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Objectives

Objectives. Finish analysis of most common HVAC Systems Learn about the psychrometric related to the cooling towers Cooling Systems Describe vapor compression cycle basics Draw cycle on T-s diagrams Compare real cycles to ideal cycles.

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Objectives

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  1. Objectives • Finish analysis of most common HVAC Systems • Learn about the psychrometric related to the cooling towers • Cooling Systems • Describe vapor compression cycle basics • Draw cycle on T-s diagrams • Compare real cycles to ideal cycles

  2. VAV Dedicated Outdoor Air System (DOAS) with occupancy sensors • Exhaust 100%OA VAV box VAV box CO2 CO2 For ventilation and humidity control Fan coil units for heating and cooling

  3. Fan Terminal Units Same as fan coil Can be with or without recirculation

  4. HVAC Systems Multi zone Single zone All Hydroinic that relay on infiltration VAV CAV CAV VAV With and without humidity control With and without reheaters Dual duct Dual duct With reheaters DOAS with fan coils DOAS with fan coils This is not the complete list !

  5. One more examples from your book

  6. Summary of HVAC Systems • Show HVAC processes on a psychrometric chart • Define SA point • Think about processes and different ways to get to SA point • Analyze HVAC processes for real buildings • Single zone • Multiple zone

  7. Cooling towes • Similarity and difference between • Evaporative coolers and • Cooing towers

  8. Direct Contact Processes • Humidification (and dehumidification – see 10.4) • Heat rejection • Water has better heat transfer properties than air • Non dimensional parameter • Lewis number, Le = α/D = hc/hD/cP • Ratio of heat transfer to mass transfer • Assume Le = 1 for evaporative coolers hc convection heat transfer coefficient hD mass transfer coefficient cp specific heat α thermal diffusivity D mass diffusivity

  9. Air Washer • Sprays liquid water into air stream • Typically, air leaves system at lower temperature and higher humidity than it enters

  10. Schematic

  11. Air Washers/Evaporative Coolers • Heat and mass transfer is mutually compensating • Can evaluatebased on temperature drop, humidification, or comparison to other energy exchangers

  12. 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

  13. Cooling Tower

  14. 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

  15. Real World Concerns • We need to know mass transfer coefficients • They are not typically known for a specific direct-contact device • Vary widely depending on packing material, tower design, mass flow rates of water and air, etc. • In reality, experiments are typically done for a particular application • Some correlations are in Section 10.5 in your book • Use with caution

  16. 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

  17. Vapor Compression Cycle Expansion Valve

  18. 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

  19. 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)

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

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