Shell and tube heat exchangers are used extensively throughout the process industry and as such a basic understanding of their design, construction and performance is important to the practicing engineer.
The objective of this paper is to provide a concise review of the key issues involved in their thermal design without having to refer to the extensive literature available on this topic.
The author claims no originality but hopes that the format and contents will provide a comprehensive introduction to the subject and enable the reader to achieve rapid and meaningful results.
The optimum thermal design of a shell and tube heat exchanger involves the consideration of many interacting design parameters which can be summarized 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.
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.
5. Setting upper and lower design limits on shell diameter, baffle cut and baffle spacing.
There are several software design and rating packages available, including AspenBJAC, HTFS and CC-THERM, which enable the designer to study the effects of the many interacting design parameters and achieve an optimum thermal design. These packages are supported by extensive component physical property databases and thermodynamic
models.
It must be stressed that software convergence and optimization routines will not necessarily achieve a practical and economic design without the designer forcing parameters in an intuitive way. It is recommended that the design be checked by running the model in the rating mode.
It is the intention of this paper to provide the basic information and fundamentals in a concise format to achieve this objective.
The paper is structured on Chemstations CC-THERM software which enables design and rating to be carried out within a total process model using CHEMCAD steady state modelling software.
However the principles involved are applicable to any software design process.
In the Attachments a Design Aid is presented which includes key information for data entry and a shortcut calculation method in Excel to allow an independent check to be made on the results from software calculations.
Detailed mechanical design and construction involving tube sheet layouts, thicknesses, clearances, tube supports and thermal expansion are not considered but the thermal design must be consistent with the practical requirements.
Source references are not indicated in the main text as this paper should be considered as a general guidance note for common applications and is not intended to cover specialist or critical applications. Sources for this paper have been acknowledged where possible.
The symbols, where appropriate, are defined in the main text. The equations presented require the use of a consistent set of units unless stated otherwise.
Shell-and-tube heat exchangers are used widely in the chemical process industries, especially in refineries, because of the numerous advantages they offer over other types of heat exchangers. A lot of information is available regarding their design and construction. The present notes are intended only to serve as a brief introduction.
Advantages
Here are the main advantages of shell-and-tube heat exchangers.
1. Condensation or boiling heat transfer can be accommodated in either the tubes or the shell, and the orientation can be horizontal or vertical.
2. The pressures and pressure drops can be varied over a wide range.
3. Thermal stresses can be accommodated inexpensively.
4. There is substantial flexibility regarding materials of construction to accommodate corrosion and other concerns. The shell and the tubes can be made of different materials.
5. Extended heat transfer surfaces (fins) can be used to enhance heat transfer.
6. Cleaning and repair are relatively straightforward, because the equipment can be dismantled for this purpose.
Basic considerations The tube side is used for the fluid that is more likely to foul the walls, or more corrosive, or for the fluid with the higher pressure (less costly). Cleaning of the inside of the tubes is easier than cleaning the outside. When a gas or vapor is used as a heat exchange fluid, it is typically introduced on the shell side. Also, high viscosity liquids, for which the pressure drop for flow through the tubes might be prohibitively large, can be introduced on the shell side.
The most common material of construction is carbon steel. Other materials such as stainless steel or copper are used when needed, and the choice is dictated by corrosion concerns as well as mechanical strength requirements. Expansion joints are used to accommodate differential thermal expansion of dissimilar materials.
Heat transfer aspects
The starting point of any heat transfer calculation is the overall energy balance and the rate equation. Assuming only sensible heat is transferred, we can write the heat duty Q as follows.
The various symbols in these equations have their usual meanings. The new symbol F stands for a correction factor that must be used with the log mean temperature difference for a countercurrent heat exchanger to accommodate the fact that the flow of the two streams here is more complicated than simple countercurrent or cocurrent flow. Consider the simplest possible shell-and-tube heat exchanger, called 1-1, which means that there is a single shell “pass” and a single tube “pass.” The sketch schematically illustrates this concept in plan view. Note that the contact is not really countercurrent, because the shell fluid flows across the bank of tubes, and there are baffles on the shell side to assure that the fluid does not bypass the tube bank. The entire bundle of tubes (typically in the hundreds) is illustrated by a single line in the sketch. The baffle cuts are aligned vertically to permit dirt particles settling out of the shell side fluid to be washed away.
The convention in shell-and-tube heat exchangers is as follows:
T1 inlet temperature of the shell-side (or hot) fluid
T2 exit temperature of the shell-side (or hot) fluid
t1 inlet temperature of the tube-side (or cold) fluid
t2 exit temperature of the tube-side (or cold) fluid
Thus,
The fraction of the circular area that is open in a baffle is identified by a “percentage cut” and we refer to the types of baffles shown as “segmented” baffles. For the shell side, in evaluating the Reynolds number, we must find the cross-flow velocity across a bundle of tubes that occurs between a pair of baffles, and determine the value of this velocity where the space for the flow of the fluid is the smallest (maximum velocity). For the length scale, the tube outside diameter is employed.
Definitions
Heat exchanger configurations are defined by the numbers and letters established by the Tubular Exchanger
Manufacturers Association (TEMA). Refer to Appendix V for full details.
For example: A heat exchanger with a single pass shell and multi-pass tube is defined as a 1-2 unit. For a fixed tubesheet exchanger with removable channel and cover, bonnet type rear head, one-pass shell 591mm (231/4in) inside diameter with 4.9m(16ft) tubes is defined SIZE 23-192 TYPE AEL.
see also Heat Exchanger Books
Tube Diameter
The most common sizes used are 3/4″od and 1″od
Use smallest diameter for greater heat transfer area with a normal minimum of 3/4″od tube due to
cleaning considerations and vibration.1/2″od tubes can be used on shorter tube lengths say < 4ft.
The wall thickness is defined by the Birmingham wire gage (BWG) details are given in Appendix XI(Kern Table 10)
Tube Number and Length
Select the number of tubes per tube side pass to give optimum velocity 3-5 ft/s (0.9-1.52 m/s) for liquids
and reasonable gas velocities are 50-100 ft/s(15-30 m/s)
If the velocity cannot be achieved in a single pass consider increasing the number of passes.
Tube length is determined by heat transfer required subject to plant layout and pressure drop constraints. To meet the design pressure drop constraints may require an increase in the number of tubes and/or a reduction in tube length.
Long tube lengths with few tubes may give rise to shell side distribution problems.
