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?
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Pressure and boiling point
Critical temperature
Latent heat of vaporization
Heat transfer properties
Viscosity
Stability
In Addition….
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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 ←
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# 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)
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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
Outdoor Air
Indoor Air
Large building system
Chiller
Large building system
Chiller
Outdoor air
95oF
53oF
Water from
building
Water to
43oF building
Heat exchangers
Air-liquid
Tube heat exchanger
Air-air
Plate heat exchanger
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
Mass flow rate

Specific capacity of fluid
THin
Heat exchanged
Maximum posible heat exchange
TCout
THout
TCin
Location B
Location A
Example:
What is the saving with the residential heat recovery system?
Outdoor Air
32ºF
72ºF
72ºF
Combustion
products
52ºF
Exhaust
Furnace
Fresh Air
Gas
For ε=0.5 and if mass flow rate for outdoor and exhaust air are the same
50% of heating energy for ventilation is recovered!
For ε=1 → free ventilation!
(or maybe not)