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Objectives

Objectives. Finish with plotting processes on Psychrometric chart Learn about evaporative cooling and cooling towers. Condition Line for a Space. HVAC Systems. Unlimited number You can make your system We will study several most common applied in practice

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Objectives

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  1. Objectives • Finish with plotting processes on Psychrometric chart • Learn about evaporative cooling and cooling towers

  2. Condition Line for a Space

  3. HVAC Systems • Unlimited number • You can make your system • We will study several most common applied in practice • http://www.ce.utexas.edu/prof/Novoselac/classes/ARE389H/Handouts.html

  4. Single-Zone Systems • Zone – any space with a thermostat

  5. Air conditioning

  6. Recirculation

  7. Reheat

  8. Evaporative Cooler

  9. Air to Air Heat Exchanger

  10. Air-to-air heat exchanger

  11. Single zone vs. Multiple zone systems

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

  13. Schematic

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

  15. 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

  16. 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 at the surface D mass diffusivity

  17. Cooling Tower

  18. 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

  19. 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

  20. 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

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