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Objectives

Objectives. Finish with compressors Learn about refrigerants and expansion valves (Ch. 4) Start with heat exchangers. Summary. Many compressors available ASHRAE Handbook is good source of more detailed information Very large industry. Expansion Valves.

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Objectives

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  1. Objectives • Finish with compressors • Learn about refrigerants and expansion valves (Ch. 4) • Start with heat exchangers

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

  3. 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

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

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

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

  7. Refrigerants

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

  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

  11. 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

  12. 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…

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

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

  15. 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

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

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

  18. Heat Exchangers

  19. Systems: residential Outdoor Air Indoor Air

  20. Large building system Chiller

  21. Large building system Chiller Outdoor air 95oF 53oF Water from building Water to building 43oF

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

  23. Some Heat Exchanger Facts • All of the energy that leaves the hot fluid enters the cold fluid • If a heat exchanger 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

  24. Heat Exchanger Effectivness (ε) C=mcp Mass flow rate Specific capacity of fluid THin TCout THout TCin Location B Location A

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

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