Code No: R41031
R10
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 23oC 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 70C 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 25oC.
[10]
2. A refrigerating unit is working between 40oC and -10oC. The load on the unit is 5 tons.
Assume the refrigerant is NH3 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 -20oC, 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.
b) With the aid of a neat line diagram showing all the component units, explain the
working of three fluid vapour absorption system.
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[5]
[10]
Code No: R41031
R10
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]
o
o
b) One kg of air at 20 C DBT and 80% RH is mixed with 2 kg of air at 30 C DBT
and 10oC dew point temperature. The mixed air is passed through the cooling coil with
a rate of 200 m3/min. The temperature of air coming out of the cooling coil is 15oC
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 m3/hr.
Outdoor conditions ………………………… …….35oC DBT and 25oC WBT.
Inside designed conditions……………………..…..27oC DBT and 60% R.H.
Minimum temperature of air supplied to room…………………….15oC DBT.
Total amount of fresh air supplied ………………………………..1600 m3/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
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R10
Code No: R41031
Set No. 2
IV B.Tech I Semester Regular/Supplementary Examinations, Nov/Dec - 2014
REFRIGERATION & AIR-CONDITIONING
(Mechanical Engineering)
Time: 3 hours
1.
2.
Max. Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
Use of psychometrics chart and refrigerant tables are permitted
**
a) Explain dense air refrigerating system and its advantages over an open air
refrigeration system.
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 7oC and after
compression the air is cooled to 27oC before entering the expansion cylinder.
Expansion and compression follow the law pv1.25=constant. Determine theoretical
C.O.P. of the machine. Take Cp= 1kJ/kg-oC and Cv = 0.7kJ/kg-oC for air.
[5]
[10]
An ice plant using NH3 as refrigerant works between -15oC and 35oC and produces 10
tons of ice per day from water supplied at 0 oC. The ice temperature is -5oC. Assuming
simple saturated cycle and using the following properties of NH3, 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]
Saturation
Temp.
o
C
-15
35
Pressure
bar
2.36
13.5
Specific
enthalpy
hf
hg
kJ/kg
kJ/kg
112.3
1426
347.5
1471
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Specific
Specific entropy
sf
kJ/kg-K
0.457
1.282
sg
kJ/kg-K
5.549
4.930
volume
vg
3
m /kg
0.509
0.096
Code No: R41031
3.
R10
Set No. 2
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: CHClF2, C2H6,CO2, CH2F-CF3.
[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.
b) What are the advantages of vortex-tube over other refrigeration systems? List out
its applications.
[8]
[7]
6. a) Derive the relation between degree of saturation and relative humidity.
[5]
b) The outdoor air (at 38oC DBT and 50% RH) is mixed with return (from room) air (at
27oC 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 l0oC. 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
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Code No: R41031
R10
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 5oC. The temperature of air used for cooling in
the heat exchangers is 32oC. The compressed air leaves the primary heat exchanger
at 64oC. Taking the following data ɳt = 85%, ɳc (secondary compressor) = 75%, ϵ
(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 15oC 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]
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Code No: R41031
R10
Set No. 3
6. a) What is sensible heat factor and what does it represent on psychrometric chart?
[6]
o
o
b) Atmospheric air at 38 C and 20% R.H. brought to a temperature of 22 C and 60%
R.H. first by adiabatic humidifying and then by cooling. If the quantity of free air
flow is 500 m3/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
100oC, 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]
o
o
b) Room conditions: 26 C DBT, 19 C WBT
Outside conditions: 35oC DBT, 27oC WBT
Room heat gains:
Sensible heat 11.1kW
Latent heat: 3.9 kW
The conditioned air supplied to the room is 50 m3/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
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Code No: R41031
R10
Set No. 4
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]
0
b) In a Bell-Coleman refrigerator, air is taken in at 1 bar and a temperature of -8 C. The
compression ratio is 4. The expansion and compression follow the law pv1.2 = constant.
The air is cooled at the upper pressure to 25oC. Find M.E.P of the cycle and C.O.P. [7]
2. A vapour compression refrigeration system of 5 ton capacity operates at 40oC condenser
and -16oC evaporator temperatures. The vapour is superheated by 5oC 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 tsat = 40oC (Psat = 1.0166 Mpa) :
hf =256.41kJ/kg, hg= 419.43 kJ/kg, sg = 1.711 kJ/kg, and for
superheated vapour cp = 1.145 kJ/kg K
At tsat = - 6oC (Psat = 0.15728 Mpa) :
hg = 389.02 kJ/kg, sg = 1.7379 kJ/kg K and for superheated vapour cp = 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 100oC, 30oC and -5oC, respectively, Find its maximum C.O.P. If the respective
temperatures of generator, condenser and evaporator are changed to 180oC, 30oC and 35oC, 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]
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Code No: R41031
R10
Set No. 4
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.
b) What are the advantages, limitations and applications of steam jet refrigeration?
[9]
[6]
6. a) Describe the psychrometric process where SH is removed and LH added?
[6]
o
o
b) Atmospheric air at 12 C and 75%R.H. is to be conditioned to a temperature of 22 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]
o
b) A hall is to be maintained at 20 C DBT and 60% R.H. when the outdoor conditions are
40oC DBT and 26oC WBT.
Sensible heat load in room…………………………60000 kJ/hr.
Latent heat load in room……………………………20000 kJ/hr.
Infiltrated air ……………………………………….30 m3/min.
DPT of the cooling coil…………………………….5oC.
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/m2K.
[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
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