1. Objectives Learn about refrigerants, compressors, and expansion valves (Ch. 4) Introduce heat exchangers (ch.11)

2. Reciprocating Compressor

3. Reciprocating 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

4. Summary Many compressors available ASHRAE Handbook is good source of more detailed information Very large industry

5. 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

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

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

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

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

10. 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

12. 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

13. 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…

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

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

16. 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

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

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

19. Air-liquid Tube heat exchanger Plate heat exchanger Heat exchangers Air-air

20. 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

21. Heat Exchanger Effectiveness (ε) C=mc p Location BLocation A T Hout T Cin T Cout T Hin Mass flow rateSpecific capacity of fluid

22. Example: What is the saving with the residential heat recovery system? Furnace 72ºF 32ºF 72ºF Outdoor Air For ε=0.5 and if mass flow rate for outdoor and exhausted air are the same 50% of heating energy for ventilation is recovered! For ε=1 → free ventilation! (or maybe not) 52ºF Exhaust Gas Combustion products Fresh Air

23. Air-Liquid Heat Exchangers Fins added to refrigerant tubes Important parameters for heat exchange? Coil Extended Surfaces Compact Heat Exchangers

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

25. Overall Heat Transfer Q = U 0 A 0 Δt m Overall Heat Transfer Coefficient Mean temperature difference

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

27. Heat Exchanger Analysis - Δt m

28. Counterflow For parallel flow is the same or