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ADVANCES IN HEAT EXCHANGERS: IMPROVING HEAT TRANSFER IN DIFFERENT DIFFE RENT HEAT HEAT EXCHANGER SAGAR K. GIRI (ID:
[email protected] [email protected])) Contact : 9096100308 HARISH K. BARI (ID:
[email protected] [email protected])) Contact : 9595649202 longitudinal flow pattern, so that tube vibration will not Abstract — In In
this manuscript, research on improving heat transfer rates by modifying the different heat exchanger in discussed. The objective of this paper is to provide a means of increase thermal efficiency with minimum maintenance and maximum runtime. Different heat exchanger describe in this paper are EM baffle technology, Double pipe heat exchanger with/without core rod, Heat recovery steam generators, Heat Wheels. The major problem of fouling in heat exchangers is greatly reduced to about 50% by using EM baffle technology. Efficiency of double pipe heat exchanger is increased by making flow turbulent using screw tape with core rod which gives 10% higher heat transfer rate as compare to screw tape without core rod. The high shear stress and induced turbulence of helically coiled exchangers reduces the tendency of fouling. This results in longer operating cycles between scheduled cleaning intervals. Waste heat recovery steam generator gives hot water or steam by utilizing waste heat of exhaust gases. Heat wheel is used where heat exchange between large amount of air having small temperature difference is required.
occur.
EM baffle heat exchanger
All these heat exchanger modifications modifications gives high heat transfer rates with low maintenance and maximum run time. THE NEW TECHNIQES IN HEAT EXCHANGERS: 1. EMBAFFLE TECHNOLOGY : The EM baffle heat exchanger is a major innovation designed
to improve performance and simultaneously reduce operating costs by reducing fouling losses in sh ell and tube heat exchanger techn ology Figure shows a conventional heat exchanger bundle and EMbaffle EMbaffle bundle. Intensive monitoring and comparative analysis of both the conventional and th e EMbaffle type type of
metal slits (tube support) Advantages:
heat exchanger has demonstrated the enhanced performance from the use of the EMbaffle EMbaffle design How it works The EMbaffle design uses expan ded metal baffles baffles (tube
supports) made of plate material that has been slit and expanded. The open structure results in a low hydraulic resistance and enhanced heat transfer. The EMbaffle allows a Fouling Fouling reduces heat transfer and increases pressure loss
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An EMbaffle test bundle 1) The fouling of heat exchanger is reduced to
half than conventional segmental heat exchanger by using EMbaffle heat exchanger technology 2) Cleaning frequency for EMbaffle heat exchanger technology is less than conventional heat exchanger 3) It has maximum run time which reduces maintenance cost 4) EMbaffle generates 24% higher thermal performance and a substantially lower pressure drop 5) 18% fewer tubes were installed in the EMbaffle heat exchanger, offering a significant cost saving on equipment as compared to conventional heat exchanger. 6) elimination of damaging flow-induced tube
heat recovery system, screw-tape can be well applied due to low Reynolds number in the system. This screw tape can help to promote higher heat transfer exchange rate because of shorter pitch length which leads to stronger swirling flow and longer residence time in the tube. The experimental investigation tells the enhancement of heat transfer in a concentric double tube heat exchanger fitted with tight-fit, regularly spaced and full-length helical screw-tape swirl generators and found that the full-length helical tape with core-rod yields the maximum heat transfer rate at about 10% better than that without core-rod but considerably higher friction loss. Effects of the helical screw-tape with core-rod on the h eat transfer are depicted in graph. It is found that the inner tube containing the helical screw-tape with core-rod gives higher heat transfer rate than that of the plain tube. This can be attributed to swirling effect created from the use of the helical screw-tape, causing higher temperature and pressure gradients in the radial direction. Due to the swirl flow or high tangential velocity component and lower flow cross-sectional area, the mixing of fluid between fluid at the wall region and fluid at the core region induced by the generated centrifugal force has significant ability to enhance heat transfer rate. Furthermore, the swirl enhances the flow turbulence, leading to even better convection heat transfer. Thus, the higher is the Reynolds number, the greater will be the heat transfer rate. Over the range investigated, the mean Nusselt numbers for the enhancement devices with the helical screw-tape (with core-rod) insert are 230% above the plain tube
vibration 7) improved heat transfer capabilities 8) applicable in combination with segmental baffle 9) lower temperature approach 10) uniform flow pattern at shell side 11) reduced weight of heat exchanger 12) Compact heat exchanger design. 2. Double pipe heat exchanger with/without core rod: Double pipe heat exchanger consist of two concentric
pipes carrying hot and cold fluid , which exchanges heat between them. But we can improve heat tran sfer rate by making flow of fluid to turbulent. Turbulent flow can be made by two ways 1. Screw tape with use of core rod. 2. Screw tape without use of core rod. The twisted-tape inserts are extensively used in enhan cement of the heat transfer rate in many heat exchangers. For the screw-tape, the swirling flow rotates in single way smooth direction of flow like a screw motion. Because of lower pressure drop and ease of manufacturing, the screw-tape is, in general, more popular having a higher heat transfer rate In a
flow in a circular tube fitted with the helical tape. (a)screw tape with core rod (b)screw tape without core rod
3 12. When an application requires equipment suitable for high operating pressure and/or extreme temperature gradients, a helical coil unit should be considered.
3. Helically Coiled Heat Exchangers : The basic and most common design consists of a series of stacked helically coiled tubes. The tube ends are connected to manifolds, which act a s fluid entry and exit locations. Th e tube bundle is constructed of a number of tubes stacked atop each other, and the entire bundle is placed inside a casing, or shell as shown in Fig . Once it is positioned in the casing, the assembly forms a helical flow path for both the casing a nd tube side fluid. Standard units have manifold and casing gaskets. Also the welded design is available for high casing-pressure service or hazardous applications. To effectively optimize thermal and hydraulic requirements, the number of tubes (coils) along with their spacing and length may be varied. Advantages: 1. High film coefficients are achieved on both the coil and casing side 2. The helical flow path imparts higher shear rates and turbulence at a given pressure drop, which can result in film coefficients 3. The high shear stresses and induced turbulence of helically coiled exchangers reduce the tendency for fouling 4. This results in longer operating cycles between scheduled cleaning intervals 5. Additionally, the lower fouling tendency permits the use of less conservative safety margins at the in itial design Stage 6. Conventional designs a llow casing-side access for cleaning and inspection. 7. True counter-current flow fully utilizes available LMTD (logarithmic mean temperature difference). 8. Helical geometry permits handling of high temperatures and extreme temperature differentials without high in duced stresses or costly expansion joints 9. High-pressure capability and the ability to fully clean the service-fluid flow area add to th e exchanger’s advantages 10. Higher film coefficients — the rate at which heat is transferred through a wall from one fluid to another — and more effective use of available pressure drop result in efficient and less-expensive designs 11. Improved thermal efficiency, compactness, easy maintenance and lower installed cost.
Spirally wound coils used in steam generator Heat Recovery steam generator.
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CONCLUSION:
1) EM baffle technology of heat exchanger reduces the tendency of fouling by 50% than conventional shell and tube heat exchanger. It has maximum run time which reduces maintenance cost. EM baffle heat exchanger uses 18% less number of tubes than conventional heat exchanger, thus it is more economical to use EM baffle heat exchanger. 2) In heat exchanger with screw tape, heat transfer rate increases due to swirl flow of fluid. Screw tape with core-rod yields the maximum heat transfer rate at about 10% better than that without core-rod. The swirl enhances the flow turbulence, leading to even better convection heat tran sfer. Thus heat exchanger without core rod is more efficient. 3) Helically coiled heat exchangers are most suitable for high pressure and extreme temperatures. The helical flow can result in heat transfer coefficients. 4) Waste heat recovery steam generators are most suitable where large amount of waste heat is available an d generally in combination with boilers. It can operate at high temperature of about 6500c. 5) Heat wheel has higher thermal efficiency up to 85 %. It A heat wheel is finding increasing applications in low to medium temperatur e waste heat recovery systems. Figure is a sketch illustrating the application of a heat wheel. It is a sizable porous disk, fabricated with material having a fairly high heat capacity, which r otates between two side-byside ducts: one a cold gas duct, the other a hot gas duct. The axis of the disk is located parallel to, and on the partition between, the two ducts. As the disk slowly rotates, sensible heat (moisture that contains latent heat) is transferred to the disk by the hot air and, as the disk rotates, from the disk to the cold air. The overall efficiency of sensible heat transfer for this kind of regenerator can be as high as 85 percent. Heat wheels have been built as large as 21 meters in diameter with 3 air capacities up to 1130 m / min. The heat or energy recovery wheel is a rotary gas heat r egenerator, which can transfer heat from exhaust to incoming gases. Its main area of application is where heat exchange between large masses of air having small temperature kudifferences is required. Heating and ventilati on systems and recovery of heat from dryer exhaust air are typical applications. .
can operate even if small temperatur e differences exist. A heat wheel is finding increasing applications in low to medium temperature waste heat recovery systems. References :
1) EMbaffle technologies Coen Gebouw, Kabelweg 37,1014 BA Amsterdam, The Netherlands www.embaffle.com 2) Enhancement of heat transfer in a tube with regularly spaced helical tape swirl generators by Smith Eiamsa-ard, Pongjet Promvonge Dept. of mechanical Engg., Faculty of Engg. King Mongkut’s Institute of Technology Ladkrabang, Bankok 10520, Thailand. 3) Heat transfer characteristics in a tube fitted with helical screw-tape with/without core rod inserts Smith Eiamsa-ard, Pongjet Promvonge a) Dept. of mechanical Engg., Faculty of Engg. Mahanakorm University of Technology Ladkrabang, b) Dept. of mechanical Engg., Faculty of Engg. King Mongkut’s Institute of Technology Ladkrabang, 4) Fuel Economy in furna ce and waste h eat recovery-PCRA Heat recovery system by D.A.Reay,E and F.fF.Span,London, 1979. www.bhes.com/frbbohome.html www.poralenergy.com www.pcra.org
