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

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? Furnace 72ºF 32ºF 72ºF Outdoor 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) 52ºF Exhaust Gas Combustion products Fresh Air

11. Heat Exchanger Effectivness (ε) C=mc p Location BLocation A T Hout T Cin T Cout T Hin Mass flow rateSpecific capacity of fluid

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

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

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

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

16. Heat Exchanger Analysis - Δt m

17. Counterflow For parallel flow is the same or

18. Counterflow Heat Exchangers Important parameters:

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

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

21. Overall Heat Transfer Q = U 0 A 0 Δt m Need to find this

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

23. Resistance model Q = U 0 A 0 Δt m 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) Criteria NTU εPRP crcr

26. 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

29. 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