8. Reviewed Mech final

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN 2249-6890 Vol. 2 Issue 4 Dec - 2012 73-82 Š TJPRC Pvt. Ltd.,

EXPERIMENTAL INVESTIGATION ON THE PERFORMANCE OF COILED ADIABATIC CAPILLARY TUBE WITH LPG AS REFRIGERANT SANJEEV SINGH PUNIA & JAGDEV SINGH Department of Mechanical Engineering, Beant College of Engineering and Technology, Gurdaspur, Punjab 143521, India

ABSTRACT This paper presented an experimental investigation of the effect of capillary tube length, capillary tube inner diameter and capillary coil diameter on the mass flow rate of refrigerant in an adiabatic helical capillary tube. In this study LPG was used as refrigerant. This paper also investigated the effect of coiling of capillary tube on the performance of vapour compression refrigeration system (VCRS). It had been observed that the coiling of capillary tube significantly influences the mass flow rate of LPG through the adiabatic helical capillary tube. Several capillary tubes with different inner diameters, length and coil diameter were selected as test sections. With the decrease in the length of capillary tube from 4.5m to 2.5m, the mass flow rate was increased by an average of 25%.

There was an average increase in mass flow rate by 106%, when the capillary inner diameter was increased from 1.12mm to 1.52mm. When the coil diameter of capillary tube was decreased from 190mm to 70mm, the mass flow rate was decreased by 13%, 7% and 9% for 1.12mm, 1.4mm and 1.52mm inner diameter of capillary tube respectively. Coiling of capillary tube effected the performance of system significantly. There was increase of COP with increase in the coil diameter of adiabatic helical capillary tube.

KEYWORDS: Helical Capillary Tube, COP, LPG, Refrigerant Flow, R134a Alternative, Eco-Friendly Refrigerant NOMENCLATURE Mass flow rate (kg/h) L

Capillary tube length (m)

COP

Coefficient of performance

D

Capillary coil diameter (mm)

d

Inner diameter of capillary tube (mm)

INTRODUCTION Capillary tubes are being used as expansion device in low capacity refrigeration machines like domestic refrigerators and window type air conditioners. Usually, the capillary tubes have diameters ranging from 0.5 mm to 2.0 mm and lengths from 2m to 6m. The advantages of the capillary tube over other expansion devices are simple, inexpensive and cause compressor to start at low torque as the pressure across the capillary tube equalise during the off-cycle. The flow characteristics of refrigerants through capillary tubes have been studied extensively in past six decades, both experimentally and numerically, most of these studies mainly focused on straight capillary tubes. However in practical applications the capillary tubes are generally coiled to make a compact unit. The studies on capillary tubes are only two decades old and still a lot of work is required to establish the effect of coiling on the flow performance of refrigerants.

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Sanjeev Singh Punia & Jagdev Singh

Since an improperly sized capillary tube can significantly reduce the performance of refrigeration system. Moreover, with the advent of new eco-friendly refrigerants, the studies on coiled capillary tubes with alternative refrigerants become all more important to select appropriate sized capillary tubes for specific applications.

The various flow aspects of the capillary tube were investigated by a number of researchers since past six decades.

Bolstad and Jordan (1948) pioneered the investigations on capillary tubes. They studied the effect of oil entrainment on the mass flow rate through the capillary tube. It was found that the use of oil separator in the system decreases the flow rate by 8% in comparison to that when no oil separator was used. Mikol (1963) carried out an extensive experimental investigation on the capillary tube to explore the various flow phenomena like metastability and choking. They developed a friction factor correlation by flowing water through the same capillary tube.

The effect of coiling on the refrigerant mass flow rate of the refrigerant has been discussed by a few investigators.

Kuehl and Goldschmidt (1990) have conducted experiments on the flow of R-22 through adiabatic capillary tubes of straight and coiled geometries. They have concluded that because of the coiling of capillary tube, the refrigerant mass flow rate was reduced by not more than 5%. Kim et al. (2002) have studied the flow of R-22 and its alternatives, viz., R-407C and R-410A through the straight and helically coiled adiabatic capillary tubes. They have observed 9% reduction in refrigerant mass flow rate through a coiled tube in comparison to that in straight tube of same length. Zhou and Zhang (2006a) conducted an experimental investigation on helically coiled capillary tubes for the flow of refrigerant R-22. In addition, a numerical model using Mori and Nakayama friction factor correlation (Mori and Nakayama, 1967) was also proposed. It was concluded that for the mean coil diameter beyond 300 mm, the change in mass flow rate was insignificant.

It was also observed that the refrigerant mass flow rate through a helical capillary tube with coil diameter of 40 mm was approximately 10% less than that of the straight capillary tube. Further, Zhou and Zhang (2006b) confirmed the hysteresis in refrigerant mass flow rate with increasing and decreasing inlet subcooling in an adiabatic helical capillary tube. The hysteresis effect was more prominent in helical capillary tube than that in straight tube due to the disturbance generated from the secondary flows caused by the centrifugal force. It was also found that in the coiled capillary tube the refrigerant flashed earlier with decreased coil diameter as reduction in coil diameter caused the pressure drop to increase.

Deodhar et al. (2006) have conducted an experimental and numerical investigation on R-134a refrigerant through straight and helically coiled capillary tube.

Experiments carried out on helically coiled capillary tubes indicate an increase in resistance to the flow due to coiling. Mass flow rate through a capillary tube of capillary tube of coil diameter of 20 mm is lower by about 9 – 19 percent than that through the straight capillary tube. Park et al. (2007) studied the flow of R-22 and its alternatives, R-407C and R-410A, through coiled adiabatic capillary. It been has revealed a slightly higher drop in mass flow rates of the coiled capillary tubes compared to those in straight capillary tubes as reported by Kim et al. (2002) and Zhou and Zhang (2006a).

It was found by Park et al. (2007) that the mass flow rate of the coiled capillary tubes was decreased by 5–16% in comparison to that for the straight capillary tubes.

They also proposed a generalized mass flow rate correlation for helically coiled capillary tubes based on Buckingham-π theorem. In addition, a numerical model based on homogenous two-phase flow model was presented using Ito (1959) friction factor correlation for single phase flow and Giri’s equation for two-phase flow (Gorasia et al., 1991).

Valladares (2007) also presented a numerical simulation model for the coiled capillary tubes based on the finite volume formulation. The solution was carried out using an implicit step-by-step numerical scheme. The calculation of mass flow rate was made iteratively using Newton–Raphson Algorithm. Khan et al. (2007) have proposed the numerical model for the

Experimental Investigation on the Performance of Coiled Adiabatic Capillary Tube with Lpg as Refrigerant

75

computation of length of adiabatic spiral capillary tube. The model for spiral capillary tube is compared with the model for straight capillary tube operating under similar conditions. It has been found that because of coiling the length of the capillary tube is reduced considerably for a given set of conditions. Khan et al. (2008a) investigated the effect of the coiling on the mass flow rate of R-134a in helical capillary tube by varying the coil pitch in the range 20-60 mm for the coil diameter of 140 mm.

They observed that the coiling of capillary tube reduces the mass flow rate by 5-15 percent as compared to those of straight capillary tube operating under similar conditions. Khan et al. (2008b) also examined the flow characteristics of R-134a in spiral capillary tube having coil pitch of 20, 40 and 60 mm. They concluded that the mass flow rate in spiral capillary tube is 5-15 percent less than that in straight capillary tube. They also developed a non-dimensional correlation to predict the mass flow rate of R-134a in spiral capillary tube. Mittal et al. (2009) proposed a homogenous model for the adiabatic flow of the refrigerant through the spiral capillary tube.

The effect of the pitch of the spiral on the mass flow rate of refrigerant. A comparison of the flow characteristics of refrigerant R22 and its alternatives, i.e., R407C and R410A has been made at different operating conditions and it has been found that the flow characteristics of R22 and R407C are almost similar for a given condenser pressure and degree of subcooling at the inlet of subcooling. Zhou et al. (2012) investigates the inlet pressure fluctuations for the coiled adiabatic capillary tubes. It has been found that pressure fluctuation for coiled capillary tubes is much more prominent than for straight ones.

The refrigerants chloro-fluoro-carbons (CFCs) and hydro-chloro-flouro-carbons (HCFCs) both have high ozone layer depleting potential (ODP) and global warming potential (GWP) which contributes to ozone layer depletion and global warming. Therefore, these two types of refrigerants are required to be replaced with environment friendly refrigerants to protect the environment. R134a is a long-term replacement refrigerant for R12. The ODP of R134a is zero, but it has a relatively high global warming potential. The issues of ozone layer depletion and global warming have led to consideration of hydrocarbon refrigerants such as propane, isobutane, n-butane or hydrocarbon blends as working fluids in refrigeration and air conditioning systems.

The efforts to explore eco-friendly alternative of R12 have brought out liquefied petroleum gas (LPG) as one of the best promising substitute of R12 to date. LPG is ozone-safe blend of three hydrocarbons (30% propane, 55% n-butane, 15% iso-butane) and exhibits properties similar to that of R12. There is not any single study reported in literature for the study of flow of LPG through coiled capillary tube. Therefore, in order to strengthen the work of LPG as refrigerant, the present study has been done by different operating conditions and different geometrical parameters of capillary tube.

EXPERIMENTAL SETUP AND PROCEDURE The schematic diagram of experimental set-up has been shown in figure1. The test section was a copper capillary tube in which the refrigerant expand from high pressure side to low pressure side. Helical coiled capillary tube was put in horizontal position. The refrigerant entered into evaporator from capillary tube consisting of copper coil submerged in a water tank. A 5 kW capacity electric heater was fitted in the evaporator tank to provide heat load to evaporator. This electric heater was connected across energy meter. Heat load given to the system was measured with the help of this energy

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Figure 1: Schematic Diagram of Experimental Setup meter. The energy consumption of system was measured with the help of energy meter of Âą0.5% accuracy which was connected across the compressor. An agitator was provided to maintain the uniform bulk temperature of water in evaporator. Refrigerant from evaporator entered into hermetically sealed compressor.

High pressure superheated vapours emerging from compressor entered into condenser. The superheated vapours were condensed in coiled condenser which was filled with water. The submergible pump was used to circulate the water of condenser through cooling unit. High pressure saturated liquid refrigerant from condenser was collected in receiver to ensure a continuous supply of refrigerant to the capillary tube. Drier-cum-filter was also installed after receiver to remove unwanted solid particles and moisture. A float type rotameter was installed after drier-cum-filter. A refrigerant sub-cooler was provided after the rotameter. The chilled water is supplied by separate refrigerating unit based on vapour compression cycle with R134a as a working fluid. The refrigerating unit consist of hermetically sealed compressor, an air cooled condenser and an evaporator. Evaporator of cooling unit is dipped in tank filled with water. Refrigerant liquid line also passes through this sub cooler tank.

This chilled water was used to subcool the refrigerant. A preheater was provided after subcooler to fix the temperature at the inlet of capillary tube. Amount of heat supplied by preheater was controlled by variac. A hand operated expansion valve was also provided after the condenser to make the pressure at the inlet of capillary tube constant by bypassing the excess refrigerant. Hand shutoff valve were provided at the both ends of the capillary tube. So that while changing the test section, no loss of refrigerant takes place. Five calibrated RTD (Pt100) temperature sensors with an accuracy of Âą0.25 K were attached to the external surface of copper tube at different locations of the setup. The working range of temperature sensors is between -200

C to 800 C. Experimental setup was instrumented with compound

pressure gauges with an accuracy of Âą0.25% at inlet and outlet of test section for measuring suction and discharge pressure.

The system was flushed with nitrogen gas before charging LPG to remove impurity and moisture. Then LPG was charged into the system. The system attains steady state condition after a run of three hours, after which experimental observations were made. The ambient temperature was maintained at 40

C throughout the work. In this investigation,

experiments were performed with capillary tubes of 1.12 mm, 1.40 mm and 1.52 mm internal diameters. Lengths of capillary tubes used in the experiment are 2.5 m, 2.9 m, 3.3 m, 3.7 m, 4.1 m and 4.5 m. These were shaped into straight as

77

Experimental Investigation on the Performance of Coiled Adiabatic Capillary Tube with Lpg as Refrigerant

well as helical coil of diameters 70 mm, 150 mm and 190mm. This makes total 72 test sections. The pressure at capillary inlet were adjusted to 760 kPa. The inlet subcooling was fixed to be 1

C.

RESULTS AND DISCUSSIONS Variation of Mass Flow Rate The thermo-fluid-dynamic behaviour of refrigerant flow in capillary tubes depends on capillary tube length, capillary tube diameter, capillary tube inlet pressure, degree of subcooling and the type of refrigerant. The effect of coil diameter, capillary tube diameter and capillary tube length on the refrigerant mass flow rate has been discussed below.

Effect of Capillary Tube Length 1.52 mm

1.40 mm

1.12 mm

3

3.5

Mass flow rate, áš (kg/h)

18

13

8

3 2

2.5

4

4.5

Capillary tube length,L (m)

Figure 2: Variation of Mass Flow Rate with Capillary Tube Length The effect of length on refrigerant mass flow rate can be studied by considering the capillary tubes which have different length but have same all other parameters such as capillary tube diameter, inlet pressure, degree of subcooling.

The variation of refrigerant mass flow rate with the length of capillary tube is shown in figure 2. It has been found that with increase in length of capillary there was decrease in mass flow rate because of increase in frictional resistance which is directly proportional to the length of capillary tube. With 1.52 mm inner diameter capillary, when the length was increased from 2.5 m to 2.9 m, 3.3 m, 3.7 m, 4.1 m and 4.5 m, the mass flow rate was decreased by 8 percent, 14 percent, 16 percent, 18 percent and 28 percent respectively. With 1.40 mm capillary diameter, for the similar increment of length of capillary tube, the mass flow rate was decreased by 6 percent, 15 percent, 18 percent, 19 percent and 20 percent respectively. With 1.12 mm capillary diameter, for the similar increment of length of capillary tube, the mass flow rate was decreased by 10 percent, 11 percent, 14 percent, 20 percent and 27 percent respectively.

Effect of Capillary Tube Inner Diameter The effect of capillary tube diameter on refrigerant mass flow rate can be studied by taking that capillaries that have different inner diameters but have same length, inlet pressure and degree of subcooling. The variation of mass flow rate with capillary tube inner diameter was shown in figure 3. As the capillary diameter was increased, there was increase in the refrigerant mass flow rate due decrease in frictional resistance. With 2.5 m capillary length, when the capillary diameter was increased from 1.12 mm to 1.40 mm and 1.52 mm, then the mass flow rate was increased by 63 percent and 108 percent respectively. With 2.9 m capillary length, when the capillary diameter was increased from 1.12 mm to 1.40

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Sanjeev Singh Punia & Jagdev Singh

mm and 1.52 mm, then mass flow rate was increased by 70 percent and 114 percen percentt respectively. With 3.3 m capillary length, when the capillary diameter was increased from 1.12 mm to 1.40 mm and 1.52 mm, then mass flow rate was increased by 55 percent and 91 percent respectively. With 3.7 m capillary length, when the capillary diamete diameter was increased from 1.12 mm to 1.40 mm and 1.52 mm, then mass flow rate was increased by 57 percent and 105 percent respectively. With 4.1 m capillary length, when the capillary diameter was increased from 1.12 mm to 1.40 mm and 1.52 mm, then mass flow rate te was increased by 67 percent and 115 percent respectively. With 4.5 m capillary length, when the capillary diameter was increased from 1.12 mm to 1.40 mm and 1.52 mm, then mass flow rate was increased by 78 percent and 103 percent respectively.

Mass flow rate, ṁ (kg/h)

2.5 m

2.9 m

3.3 m

3.7 m

4.1 m

4.5 m

15

10

5 1

1.1

1.2

1.3

1.4

1.5

1.6

Capillary tube inner diameter, d (mm)

Figure 3: Variation of Mass Flow Rate with h Capillary Tube Inner Diameter Effect of Coil Diameter The flow through the helical capillary tube is complicated as compared to straight capillary tube. The frictional resistance of single phase fluid flow through curved curved tube is larger than that for flow through straight capillary tube under similar conditions. The fluid flowing in the tube undergoes a centrifugal force, which results in secondary flow. The existence of secondary flow is called the Dean effect. The variation variation of mass flow rate with capillary coil diameter is shown in figure 4. As the straight capillary tube was coiled to diameter 190 mm, 150 mm and 70 mm, the average decrease in mass flow rate was 5 percent, 9 percent and 12 percent respectively.

1.52 mm

1.40 mm

1.12 mm 1.52 mm

L = 2.5 m Mass flow rate, ṁ (kg/s)

Mass flow rate, ṁ (kg/s)

19

14

9

4

19

1.40 mm

1.12 mm

L=2.9m

14

9

4 50

100

150

Capillary coil diameter, D(mm)

200

50

100

150

Capillary coil diameter, D(mm)

200

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Experimental Investigation on the Performance of Coiled Adiabatic Capillary Tube with Lpg as Ref Refrigerant

1.52 mm

1.12 mm

1.52 mm

L=3.3 m Mass flow rate, ṁ (kg/s)

Mass flow rate, ṁ (kg/s)

14

1.40 mm

9

14

1.12 mm

L=3.7 m

9

4

4 50

100

150

200

50

Capillary coil diameter, D(mm)

100

1.40 mm

150

200

Capillary coil diameter, D(mm)

1.52 mm 1.52 mm

1.40 mm

1.12 mm

1.12 mm 14

L=4.5m

14 L=4.1 m

Mass flow rate, ṁ (kg/s)

Mass flow rate, ṁ (kg/s)

1.40 mm

9

4

9

4 50

100

150

200

Capillary coil diameter, D(mm)

50

100

150

200

Capillary coil diameter, D(mm)

Figure 4: Variation of Mass Flow Rate with Capillary Coil Diameter

Effect of Coil Diameter on Compressor Energy Consumption The compressor energy consumption is plotted against different daimeters of coil as well as straight capillary tube.

These observ

C fixed ambient temperature. Figure 5 and 6 shows the variation of the compressor

energy consumption with coil diameter of capillary tube. Figure 5 shows the plot of compressor energy consumption with capillary coil diameter for 1.12 2 mm internal diameter capillary tube. Figure 6 shows the plot of compressor energy consumption with capillary coil diameter for 1.40 mm internal diameter capillary tube. In all these observations, the refrigeration effect was fixed. The refrigerating effe effect ct was 316.8 KJ for every observation. Compressor work was decreased by 72 percent, when capillary geometery was changed from 70 mm coil to straight for 1.12 mm capillary internal diameter. For similar change in geometery of capillary tube compressor work is decreased by 85 percent for 1.40 mm diameter.

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Sanjeev Singh Punia & Jagdev Singh

Compressor Work (KJ)

800

720

700 600

504

500 360

400 300

204

200 100 0 70

150

190

Straight Coil Diameter of capillary tube (mm)

Figure 5: Variation off Compressor work with Coil Diameter of 1.12 mm m Capillary Tube

Compressor Work (KJ)

1200

d=1.40m

1008

1000 800

612

600 306

400

144

200 0 70

150

190

Straight Coil diameter of capillary tube (mm)

Figure 6: Variation off Compressor work with Coil Diameter of 1.40 mm m Capillary Tube Effect of Coil Diameter on COP of the he System The COP is plotted against different diameters of coil as well as C ambient temperature. Figure 7 shows the plot between COP and coil diameter for 1.12 mm capillary tube and Figure 8 shows the similar plot for 1.40 mm capillary tube. When geometery of capillary tube was changed from 70mm coil to straight, COP of system increased by 250 percent and 600 percent for 1.12 mm capillary and

COP

1.40 mm capillary respectively.

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

1.55

d=1.12

0.88 0.63 0.44

70

150

190

Straight Diameter of helical coil capillary tube, d (mm)

Figure 7: Variation off COP with Coil Diameter with 1.4 mm Capillary Diameter

Experimental Investigation on the Performance of Coiled Adiabatic Capillary Tube with Lpg as Ref Refrigerant

2.5

d=1.4 mm

81

2.20

COP

2.0 1.5 1.04 1.0 0.52 0.5

0.31

0.0 70

150

190

Straight

Diameter of helical coiled capillary tube, D (mm)

Figure 8: Variation off COP with Coil Diameter with 1.12 mm m Capillary Diameter

CONCLUSIONS This investigation has studied the effects of capillary diameter, length and capillary coil diameter on the mass flow rate of LPG. Out of these parameters parameters,, capillary tube diameter effects the mass flow rate more significantly than the other variables. Mass flow rate increases with increase in capillary inner diameter and coil diameter where as mass flow rate decreases with increase in length. Performance of system was also calculated for different coil diameter of helical coiled capillary tube. It was observed that the COP of system increases with increase in coil diameter of capillary tube. For similar change in geometry of capillary tube, 1.40 mm diameter capillary effected the system more as compared to 1.12 mm diameter capillary tube.

REFRENCES 1.

Bolstad, M.M., Jordan, R.C. (1948). Theory and use of the capillary tube expansion device. Refrigerating Engineering, 519-523.

2.

Dean, W.R. (1927). Note on the mot motion of fluid in a curved pipe. Phil. Mag. 4, 208-223.

3.

Deodhar, D.S., Prabhu, V.S., Iyer, K.N. (2006). Experimental and numerical investigations of two two-phase flow through (adiabatic and heated) and helically coiled (adiabatic) capillary tubes. 18th National and 7th ISHMTASME Heat and Mass Transfer conference, IIT Guwahati, India.

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Douglas, J.F., Gasiorek, J.M., Swaffield, J.A. (1995). Fluid Mechanics. Addison-Wesley Addison Wesley Longman Inc., Edinburg, UK, 299 pp.

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Khan, M.K., Sahoo, P.K., Kumar, R. (2008a). Experimenta Experimental study of the flow of R-134a 134a through a adiabatic helical coiled capillary tube. HVAC&R Res. 14(5), 749 749-762.

6.

Khan, M.K., Sahoo, P.K., Kumar, R. (2008b). An experimental study of the flow of R R-134a inside inside an adiabatic spirally coiled capillary tube. International Journal of Refrigeration 31(6), 970-978.

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Kuehl, S.J., Goldschmidt, V.W. (1990). Steady flows of R R-22 22 through capillary tubes: test data. ASHRAE Trans.

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McLinden, M.O., Klien, S.A., Lemmon, E.W. (2002). REFPROP REFPROP-Version 7- Thermodynamic Thermodynami and transport properties of refrigerants and refrigerant mixtures. NIST Standard Reference Database.

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9.

Milkol, E.P. (1963). Adiabatic single and two-phase flow in small bore tubes. ASHRAE Journal, 75-86.

10. Mittal, M.K., Kumar, R., Gupta, A. (2009). Numerical analysis of adiabatic adiabatic flow of refrigerant through a spiral capillary tube. International Journal of Thermal Sciences, 48, 1348-1354.

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12. Zhou, G., Zhang, Y. (2012). Inlet pressure fluctuation characteristics of coiled capillary tubes. Applied Thermal Engineering, 33-34, 183-189.

13. Zhou, G., Zhang, Y. (2006). Numerical and experimental investigations on the performance of coiled adiabatic capillary tube. Applied Thermal Engg. 26, 1106-1111.

14. Zhou, G., Zhang, Y. (2006). Experimental investigation on the hysteresis effect of refrigerant flowing through a coiled adiabatic capillary tube. Energy Conservaion Manag. 47, 3084-3093.