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.html

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 coefficienthD mass transfer coefficientcp specific heatα thermal diffusivity at the surfaceD 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