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In all these applications, the purpose is to cool the fluid or heat the fluid. However, sometimes the temperature of one fluid increases and the temperature of another decreases, both of which are intended to be used interchangeably. Some heat exchangers are also used to generate steam (evaporate) or convert steam to liquid (condensate).

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In all these applications, the purpose is to cool the fluid or heat the fluid. However, sometimes the temperature of one fluid increases and the temperature of another decreases, both of which are intended to be used interchangeably. Some heat exchangers are also used to generate steam (evaporate) or convert steam to liquid (condensate).

Heat exchangers are classified according to a variety of criteria, most notably: structure, fluid flow arrangement, compression rate of heat transfer, heat transfer process, heat transfer mechanism and fluid number. Exchanges are also categorized from different points of view. Generally fixed heat exchangers are divided into two groups of tubes and plates. The pipe exchanges are in turn made in a variety of ways, depending on their properties, preferring each in particular applications to others. But in general, heat exchangers are the most commonly used pipe heaters in industry.

Since heat exchangers are used to transfer heat between two fluid streams, much attention has been paid to designing and manufacturing heat transfer methods in addition to operational issues. Especially nowadays, with the importance of energy, methods of increasing heat transfer in heat exchangers have become more and more popular. Increasing the heat transfer rate reduces the size of the exchanger and consequently reduces the cost of its construction. Generally the methods of increasing heat transfer in the exchangers are divided into two categories of active and passive methods.

Active methods require external forces. Generating electrostatic fields, surface vibration, fluid vibration, injection and suction are examples of active methods. In passive methods that are more applicable, changes in the geometry of the surface of the heat transfer or the use of additives to the operating fluid increase the rate of heat transfer. Rough surfaces, expanded surfaces, coils, rotational flow, increased surface tension are examples of passive methods.

Spiral exchanges [3]

One of the passive ways to improve heat transfer is to use a spiral structure for pipes instead of regular pipes. Compared to torsion tubes, they have a more compact surface and have a higher heat transfer coefficient and friction coefficient. As the fluid flows through the winding tubes, it is affected by the centrifugal force. This centrifugal force causes a secondary flow in the fluid, thereby increasing the axial velocity of the flow near the outer wall of the tube. Increasing the axial speed will reduce the thermal resistance and consequently increase the heat transfer coefficient. However, the increase in speed also increases the coefficient of friction and consequently increases the pressure drop of the fluid relative to the straight pipe. In torsion tubes, the coefficient of heat transfer and pressure drop in addition to the usual factors such as Reynolds number, Prandtl number, Newtonian or non-Newtonian fluid, wall boundary conditions also depend on such factors as the ratio of tube radius to coil radius and step coil. In general, due to the compact structure and high coefficient of heat transfer in coils, coil tube heat exchangers are used in various industries such as cryogenic industries, liquefied natural gas, food and pharmaceutical industries, refrigeration and air conditioning. Figure (1-1) shows a view of a simple spiral exchange and a binary spiral exchange.

Figure (1-1) Side view and top of two types of spiral exchanges (simple and binary)

second chapter

2-1- Flow through the spiral tubes

Due to the secondary flow created by the centrifugal force in the spiral tubes, velocity components are created in all three coordinate directions even in fully developed flow. In recent years, due to the complexity of the analysis, there are many theoretical and experimental studies on the flow and heat transfer of fluids inside curved tubes that will be reviewed.

Dean first noticed a secondary flow inside the curved tubes and introduced a dimensionless flow control group called the Dean number:

(2-1)

Where d is the diameter of the pipe and R is the radius of curvature of the coil. The geometric parameters of a spiral coil can be seen in Fig. 2-1. P is the vertical step of the coil and the angle of coil twisting. It is worth noting that in this type of coil the geometrical properties mentioned throughout the coil are fixed.

Figure (2-1) Geometric Parameters of a Coil [1]

In torsion tubes, the effective curvature radius in each round is affected by the coil step and is defined as a relation (2-2). Using the curvature radius effective in defining the number of the religion rather than the radius of curvature, a new dimensionless group is created, which is a number. This is called the coil twist (relationship (2-3)).

(2-2) (2-3)

For smooth tubes, one discontinuity appears in the friction factor curve in terms of Reynolds number from which to determine the critical Reynolds number

[1]. Active

[2]. Passive

[3]. Helical Heat Exchangers

[4]. Dean

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