1. Objectives
Solve one more example related to the psychometrics in AHU and building systems
Learn about the psychometrics related to the cooling towers
Analize cooling cycles

2. 2

3. Summary of HVAC Systems
Show HVAC processes on a psychrometric chart
Draw condition lines
Define sensible heat ratio
Draw HVAC processes for real buildings
Single zone
Multiple zone

4. Cooling towes Similarity and difference between
Evaporative coolers and
Cooing towers

5. 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

6. Air Washer Sprays liquid water into air stream
Typically, air leaves system at lower temperature and higher humidity than it enters

7. Schematic

9. Cooling Tower

11. Solution Can get from Stevens diagram (page 272)
Can also be used to determine
Minimum water temperature
Volume of tower required

12. 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

13. 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

14. Reading assignments related to the HVAC systems Chapters 7 & 8 and Handout Section on the course website

15. Vapor Compression Cycle
Expansion Valve

17. 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

18. 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)

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

20. Example
R-22 condensing temp of 30 °C (86F) and evaporating temp of 0°C (32 F)
Determine
qcarnot wcarnot
Diminished qR and excess w for real cycle caused by throttling and superheat horn ηR

22. Comparison Between Single-Stage and Carnot Cycles
Figure 3.6

23. Subcooling and Superheating
Refrigerant may be subcooled in condenser or in liquid line
Temperature goes below saturation temperature
Refrigerant may be superheated in evaporator or in vapor (suction) line
Temperature goes above saturation temperature

24. Two stage systems

25. Multistage Compression Cycles
Combine multiple cycles to improve efficiency
Prevents excessive compressor discharge temperature
Allows low evaporating temperatures (cryogenics)