or mechanically. Here the heat present in water is rejected to atmosphere.

4-What are the various types of cooling towers used in power plants or chemical plants?

Natural draft cooling towers: These are tall & concrete made cooling towers, used for water circulation more than 50000 m3/hr

Mechanical draft cooling towers: Mechanical draft cooling towers use fans which suck or force the air for heat transfer.

5-What are the types of Mechanical draft cooling towers?

  • Counter flow induced draft
  • Counter flow induced draft
  • Cross flow induced draft

5a-Whar do you mean by counter & cross flow type cooling towers?

In counter flow, air flow & water falling directions are opposite to each other. Air enters from bottom to the top & water falls from top to the bottom.

In cross flow, air & water flow intersect, where air is blown horizontally & water flows vertically.

6-What are the other accessories of cooling towers?












  1. Sump
  2. Fore bay (cold water basin)
  3. Louvers
  4. Fans
  5. Drift eliminators
  6. Fills (Splash type & film type)
  7. Water distribution pipe lines
  8. Water nozzles
  9. Hot water basin
  10. Cooling water inlet & out lines
  11. Sump over flow & drain lines
  12. Sluice gate valve

7-What is the function of drift eliminators?

Drift eliminators fitted at the top of the tower capture water droplets trapped in air & water vapour mixture.

Drift is water that is carried away from the tower in the form of droplets with the air discharged from the tower.

8-What is the function of fills?

Fills situated just below the drift eliminator facilitate the heat transfer by maximizing the contact between air & water particles.

9-What are the two types fills used in cooling towers?

Splash type fills & Film type fills

Splash type fills: Are made up of plastics or wooden materials. These are fitted on splash bars .These splash break the water particles into small particles to increase the surface contact area with air.








Film type fills: These are made of plastic materials, the water particle falling on this forms small films, which increases heat transfer by making contact area larger.

Films fills may be flat, corrugated or horizontal type. Film type fills are more efficient than splash type fills










10-What is the function of lower in cooling towers?

  • Lowers equalize the air flow into the fills
  • Retain the water falling within the sump of tower

11-What are the different types fans used in cooling tower?

  • Centrifugal fans for forced type
  • Propeller type for both induced & forced draft cooling towers



12-What type of blades used for propeller type fans?

Fixed pitch & variable pitch blades.

13-What is the material of construction of CT blades?

Generally Blades are of Aluminium or FRP (Fibre Reinforced Plastic)

14-Why do the cooling towers used in power plant

Cooling towers used for cooling

  • Generator air
  • Turbine lube oil
  • Reciprocating air compressor air
  • Boiler feed pumps bearings
  • Coal feeding system bearings
  • Ash handling domes & surge vessels
  • Air conditioners refrigerant

15-Why do the Mechanical draft cooling towers some time installed inside the buildings?

Because, mechanical draft cooling towers do not depend on atmospheric air

16-Briefly explain the cooling tower working procedure?

Hot water from various plant appliances is sent to cooling tower hot basin, where water distributed into various cells through pipe lines & water is being sprayed on fills. While falling downward water, comes in contact with cold air that was sucked, forced or naturally drafted by cooling tower. Thus exposure of hot water to cold air converts water vapour & the remaining water falls down in sump for recirculation.

The vapour is pulled by fans & expelled to atmosphere. Such loss of water due to vapour is added through fresh same quality make up water.

17-What is the function of sluice valves















Sluice valves are used to isolate the cooling towers sumps of different cells for cleaning or any maintenance purpose.

18-What are the various pipe lines connected to cooling towers?

  • Cooling water pumps suction line
  • Cooling water return lines
  • Cooling water make up lines
  • Cooling water pumps recirculation lines
  • Side stream filter outlet lines
  • Cooling water cell drain & over flow lines
  • Cooling water corrosion analysis line










19-What is the blade angle of CT fans usually set?

It is in the range of 12 to 140















20-What is the function of fore bay in cooling tower?

It is a water sump or canal provided to connect the different cells & to provide suction water to cooling water pumps.

21-What are the reasons for drift loss in cooling towers?

  • Improper designed cooling towers & their lowers
  • Damaged drift eliminators
  • Improper set of CT fan blade angles

Note: 

  1. Natural draft Cooling towers have more drift losses around 0.3 to 1%
  2. Cooling towers without drift eliminators have drift losses around 0.1 to 0.3%
  3. Induced CT with drift eliminators has drift losses around 0.005%

22-For which type fill height required is very less?

Film type fills

26-Distinguish between cooling towers having film type fills & flash type fills

Sl No.

Film type fills

Flash type fills

1

Fill height required less

Fill height required more

2

Pump static head required is less

Pump static head required is more

3

Motor power consumption is less

Motor power consumption is more

 

27-What is the maximum speed of cooling tower fans for induced counter flow film type CTs?

It’s generally 125 rpm to 175 rpm

28-Define Range & Approach in cooling towers?

Range: It is the difference between cooling tower inlet and out let water temperature

Range = T2-T1

Where T1 = Cooling tower outlet cold water temperature in deg C

T2 = Cooling tower inlet hot water temperature in deg C

Approach: It is the difference between cooling tower outlet cold water temperature (T2 or Tc) and wet bulb temperature (Twb)

Approach = T2-Twb or Tc-Twb

29-Define the term cooling tower effectiveness?

CT effectiveness = Range X 100 / (Range + Approach)

30-How do you measure the Cooling tower capacity?

It is measured in terms of heat rejected.

Heat rejected = Mass of circulating water X Specific heat of water Cp X Range

40-What do you mean by evaporation loss in cooling towers?

It is the evaporation of circulating water during cooling duty

Evaporation loss = 0.00085 X 1.8 X Water circulation rate X Range            

1.8 is taken as 1.8 m3 of water is rejected on every 10,00000 kcal heat rejected

Evaporation Rate is the fraction of the circulating water that is evaporated in the cooling process.

A typical design evaporation rate is about 1% for every 12.5°C range at typical design conditions.

It will vary with the season, in colder weather there is more sensible heat transfer from the water to the air, and therefore less evaporation.

The evaporation rate has a direct impact on the cooling tower makeup water requirements.

41-What do you mean by cycles of concentrations (COC) in CT

It is the ratio of dissolved solids in circulating cooling water to the dissolved solids in makeup water

It is given as

COC = Conductivity in circulation water / Conductivity in makeup water

OR

COC = Chloride in circulation water / Chloride in makeup water

42-How do you calculate the blow down loss in CT?

Blow down loss = Evaporation loss / (COC-1)

43-Heat rejection of a counter flow induced draft cooling tower is 57000000 kcal/hr & circulation rate of cooling water is 5500 m3/hr. Calculate the cooling water temperature differences (range)

We have Range = Heat load (heat rejection) / Circulation rate X Specific heat Cp

Range = 57000000 / (5500 X 1000 X 1)

Range = 10.36

44-Calculate the approach of cooling tower having effectiveness 75% & cooling water temperature difference 7 deg C

We have,

Effectiveness = Range / (Range + Approach)

0.75 = 7 / (7+Approach)

Approach = 2.33

45-Inlet & outlet temperatures of circulating cooling water of a induced draft cooling towers are 38 deg C & 31 deg C respectively. The hygrometer shows 27 deg C wet bulb temperature, calculate the cooling tower efficiency.

We have,

CT effectiveness = Range X 100 / (Range + Approach)

Range = 38-31 = 7 deg C

Approach = 31-27 = 4 deg C

CT effectiveness = 7 X 100 / (7+4) = 63.63%

46-A cooling tower of circulation water flow 2500 M3/hr & temperature difference (range) 6 deg Calculate the heat load of a cooling tower

Heat rejected = Mass of circulating water X Specific heat of water Cp X Range

Heat load of cooling tower = 2500 X 1000 X 1 X 6 =15000000 kcal

47-A cooling tower having circulation water flow 3475 m3/hr & inlet and outlet circulating water temperatures are 36 & 29 deg C respectively. Calculate the evaporation loss in %

We have,

Evaporation loss = 0.00085 X 1.8 X Water circulation rate X Range              

Range = 36-29 = 7 deg C

Evaporation loss = 0.00085 X 1.8 X 3475 X 7 = 37.21 M3/hr

% of evaporation loss = 37.21 X 100 / 3475 =1.07%

48-Calculate the blow blown loss of cooling tower, if its evaporation loss & COC are 0.8% & 5 respectively.

Blow down loss = Evaporation loss X 100 / (COC-1)

Blow down loss = 0.8 / (5-1) =0.2%

49-Calculate the COC of a cooling tower if Chloride & conductivity of circulating water are 147 ppm & 550 micS/cm and that of makeup water are 33 ppm & 90 micS/cm respectively

We have,

COC = Chloride in circulation water / Chloride in makeup water = 147 / 33 =4.45

COC = Conductivity in circulation water / Conductivity in makeup water = 550/90 =6.11

49- A cooling tower having circulation water flow 6400 m3/hr & inlet and outlet circulating water temperatures are 39 & 31 deg C respectively. Calculate the evaporation loss & blow down loss in %. Consider chloride level in circulating water & make up water are 155 ppm & 35ppm respectively.

We have

Evaporation loss = 0.00085 X 1.8 X Water circulation rate X Range              

Range = 39-31 = 8 deg C

Evaporation loss = 0.00085 X 1.8 X 6400 X 8 = 78.34 M3/hr

% of evaporation loss = 78.34 X 100 / 6400 =1.22%

Blow down loss = Evaporation loss X 100 / (COC-1)

COC = Chloride in circulation water / Chloride in makeup water

 COC = 155 / 35 = 4.4

Blow down loss = 1.22 / (4.4-1) =0.36%

48-A induced draft CT having cooling water circulation flow 7200 M3/hr, Calculate the quantity of makeup water required  in a day. Assume evaporation, blow down & drift losses 0.9%, 0.2% & 0.003% respectively.

We have,

Evaporation loss = 7200 X 0.9/100 =64.8 M3/hr

Blow sown loss = 7200 X 0.2/100 =14.4 M3/hr

Drift loss = 7200 X 0.003/100 =0.00216 M3/hr

Total make up water quantity = 64.8 + 14.4 + 0.00216 =79.2 M3/hr

49-A Mechanical cooling tower operating at 5 COC is used to cool 8500 M3 of water required for the power plant auxiliaries from 37 deg C to 28 deg C at 24 deg C WBT.Calculate Range, approach, evaporation loss, blow down loss and make up water requirement.

We have,

1-Range = 37-28 = 9 deg C

2-Approach = 28-24 = 4 deg C

3-Evaporation water loss = 0.00085 X 1.8 X 8500 X 9 =117.045 M3/hr

4-Blow down water loss =117.045 / (5-1) =29.26 M3

5-Make up water = 117.045 + 29.26 = 146.30 M3/hr

50-What do you mean by CT hold up of volume?

It is the operating water volume of CT including all cells & fore bays.

51-How do you decide the cooling tower hold up capacity?

Generally it is 25% of the circulating flow for safe operation.

That is if cooling tower required circulation water flow is 5000 M3/hr, then its hold up volume will be

5000 X 25 / 100 = 1250 M3

52-What do you mean by liquid & gas ratio (L/G) in cooling towers?

It is the mass ratio of water (Liquid) flowing through the tower to the air (Gas) flow. Each tower will have a design water/air ratio.

An increase in this ratio will result in an increase of the approach, that is, warmer water will be leaving the tower.

L/G = (h2-h1)/(T2-T1)

L/G = liquid to gas mass flow ratio (kg/kg)

 T2 = hot water temperature (°C) 

T1 = cold-water temperature (°C) 

h2 = enthalpy of air-water vapor mixture at exhaust wet 

 h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature.

53-What are the types of heat transfer that occur in cooling towers?

Heat transfer in cooling towers occurs by two major mechanisms:

  1. Sensible heat from water to air (convection)
  2. Latent heat by the evaporation of water (diffusion).

54-What are the various chemicals used in cooling water treatment?

SL No.

Chemicals

Use

1

Scale inhibitors

To prevent scaling

2

Corrosion inhibitor

To prevent corrosion

3

Bio dispersant

Bio-Dispersion

4

Sulphuric acid

To maintain pH level of water

5

Chlorine granules

To kill the bacteria

6

Oxidising biocides

Controls bio fouling in heat exchangers like Condensers, oil coolers)

7

Non- Oxidising biocides

To kill microorganisms

 

55-What are the various tests conducted for cooling water in CT?

  • pH
  • Conductivity
  • Hardness
  • Chloride as cl
  • Phosphate
  • Silica
  • Iron as Fe
  • Turbidity
  • COC
  • TBC (Total bacteria count) test
  • SRB (Sulphur reducing bacteria) test
  • Corrosion coupon test

 

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