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Department of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 29 The Vapor Compression Refrigeration (VCR) Cycle

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Refrigerator used for Cooling 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 (COP C ). A refrigeration system that is used for cooling is called a refrigerator. Observation: 2

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Refrigerator used for Heating 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 (COP H ). A refrigeration system that is used for cooling is called a heat pump. Observation: 3

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

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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 pump define the goal of the cycle. 5

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The Vapor Compression Refrigeration Cycle 6

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Two phase changes –Boiling (evaporator) –Condensing (condenser) Low temperature boiling fluids Working fluid = Refrigerant 7

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

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The Pressure-Enthalpy Diagram 9

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Vapor Compression Refrigeration Analysis Performance 10

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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 11

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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 12

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

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Practical VCR Cycle SCT = Saturated Condensing Temperature DSC = Degrees of Subcooling = SCT – T 3 SET = Saturated Evaporating Temperature DSH = Degrees of Superheat = T 1 – SET Subcooling increases the refrigeration capacity Superheating provides a dry vapor at the compressor inlet 14

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Example 15 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

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Example 16

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Example 17

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Example 18 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!

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Example 19

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Example 20 The property table is complete!

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

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