Lect29 - Lecture notes 29 PDF

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Vapor Compression Refrigeration System Purpose of the system is to maintain a cold region at a temperature below the temperature of its surroundings Warm surroundings TH < T3 Cold region TC > T1 Note: Q& in referred to as the refrigeration capacity, W (u...

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Vapor Compression Refrigeration System Purpose of the system is to maintain a cold region at a temperature below the temperature of its surroundings Warm surroundings TH < T3

Cold region TC > T1

Note: Q& in referred to as the refrigeration capacity, W (units: 1 ton = 711 KJ/min) Working fluid: up to 1990’s most common refrigerants were chlorinated fluorocarbons (CFCs), e.g., Freon-12 (CCl2F2), Freon-22 (CHClF2) Æ chlorine destroys earth’s ozone layer so CFCs banned A common refrigerant used in most hockey rinks in Canada is ammonia (NH3) 204

Ideal refrigeration cycle: 1 Æ 2 Isentropic compression 2 Æ 3 Constant pressure heat removal 3 Æ 4 Constant enthalpy (throttling) 4 Æ 1 Constant pressure heat addition

2

Q& out Temperature of warm region, TH Temperature of cold region, TC

Q& in

1Æ2

W& in = m& (h1 − h4 )

3 Æ 4 Throttling process 4Æ1

2Æ3

& (h2 − h3 ) Q& out = m

h3 = h4

Q& in = m& (h1 − h4 ) 205

Coefficient of Performance (COP): Q& in

β =

m& = h1 − h4 > 1 W&C h2 − h1 m&

Actual vapor-compression cycle may include: state 1 in superheated vapor region and state 3 in the compressed liquid region

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