Cooling Towers Design

Topics: Temperature, HVAC, Atmospheric thermodynamics Pages: 7 (1546 words) Published: May 6, 2012
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. 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

|[pic] |
|Figure 1: Closed Loop Cooling Tower System |

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 generally used for water flowrates 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 are much more widely used. These towers utilize large fans to force air through circulated water. The water falls downward over fill surfaces which help increase the contact time between the water and the air. This helps maximize heat transfer between the two.

Mechanical Draft Towers

Mechanical draft towers offer control of cooling rates in their fan diameter and speed of operation. These towers often contain several areas (each with their own fan) called cells.

|[pic] |
|Figure 2: Mechanical |
|Draft Counterflow |
|Tower |

|[pic] |
|Figure 3: Mechanical |
|Draft Crossflow Tower|

Cooling Tower Theory

Heat is transferred from water drops to the surrounding air by the transfer of sensible and latent heat. |[pic] | |Figure 4: Water Drop with Interfacial Film |

This movement of heat can be modeled with a relation known as the Merkel Equation: |[pic] | |

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 (°F or °C)
T2 = cold water temperature (°F or °C)
T = bulk water temperature (°F or °C)
hw = enthalpy of air-water vapor mixture at bulk water temperature (J/kg dry air or Btu/lb dry air) ha = 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 surrounding air: |[pic] | |

|[pic] | |

L/G = liquid to gas mass flow ratio (lb/lb or kg/kg)
T1 = hot water temperature (0F or 0C)
T2 = cold water temperature (0F or 0C)
h2 = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature (same units as above) h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature (same units as above)

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

|[pic] | |

Δh1 = value of hw-ha at T2+0.1(T1-T2)
Δh2 = value of hw-ha at T2+0.4(T1-T2)
Δh3 = value of hw-ha at T1-0.4(T1-T2)
Δh4 = value of hw-ha at T1-0.1(T1-T2)
Figure 5 below shows a graphical representation of how heat is tranferred between the warm water and cooler surrounding air: |[pic] | |Figure 5: Graphical Representation of the Tower Characteristic |

The following represents a key to Figure 5:
C' = Entering air enthalpy at wet-bulb...

References: 1. The Standard Handbook of Plant Engineering, 2nd Edition, Rosaler, Robert C., McGraw-Hill, New York, 1995
2. Perry 's Chemical Engineers ' Handbook, 6th Edition, Green, Don W. et al, McGraw-Hill, New York, 1984
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