1. Objectives Finish with plotting processes on Psychrometric chart
Learn about evaporative cooling and cooling towers

2. Condition Line for a Space

3. HVAC Systems Unlimited number
You can make your system
We will study several most common applied in practice

4. Single-Zone Systems
Zone – any space with a thermostat

5. Air conditioning

6. Recirculation

7. Reheat

8. Evaporative Cooler

9. Air to Air Heat Exchanger

10. Air-to-air heat exchanger

11. Single zone vs. Multiple zone systems

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

16. Schematic

17. Air Washers/Evaporative Coolers
Heat and mass transfer is mutually compensating
Can evaluate based on temperature drop, humidification, or comparison to other energy exchangers

18. 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

19. Direct Contact Processes
Humidification (and dehumidification – see 10.4)
Heat rejection
Water has better heat transfer properties than air
Non dimensional parameter
Lewis number, Le = α/D = hc/hD/cP
Ratio of heat transfer to mass transfer
Assume Le = 1 for evaporative coolers
hc convection heat transfer coefficient
hD mass transfer coefficient
cp specific heat
α thermal diffusivity at the surface
D mass diffusivity

21. Cooling Tower

23. 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

24. 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

25. 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