1 / 26

Objectives

Objectives. Compare real and ideal compression process Learn about expansion valves (Ch. 4) Compare residential and commercial systems Introduce heat exchangers (ch.11) Next two weeks. Real vs. Ideal Compression ( Example with Reciprocating Compressor). Reciprocating Compressor.

mechelle-gonzales
Download Presentation

Objectives

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Objectives • Compare real and ideal compression process • Learn about expansion valves (Ch. 4) • Compare residential and commercial systems • Introduce heat exchangers (ch.11) • Next two weeks

  2. Real vs. Ideal Compression(Example with Reciprocating Compressor)

  3. Reciprocating Compressor • Piston compressing volume • PVn = 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 n and volumetric efficiency ηv (book page 82-86, Fig 4.6) Define how isentropic is our compression

  4. 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

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

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

  7. Summary • Expansion valves make a big difference in refrigeration system performance • Trade-offs • Cost, refrigerant amount • Complexity/moving parts

  8. Refrigerants

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

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

  11. 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

  12. 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

  13. 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…

  14. Carbon Dioxide (R744) • Cheap, non-toxic, non-flammable • Critical temp? • Huge operating pressures

  15. Water (R718) • Two main disadvantages? • ASHRAE Handbook of Fundamentals Ch. 20

  16. 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

  17. Oil • Miscible refrigerants • High enough velocity to limit deposition • Especially in evaporator • Immiscible refrigerants • Use a separator to keep oil contained in compressor • Intermediate

  18. The Moral of the Story • No ideal refrigerants • Always compromising on one or more criteria

  19. Example Problem Explain the principle of operation of vapor compression based dehumidifier and show how it affects the indoor environment. If space conditions are T=25ºC, RH=70% and flow rate through humidifier is 360 m3/h, calculate T and RH in the dehumidifier discharge jet and amount of energy that this dehumidifier uses. Assume that: • Temperature of R22 in the evaporator is 2ºC, • Average surface temperature of cooling coil is 10ºC above temperature of evaporation, • Temperature of air leaving evaporator is 15ºC, • Temperature of condensation is 10ºC above temperature of air that leaves condenser, • We have isentropic compression and compressor motor efficiency 80%, • Air pressure drop in evaporator is 80 Pa and in condenser is 50Pa, • Fan motor efficiency of 50%.

  20. Coil Extended Surfaces Compact Heat Exchangers • Fins added to refrigerant tubes • Important parameters for heat exchange?

  21. 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

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

  23. Overall Heat Transfer • Q = U0A0ΔTm

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

  25. Heat Exchanger Analysis Counterflow Parallel

More Related