Objectives
• Finish with plotting processes on Psychrometric chart
• Learn about evaporative cooling and cooling towers
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
Air Washer
• Sprays liquid water into air stream
• Typically, air leaves system at lower temperature and higher humidity than it enters
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 coefficienthD mass transfer coefficientcp specific heatα thermal diffusivity at the surfaceD mass diffusivity
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