A Heat Exchanger Device Biology Essay

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PART 2

Condenser is a heat exchanger device in which the heat generated will be removed from the system by condensing air. It is also known as two-phase flow heat exchanger. Basically in heat exchangers, the two fluids are separated by solid walls, so they are not directly in contact with one another. There are two modes of energy transfer in heat exchanger, which is convection and conduction. Convection occurs in the boundary layer of fluid on each side of the solid wall, while the conduction occurs in the wall itself. Kakac et al. (2002) generally classified condenser into two main types:

Condenser in which the condensate stream and coolant are separated by a solid surface.

Condenser in which the condensing vapor and coolant are brought into direct contact.

Direct contact type of condensers normally made up of a steam bubble forms in a pool of liquid, the liquid is sprayed into the steam. In other words, the liquid flows downwards as a film for packaging materials against the upward flow of steam. Condensers in which the condensate streams are separated into three main types:

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Shell-and-tube

Plate

Air-cooled

In the shell-and-tube heat exchanger, condensation may occur inside or outside the tubes. The orientation of the device can be vertically or horizontally. They are classified according to whether they are coils or shell-and-tube condenser. Evaporator and condenser coils are used when the second fluid is air because of low coefficient of heat transfer in the air. Condensation occurs inside tubes in the air-cooled type with blowing or sucking air in the tubes for cooling. Fins with large surface areas are usually provided in the air to offset the lower heat exchange coefficient of air-side.

For the purpose of effective heat transfer enhancement, the selection of a shell-and-tube heat exchanger as a heat transfer device is the best way.

2.2 Shell-and-tube heat exchanger

Shell-and-tube heat exchangers are commonly used in refrigeration, power production, air conditioning, and chemical processing. It also used in nuclear power plants as condensers, steam power generators in pressurized water reactor, as well as feed water heater. Shell-and-tube heat exchangers are probably the most versatile and common type of heat exchangers in use. Kakac et al. (2002) indicated that shell-and-tube heat exchanger have larger heat transfer surface area to volume ratios than double pipe heat exchangers, and easy to produce in a variety of sizes and flow configurations. It can operate at high pressures on the environment and can be easily cleaned.

2.2.1 Basic components of shell-and-tube heat exchanger

A shell-and-tube heat exchanger is an extension of a double pipe configuration. It consists of a closed tube within a cylindrical shell. It operates with a flow of fluid through the tube, while the other fluid flows within the space between the tubes and shell. The basic components of the shell-and-tube heat exchanger are tubes, shell, front-end head, rear-end head, baffles, and tube sheets. As shown in Figure 2.1, the baffles are placed along a bundle of tubes to force the fluid between the tubes and shell to flow across the tubes.

Figure 2.1: A shell-and-tube heat exchanger consists of a bundle tubes surrounded by a shell. (http://beta.cheresources.com/)C:\Users\Shahrin\Desktop\Untitled.jpg

There are many variations of the shell-and-tube heat exchanger design, which is one-tube pass, two-tube pass, four-tube pass, one-shell pass, and two-shell pass heat exchanger. Fraas et al. (1998) indicated several numbers of basic components of shell-and-tube heat exchanger:

Tubes - the tubes are the basic component of the shell-and-tube heat exchanger. It provides heat transfer between a fluid flowing in the tube and the other fluid flowing out of the tubes. It mostly made of copper or steel. For specific applications, nickel, titanium, or aluminum may also be used. Most of the surface of the tube is extended or enhanced. Extended or enhanced surface tubes are used when one fluid has a much lower heat transfer coefficient of the other fluid. Double enhanced tubes, the improvement both inside and outside, so as to reduce the size and cost of the heat exchanger. Extended surfaces (fins and tubes) to provide two to four times the area of heat transfer at the outside of the bare tube is appropriate. With the ratio, it helps to offset the lower heat transfer coefficient outside.

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Tube sheets - the tubes are held in place by inserted into the hole in the tube sheet. Tube sheet is usually a single round metal plate that was based on drilled and grooved to take the tubes, the gaskets, the spacer rods, and the bolt circle in which they are tied to the shell. The space between the tube sheets is open to the atmosphere so that any leaks can be quickly detected. To allow any liquid leaking into the atmosphere separately without mixing, the triple tube sheets are used in cases of extreme hazard or high value of the fluid. In addition, the tube sheet should withstand corrosive attack by both fluids in a heat exchanger. Other than that, the tube sheet also should be electrochemically compatible with all the tube and tube-side material. Tube sheets are usually made from low carbon steel with a thin layer of corrosion-resisting joint metallurgically bound on one side.

Shell and shell-side nozzles - the shell is usually has a circular cross section and generally made by rolling a metal plate of the suitable dimensions. Shell is the container for the shell-side fluid, while the nozzles are the ports of inlet and outlet. The roundness of the shell is important in improving the maximum diameter of the baffles that can be inserted and therefore the effect of shell-to-baffle leakage. For the large exchangers, the shell usually made out of low carbon steel, although other combination can be and are used when corrosion or high temperature strength demands must be fulfilled.

Tube-side channels and nozzles - tube-side of the channels and nozzles are used to control the tube-side fluid flow into and out of the tubes of the heat exchanger. These channels and nozzles are normally made out of alloy material which is compatible with the combination of the tubes and tube sheets from the tube-side fluid are generally more corrosive.

Channel covers - the channel covers are round plates that bolt to the channel flanges and can be removed for inspection without disturbing the tube-side piping as well. For smaller heat exchangers, bonnets with flanged nozzles or threaded connections to the tube-side piping are often used instead of channels and channels covers.

Pass divider - in one channel or bonnet, a pass divider is needed for an exchanger having two tube-side passes, and they are required both in channels and exchanger have a bonnets for more than two passes.

Baffles - baffles provides two functions, firstly, to support the tubes in proper position during assembly and operation of the tubes and prevent vibration caused by vortex induced flow, and secondly, they guide the flow of the shell-side back and forth in the tube, thus increasing the speed and the coefficient of heat transfer.

2.2.2 Type of shell-and-tube heat exchanger

Hagen et al. (1999) indicated that if the fluid in the tubes flows from one end of the heat exchanger to the other end only once, it is called one-tube pass heat exchanger, as shown in Figure 2.2.

Figure 2.2: A one-tube pass heat exchanger. (http://en.wikipedia.org/)C:\Users\Shahrin\Desktop\asa.jpg

If the fluid in the tubes flows from one end to the other end and returns to the inlet end, it is called a two-tube pass heat exchanger, as shown in Figure 2.3.

Figure 2.3: A two-tube pass heat exchanger. (http://en.wikipedia.org/)C:\Users\Shahrin\Desktop\2 tube.jpg

A four-tube pass configuration is also common. For the shell-side flow configuration, if the fluid between the tubes and the shell flows from one end to the other only once, it is called one-shell pass heat exchanger. If the fluid makes two passes through the shell side, it is called two-shell pass heat exchanger. The tubes basically straight in shape, but shell-and-tube heat exchanger also have shape of U, called U-tubes.

U-tubes mostly used in nuclear power plants to boiling water recycled from a surface condenser into steam to drive a turbine to produce power as shown in Figure 2.4.

Figure 2.4: A U-tube heat exchanger. (http://en.wikipedia.org/)

Shell-and-tube heat exchangers are available in a various sizes to fit specific applications. A typical industrial size of shell-and-tube heat exchanger is shown in Figure 2.5 and for miniature shell-and-tube heat exchanger is shown in Figure 2.6.

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Figure 2.5: Shell-and-tube heat exchangers are commonly used in an industrial application. (http://www.secshellandtube.com/)

Figure 2.6: Miniature shell-and-tube heat exchangers are used in low-flow-rate and research applications. (http://stms.kr/)C:\Users\Shahrin\Desktop\miniature.jpg

Tubular Exchanger Manufacturers Association (TEMA) published standards for the design, manufacture, testing, installation, operation, and maintenance of shell-and-tube heat exchangers. They are identified by an alphabetic character, as shown in Figure 2.7 (TEMA, 1998).

Figure 2.7: Standard shell type and front end rear end head types. (http://pump-heat-exchanger.blogspot.com/)C:\Users\Shahrin\Desktop\journal fyp\pics\Heat Exchanger Type.JPG

Advantages of shell-and-tube heat exchanger

Wang et al. (2009) indicated the reasons for the shell-and-tube heat exchanger had been widely used are; for instance, it provides a relatively large ratio of heat transfer area to volume and weight. Most of the parts of shell-and-tube heat exchanger are easily replaced if damaged occur and also easily to cleaned, such as baffles. Baffles are used in shell-and-tube heat exchanger to stimulate flow across the tubes, thus increasing heat transfer performance. In addition, shell-and-tube heat exchangers are robustness to high-pressure when operates. The surface of shell-and-tube heat exchanger is easy to construct in a variety of sizes and mechanical rough.

2.3 Nanofluid

Nanofluid is a conventional coolant containing nanometer-sized particles suspension. The nanofluids have a high thermal conductivity than conventional coolants. The performance of nanofluids depends on various parameters such as temperature, particle size, and volume fraction. The conventional coolants has been widely used in industrial processes involving the electronic chips, heat exchangers, automotives, manufacturing, refrigerant and air conditioning, aircraft, and laser applications. Due to the low performance of conventional coolants such as water, ethylene glycol, and oil, a variety of methods and studies have been undertaken to improve the thermal conductivity of fluids by suspending nanoparticles in these conventional coolants.

Nanofluids are suitable for engineering applications and have shown some advantages compared with conventional coolant, such as better stability, higher thermal conductivity, and no extra pressure drop. Since thermal conductivity is the most important parameters for enhanced heat transfer, numerous research have been conducted on the thermal conductivity of nanofluids. All experimental results have indicated the enhancement of thermal conductivity with the addition of nanoparticles.

2.3.1 Production of nanofluids

Yu et al. (2008) stated nanofluids have been produced by two techniques, which are a two-step process and a one-step process. The two-step process starts with nanoparticles produced whether the physical or chemical synthesis techniques, and dispersed them into a base fluid. The one-step process directly disperses the nanoparticles into a base fluid.

2.3.1.1 Two-step process

An advantage of this step is the technique used in the commercialization of nanofluids. By the two-step method, production of nanofluids can be made if the agglomeration problem can be overcome using such nanopowders produced economically in bulk. Producing nanofluids using the two-step method is quite challenging because individual particles tend to quickly agglomerate before achieving dispersion. This agglomeration is occurring due to attractive van der Waals forces between nanoparticles, and the agglomerations of particles tend to quickly settle out of liquids. A key step to success in achieving great performance of heat transfer is to produce and suspend nearly monodispersed or non-agglomerated nanoparticles in liquids. The agglomeration problem becomes worse at high volume concentrations. Some surface-treated nanoparticles show excellent dispersion in base fluid and good thermal properties. To produce well-dispersed nanofluids in large volumes, the challenge is to develop innovative ways to improve the two-step process.

2.3.1.2 One-step process

Yu et al. (2008) states that a one-step process more preferred than two-step process to prevent oxidation of the particles to the high conductivity of nanofluids containing metal such as copper. Nanoparticles are formed and dispersed in the fluid in a single process with this method. One-step method that involves direct evaporation was used to produce copper nanoparticles fixed number of non-uniformly dispersed and stable depends on ethylene glycol. By using this method, the nanophase powders, condensed from the vapor phase flowing directly to the low vapor pressure ethylene glycol in the vacuum chamber. Although a one-step process have produced nanofluids in small quantities of nanofluids for experimental purposes, they are unlikely to be the mainstay of the production of nanofluids. There are two problems with this method. First, to produce nanofluids with these one-step processes is expensive. Second, a process involving vacuum significantly slows the production of nanoparticles and the nanofluids, thereby affecting production leves.

Polyvinylpyrrolidone was added as a protector and as a stabilizer that prevents the agglomeration. With this one-step chemical method, copper nanofluids was produced, shows that nearly the same increase in thermal conductivity of nanofluids produced by the one-step physical method. This method has the potential to produce large amounts of nanofluids and faster than one-step physical process.

2.3.2 Thermal properties of nanofluids

Namburu et al. (2008) indicated that the heat transfer coefficient not only depends on the thermal conductivity, but it also depends on other fluid properties such as the specific heat, density, and viscosity of nanofluids. In addition, it also depends on the geometry and the roughness of the solid surface. The thermophysical properties are listed in table 2.1.

Table 2.1: Thermophysical properties of nanofluids of 293 K. (Namburu et al, 2008)

Properties

Volume concentration (%)

Viscosity (mPa s)

Density (kg/m3)

Specific heat

(J/kg K)

Thermal conductivity (W/m.K)

Mixture of ethylene glycol/water (base fluid)

0

5.38

1086.27

3084.00

0.349

Mixture of copper oxide/ethylene glycol water

6

18.75

1399.69

2393.99

0.424

Mixture of silicon dioxide/ ethylene glycol water

(20 nm diameter)

6

7.58

1154.29

2814.09

0.381

Mixture of silicon dioxide/ ethylene glycol water

(50 nm diameter)

6

7.16

1154.29

2814.09

0.381

Mixture of silicon dioxide/ ethylene glycol water (100 nm diameter)

6

6.53

1154.29

2814.09

0.381

Mixture of alumina/ethylene glycol water

6

9.67

1259.29

2645.35

0.425

Namburu et al. (2008) indicated that CuO, Al2O3, and SiO2 nanofluids for the same concentration and at a particular Reynolds number, CuO nanofluids have higher heat transfer performance followed by Al2O3 and SiO2. For the viscosity of nanofluids, it will increase as the nanoparticle diameter decreases. For 6% CuO nanofluids, it increases Nusselt number by 1.35 times and heat transfer coefficient by 1.75 times over the base fluid at fixed Reynolds number of 20000. Heat transfer coefficient of nanofluids increases when increasing the volume concentration of nanofluids and Reynolds number.

2.3.3 Experimental studies on nanofluids

Das et al. (2009) obtained that the thermal conductivity ratio is higher for smaller size of nanoparticles as shown in Figure 2.8.

Figure 2.8: Effect of nanoparticle size on thermal conductivity ratio of nanofluid for varying temperature at two different particle volumetric concentrations of ZnO nanofluid in 60:40 ethylene glycol/water. (Das et al, 2009)

Temperature KC:\Users\Shahrin\Desktop\graf.jpg

This behavior intuitively correct as the transfer of thermal energy depends on surface area and small particle volumetric concentration at providing more surface area to transfer thermal energy. Therefore, the effectiveness of thermal conductivity is higher for smaller particles. To get a sense of order size increases, the thermal conductivity ratio is 3% higher in 305 K for 29 nm particle over that of 77 nm particle at 2% of volumetric concentration. For the 4%, volumetric concentration the thermal conductivity ratio is 3.3% higher for particle of 29 nm that of 77 nm particles.

Nguyen et al. (2007) was investigated the effect of nanoparticles size on thermal conductivity. Several experiments were done for the Al2O3/water nanofluid with 47 nm average diameter. The comparative was done only for a certain particle volume concentration of 6.8%. As shown in Figures 2.9 and 2.10, a comparative on result obtained for the waterblock coefficient of heat transfer and the convective Nusselt numbers.

Figure 2.9: Effect of particle size on hw-block for 6.8% particle volume concentration. (Nguyen et al, 2007)C:\Users\Shahrin\Desktop\journal fyp\pics\graff.jpg

Figure 2.10: Effect of particle size on Nu for 6.8% particle volume concentration. (Nguyen et al, 2007)C:\Users\Shahrin\Desktop\journal fyp\pics\figure 7.jpg

The result has revealed that for the same nanofluids family (the same type of constituents), a nanofluid with smaller particle size does provide a better heat transfer. So, for the range of mass flow rate considered in this study and according to the tendency of the curve shown in Figure 2.9, one can clearly see that the value of heat transfer waterblock obtained with 36 nm particle size are consistently higher than 47 nm particles. As result, one can simply explained that for a given volume concentration and with finer particles, their total contact area are higher and the number of particles, will provide a better heat transfer. From the results of Nusselt number as shown in Figure 2.10, for a given Reynolds number, the value of Nu obtained for 36 nm particle nanofluids is clearly higher than the one corresponding to nanofluid with 47 nm particles.

Namburu et al. (2008) obtained from his experiment about the effect of nanoparticles diameter on the Nusselt number for SiO2 nanofluids of 6% volume concentration and revealed that the fluid containing 20 nm particles diameter have higher Nusselt numbers. It is followed by 50 nm and 100 nm particles diameter for a given same Reynolds number. These results achieved because of the viscosity values of 20 nm nanofluid are higher, followed by 50 nm and 100nm. With lower diameter of particles for the same volume concentration, it provides large surface area of interaction with the fluid. Therefore, the higher the viscosity, the higher the Prandtl number for same concentration of SiO2 nanofluids.

Figure 2.11 shows the effect of nanoparticle diameter on the Nusselt number for a 6% volume concentration of SiO2 nanofluids.

Figure 2.11: Effect of nanoparticles diameter on the Nusselt number for a 6% volume concentration of SiO2 nanofluids. (Namburu et al, 2008)C:\Users\Shahrin\Desktop\SiO2.jpg

2.3.4 Advantages of nanofluids

Kondaraju et al. (2009) reveals that the advantages of nanofluids are:

No extra pressure drop

Minimal clogging

Increasing heat transfer

Reducing the pumping power

High-speed flow can be reduced

Lack of erosion of piping components

Lubrication and cooling for the engine operation can be improved

Reduce the size of the heat exchanger

2.4 Summary

In this part, we may conclude that shell-and-tube heat exchanger have many advantages compared to other heat exchanger, which is provides large ratio of heat transfer area to volume and weight, can be easily cleaned, and robustness to high-pressure operations. For the performance of nanofluids, it is basically depends on several parameters such as temperature, particle size and volume fraction. Nanofluids are produced by two techniques, which is one-step process and two-step process. From the experimental results researchers, one can conclude that for CuO, Al2O3, and SiO2 nanofluids at a same concentration, CuO nanofluid give higher heat transfer performance compared to those two. In addition, the effectiveness of thermal conductivity is higher for finer particles use.