Cooling Towers: Design and Operation Considerations

Cooling towers are a very important part of many chemical plants. They represent a relatively inexpensive and dependable means of removing low grade heat from cooling water.

Figure 1: Closed Loop Cooling Tower System

The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling.

Types of Cooling Towers

Cooling towers fall into two main sub-divisions: natural draft and mechanical draft. Natural

draft designs use very large concrete chimneys to introduce air

through the media. Due to the tremendous size of these towers (500

ft high and 400 ft in diameter at the base) they are gen

erally used for water f

lowrates above 200,000 gal/min. Usually these types of towers are only used by utility power stations in the United States. Mechanical draft cooling towers a

re much more widely used. These towers utilize large fans to for

ce air through circulated water. The water falls downward over fill surfaces which help increas

e the contact time between the water and the air. This helps maximize heat transfer between the two.

Types of Mechanical Draft Towers

Figure 2: Mechanical Draft Counterflow Tower Figure 3: Mechanical Draft Crossflow Tower

Mechanical draft towers offer control of cooling rates in t

heir fan dia

meter and speed of operation. These towers often contain several areas (eac

h

with their own fan) called cells.

Cooling Tower Theory

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

Figure 4: Water Drop with Interfacial Film

where:

KaV/L = tower characteristic

K = mass transfer coefficient (lb water/h ft²)

a = contact area/tower volume

V = active cooling volume/plan area

L = water rate (lb/h ft²)

T₁ = hot water temperature (⁰F or ⁰C)

T₂ = cold water temperature (⁰F or ⁰C)

T = bulk water temperature (⁰F or ⁰C)

(J/kg dry air or Btu/lb dry air)

h_a = enthalpy of air-water vapor mixture at wet bulb temperature

(J/kg dry air or Btu/lb dry air)

Thermodynamics also dictate that the heat removed from the water must be equal to the heat absorbed by the su

rrounding air:

The tower characteristic value can be calculated by solving Equation 1 with the Chebyshev numberical method:

Figure 5: Graphical Representation of Tower Characteristic

The following represents a key to Figure 5:

C' = Entering air enthalpy at wet-bulb temperature, T_wbBC = Initial enthalpy driving force

CD = Air operating line with slope L/G

DEF = Projecting the exiting air point onto the water operating line and then onto the

temperature axis shows the outlet air web-bulb temperature

As shown by Equation 1, by finding the area between ABCD in Figure 5, one can find the tower characteristic. An increase in heat load would have the following effects on the diagram in Figure 5:

1. Increase in the length of line CD, and a CD line shift to the right

2. Increases in hot and cold water temperatures

3. Increases in range and approach areas

The increased heat load causes the hot water temperature to increase considerably faster than does the cold water temperature. Although the area AB

CD should remain constant, it actually decreases about 2% for every 10 ⁰F increase in hot water temperature above 100 ⁰F. To account for this de

crease, an "adjusted hot water temperature" is usd in cooling tower de

sign.

Figure 6: Graph of Adjusted Hot Water Temperatures