EXPERIMENTAL INVESTIGATION OF A HEAT EXCHANGER WITH A HELICAL COIL MADE OF CORRUGATED TUBES

Transcription

1 EXPERIMENTAL INVESTIGATION OF A HEAT EXCHANGER WITH A HELICAL COIL MADE OF CORRUGATED TUBES Yordam Kocatepe 1, Hojin Ahn 2, Cafer Aydın 3, Aydın Karacasu 3 1 Yeditepe University, Department of Mechanical Engineering 2 Yeditepe University, Department of Mechanical Engineering, corresponding author Telefon: (216) /3773, Fax: (216) , erdeman@yeditepe.edu.tr 3 Intermetalflex, Dudullu Organize San. Bölg. 1.Cadde No.30 Ümraniye/Istanbul Telefon: (216) , Fax: (216) , akaracasu@intermetalflex.com ABSTRACT Characteristics of heat transfer and pressure drop of a helically coiled corrugated flex tube in a heat exchanger were experimentally investigated in this study. A corrugated flex tube, 13 m long and m in diameter, is helically coiled in the heat exchanger. Water is selected as working fluid. Hot water flows through the tube, and cold water in the boiler is either stationary or in motion by a pump. Temperatures and pressures are measured at both inlet and outlet of the coiled tube. Flow rates are controlled by a control valve and measured by a flowmeter. The results show that friction coefficients in the present setup are considerably high (by a factor of 2.2 to 3.6) compared to the data available in the literature for a straight corrugated tube. In addition, the ratio of the friction coefficient of the present setup to that of the straight corrugated tube appears to increase as the flow rate decreases. For the case of stationary cold water in the boiler where natural convection takes place outside the tube wall, the overall heat transfer coefficient is measured to be approximately 400 W/m 2 K. The overall heat transfer coefficient increases only slightly as the flow rate inside the tube increases. When cold water in the boiler is set to a motion by a pump, on the other hand, the overall heat transfer coefficient is measured to be 800 to 1000 W/m 2 K, depending on the flow rate inside the tube. When the surface area of an imaginary smooth tube based on the outer diameter is employed in data analysis, however, the overall heat transfer coefficient is calculated to be much higher. This shows that the corrugated tube considerably enhances heat transfer rate per unit length of the tube, compared to a smooth tube. Keywords: heat exchanger; corrugated; helical coil 1. INTRODUCTION Helical coils are characterized by their compactness and high heat transfer coefficient. When fluid flows through a helically coiled tube, the curvature of the coil induces centrifugal force, causing the development of secondary flow. This secondary flow enhances fluid mixing and thus heat transfer. Fluid flow in a helical tube is characterized by the Dean number, which is defined as De Re r R where Re is the Reynolds number, r is the inner radius of the tube and R is the helix radius. The Dean number is a measure of the ratio of geometric average of inertial and centrifugal forces to the viscous force, and thus a measure of the magnitude of the secondary flow. For laminar flow and small aspect ratio, r/r, frictional loss in a curved tube may be represented as a function of the Dean number. Helical coils are widely used in applications such as heat recovery system, chemical processing, food processing, nuclear reactors, and high-temperature gas cooling reactors. Helical coils have been widely studied both experimentally (for example, Rozzi et al., 2007) and numerically (for example, Jayakumar et al. 2008). Heat transfer and pressure drop through a curved tube have comprehensively been reviewed by Berger et al. (1983), Shah and Joshi (1987), and Naphon and Wongwises (2006). Most of studies have been conducted with constant wall temperature or constant wall heat flux conditions. Some studies are available with a fluid-to-fluid helical heat exchanger setting (Prabhanjan et al., 2002). In most cases, helical coils are made of smooth tubes. On the other hand, a corrugated tube is employed instead of a smooth tube in an attempt to enhance heat transfer rate. However, most of

2 researches have been conducted in the configuration of a straight corrugated tube to determine the effect of corrugation in heat transfer and pressure drop (see, for example, Dong et al., 2001). On the other hand, no or little literature is available on heat exchangers with helically coiled corrugated tubes. The use of corrugated flex tubes in the helical coil heat exchanger has grown since several years ago. For example, they provide connection between solar panels and a boiler inside a house, and they are also used for helical coils inside the boiler. Corrugated flex tubes made of stainless steel have several advantages over smooth copper tubes in their application to helical coils. First, stainlesssteel corrugated tubes provide considerable cost saving compared to copper tubes. Second, since corrugated tubes can be easily bended, a pipe system can be set up just by a long corrugated flex tube, not requiring 90º and 180º bends, whereas various fittings are necessary for copper tubes to fit into the system. In addition, the use of corrugated flex tubes reduces the cost of labor in installation. While the use of corrugated flex tubes is expected to enhance heat transfer characteristics, the pumping cost is expected to be high due to large pressure drops along the tubes. In the present work, preliminary investigation has been conducted on heat transfer and pressure drop in a heat exchanger with a helically coiled corrugated tube. Figure 2. Detailed two-dimensional sketch of the boiler (dimensions in mm). 2. EXPERIMENTAL SETEP AND PROCEDURE In this study, a helical coil made of a stainless-steel corrugated tube is employed to investigate characteristics of heat transfer and pressure drop in the heat exchanger. A 13-m long corrugated flex tube, whose cross-sectional view is shown in Figure 1, is helically coiled in the heat exchanger or boiler as shown in Figure 2. The helix radius of the coil is m, and the total surface area of the coil is m 2. In order to support the helical coil, a frame work is built inside the cylindrical boiler, m in diameter and m in height. Figure 3. Schematic of the test setup. Figure 1. Cross-sectional view of the corrugated tube (dimensions in mm). Water is selected as a working fluid. The entire test setup comprises four water tanks as shown in Figure 3: the heater tank where an electrical heater prepares hot water, the hot water tank which collects heated water from the heater tank and provides the hot water to the coiled tube, the boiler where cold water is heated by heat transfer from the coil, and the cold water tank which provides cold water to the boiler. Each tank except for the cold water tank is fully insulated to minimize heat loss. Thermometers are installed in the heater tank, the

3 hot water tank and the boiler to monitor water temperatures. All the tanks are built on a metal frame. Hot water flow rates through the coiled tube are controlled by a control valve and measured by a flowmeter. Temperatures and pressures at both inlet and outlet of the tube are measured by thermocouples and pressure transducers which are connected to a data acquisition (DAQ) system. Cold water in the boiler is either stationary or in motion by the cold water pump. Experimental procedures are as follows: Water is filled in the heater tank while the hot water tank below is nearly empty. The valve between the heat tank and the hot water tank remains closed. The heater in the heater tank is turned on until water temperature becomes approximately 40 C. The valve between the hot water tank and the heater tank is opened, and heated water pours down to the hot water tank. This procedure helps hot water to be well mixed in the bottom tank and thus uniform in temperature. In the meantime, the cold water pump is operated to mix cold water inside the boiler with the water in the cold water tank. This procedure makes it sure that cold water inside the boiler is uniform in temperature before the hot water flows through the coil. For the case of runs with stationary cold water in the boiler, the cold water pump is stopped and the valve between the boiler and the cold water tank is closed when hot water is fully collected in the hot water tank. And then a few minutes are waited before starting the hot water pump. This allows cold water in the boiler to settle down into no motion. For the case of runs with cold water in motion, the valve between the boiler and the cold water tank remains open and the cold water pump keeps operated. The control valve is set to yield a desired flow rate. The data acquisition (DAQ) system is started to measure temperatures and pressures. The hot water pump is started. The hot water pump remains in operation until water in the hot water tank is run out and transferred to the heater tank. Data collected by the DAQ system are processed and analyzed. 3. DATA PRESENTATION AND DISCUSSION In the present study, pressure drop along the helically coiled corrugated tube is investigated at three different flow rates. Pressure measurements by the pressure transducers at the inlet and outlet of the tube are compensated by head due to gravity and head loss at the 90º bend, and thus only frictional pressure drop is considered in the present study. PRESSURE DROP (BAR) AVERAGE CURRENT SETUP STRAIGHT CORRUGATED TUBE STRAIGHT SMOOTH TUBE VELOCITY (M/S) Figure 4. Pressure drops of water flow in 13-m long tubes as a function of average velocity in the tubes. Frictional pressure drop as a function of average velocity in the coiled tube is plotted in Figure 4. The pressure drop ranges from 0.25 to 0.36 bars, and increases as flow rate increases. For the purpose of comparison, the estimate of pressure loss for a straight corrugated tube with the same conditions, which is available in Witzenmann (2003), is also plotted in Figure 4. Pressure loss along the helically coiled corrugated tube in the present setup is substantially higher than that of the straight corrugated tube. Pressure loss for a straight smooth tube is obtained from the Moody chart and also plotted in the same figure for comparison. Pressure drops investigated in Figure 4 are examined in terms of friction coefficients in Figure 5. The friction coefficient is defined as: P f L 1 V 2 d 2 where L and d are the length and diameter of the tube, respectively. The friction coefficient of the helically coiled corrugated tube in the present setup ranges from 0.15 to 0.23, depending on the Reynolds number. Friction coefficients of both the helically coiled corrugated tube and the straight smooth tube decrease as the Reynolds number increases, whereas that of the straight corrugated 3.1. Pressure Drop

4 tube appears to be independent of the Reynolds number. The ratio of the friction coefficient measured in the helically coiled corrugated tube to that in the straight corrugated tube in Witzenmann (2003) is examined in Figure 6. For the range of the Reynolds number from 24,000 to 36,000, the ratio of friction coefficients varies from 3.6 to 2.2, respectively. It is noted that the ratio of friction coefficients decreases as the Reynolds number increases. FRICTION COEFFICIENT CURRENT SETUP STRAIGHT CORRUGATED TUBE STRAIGHT SMOOTH TUBE REYNOLDS NUMBER Figure 5. Friction coefficient against the Reynolds number. RATIO OF FRICTION COEFFICIENTS REYNOLDS NUMBER Figure 6. Ratio of the measured friction coefficient in the helically coiled corrugated tube to that in the straight corrugated tube in Witzenmann (2003) Heat Transfer Temperatures at both inlet and outlet of the tube are measured as a function of time by T-type thermocouples as shown in Figure 7. As the hot water pump starts, hot water starts filling the evacuated coiled tube, resulting in a time leg between inlet and outlet temperature rises. The hot water tank is emptied approximately in 240 s. During this period, the inlet temperature appears to be steady while the outlet temperature slowly increases. The increase of the outlet temperature is due to the local temperature increase of cold water at the vicinity of the coiled tube in the cold water tank. The local temperature increase of cold water reduces heat transfer rate between hot water flowing in the tube and cold water outside the tube, resulting in higher outlet temperature as time elapses. TEMPERATURE (DEG C) INLET TEMPERATURE OUTLET TEMPERATURE TIME (S) Figure 7. Typical temperature measurement at inlet and outlet of the coiled tube. The arrow indicates the moment at which heat transfer calculation is performed. Initially cold water in the tank is uniform in temperature. This is achieved by letting the cold water pump run sufficiently long to mix water well in the cold water tank before the testing starts. On the other hand, the local temperature of the cold water at the vicinity of the tube was not measured throughout a run and thus not defined. In this study, therefore, heat transfer rates are investigated at the moment when the outlet temperature becomes momentarily stable right after the pump starts (as indicated by the arrow in Figure 7). The temperature difference, ΔT, between inlet and outlet temperatures at this moment is taken to calculate the total heat transfer rate Q as follows: Q mc PT On the other hand, the log mean temperature difference ΔT m is calculated assuming no

5 temperature change in cold water. Thus the overall heat transfer coefficient, U, is obtained as follows: Q U A T m In this study, the overall heat transfer coefficient can be examined based on two surface areas. One is the total surface area of the 13-m long corrugated tube, which is A t = m 2. This includes the surfaces of all the ridges and furrows of the corrugated tube. The overall heat transfer coeficient based on the total surface area, denoted by U t, gives actual heat transfer rate from the hot water to the cold water through the tube wall. The other is the area of an imaginary smooth tube based on the outer diameter of the corrugated tube. This value is A o = m 2. The overall heat transfer coefficient based on A o, denoted by U o, is an indicative for the performance of the coiled tube per unit length. Overall Heat Transfer Coefficient (w/m 2 K) COLD WATER STATIONARY COLD WATER IN MOTION DEAN NUMBER Figure 8. Overall heat transfer coefficient based on the total surface area against the Dean number. During the present experiments, cold water in the boiler is either stationary or in motion by the cold water pump. The overall heat transfer coefficient, U t, based on the total surface area is examined as a function of the Dean number in Figure 8. For the case of stationary cold water in the boiler where natural convection takes place outside the tubes, the overall heat transfer coefficient, U t, is measured to be approximately 400 W/m 2 K as shown in Figure 8. The overall heat transfer coefficient increases only slightly as the Dean number increases. Little dependency of the overall heat transfer rate on flow rates inside the tube is most likely from the fact that the overall thermal resistance must be dominated by the thermal resistance due to the natural convection at the outside wall of the tube. This may be discussed as follows: Jayakumar (2008) shows that, for helically coiled smooth tube, the inner Nusselt number at Dean number equal to 10,100 is about 130, thus resulting in the convection heat transfer coefficient for the inner tube equal to 3100 W/m 2 K. Note that the helically coiled corrugate tube expects to have a convection heat transfer coefficient higher than that for the helically coiled smooth tube. On the other hand, it can be shown that, with the same conditions as the present tests, the natural convection heat transfer coefficient on a horizontal tube is approximately 350 W/m 2 K which is much smaller than the convection heat transfer coefficient at the inner tube wall. In the present setup, therefore, the dominant thermal resistance is from the thermal resistance due to natural convection at the outside wall of the tube. Therefore, the overall heat transfer is little affected by the convection heat transfer at the inner tube, and thus by flow rates in the tube. It is noted that the natural convection heat transfer coefficient of 350 W/m 2 K is close to the present experimental data shown in Figure 8. Prabhanjan et al. (2002) reports that the overall heat transfer coefficient for the helically coiled smooth tube placed in the heat exchanger is approximately 350 to 500 W/m 2 K with stationary cold water bath, and also that the overall heat transfer coefficient is not affected by flow rates in the tube. When cold water in the boiler is set to a motion by a pump, as shown in Figure 8, the overall heat transfer coefficient is measured to be 800 to 1000 W/m 2 K, which is much higher than that for the case of cold water being stationary in the boiler. The cold water pump is estimated to provide the flow rate of 40 lt/min, that is, the average velocity of only m/s across the cross section of the boiler. It can be said, therefore, that the overall heat transfer coefficient appears to be very sensitive to any fluid motion outside the tube. It is also observed in Figure 8 that the overall heat transfer coefficient increases as the Dean number increases, indicating that the overall heat transfer is affected by flow rates inside the tube. This is contrary to the case of stationary cold water where the overall heat transfer coefficient is little sensitive to the Dean number. Note that Jayakumar (2008) reports the overall heat transfer coefficient in a helically coiled smooth tube to be approximately 1250 W/m 2 K with cold fluid in motion. On the other hand, the overall heat transfer coefficient, U o, can be calculated from the surface area of an imaginary smooth tube based on the outer diameter, instead of the total surface area of the corrugated tube. This allows the performance comparison of the heat exchanger, not based on the surface area of the tube but based on the length of the tube. In this case, the overall heat transfer coefficient would be 650 W/m 2 K with the stationary cold water and 1300 to 1600 W/m 2 K

6 with cold water in motion. This shows that the corrugated tube considerably enhances heat transfer rate per unit length of the tube, compared to a smooth tube. Therefore, the use of the helically coiled corrugate tube makes it possible to handle the same heat load with a smaller size of the heat exchanger. 4. CONCLUSION In this study, characteristics of heat transfer and pressure drop of a helically coiled corrugated tube in a heat exchanger were experimentally investigated. The results show that friction coefficients in the present setting are considerably high (by a factor of 2.2 to 3.6) compared to the data available in the literature for a straight corrugated tube. In addition, the ratio of the friction coefficient of the helically coiled corrugated tube to that of the straight corrugated tube appears to increase as the flow rate decreases. For the case of stationary cold water in the boiler where natural convection takes place outside the tubes, the overall heat transfer coefficient based on the total surface area including the ridges and furrows of the corrugated tube is measured to be approximately 400 W/m 2 K. The overall heat transfer coefficient increases only slightly as the flow rate inside the tube increases. This is expected because the highest thermal resistance is from the natural convection at the outside tubes. When cold water in the boiler is set to a motion by a pump even at a low flow rate, on the other hand, the overall heat transfer coefficient is measured to be 800 to 1000 W/m 2 K, depending on the flow rate inside the tube. When the surface area of an imaginary smooth tube based on the outer diameter is employed in data analysis, however, the overall heat transfer coefficient is calculated to be 650 W/m 2 K with the stationary cold water and 1300 to 1600 W/m 2 K with cold water in motion. These values are much higher than those based on the total surface area of the corrugated tube. In this case, the performance of the heat exchanger is evaluated, not based on the surface area of the tube, but based on the length of the tube. Therefore, the use of the helically coiled corrugate tube makes it possible to handle the same heat load with a shorter length of the tube and thus with a smaller heat exchanger. Dong, Y., Huixiong, L., Tingkuan C., Pressure drop, heat transfer and performance of single-phase turbulent flow in spirally corrugated tubes, Exp. Therm. Fluid Sci. 24, , J.S. Jayakumar, S.M. Mahajani, J.C. Mandal, P.K. Vijayanb, Rohidas Bhoia, Experimental and CFD estimation of heat transfer in helically coiled heat exchangers, Chemical Engineering Research and Design 86, , Ju, H., Huang, Z., Xu, Y., Duan, B., Yu, Y., Hydraulic performance of small bending radius helical coil-pipe, J. Nuclear Science and Tech. 38, , Naphon, P., Wongwises, S., A review of flow and heat transfer characteristics in curved tubes, Renewable and Sustainable Energy Reviews 10, , Prabhanjan, D. G., Raghavan, G. S., Rennie, T. J., Comparison of heat transfer rates between a straight tube heat exchanger and a helically coiled heat exchanger, Int. Comm. Heat Mass Tranfer, Vol.29, No.2, , Rozzi, S., Massini, R., Paciello, G., Pagliarini, G., Rainieri, S., Trifiro, A., Heat treatment of fluid foods in a shell and tube heat exchanger: Comparison between smooth and helically corrugated wall tubes, J. Food Eng. 79, , Salimpour, M.R., Heat transfer coefficients of shell and coiled tube heat exchangers, Exp. Therm. Fluid Sci., oi: /j.expthermflusci , Shah, R.K. and Joshi, S.D., Convective heat transfer in curved ducts, in Handbook of singlephase convective heat transfer, Kakac, S., Shah, R.K., & Hung, W. (eds). (Wiley Interscience, New York) (Chapter 3), Witzenmann GmbH, Metal Hoses Manual, p. 64, REFERENCES Berger, S.A., Talbot, L. and Yao, L.S., Flow in curved pipes, Ann Rev Fluid Mech, 15: , 1983.