1. Objectives Finished Cooling Towers and Adiabatic Humidifiers
Cooling Cycles
Refrigerants

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

3. Schematic

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

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

7. Cooling Tower

9. 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

10. 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

11. 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

12. Vapor Compression Cycle
Expansion Valve

14. 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

15. 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)

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

17. 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

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

20. 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

21. Two stage systems

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

23. What are desirable properties of refrigerants?
Pressure and boiling point
Critical temperature
Latent heat of vaporization
Heat transfer properties
Viscosity
Stability

24. In Adition…. Toxicity Flammability Ozone-depletion
Greenhouse potential
Cost
Leak detection
Oil solubility
Water solubility

25. Refrigerants What does R-12 mean? ASHRAE classifications
From right to left ←
# fluorine atoms
# hydrogen atoms +1
# C atoms – 1 (omit if zero)
# C=C double bonds (omit if zero)
B at end means bromine instead of chlorine
a or b at end means different isomer (b is generally less symmetric)

28. Refrigerant Conventions
Mixtures show mass fractions
Zeotropic mixtures
Change composition/saturation temperature as they change phase at a constant pressure
400 series (if commercialized)
Azeotropic mixtures
Behaves as a monolithic substance
Composition stays same as phase changes
500 series (if commercialized)

29. More Refrigerant Arcana
Organic refrigerants – 600 series
Inorganic refrigerants molecular weight

30. Inorganic Refrigerants
Ammonia (R717)
Boiling point?
Critical temp = 271 °F
Freezing temp = -108 °F
Latent heat of vaporization?
Small compressors and linesets
Excellent heat transfer capabilities
Not particularly flammable
But…

31. Carbon Dioxide (R744) Recent ASHRAE papers
Evaluation of carbon dioxide as R-22 substitute for residential air-conditioning Brown, J. Steven (Department of Mechanical Engineering, Catholic University of America); Kim, Yongchan; Domanski, Piotr A. Source: ASHRAE Transactions, v 108 PART 2, 2002, p Abstract: This paper compares the performance of CO2 and R-22 in residential air-conditioning applications using semi-theoretical vapor compression and transcritical cycle models. The simulated R-22 system had a conventional component configuration, while the CO2 system also included a liquid-line/suction-line heat exchanger. The CO2 evaporator and gas cooler were microchannel heat exchangers originally designed for CO2. The R-22 heat exchangers employed the same microchannel heat exchangers as CO2 with the difference that we modified the refrigerant passages to obtain reasonable pressure drops. The study covers several heat exchanger sizes. The R-22 system had a significantly better coefficient of performance (COP) than the CO2 system when equivalent heat exchangers were used in the CO2 and R-22 systems, which indicates that the better transport properties and compressor isentropic efficiency of CO2 did not compensate for the thermodynamic disadvantage of the transcritical cycle in comfort cooling applications. An entropy generation analysis showed that the CO2 evaporator operated with fewer irreversibilities than did the R-22 evaporator. However, the CO2 gas cooler and expansion device generated more entropy than their R-22 counterparts and were mainly responsible for the low COP of the CO2 system. (33 refs.)
Cheap, non-toxic, non-flammable
Critical temp?
Huge operating pressures
Often no phase change

32. Water (R718) Two main disadvantages?
ASHRAE Handbook of Fundamentals Ch. 20

33. Water in refrigerant
Water + Halocarbon Refrigerant = (strong) acids or bases
Corrosion
Solubility
Free water freezes on expansion valves
Use a dryer (desiccant)
Keep the system dry during installation/maintenance

34. Oil Miscible refrigerants (11,12, 21,113)
High enough velocity to limit deposition
Especially in evaporator
Immiscible refrigerants (717,744,13,14)
Use a separator to keep oil contained in compressor
Intermediate (22, 114)

35. The Moral of the Story No ideal refrigerants
Always compromising on one or more criteria
Should be able to look up properties and analyze good candidates for refrigeration cycles