Cooling
tower basics calculation formulas | Cooling Tower Efficiency
In this article explained about basic
concepts of cooling tower, types of cooling towers, formula for cooling tower
efficiency. Also brief about Cooling tower mass balance of make-up water
requirement in system, Drift Losses or Windage, Evaporation losses &
Blowdown or Draw off.
Types of cooling tower | Cooling Tower Basic Calculations
Cooling towers are a very important part of
many process plants and power plants. The make-up water source is used to
refill water lost to evaporation. Hot water from heat
exchangers/condenser is sent to the cooling tower. After reducing the
temperature of water in cooling tower and is sent back to the condenser /exchangers
or to other units for further process.
Drift: From a cooling tower water lost due to liquid
droplets entrained in the exhaust air. It is independent of water lost by
evaporation.
Heat exchange/ Condenser – It
is a device for transferring heat from one substance to another.
Concentration : The
process of increasing solids per unit volume of solution also the amount of
material dissolved in a unit volume of solution.
Usually concentration of liquid in cooling
tower occurs due to evaporation that cools the water. It is normally expressed
directly as (parts per million) ppm or indirectly as mhos conductivity.
Air Blows: By the opening in cooling tower through which air
enters a tower.
Blowdown: Water
discharged from the system of cooling tower to control concentration of salts
or other impurities in the circulating water
Evaporation loss: In the
process of cooling tower system, water evaporated from the circulating
water into the atmosphere.
Makeup : Water
added to the circulating water in cooling tower system to replace water lost by
evaporation, blowdown, drift and leakage
Drift eliminators: An
assembly constructed of plastic, cement board, wood, or other material that
minimize the entrained water moisture from the discharged air.
Cooling water : Water
circulated through a cooling system to remove heat from certain areas.
Exhaust air : The
mixture of air and its associated vapor leaving the cooling tower system
Louvers : Members
installed horizontally in a cooling tower wall to provide openings through
which the air enters into the system while also containing the falling water
within the tower. Usually Louvers are installed at an angle to the direction of
air flow to the cooling tower.
Fundamental Concept of
Cooling Tower :
Heat is transferred from water drops to the surrounding atmospheric air by the
transfer of latent heat and sensible heat.
Types of Cooling Towers
Cooling towers mainly classified into two
sub-divisions
1. Mechanical draft cooling tower
2. Natural draft cooling tower
Mechanical draft water-cooling tower:
Mechanical draft cooling towers are more
commonly used for cooling of water. These mechanical draft cooling towers
utilize large fans to force air through circulated water. The water
falls downward over surface of fills, which help increase the contact time
between the air and water. This helps maximize heat transfer between
the air and water.
Mechanical draft water-cooling tower consists
mainly two types
a) Forced draft – In this cooling tower fans
located at the air inlet
b) Induced draft – In this cooling tower fans
located at the air exhaust.
Forced draught cooling tower
These take the form of a large rectangular
concrete structure. The water is pumped to the top and broken up into sprays.
It falls as rain on to successive stages of boards in the form of louvres of
wood, polystyrene or metal, with notches and ribs to break up the flow. The
water flows from one stage to the next and finally arrives at the pond which
forms the base of the structure, from which it is taken by the cold water pump.
In a forced draught cooling tower, as shown
in figure. The circulation of air in forced draft cooling tower is produced by
means of fans
placed at the base of the tower
Induced draught cooling tower
In an induced draught cooling tower, as shown in below figure. The circulation of air is
provided by means of fans placed at the top of the tower.
They are generally induced draft cooling
tower classified as counter
current or cross
flow tower, descriptive of the direction of air flows relative
to that of the water.
The counter current tower is expected to be
more efficient, but the cross flow tower can be operated with lower power
requirement or higher vapour velocities.
Induced Draft Cooling Tower – Counter Current Flow type
Induced Draft Cooling Tower – Cross Flow type
Natural draft water-cooling tower
The cooling tower, generally packed with
brushwood or wooden laths. This is a very large tower in which the hot water
encounters a natural draught which promotes cooling of the water by convection
and evaporation.
In a natural draught cooling tower, as shown
in figure the circulation of air is produced by the pressure difference of air
inside and outside the cooling tower
Design Considerations of cooling tower
The efficiency of this device depends
essentially on climatic conditions, in particular the wet-bulb temperature and
the relative humidity of the ambient air.
The required cooling tower design and size
will be a function of following values
a) Cooling range
b) Web bulb temperature
c) Approach to wet bulb temperature
d) Mass flowrate of water (Circulation rate)
e) Air velocity through tower or individual
tower cell
f) Tower height
The performance of cooling towers is
estimated by approach and range. During the performance assessment, portable
monitoring instruments are used to measure the following parameters:
a) Dry bulb temperature of air
b)
Wet bulb temperature of air
c) Cooling tower inlet water temperature
d) Cooling tower outlet water temperature
e) Electrical readings of pump and fan motors
f) Exhaust air temperature
g) Water flow rate
h) Air flow rate
Operation Considerations of cooling tower (Cooling tower mass
balance)
Cooling tower mass balance gives an
indication about make-up water requirement in system. Cooling Tower Makeup is
required due to water losses resulting from Drift, Evaporation & Blowdown.
Water make-up (M ) = Total water losses = Drift Losses ( D) +
Evaporation Losses (E ) + Blow down Losses (B)
M = D + E + B
M = Make up water Requirement in m3/hr
D = Drift Loss
in m3/hr
E = Evaporation Loss in m3/hr
B = Blow Down in m3/hr
Drift Losses (D) or Windage
Drift losses may be assumed to be:
D = 0.3
to 1.0 percent of Circulating water (C ) for a natural
draft cooling tower
D = 0.1 to 0.3 percent
of Circulating water (C ) for an induced draft cooling tower
D = about 0.01 percent
or less of Circulating water (C ) if the cooling tower has windage drift
eliminators
Evaporation Losses (E )
It is calculated on the basis of heat balance
around the cooling tower
where:
C = Circulating water in m3/hr
Ti – To = water temperature difference from tower top to tower bottom in
°C ( cooling tower inlet hot water and outlet cold water temperature
difference)
Cp = specific heat of water = 1 kcal/kg / °C (or) 4.184 kJ / kg / °C
Alternatively to find the evaporation loss by
the following formula (Reference:
Perry’s Chemical Engineers Hand Book)
Evaporation loss in m3/hr = 0.00085 x 1.8 x circulation rate in m3/hr x (Ti-To) in 0C
( Hint: Theoretically the evaporation quantity for every 1,000,000 kCal
heat rejected to evaporate 1.8 m3 of
water)
Blowdown or Draw off
The process of cooling water circulates the
cooling tower part of water evaporates thereby increasing solids per unit
volume of solution also the amount of material dissolved in a unit volume of
solution.
To control the Cycle of Concentration blow
down or Draw off is given. Blowdown can be calculated from the formula
B = E/ (COC-1)
B = Blow Down (m3/hr)
E = Evaporation Loss (m3/hr)
COC = Cycle of Concentration.
Cycle of Concentration (COC)
The cycle of concentration is a dimensionless
number. It is a ratio between parameter in Cooling Water to the parameter in
Makeup water. It can be calculated from any the following formulae.
Cycles of concentration (COC) = Silica in
Cooling Water / Silica in Makeup Water
Cycles of concentration (COC) = Ca
Hardness in Cooling Water/ Ca Hardness in Makeup water
Cycles of concentration (COC) =
Conductivity of Cooling Water / Conductivity of Makeup water
Cycles of concentration (COC) = Concentration
of chlorides in circulating water /Concentration of chlorides in make-up water
The cycle
of concentration (COC) normally varies from 3.0 to 7.0 depending
on the Process Design and Manufactures Guidelines. In some large power plants,
concentration cycles of the cooling tower may be much higher.
Cooling Tower Efficiency Calculations
The efficiency of this cooling tower system
depends on climatic conditions, in particular the wet-bulb temperature and the
relative humidity of the ambient air.
The required cooling tower design and size
will be a function of following values
a) Cooling range
b) Web bulb temperature
c) Approach to wet bulb temperature
d) Mass flowrate of water (Circulation rate)
e) Air velocity through tower or individual
tower cell
g) Tower height
In this calculation Cooling Tower Approach &
Cooling Tower Range are important factors
Cooling Tower Approach
Approach: The difference between the Cooling Tower Outlet water
(Cold Water Temperature) and ambient Wet Bulb Temperature is called as Approach
of Cooling Tower.
Approach of cooling tower = Cooling tower outlet water –
Wet bulb temperature
Cooling Tower Range
Range: The difference between the cooling tower inlet temperature
(Hot Water Temperature) and cooling tower outlet temperature (Cold water
temperature) is called Range of Cooling Tower.
Range of cooling tower = Cooling tower inlet temperature
– Cooling tower outlet temperature
Cooling Tower Efficiency
Formula:
Cooling Tower
Efficiency
Or Simply
Cooling Tower
Efficiency
In practice, the cooling tower efficiency
will be in between 65 to 70%. In summer season the ambient air wet bulb
temperature raises when compared to winter season so cooling tower efficiency
limiting in summer season.
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