Objectives Finish with compressors Learn about refrigerants and

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 (R 717) • • 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 (R 744) • Cheap, non-toxic, non-flammable • Critical temp? • Huge operating pressures

Water (R 718) • 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 95 o. F 53 o. F Water from building Water to 43 o. F 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 TCout THout TCin Location B Location A

Example: What is the saving with the residential heat recovery system? Outdoor Air 32ºF 72ºF Combustion products 52ºF Exhaust Furnace Gas Fresh Air 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)