The document discusses the history and scientific development of cooling tower design theory. It begins by explaining how Merkel developed the first scientific theory for evaluating cooling tower performance in 1925. It then provides definitions of key cooling tower concepts like approach, range, and heat transfer methods. The document goes on to describe parameters like tower characteristics, fan power requirements, and water loss factors. It also summarizes Merkel's assumptions and the development of generalized supply equations from manufacturer curves.

1. By- alok Kumar swain
Mechanical engineer
BrmIIT
Bbsr

2. Artistic to Scientific Design of Cooling Towers
• The art of evaporative cooling is quite ancient, although it
is only relatively recently that it has been studied
scientifically.
• Merkel developed the theory for the thermal evaluation
of cooling towers in 1925.
• This work was largely neglected until 1941 when the
paper was translated into English.
• Since then, the model has been widely applied.
• The Merkel theory relies on several critical assumptions to
reduce the solution to a simple hand calculation.
• Because of these assumptions, the Merkel method does
not accurately represent the physics of heat and mass
transfer process in the cooling tower fill.

3. What Does A Cooling Tower Do?
• Cooling Towers are used
to transfer heat from
cooling water to the
atmosphere.
– Promotes efficient water
usage
– Prevents environmental
damage

4. What Does A Cooling Tower Do?
• Cooling Towers are used
to transfer heat from
cooling water to the
atmosphere.
– Promotes efficient water
usage
– Prevents environmental
damage

5.  Cooling water is continuously circulated
through heat exchangers to absorb heat from
process material and machinery.
 Because it's cost efficient to reuse water and
plants can't dump excessive amounts of hot water
into rivers and lakes, cooling towers are used to
remove the heat from the water, so it can be
recirculated.
 Used in power stations, oil refineries,
petrochemical plants and natural gas plants.

6. Heat Transfer
Methods
• Wet Cooling Tower
– Uses evaporation to transfer heat
– Water can be cooled to a temperature lower than
the ambient air “dry-bulb” temperature
• Dry Cooling Tower
– Uses convection to transfer heat
– Heat is transferred through a surface that separates
the water from ambient air, such as in a heat
exchanger.

7. Air Flow Generation Methods
• Natural Draft
– Warm air naturally rises due to
the density differential to the
dry, cooler outside air. This
moist air buoyancy produces
an airflow through the tower.
• Mechanical Draft
– A fan induces airflow through
a tower.

8. NATURAL DRAFT

9. MECHANICAL DRAFT

11. Parameters of Cooling Towers
• A number of parameters describe the performance of a
cooling tower.
• Range is the temperature difference between the hot
water entering the cooling tower and the cold water
leaving.
• The range is virtually identical with the condenser rise.
• Note that the range is not determined by performance of
the tower, but is determined by the heat loading.

12. • Approach is the difference between the temperature of the
water leaving the tower and the wet bulb temperature of the
entering air.
• The approach is affected by the cooling tower capability.
• For a given heat loading, water flow rate, and entering air
conditions, a larger tower will produce a smaller approach;
i.e., the water leaving the tower will be colder.
• Water/Air Ratio (mw/ma) 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.
• A test ratio is calculated when the cooling tower performance
is evaluated.

13. Cooling Tower Mass Balances

14. Water and Salt Balances
A water balance around the entire system is:
M = E + D + W
Since the evaporated water (E) has no salts, a chloride balance
around the system is:
M (XM) = D (XC) + W (XC) = XC (D + W)
and, therefore:
XC/XM = Cycles of concentration
= M/(D + W)
= M/(M – E) = 1 + [E ÷ (D + W)]

15. Heat Balance
From a simplified heat balance around the cooling tower:
E HV = C · ΔT · cp
Hv = Latent heat of water evaporation
= 2260 kJ/kg
Δ T = Temperature difference top to bottom
Cp = Specific heat of water = 4.184 kJkg-1C-1

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Thermodynamics of Air Water Systems
Humidity Ratio:

17. Local Cooling Tower Theory
Heat is transferred from water drops to the surrounding air by the transfer of sensible and latent
heat

18. Mechanism of Heat Transfer in Cooling Towers
• Heat transfer in cooling towers occurs by two major
mechanisms:
• Sensible heat from water to air (convection) and
• transfer of latent heat by the evaporation of water
(diffusion).
• Both of these mechanisms operate at air-water boundary
layer.
• The total heat transfer is the sum of these two boundary
layer mechanisms.
• The total heat transfer can also be expressed in terms of the
change in enthalpy of each bulk phase.
• A fundamental equation o f heat transfer in cooling towers
(the Merkel equation) is obtained.
  airairasaWWCW dhmdVhhKAdTCm  

19. The Merkel Method
• The Merkel method, developed in the 1920s, relies on
several critical assumptions to reduce the solution to a
simple manual iteration.
• These assumptions are:
• The resistance for heat transfer in the water film is
negligible,
• The effect of water loss by evaporation on energy balance
or air process state is neglected,
• The specific heat of air-stream mixture at constant
pressure is same as that of the dry air, and
• The ratio of hconv/hdiff (Lewis factor) for humid air is unity.
• Merkel combined equations for heat and water vapor
transfer into a single equation similar as

20. where:
kAV/mw = tower characteristic
k= mass transfer coefficient
A = contact area/tower volume
V = active cooling volume/plan area
mw = water flow rate
T1 = hot water temperature
T2 = cold water temperature
T = bulk water temperature
hsa = enthalpy of saturated air-water vapor mixture at bulk
water temperature
(J/kg dry air)
ha = enthalpy of air-water vapor mixture (J/kg dry air )
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21. Tower Characteristics
• Tower Characteristic (MeM or NTU) is a characteristic of
the tower that relates tower design and operating
characteristics to the amount of heat that can be
transferred.
• For a given set of operating conditions, the design
constants that depend on the tower fill.
• For a tower that is to be evaluated using the characteristic
curve method, the manufacturer will provide a tower
characteristic curve.
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22. SUPPLY TOWER CHARACTERISTIC
• The supply tower characteristic of the cooling tower can
be evaluated with the help of cooling tower fill
characteristics curves provided by manufacturer which
takes into account the effect of rain and spray zones as
well as fill fouling.
• These curves are certified by the cooling tower institute.

23. Generalized Equation for Cooling Tower Supply
• A generalized equation for cooling tower supply can be
developed from the manufacturer curves (known as the
supply equation) and is of the form:
m
a
wn
air
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KAV
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24. BHP OF THE FAN
• The total pressure drop (PD) across the cooling
tower which is the summation of the pressure
drops across the drift eliminators, inlet louvers
and the fill packing (constituting the static
pressure drop) and also the velocity pressure
drop is calculated.
• Now, the total fan power required is calculated as
BHP = (CFM * PD)/ (n * 6356)
where n is the efficiency of the fan.

25. Loss of Water
• 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, since 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.

26. • Drift is water that is carried away from the tower in the form
of droplets with the air discharged from the tower.
• Most towers are equipped with drift eliminators to minimize
the amount of drift to a small fraction of a percent of the
water circulation rate.
• Drift has a direct impact on the cooling tower makeup water
requirements.
• Recirculation is warm, moist air discharged from the tower
that mixes with the incoming air and re-enters the tower.
• This increases the wet bulb temperature of the entering air
and reduces the cooling capability of the tower.
• During cold weather operation, recirculation may also lead to
icing of the air intake areas.