The Design of Cooling Towers_narrated_a.pptx

Cooling Tower Design

1. INGM 427 Termovloeierstelsel Ontwerp Thermal-fluid System Design Hulpstelsels Auxiliary Systems Prof. M du Toit Adapted from Mr Marinus Potgieter 2019

2. Piping and Auxiliary System Design Standard pipe diameters and classification Pressure vessel and pressurised system considerations Valve selection Pump selection Cooling tower selection INGM 427

3. Cooling Tower A cooling tower is a type of heat exchanger, namely a direct contact heat exchanger. This means the two process fluids are in direct contact and not separated by some solid material. Cooling towers can be either: Forced draught (a mechanical fan induces flow) Natural draught (natural flow is induced by density changes in air and from tower geometry)

4. Cooling Tower Hyperbolical shape

5. Direct contact cooling towers are commonly referred to as “wet” cooling towers. Due to the latent heat of water ( ) being so large, a wet cooling tower is able to cool water to below the (dry bulb) ambient air temperature.   Heat Exchanger Makeup water Saturated air Cooling Tower

6. Cooling Tower https://www.researchgate.net/publication/327830964_A_Review_Study_On_Cooling_Towers_Types_Performance_and_Application

7. Thermodynamics of cooling towers As with any heat exchanger there are two fluid streams, one removes heat from the other Warm water is fed to the cooling tower, this is the “hot fluid inlet” Atmospheric air flows into the tower, this is the “cold fluid inlet” The water is sprayed (on a packing medium) and as it makes contact with the moving air some of this water evaporates in the process Wet saturated air leaves the cooling tower, this is the “cold fluid outlet” Cooled water is removed by an extraction pump, the “hot fluid outlet” The water which was evaporated in the process is refilled as makeup water from a source https://www.youtube.com/watch?v=2htplQVEU7g https://www.youtube.com/watch?v=O336i2HMdQM

8. Thermodynamics of cooling towers Air-vapor mix Water Makeup water Hot fluid in Hot fluid out Cold fluid in Cold fluid out Water evaporates Cooling tower Q The water releases latent heat of vaporization ( ) which cools the water by turning a certain amount of water to steam which is absorbed by the air.  

9. Mass conservation The mass flow of dry air is constant The mass flow of the water is constant when it is assumed the makeup water fully replenishes any evaporated water. Energy conservation To calculate unknown inlet / outlet conditions rely on enthalpies. For the water side the specific heat c p value remains fairly constant and we therefore may use temperatures difference with a constant c p Momentum conservation The pressure drop for an air stream through a cooling tower may be modelled simply as a secondary loss using a K -factor Thermodynamics of cooling towers

10. When can I use Cp∆T in stead of ∆h Enthalpy Definition of Cp If is a weak function of Or the change in is small then: If is a weak function of in the specific temperature range  

11. When can I use Cp∆T in stead of ∆h For water in the subcooled regime is a weak function of stays constant as the pressure changes  

12. When can I use Cp∆T in stead of ∆h Cp is a weak function of T at atmospheric conditions (0-50°C)

13. Thermodynamics of cooling towers Mass conservation Water: Where:   Air Water Makeup water Water evaporates

14. makeup water evaporated water

15. Mass conservation Dry air: Vapour in air: Remember Specific humidity   Thermodynamics of cooling towers Air Water Makeup water Water evaporates

16. Vapour in air: Divide by   Thermodynamics of cooling towers

17. Thermodynamics of cooling towers Energy conservation     Air Water Makeup water Water evaporates

18. Thermodynamics of cooling towers  

19. Thermodynamics of cooling towers What about Q? Adiabatic saturation process… A simple definition of a cooling tower is where heat is moved from the hot water to the air Assume heat released from water is absorbed by the air Equation [1] can also be written as: Heat released from water: (In=Out) +   Air Water Makeup water Water evaporates

20. Thermodynamics of cooling towers Heat absorbed by air: Or Air Vapour Air Water Makeup water Water evaporates  

21. The final and most important assumption is that the air leaving the cooling tower is completely saturated The local bulk water temperature of the water droplets is also an important value, it will lie somewhere in between the inlet warm water and outlet cold water temperatures. Specific humidity's and enthalpies of air may be determined by measuring wet and dry bulb temperature and using a psychrometric chart. Water temperatures can be measured. Thermodynamics Cooling Tower Air Water Makeup water