Objectives Learn about Cooling towers Cooling cycles

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

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

Vapor Compression Cycle Expansion Valve

Efficiency First Law Second law Coefficient of performance, COP COP = useful refrigerating effect/net energy supplied COP = qr/wnet Second law Refrigerating efficiency, ηR ηR = COP/COPrev Comparison to ideal reversible cycle

Carnot Cycle No cycle can have a higher COP All reversible cycles operating at the same temperatures (T0, TR) will have the same COP For constant temp processes dq = Tds COP = TR/(T0 – TR)

Carnot Vapor-Compression Cycle Figure 3.2

Get Real Assume no heat transfer or potential or kinetic energy transfer in expansion valve COP = (h3-h2)/(h4-h3) Compressor displacement = mv3

Area Analysis of Work and Efficiency

Comparison Between Single-Stage and Carnot Cycles