A STUDY OF THE FLOW OF REFRIGERANT THROUGH A CAPILLARY TUBE

Transcription

1 A STUDY OF THE FLOW OF REFRIGERANT THROUGH A CAPILLARY TUBE A DISSERTATION Submitted in partial fulfillment of the requirements for the award of the degree of MASTER OF TECHNOLOGY In MECHANICAL ENGINEERING (With Specialization in Thermal Engineering) 0 NITIN RAI EpF TECMxp~ 5 1,o - ~' ~' r RdO~K~ DEPARTMENT OF MECHANICAL AND INDUSTRIAL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY ROORKEE ROORKEE (INDIA) JUNE, 2012

2 INDIAN INSTITUTE OF TECHNOLOGY ROORKEE CANDIDATE'S DECLARATION I hereby declare that the work carried out in this thesis entitled "A STUDY OF TIIE FLOW OF REFRIGERANT TIIROUGII A CAPILLARY TUBE ", submitted in partial fullilhnent of the requirements for the award of degree of Master of Technology with specialization in Thermal Engineering, submitted to the Department of Mechanical & Industrial Engineering, Indian Institute of Technology Roorkee (India), is an authentic record of my own work carried out under the supervision of Dr. Ravi kumar, MIED, IIT Roorkee, India. Date: June 2012 Place: Roorkee (NITIN RAI) CERTIFICATION This is to certify that the above statement made by the candidate is correct to the best of our knowledge and belief. (Ravi-Kumar)'\ L 1 ) Professor M.I. E. D. SIT Roorkee (Akh esh Gupta Professor M.I.E.D. IIT Roorkee

3 ACKNOW LED GEMENT I wish to express my immense pleasure and sincere thanks to Prof. Ravi Kumar, Professor and Prof. Akhilesh Gupta, in the department of Mechanical & Industrial Engineering, IIT, Roorkee for their valuable guidance and support. This work is simply the reflection of their thoughts, ideas and concepts. Working under their guidance was a privilege and an excellent learning experience that I will cherish for life time. I express my sincere thanks to all the teaching and non teaching staff members of the department who have contributed directly or indirectly in successful completion of my dissertation work. I am extremely grateful to friends and well-wishers for their candid help, meaningful suggestions and persistent encouragement given to me at different stages of my work. Finally, I would like to say that I am indebted to my parents for everything that they have given to me. I thank them for the sacrifices they made so that I could grow up in a learning environment. They have always stood by me in everything I have done, providing constant support, encouragement and love. Al Date:~IJune Place: Roorkee. Nitin Rai M. Tech. (Thermal Engg.) Enrollment No MIED, IIT-Roorkee.

4 Table of Contents Candidate's Declaration Acknowledgements Table of Contents Nomenclature Abstract i ii iii vii viii Chapter 1: Introduction Capillary tube Types of capillary tube Adiabatic straight capillary tube Diabatic straight capillary tube Selection of Refrigerant on The Basis of environmental Concern Need of Present Study Objectives 6 Chapter 2: Literature review 7-12 Chapter 3: Experimental Set-up and procedure 3.1 Structural Layout Major Components Compressor Condenser Evaporator Subcooler Chiller unit 20

5 3.2.6 Pre-heater Test-section Straight capillary tube test-section Measurements Temperature measurement Mass flow rate measurement Pressure measurement Calibration of measuring instruments Calibration of Pressure gauge Calibration of thermocouple Leak detection Experimental procedure 31 Chapter 4: Result and discussion 4.1 Effect of inlet sub-cooling on mass flow rate Effect of inlet quality on mass flow rate Effect of tube length on mass flow rate Effect of tube diameter on mass flow rate Temperature profile Temperature profile for inlet subcooling Temperature profile for inlet quality 0i Chapter 5: conclusion 47 References 48 Appendix A: Uncertainty analysis 50 Appendix B: Experimental data 53 iv

6 List of figures 1.1 Adiabatic capillary tube (a) block diagram (b) p-h diagram Diabatic capillary tube (a) block diagram (b) P-h diagram (a) Lateral arrangement (b) Concentric arrangement Structural lay out of the experimental set-up Photographic view of the experimental set-up Details of the compressor of the experimental set-up (a) Water-cooled condenser (b) Details of the water- cooled condenser Details of the evaporator of experimental set-up Details of subcooler Photographic view of chiller unit straight capillary tube test-section Temperature measurement details Mass flow measurement details Dead weight pressure calibrator Pressure calibration curve Thermocouples calibration curve (a) effect of inlet subcooling on mass flow rate for L = 2 m and D = 1.12 mm (b) effect of inlet subcooling on mass flow rate for L = 1.6 m and D = 1.12 mm (c) effect of inlet subcooling on mass flow rate for L = 1 m and D = 1.12 mm (d) effect of inlet subcooling on mass flow rate for L = 2 m and D = 1.27 mm (e) effect of inlet subcooling on mass flow rate for L = 1.6 m and D = 1.27 mm 36 u

7 4.1 (f) effect of inlet subcooling on mass flow rate for L = 1 m and D = 1.27 mm (a) effect of inlet quality on mass flow rate for L = 2 m and D = 1.12 mm (b) effect of inlet quality on mass flow rate for L = 1.6 m and D = 1.12 mm (c) effect of inlet quality on mass flow rate for L = 1 m and D = 1.12 mm (d) effect of inlet quality on mass flow rate for L =2 m and D = 1.27 mm (e) effect of inlet quality on mass flow rate for L = 1.6 m and D = 1.27 mm (f) effect of inlet quality on mass flow rate for L = I m and D = 1.27 mm (a) variation of mass flow rate with inlet subcooling for different length (b) variation of mass flow rate with inlet quality for different length (c) variation of mass flow rate with inlet quality for different length (d) variation of mass flow rate with inlet quality for different length (a) variation of mass flow rate with inlet subcooling for different length and Diameter (b) variation of mass flow rate with inlet quality for different length and Diameter (a) variation of temperature with position for different inlet subcooling (b) variation of temperature with position for different inlet quality 46 List of tables A. 1 Experimental data of adiabatic capillary tube D = 1.12 mm 53 A.2 Experimental data of adiabatic capillary tube D = 1.27 mm 54 vi

8 Nomenclature D L capillary tube diameter capillary tube length Tsub inlet subcooling at the inlet of capillary tube, C Pin h CFC inlet pressure at the capillary tube inlet, Pa enthalpy, J /kg Chlorofluorocarbons vii

9 Abstract Present experimental work has been done to study the flow of refrigerant R-134a through the straight adiabatic capillary tube with different lengths and diameters. The effect of tube diameter and tube length on the mass flow rate has been studied. The mass flow rate increases with increase of the inlet sub-cooling and tube diameter and mass flow decreases with increase in inlet sub-cooling and tube length. The mass flow of the refrigerant is more sensitive to the tube diameter as compared to the other parameters i.e. when tube diameter increases, the mass flow rate of the refrigerant increases. The mass flow rate is a function of the inlet sub-cooling and inlet quality. The temperature at the end of the capillary tube is less in case of inlet quality as compared to the inlet sub-cooling. Temperature remains almost constant when refrigerant in liquid phase (single phase). The temperature at the end of the capillary tube is less in case of small tube length and larger tube diameter. As refrigerant goes into two-phase (liquid vapor mixture) the temperature of the refrigerant decreases continuously. This report presents some basic features of flow of refrigerant through capillary tube as expansion device, the details of the components of the. experimental set-up, calibration of pressure gauges and thermocouples and results of the present study. VIII

10 Chapter 1 INTRODUCTION An expansion device is another basic component of a refrigeration system. The basic functions of an expansion device used in refrigeration systems are to: 1. Reduce pressure from condenser pressure to evaporator pressure, and 2. Regulate the refrigerant flow from the high-pressure liquid line into the evaporator at a rate equal to the evaporation rate in the evaporator. Under ideal conditions, the mass flow rate of refrigerant in the system should be proportional to the cooling load. Sometimes, the product to be cooled is such that a constant evaporator temperature has to be maintained. In other cases, it is desirable that liquid refrigerant should not enter the compressor. In such a case, the mass-flow rate has to be controlled in such a manner that only superheated vapor leaves the evaporator. Again, an ideal refrigeration system should have the facility to control it in such a way that the energy requirement is minimum and the required criterion of temperature and cooling load are satisfied. Some additional controls to control the capacity of compressor and the space temperature may be required in addition, so as to minimize the energy consumption. The expansion devices used in refrigeration systems can be divided into fixed opening type or variable opening type. As the name implies, in fixed opening type the flow area remains fixed, while in variable opening type the flow area changes with changing mass flow rates. There are basically seven types of refrigerant expansion devices. These are: 1. Hand (manual) expansion valves 2. Capillary Tubes 3. Orifice 4. Constant pressure or Automatic Expansion Valve (AEV) 5. Thermostatic Expansion Valve (TEV) 6. Float type Expansion Valve a) High Side Float Valve b) Low Side Float Valve 7. Electronic Expansion Valve 1

11 Of the above seven types, Capillary tube and orifice belong to the fixed opening type, while the rest belong to the variable opening type. Of the above seven types, the hand operated expansion valve is not used when an automatic control is required. The orifice type expansion is used only in some special applications. Hence these two are not discussed here. 1.1 Capillary tube: A capillary tube is a long, narrow tube of constant diameter. The word "capillary" is a misnomer since surface tension is not important in refrigeration application of capillary tubes. Typical tube diameters of refrigerant capillary tubes range from 0.5 mm to 3 mm and the length ranges from 1.0 m to 6 m. The pressure reduction in a capillary tube occurs due to.the following two factors: 1. The refrigerant has to overcome the frictional resistance offered by tube walls. This leads to some pressure drop, and 2. The liquid refrigerant flashes (evaporates) into mixture of liquid and vapor as its pressure reduces. The density of vapor is less than that of the liquid. Hence, the average density of refrigerant decreases as it flows in the tube. The mass flow rate and tube diameter (hence area) being constant, the velocity of refrigerant increases since m = pva. The increase in velocity or acceleration of the refrigerant also requires pressure drop. -Capillary tubes are used as expansion device in low capacity refrigerating machines like domestic refrigerators and window type room air conditioners. In general, the capillary tubes are used where the cooling load is fairly constant and the cooling capacity id below 3 TR. Capillary tube is a long narrow hollow drawn copper tube with an internal diameter ranging from.5 to 2.0 mm and length from 2 to 6 m. Together with evaporator, compressor and condenser, capillary tube forms a closed circuit in a vapour compression cycle, shown in figure 1.1. The vapour compression systems are widely adopted in refrigeration as well as comfort and process air conditioning. As compared to other expansion devices, viz., automatic and thermostatic expansion valves, the capillary tube is simple in construction,, has no moving parts and all above it is inexpensive and capillary tube permits the 2

12 compressor to start in an unloaded condition by allowing the pressure between the condenser and evaporator to equalize during the off cycle, thus, reducing the starting torque of the compressor. Hence, low starting torque motor can be used with the vapor compression system having capillary tube as an expansion device. 1.2 Types of capillary tube A capillary tube can be classified on the basis of the flow arrangement adiabatic and diabatic (also known as non adiabatic) and on the basis of geometrical shape straight, helical and spiral. In adiabatic arrangement, the capillary tube is insulated and, thus, no heat transfer takes place between the capillary tube and its surroundings, whereas, in the diabatic arrangement, the capillary tube is soldered or brazed on the compressor suction line and the heat exchange between the two tubes take place. The diabatic arrangement is preferred over adiabatic arrangement because of the increased refrigerating effect achieved in the former arrangement. Capillary tube of each of the above stated geometries is possible for both the flow arrangement i.e. adiabatic and diabatic Adiabatic straight capillary tubes [10] I h Fig 1.1 Vapor compression refrigeration system employing adiabatic capillary tube (a) block diagram (b) p-h diagram In adiabatic capillary tube, as shown in Fig 1.1 (a), the refrigerant expands from high pressure side to low pressure side adiabatically. The refrigerant enters the capillary in a subcooled liquid state. As the liquid refrigerant flows through the capillary, the pressure 3

13 drops linearly due to friction while the temperature remains constant. As the pressure falls below the saturation pressure corresponding refrigerant temperature, a fraction of liquid refrigerant flashes into vapor. The inception of vaporization gives rise to two-phase flow in the capillary tube. This causes an increase in the vapour quality and fluid velocity, resulting in an additional pressure drop called acceleration pressure drop. The increased pressure causes the temperature of the refrigerant to fall rapidly as in the two-phase region the temperature is a function of pressure. Fig. 1.1(a) shows the vapour compression system employing the adiabatic capillary tube. The process 3-4 in Fig. 1.1(b) represents the adiabatic expansion of the high pressure liquid refrigerant. Since the flow is adiabatic inside capillary tube, the refrigerant temperature remains constant as long as it is in liquid state and as soon as the flashing occurs the temperature falls rapidly. The process is adiabatic rather than isenthalpic and as such the enthalpy is constant till the flashing occurs. As a result of flashing, the a part of total energy gets converted to kinetic energy and as such the enthalpy falls in the latter part of the capillary tube Diabatic straight capillary [10] In a diabatic flow arrangement, shown in Fig.1.2 (a), the capillary tube is bonded with the cold compressor suction line in a counter flow heat exchange arrangement. The advantage of employing diabatic capillary tube results in higher refrigerating effect and thus a better system performance. a Condenser b Diabatic 2 Capillary Compressor p Evaporator h Fig. 1.2 Diabatic capillary tube (a) block diagram (b) P-h diagram 0

14 As can be seen from the Fig. 1.2(b), the process of expansion as shown by the process 4-- 5, the temperature of the hot liquid refrigerant emerging from the condenser continues to fall due to the thermal contact with the cold suction line. Consequently, the enthalpy of the refrigerant falls continuously throughout the capillary tube length. The heat transfer from the capillary tube to the compressor suction line also causes a delay in flash point and increase in the refrigerant effect. On the other hand, on receiving heat from the capillary tube, the cold vapour in the suction line gets superheated and thus diminishing the chances of liquid entering the compressor. The thermal contact between the capillary tube and the compressor suction line can be attained in two ways i.e. lateral and concentric arrangement. The two arrangements are shown in figure 1.3. a capillary tube T capillary -_. suction line Fig 1.3 (a) Lateral arrangement (b) Concentric arrangement [10] In lateral configuration, the capillary tube is bonded with the compressor suction line by means of a solder or brazing joint. Further, the copper tape is wrapped around the two bonded tubes, so that, the walls of the tube could be attain the thermal equilibrium. In concentric arrangement, the capillary tube occupies the core of compressor suction line as shown in figure 1.3 (b). 1.3 Selection of The Refrigerant On The Basis Of Environmental Concern The use of chloro-flouro carbons (CFCs) has been banned because it has high ozone depletion potential. In fact the earth is enveloped in a thin shell of ozone layer. The ozone layer prevents the earth from harmful ultra-violet radiation coming from the sun CFCs are believed to be the most detrimental to the protective layer of ozone in the upper atmosphere, the strong solar radiation break down the CFCs freeing chlorine atom from the 5

15 structure. This chlorine reacts with ozone and converts it to oxygen. The conversion of ozone into oxygen will ultimately cause thinning of the layer to the extent that a hole is formed. Keeping in view of environmental concerns, the refrigerant R-134a has been selected for the present experiment. 1.4 Need of Present Study The conventional refrigerants have varying degree of ozone depletion potential. The phase-out of chlorofluorocarbons (CFC's), as mandated by the Montreal Protocol, has created great interest in the development of new ozone-friendly refrigerants. These new refrigerants have made it necessary to develop new rating methods for the prediction of adiabatic capillary tube flow rates. 1.5 Objectives The present experimental study has been conducted to investigate the flow of refrigerant R-134a through capillary tube with different length and diameter under adiabatic flow condition. The objectives of present study have been discussed below. > To investigate the effect of inlet subcooling, inlet quality, capillary tube diameter and length on the mass flow rate of the refrigerant. > To investigate the temperature profile of refrigerant flowing through capillary tube. L

16 Chapter 2 LITERATURE REVIEW This chapter presents a review of the literature on the flow of refrigerants through a capillary tube. The capillary tube is a major component of the small capacity vapor compression refrigeration systems. In these systems, the capillary tube is used as an expansion device. Kim et al. (2001) has developed a dimensionless correlation on the basis of experimental data of adiabatic capillary tubes for R-22 and its alternatives, R-407c (R32/125/134a, 23/25/52 w.t %) and R-410a ((R32/125, 50/50 wt. %). Several capillary tubes with different length and inner diameter were selected as test sections. Mass flow rate through the capillary tube was measured for several condensing temperatures and various degrees of subcooling at the inlet of each capillary tube. Experimental conditions for the condensing temperatures were selected as 40, 45 and 50 C, and the degrees of subcooling were adjusted to 1.5, 5 and 10 -C. Mass flow rates of R407C and R410A were compared with those of R22 for - the same test conditions. The results for straight capillary tubes were also compared with those of coiled capillary tubes. A new correlation based on Buckingham it theorem to predict the mass flow rate through the capillary tubes was presented based on extensive experimental data for R22, R407C and R410A. Dimensionless parameters were chosen considering, the effects of tube geometry, capillary tube inlet conditions, and refrigerant properties. The Performances of the adiabatic capillary tubes with several length and inner diameter combinations for R22 and its alternatives, R407C and R410A were experimentally investigated. The mass flow rates of R407C are greater by 4.0%, and those of R410A are greater by 23% as an average, than those of R22. The mass flow rates for coiled capillary tubes are quite reduced when compared with those for straight capillary tubes especially for the cases where the coiled diameter is reduced. The mass flow rates in capillary tubes with coiled diameter of 40 mm are approximately 9% less than those of straight capillary tubes. Dimensionless correlation was developed to predict mass flow rates through adiabatic capillary tubes as a function of several dimensionless parameters based on the Buckingham f) theorem. The deviation of experimental results for R22, R407C and R410A from the dimensionless correlation in this study lies between -12% and +12% for all FA

17 test conditions, and RMS (root mean square) of relative errors is 3.2% on average. Fiorelli et al. has done an experimental study on HCFC 22 alternative refrigerant mixtures flow through capillary tubes. Results for R-410A and R-407C showed that the main operational parameters affect in a similar way the performance of capillary tubes for both refrigerants. The influence of geometry on the behavior of capillary tubes in refrigeration systems is characterized. An analysis on the differences in R-410A and R-407C flow through capillary tubes is also performed. This work presents the results of an extensive experimental survey on R-407C and R-410A flow through capillary tubes. Such survey, which was carried out for both subcooled and two-phase inlet conditions, characterized the influence of these refrigerants, as well as the several operating and geometric parameters on the behavior of capillary tubes used in refrigeration systems. It could be seen that the main operational parameters (inlet and outlet conditions) affects in a similar way the performance of capillary tubes for both refrigerants. The study characterised the influence of geometry (diameter and length) on the behaviour of capillary tubes used in small refrigeration systems, and analysed the differences in R-410A and R-407C flow through capillary tubes. Jiraporn et al. (2002) developed a mathematical model to study the flow characteristics in non-adiabatic capillary tubes. The theoretical model is based on conservation of mass, energy and momentum of fluids in the capillary tube and suction line. The mathematical model is categorized into three different cases, depending upon the position of the heat exchange process. The first case is considered when the heat exchange process starts in the singlephase flow region, the second case is determined when the heat exchange process starts at the end of the single-phase flow region, and the last case is considered when the heat exchange process takes place in the two-phase flow region. A set of differential equations is solved by the explicit method of finite-difference scheme. The model is validated by comparing with the experimental data obtained from previous works. The following conclusions are obtained from the present study: Case 1: The heat exchange process starts in the single-phase flow region with the refrigerant leaving the heat exchanger as subcooled or saturated liquid. The present model is validated with the measured mass flow rate of Bittle et al. [1] for R152a as well as Liu and Bullard [3] and Peixoto [2] for R134a and is found that the average discrepancies are 3.2%, 3.5% and 4.8%, respectively. The average discrepancy of the capillary tube length obtained from the 8

18 comparison with the experimental data of Melo et al. [27] for R134a is 4.8%. Case 2: The heat exchange process starts at the end of the single-phase flow region. The comparison that the linear vapour quality model gives complete agreement for the pressure variation along the capillary tube. Case 3: The heat exchange process starts in the two-phase flow region. The linear vapour quality model is used in the heat exchange region. The present model gives fair agreement with the measured mass flow rate of Peixoto [2] for R134a by giving a discrepancy of 14.3%. It is also compared with the experimental data of Pate and Tree [18] for R12 and gives a discrepancy in length of 9.50%. Numerical results reveal that, it is possible to use the present calculation to predict the flow characteristics of fluids in capillary tube and suction line, according to the position of the starting point of the heat exchange process. The present model allows for a number of parameters to be included for any given computation. Type of refrigerant, condenser temperature, evaporator temperature, suction line inlet temperature, degree of sub--cooling, inner diameter of capillary tube and suction line, initial length of capillary tube, heat exchange region length, relative roughness and an appropriate two-phase viscosity model can be varied in turn for different mass flow rate of refrigerant to determine the final length of capillary tube. Jabaraj et al. (2006) they have done an experimental study on the flow characteristics of HFC407C/HC600a/HC290.a new refrigerant mixture in adiabatic capillary tubes. The experimental work investigates the effect of capillary length, capillary diameter, condensing pressure, and refrigerant sub-cooling on the mass flow rate through the capillaries. They have developed a new correlation to predict the mass flow rate through the adiabatic capillary tubes for different operating conditions, capillary tube geometry and refrigerant properties based on the experimental data for HCFC22 and HFC407C/HC600a/HC290 refrigerant mixture. The mass flow rates through the adiabatic capillary tubes with different tube inlet conditions and tube geometries were measured for the refrigerants HCFC22 and M20. The mass flow rates of M20 are higher than those of HCFC22 by 3.825% on an average. Based on the measured mass flow rates of HCFC22 and M20 a non-dimensional correlation was developed to predict the mass flow rates through the adiabatic capillary tubes as a function of several non-dimensional parameters using the Buckingham it theorem. The developed correlation for mass flow rate has good agreement with the measured data for HCFC22 and M20 with average deviations of 0.618% and -0.11%.and mean deviations of 5.536% and 4.448%, respectively. Also the present correlation matches a

19 very well with the results in published literature reported by previous investigators. Park et al. (2007) have been developed a generalized correlation for the prediction of mass flow rate through the both straight and coiled capillary tubes by implementing dimensionless parameters that were generated using the Buckingham it-theorem considering the effects of tube inlet conditions, coiled tube geometries, and refrigerant properties. The coiled effects of the coiled capillary tube were considered by introducing the capillary equivalent length Le. This correlation deviates from the present database for R-22, R-410a and R-407c were between -8.3% and 8.8%. The mass flow rate of R22 through the coiled capillary tubes and straight capillary tubes was measured for various operating conditions and tube geometries. It has been found from the experiment that the mass flow rates of the coiled capillary tubes decreased by 5 to 16% more than those of the straight capillary tubes under the same operating conditions due to increased flow friction resulting from strong coiled effects. Khan et al. (2008) performed an experiment on the flow of refrigerant R-134a inside an adiabatic spirally coiled capillary tube and developed the correlation to predict the mass flow rate. The effect of various geometric parameters like capillary tube diameter, length and coil pitch for different capillary tube inlet sub-cooling on the mass flow rate of R-134a through the spiral capillary tube geometry has been investigated. It has been found that the coil pitch significantly influences the mass flow rate of R- 134a through the adiabatic spiral capillary tube. The effect of coiling of capillary tube reduces the mass flow rate by 5-15% as compared to those of the straight capillary tube operating under similar conditions. The proposed correlation predicts more than 91% of the mass flow rate which is in agreement with measured data in an error band of ±10%. Khan et al. (2009) has carried out an experimental investigation to investigate the effects of various geometric parameters on the mass flow rate of R-134a through diabatic spiral capillary tube. The lateral type of diabatic capillary tube has been investigated in the present experimental study. The major geometric parameters investigated are capillary tube diameter, capillary tube length and coil pitch. In addition, effect of inlet sub-cooling on the mass flow rate through diabatic spiral capillary tube is also done. A comparison of the performance of diabatic spiral capillary tube has been made with adiabatic spiral capillary tube. Generalized empirical correlation for-diabatic spiral capillary tube has also been proposed. It has been found that the flow behaviour of the diabatic capillary tubes is entirely different from those of adiabatic capillary tubes. In case of diabatic capillary tube, the refrigerant mass flow has been found to be the function 10

20 of suction-line inlet superheat and heat exchange length in addition to capillary tube diameter, capillary tube length, coil pitch and capillary inlet sub-cooling. An empirical correlation for the refrigerant mass flow rate through diabatic spiral capillary tube geometry 'has been developed. It has been found the proposed correlation predicts the refrigerant mass flow rate in the error band of -7% of the measured experimental mass flow rate. Mittal et al. (2010) has done an experiment on the coiling effect on the flow of R-407C in an adiabatic helical capillary tube. It has been observed that the coiling of capillary tube significantly influences the mass flow rate of R-407C through the adiabatic helical capillary tube. For the sake of comparison, he has also been conducted experiments for the straight capillary tubes and he has been found that the mass flow rates in coiled capillary tube are 5-10 percent less than those in a straight one. The experiments data have been analyzed and non-dimensional correlation have been developed, for the prediction of mass flow rate of R-407C in straight and helical capillary tube. He has been found that the mass flow rate variation with sub-cooling is independent of capillary length, inlet pressure and coil diameter. As compared to the mass flow rate of R-407C in straight capillary tube, the mass flow rate in Coiled capillary with coil diameter of 60mm, 100mmand 140mmis reduced by an average of 10 percent, 7 percent and 5 percent respectively. The developed correlations for mass flow rate yield good agreement with the measured data of present study with deviations of ±10 percent. Also, the proposed correlations predict very well the results reported in the literature by previous investigators. Chingulpitak et al. (2011) they have done numerical investigation of the flow characteristics of helical capillary tubes compared with straight capillary tubes. The homogenous two-phase flow model developed is based on the conservation of mass, energy, and momentum of the fluids in the capillary tube. A homogeneous two-phase flow model has been developed to determine the refrigerant flow characteristics in adiabatic helical capillary tubes. The present model was validated by comparing with the experimental data of Zhou and Zhang [3] for R-22 and was found to give an average discrepancy at around 3%. The mass flow rates of refrigerant flowing through the helical capillary tubes are lower compared with those for the straight capillary tubes, especially for lower coil diameters. The mass flow rates of the helical capillary tubes with coil diameters of 40 mm, 80 mm, and 120 mm are decreased by 5-9% more than those of the straight capillary tubes under given conditions due to increased flow friction resulting from strong coil effects. Under the same operating conditions, the length of the helical 11

21 capillary tube is 20% shorter than that of the straight capillary tube. Finally, the numerical results for straight capillary tubes give reasonable predictions compared with the measured data of straight capillary tubes. Therefore, the present model can be applied to predict the flow characteristics of refrigerant flowing through straight capillary tubes using the Churchill friction factors equation. 12

22 The experimental set-up was fabricated in the Refrigeration and Air conditioning Laboratory to the study the flow of refrigerant R-134a through capillary tubes of different sizes. In this experimental set-up we assemble different components to form a vapor compression refrigeration cycle. The major components of this cycle are evaporator, compressor, condenser and expansion device, i.e., capillary tube. Degree of subcooling was attained bt the subcooler and varied by heating of refrigerant in the preheater. Both subcooler and preheater was installed after the condenser. The basic flow line of the refrigerant is made of copper tubes supported with hand shut off valves. The experiment were carried out at adiabatic flow through capillary tubes of different internal diameters and lengths. 3.1 Structural layout The schematic diagram of experimental set-up is shown in Fig 3.1. Refrigerant expands from high pressure to low pressure in the test section of copper capillary tube. After expansion, refrigerant entered into the evaporator coil which is made of copper. This copper coil was submerged into the water tank. An electric heater was used to given the heating load to the evaporator. This heating load was varied with the help of variac. An agitator was also used in the tank to maintain the uniform bulk temperature of water. The vapours coming out from the evaporator entered into the liquid accumulator so that liquid refrigerant cannot be entered into the compressor. Three phase electric motor was used to run the compressor with belt and pulley type arrangement. The saturated vapour coming out from the accumulator entered into the compressor where it was compressed to high pressure and temperature. The high pressure superheated vapours emerging from the compressor entered the oil separator. The refrigerants vapours coming out from oil separator entered into the water cooled condenser where it was condensed. The condenser pressure will varied by adjusting the temperature and mass flow rate of water circulating in the condenser. After condensation, liquid refrigerant sent to the receiver in order to continuous supply of refrigerant to the capillary tube. Drier-cum filter was used to remove the moisture and dust in the refrigerant. A hand operated expansion valve also used between the condenser and evaporator to bypass the refrigerant into the. evaporator to avoid the accumulation of refrigerant at the inlet of capillary tube and to control the evaporator 13

23 pressure. The refrigerant from the filter entered to the subcooler where the refrigerant gets subcooled. The chilled water was supplied to the subcooler by the separate chiller unit which was based on vapour compression cycle with R-22 as working fluid. The mass flow rate of the high pressure liquid refrigerant was measured by the roatameter. A preheater after'the subcooler was installed to vary the degree of subcooling at the inlet of capillary tube. The heat input to the preheater was controlled by the variac. A needle-type control valve was used before the inlet of the capillary tube to regulate the refrigerant pressure precisely at the inlet of capillary tube. A sight glass was used to visualise evaporator compressor oil seperator condensor liquid accumulator expansion drier cum filter Reciver sight glass Ii subcooler _ --- I Test-section chiller variac i = Rotameter Y 1 preheater pump Fig. 3.1 Structural layout of the experimental set-up the state of refrigerant at the inlet of capillary tube. A number of hand shutoff valves were also used in between the major components of the experimental set-up. Therefore, in case of leakage or any repair, the damaged component will retrieved with ease. The temperature at different locations of the set-up and the test section were measured by the T-type thermocouples. The pressure of refrigerant will be measured by the pressure gauges. 14

24 Accrte`... 0a1o. ~!1uj~:... 1J TTLTg ii) ('T Figure 3.2 photographic view of experimental set-up 15

25 3.2 MAJOR COMPONENTS The experimental set-up is a mechanical assembly of different components. The main components are evaporator, condenser, subcooler, chiller unit, preheater and all the test sections of straight and helical capillary tube. The test sections were designed and fabricated in the Refrigeration and Air conditioning laboratory. The details of major components of experimental set-up are described below Compressor aice.e toe suction em to condenser a from evaptxator Electric a olo! comprr.,o1,~ L,. Figure 3.3 Details of the compressor of the experimental set-up A reciprocating compressor is a positive displacement compressor that used piston driven by crank shaft to deliver gases at high pressure. The intake gas enters the suction manifold then flows into the compression cylinder where it gets compressed by a piston driven in a reciprocating motion by a crank shaft and is then discharged. In open pressurised system can leak its operating gases if it is not operated frequently enough. Open systems reloi on lubricant in the system to splash on pump components and seals slowly evaporates and then the seals begin to leak until the system is no longer functional and must be recharged and advantaged of an open compressor is that they can be driven by non electric power sources such as i.c. engine or turbine however open compressor that drives refrigeration system are generally not totally maintenance free through the life of the system since some gases leakage will occur over time The compressor is a major component of the experimental set-up. The compressor used in the experimental set-up is a single cylinder 16

26 open type R-134a reciprocating compressor of 1.5 TR capacity. The rated speed of the compressor is 550 rpm. It will run by three phase, 2.0 hp induction motor by means of belt and pulley arrangement as shown in figure3.3. The saturated vapour coming out from the evaporator will compress to the high pressure and temperature by this compressor Condenser F ~ P 9534' water. out 3.4 (a) Water-cooled condenser Condesor is a device which is desinged to condese a gas into a liquid. The Liebig condenser is the most basic water-cooled design. The inner-tube is straight, making it cheaper to manufacture. Though named after the German chemist Justus Baron von Liebig, he cannot be given credit for having invented it because it had already been in use for some time before him. However, it is believed that he popularized the device. The true inventors, all of them inventing it independently, were the German chemist Christian Ehrenfried Weigel in 1771, the French scientist, P.J. poisonnier in 1779, and the Finnish chemist Johan Gadolin in 1791.Liebig himself incorrectly attributed the design to the German pharmacist Johann Friedrich August Gottling who had made improvements to the Weigel design in The Liebig condenser is much more efficient than a simple retort due to its use of liquid for cooling. Water can absorb much more heat than the same volume of air, and its constant circulation through the water jacket keeps the condenser's temperature constant. Therefore, a Liebig condenser can condense a much greater flow of incoming vapour than an air condenser or retort. Additionally, stainless steel 'wool' or another heat-conductive, nonreactive material can be loosely placed in the inner cylinder of a Liebig condenser, 17

27 substantially increasing the reflux effect, and the overall efficiency of the distillation. The cooling water should flow from top to bottom in this instance, as that also increases the efficiency and helps prevent thermal shock to the glassware. The water cooled condense is using for the present experimental investigation, The water cooled. condenser is coil-in-shell type in which refrigerant flowing inside the coiled tube and tap water flowing in the in shell forming a counter flow heat exchanger. The high temperature and high pressure refrigerant will condense inside a water cooled condenser as shown in figure 3.4(b) : 12.54J 760 min Fig 3.4 (b) Details of the water- cooled condenser. The condenser was fabricated with a mild steel cylinder shell of 200 mm diameter and 760 mm length. Coiled copper tube which is inside the shell has 9.53 mm diameter and 12 m length. The condensing pressure of the refrigerant is controlled by the mass flow rate of the cooling water inside the condenser Evaporator An evaporator is used in an air conditioning system to allow the compressed cooling chemical, Freon, to evaporate from liquid to gas while absorbing heat in the process. It can also be used to remove water or other liquids from mixtures. The process of evaporation is widely used to concentrate foods and chemicals as well as. salvage solvents. In the concentration process, the goal of evaporation is to vaporize most of the water from a 18

28 solution containing the desired product. In the case of desalination of sea water, the reverse purpose applies; evaporation removes the desirable drinking water from the undesired product, salt. One of the most important applications of evaporation is in the food and beverage industry. Foods or beverages that need to last for a considerable amount-of time or need to have certain consistency, like coffee, go through an evaporation step during processing. Evaporator which is used in this set-up consisted of mild steel tank of dimensions 550 x 550 x 440 mm in which coiled copper tube of 11.5 mm diameter and 8 m length. The coiled copper tube was submerged into tank which was filled with Water up to 350 mm. the tank was insulated by the ceramic wool from outside. An electric heater of kw was used to provide the necessary heating-load to the water tank. A variac was used to control the heat input to the tank water. An agitator was also used to maintain the uniform bulk temperature in the tank water. After expansion, refrigerant entered in to the evaporator where it was heated by the hot water of the tank. Before entering in to the compressor, refrigerant vaporised in the evaporator. The details arrangement of the evaporator is shown in fig to compressor from capillary tube All dimensions in mm Figurer 3.5 Details of the evaporator of experimental set-up 19

29 3.2.4 Subcooler 925 mm z ~ f y '22O mm _ 1' H: condenser chilled -water in to chiller Figure 3.6 Details of subcooler The subcooler is shown in figure 3.6. The subcooler is fixed after the water- cooled condenser. The construction of subcooler is similar to water-cooled condenser. Inside the subcooler, there is a coil which is made by the copper with 12.7 mm diameter and 8.0 m length. The shell of the subcooler is made of a 3 mm thick mild steel sheet having 220 mm diameter and 925 mm length. The cooling in subcooler is attained by circulating the chilled water from the chiller unit Chiller unit A chiller is a machine that removes heat from a liquid via a vapor-compression. This liquid can then be circulated through a heat exchanger to cool air or equipment as required. In air conditioning systems, chilled water is typically distributed to heat exchangers, or coils, in. air handling units, or other type of terminal devices which cool the air in its respective space(s), and then the chilled water is re-circulated back to the chiller to be cooled again. These cooling coils transfer sensible heat and latent heat from the air to the chilled water, thus cooling and usually dehumidifying the air stream. There are four basic types of compressors used in vapor compression chillers: Reciprocating compression, scroll compression, screw-driven compression, and centrifugal compression are all mechanical 20

30 machines that can be powered by electric motors, steam, or gas turbines. They produce their cooling effect via the "reverse-rankine" cycle, also known as 'vapor-compression'. With evaporative cooling heat rejection, their coefficients-of-performance (COPs) are very high; typically 4.0 or more. A separate chiller unit is shown in figure 3.7. It was fabricated for circulating chilled water in the subcooler. This separate chiller unit works on vapor compression cycle and it was designed to achieve a high degree of subcooling of the refrigerant at capillary tube inlet. A R-22 compressor of 1 TR capacity was employed in the unit. The air cooled condenser is used in this chiller unit and capillary tube is used as a expansion device. The water will be chilled inside the evaporator tank of 400 mm x 400 mm x 800 mm after coming in contact with the cold evaporator coils. The evaporator coil was made up of copper tube of 12.7 mm diameter and 6 m long. The chilled water will circulate through the subcooler by a mono-block centrifugal pump. Figure 3.7 photographic view of chiller unit 21

31 3.2.6 Preheater A preheater is used to vary the degree of subcooling of the refrigerant at the capillary tube inlet. The heat flux in the preheater is controlled by the variac to obtain a wide degree subcooling at the capillary tube inlet. The refrigerant in a subcooled liquid state from the subcooler enters the preheater absorbs the heat from the preheater resulted in reduced subcooling. Higher the heat flux the degree of subcooling will be lower Test-Section condition. Straight is using for the present experimental investigation under adiabatic flow The test-section of the following geometry is discussed below: 1- Straight capillary tube a Straight capillary tube from condenser to evaporator Figure 3.8 straight capillary tube test-section 22

32 Straight capillary tube as a test-section is using with different length i.e. 2.0 m, 1.6 m and 1.0 m and with different diameters of mm and 1.27 mm for the present investigation. The schematic diagram of the straight capillary tube test-section is shown in figure 3.8 with pressure transducer and thermocouple connections for the measurements of pressure and temperature. The straight capillary tube test-section is fixed on the wooden platform. The copper-constantan (T-type) thermocouple is using for the temperature measurement at different locations on the capillary tube. All the thermocouple is attached with the temperature indicator. Pressure gauges is using for the pressure measurement at the inlet and outlet of the capillary tube. 3.3 Measurement The details of temperature, pressure and flow measurements are given below Temperature Measurement thermocouple temperature - { ---_ indicator Display Power supply Figure 3.9 Temperature measurement details Temperature will measure with the help of thermocouple. T-type thermocouple is using for the measuring the temperature at different locations of capillary tube length. These thermocouples were made by diffusing the ends of two wires of 36 SWG (one copper and other constantan) in the mercury well by the application of controlled current. These thermocouples were soldered on the outer wall of the capillary tube to measure the refrigerant temperature at different locations of capillary tube length. These thermocouples 23

33 will be connected to the temperature indicator. Figure 3.9 shows the arrangement for the measurement of temperature Mass flow Rate Measurement. f L P, )tanieter at cale ioatameter scale float Figure 3.10 Mass flow measurement details A rotameter is a device that measures the flow rate of liquid or gas in a closed tube. It belongs to a class of meters called variable area meters, which measure flow rate by allowing the cross-sectional area the fluid travels through to vary, causing some measurable effect. A rotameter consists of a tapered tube, typically made of glass with a 'float', actually a shaped weight, inside that is pushed up by the drag force of the flow and pulled down by gravity. Drag force for a given fluid and float cross section is a function of flow speed squared only, see drag equation. A higher volumetric flow rate through a given area results in increase in flow speed and drag force, so the float will be pushed upwards. However, as the inside of the rotameter is cone shaped (widens), the area around the float through which the medium flows increases, the flow speed and drag force decrease until there is mechanical equilibrium with the float's weight. Floats are made in many different shapes, with spheres and ellipsoids being the most common. The float may be diagonally grooved and partially colored so that it rotates axially as the fluid passes. This shows if the float is stuck since it will only rotate if it is free. Readings are usually taken at the top of the widest part of the float; the center for an ellipsoid, or the top for a cylinder. Some manufacturers 24

34 use a different standard. A rotameter requires no external power or fuel, it uses only the inherent properties of the fluid, along with gravity, to measure flow rate. A rotameter is also a relatively simple device that can be mass manufactured out of cheap materials, allowing for its widespread use Pressure measurement Pressure gauges are used for the measurement of the pressure at the different locations of the experimental set-up. In order to measure the pressure at a particular position in the experimental set-up, a needle size hole was created on the copper tube at a point where the pressure is to be measured. The capillary tube of 1.27 mm is used to connect this fine hole with the pressure transducer. 3.4 Calibration of Measuring Instrument The procedure for the calibration of the pressure gauges and thermocouples are described below Calibration of Pressure gauges Pressure gauges were calibrated using a dead weight pressure gauge testing system which is shown in figure The 'pressure gauge was mounted on the tester and the standard weights were placed in the pan. As a handle was rotated, the indicator pressure started rising and a point was reached at which the red mark was visible. The indicated reading of the transducer shown in the display unit and the corresponding weight on the pan was recorded. This was repeated for the other standard weights. The indicated and the actual pressures were plotted and a calibration curve was drawn. 25

35 Figure3.11 Dead weight pressure calibrator 2 C 40 ec K indicated pressure, bar indicated pressure, bar 26