Set No. 1 IV B.Tech I Semester Regular/Supplementary Examinations, Nov/Dec - 2014 REFRIGERATION & AIR-CONDITIONING (Mechanical Engineering) Time: 3 hours Max. Marks: 75 Answer any FIVE Questions All Questions carry equal marks Use of psychometrics chart and refrigerant tables are permitted ***** 1. a) Explain Bell-Colemann air refrigerator with diagrams. [5] b) A simple air-cooled system is used for an aero plane to take the load of 20 tons. Atmospheric temperature and pressure conditions are 23 o C and 0.9 bar respectively. The pressure of air is increased due to isentropic ramming from 0.9 bar to 1 bar. The pressure of air leaving the main compressor is 3.5 bar and its 60% heat is removed in the air-cooled heat exchanger and then it is passed through an evaporator for further cooling. The temperature of air is reduced by 7 0 C in the evaporator. Lastly the air is passed through cooling turbine and then it is supplied to the cooling cabin where the pressure is maintained at 1.03 bar. Assuming isentropic efficiencies of the compressor and turbine are 80% and 75%, determine, i) kw capacity required to take the load in the cooling cabin. ii) C.O.P. of the system. The temperature of the air leaving the cabin should not exceed 25 o C. [10] 2. A refrigerating unit is working between 40 o C and 10 o C. The load on the unit is 5 tons. Assume the refrigerant is NH 3 which is dry and saturated vapour leaving the evaporator and compression is isentropic. Find (i) COP of the system (ii) Power required to run the system. If the temperature of the refrigerant required in the evaporator is 20 o C, then find the change in C.O.P. of the system and power required. [15] 3. a) Explain the working of thermostatic expansion valve with the help of neat sketch. [8] b) What are the desirable properties and importance of an ideal refrigerant? [7] 4. a) Explain how an absorption refrigeration system differs from a mechanical refrigeration system and write any two examples. [5] b) With the aid of a neat line diagram showing all the component units, explain the working of three fluid vapour absorption system. [10] 1 of 2
Set No. 1 5. a) Draw a neat sketch and explain working principle of vortex tube refrigeration. b) Explain the working of steam ejector with a diagram. [8] [7] 6. a) Define the term bypass factor used for cooling or heating coil and find the expression for bypass factor. [6] b) One kg of air at 20 o C DBT and 80% RH is mixed with 2 kg of air at 30 o C DBT and 10 o C dew point temperature. The mixed air is passed through the cooling coil with a rate of 200 m 3 /min. The temperature of air coming out of the cooling coil is 15 o C DBT. Calculate the following: i) DBT and specific humidity of air after mixing. ii) Cooing load on the cooling coil in tons of refrigeration. [9] 7. a) Explain the concepts of ESHF and GSHF. [5] b) An AC is designed for Restaurant for the following conditions: [10] Total heat flow through the walls, roof..21000 kj/hr. Solar heat gain through glass 6000 kj/hr. Equipment sensible & latent heat gain..10,000 kj/hr. & 2500 kj/hr. Total infiltrated air. 400 m 3 /hr. Outdoor conditions.35 o C DBT and 25 o C WBT. Inside designed conditions....27 o C DBT and 60% R.H. Minimum temperature of air supplied to room.15 o C DBT. Total amount of fresh air supplied..1600 m 3 /hr. The seating capacity 100 Employees serving the meals 10 If the fan is situated before the conditioner then find the following: i). Amount of air delivered to the room in cu.m. per hour. ii). Percentage of recirculated air. iii). Refrigeration load on the coil in tons of refrigeration. Take motor power connected to fan a 10 kw iv). DPT of the cooling coil and its bypass factor, neglect the heating of air passing through the fan. 8. Answer any three of the following: a) Comfort chart b) Reduced ambient air craft refrigeration system c) Different heat pump circuits d) Capillary tube and its sizing [3x5=15] 2 of 2
Set No. 2 IV B.Tech I Semester Regular/Supplementary Examinations, Nov/Dec - 2014 REFRIGERATION & AIR-CONDITIONING (Mechanical Engineering) Time: 3 hours Max. Marks: 75 Answer any FIVE Questions All Questions carry equal marks Use of psychometrics chart and refrigerant tables are permitted ***** 1. a) Explain dense air refrigerating system and its advantages over an open air refrigeration system. [5] b) An air refrigerator working on the reversed joule cycle works between pressure of 1bar and 8 bar. The temperature of the air entering the compressor is 7 o C and after compression the air is cooled to 27 o C before entering the expansion cylinder. Expansion and compression follow the law pv 1.25 =constant. Determine theoretical C.O.P. of the machine. Take C p = 1kJ/kg- o C and C v = 0.7kJ/kg- o C for air. [10] 2. An ice plant using NH 3 as refrigerant works between 15 o C and 35 o C and produces 10 tons of ice per day from water supplied at 0 o C. The ice temperature is 5 o C. Assuming simple saturated cycle and using the following properties of NH 3, determine i) The capacity of the refrigeration system required. ii) The discharge temperature. iii) The diameter and stroke of the compressor cylinder if its speed is limited to 1250 r.p.m. Take L/D as 1.2 and volumetric efficiency of the compressor as 0.75. iv) The power of the motor required to run the compressor if the isentropic efficiency is 85% and mechanical efficiency of 95%. v) The theoretical and actual C.O.P. [15] Specific Specific Saturation Specific entropy Pressure enthalpy volume Temp. h f h g s f s g v g o C bar kj/kg kj/kg kj/kg-k kj/kg-k m 3 /kg -15 35 2.36 13.5 112.3 347.5 1426 1471 0.457 1.282 5.549 4.930 0.509 0.096 1 of 2
Set No. 2 3. a) List various types of compressors used in refrigeration units? Discuss the advantages of hermetically sealed compressors. [6] b) Define primary refrigerant and discuss desirable properties of primary refrigerants. Write the refrigerant number for the following: CHClF 2, C 2 H 6,CO 2, CH 2 F-CF 3. [9] 4. a) Distinguish vapour compression refrigeration system and vapour absorption refrigeration system with their neat diagrams. [8] b) With a neat diagram explain practical Ammonia water absorption refrigeration system. [7] 5. a) Explain the operation of the steam jet refrigeration system with diagrams. [8] b) What are the advantages of vortex-tube over other refrigeration systems? List out its applications. [7] 6. a) Derive the relation between degree of saturation and relative humidity. [5] b) The outdoor air (at 38 o C DBT and 50% RH) is mixed with return (from room) air (at 27 o C DBT and 40% RH) in the ratio of 1:2 before entering the cooling coil. The bypass factor for the cooling coil is 0.25 and the ADP is l0 o C. Air flow rate (total) the cooling coil is l0 kg/s. Determine: i). condition of air at inlet and exit of the cooling coil ii). RSHF (room sensible heat factor) iii). Tonnage of the plant iv). Rate of condensation [10] 7. Explain briefly the different types of heat loads which are to be taken into account to considerations. [15] 8. Answer any three of the following: a) Design of AC system b) Selection and location of supply outlets c) Bypass factor of multi-depth coils d) Summer air-conditioning [3x5=15] 2 of 2
Set No. 3 IV B.Tech I Semester Regular/Supplementary Examinations, Nov/Dec - 2014 REFRIGERATION & AIR-CONDITIONING (Mechanical Engineering) Time: 3 hours Max. Marks: 75 Answer any FIVE Questions All Questions carry equal marks Use of psychometrics chart and refrigerant tables are permitted ***** 1. a) What is the necessity of cooling the aeroplane? Explain major heat sources. [6] b) A boot-strap system is used for an air plane. The pressure in the cabin is maintained at 1 bar and the air enters the cabin at 5 o C. The temperature of air used for cooling in the heat exchangers is 32 o C. The compressed air leaves the primary heat exchanger at 64 o C. Taking the following data t = 85%, c (secondary compressor) = 75%, (secondary H.E.) = 0.9. Determine (i) the temperature of air entering the cooling turbine and (ii) the pressures of air at discharge from primary and secondary compressors. [9] 2. An ideal F-12 compression refrigerating cycle operates between 1 bar and 7 bar. Dry saturated vapour enters the compressor and F-12 at 7 bar and 15 o C enters the expansion valve. Compression is isentropic. Determine the power input in kw for 40 tons refrigerating capacity. [15] 3. a) Explain the working principle of evaporative condenser with the help of neat labeled sketch. [8] b) What are secondary refrigerants? Why are these used in refrigeration and air conditioning industry? Name a few substances which are commonly used as secondary refrigerants. [7] 4. a) Enumerate the desirable properties of solvent and refrigerant-solvent combination for the vapour absorption system. [5] b) Draw a neat Diagram of Lithium bromide water absorption refrigeration system and explain its working. List out the applications of this refrigeration system. [10] 5. a) Under what circumstances, the steam jet refrigeration system is more preferable over other systems? [8] b) Define a semi-conductor and explain its properties from thermo-electric refrigeration point of view. [7] 1 of 2
Set No. 3 6. a) What is sensible heat factor and what does it represent on psychrometric chart? [6] b) Atmospheric air at 38 o C and 20% R.H. brought to a temperature of 22 o C and 60% R.H. first by adiabatic humidifying and then by cooling. If the quantity of free air flow is 500 m 3 /min, then find the following: i) Capacity of the humidifier. ii) Capacity of the cooling coil in tons of refrigeration. If the required condition is achieved first by injecting the saturated steam at 100 o C, then find the quantity of steam required per hour and change in the capacity of the cooling coil. [9] 7. a) With neat sketch explain working of summer air conditioning for hot and wet weather. [5] b) Room conditions: 26 o C DBT, 19 o C WBT Outside conditions: 35 o C DBT, 27 o C WBT Room heat gains: Sensible heat 11.1kW Latent heat: 3.9 kw The conditioned air supplied to the room is 50 m 3 /min and 25% fresh air and 75% recirculated room air. Determine the following: i). The DBT and WBT of supply air. ii). The DBT and WBT of mixed air before the cooling coil. iii). The apparatus dew point and bypass factor of the coil. iv). The refrigeration load on the cooling coil and the moisture removed by the coil. [10] 8. Answer any three of the following: a) Window air conditioner b) Surging phenomenon in centrifugal compressors c) Fans and blowers d) Psychrometric relations [3x5=15] 2 of 2
Code No: IV B.Tech I Semester Regular/Supplementary Examinations, Nov/Dec - 2014 REFRIGERATION & AIR-CONDITIONING (Mechanical Engineering) Time: 3 hours Max. Marks: 75 Answer any FIVE Questions All Questions carry equal marks Use of psychometrics chart and refrigerant tables are permitted ***** 1. a) Explain regenerative cooling aero plane and air condition system with schematic and T-s diagrams. [8] b) In a Bell-Coleman refrigerator, air is taken in at 1 bar and a temperature of 8 0 C. The compression ratio is 4. The expansion and compression follow the law pv 1.2 = constant. The air is cooled at the upper pressure to 25 o C. Find M.E.P of the cycle and C.O.P. [7] 2. A vapour compression refrigeration system of 5 ton capacity operates at 40 o C condenser and 16 o C evaporator temperatures. The vapour is superheated by 5 o C at the entry to the compressor. Determine COP and power requirement. Use the following properties of the refrigerant (do not use other property tables as refrigerant is not known) : At t sat = 40 o C (P sat = 1.0166 Mpa) : h f =256.41kJ/kg, h g = 419.43 kj/kg, s g = 1.711 kj/kg, and for superheated vapour c p = 1.145 kj/kg K At t sat = 6 o C (P sat = 0.15728 Mpa) : h g = 389.02 kj/kg, s g = 1.7379 kj/kg K and for superheated vapour c p = 0.831 kj/kg K [15] 3. a) Compare the utility of reciprocating compressors and centrifugal compressors. Derive an expression for the volumetric efficiency of reciprocating compressor. [8] b) What is a refrigerant? Give the classification of refrigerants. [7] 4. a) The temperature of generator condenser and evaporator of a vapour absorption system are 100 o C, 30 o C and 5 o C, respectively, Find its maximum C.O.P. If the respective temperatures of generator, condenser and evaporator are changed to 180 o C, 30 o C and 35 o C, respectively, find the new C.O.P and the percentage change in C.O.P. [6] b) With a diagram explain two shells Li-Br water absorption system. [9] 1 of 2
Code No: 5. a) Explain the working principle of thermo-electric refrigeration system. Define the figure of merit and explain its effect on C.O.P. of the system. [9] b) What are the advantages, limitations and applications of steam jet refrigeration? [6] 6. a) Describe the psychrometric process where SH is removed and LH added? [6] b) Atmospheric air at 12 o C and 75%R.H. is to be conditioned to a temperature of 22 o C and 60% R.H. The amount of air supplied is 200 cum. /min. The required condition is achieved first by heating and then by adiabatic humidification. Find the following: i) Amount of steam required I kg per hour through the heating coil at pressure 2 bar and 0.96 dry. Assume only latent heat of steam is used for heating. ii) The quantity of water required per hour in the humidifier. [9] 7. a) What are the different factors which must be considered evaluating cooling load? [5] b) A hall is to be maintained at 20 o C DBT and 60% R.H. when the outdoor conditions are 40 o C DBT and 26 o C WBT. Sensible heat load in room 60000 kj/hr. Latent heat load in room 20000 kj/hr. Infiltrated air.30 m 3 /min. DPT of the cooling coil.5 o C. If the 60% of the total air is recirculated from the hall and is mixed with the conditioned air after the conditioner, then find the following: i). Conditions of air leaving the conditioner and just before entering the hall. ii). Mass of fresh air entering the cooler. iii). Bypass factor of the conditioner coil. iv). Refrigeration load in tons of refrigeration on conditioner coil. v). Area of the cooling coil if the overall heat transfer coefficient is 50 W/m 2 K. [10] 8. Answer any three of the following: a) Industrial applications of air conditioning system b) Classification of AC equipment c) Electrolux refrigeration system d) Requirements of comfort air-conditioning [3x5=15]