AN EXPERIMENTAL STUDY ON THE FLOW OF REFRIGERANTS THROUGH A CAPILLARY TUBE

Abstract

The capillary tube is an expansion device of a refrigeration system and its certain advantages are pushing the limits of researchers to a great extent. The capillary tube design and installation is always depends upon a combination of both i.e., geometry of the capillary tube and the type of refrigerant. The researchers made several attempts to reveal the flow characteristics of such combinations. An exhaustive literature survey of recent past decade reveals that, the research had inclined towards the flow of natural refrigerants inside the capillary tubes due to their eco-friendly characteristics. In addition to the type of refrigerant, other interesting portion is geometry of the capillary tube. The research trends of seven decades comprises of the flow characteristics of straight and helical and spiral coiled capillary tubes with various refrigerants. Amongst all the refrigerants, the natural refrigerants exhibits better eco-friendly characteristics. The present study comprises of an experimental study for the adiabatic flow of R-600a through a helically coiled capillary tube. The experiments for testing the capillary tubes were proceeded by an integrity test. The details of experimental facility for testing a capillary tube with different inlet sub-cooling degree and varying pressure are discussed. The effect of coil diameter, capillary length, capillary tube diameter, sub-cooling degree and inlet pressure on mass flow rate are presented. The degree of sub-cooling at the inlet of both straight and helical capillary tube is varied from 3-15°C. From the experimental investigations it is noted that the mass flow rate in straight capillary tube is 1.5-16 percent higher than in coiled capillary tube. The following non-dimensional correlation to predict the mass flow rate inside a straight and helical coiled capillary tube has been developed. 𝜋1,coil=0.124.𝜋22.33.𝜋30.55.𝜋5−0.16.𝜋60.148.𝜋70.16; 𝜋1,str=0.138.𝜋22.06.𝜋30.66.𝜋5−0.32.𝜋60.12; The above correlation shows an agreement with the experimental mass flow rate within an error band of ±20 percent. The flow characteristics of a diabatic flow of R-600a through a concentric configured helically coiled capillary tube suction line heat exchanger have also been discussed. The degree of sub-cooling at the inlet of both straight and helical capillary tube is varied from 3-20°C. In case of diabatic flow, the coiling reduces the mass flow rate of R-600a by 3-12 percent as compared to straight capillary tube. The refrigerant mass flow rate is scattered up with rise of pressure. The following semi empirical correlation to predict the mass flow rate of R-600a flowing through a diabatic helical coiled capillary tube is proposed for sub-cooled inlet conditions. ii 𝜋1=K.𝜋2−0.5296.𝜋30.7605.𝜋41.0711.𝜋5−0.9015.𝜋60.2895.𝜋7−0.1234.𝜋8−0.1199.𝐹; Straight capillary tube: K = 0.164 & F = 1; Helical capillary tube: K = 0.0439 & F = 𝜋90.3344; The above correlation is valid for 3°C ≤Δ𝑇𝑠𝑢𝑏≤ 20°C; 600 kPa ≤𝑝𝑖𝑛≤ 700 kPa; 1 mm ≤𝑑𝑐≤ 1.6 mm; 2.5 m ≤𝐿≤ 5 m; 1.69 𝜇m ≤𝜖≤ 2.6 𝜇m; 𝐿𝑎𝑑𝑖𝑎= 1.0 m; 2.7 m ≤𝐿ℎ𝑥≤ 4.5 m; The proposed correlation predicts measured data of diabatic capillary tube, with an error band of ±20 percent. In addition to the sub-cooled inlet conditions of capillary tube, the two-phase inlet conditions have also included as a part of the present study. This study carried out for R-134a flowing inside an adiabatic straight capillary tube. The vapour quality varied from 0.05-0.25. In this case, all the experiments were done by varying the pressure from 655 to 724 kPa. The effect of two phase inlet conditions on mass flow rate of R-134a through an adiabatic capillary tube is observed. The following correlation to predict the mass flow rate of partially condensed R-134a flowing through an adiabatic capillary tube is proposed. 𝜋1=321.12 𝜋2−0.142.𝜋30.514. 𝜋4−0.31 The above proposed correlation predicts the measured data with an error band of ±25 percentage.