389H NO 7 Cooling towers by 2sGRZLZ

VIEWS: 3 PAGES: 25

									               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
  – http://www.ce.utexas.edu/prof/Novoselac/classes/ARE389H/Handouts.h
    tml
       Single-Zone Systems
• Zone – any space with a thermostat
Air conditioning
Recirculation
Reheat
Evaporative Cooler
Air to Air Heat Exchanger
Air-to-air heat exchanger
Single zone vs. Multiple zone
          systems
             Air Washer
• Sprays liquid
  water into air
  stream
• Typically, air
  leaves system
  at lower
  temperature and
  higher humidity
  than it enters
Schematic
Air Washers/Evaporative
        Coolers
           • Heat and mass transfer is
             mutually compensating
           • Can evaluate based on
             temperature drop,
             humidification, or
             comparison to other
             energy exchangers
             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
         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
Cooling Tower
                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
       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
                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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