COOLING TOWER 

Reference:

Basic Heat and Mass Transfer by A.F.Mils

Psychrometric chart

Psychrometric ChartNeed two quantities for a state point

Can get all other quantities from a state point

Dry-bulb temperature

Wet-bulb temperature

Dew-point temperature

Temperature

Cooling Towers At Nuclear Plants

What Does A Cooling Tower Do?

Cooling Towers are used to transfer heat from Hot water to the atmosphere.– Promotes efficient water

usage– Prevents environmental

damage

Mass flow:

Energy flow:

The mass flow rate of the makeup water is given by the difference in specific humidity ω at the inlet and outlet air streams multiplied by the mass flow rate of the dry air.

Example 1: Compute air folwrate and Water Make up

Driving force diagram (operating diagram) for a cooling tower.

J.R. Picardo, J.E. Variyar

The Merkel equation revisited: A novel method to compute the packed height of a cooling tower

Energy Conversion and Management, Volume 57, 2012, 167–172

http://dx.doi.org/10.1016/j.enconman.2011.12.016

Merekel equation: A Method to compute the packed height of a cooling tower

To determine the number of transfer unit (NTU):

𝑁𝑇𝑈=𝑀𝑒=∫𝑇 𝑤𝑜

𝑇 𝑤𝑖 𝑐𝑝𝑤𝑑𝑇𝑤

h𝑠−h𝑎

𝐻𝑇𝑈=𝐺𝑤

h𝑑𝑎𝑓𝑖

𝑍=(𝐻𝑇𝑈 )×(𝑁𝑇𝑈 )

To calculate height of transfer unit (HTU):

To calculate height of packing:

Example 1: A performance test on a counterflow cooling tower fill

materiall(water flows downward and air flows upwaqrd) shows that

for an inlet water temperature

the outlet water temperature

The enthalpies of saturated air corresponding to these temperature are

The air enthalpy

Air inlet enthalpy

Air outlet enthalpy

Use of Merkel’s Method

The specific heat of water at mean temperature: =Is

Determine the Merkel number for this fill

Solution:

=0.6676

Exaple 2: A wet cooling tower is required to cool water from 40 to 26 when the inlet air is at 10, 1 atm , and saturated. Calculate the required number of transfer units for balanced flow, that is, .

𝑁𝑡𝑢= ∫h 𝐿,𝑜𝑢𝑡

h 𝐿, 𝑖𝑛 𝑑h𝐿h𝑠−h𝐺

Using Table,

also obtained from Table

=+()

kJ/kg kJ/kg kJ/kg kJ/kg

26 109.07 29.15 79.12 49.97 0.02001`

28 117.43 37.51 87.86 50.35 0.01986

30 125.79 45.87 97.36 51.49 0.01942

32 134.15 54.23 107.67 53.44 0.01871

34 142.50 62.58 118.89 56.31 0.01776

36 150.86 70.94 131.10 60.16 0.01662

38 159.22 79.30 144.39 65.09 0.01536

40 167.58 87.66 158.86 71.20 0.01404

using table

Example 3:A counter current tower should cool water flowing at 3.999 kg/s. The water enter at 39.67 and should be cooled to 25.Air will be fed to the tower 4.134 kg/s with a dry bulb temperature of 9.7 and wet bulb temperature of 8.23. [HTU]=2.747. Find the height of the packing