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Understanding HVAC Systems: Psychrometrics, Cooling Towers, and Evaporative Cooling

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This guide explores key objectives in HVAC systems including psychrometric processes for efficient cooling. Learn about evaporative cooling and cooling towers that facilitate heat rejection in air conditioning systems. We compare single-zone and multi-zone systems, dive into the mechanics of air washers, and direct contact cooling. Additionally, we address real-world concerns surrounding mass transfer coefficients in these systems and how they affect performance. Gain essential insights through practical applications and analytical techniques relevant to HVAC professionals.

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Understanding HVAC Systems: Psychrometrics, Cooling Towers, and Evaporative Cooling

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Presentation Transcript

  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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