Objectives Differentiate CAV, VAV, dual-duct systems Finish psychrometric process (Chapter 8) Calculate heat transfer for direct air – liquid water contact devices Heat rejection Cooling towers Sections 10.1-10.3,10.5

Humidification hw Specific enthalpy of water added to system hg Specific enthalpy of saturated water vapor

Evaporative Cooling (aka air washing)

Condition Line for a Space

Single-Zone Systems Zone – any space with a thermostat

Air conditioning

Recirculation

Reheat

Evaporative Cooler

Air-to-air heat exchanger

Comment Look at material on CAV w/ reheat, VAV, and dual-duct systems in text Will form basis of first take home exam problem Skipping Chapter 9

Why do we have 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 D mass diffusivity

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

Solution Requires f-factor 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