Heat exchangers concentric tube

Concentric tube heat exchangers are widely used because of their simplicity of construction and the ease with which additions may be made to increase the area. They also give turbulent conditions at low volumetric flowrates. 

A power-law non-Newtonian solution of a polymer is to be heated from 288 K to 303 K in a concentric-tube heat exchanger. The solution will flow at a mass flow rate of 210 kg/h through the inner copper tube of 31.75 mm inside diameter. Saturated steam at a pressure of 0.46 bar and a temperature of 353 K is to be condensed in the armulus. If the heater is preceded by a sufficiently long unheated section for the velocity profile to be fully established prior to entering the heater, determine the required length of the heat exchanger. Physical properties of the solution at the mean temperature of 295.5 K are ... 

Fig. 5.2. Several concentric tube heat exchanger configurations, (a) simple heat exchanger (b) multiple-tube heat exchanger (c) concentric tube with a wire spacer or turbulator and (d) bundle heat exchanger. LP refers to the general location of low-pressure flow and HP refers to the normal location of high-pressure flow.

The heat transfer coefficient can be enhanced in the concentric tube heat exchanger by inserting a wire spacer or turbulator in the annular space as shown in Fig. 5.2c. The resulting increase in turbulence increases the Reynolds number, which in turn favorably affects the heat transfer coefficient. The heat transfer can be further improved by soldering the wire spacer to the outside surface of the inner pipe since this increases the effective heat-transfer area for the low-pressure side of the exchanger. 

Example 5.5, If the finned tube in Example 5.4 is placed inside another tube with an inside diameter of 30 mm to form a countercurrent concentric tube heat exchanger, determine the overall heat transfer coefficient for the exchanger when the mass flow rate of the return air at 0.101 MPa and 175 K is 0.094 kg/s. Base the overall heat transfer coefficient on the outside area of the finned tube. 

A standby air liquefaction plant uses a cold carbon dioxide gas to countercurrently precool a 20.2-MPa air stream from 300 to 260 K in an insulated concentric tube heat exchanger. The cold carbon dioxide gas enters the outer annulus at 220 K and exits at 280 K. The mass flow rate of the carbon dioxide gas is 1 kg/s. The outside diameter of the inner pipe is 80 mm, while the inside diameter of the outer pipe is 155 mm. Assuming comparable resistances to heat transfer through the carbon dioxide and air films, evaluate the heat transfer coefficient for the carbon dioxide side of the heat exchanger. At a mean film temperature of 257 K, carbon dioxide gas has the following properties /i= 1.28 x 10 kg/m s Cp = 795.7 J/kg K p = 2.11 kg/ml... 

If the modified concentric-tube heat exchanger of Problem 5.13 is coiled into a helix with an average diameter of 0.5 m, evaluate the overall heat transfer coefficient, the required... 

The per pass ethylene conversion in the primary reactors is maintained at 20—30% in order to ensure catalyst selectivities of 70—80%. Vapor-phase oxidation inhibitors such as ethylene dichloride or vinyl chloride or other halogenated compounds are added to the inlet of the reactors in ppm concentrations to retard carbon dioxide formation (107,120,121). The process stream exiting the reactor may contain 1—3 mol % ethylene oxide. This hot effluent gas is then cooled ia a shell-and-tube heat exchanger to around 35—40°C by usiag the cold recycle reactor feed stream gas from the primary absorber. The cooled cmde product gas is then compressed ia a centrifugal blower before entering the primary absorber. 

Longitudinal fins can also be used, but their application is restricted to small heat exchangers in the form of a concentric pipe heat exchanger, similar to the schematic in Figure 15.5a. In this arrangement, the inner tube would be the extended surface tube with the fins in the annular space to enhance the heat transfer. Longitudinal fins can increase the surface area by a factor of 14 to 20 relative to plain tubes. 

The tube-in-tube or multitube-in-tube heat exchangers are useful in small Linde lique-fiers or in the final Joule-Thomson stage of any liquefier. The performance of Linde-type exchangers is easy to calculate, and their realization is simple. In the examples shown in Fig. 5.12 (a)-(c), the tubes are concentric and the outer wall contributes appreciably to the pressure drop in the outer stream without contributing to the heat transfer. Usually, the smaller inner tube is used for the high-pressure stream and the low-pressure stream flows through the outer annular space. The tubes in Fig. 5.12 (d) and (e) are solder bonded while that in (f) is flattened and twisted before insertion into an outer tube. 

Evaporation. The evaporator is normally needed to remove water from the AN solution. It must produce a solution with the required concentration at a temperature that avoids crystallization. The acceptable water content is normally below l percent for a prilled product and up to 8 percent for the feed to some granulation processes. Evaporators in commercial use include circulatory systems, shell and tube heat exchangers, and falling film-type evaporators.103... 

Some simple heal transfer equipments consist of two concentric tubes, and are properly called double-tube heat exchangers (Fig. 8-27). In such devices, one fluid flows through the tube while the other flows through the aunular space. The governing differential equations for both flow.s are identical. I herefore, steady laminar flow through an annulus can he studied analytically by using suitable boundary conditions. 

FIGURE 8-27 A double-tube heat exchanger that consists of two concentric tubes. 

The mass transfer coefficients Kg and K/ are overall coefficients analogous to an overall heat transfer coefficient in a shell-and-tube heat exchanger. The overall coefficient in a heat exchanger has three components, an inside coefficient, a wall resistance, and an outside coefficient. Analogs exist in mass transfer. For the inside coefficient, we consider the driving force between the bulk liquid concentration and liquid concentration at the interface ... 

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