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HW2

HW2. AHU problems: Book: 8.5, 8.25, 8.27, 8.28, 8.22 Cooling Cycles Problems: - Book: 3.1 (page 69),  - Book: 3.5 ((page 70), - Out of book: Same like 3.5 for R22 with no intercooler - Book 3.9 (pages 70-71). Objectives. Learn about Cooling towers Cooling cycles. Cooling Tower.

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HW2

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  1. HW2 AHU problems: Book: 8.5, 8.25, 8.27, 8.28, 8.22 Cooling Cycles Problems: - Book: 3.1 (page 69),  - Book: 3.5 ((page 70), - Out of book: Same like 3.5 for R22 with no intercooler- Book 3.9 (pages 70-71)

  2. Objectives Learn about Cooling towers Cooling cycles

  3. Cooling Tower • Similar to an evaporative cooler, but the purpose is often to cool water • Widely used for heat rejection in HVAC systems • Also used to reject industrial process heat

  4. Cooling Tower

  5. Solution • Can get from Stevens diagram (page 272) • Can also be used to determine • Minimum water temperature • Volume of tower required • Can be evaluated as a heat exchanger by conducting NTU analysis

  6. Summary • Heat rejection is often accomplished with devices that have direct contact between air and water • Evaporative cooling • Can construct analysis of these devices • Requires parameters which need to be measured for a specific system

  7. Vapor Compression Cycle Expansion Valve

  8. 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

  9. 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

  10. 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)

  11. Carnot Vapor-Compression Cycle • Figure 3.2

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

  13. Area Analysis of Work and Efficiency

  14. Comparison Between Single-Stage and Carnot Cycles

  15. 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

  16. 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

  17. Two stage systems

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