Key Points In The Design Of Cooling Tower And Step By Step Design Procedure Of Cooling Tower | Mass Transfer - Chemical Engineering PDF Download

Key points in the design of cooling tower: 
(I) An increase or decrease in wet-bulb temperature of entering water (mainly due to atmospheric condition) cannot change tower characteristic

(II) An increase in ‘cooling range’ can not change tower characteristic  It increases ‘approach’ only.
(III) A change in L/G can change tower characteristic
Fill height (FH) depends on tower characteristic, L/G and correlated by the following equation:

where, C and n are constants and solely dependent on tower fill.

Step-by-step design procedure of cooling tower 
1. Specify the inlet and outlet temperatures and flow rate of warm water.
2. Select the design value of dry-bulb and wet-bulb temperatures of air (at the proposed geographical location).
3. Draw the ‘equilibrium line curve’ i.e., saturation humidity curve [H^/ vs T]. The enthalpy data are calculated using vapor pressure equation for water and physical properties of air and water vapor
4. Locate the lower terminal of the operating line, ‘B’ on T_L-H plane by the point (T_L1, H^/1 ). This point indicates the condition at the bottom of the tower.
5. Draw a tangent to the equilibrium line through the point ‘B’. The slope of the tangent gives the ratio of the liquid and minimum gas flow rate. Hence, minimum air rate is calculated. Actual air rate taken is usually 1.25 to 1.5 times the minimum [not required if air rate is given].
6. The upper terminal of the operating line is located by the point ‘A’ (T_L2, H^/2 ). It is the point where the operating line of the slope determined in step 5 meets the vertical line through T_L2. It can also be located by calculating the top end enthalpy H^/2 from Equation (6.18) as
7. Evaluate the integral in Equation (6.27)  number of gasphase enthalpy transfer units and calculate height gas-phase enthalpy transfer units, H_tG as   are required. A set of parallel lines (tie lines) of slope  is drawn between the operating line and equilibrium line. H^/ and  are taken from terminals. Integral is calculated numerically or graphically.
8. If the overall enthalpy transfer coefficient K^/Y is known and used, ‘tie lines’ are vertical. For a given value of H^/ , value of H^*/ is given by the point on the equilibrium line vertically above it. The integral of Equation   gives the number of overall transfer units.
9. The height of a transfer unit     is calculated. The packed height is the product of height of transfer unit and number of transfer units.

Approach: It is the difference between cooling water temperature leaving cooling tower and wet-bulb temperature of inlet air which is approach to wet bulb temperature (^ºF), (T_L1-T_as). For getting small approach, cooling tower height must be increased. To achieve zero (0) approach theoretically, infinite packing height is needed.

Range: ‘Cooling range’ or purely ‘range’ is the difference in the inlet hot water and outlet cooled water temperature (^ºF) (T_L2-T_L1).