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Objectives

Objectives. Learn about refrigerants, compressors, and expansion valves (Ch. 4) Introduce heat exchangers (ch.11). Reciprocating Compressor. Reciprocating. Piston compressing volume PV n = constant = C For all stages, if we assume no heat transfer

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Objectives

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

  2. Reciprocating Compressor

  3. Reciprocating • Piston compressing volume • PVn = 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

  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 Air-liquid Tube heat exchanger Air-air Plate heat exchanger

  19. 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

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

  21. 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 exhausted air are the same 50% of heating energy for ventilation is recovered! For ε=1 → free ventilation! (or maybe not)

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

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

  24. Overall Heat Transfer Q = U0A0Δtm Overall Heat Transfer Coefficient Mean temperature difference

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

  26. Heat Exchanger Analysis - Δtm

  27. Heat Exchanger Analysis - Δtm Counterflow For parallel flow is the same or

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