Refrigeration Cycles Dr. Md. Zahurul Haq Professor Department of Mechanical Engineering Bangladesh University of Engineering & Technology (BUET) Dhaka-1000, Bangladesh zahurul@me.buet.ac.bd http://teacher.buet.ac.bd/zahurul/ ME 203: Engineering Thermodynamics T266 T273 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 1 / 24 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 2 / 24 Refrigerators, Air-conditioners & Heat Pumps Refrigeration Capacity/Performance 1 ton refrigeration: heat absorbed by 1 ton (2000 lb) of ice melting at 0 o C in 24 hours. 1 ton refrigeration (TR) 3.516 kw 12000 BTU/hr 200 BTU/min Coefficient of Performance, R Refrigeration Effect Net Work Required kw/ton power required per ton of refrigeration kw/ton 3.516 T270 R Q L W R : AC Q L W AC : HP Q H W HP HP R + 1 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 3 / 24 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 4 / 24
Example: T639 Q in T C s W c W t (T H T C ) s T C T H T C c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 5 / 24 T271 η Carnot 1 TL T H 1 293.15 473.15 0.38 38± R/AC TL T H T L 293.15 473.15 293.15 1.63 HP TH T H T L 473.15 473.15 293.15 2.63 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 6 / 24 Problems: Wet Compression in Carnot Cycle During compression, droplets in liquid are vaporised by the internal heat transfer process which requires finite time. High-speed compressors are susceptible to damage by liquid because of the short time available. In wet compression, the droplets of the liquids may wash the lubricating oil from the walls of the cylinder, accelerating wear. Dry compression takes place with no droplets and is preferable. Liquid refrigerants may be trapped in the head of reciprocating compressor by the rising piston, possibly damaging the valves or the cylinder head. Problems: Expansion Process in Carnot Cycle Carnot cycle demands that the expansion take place isentropically and that the resulting work be used to help drive the compressor. Practical difficulties, however, militate against the expansion engine: the possible work that can be derived from the engine is small fraction that must be supplied to the compressor. practical problems such as lubrication intrude when a fluid of two phases drives the engine. the economics of the power recovery have in past not justified the cost of the expansion engine. A throttling device, such as a valve or other restriction, is almost universally used for this purpose. c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 7 / 24 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 8 / 24
Basic Refrigeration System using 2-Phase Refrigerant R-134a 1400 1200 1000 (kpa) P sat 800 600 400 200 T265 0-50 -40-30 -20-10 0 10 20 30 40 50 T sat ( o C) T264 T641 T640 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 9 / 24 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 10 / 24 Processes of Vapour Compression Refrigeration System Q L Q 41 m(h 1 h 4 ) Q H Q 23 m(h 2 h 3 ) W in W 12 m(h 2 h 1 ) Q L /W in T257 T255 T256 1 2: Isentropic compression, P evap P cond 2 3: Isobaric heat rejection, Q H 3 4: Isenthalpic expansion, P cond P evap 4 1: Isobaric heat extraction, Q L c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 11 / 24 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 12 / 24
Example: A theoretical single stage cycle using R134a as refrigerant operates with a condensing temperature of 30 o C and an evaporator temperature of -20 o C. The system produces 50 kw of refrigeration effect. Estimate: 1 Coefficient of performance, 2 Refrigerant mass flow rate, m Q L W in Q 41 m(h 1 h 4 ) 50 kw m 0.345 Kg/s W 12 m(h 2 h 1 ) 12.5 kw Q L /W in 50.0/12.54.0 Effect of Evaporator Temperature 10 9 8 7 6 5 4 3 2 1 R134a: RE 50 kw, 30 o C Ref. flow rate 0.40 0.39 0.38 0.37 0.36 0.35 0.34 0.33 0.32 0.31 Refrigerant flow rate (kg/s) T257 T259 0-50 -45-40 -35-30 -25-20 -15-10 -5 0 T evap ( o C) 0.30 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 13 / 24 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 14 / 24 Effect of Condenser Temperature Effect of Evaporator & Condenser Temperatures R134a: RE 50 kw, T 12 evap -20 o C 11 10 9 8 7 Ref. flow rate 6 5 4 3 c 2 0.24 0 5 10 15 20 25 30 35 40 45 50 ( o C) T261 Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 15 / 24 0.44 0.42 0.40 0.38 0.36 0.34 0.32 0.30 0.28 0.26 Refrigerant flow rate (kg/s) T260 12 11 10 9 8 7 6 5 4 3 2 1 20 o C 30 o C 40 o C 0-50 -45-40 -35-30 -25-20 -15-10 -5 0 T evap ( o C) c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 16 / 24
Deviation from Simple Cycle Deviations from Ideal Cycle Deviation from Simple Cycle Super-heating & Sub-cooling 1 Refrigerant pressure drop in piping, evaporator, condenser, receiver tank, and through the valves and passages. 2 Sub-cooling of liquid leaving the condenser. 3 Super-heating of vapour leaving the evaporator. 4 Compression process is not isentropic. T263 T262 Sub-cooling of liquid serves a desirable function of ensuring that 100% liquid will enter the expansion device. Super-heating of vapour ensures no droplets of liquid being carried over into the compressor. Even through refrigeration effect is increased, compression work is greater & probably has negligible thermodynamic advantages. c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 17 / 24 Deviation from Simple Cycle c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 18 / 24 Deviation from Simple Cycle Example: Example: A dual-evaporator refrigeration system: T F -18.0 o C, T R 4.0 o C, T c 30.0 o C & η c 80±. h 1 h(p 0.14 MPa,T 10 o C) h 2 h(p 0.80 MPa,T 50 o C) R? x 5? h 3 h f,26o C h 4 h 3 m Q R+ Q F h 1 h 4h W c m(h2s h1) η c 1.62 10 3 kg/s T258 Q L (h 1 h 4 ) 158.53 kj/kg R Q R+ Q F W c 3.37 W in (h 2 h 1 ) 40.33 kj/kg R QL W in 3.93 Q R m(h 5 h 4h ) h 5 h 5 x 5 0.56 η c h2s h1 h 2 h 1 93.6± T274 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 19 / 24 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 20 / 24
Miscellaneous Refrigeration Systems Miscellaneous Refrigeration Systems Vapour Absorption Refrigeration System T267 T272 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 21 / 24 Ammonia absorption refrigeration system c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 22 / 24 Miscellaneous Refrigeration Systems Vapour-Compression Heat Pump (HP) System Miscellaneous Refrigeration Systems Gas Refrigeration System: Brayton Cycle T268 T269 Q out W c + Q in HP Q out W c 1+ Ref Q in (h 1 h 4 ) W c W t (h 2 h 1 ) (h 3 h 4 ) c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 23 / 24 c Dr. Md. Zahurul Haq (BUET) Refrigeration Cycles ME 203 (2017) 24 / 24