Objectives Learn about Cooling towers Cooling cycles Cooling

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 directcontact 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 • 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 (T 0, TR) will have the same COP • For constant temp processes • dq = Tds • COP = TR/(T 0 – TR)

Carnot Vapor-Compression Cycle • Figure 3. 2

Get Real • Assume no heat transfer or potential or kinetic energy transfer in expansion valve • COP = (h 3 -h 2)/(h 4 -h 3) • Compressor displacement = mv 3

Area Analysis of Work and Efficiency

Comparison Between Single-Stage and Carnot Cycles