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 are 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?
- Sump
- Fore bay (cold water basin)
- Louvers
- Fans
- Drift eliminators
- Fills (Splash type & film type)
- Water distribution pipe lines
- Water nozzles
- Hot water basin
- Cooling water inlet & out lines
- Sump over flow & drain lines
- 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 lowers 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:
- Natural draft Cooling towers have more drift losses around 0.3 to 1%
- Cooling towers without drift eliminators have drift losses around 0.1 to 0.3%
- 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:
- Sensible heat from water to air (convection)
- 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