1. Objectives
Learn about Cooling towers
Cooling cycles

2. Cooling Tower
Similar to an evaporative cooler, but the purpose is often to cool water
Widely used for heat rejection in HVAC systems
Also used to reject industrial process heat

4. Cooling Tower

6. Solution Can get from Stevens diagram (page 272)
Can also be used to determine
Minimum water temperature
Volume of tower required
Can be evaluated as a heat exchanger by conducting NTU analysis

8. Real World Concerns We need to know mass transfer coefficients
They are not typically known for a specific direct-contact device
Vary widely depending on packing material, tower design, mass flow rates of water and air, etc.
In reality, experiments are typically done for a particular application
Some correlations are in Section 10.5 in your book
Use with caution

9. Summary
Heat rejection is often accomplished with devices that have direct contact between air and water
Evaporative cooling
Can construct analysis of these devices
Requires parameters which need to be measured for a specific system

10. Vapor Compression Cycle
Expansion Valve

12. Efficiency First Law Second law Coefficient of performance, COP
COP = useful refrigerating effect/net energy supplied
COP = qr/wnet
Second law
Refrigerating efficiency, ηR
ηR = COP/COPrev
Comparison to ideal reversible cycle

13. Carnot Cycle No cycle can have a higher COP
All reversible cycles operating at the same temperatures (T0, TR) will have the same COP
For constant temp processes
dq = Tds
COP = TR/(T0 – TR)

14. Carnot Vapor-Compression Cycle
Figure 3.2

15. Get Real
Assume no heat transfer or potential or kinetic energy transfer in expansion valve
COP = (h3-h2)/(h4-h3)
Compressor displacement = mv3

16. Area Analysis of Work and Efficiency

17. Comparison Between Single-Stage and Carnot Cycles