1 / 21

Lecture 29

Lecture 29. The Vapor Compression Refrigeration (VCR) Cycle. Refrigerator used for Cooling. R. Observation:.

lise
Télécharger la présentation

Lecture 29

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. Lecture 29 The Vapor Compression Refrigeration (VCR) Cycle

  2. Refrigerator used for Cooling R Observation: The concept of an efficiency being greater than 100% makes people uneasy. Therefore, the conversion efficiency for a refrigerator is called the Cooling Coefficient of Performance (COPC). A refrigeration system that is used for cooling is called a refrigerator.

  3. Refrigerator used for Heating R Observation: The concept of an efficiency being greater than 100% makes people uneasy. Therefore, the conversion efficiency for a refrigerator is called the Heating Coefficient of Performance (COPH). A refrigeration system that is used for cooling is called a heat pump.

  4. Analysis of the Carnot Refrigerator For the Refrigeration cycle … R For the Heat Pump cycle …

  5. Terminology • Refrigeration cycle • The cycle is operating in a refrigeration cycle • The goal is to keep the cold space cold • Transfer heat from a low-temperature source to the cycle • Heat pump cycle • The cycle is operating in a refrigeration cycle • The goal is to keep the hot space hot • Transfer heat to a high-temperature sink from the cycle The words refrigeration or heat pumpdefine the goal of the cycle.

  6. The Vapor Compression Refrigeration Cycle R

  7. The Vapor Compression Refrigeration Cycle • Two phase changes • Boiling (evaporator) • Condensing (condenser) • Low temperature boiling fluids Working fluid = Refrigerant

  8. Refrigerants Most refrigerants are halogenated hydrocarbons. The naming convention adopted by ASHRAE is, R(a-1)(b+1)d = CaHbClcFd c = 2(a – 1) – b – d Example: R22 (R022) H chlorodifluoromethane C F F Cl

  9. The Pressure-Enthalpy Diagram

  10. Vapor Compression Refrigeration Analysis Performance

  11. The Ideal VCR Cycle on the P-h Diagram 1-2: Isentropic compression 2-3: Isobaric heat rejection 3-4: Isenthalpic expansion 4-1: Isobaric heat addition

  12. Refrigeration Effect and Capacity Refrigeration Effect: Refrigeration Capacity: Refrigeration capacity is often expressed in tons of refrigeration. Definition … 1 ton of refrigeration is the steady state heat transfer rate required to melt 1 ton of ice at 32°F in 24 hours. 1 ton = 12,000 Btu/hr = 3.516 kW

  13. VCR Cycle Irreversibilities Pressure drop through the condenser Isentropic efficiency of the compressor Pressure drop through the evaporator

  14. Practical VCR Cycle SCT = Saturated Condensing Temperature DSC = Degrees of Subcooling = SCT – T3 Subcooling increases the refrigeration capacity Superheating provides a dry vapor at the compressor inlet SET = Saturated Evaporating Temperature DSH = Degrees of Superheat = T1 – SET

  15. Example Given: A vapor compression refrigeration cycle is operating with a saturated evaporating temperature of -20°F and a saturated condensing temperature of 80°F. The refrigerant, R22, leaves the condenser as a saturated liquid and enters the compressor with 5 degrees of superheat. The pressure drops through the evaporator and condenser can be considered negligible. The compressor has an isentropic efficiency of 85%. The cycle has a capacity of 15 tons. Find: (a) the mass flow rate of the R22 (lbm/hr) (b) the power requirement of the compressor (hp) (c) the coefficient of performance of the cycle

  16. Example

  17. Example

  18. Example Strategy: Build the property table then do the thermodynamics! The high and low pressures in the cycle can be found since the saturation temperatures are given. The pressures at all four states are known!

  19. Example

  20. Example The property table is complete!

  21. Example EES Solution (Key Variables) Comparison (same units) …

More Related