Objectives Learn about refrigerants, compressors, and expansion valves (Ch. 4) Introduce heat exchangers (ch.11)

Reciprocating Compressor

Reciprocating Piston compressing volume PV n = constant = C For all stages, if we assume no heat transfer Can measure n, but dependent on many factors Often use isentropic n in absence of better values R-12 n =1.07 R-22 n = 1.12 R-717 n = 1.29

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

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

Air-liquid Tube heat exchanger Plate heat exchanger Heat exchangers Air-air

Some HX (Heat Exchanger) truths All of the energy that leaves/enters the refrigerant enters/leaves the heat transfer medium If a HX 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 Effectiveness (ε) C=mc p Location BLocation A T Hout T Cin T Cout T Hin Mass flow rateSpecific 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 exhausted air are the same 50% of heating energy for ventilation is recovered! For ε=1 → free ventilation! (or maybe not) 52ºF Exhaust Gas Combustion products Fresh Air

Air-Liquid Heat Exchangers Fins added to refrigerant tubes Important parameters for heat exchange? Coil Extended Surfaces Compact Heat Exchangers

What about compact heat exchangers? Geometry is very complex Assume flat circular-plate fin

Overall Heat Transfer Q = U 0 A 0 Δt m Overall Heat Transfer Coefficient Mean temperature difference

Heat Exchangers Parallel flow Counterflow Crossflow Ref: Incropera & Dewitt (2002)

Heat Exchanger Analysis - Δt m

Counterflow For parallel flow is the same or