objectives finish with compressors learn about refrigerants and expansion valves (ch. 4) start with...
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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
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