HW2 AHU problems: Book: 8.5, 8.25, 8.27, 8.28, 8.22 Cooling Cycles Problems: - Book: 3.1 (page 69), - Book: 3.5 ((page 70), - Out of book: Same like 3.5 for R22 with no intercooler - Book 3.9 (pages 70-71)

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

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 = q r /w net Second law Refrigerating efficiency, η R η R = COP/COP rev Comparison to ideal reversible cycle

Efficiency First Law Coefficient of performance, COP COP = useful refrigerating effect/net energy supplied COP = q r /w net Second law Refrigerating efficiency, η R η R = COP/COP rev Comparison to ideal reversible cycle

Carnot Cycle No cycle can have a higher COP All reversible cycles operating at the same temperatures (T 0, T R ) will have the same COP For constant temp processes dq = Tds COP = T R /(T 0 – T R )

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

Example R-22 condensing temp of 30 °C (86F) and evaporating temp of 0°C (32 F) Determine a) q carnot w carnot b) Diminished q R and excess w for real cycle caused by throttling and superheat horn c) η R

Subcooling and Superheating Refrigerant may be subcooled in condenser or in liquid line Temperature goes below saturation temperature Refrigerant may be superheated in evaporator or in vapor (suction) line Temperature goes above saturation temperature

Two stage systems