1. Objectives Compare real and ideal compression process Learn about expansion valves (Ch. 4) Compare residential and commercial systems Introduce heat exchangers (ch.11) Next two weeks

2. Real vs. Ideal Compression ( Example with Reciprocating Compressor)

3. Reciprocating Compressor Piston compressing volume PV n = constant = C For all stages, if we assume no heat transfer Can measure n, but dependent on many factors Often use isentropic n in absence of better values R-12 n =1.07 R-22 n = 1.12 R-717 n = 1.29 n and volumetric efficiency η v (book page 82-86, Fig 4.6) Define how isentropic is our compression

4. Expansion Valves Throttles the refrigerant from condenser temperature to evaporator temperature Connected to evaporator superheat Increased compressor power consumption Decreased pumping capacity Increased discharge temperature Can do it with a fixed orifice (pressure reducing device), but does not guarantee evaporator pressure

5. Thermostatic Expansion Valve (TXV) Variable refrigerant flow to maintain desired superheat

6. AEV Maintains constant evaporator pressure by increasing flow as load decreases

7. Summary Expansion valves make a big difference in refrigeration system performance Trade-offs Cost, refrigerant amount Complexity/moving parts

8. Refrigerants

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

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

11. 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

13. Refrigerant Conventions Mixtures show mass fractions Zeotropic mixtures Change composition/saturation temperature as they change phase at a constant pressure Azeotropic mixtures Behaves as a monolithic substance Composition stays same as phase changes

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

15. Carbon Dioxide (R744) Cheap, non-toxic, non-flammable Critical temp? Huge operating pressures

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

17. 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

18. Oil Miscible refrigerants High enough velocity to limit deposition Especially in evaporator Immiscible refrigerants Use a separator to keep oil contained in compressor Intermediate

19. The Moral of the Story No ideal refrigerants Always compromising on one or more criteria

20. Example Problem Explain the principle of operation of vapor compression based dehumidifier and show how it affects the indoor environment. If space conditions are T=25ºC, RH=70% and flow rate through humidifier is 360 m3/h, calculate T and RH in the dehumidifier discharge jet and amount of energy that this dehumidifier uses. Assume that: Temperature of R22 in the evaporator is 2ºC, Average surface temperature of cooling coil is 10ºC above temperature of evaporation, Temperature of air leaving evaporator is 15ºC, Temperature of condensation is 10ºC above temperature of air that leaves condenser, We have isentropic compression and compressor motor efficiency 80%, Air pressure drop in evaporator is 80 Pa and in condenser is 50Pa, Fan motor efficiency of 50%.

21. Coil Extended Surfaces Compact Heat Exchangers Fins added to refrigerant tubes Important parameters for heat exchange?

22. Some HX (Heat Exchanger) truths All of the energy that leaves/enters the refrigerant enters/leaves the heat transfer medium If a HX surface is not below the dew point of the air, you will not get any dehumidification Water takes time to drain off of the coil Heat exchanger effectivness varies greatly

23. What about compact heat exchangers? Analysis is very complex Assume flat circular-plate fin

24. Overall Heat Transfer Q = U 0 A 0 ΔT m

25. Heat Exchangers Parallel flow Counterflow Crossflow Ref: Incropera & Dewitt (2002)

26. Heat Exchanger Analysis Counterflow Parallel