1. Objectives Finish analysis of most common HVAC Systems
Learn about the psychrometric related to the cooling towers
Cooling Systems
Describe vapor compression cycle basics
Draw cycle on T-s diagrams
Compare real cycles to ideal cycles

2. VAV Dedicated Outdoor Air System (DOAS) with occupancy sensors
Exhaust
100%OA
VAV box
VAV box
CO2
CO2
For ventilation
and humidity
control
Fan coil units for
heating and cooling

3. Fan Terminal Units Same as fan coil
Can be with
or without
recirculation

4. HVAC Systems Single zone Multi zone All Hydroinic that relay on
infiltration
VAV
CAV
CAV
VAV
With and
without
humidity
control
With and
without
reheaters
Dual
duct
Dual
duct
With
reheaters
DOAS with
fan coils
DOAS with
fan coils
This is not the complete list !

5. One more examples from your book

6. Summary of HVAC Systems
Show HVAC processes on a psychrometric chart
Define SA point
Think about processes and different ways to get to SA point
Analyze HVAC processes for real buildings
Single zone
Multiple zone

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

8. 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

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

10. Schematic

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

12. 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

14. Cooling Tower

16. 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

17. 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

18. 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

19. Vapor Compression Cycle
Expansion Valve

21. 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

22. 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)

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