CC BY
10
UDK 536.27
Mukhammadzhonova I. trainee applicant Namangan Engineering and Technology Institute
Uzbekistan, Namangan city Sayidmuradov M. senior lecturer
Department of Process Machines and Equipment Namangan Engineering and Technology Institute
Uzbekistan, Namangan city Xudayberduyev A.
Associate Professor Namangan Engineering and Technological Institute
Uzbekistan, Namangan city
ANALYSIS OF EXISTING METHODS OF INTENSIFICATION OF HEAT EXCHANGE IN PIPE HEAT EXCHANGERS
Abstrakt: The results of an experimental study are presented to study the intensification of the heat transfer process during air cooling of hydrocarbon vapors and oil distillates in tubular apparatuses and the development of recommendations for increasing the energy efficiency of industrial air and water cooling apparatuses that are part of the primary oil distillation unit.The intensification of heat exchange processes will lead to an increase in the productivity of the operation of oil refineries, a decrease in overall dimensions and production areas. In turn, this allows to reduce the cost of repair and operation of heat exchangers.
Key words: heat exchange processes, heat exchangers, intensification of heat exchange processes, primary oil distillation unit, cooling equipment, heat transfer agents, cooling agents.
Introduction. The technical and technological re-equipment of enterprises, the intensification of technological processes, the improvement of the existing production technology and the introduction of new, energy and resource-saving technologies for the processing of hydrocarbon raw materials are priority areas for the further accelerated development of the oil refining industry of the republic's economy.
Primary distillation plants, which are large-scale technological objects, consume a large amount of heat and electricity. Therefore, in the conditions of constant growth of tariffs for energy carriers, these installations do not always meet modern criteria for the efficiency of thermal energy use. This circumstance indicates the need to find ways to improve the efficiency of oil refineries by reducing energy costs, increasing the use of secondary energy resources,
maximizing the use of heat recovery and optimizing the technological regime of heat exchange equipment.
With regard to the oil refining industry, heat exchangers are classified according to the method of heat transfer and by purpose. By the method of heat transfer, a distinction is made between surface heat exchangers, where heat transfer between heat carriers is carried out through the surface separating them, and mixing devices, in which heat transfer between media occurs through their direct contact. It should be noted that surface heat exchangers are mainly used at refineries, since the mixing of coolant flows in many technological stages of oil processing is excluded [2].
Heat exchangers are also classified according to the direction of movement of the coolant, design features and manufacturing method. In the direction of movement of heat carriers, heat exchangers are distinguished direct-flow, counter-flow, as well as devices with cross-flow.
Air coolers are widely used in the industry of oil refineries, in which a stream of atmospheric air is used as a cooling agent, forced by specially installed fans.
The use of devices of this type allows for significant savings in cooling water, reducing the amount of waste water, eliminates the need to clean the outer surface of the heat exchange tubes. Such devices are used as condensers and refrigerators.
The relatively low heat transfer coefficient from the side of the air flow, characteristic of these devices, is compensated by the significant ribbed outer surface of the pipes, as well as by the relatively high speeds of the air flow.
Air coolers of various types are manufactured according to the relevant standards, which provide for large ranges in terms of surface size, degree of ribbing and type of structural material used for their manufacture [2,3].
The intensity of the heat exchange process in the apparatus is determined by the ratio of the apparatus's thermal performance to the main values that characterize the driving force of the process and the size of the apparatus.
Therefore, intensification of heat exchange is an effective way to solve the problem of reducing the mass and dimensions of heat exchangers, contributes to the design of more efficient and compact devices that provide significant savings in energy, metal and labor costs. As the unit capacity of power plants increases (the main trend in their development), the absolute weight and size characteristics of the heat exchangers included in the plants are increasing. The conclusion is quite obvious that at present and in the future one of the main, technically and economically most affordable and justified ways to reduce the mass and increase the efficiency of power plants is to improve heat exchangers, which can be carried out through the use of effective methods of intensifying heat transfer.
Main part. The problems of improving heat exchangers, i.e. reducing their size and weight (metal consumption), reducing the power of pumping heat carriers through the apparatus under the condition of a fixed heating capacity [4].
In most cases of practical application of methods for intensifying heat transfer, the developers of heat exchangers, in addition to meeting the technical conditions and ensuring the specified performance characteristics of heat exchangers, pursue the following goals:
1.Increase in the thermal power of the existing heat exchanger without changing the power for pumping heat carriers (or pressure losses) at a fixed flow rate of the heat carriers.
2.Reduction of the temperature difference between the heat carriers to ensure the specified heat output with fixed dimensions of the heat exchanger.
3.Reduction of the weight and size parameters of the heat exchanger while maintaining the heat capacity of the heat exchanger and the level of pressure losses in its ducts.
4. Decrease in the power for pumping the coolant with a fixed heat power and maintaining the heat exchange surface area.
Note that goals 1, 2 and 4 correspond to the tasks of energy saving, and goal 3 is resource saving (reduction of metal consumption and cost).
Intensification techniques essentially reduce the thermal resistance of the near-wall layers during convective heat transfer in the heat exchanger, helping to increase the heat transfer coefficient with or without increasing surface area. Sixteen different methods of heat transfer enhancement have been classified by A.E. Bergles et al. [5, 6] and are divided into passive (do not require external energy supply for intensification) and active methods (require external energy supply).
In complex methods of heat transfer enhancement, any two or more of the listed methods (passive and / or active) are used simultaneously.
Tubular heat exchangers of various types and purposes account for 80-90% of the world and domestic market for heat exchangers. The main advantage of tubular heat exchangers is a wide range of operating temperatures and pressures, the ability to use in various industries and types of technical devices and technologies. In this regard, the results of testing these heat exchangers with various heat transfer intensifiers are considered below.
To date, various methods of intensifying convective heat transfer have been proposed and investigated, which can be combined into the following three main groups [4]:
1. The method of artificial turbulization of the flow in the near-wall zone of tubular heat exchangers (E.K. Kalinin, G.A. Dreitser, S.A. Yarkho and S.G. Zakirov), based on the periodic creation of small vortex zones near the wall, which are a source additional turbulization of the flow.
2. Method of swirling flow inside coiled oval pipes (VM Ievlev, Yu.V. Vilemas and BV Dzyubenko) with longitudinal and transverse flow around close-packed bundles of coiled pipes.
3. Method of controlled separation of the boundary layer (AA Zhukauskas and AA Shlanciauskas) in the case of transverse flow around tube bundles by installing special turbulators on their surface.
With regard to the flow of single-phase heat carriers, flow turbulators on the surface of pipes, rough surfaces and ribbed surfaces are used; swirling the flow with spiral ribs, screw devices and swirlers installed at the channel inlet; mixing gas bubbles to the liquid flow, and solid particles or liquid droplets to the gas flow; rotation and vibration of the heat exchange surface; pulsation of the coolant flow, impact on the flow of electrostatic fields, suction of flow from the boundary layer, jet systems, etc.
To intensify heat transfer in air-cooled devices, pipes with external spiral finning are used. In hot climates, to improve the heat transfer coefficient, the air must be humidified before entering the tube bundles.
The relatively low coefficients of heat transfer from the air side compared to the coefficients for cooled or condensed process fluids can be partially compensated by the development of surfaces on the air side. This is done through the use of bundles of finned tubes.
Due to the ribbing, the heat exchange surface can be increased 10-25 times compared to the surface of smooth pipes. The degree of surface development is optimized taking into account economic considerations and manufacturing technology.
One of the optimization criteria is the parameter characterizing the growth of heat transfer during finning per unit of cost, which initially increases with the growth of the A / A surface development, but after reaching the optimal values it starts to decrease (Fig. 1). The maximum value of this function gives the optimum value of the degree of surface development, which increases with an increase in the heat transfer coefficient in the pipes [2].
In the optimized parameter UA/S/C in accordance with Fig. 1, both A / S and U depend on the degree of surface development. The parameter A / S-surface area in contact with air per 1 m2 of the flow area of the air flow can be easily found for the selected type of finned tubes (Fig. 1).
The heat transfer coefficient U depends on a large number of parameters and is determined by the equality:
11 1 _
— =-+-+ R
UA r/Fa0A a A
Where a0 - is the average value of the variable heat transfer coefficient
from the air side; r - efficiency of ribbing; ai - coefficient of heat transfer from
the side of the coolant in the pipes; Rj -thermal resistance, including the contact
resistance between the rib and the supporting pipe, the resistance of deposits inside the pipes, pipe walls and deposits on the outer surface of the pipes, the last two resistances being negligible; A is the total area of the heat exchange surface on one side.
\
\
aj= J00(. - - t '/ N V \
af 200. N / u V \
> / h
/ / \
1 \
!
/
1 2 4 6 8 10 20 40 A/A i Figure: 1. Optimal value of finned tube surface.
A. The most common finned tubes. In fig. 2. Shows typical finned tubes for air cooled heat exchangers and various methods for attaching the fins to the tubes.
The contact resistance at the base of the fin is the limiting factor when using pipe finning.
Aluminum ribs fitted with an interference fit on a steel pipe (Fig. 2, a, i, d) have high contact resistances, which rapidly increase with increasing temperature. Therefore, their use is limited to temperatures up to 100 °C, since at higher temperatures the fastening of the fins to the pipes is weakened due to the greater thermal expansion of aluminum [3].
Ribs installed in grooves and secured as shown in fig. 2.f, are applicable up to a temperature of 350 °C, but this requires pipes with a wall thickness increased by the depth of the groove.
Ribs extruded from thin-walled aluminum pipes (Fig. 2.g) ensure good contact even when using thin-walled pipes, so that operating temperatures up to 250 °C are quite acceptable. Fastening of fins to pipes, shown in fig. 2, b, h, and, lead to lower contact resistances and are used at temperatures not exceeding the melting temperature of the solder. For a pipe with flat fins (Fig. 2, b), it is allowed to use any type of attachment to pipes of any shape and for any size of fins. For plate type fins, it is allowed to use turbulators, which increase heat transfer from the air side at low air speeds and pressure drops.
In hot-dip galvanizing, a uniform metal strip is applied to the finned pipes, which also serves as an additional protection against corrosion. Welded ribs (Fig. 2, c) are used at high temperatures (over 400 ° C), as well as in the absence of the possibility to apply the above-described fastening methods.
Fig. 2. Various geometries and methods of fastening finned tubes:
a - round ribs put on with an interference fit; b-rectangular ribs soldered to round or elliptical pipes; c-bundles of pipes with soldered or stretched rectangular ribs; z-welded single L-shaped ribs; d-superimposed on each other L-shaped ribs; e-ribs inserted into the grooves; w-ribs formed by the extrusion method; h-welded or soldered ribs; i-ribs with metal coating.
Fin efficiency and temperature distribution in finned tubes. The
efficiency of the rib is estimated in accordance with the scheme shown in Fig. 3.
Fig. 3. Temperature distribution in the rib (to determine the efficiency of the
rib).
For flat ribs with constant thickness:
rjF = thX / X here
X = h(2a0 / XF8F )1/2
where h-is the rib height (often not constant); a0 -the heat transfer coefficient (changes along the rib); ap - thermal conductivity of the rib material (for galvanized ribs, the thermal conductivity is selected as a combination of values for the rib material and the zinc layer); sP - rib thickness (often unstable)
[4].
Thus, the chosen method of heat transfer intensification should be effective while maintaining the lowest energy costs required for a known heat exchanger (if the task is to reduce the size) or it should provide a significant reduction in the energy costs for pumping the coolant (if the overall dimensions of the heat exchanger are preserved), or reduce as required ratio and dimensions and energy costs. In addition, when choosing a method for intensifying heat transfer, it is necessary to take into account not only the efficiency of the heat transfer surface itself, but also its manufacturability during manufacture and assembly, as well as the features of the apparatus operation.
References:
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[2]. Технологические расчеты установок переработки нефти: Учеб. пособие для вузов/Танатаров М. А., Ах- метшина М. Н., Фасхутдинов Р. А. и др. М.: Химия, 1997. 352 с. (Technological calculations of oil processing plants: Textbook. manual for universities / Tanatarov M.A., Akhmetshina M.N., Faskhutdinov R.A. et al. M .: Chemistry, 1997.352 p.)
[3]. Справочник по теплообменникам: В 2-х т. Т. 2 /С 74 Пер. с англ. под ред. О. Г. Мартыненко и др.— М.: Энергоатомиздат, 2007. 352 c. (Handbook on heat exchangers: In 2 t. T. 2 / S 74 Per. from English. ed. OG Martynenko et al. -M .: Energoatomizdat, 2007. 352 p.)
[4]. Сайидмуродов М., Эргашев О., Розикова Д. Экспериментальные исследования интенсификации теплообмена с использованием ленточных турбулизаторов при движении двухфазного потока внутри горизонтальных труб. Фергана. Научно-технический журнал Фер.ПИ. 2014 г., №3. 33-37 с. (Sayidmurodov M., Ergashev O., Rozikova D. Experimental studies of heat transfer intensification using belt turbulators during two-phase flow movement inside horizontal pipes. Fergana. Scientific and technical journal Fer.PI 2014, no. 3. 33-37 p.)
