Chapter 11 Flashcards
The transfer of heat from a low-temperature region to a high-temperature one requires special devices called ________
Refrigerators
Device that transfers heat from a low-temperature medium to a high-temperature one is the _______
Heat pump
Objective of refrigerator
Remove heat (QL) from cold medium
Objective of heat pump
Supply heat (QH) to warm medium
Most efficient refrigeration cycle operating between TL and TH
Reversed Carnot Cycle
Why reversed Carnot cycle is not practical
2-3 involves compression of liquid-vapor mixture, requiring a compressor that handles 2 phases
4-1 involves the expansion of high-moisture content refrigerant in turbine
Ideal model for refrigeration systems
Vapor Compression Refrigeration Cycle
Vapor compression refrigeration cycle
Refrigerant vaporized completely before it is compressed and the turbine is replaced with a throttling device
1-2 isentropic compression in a compressor
2-3 Constant-pressure heat rejection in a condenser
3-4 throttling in an expansion device
4-1 constant pressure heat absorption in an evaporator
Vapor compression refrigeration cycle: most widely used cycle for _____
refrigerators
A-C systems
Heat pumps
The ideal vapor-compression refrigeration cycle involves an _____ process to make it a more realistic model for the actual systems
Irreversible (throttling)
Replacing the expansion valve by a turbine is not practical since the added benefits cannot justify the added _____
Cost and complexity
Actual vapor compression refrigeration cycle irreversibilities
Fluid friction (causes pressure drops)
Heat transfer to or from the surroundings
Actual vs. ideal
Non-isentropic compression
Superheated vapor at evaporator exit
Subcooled liquid at condenser exit
Pressure drops in condenser and evaporator
The COP _____ as a result of irreversibilities
Decreases
