objectives
DESCRIPTION
Objectives. Finish with compressors Learn about refrigerants and expansion valves (Ch. 4) Start with heat exchangers. Summary. Many compressors available ASHRAE Handbook is good source of more detailed information Very large industry. Expansion Valves. - PowerPoint PPT PresentationTRANSCRIPT
Objectives
• Finish with compressors
• Learn about refrigerants and expansion valves (Ch. 4)
• Start with heat exchangers
Summary
• Many compressors available• ASHRAE Handbook is good source of more
detailed information• Very large industry
Expansion Valves
• Throttles the refrigerant from condenser temperature to evaporator temperature
• Connected to evaporator superheat• Increased compressor power consumption• Decreased pumping capacity• Increased discharge temperature
• Can do it with a fixed orifice (pressure reducing device), but does not guarantee evaporator pressure
Thermostatic Expansion Valve (TXV)
• Variable refrigerant flow to maintain desired superheat
AEV
• Maintains constant evaporator pressure by increasing flow as load decreases
Summary
• Expansion valves make a big difference in refrigeration system performance
• Trade-offs• Cost, refrigerant amount• Complexity/moving parts
Refrigerants
What are desirable properties of refrigerants?
• Pressure and boiling point
• Critical temperature
• Latent heat of vaporization
• Heat transfer properties
• Viscosity
• Stability
In Addition….
• Toxicity• Flammability• Ozone-depletion• Greenhouse potential• Cost• Leak detection• Oil solubility• Water solubility
Refrigerants
• What does R-12 mean?• ASHRAE classifications• From right to left ←
• # fluorine atoms
• # hydrogen atoms +1
• # C atoms – 1 (omit if zero)
• # C=C double bonds (omit if zero)
• B at end means bromine instead of chlorine• a or b at end means different isomer
Refrigerant Conventions
• Mixtures show mass fractions
• Zeotropic mixtures• Change composition/saturation temperature as
they change phase at a constant pressure
• Azeotropic mixtures• Behaves as a monolithic substance• Composition stays same as phase changes
Inorganic Refrigerants
• Ammonia (R717)• Boiling point• Critical temp = 271 °F• Freezing temp = -108 °F• Latent heat of vaporization
• Small compressors
• Excellent heat transfer capabilities• Not particularly flammable
• But…
Carbon Dioxide (R744)
• Cheap, non-toxic, non-flammable
• Critical temp?
• Huge operating pressures
Water (R718)
• Two main disadvantages?
• ASHRAE Handbook of Fundamentals Ch. 20
Water in refrigerant
• Water + Halocarbon Refrigerant = (strong) acids or bases• Corrosion
• Solubility• Free water freezes on expansion valves
• Use a dryer (desiccant)
• Keep the system dry during installation/maintenance
Oil
• Miscible refrigerants
• High enough velocity to limit deposition• Especially in evaporator
• Immiscible refrigerants • Use a separator to keep oil contained in
compressor
• Intermediate
The Moral of the Story
• No ideal refrigerants
• Always compromising on one or more criteria
Heat Exchangers
Systems: residential
Indoor Air
Outdoor Air
Large building system Chiller
Large building system Chiller
Outdoor air
53oF
43oF
Water to building
Water from building
95oF
Air-liquid Tube heat exchanger
Plate heat exchanger
Heat exchangers
Air-air
Some Heat Exchanger Facts
• All of the energy that leaves the hot fluid enters the cold fluid
• If a heat exchanger surface is not below the dew point of the air, you will not get any dehumidification• Water takes time to drain off of the coil
• Heat exchanger effectivness varies greatly
Heat Exchanger Effectivness (ε)
C=mcp
exchangeheatposible Maximum
exchangedHeat
Location B Location A
THout
TCin
TCout
THin
Mass flow rate Specific capacity of fluid
Example:What is the saving with the residential heat recovery system?
Furnace
72ºF
32ºF 72ºF
Outdoor Air
For ε=0.5 and if mass flow rate for outdoor and exhaust air are the same50% of heating energy for ventilation is recovered!
For ε=1 → free ventilation! (or maybe not)
52ºF Exhaust
Gas
Combustion products
Fresh Air