IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE)
e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 11, Issue 3 Ver. II (May- Jun. 2014), PP 101-113
www.iosrjournals.org
www.iosrjournals.org 101 | Page
An experimental study of the Effect of capillary tube diameter
and configuration on the performance of a simple vapour
compression refrigeration system
Shashank shekhar pathak
1
, Prakhar shukla
2
, Sanjeev chauhan
3
,A.K.Srivastava
4
1,2,3
Student, Department of Mechanical Engineering, S.R.M.G.P.C, Lucknow, India
4
Head of Department, Department of Mechanical Engineering, S.R.M.G.P.C Lucknow,India
Abstract: The study of the expansion device in the simple vapour compression refrigeration system is
necessary in order to understand the parameters which can enhance the overall performance of the system. The
experimental study was done on the capillary tubes of 31 gauge, 36 gauge and 40 gauge and each test section
was studied with three distinct configurations i.e. helical coiled, straight coiled and serpentine coiled
configuration. The effect of the configuration and the capillary tube diameter on the overall performance of the
system was studied. The findings of the experimental study revealed that the mass flow rate is maximum for the
straight configuration and is least for the helical coiled configuration. The refrigeration effect was found to be
maximum for the helical coiled configuration and was found to be least for straight coiled configuration. The
compressor work was found to reduce as the load was increased on the system. Decreasing the capillary tube
diameter increased the mass flow rate in the system and decreased the refrigeration effect produced.
Keywords: Capillary Tube, configuration, diameter, refrigeration system.
I. Introduction
A simple vapour compression refrigeration system consists of mainly five components namely
compressor, condenser, expansion device, evaporator and a filter/drier. The following study is focused towards
finding out the effect of the capillary tube on the performance of the refrigeration system. A capillary tube is a
small diameter tube which is used for the expansion of the flowing fluid. The pressure difference between the
entry and exit ends of the capillary tube is always equal to the pressure difference between the condenser and the
evaporator. The diameter of the capillary tube used in the refrigeration appliances varies from 0.5mm to 2.3mm.
the effect of the capillary tube has been investigated by many researchers in the past and encouraging results
were obtained.
Hirendra Kumar Paliwal and Keshav Kant
[1]
(2006) developed a flow model for designing and studying
the performance of helical coiled capillary tubes and to mathematically simulate a situation closer to one
existing in real practice. Homogeneous flow of two phase fluid was assumed through the adiabatic capillary
tube. The model included the second law restrictions. The effect of the variation of different parameters like
condenser and evaporator pressures, refrigerant flow rate, degree of sub cooling, tube diameter, internal
roughness of the tube, pitch and the diameter of the helix and the length of the capillary tube were included in
the model. Theoretically predicted lengths of helical coiled capillary tube for R-134a are compared with the
length of the capillary tube actually required under similar experimental conditions and majority of predictions
were found to be within around 10% of the experimental value.
M.Y.Taib
[2]
et al. (2010) studied the performance of a domestic refrigerator and developed a test rig
from refrigerator model NRB33TA. The main objective of the performance analysis was to obtain the
performance of the system in terms of refrigeration capacity, coefficient of performance (cop), and compressor
work by determining three important parameters which are temperature, pressure and refrigerant flow rate. The
analysis of the collected data gave the cop of the system as 2.75 while the refrigeration capacity was ranging
from 150 watt to 205 watt.
J.K.Dabas
[3]
et al. (2011) studied the behavior of performance parameters of a simple vapour
compression refrigeration system while its working under transient conditions occurred during cooling of a
fixed mass of brine from initial room temperature to sub-zero refrigeration temperature. The effects of different
lengths of capillary tube over these characteristics were also investigated. The investigation showed that with
constantly falling temperature over evaporator, refilling of it with more and more liquid refrigerant causes
increase in heat transfer coefficient which maintains the refrigeration rate at falling temperature. The study
revealed that larger capillary tubes decreases the tendency of refilling but offers less evaporator temperature
while shorter capillary tubes ensure higher cop initially but it deteriorates at a faster rate in lower temperature
range.
An experimental study of the Effect of capillary tube diameter and configuration on the performance
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Ankush Sharma and Jagdev Singh
[4]
(2013) experimentally investigated about the effects simple and
twisted spirally coiled adiabatic capillary tubes on the refrigerant flow rate. Several capillary tubes with
different bore diameters, lengths and pitches were taken as test sections. LPG was used as an alternative for
R134a. mass flow rates for different capillary tubes were measured for different degrees of subcooling with
constant inlet pressure of the capillary tube. Experiments were conducted on straight capillary tubes as well so
as to facilitate proper comparison. The test results showed that mass flow rate is greater in straight capillary
tube and least in twisted spirally coiled capillary tube.
Sudharash Bhargava and Jagdev Singh
[5]
(2013) experimentally investigated the of pitch and length of
the serpentine coiled adiabatic capillary tube on the flow of a eco friendly gas. The azeotropic blend ( 30%
propane, 55% n-butane, 15% iso-butane) is used as refrigerant in the experiment. Various capillary tubes with
distinct lengths, pith and bore diameter were used as the test sections in the experiment. Inlet pressure of the
capillary tubes was kept constant and then mass flow rates for different capillary tubes with different lengths
and pitches were measured. Straight capillary tubes were also investigated. The data from the experiments
showed that mass flow rate of the refrigerant in the system was less for serpentine coiled capillary tubes and
was grater for straight capillary tubes.
Thamir K. Salim
[6]
(2012) experimentally investigated the performance of the capillary tube expansion
device using R134a as the refrigerant in the system. All the properties of the refrigeration system was measured
for the mass flow rate ranging from 13 kg/hour to 23 kg/hour and capillary tube coil number ( 0-4) with fixed
length (150 cm) and capillary tube bore diameter (2.5 mm). the test results showed that the theoretical
compression power increases by 65.8% as the condenser temperature increases by 2.71% and the theoretical
compression power decreases by 10.3% as the capillary tube coil number increases. The test results also showed
that cooling capacity increases by 65.3% as evaporator temperature increases by 8.4% and the cooling capacity
increases by 1.6% as the capillary tube coil number increases in the range (0-4). The cop decreases by 43.4% as
the mass flow rate increases by 76.9% and the cop of the system increases by 13.51% as the capillary tube coil
number increases in the range (0-4). The study showed that coil number 4 was the best for the lowest mass flow
rate (13 kg/hour) and the highest mass flow rate ( 23 kg/hour).
M.A. Akintunde
[7]
(2007) investigated the effects of various geometries of capillary tubes based on the
coil diameters and lengths alone. There was no any particular attention paid on the effect of coil pitch. This
paper examined the effects that the pitches of both helical coiled and serpentine coiled capillary tubes have on
the performance of a vapor compression refrigeration system. Several capillary tubes of equal lengths (2.03 m)
and varying pitches, coile diameters and serpentine heights were used. Both the inlet and outlet pressure and the
temperature of the test section (capillary tube) were measured and were used to estimate the COP of the system.
In the case of helical coiled capillary tubes, the pitch did not have any significant effect on the system
performance, while in the case of serpentine coiled capillary tubes , both pitch and height of the serpentine
influences the system performance. Performance improved with increase in both the pitch and the height.
Correlations were proposed to describe the relationships between straight and coiled capillary tubes and
between helical coiled capillary tubes and serpentine coiled capillary tubes. The coefficient of correlation
proposed was 0.9841 for the mass flow rates of helical and serpentine with straight tubes and 0.9864 for the
corresponding COPs and 0.9996 for the mass flow rates of helical and serpentine coiled tubes.
II. Methodology
The experimental study was done in the refrigeration and air conditioning laboratory of Shri
Ramswaroop Memorial Group of Professional Colleges, Lucknow, India in the best possible controlled
environment. Hermetic sealed compressor unit and tubular condenser unit were used. The evaporator unit was
properly insulated to the best of the effort so as to minimize the heat leakage into the system from the
surrounding. Copper tubes of diameter ¼ inches were used for providing the supply and return lines to the
flowing fluid in the system. Refrigerant R134a was used as the cooling fluid. A filter/drier, specific for R134a,
was installed just after the condenser unit in order to avoid any situation of choking of the flow lines. The
filter/drier does not allow the ice to be formed in the flow lines by absorbing all the moisture particles present
in the flowing fluid. Two analogue pressure gauges were used to determine the pressure of the flowing fluid in
the high pressure and the low pressure line. The pressure gauge in the high pressure line was installed just after
the filter/drier and just before the capillary tube. Another pressure gauge was installed in the low pressure retun
line to measure the pressure of the fluid returning back to the compressor. A digital temperature meter was used
to determine the temperatures that were to be used in the analysis of the system. The readings of the
temperature and pressure were plotted on the PH chart and the corresponding enthalpies were noted down and
from the obtained values of the enthalpies, the parameters like the refrigeration effect and the compressor work
were determined. The carnot COP of the system was determined by using the temperature limits of the system
and the actual COP of the system was determined by taking the ratio of the refrigeration effect and the
compressor work obtained from the PH chart.
An experimental study of the Effect of capillary tube diameter and configuration on the performance
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III. EXPERIMENTAL OBSERVATION AND RESULT DISCUSSION
Capillary tubes of 31 gauge, 36 gauge and 40 gauge were used as the test sections. The length of each test
section was kept constant to 3.5m. For each test section, readings were taken for three distinct configurations
i.e. helical coiled configuration, straight coiled configuration and serpentine coiled configuration. Every set of
readings consists of at least five readings, two for no load condition and one each for loaded condition of
600ml, 1200ml and 1800ml.
Readings for 31 gauge – helical coiled capillary tube:-
S.No.
Condition
P
1
(psi)
P
2
(psi)
P
2
/P
1
T
e
(
o
C)
T
c
(
o
C)
COP
carno
t
R.E.
(KJ/Kg)
W
c
(KJ/Kg)
COP
actual
Mfr
theo.
(Kg/hr)
1.
No Load
32
250
7.8
-6.5
68
3.58
108.8
49
2.22
92.65
2.
No load
32
250
7.8
-6.2
68.2
3.59
109.4
48.8
2.24
92.14
3.
No load
25
240
9.6
-12.8
66
3.30
112.4
48.9
2.29
89.68
4.
600ml
Load
28
239
8.54
-9.1
65.5
3.54
109.2
47.4
2.30
92.30
5.
1200ml
Load
38
252
6.63
1.1
70
3.98
110.9
34.6
3.20
90.89
6.
1800ml
Load
42
262
6.24
14.3
73
4.89
113.5
27.6
4.11
88.81
Table 1: Readings for 31 gauge – helical coiled capillary tube
Readings for 31 gauge – straight coiled capillary tube:-
S.No.
Condition
P
1
(psi)
P
2
(psi)
P
2
/P
1
T
e
(
o
C)
T
c
(
o
C)
COP
carnot
R.E.
(KJ/Kg)
W
c
(KJ/Kg)
COP
actual
Mfr
theo.
(Kg/s)
1.
No Load
38
274
7.21
-5
73
3.44
94.67
36.09
2.62
106.48
2.
No load
34
270
7.94
-6.5
72
3.39
94.23
40.38
2.33
106.97
3.
600ml
Load
40
274.5
6.86
3
73.5
3.88
96.79
32.11
3.01
104.14
4.
1200ml
Load
40
275
6.87
10
74
4.42
94.87
26.28
3.61
106.25
5.
1800ml
Load
40
277
6.92
14
75
4.70
98.77
21.79
4.53
102.05
Table 2: Readings for 31 gauge – straight coiled capillary tube
Readings for 31 gauge – serpentine coiled capillary tube:-
S.No.
Conditio
n
P
1
(psi)
P
2
(psi)
P
2
/P
1
T
e
(
o
C)
T
c
(
o
C)
COP
carnot
R.E.
(KJ/Kg)
W
c
(KJ/Kg)
COP
actual
Mfr
theo.
(Kg/s)
1.
No Load
28
250
8.92
-8.5
68
3.34
104.67
36.67
2.85
96.30
2.
No load
32
255
7.96
-8.2
69
3.43
102.33
36.33
2.82
98.50
3.
600ml
Load
38
274
7.21
-1.4
72
3.70
100.00
29.67
3.37
100.80
4.
1200ml
Load
40
278
6.95
4.6
74
4.00
101.33
29.34
3.45
99.48
5.
1800ml
Load
40
280
7.00
5
74.5
4.01
102.00
29.00
3.52
98.82
Table 3: Readings for 31 gauge – serpentine coiled capillary tube
Readings for 36 gauge – helical coiled capillary tube:-
S.No.
Condition
P
1
(psi)
P
2
(psi)
P
2
/P
1
T
e
(
o
C)
T
c
(
o
C)
COP
carnot
R.E.
(KJ/Kg)
W
c
(KJ/Kg)
COP
actual
Mfr
theo.
(Kg/s)
1.
No Load
30
260
8.67
-12
70
3.18
97.95
38.19
2.57
102.91
2.
No load
30
260
8.67
-13
70
3.12
97.32
38.33
2.54
103.58
3.
600ml Load
32
265
8.28
-10
71
3.25
99.89
36.99
2.70
100.91
4.
1200ml
Load
33
274
8.30
-1.8
72
3.67
100.61
35.19
2.86
100.19
5.
1800ml
Load
34
275
8.09
1
73
3.81
102.90
28.67
3.59
97.96
Table 4: Readings for 36 gauge – helical coiled capillary tube
An experimental study of the Effect of capillary tube diameter and configuration on the performance
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Readings for 36 gauge – straight coiled capillary tube:-
S.No.
Condition
P
1
(psi)
P
2
(psi)
P
2
/P
1
T
e
(
o
C)
T
c
(
o
C)
COP
carnot
R.E.
(KJ/Kg)
W
c
(KJ/Kg)
COP
actual
Mfr
theo.
(Kg/s)
1.
No Load
30
264
8.8
-8.3
71
3.38
96.64
36.34
2.66
104.31
2.
No load
30
265
8.83
-8.7
71
3.32
97.01
36.20
2.68
103.91
3.
600ml Load
28
260
9.29
-1.9
70
3.77
99.32
35.38
2.81
101.50
4.
1200ml
Load
30
262
8.73
3.4
70.5
4.12
100.34
30.35
3.31
100.49
5.
1800ml
Load
32
270
8.44
6.9
72
4.30
101.59
28.91
3.48
99.22
Table 5: Readings for 36 gauge – straight coiled capillary tube
Readings for 36 gauge – serpentine coiled capillary tube:-
S.No.
Condition
P
1
(psi)
P
2
(psi)
P
2
/P
1
T
e
(
o
C)
T
c
(
o
C)
COP
carnot
R.E.
(KJ/Kg)
W
c
(KJ/Kg)
COP
actual
Mfr
theo.
(Kg/s)
1.
No Load
32
264
8.25
-9
71
3.30
97.21
33.60
2.89
103.69
2.
No load
32
264
8.25
-8.5
71
3.32
97.54
33.25
2.93
103.34
3.
600ml Load
33
275
8.33
-3.7
73
3.51
99.41
32.34
3.07
101.40
4.
1200ml
Load
34
276
8.12
2
74
3.82
100.48
29.60
3.39
100.32
5.
1800ml
Load
34
276.5
8.13
4.8
74.5
4.01
102.16
27.94
3.66
98.67
Table 6: Readings for 36 gauge – serpentine coiled capillary tube
Readings for 40 gauge – helical coiled capillary tube:-
S.No.
Condition
P
1
(psi)
P
2
(psi)
P
2
/P
1
T
e
(
o
C)
T
c
(
o
C)
COP
carnot
R.E.
(KJ/Kg)
W
c
(KJ/Kg)
COP
actual
Mfr
theo.
(Kg/hr)
1.
No Load
30
265
8.83
-
13.3
71
3.08
95.68
41.33
2.32
105.35
2.
No load
30
265
8.83
-13
71
3.10
96.54
40.61
2.38
104.41
3.
600ml Load
32
275
8.59
-11.2
73
3.11
97.31
38.4
2.53
103.59
4.
1200ml
Load
34
273
8.03
-4.3
75
3.38
98.38
35.34
2.78
102.46
5.
1800ml
Load
35
280
8
-2.5
77
3.40
101
33
3.06
99.80
Table 7: Readings for 40 gauge – helical coiled capillary tube
Readings for 40 gauge – straight coiled capillary tube:-
S.No.
Condition
P
1
(psi)
P
2
(psi)
P
2
/P
1
T
e
(
o
C)
T
c
(
o
C)
COP
carnot
R.E.
(KJ/Kg)
W
c
(KJ/Kg)
COP
actual
Mfr
theo.
(Kg/s)
1.
No Load
32
268
8.38
-9.1
72
3.25
95.06
38.93
2.44
106.04
2.
No load
32
269
8.41
-9.3
72
3.24
95.89
38.72
2.48
105.12
3.
600ml
Load
33
275
8.33
-4.5
73
3.47
97.12
37.54
2.59
103.79
4.
1200ml
Load
35
278
7.94
-3.5
74
3.48
98.04
33.45
2.93
102.82
5.
1800ml
Load
37
280
7.57
-1
75
3.58
99.12
31.89
3.11
101.70
Table 8: Readings for 40 gauge – straight coiled capillary tube
Readings for 40 gauge – serpentine coiled capillary tube:-
S.No.
Condition
P
1
(psi)
P
2
(psi)
P
2
/P
1
T
e
(
o
C)
T
c
(
o
C)
COP
carnot
R.E.
(KJ/Kg)
W
c
(KJ/Kg)
COP
actual
Mfr
theo.
(Kg/s)
1.
No Load
30
268
8.93
-9.6
71
3.27
95.64
38.23
2.50
105.40
2.
No load
30
270
9
-9.9
71
3.21
96.01
38.33
2.51
104.99
3.
600ml Load
32
272
8.5
-4.9
72
3.49
97.34
36.98
2.63
103.55
4.
1200ml
Load
33
276
8.36
-3.9
73
3.50
98.12
35.98
2.73
102.73
5.
1800ml
Load
35
278
7.94
-1.5
74
3.60
99.34
34.89
2.85
101.47
Table 9: Readings for 40 gauge – serpentine coiled capillary tube
The above shown readings were used to plot bar graphs for comparing the system performance for different
configurations for each capillary tube. The bar graphs were used to compare the effect of the capillary tube
configuration and its diameter on the system performance. The bar graphs for the different test sections and
An experimental study of the Effect of capillary tube diameter and configuration on the performance
www.iosrjournals.org 105 | Page
their different configurations are drawn and shown below:-
Effect of configuration on the mass flow rate for 31 gauge capillary tube:-
Y axis in the following bar graph shows the mass flow rate in the system in kg/hr.
Bar graph 1: Effect of configuration on the mass flow rate for 31 gauge capillary tube
Effect of configuration on the refrigeration effect for 31 gauge capillary tube:-
Y axis in the following bar graph shows the refrigeration effect produced in the system in kj/kg.
Bar graph 2: Effect of configuration on the refrigeration effect for 31 gauge capillary tube
Effect of configuration on the compressor work for 31 gauge capillary tube:-
Y axis in the following bar graph shows the compressor work in kj/kg.
Bar graph 3: Effect of configuration on the compressor work for 31 gauge capillary tube
Effect of configuration on the mass flow rate for 36 gauge capillary tube:-
Y axis in the following bar graph shows the mass flow rate in the system in kg/hr.
0
20
40
60
80
100
120
no load no load 600ml load 1200ml load 1800ml load
helical
straight
serpentine
0
20
40
60
80
100
120
no load no load 600ml load 1200ml load 1800ml load
helical
straight
serpentine
0
10
20
30
40
50
60
no load no load 600ml load 1200ml load 1800ml load
helical
straight
serpentine