# 389H NO 7 Cooling towers by 2sGRZLZ

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• pg 1
```									               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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