a review on effect of augmentation techniques on performance

A REVIEW ON EFFECT OF AUGMENTATION TECHNIQUES ON PERFORMANCE PARAMETERS OF SHELL AND TUBE HEAT EXCHANGERS 1

2

3

Kiran K , Nagaraj Y , Rajgopal Asst. Professor, Dept. of Mechanical Engineering, Faculty of Engineering, Christ University, Bengaluru1, 2 Asst. Professor, Dept. of Mechanical Engineering, Basavakalyana Engineering College, Basavakalyana 3

Abstract: The present research review emphasizes on the performance of Heat Exchanger with various types of augmentation techniques. Since Heat Exchanger are widely used in different industries, hence there is a need of utilization of various types of Heat Transfer enhancement Techniques (Active, Passive, combined), which in turns optimizes the performance of available Heat Exchanger setup with different types of turbulators. This results in transformation of fluid flow characteristics from Laminar to Turbulent flow which leads to increase in Heat Transfer characteristics. Finally the conclusion will be drawn by summarizing the literature study and suggesting the new design and modifications in the present turbulators. Index Terms: Heat Exchanger, Pasiive& Active Techniques, Turbulators, Enhancement of heat transfer.

1. INTRODUCTION Due to the scarcity of fossil fuel in the world. The subject of energy consumption and optimization in various industrial processes becomes essential need. In chemical industries most of the processes and the devices related to energy and heat transfer are heat exchangers[1]. Heat exchangers are the devices which are used to transfer thermal energy (enthalpy) between two or more fluids or between a solid surface and a fluid or between solid particulates and a fluid which are at different temperatures. Heat exchanger is based on the concept that the loss of heat on the high temperature side is exactly the same as the heat gained in the low temperature side after the heat and mass flows through the heat exchanger machine. This results in decreasing the temperature of higher temperature side and increasing the temperature of lower temperature side. In a heat exchanger, the temperature of fluid keeps on changing as it passes through the tubes and also the temperature of the dividing wall located between the fluids varies along the length of heat exchanger [21-26]. Heat exchangers are used in different processes like conversion, utilization & recovery of thermal energy in various industrial, domestic applications & commercial process. Most common examples include steam generation & condensation in power & cogeneration plants. Heating & cooling in thermal processing of chemical, pharmaceutical

products. Fluid heating in manufacturing & waste heat recovery etc. The increase in Heat

exchanger’s performance can lead to more economical design of heat exchanger which can help to make savings

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w.r.t. energy, material & cost related to a heat exchange process. The need to increase the thermal performance of heat exchangers effects energy, material & cost savings have led to development & use of many techniques termed as Heat transfer Augmentation. The main idea of Heat transfer enhancement or augmentation techniques are referred to the Optimization of thermo-hydraulic performances[2]. In recent years, heat transfer technology has been wide applied to device applications in refrigeration, automotive, method industries etc. Webb [1994], Bergles [3].The goal of increased heat transfer is to encourage or accommodate high heat fluxes. This leads to reduction of the warmth money handler size, that usually ends up in less opportunity cost. Another advantage is that the reduction of the temperature drive, that reduces the entropy generation and will increase the second law potency Bejan [4]. additionally, the warmth transfer improvement permits the warmth exchangers to control at smaller rate, however still accomplish a similar or maybe higher heat transfer rate. this suggests that a discount of the pressure drop, like less operating expense, is also achieved. of these blessings have created the warmth transfer improvement technology engaging within the device applications notably for the retrofit state of affairs in shell-and-tube heat exchangers.

With tube insert

technology, extra exchangers will usually be avoided, and therefore important price saving becomes potential. Tube insert devices embrace twisted tape inserts, wire

coil

inserts,

extended

surface

inserts, mesh

inserts, etc. Twisted tape inserts cause the flow to spiral on the tube length. they typically don't have smart thermal contact with the tube wall. Wire coil inserts accommodates a turbinate whorled spring that functions as a non-integral roughness. a number of the inserts scale back the hydraulic diameter Associate in Nursingd act as an extended surface. the choice of the tube inserts depends on 2 factors: performance and prices. The performance comparison for various tube inserts could be a helpful complement to the retrofit style of each heat exchangers and warmth exchangers networks. In fact, twisted tape inserts and wire coil inserts area unit additional wide applied than the others. Therefore, this paper concentrates on the comparison of the thermal and hydraulic performance for twisted tape insert and wire coil insert, within the retrofit state of affairs. Firstly, the tube inserts area unit delineated , and general comments on their applications area unit given. Secondly, correlations for twisted tape and wire coil inserts area unit bestowed within the turbulent region, and performance comparison is provided. Thirdly, a comprehensive comparison of the thermal and hydraulic performance for these tube inserts has been administered supported the extended PEC [21-26]. The performance of Associate in Nursing increased device relative to a reference one has been portrayed by many various ratios embodying a good style of the look constraints. On the idea of the first- law analysis many authors Bergles et al.[5], Webb [6] have planned performance analysis criteria (PEC) that outline the performance advantages of Associate in Nursing device having increased surfaces relative to a typical one having swish surfaces subject to numerous style constraints. A solid thermodynamical basis to guage the benefit of augmentation techniques by second law analysis has been planned by Bejan developing the entropy generation reduction (EGM)

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technique. Extended PEC equations are recently developed by Zimparov[7]. Heat transfer Enhancement (Augmentation) techniques are classified into three different categories.

1.1.

1.

Passive Techniques

2.

Active Techniques

3.

Compound Techniques.

Passive Techniques Passive techniques generally use surface or geometrical modifications to the flow channel by incorporating

additional devices. This additional devices promote higher heat transfer coefficients

by disturbing or altering the

existing flow behaviour may be from Laminar to Turbulent, which also leads to increase in the pressure drop. But in some cases like extended surfaces, the effective heat transfer area of extended surface is increased. These Passive techniques hold the advantage over the active techniques as they do not require any direct input of external power. The Heat transfer augmentation by these techniques can be achieved by using: Treated Surfaces: This technique involves using pits, cavities or scratches like alteration in the surfaces of the heat transfer area which may be continuous or dis continuous. They are primarily used for boiling and condensing duties. Rough surfaces: These surface modifications particularly create the disturbance in the viscous sub-layer region. These techniques are applicable primarily in single phase turbulent flows. Extended surfaces: Plain fins are one of the earliest types of extended surfaces used extensively in many heat exchangers. Finned surfaces have become very popular now a days owing to their ability to disturb the flow field apart from increasing heat transfer area. Displaced enhancement devices: These inserts are used primarily in confined forced convection. They improve heat transfer indirectly at the heat exchange surface by displacing the fluid from the heated or cooled surface of the duct with bulk fluid from the core flow. Swirl flow devices: They produce swirl flow or secondary circulation on the axial flow in a channel. Helical tape, & various forms of altered(tangential to axial direction) are common examples of swirl flow devices. They can be used for both single phase and two-phase flows. Coiled tubes: In these devices secondary flows or vortices are generated due to curvature of the coils which promotes higher heat transfer coefficient in single phase flows and in most regions of boiling. This leads to relatively more compact Hear Exchanger. Surface tension devices: These devices direct and improve the flow of liquid to boiling surfaces and from condensing surfaces. Examples include wicking or grooved surfaces.

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1.2. Additives for liquids: This technique involves addition of solid particles, soluble trace additives and gas bubbles added to the liquids to reduce the drag resistance in case of single phase flows. In case of boiling systems, trace additives are added to reduce the surface tension of the liquids. 1.3. Active Techniques These techniques are more complex from the use and design point of view as the method requires some external power input to cause the desired flow modification and improvement in the rate of heat transfer. It finds limited application because of the need of external power in many practical applications. In comparison to the passive techniques, these techniques have not shown much potential as it is difficult to provide external power input in many cases. Various active techniques are as follows: Mechanical Aids: Examples of the mechanical aids include rotating tube exchangers and scrapped surface heat and mass exchangers. These devices stir the fluid by mechanical means or by rotating the surface. Surface vibration: They have been used primarily in single phase flows. A low or high frequency is applied to facilitate the surface vibrations which results in higher Convective heat transfer coefficients. Fluid vibration: Instead of applying vibrations to the surface, pulsations are created in the fluid itself. This kind of vibration enhancement technique is employed for single phase flows. Electrostatic fields: Electrostatic field like electric or magnetic fields or a combination of the two from DC or AC sources is applied in heat exchanger systems which induces greater bulk mixing, force convection or electromagnetic pumping to enhance heat transfer. This technique is applicable in heat transfer process involving dielectric fluids. Injection: In this technique, same or other fluid is injected into the main bulk fluid through a porous heat transfer interface or upstream of the heat transfer section. This technique is used for single phase heat transfer process. Suction: This technique is used for both two phase heat transfer and single phase heat transfer process. Two phase nucleate boiling involves the vapour removal through a porous heated surface whereas in single phase flows fluid is withdrawn through the porous heated surface. Jet impingement: This technique is applicable for both two phase and single phase heat transfer processes. In this method, fluid is heated or cooled perpendicularly or obliquely to the heat transfer surface.

1.4.

Compound Techniques A compound augmentation technique is the one where more than one of the above mentioned techniques is

used in combination with the purpose offurther improving the thermo-hydraulic performance of a heat exchanger.

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1.5.

Performance Analysis Criteria In most sensible applications of sweetening techniques, the subsequent performance objectives, beside a

group of operative constraints and conditions, are sometimes thought-about for optimizing the utilization of a heat exchanger: Increase the warmth duty of Heat Exchanger existing device while not fixing the pumping power (or pressure drop) or flow ratenecessities. Reduce the approach temperature distinction between the 2 heat-exchanging fluid streams for a nominal heat load and size of Heatexchanger. Reduce the scale or heat transfer extent necessities for a nominal heat duty and pressure drop or pumping power. Reduce the process stream’s pumping power requirements for a given heat load and exchanger surface area. 2. TYPES OF TUBE INSERTS USED FOR HEAT TRANSFERENHANCEMENT: 2.1.

Twistedtapes: Swirl flow devices causes swirl flow or secondary flow in the fluid .A variety of devices can be employed

to cause this effect which includes tube inserts, altered tube flow arrangements, and duct geometry modifications. Dimples, ribs, helically twisted tubes are examples of duct geometry modifications. Tube inserts include twistedtape inserts, helical strip or cored screw–type inserts and wire coils. Periodic tangential fluid injection is type of altered tube flow arrangement. Among the swirl flow devices, twisted- tape inserts had been very popular owing to their better thermal hydraulic performance in single phase, boiling and condensation forced convection, as well as design and application issues. Fig.1. shows a typical configuration of twisted tape which is used commonly.

Fig.1. A typical view of Twisted tapes

Twisted tape inserts increases the heat transfer coefficients with relatively small increase in the pressure drop. They are known to be one of the earliest swirl flow devices employed in the single phase heat transfer processes. Because of the design and application convenience they have been widely used over decades to generate the swirl flow in the fluid. Size of the new heat exchanger can be reduced significantly by using twisted tapes in the new heat exchanger for a specified heat load. Thus it provides an economic advantage over the fixed cost of the equipment. Twisted tapes can be also used for retrofitting purpose. It can increase the heat duties of the existing shell and tube heat exchangers. Twisted tapes with multitube bundles are easy to fit and remove, thus enables

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tube side cleaning in fouling situations. Durmus [12] also studied the effect of cutting out conical turbulators, placed in a heat exchange tube, on the heat transfer rate with four different types of turbulators and different conical-angles and reported that the heat transfer improvement depends on

Fig.2. A detailed view of topologies of Twisted tapes

Eiamsa et al, 2007[8] revealed that the lowest value of regularly spacing twisted tape gives the heat transfer lower than full length twisted tape around (5- 15 %) while it can be decreased the pressure drop around 90%. Sivashanmugam and Nagarjan, 2007[9] proved that the heat transfer coefficient enhancement through a circular tube fitted with right and left helical screw inserts is higher than that for straight helical twist inserts of equal and unequal length for a given twist ratio. 2.2. V NozzelsInserts:

Fig.3. A Typical view of V Nozzels Inserts Yakut et al. [10] experimentally investigated the effect of conicalringturbulators on the turbulent heat transfer, pressure drop and flow-induced vibrations. Their experiments were analyzed and presented in terms of the thermal performances of the heat-transfer promoters with respect to their heat-transfer enhancement efficiencies for a constant pumping power. Yakut and Sahin [11] again reported the flow- induced vibration characteristics of conical-ring turbulators used for heat transfer enhancement in heat exchangers. They found that the Nusselt number increases with the rise of Reynolds number and the maximum heat transfer is obtained for the smallest pitch arrangement.

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Figure-4 2.1. Helicaltapes: Helical inserts are another classification in the list of inserts used for heat transfer augmentation. The helices provided on the core rod creates turbulence and swirl in the flow and hence assists in heat transfer enhancement. Eiamsa et al. (2007) conducted experimental studies and investigated the heat transfer and pressure drop characteristics in single phase fluid by inserting helical inserts in the core region of the tube.

Fig.5. A detailed view of topologies of Helical tapes. Extensive studies have been performed to modify the helical tapes in order to improve their performance with regard to the typical one. Gul and Evin [13] experimentally studied the heat transfer and fluid friction characteristics for turbulent flow using short helical swirl generators with different helix angles of 30°, 45° and 60° placed at the entrance of the test section for the Reynolds number range of 5000 to 30,000. Zohir et al. [14] investigated the heat transfer and pressure drop characteristics for turbulent flow in a sudden expansion pipe fitted with propeller type swirl generators for the Reynolds range between 7500 and 18,500 with several pitch ratios under a uniform heat flux condition. Eiamsa-ard and Promvonge [15] studied on the heat transfer characteristics in a tube fitted with helical screw tape with/without core-rod inserts. Kurtbas et al. [16] investigated the performances of heat transfer and pressure drop through a tube with different swirl generators for Reynlods number range of 10,000 to 35,000 under a constant heat flux condition. Sarkar et al. [17] experimentally studied the heat transfer in turbulent flow through a tube with wire-coil inserts.

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2.3.

hi TRAN wirematrix: Sometimes the problem to be solved is simple- poor thermal performance. Although the heat exchanger

designer always aim for high heat transfer coefficient this can sometimes be difficult to achieve with a conventional plain tube design. In many cases this due to the properties of tube side fluid such as high viscosity and low thermal conductivity. Occasionally low heat transfer rates are a consequence of the arrangement of the exchanger such as when single pass tube bundles are require. Whatever the reason, poor tube side performance can usually be avoided by considering the use of heat transfer enhancement technologies. Engineering devices such as hiTRAN matrix elements in variably provides increased heat transfer relative to the plain tube. When fluid flow through the plain tube the fluid nearest to the wall is subjected to the frictional drag which has the effect of slowing down the fluid at the wall this laminar boundary layer can significantly reduce the tube side heat transfer coefficient and consequently the performance of heat exchanger. Inserting correctly the profiled hiTRAN wire matrix element into the tube will disrupt the laminar boundary layer, creating the additional fluid shear and mixing, her by minimizing the effect of frictional drag. hiTRAN wire matrix tabulators are particularly effective at enhancing heat transfer efficiency in tubes operating at low Reynolds number (Laminar to transitional flow). Although heat transfer increase is greatest in the laminar flow region (up to 16 times) significant benefits can be obtained in the transitional flow regime (up to 12 times) and turbulent regime (up to 3 times). Whilst there is an increase in frictional resistance associated with hiTRAN system, the amount of enhancement such that solution can be found which offer increased heat transfer at equivalent or low pressure drop than a plaintube.

Fig.6. A typical view of hi TRAN wire matrix The mechanism of laminar heat transfer in horizontal tube is complex as they can be forced, natural and mixed convection. The dominant mechanism depends on the condition and physical properties of the fluid being heated or cooled [18] (Holmen 1992). The fluid is forced through the tube at low enough velocities the natural convection buoyancy force still have effect on flow pattern inside the tube. Metais and Exkert (1964) [19] have

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proposed the forced mixed and free convection regimes in horizontal tube. Nusselt number correction by Sieder and Tate (1936) and Oilver (1962) for laminar, forced and mixed convection are used to compare the result from with various heat transfer parameter in case hiTRAN tube insert Chandrasker et al (2010-2011) conducted the experiments which involve usage of a wire coil insert fitted in circular tube which showed that there was rise in heat transfer rate with insignificant rise in friction factor in plain tube and tube with wire coilinserts.

2.4.

Solded wire woundinserts: Selvam, S., et al [20] showed the effect of bonding and without bonding of wire coiled coil matrix turbulator

on the heat transfer for a fully developed turbulent flow. Experiments are conducted by maintaining constant wall temperature. Tests are performed on three different wire coiled coil matrix turbulators of different pitches of 5, 10, and 15 mm without bonding of the turbulator. Three similar types of heat exchangers are fabricated and the wire coiled coil matrix turbulators with different pitches of 5, 10, and 15 mm are inserted in the heat exchangers and bonding is done on the surface of the tube section. Results have indicated that the heat transfer rate enhances inversely with the pitch of the wire coiled coil matrix turbulator with bonding. With a pitch of 5 mm, the turbulators without bonding have resulted in almost 25.4% enhancement when compared with plain tube. On the other hand, for pitches of 10 mm and 15 mm the enhancement were 20.7% and 16.8%, respectively. The empirical correlations developed for turbulators with and without bonding results in ±6% deviation for Nusselt number and ±3% for friction factor. Similarly with a pitch of 5 mm, the turbulators with bonding have resulted in almost 42% enhancement. For pitches of 10 mm and 15 mm the enhancements were 34.7% and 25%, respectively.

Fig.7. A typical view of Solded wire wound inserts wire matrix 3. FUTURE WORK TO BE CARRIEDOUT: All the literature reviews that using various kinds of inserts results in enhancement of Heat transfer characteristics. Above works are carried out with water. So, there is a need to carry out research by using different fluid medium, nanoflids with incorporation of inserts. Modifications like holes in cones, baffles, making holes, Ucut, etc., can be employed in twisted tapes. REFERENCES

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