lecture objectives:
DESCRIPTION
Lecture Objectives:. Summarize AHUs Learn more about cooling cycles. VAV Dedicated Outdoor Air System (DOAS) with occupancy sensors. Exhaust. 100%OA. VAV box. VAV box. CO 2. CO 2. For ventilation and humidity control. Fan coil units for heating and cooling. - PowerPoint PPT PresentationTRANSCRIPT
Lecture Objectives:
• Learn more about cooling cycles
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 (T0, TR) will have the same COP
• For constant temp processes
• dq = Tds
• COP = TR/(T0 – TR)
Get Real
• Assume no heat transfer or potential or kinetic energy transfer in expansion valve
• COP = (h3-h2)/(h4-h3)
• Compressor displacement = mv3
Example
• R-22 condensing temp of 30 °C (86F) and evaporating temp of 0°C (32 F)
• Determinea) qcarnot wcarnot
b) Diminished qR and excess w for real cycle caused by throttling and superheat horn
c) ηR
Comparison Between Single-Stage and Carnot Cycles
• Figure 3.6
carnot
carnotR
wAA
qA
21
2
1
1
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
Multistage Compression Cycles
• Combine multiple cycles to improve efficiency• Prevents excessive compressor discharge
temperature• Allows low evaporating temperatures (cryogenics)