Research Paper: Shell and Tube Heat Exchangers

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Shell and Heat Tube Exchangers: Shell Tube Heat Excanger Double Pass

The objective of this work is to analyze a horizontal conventional double pass shell and tube heat exchanger with a focus on the features as follows: (1) use of distribution pressure not requiring steam regulating valve or steam control valve; (2) Using a higher temperature steam to reduce the heat exchanger size; 3) Providing condensate sub-cooling in a single heat exchanger; 4) Using discharge control valve to flood heat exchanger to modulate capacity; 5) Eliminates/minimizes water hammer because steam is not traveling over water parallel to the water/steam surface; 6) Uses distribution pressure to push condensate back to the steam plant thereby eliminating a separate condensate pump; 7) Eliminates steam flashing from discharged condensate; 8) Provides a closed steam-water system thereby eliminating air and oxygen introduction into the condensate return; and (9) Flood system rather than a conventional drain system.

Introduction

Shell and tube heat exchangers are used throughout the process industry and extensively. The thermal design of a shell and tube heat exchanger includes many interacting design parameters which are those as follows:

Process

1. Process fluid assignments to shell side or tube side;

2. Selection of stream temperature specifications;

3. Setting shell side and tube side pressure drop design limits;

4. Setting shell side and tube side velocity limits;

5. Selection of heat transfer models and fouling coefficients for shell side and tube side. (Edwards, 2008)

Mechanical

1. Selection of heat exchanger TEMA layout and number of passes;

2. Specification of tube parameters -- size, layout, pitch and material;

3. Setting upper and lower design limits on tube length;

4. Specification of shell side parameters -- materials, baffle cut, baffle spacing and clearances; and

5. Setting upper and lower design limits on shell diameter, baffle cut and baffle spacing. (Edwards, 2008)

There are four basic types of heat exchangers including: (1) tubular; (2) plate and frame; (3) Jacketed; and (4) Coil. (U.S. Department of Energy, nd) Tubular heat exchangers are stated to be "...tube bundles that are surrounded by the heated or heating medium. This type of heat exchanger includes finned tube and shell and tube designs." (U.S. Department of Energy, nd) Finned tube heat exchangers are stated to be most often used in heating air of "drying and space heating applications. Shell and tube heat exchangers are often used for liquid heating and evaporation. Since the tube side of shell and tube heat exchangers can be designed to withstand high pressures, sometimes exceeding 1,500 psig, heat exchangers of this type are often used in high temperature and high-pressure applications." (U.S. Department of Energy, nd) Lastly, jacketed heat exchangers are stated to "...use an enclosure to surround the vessel that contains the heated product. A common example of a jacketed heat exchanger is the jacketed kettle." (U.S. Department of Energy, nd) Coil heat exchangers are stated to "...characteristically use a set of coils immersed in the medium that is being heated. Coil heat exchangers are generally compact, offering a large heat transfer area for the size of the heat exchanger." (U.S. Department of Energy, nd)

The work of Bose (2009) states that a shell and tube heat exchanger "...contains a bundle of tubes inside a large pressure vessel which is referred to as the shell. The tubes on each end are attached to tube sheets. Two different fluids run through the shell and the tube heat exchanger, one through the tubes and the other outside the bundle of tubes within the shell. As the fluids flow through the system, transfer of heat takes place from the fluid that is at a higher temperature to that at a lower temperature. The fluid inside the tubes is called the tube side fluid whereas that which is flowing outside the tubes and within the shell is called the shell side fluid. This heat exchange takes place through the tube walls. The fluid that has to be heated or cooled runs through the tubes. The fluid that runs through the shell outside the tube bundle is heated or cooled depending on whether the fluid inside the tubes have to be cooled or heated. Use of a number of tubes instead of one increases the efficiency of the heat exchanger as circular surfaces of all the tubes increases the net area of transfer of heat." The fluids in the single or one phase heat exchanger are either only gas or only liquid. Two phase heat exchangers involve the transfer of heat between the two fluids of two different phases or the exchange of heat between a liquid and a gas. Boilers are stated to be "two phase heat exchangers in which liquids are heated to a gas." (Bose, 2009) The gas is cooled down into its liquid phase in condensers. The U-Tube Heat Exchanger is comprised by the tubes in the bundle bending and forming a U. It is related that the tube ends "open into plenums or water boxes through holes in the tube sheet. There is an inlet plenum and an outlet plenum. The fluid enters through the inlet plenum, runs the entire length of the U-tube and leaves the heat exchanger through the outlet plenum. As the fluid flows inside the U-tube, heat transfer takes place between the fluid in the tube side and that in the shell side. Two phase U-tube heat exchangers are commonly used in nuclear power plants called pressurized water reactors. Using these heat exchangers, water that has been obtained by condensing steam is heated back to steam that is used to turn steam turbines. As the steam turbines rotate, power is generated." (Bose, 2009)

The work entitled: "Heat Exchangers: Design Considerations" states that heat exchangers are "...ubiquitous to energy conversion and utilization. They involve heat exchange between two fluids separated by a solid and encompass a wide range of flow configurations." The following illustration shows Concentric-Tube Heat Exchangers both the parallel flow and counterflow.

Figure 1

Heat Exchangers: Design Considerations, 2009

The following figure illustrates the cross-flow heat exchangers.

Figure 2

Heat Exchangers: Design Considerations, 2009

The following illustration represents the Shell and tube Heat Exchanger. The Baffles are the method by which a cross-flow is established for induction of turbulent mixing of the shell-side fluid. (Heat Exchangers: Design Considerations, 2009)

Figure 3 Shell and Tube Heat Exchangers

Heat Exchangers: Design Considerations, 2009

Baffles are stated to be used for the purpose of establishing a cross-flow and for induction of turbulent mixing of the shell-side fluid, both of which enhance convection. "(Heat Exchangers: Design Considerations, 2009)

Figure 4

One Shell Pass, Two Tube Passes

Heat Exchangers: Design Considerations, 2009

Figure 5

Two Provisions for Thermal

Two Shell Passes, Four Tube Passes

Heat Exchangers: Design Considerations, 2009

Compact Heat Exchangers are stated to be widely used to achieve large heat rates per unit volume, particularly when one or both fluids is a gas. Compact heat exchangers are characterized by "...large heat transfer surface areas per unit volume, small flow passages, and laminar flow." (Heat Exchangers: Design Considerations, 2009) Included are the following:

(1) Fin-tube (flat tubes, continuous plate fins);

(2) Fin-tube (circular tubes, continuous plate fins);

(3) Fin-tube (circular tubes, circular fins);

(4) Plate-fin (single pass); and (5) Plate-fin (multipass) (Heat Exchangers: Design Considerations, 2009)

Figure 6

Five Types of Fins

Heat Exchangers: Design Considerations, 2009

It is related in the Wolverine Tube, Inc. "Engineering Data Book II" that enhanced tubes are used in the refrigeration, air-conditioning and commercial heat pump industries extensively however, the use of enhance tubes in the chemical, petroleum and other industries is "still not standard practice" although it is a practice that is on the increase. The design of enhanced tubular heat exchangers allows for a more compact design than are able to be used in the conventional plain tube units "obtaining not only thermal, mechanical and economical advantages for the heat exchanger, but also for the associated support structure, piping and/or skid package unit, and also notably reduced cost for shipping and installation of all these components" and this results in a cost factor of approximately two to three times that of the exchanger itself in petrochemical applications." (Wolverine Tube, Inc. 2009) It is additionally related that enhanced evaporators and condensers "enjoy much larger tube-side water velocities, which increases the internal heat transfer coefficient and reduces fouling and scale formation." (Wolverine Tube, Inc. 2009)

According to the Wolverine Tube Heat Transfer Data Book entitled: Construction of Shell and Heat Tube Exchangers" Shell and Tube Heat Exchangers are the most widespread and most often used basic heat exchanger configuration in the process industries. Stated to be the reasons for the general acceptance of these are as follows: (1) The shell and tube heat exchanger provides a comparatively large ratio of heat transfer area to volume and weight. It provides this surface in a form which is relatively easy to construct in a wide range of sizes and which is mechanically rugged enough to withstand normal shop fabrication stresses, shipping and… [END OF PREVIEW]

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