1. Objectives Differentiate CAV, VAV, dual-duct systems
Finish psychrometric process (Chapter 8)
Calculate heat transfer for direct air – liquid water contact devices
Heat rejection
Cooling towers
Sections ,10.5

2. Humidification hw Specific enthalpy of water added to system
hg Specific enthalpy of saturated water vapor

3. Evaporative Cooling (aka air washing)

4. Condition Line for a Space

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

6. Air conditioning

7. Recirculation

8. Reheat

9. Evaporative Cooler

10. Air-to-air heat exchanger

15. Comment
Look at material on CAV w/ reheat, VAV, and dual-duct systems in text
Will form basis of first take home exam problem
Skipping Chapter 9

16. Why do we have 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
D mass diffusivity

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

18. Schematic

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

20. 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

24. Solution Requires f-factor 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

25. 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

26. 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