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Objective

Objective. Discuss Expansion Valves and Refrigerants Heat Exchangers Learn about different types Define Heat Exchanger Effectiveness ( ε ). AEV. Maintains constant evaporator pressure by increasing flow as load decreases. Thermostatic Expansion Valve (TXV).

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Objective

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  1. Objective • Discuss Expansion Valves and Refrigerants • Heat Exchangers • Learn about different types • Define Heat Exchanger Effectiveness (ε)

  2. AEV • Maintains constant evaporator pressure by increasing flow as load decreases

  3. Thermostatic Expansion Valve (TXV) • Variable refrigerant flow to maintain desired superheat

  4. Refrigerants

  5. What are desirable properties of refrigerants? • Pressure and boiling point • Critical temperature • Latent heat of vaporization • Heat transfer properties • Viscosity • Stability

  6. In Addition…. • Toxicity • Flammability • Ozone-depletion • Greenhouse potential • Cost • Leak detection • Oil solubility • Water solubility

  7. 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

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

  9. 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

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

  11. Heat Exchanger Effectivness (ε) C=mcp Mass flow rate Specific capacity of fluid THin TCout THout TCin Location B Location A

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

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

  14. Overall Heat Transfer Q = U0A0Δtm Overall Heat Transfer Coefficient Mean temperature difference

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

  16. Heat Exchanger Analysis - Δtm

  17. Heat Exchanger Analysis - Δtm Counterflow For parallel flow is the same or

  18. Counterflow Heat Exchangers Important parameters:

  19. What about crossflow heat exchangers? Δtm= F·Δtm,cf Correction factor Δt for counterflow Derivation of F is in the book: ………

  20. Example: Calculate Δtm for the residential heat recovery system if : mcp,hot= 0.8· mc p,cold th,i=72 ºF, tc,i=32 ºF For ε = 0.5 → th,o=52 ºF, th,i=48 ºF → R=1.25, P=0.4 → F=0.89 Δtm,cf=(20-16)/ln(20/16)=17.9 ºF,Δtm=17.9 ·0.89=15.9 ºF

  21. Overall Heat Transfer Q = U0A0Δtm Need to find this

  22. Heat Transfer tP,o From the pipe and fins we will find t tF,m

  23. Resistance model • Q = U0A0Δtm • Often neglect conduction through tube walls • Often add fouling coefficients

  24. Heat exchanger performance (Book section 11.3) • NTU – absolute sizing (# of transfer units) • ε – relative sizing (effectiveness)

  25. Fin Efficiency • Assume entire fin is at fin base temperature • Maximum possible heat transfer • Perfect fin • Efficiency is ratio of actual heat transfer to perfect case • Non-dimensional parameter

  26. Summary • Calculate efficiency of extended surface • Add thermal resistances in series • If you know temperatures • Calculate R and P to get F, ε, NTU • Might be iterative • If you know ε, NTU • Calculate R,P and get F, temps

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