SCOPE OF MECHANICAL DESIGN
A designer must know how to apply certain codes and standards, which are available for heat exchanger. In mechanical designing of heat exchanger, first of all the designer decides the type of heat exchanger. Following are the important points, which should be kept in mind while doing mechanical design of heat exchanger.
To calculate shell thickness against design pressure.
To check the thickness of tube against design pressure.
To calculate the channel thickness that is of both front and rear ends.
Flange calculations.
Tube sheet calculations.
To design and calculate the thickness and dia of baffles.
To calculate number of tie-rods and dia of tie-rods.
To calculate the number of spacers, dia and length of spacer.
To calculate the size and thickness of partition plates.
Location of Impingement plate.
To design and calculate size and thickness of inlet and outlet nozzles.
To select and calculate the size of gasket.
Calculation of Hard-Ware items that is bolts, Nuts, washer etc.
Material selection.
MECHANICAL DESIGN CODES
Introduction
Pressure vessel codes, which also cover the mechanical design of shell and tube exchangers, fulfill various functions; in several countries a national code is legally enforced, and compliance with the code is mandatory for items supplied to that country, whether built there or imported. Table 6.1 shows the status of the principal codes covering exchanger design. Where no code is shown for a country, the table shows the codes generally accepted.
The requirements of U.S., U.K., and German codes, as they affect shell-and-tube exchangers, are listed. The codes aim to achieve safe construction and give rules for design and fabrication, which are based on experience with conventional plant. However, when the components differ from conventional design or are outside the range of the code rules, or when more certain assurance of safety is required, it may be necessary to justify the design using alternative codes or a stress analysis or component testing. These alternatives must be agreeable to the purchaser and the inspecting authority.
Codes also provide a useful tool for the education of junior engineers or those entering the vessel and exchanger field, and this aspect should not be overlooked when new codes or rules are being formulated. As well as giving rules for design and fabrication, most codes are specific as to acceptable materials, but usually the range of materials is wide enough to allow the designer a choice.
Once the designer has made a choice of material, the design code gives an allowable design stress that is used to dimension the pressurized components, and the code specifies any special fabrication requirements for that material, such as heat treatment, for example.
Although codes do list acceptable materials, they also permit other materials.
ASME VIII
The listed materials are taken from specifications of the ASTM. In order to use other materials for ASME-coded vessels, special application must be made to, and approval secured from, the ASME Code Committee.
Table 0.1 Principal pressure vessel codes BS 5500
Materials other than those listed in the code may be used by agreement between purchaser and manufacturer provided that they are covered by a written specification as comprehensive as the BS specification for the equivalent material and that, the design stresses are determined in a manner consistent to BS.
A. D. Merkblatter
The A. D. Merkblatter W series of specifications lists acceptable materials, but others may be authorized with the agreement of the inspecting authority. In the latter case the W specifications give requirements that must be satisfied.
ASME Boiler and Pressure Vessel Code Section VIII, Div. 1
This code gives minimum requirements for the design, fabrication, inspections and certification of vessels with design pressures between 1.03 bar g (15 psig) and 206 bar (3000 psig). Where the design needs to be justified by a full stress analysis, Div. 2 of this code
should be used. New editions of the code are usually issued every 3 year, but interim revisions are made twice yearly in the form of addenda. The ASME code references used refer to the 1977 edition.
TEMA: (Standards of Tubular Exchanger Manufacturers Association)
These standards serve to supplement and define the ASME Pressure Vessel Code for all shell-and-tube exchanger applications (double-pipe exchangers are not included).
Although TEMA is linked specifically to ASME VIII, it is a useful standard that can be used to supplement other national codes. Recommendations for construction are given in three classes, the class being specified by the purchaser. The design rules for each class are the same, the only difference being in dimensions and details of construction.
Class R is for the generally severe requirements of petroleum and related processing applications. Class C is for the generally moderate requirements of commercial and general process service. Class B is for chemical process service.
The numbering system is common to all classes, and the TEMA references used refer to class R, C, or B of the 1978 edition. This edition carries for the first time a section entitled
"Recommended Good Practice" relating to aspects not covered in the main sections of the standard, particularly the requirements for exchangers with shell diameters from 1524 to 2540 mm (60 to 100 in). The numbering system used is the same as in the three main sections of TEMA, and an * is used throughout to denote that there is an additional recommended good practice.
BS 5500: The British Standard Specification for Unfired Fusion-Welded Pressure Vessels This recently introduced code (41 replaces BS 1500 and 351515 and is intended to unify the U.K. requirements for all pressure vessels. A major departure for this code is that the purchaser now specifies the construction category (BS 5500 3.4), which then defines the amount of non-destructive testing and restricts the permitted materials. The degree of testing is not now reflected by the use of design stress-reduction factors. The code references used in Sec. 6.3 refer to the third issue of BS 5500.
A.D. Merkblatter: German Pressure Vessel Regulations
These regulations are in the form of data sheets covering different aspects of vessel design and construction, and are produced by a group of associations. Revisions are made from time to time to keep up with advances in the knowledge. Some aspects of vessel and exchanger design are not covered, and the method is agreed upon by the purchaser, inspecting authority, and designer. The code references used refer to the 1977 edition of A. D.
Merkblatter.
International codes
The International Standards Organization (ISO) has been endeavoring for some time to write an international code for pressure vessels. Various working groups have written draft sections of the code, and these were submitted in late 1973 to form a proposed draft standard DIS 2694, this draft was considered by the subscribing countries to ISO, but when votes were cast in 1974 many major countries voted against its acceptance. The draft was then sent back to the organizing secretariat and working groups with comments, but there has been no action to date.
In Europe, the EEC has published a Council Directive of July 27, 1976, giving a general framework to common provisions for vessels and methods of inspecting them. The intention is that separate directives for different categories of vessels will lay down the technical requirements for design and methods of inspection, and thus will achieve free movement of vessels within the EEC and avoid multiple inspections.
European standards will be organized by CEN (Comite European de Normalisation), and a draft for "simple" pressure vessels has been written. Unfired pressure vessels are being considered by CEN working group 54, but the EEC Directive is awaited.