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Many industrial processes involve the heat transfer mechanism. Heat transfer is a process by which internal energy from one substance transfers to another substance. The heat transfer occurs when existing of temperature difference between two regions. The heat transfer is normally occurs from a high temperature area to a lower temperature area. There are three modes of heat transfer. There are conduction, convection and radiation. The conduction mode is the transfer of heat in solids or fluids at rest conditions. The convection mode is occurs at fluids near a solid surface. While the radiation mode is the transfer of heat through the electromagnetic waves without medium required. The processes of heat transfer cover a large range of temperatures and pressure. There are devices to perform heat transfers process called heat exchangers.
Heat exchangers are devices that conducting heat transfer between two fluids at difference temperatures. In many types of heat exchanger, the two fluids are separated by a solid wall to prevent them from direct contact with each other. The primary heat transfer modes in heat exchangers are convection and conduction. The convection occurs in the boundary layer between the fluids on each side of the solid wall and the conduction occurs in the solid wall itself. Double pipe is one of the examples of the heat exchanger applications. To maximize conduction heat transfer, the heat exchanger devices such as double pipe heat exchanger uses liquid coolants to transfer the heat. Thus, heat exchanger performance depends on the effectiveness of the heat transfer fluids.
The heat transfer fluids play an important role in the development of heat exchanger equipment. However, the conventional coolant fluids such as ethylene glycol, water and oil offer low heat transfer properties. Thus, the advanced heat transfer fluids development with higher heat transfer properties is in a strong demand. Therefore, to obtain higher heat transfer properties, a new generation of heat transfer fluids have been developed which is called nanofluid.
2.2 Double pipe heat exchanger
Double pipe is the one of heat exchanger applications. Double pipe heat exchanger consists of two pipes with one of the pipe placed concentrically inside another one pipe which has larger diameter. The larger diameter pipe served as the shell to lead the flow from one section to another section (Sadik et al, 2002). In the double pipe, the cold and hot liquid continuously flows in the gap of inner pipe and exchanges heat at the same time. At the end of the pipe, U bends are used for inter connection inner pipe. Hence, the double pipe required less space for installation. Since the structure of double pipe heat exchanger is simple, the heat transmission conducting is in large scale. Figure 2.1 shows the double pipe heat exchanger used in industry.
Figure 2.1 Double pipe heat exchanger in industry (Harlon, 2010)
There are several arrangements for double pipe. The simplest arrangement is inserting a small diameter pipe into a lager diameter pipe. This arrangement is required a large relative length. This arrangement is shown in Figure 2.2.
Figure 2.2 A Single pass double pipe heat exchanger (Sadik et al,2002)
Another arrangement for double pipe is the end of the pipes connected with a return U-bend pipe. This return U-bend is covered with housing. The housing has a removable cover to allow removal or inner tubes. This double pipe is arranged as in Figure 2.3.
Figure 2.3 The structure of double pipe heat exchanger (Paul, 2009)
There are two shells joined at one end through return U-bend housing. In the inside of return bend housing, the central tube is bent or welded into a U-shape with the U-bend for this arrangement. It will cause the shell side fluid to flow in series through each of two shells. This arrangement also can be arrange in series as shown in Figure 2.4. Figure 2.5 showed the double pipe heat exchanger in parallel or series parallel combinations.
Figure 2.4 Oil cooler with two hairpin sections arranged in series (Sadik et al, 2002).
Figure 2.5 Oil cooler with two double pipe units in series on the shell side and parallel on the tube side (Sadik et al, 2002).
2.2.1 Purpose of double pipe heat exchanger
Generally, the double pipe heat exchanger used for high flow rate, temperature and pressure conditions. The heat transfer rate is high because the current in double pipe is counter flow. The double pipe heat exchanger was developed to fit applications that are too small area and sections. They can be use in fouling conditions for easy to clean and maintenance. When small heat transfer areas are required, double pipe heat exchanger is excellent for heating and cooling fluids process. They are also very suitable for the fluids at high pressure due to the smaller diameter of the pipes.
2.2.2 Type of double pipe heat exchanger
The double pipe heat exchanger made by joining tubes. The design constructions produce difference types of double pipe units. The types of double pipe heat exchangers are listed in Figure 2.6.
Figure 2.6 Types of double pipe heat exchanger (Sarit, 2005).
Sarit, 2005 has listed the details some types of double pipe heat exchanger. There are listed below:
Single pass unit
This is the simplest type of the double pipe heat exchanger. It made by inserting one tube into another pipe and sealing properly. This unit is not very popular because of relatively large length for a given duty and required large space even though the maintenance is easier. Usually, this type is used in small application such as inline heating of furnace oil for small domestic boiler. Figure 2.7 shows one of examples for single pass unit.
Figure 2.7 A Single pass double pipe heat exchanger (Leonard, 2009).
This is the most popular design of double pipe heat exchanger. Figure 2.8 shows a U-tube unit. It is arranged by two inner tube are jointed at the end pipe with U-bend by welding. The two outer tubes are joined by return bend housing. This type is allowed thermal expansion since the end of the pipe is joined with U-bend.
Figure 2.8 A U-tube double pipe heat exchanger (Leonard, 2009).
Multi-tube double pipe are similar to the U-tube but different in using one inner pipe to a bundle of tube. For low pressure application, the tube sheet is sealed both on the shell side fluids. This type is suitable for low pressure and non-hazardous units. This type is shown in Figure 2.9.
Figure 2.9 A multi tube double pipe heat exchanger (Leonard, 2009).
2.2.3 The advantages of double pipe heat exchanger
Hewit et al. (1994) have listed the advantages of double pipe heat exchanger. There are many the advantages for double pipe heat exchanger which are:
They are cheap and readily available due to simplicity construction in its typical applications.
Thermal expansion did not occurs because of the U-tube type of construction
Easy of maintenances since shell circuit can be inspected and steam or mechanically cleaned due to simple construction and flanged joint.
They are flexible in any condition because come in with multi-pass tube circuit arrangement that can be adjusted either add up the joining or remove it.
Capable of withstanding thermal applications and support in high temperature and pressure applications.
They allow counter flow current heat exchange in which the hot fluids can be cooled to a lower temperature since there are the hot fluids at the exit.
2.2.4 The applications of double pipe heat exchanger
Based on the purpose of double pipe heat exchanger, there are several applications that can be fit to use this type of exchanger. The applications are listed below:
Oil and gas production.
Engine and transmission oil coolers.
Chemical and water heating applications.
Steam to liquid applications.
Marine fuel cooler.
2.2.5 Application of double pipe heat exchanger in cooling lubricating oil
In this study, the application of double pipe heat exchanger used in cooling lubricating oil in gas turbine at steam power plant sector. The purpose of the double heat exchanger is to take the heat from the lubricating oil before recycle back to the system. During the operation, temperature from combustion chamber is extremely high. The heat is conducted from turbine blade to shaft and bearing. Therefore, lubricating oil extract heat generates by bearing and takes away the heat to double pipe heat exchanger for heat transfer process. Figure 2.10 shows the double pipe heat exchanger place in lubricating oil circulation system. Figure 2.11 shows the bypass the cooling water through double pipe heat exchanger.
Turbine Lube Oil Tank
Figure 2.11 Bypass the cooling water through double pipe heat exchanger (Sarit, 2005)
Argonne National Laboratory in US has conceived the name nanofluids to describe a fluid in which nanometer sized particles are suspended. The nanofluids are new kind of heat medium containing nanoparticles which is uniformly and stably distributed in a base fluid (Mohorianu et al, 2006). Nanofluids are engineered by suspending nanoparticles with average sizes below 100nm in common base fluids (Wenghua et al. 2007). Common base fluids are usually conventional coolants such as water, oil and ethylene glycol. Figure 2.12 shows that the one example of nanofluid type, alumina, Al2O3 is smaller than 30 ± 5 nm.
Figure 2.12 Alumina, Al2O3 nanoparticles dispersed in water (Lee et al. 2007).
2.3.1 Production of nanofluids
The development of modern technology allows the fabrication of materials at the nanometer scale. Nanoparticles used in nanofluids have been made from many materials. The example of nanoparticles materials are oxide ceramics, nitride ceramics, carbide ceramics, metals and composite materials.
There are two techniques to produce nanofluids which are the two step technique and the single step technique (Wenghua et al. 2007). The two step technique is produced the nanoparticles by the physical or chemical synthesis. Then, the nanoparticles are dispersed into base fluids. Meanwhile, the single step technique is made and dispersed the nanoparticles directly into the base fluid.
2.3.2 Nanofluids in enhancing thermal performance
Nanofluids are nanotechnology-based heat transfer fluids that are derived by stably suspending nanometer sized particles with typical length scales of 1 to 100nm in conventional heat transfer fluids. Numerous studies found that nanofluids offer better thermal properties that are different from conventional heat transfer fluids.
Several studies focused on the thermal conductivity of nanofluids. Masuda et al. (1993) has showed that the thermal conductivity of ultra fine suspensions of alumina, silica and other oxides in water increased by up to 30% for a volume fraction of 4.3%. Choi, et al. (1995) stated that the addition of a small amount of nanoparticles to conventional heat transfer liquids increased the thermal conductivity of the fluids up to approximately two times. Wang et al. (1999) reported that thermal conductivity of base fluid is greatly enchanted with addition of alumina and cupric oxide. They observed a maximum of 12% increase in the conductivity with a volume fraction of 3% of alumina particles. On the other hand, the viscosity showed an increase of 20%-30% for the same volume fraction.
Eastman et al. (2001) found that the thermal conductivity of ethylene glycol nanofluid containing 0.3% volume fraction of copper particles can increased up to 40%. Mintsa et al. (2009) studied the effect of temperature, particle size and volume fraction on thermal conductivity of water based nanofluids of copper oxide and alumina. They have suggested that the improvement of the thermal conductivity was due to the particles volume fraction, temperature and particles size. When smaller particles are used in the same volume fraction, the contact surface area of particles with fluid and Brownian motion can be increased. Thus, this increased thermal conductivity of nanofluids. Pantzali et al. (2009) have conducted investigation the efficacy of nanofluids in plate heat exchanger. They found that when the nanofluid is added in coolant, the thermal conductivity is increased.
The convective heat transfer coefficient also has studied and investigated by the researches. Choi (1995) reported a possibility to increase convective heat transfer coeffients is by using nanoparticles suspended in liquids. Byung et al. (2008) have conducted an experiment on effect of alumina in double pipe heat exchanger. They found that the addition of nanoparticles in the fluid increase the average heat transfer coefficient of the system in laminar flow. They also stated that the factors of enhancing the heat transfer of nanofluids are surface properties of nanoparticles, particles loading and particle shape. Leong et al. (2010) have investigated thermal performance of an automotive car radiator operated with nanofluids based coolants. They have concluded that with the increasing in volume concentration of nanofluids, the heat transfer rate is increased.
The researches have performed the investigation on the effect of nanofluids properties such as volume concentration to improve the thermal performance. Xuan and Li (2003) performed experimental study on Copper based water nanofluids up to 2% volume concentration and developed a Nusselt number correlation. They found that Copper based water nanofluids have almost same pressure drop as water under same Reynolds number. Hwang et al. (2007) reported that thermal conductivity of the nanofluids depends on the volume fraction of particles and thermal conductivity of particles and base fluids. Praven et al. (2007) stated that with 6% volume concentration of Copper oxide, CuO over the base fluids, the Nusselt number increase 1.35 times. The thermal conductivity of aqueous alumina (Al2O3) nanofluids with low concentration has been investigated by Lee et al. (2008). This alumina (Al2O3) nanofluids produced by two step technique. They indicated that the thermal conductivity of aqueous nanofluids increases linearly with the addition of alumina particles. The summary of literature studies in thermal performance operated with nanofluids is listed in Table 2.1.
Table 2.1 The summary of literature studies in thermal performance operated with nanofluids.