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1 HVAC PROBLEM SHEET # 02(REVERSED BRAYTON CYCLE) kg of atmospheric air is circulated per hour in an open type of refrigeration installation. The air is drawn from the cold chamber at temperature ͺ and 1bar, and then compressed isentropically to 5 bars. It is cooled at this pressure to ʹͺ and then led to the expander where it expands isentropically down to atmospheric pressure and is discharged to cold chamber. Determine: (1) Heat extracted from cold chamber per hour; (2) Heat rejected to cooling water per hour; (3) C.O.P. of the system.(45837kj/h, 72316kJ/h, 1.73) 2. An open air cycle operated by air-refrigeration system is required to produce 6 tonnes of refrigerating effect with a cooler pressure of 11 bar abs. and a refrigerated space or region at a pressure of 1.05 bar. The temperature of air leaving the cooler is ͺ and the air leaving the room isͳ. Calculate: (1) Mass of air circulated per minute; (2) Compressor displacement required per minute; (3) Expander displacement required per minute; (4) C.O.P.; (5) Power required per tonne of refrigeration.(10.57kg, 8.35m 3 /min, 4.54 m 3 /min, 1.02, 3.8kW) 3. A Bell-Coleman refrigerator operates between pressure limits of 1 bar and 8 bars. Air is drawn from the cold chamber at ͻ, compressed and then it is cooled to ʹͻ before entering the expansion cylinder. Expansion and compression follow the law PV 1.25 =Constant. Calculate the theoretical C.O.P. of the system. For air take ã=1.4and C p =1.003KJ/KgK (1.27) 4. An air-refrigeration system operating on Bell-Coleman cycle takes in air from cold room at and compresses it from 1.04 bars to 6.2bar. The index of compression being The compressed air is cooled to ʹͷ.The ambient temperature is 18 C. Air expands in an expander where index of compression is Determine: (1) C.O.P. of the system; (2) Quantity of air circulated per minute for production of 1500 kg of ice per day at Ͳ from water atͳͺ; (3) Capacity of the plant. Take C pw =4.18KJ/KgK (for water); C pa = 1.003KJ/KgK for air and latent heat of ice =335KJ/KgK (1.837, 5.02kg/min, 1.83tonnes) 5. A refrigerating machine of 6 tonnes capacity working on Bell-Coleman cycle has an upper limit of pressure of 5.2 bars. The pressure and temperature at the start of the compression are 1.0 bar and ͳ respectively. The compressed air cooled at constant pressure at a temperature of Ͷͳ enters the expansion cylinder. Assuming both expansion and compression processes to be adiabatic with ߛ ͳǥͷǡ calculate: (1) C.O.P.; (2) Quantity of air in circulation per minute; (3) Piston displacement of compressor and expander; (4) Bore of compressor and expansion cylinders. The unit runs at 240 rpm and is double-acting. Stroke length =200 mm; (5) Power required to drive the unit; for air take ã =1.4and C p =1.003KJ/KgK (1.67, 15.05kg/min, m 3 /min, 407, m 3 /min, 335mm, 13.97kW) 6. A dense air refrigeration cycle operates between 5 bars and 20 bars. The air temperature after heat rejection to surroundings is and air temperature at exit of refrigerator is. The isentropic efficiencies of compressor and turbine are 0.84 and 0.82 respectively. Determine: (1) Compressor and turbine work per tonne of refrigeration; (2) C.O.P.; (3) Power per tonne of refrigeration. Take ã =1.4and C p =1.003KJ/KgK (710.06kJ/min, 364.9kJ/min, 0.676, 5.75kW) 7. A dense air closed Bell-Coleman refrigeration system working between 4 bars and 16 bars extracts 125 MJ/h. The air enters the compressor at ͷ and enters the expander at 23 C. The compressor is double-acting and its stroke = 30cm; ã=1.4and C p =1.003KJ/KgK; ç=87%; C p =1.005; R air =0.287KJ/KgKǤAssuming the unit runs at 300 rpm. Find: (1) Power required running the unit; (2) Bore of the compressor; (3) Refrigerating capacity in tonnes. Assume isentropic compression and expansion.(35.4kw, 19.26cm, 8.93tonnes) 1/1

2 HVAC PROBLEM SHEET # 03 (VAPOURE COMPRESSION CYCLE) 1. An ammonia refrigerator produces 20 tonnes of ice per day from and atͳ. The condensation and evaporation take place at 20 and -20 respectively. The temperature of vapour at the end of isentropic compression is 50 and there is no under-cooling of the liquid. The actual COP is 70% of the theoretical COP. Determine; (1) The rate of ܪ ଷ circulation; (2) The size of single acting-compressor when running at 240 r.p.m., assuming L=D and volumetric efficiency of 80%. Take (fusion of ice) 335 kj/kg. Use the properties of ܪ ଷ as listed below: Take v g at -20 =0.624m 3 /kg and Cps=2.8kJ/kg (0.097kg/s, 28.9cm, 28.9cm) Temp. () Enthalpy (kj/kg) Entropy (kj/kg K) ሺ ሻ ( ሻ ݏ) ሻ ݏ) ሻ A food storage locker requires a refrigeration capacity of 50 kw. It works between a condenser temperature of 35 and an evaporator temperature of 10. The refrigerant is ammonia. It is sub cooled by 5 before entering the expansion valve by the dry saturated vapour leaving the evaporator. Assuming a single- cylinder single acting compressor operating at 1000 r.p.m. with stroke equal to 1.2 times the bore, Determine: (1) The power required, and (2) The cylinder dimensions. Properties of ammonia are: (10.1kW, 0.19m, 0.228m) T () sbar) Enthalpy (kj/kg) Entropy (kj/kg K) Specific Volume Specific heat (kj/kgk) Liquid Vapour Liquid Vapour Liquid Vapour A food storage locker requires a refrigeration system of 2400 kj/min. capacity at an evaporator temperature of 263 K and a condenser temperature of 303 K. The refrigerant used is Freon-12 and sub cooled by before entering the expansion valve and vapour is superheated by before leaving the evaporator coil. The compression of refrigerant is reversible adiabatic. The refrigeration compressor is two cylinders single-acting with stroke equal to 1.25 times the bore and operates at 1000 r.p.m. Properties of Freon-12 T(K) P(bar) v h f (kj/kg) h g (kj/kg) s f (kj/kg K) s g (kj/kg K) Take: Liquid specific heat = kj/kgk, vapour specific heat = kj/kgk. Determine: (1) Refrigerating effect per kg, (2) Mass of refrigerant to be circulated per minute, (3) Theoretical piston displacement per minute, (4) Theoretical power required to run the compressor, in kw, (5) Heat removed through condenser per min., and (6) Theoretical bore and stroke of compressors. (131.14kJ/kg, 18.3kg/min, 1.441m 3 /min, 6.79kW, kJ/min, m 3 /min, 112.5mm) 4. In an ammonia vapour compression refrigerator condensation and evaporation take place at bar and 2.57 bar respectively. The temperature at the end of compression is ͷͳ and there is no undercooling. One tone of ice is to be formed per hour at ͷ from water at ͳͳ. Assuming specific heat of ice as 2.09 the latent heat 335 kj/kg and of the super heated ammonia vapour as 2.93, calculate the power required to drive the machine; neglect mechanical losses. For ammonia: (20.6kw) 1/3

3 P (bar) T(ሻ Enthalpy kj/kg Entropy kj/kg K Liquid Latent Liquid Latent A single-cylinder, single-acting compressor having bore and stroke of 16cm and 24 cm respectively runs at a speed of 110 r.p.m. and the indicated mean effective pressure is 2.06 bars. The pressure limits of the refrigerant are 9.66 bar and 2.66 bar and the temperatures at entry to and at exit from condenser are Ǥ and ͳ, flow of cooling water is 14 kg/min and ther inlet and outlet termperature are ͳ andʹͷ. The weight of ice produced per hour from water at ʹͲ is 56 kg. Assuming the latent heat of ice as 335 kj/kg and using the following table, find the following: (1) The coefficient of performance; (2) The mass of flow of ammonia/min, and; (3) The condition of ammonia entering the compressor, neglecting leakage.(3.57, 0.376kg/min, 0.899) Pressure (bar) Saturation temp.(ሻ Enthalpy kj/kg Specific heat Liquid Vapour Liquid Vapour Following results were obtained in a test conducted on a vapour compression refrigerator: Evaporator temperature = ʹͺǤͷ, condenser pressure = 2.75 bar; Refrigerant entering the condenser is superheat, refrigerant leaving the condenser is at ͳʹǥͺ. Determine the C.O.P. The following properties are given: (5.32) P (bar) T(ሻ Enthalpy kj/kg Entropy vapour Specific heat at constant pressure Liquid Vapour (kj/kg K) Liquid Vapour A vapour compression heat pump is driven by a power cycle having a thermal efficiency of 25%. For the heat pump, refrigerant-12 is compressed from saturated vapour at 2.0 bars to the condenser pressure of 12 bars. The isentropic efficiency of the compressor is 80%. Saturated liquid enters the expansion valve at 12 bars. For the power cycle 80% of the heat rejected by it is transferred to the heated space which has a total heating requirement of 500 kj/min. Determine the power input to the heat pump compressor. The following date for refrigerant-12 may be used: (3.2kW) P (bar) T(ሻ Enthalpy kj/kg Entropy (kj/kg K) Liquid Vapour Liquid Vapour A refrigerator operating on standard vapour compression cycle has a coefficient of performance of 6.5 and is driven by a 50kW compressor. The enthalpies of saturated liquid and saturated vapor refrigerant at the operating condensing temperature of ͷ are kj.kg and kj/kg respectively. The saturated refrigerant vapor leaving evaporator has an enthalpy of kj/kg. Find the refrigerant temperature at compressor discharge. The of refrigerant vapour may be taken to be kj/kgǥ (41.87 C) 9. A refrigeration cycle uses Freon-12 as the working fluid. The temperature of the refrigerant in the evaporator is -10 C. The condensing temperature is 40 C. The cooling load is 150 W and the volumetric efficiency of the compressor is 80%. The speed of the compressor is 720 r.p.m Calculate the mass flow rate of the refrigerant and the displacement volume of the compressor. Properties of Freon-12:( kg/s, m 3 /s 2/3

4 T ( C) P (MPa) h f (kj/kg) h g (kj/kg) V g A refrigerator is to be designed to operate between -45 and 0. You are asked to select one among the three given refrigerants, namely Freon-12, and ܪN ଷ and C ଶ, on the basis of the factors (1) COP, (2) Power required per ton, and (3) the condenser and evaporator pressures. The properties of the refrigerants are as given below: Refrigerants ௦ሺሻ ௦ሺ ݎሻ h f (kj/kg) h g (kj/kg) s f (kj/kgk) s g (kj/kgk) F ܪN ଷ C ଶ Determine the values for the above factors and suggest your choice for the most suitable refrigerant. 11. For a vapour compression refrigeration system using R-22 as refrigerant, condenser outlet temperature is ͶͲ and evaporator inlet temperature is ʹͲ. In order to avoid flashing of refrigerant, a liquid-suction vapour heat-exchanger is provided where liquid is sub cooled toʹ. The refrigerant leaves the evaporator as saturated vapor. The compression process is isentropic. Find the power requirement and coefficient of performance if capacity of the system is 10kW at ʹͲ. Show cycle on pressure-enthalpy diagram. of vapour is 1.03 kj/kg K. The thermodynamic properties are given below: ሺሻ (bar) ݒ ݒ (kj/kg) (kj/kg) ݏ (kj/kg K) ݏ (kj/kg K) A compressor having a stroke volume of 500 c.c. runs at 500 r.p.m. and works with. ଶrefrigerant ܥ The evaporator and condenser temperatures are ͳͷ andʹͷ, respectively. The condenser liquid is sub cooled to ͳͷ before passing on to the expansion valve. Assume wet compression with an initial quality x of ଶ ܥ as 0.9. Assuming isentropic compression and volumetric efficiency of the compressor as 0.85, calculate the following: (1) The COP, (2) The power required by the compressor (kw), and (3) The refrigerating capacity of the compressor in tones of refrigeration. The following date for ଶmay ܥ be used: ሺሻ (bar) V h(kj/kg) s(kj/kg K) Liquid Vapour Liquid Vapour Liquid Vapour 3/3

5 Take specific heat ܥ of ଶgas ܥ = 2.4 kj/kg K. 4/3

RAC. Unit 1. Previous year Questions

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