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Objectives

Objectives. Cooling Cycles Examples Cooling system components Refrigerants. Efficiency. First Law Coefficient of performance, COP COP = useful refrigerating effect/net energy supplied COP = q r /w net Second law Refrigerating efficiency, η R η R = COP/COP rev

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Objectives

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  1. Objectives • Cooling Cycles • Examples • Cooling system components • Refrigerants

  2. Efficiency • First 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

  3. 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)

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

  5. 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

  6. Comparison Between Single-Stage and Carnot Cycles • Figure 3.6

  7. 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

  8. Two stage systems

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

  10. Example 1 • R-22 condensing temp of 30 °C and evaporating temp of 0°C • Determine • qcarnot wcarnot • COPcarnot • ηR

  11. Example 2 • R-22 condensing temp of 60 °C and evaporating temp of 0°C • Determine the ηR

  12. Example 3 • Two stage compressor • R-22 condensing temp of 60 °C • Evaporating temp of compressor A 30°C and for compressor B 0°C. • Determine the ηR Assume that condenser temperature for compressor B is the same like evaporation temperature of compressor A

  13. Systems: residential Outdoor Air Indoor Air

  14. Large building cooling systems(chillers with cooling towers) Cooling tower Weather parameters DBT=30oC WBT=20oC 25oC 35oC Water P refrigerant 6oC 11oC

  15. System componentsLarge building system Water to cooling tower Water form cooling tower 25oC 35oC Plate heat exchanger 6oC 11oC Water to building Water from building

  16. Chiller

  17. Compressors

  18. Reciprocating compressor

  19. Scroll compressor http://www.youtube.com/watch?v=f_6xolDoqs0

  20. Screw compressor http://www.youtube.com/watch?v=xO7IhhzImMU&feature=related

  21. Rotary Compressor

  22. Refrigerants

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

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

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