Performance Evaluation of a Triple Concentric Tube Heat Exchanger Using Deionized Water and Oil-40

: This study examines experimentally the performance of a horizontal triple concentric tube heat exchanger TCTHE made of copper metal using water as cooling fluid and oil-04 as hot fluid. Hot fluid enters the inner annular tube of the TCTHE in a direction at a temperature of 50, 60 and 70 o C and a flow rate of 20 l/hr. On the other hand, the cooling fluid enters the inner tube and the outer annular tube in the reverse direction (counter current flow) at a temperature of 25 o C and flow rates of 10, 15, 20, 25, 30 and 35 l/hr. The TCTHE is composed of three copper tubes with outer diameters of 34.925 mm, 22.25 mm, and 9.525 mm, and thicknesses of 1.27 mm, 1.143 mm, and 0.762 mm, respectively. TCTHE tube's length was 670 mm. Nusselt number, overall heat transfer coefficient, convective heat transfer coefficient (CHTC), friction factor and pressure drop were measured from the obtained experimental results and plotted in graphs against Reynold number and volumetric flow rate of water. These parameters appeared good results in the cooling process. Nusselt numbers increased linearly with DIW flow rate for both C1 and C2 reaching maximum values of 38.25 and 14.64 respectively. CHTC increased linearly with the DIW flow rate for both C1 and C2 reaching maximum values of 2934.3 and 871.7 respectively. Overall heat transfer coefficient of DIW reached maximum values of 296.36 and 251.4 at 35 l/hr for C1 and C2, respectively. Friction factor DIW in C1 and C2 decreased with the volumetric flow rate increases, reaching minimum values of 0.04 and 0.25 respectively. Pressure drop of DIW increased linearly with flow rate reaching maximum values of 81.4 and 4.31 for C1 and C2 respectively. This in turn leads to reduced TCTHE length and size leading to a decrease in the construction cost of the heat exchanger.


Introduction:
The double pipe heat exchanger (DPHE) is a device used to transfer heat energy between two fluids depending on the temperature gradient between them. It is a basic unit in many industries: a tulla i food, pharmaceutical, dairy pasteurization, evaporation, refrigeration, chilling, freezing, etc. Heat is transferred between two fluids in DPHE through the tube wall, thus to increase the heat transfer rate it is necessary to increase the tube surface area by increasing cgs us icg io cgs cless. This in turn leads to an increase in the heat exchanger (HE) size and cost. For that reason, researchers tried to modify DPHE and reduce its size by using a triple concentric tube heat exchanger (TCTHE). TCTHE is designed of three tubes connected concentrically with each other. It consists of an inner tube, inner annular, and outer annular. For an effective heat transfer rate the hot fluid should ouif through the inner annular on the other hand, the cold or normal temperature fluid flow through the inner tube and the outer annular. The heat transfers take place along the outer surface area of the inner tube and the inner surface area of the inner annular.
The flow arrangements in DPHE could be divided into two types co-current and counter current. On the other hand, TCTHE consists of three flowing fluids as a result the flow arrangements could be divided into four types. Using TCTHE could enhance heat transfer by reducing HE size and cost. Water is an essential fluid used as HTF in different heat transfer units such as solar collectors, it is cheap, available thermophysical properties, high thermal conductivity with moderate viscosity. Studying TCTHE began with C. Zuritz in 1990 by deriving three first order differential equations using Laplace transformation for three fluids. By comparing the obtained results, he concluded that using TCTHE causes an overall heat transfer (OHT) efficiency improvement and a HE length reduction by 25% 1 . Radulescu et al. 2 studied experimentally TCTHE by using mineral oil as the heating medium in the inner annular and water as cooling in an inner tube and outer annular. They obtained a deviation range of 3.5 to 4.8 % for temperature prediction. Ghiwala and Matawala 3 , studied numerically TCTHE by comparing TCTHE with DTHE and found that the TCTHE shows better heat transfer efficiencies and a large heat transfer area per unit exchanger length. This means a reduction in the space requirement. Hossain et. al 4 studied experimentally TCTHE by designing it and studying its performance using different inlet temperatures and mass flow rates. He found that the overall heat transfer coefficient increases with the increasing mass flow rate for hot and cold fluid In the same year, Pancholi et al. 5 found in their research that TCTHE performs better using counter flow than using co-current flow.
Tamkhade et al. 6 investigated experimentally TCTHE using water as a cooling medium and oil as a hot fluid medium in counter current flow. They found that heat transfer rates increased with increasing water flow rates in the inner tube and outer annular. Furthermore, the heat transfer rate increased with increasing Reynolds number of oil. Dwivedee et. al 7 admitted a review on TCTHE and concluded that it has better performance than double tube heat exchangers. Amanuel and Mishra 8 employed CFD software ANSYS Fluent v17.0 to study heat transfer and pressure drop performance in TCTHE and found that increasing Reynolds numbers cause huge growth in Nusselt number and decreasing friction factor. On the other hand, increasing the length/hydraulic diameter of the tube causes a large decrease in both Nusselt number and friction factor.
Reddy et. al 9 studied experimentally TCTHE made of stainless steel tubes using TiO2/water nanofluid as a cooling medium in the inner tube and outer annular and water as a hot fluid medium. They found that the heat transfer rate increased with TiO2 nanoparticle concentration in nanofluid. Nayak et. al 10 studied experimentally the performance in a double and triple concentric tube heat exchanger and found that the effectiveness of the cocurrent flow in TCTHE was not always greater than that of the theoretical current flow in a double tube heat exchanger . Jradi et. al 11 developed and validated theoretically a numerical model for the performance evaluation of TCTHE. Tamkhade and Mandar 12 calculated theoretically heat transfer coefficient of TCTHE using an established empirical correlation using 'Oil ISO VG 22' as a hot medium and water as a cooling medium in an inner tube and the outer annular in TCTHE under adiabatic condition and counter current flow.
Rajab et. al 13 investigated and analysed the heat transfer process using water and titanium dioxide (TiO2) as a cooling medium and air passing through the central tube and hot water circulating in the spiral tube of a triple tube heat exchanger and found that using TiO2/water nanofluid lead to increase the overall heat transfer. Kilinҫ 14 prepared 2% by weight graphene/water nanofluid with 0.2% of sodium dodecyl benzene sulfonate as surfactants and found that using this nanofluid as a cooling medium in TCTHE in counter current flow lead to enhancing the heat transfer coefficient by 9.6%. Hussien et. al 15 studied experimentally the performance by using Alumina (Al2O3) and Copper oxide (CuO) nanoparticles mixed with engine oil nanofluid as a cooling medium in a double pipe heat exchanger and found an enhancement of thermal conductivity of the base fluid (oil) this in turn improved the heat transfer rate and thermal performance of the heat exchanger. For that reason, it is recommended to prepare nanofluids using the nanoparticle silver and Pomegranate peel extract (MWCNTs-SNPs -NPGPE) prepared by Hamza et. al 16 and Gold nanoparticles prepared by Saeed et. al 17 separately with deionized water as base fluid and use them as a cooling medium in TCTHE to study the heat transfer performance.
The main objectives of this study are: 1. Studying the performance of TCTHE to give an idea about the efficiency obtained by this type of HE. 2. Using water as a cooling fluid in the inner tube and the outer annular and Oil-40 as hot fluid in the inner annular using a counter current flow arrangement. 3. Investigate counter current flow arrangement. 4. Measuring Nusselt number, pressure drop, heat transfer coefficient, overall heat transfer coefficient and plots them in graphs versus flow rate and Reynold number. 5. Reducing the size and the construction cost of the heat exchanger.

The novelty of this study:
The novelty of this study is fabricating TCTHE using copper tubes with different and small diameters compared with other research. Furthermore, studying the heat transfer behavior using oil-40 as hot fluid and DIW as cooling fluid using laminar flow.

Materials
Three liters of deionized water (DIW) were used as a cooling fluid medium and three liters of Oil-40 were used as a hot fluid medium. Table. 1 shows the physical properties of Oil-40 measured in Laboratory Research of Oil in Al-Doura Refinery.  Figure 1. shows the front view of the designed TCTHE system. It is shown clearly the TCTHE tubes that are made of copper. TCTHE system consists of: 1-Two stainless steel tanks each of them of 4 liters capacity, one for the Oil-40 and the other for the DIW tanks. 2-Four temperature sensors are used supplied, two of them inside each tank and two to detect the outlet (hot and cold) fluid temperature. 3-Two pumps are used in order to pump the liquids to tubes of TCTHE. 4-Two volumetric flow rate controller is used to control the flow rate of the liquids. 5-Fin and tube heat exchanger for cooling the DIW after leaving the TCTHE. 6-Heater inside the hot fluid tank in order to heat Oil-40 to the desired temperature.

Figure 1. The front view of the TCTHE
In this study DIW starts flowing from the water tank through pump 1 passing through volumetric flow rate controller 1, then enters the TCTHE through the inner tube and the outer annular. At the same time, oil-40 starts flowing from the oil tank through pump 2 passing through volumetric flow rate controller 2, then enters the TCTHE through the inner annular in a countercurrent flow. Fig. 2 shows the schematic fluid diagram of TCTHE used in the experiment. The oil and water temperature was measured by a thermocouple inserted in different locations: the oil tanks, the water tank, the outlet Oil-40 and DIW from the TCTHE. The equations in Table 3. are used to calculate TCTHE parameters. The TCTHE program measured the inlet temperatures, the outlet temperatures and flow rates of the cooling and the heating mediums spontaneously. In addition, the TCTHE computer program calibrated automatically tgs csrusriclrss and the flow rate. The linear velocity was calculated using Eqs 4-6. The convective heat transfer coefficient was calculated using eqs 10-12. The frictional pressure loss was calculated using Eqs 13-15. In addition, the entrance and exit pressure losses were calculated using eqs 16-18. Since the flow was laminar (Re< 2300) Nusselt number of the cooling fluid was calculated using Eq 22.       Over all heat transfer coefficient of DIW in TCTHE: Figures. 9 and 10 show that the overall heat transfer coefficient of DIW increased with flow rate. On the other hand, the overall heat transfer coefficient in C1 was increased by about 29% over that in C2. Fig. 9 shows that the overall heat transfer coefficient of DIW reached maximum values of 296.36 and 251.4 at 35 l/hr for C1 and C2, respectively. Again Figs. 11 and 12 show that the overall heat transfer coefficient of DIW increased with flow rate. On the other hand, the temperature has no clear effect on the overall heat transfer coefficient for both C1 and C2. That could be related to the constant and stable thermophysical properties including viscosity, density, specific heat, and thermal conductivity with the temperature variation range used in the experiment.     Figures. 13 and 14 show clearly that the DIW friction factor ( -factor) in C1 and C2 was decreased with the volumetric flow rate increasing reaching minimum values of 0.04 and 0.25 respectively. But friction factor slightly reduced with the oil inlet temperature increased. This could be resulted from increasing the DIW velocity. Generally, -factor of DIW in C2 increased by 5.5 times the -factor in C1. The -factor in the outer annular of the TCTHE shows a high value compared with the inner tube. That could be resulted from increasing the velocity in the inner tube, compared with the outer annular, sincefactor is related inversely to velocity as in Eq. 23 shown in Table. 3.

Conclusions:
In this study, the performance of a TCTHE was investigated using DIW as a cooling fluid medium and Oil-40 as a hot fluid medium. Depending on the used volumetric flow rate and temperature range different heat transfer parameters were measured. These parameters include: These parameters appeared good results in the cooling process. This in turn leads to reduced TCTHE length and size leading to a decrease in the construction cost of the heat exchanger compared with a double pipe heat exchanger. It is also shown a low values of pressure drop in the outer annular compared with the inner tube. In contrast the friction factor in the outer annular of the TCTHE shows a high value compared with the inner tube. That could be resulted from increasing the velocity in the inner tube, compared with the outer annular, since the friction factor is related inversely to velocity as in Eq. 23, shown in Table 3. On the other hand, pressure drop depends directly on the velocity as shown in Eqs 13, 15, 16 and 18 Table 3.