By- alok Kumar swain Mechanical engineer

BrmIIT Bbsr

ASCENTRIC COOLING TOWER

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.

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

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

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.

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.

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.

NATURAL DRAFT

MECHANICAL DRAFT

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.

• 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.

Cooling Tower Mass Balances

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)]

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

a

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AirDry of Flow Mass

Vapour Water of Flow Mass

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TRmTRmVp

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622.0

Thermodynamics of Air Water Systems

Humidity Ratio:

Local Cooling Tower Theory

Heat is transferred from water drops to the surrounding air by the transfer of sensible and latent heat

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

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

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 )

1

2

T

T asawM hh

dTmkAVMe

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.

n

a

w

mmCNTU

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.

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

wnair m

muCLKAV

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.

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 12.5C 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.

• 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.