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