Mechanical Constraints on Thermal Design of Shell and Tube Exchangers

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Process Engineering Guide: GBHE-PEG-HEA-512

Mechanical Constraints on Thermal Design of Shell and Tube Exchangers Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Mechanical Constraints on

Thermal Design of Shell and Tube Exchangers

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 STANDARD DIMENSIONS 3 4.1 Shell Diameters 3 4.2 Tube Lengths 4 4.3 Tube Diameters 4 4.4 Tube Wall Thicknesses 4 5 CLEARANCES 5 5.1 Tube Pitch 5 5.2 Pass Partition Lane Widths 6 5.3 Minimum 'U' Bend Clearance 6 5.4 Tube-to-Baffle Clearance 6 5.5 Baffle-to-Shell Clearance 6 5.6 Bundle-to-Shell Clearance 7



6 TUBESHEET THICKNESS 8

7 END ZONE LENGTHS 12 8 TUBE COUNTS 14 8.1 Program Correlations 15 8.2 Use of Tube count Tables 15 8.3 Graphical Layout 15 8.4 Use of Computer Programs 15 8.5 Tie Rods 15 TABLES 1 HEAT EXCHANGER SHELLS - GEOMETRICAL DATA

FOR INLET & OUTLET BRANCHES: PIPE WITH ANSI 150 FLANGE 13

2 HEAT EXCHANGER SHELLS - GEOMETRICAL DATA

FOR INLET & OUTLET BRANCHES: PIPE WITH ANSI 300 FLANGE 14

3 TEMA TIE ROD STANDARDS 16



FIGURES 1 DEFINITION OF TUBE PITCH, LIGAMENT THICKNESS

& PASS PARTITION LANE WIDTH 5

2 DEFINITION OF PASS PARTITION LANE WIDTH FOR

U-TUBES 7

3 BUNDLE TO SHELL CLEARANCES FOR DIFFERENT

BUNDLE TYPES 8

4 ESTIMATED TUBESHEET THICKNESS FOR FIXED TUBE CONSTRUCTION 10

5 ESTIMATED TUBESHEET THICKNESS FOR U-TUBE

CONSTRUCTION 11

6 END ZONE 13 7 EXAMPLE OF OPTU3 GRAPHICAL OUTPUT 16 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 17



0 INTRODUCTION/PURPOSE The thermal design of a shell and tube exchanger is influenced by mechanical requirements such as materials thicknesses and clearances necessary for fabrication purposes. Although these are ultimately determined by the Mechanical Designer, with reference to the appropriate codes, it is helpful if the thermal designer is aware of the constraints, to avoid unnecessary recycling in the design process. 1 SCOPE This document gives guidelines on dimensions to be used in the thermal design of shell and tube heat exchangers. Dimensions given in this guide are only for use in thermal calculations. 2 FIELD OF APPLICATION This Guide applies to the process engineering community in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide, the following definitions apply: HTFS Heat Transfer and Fluid Flow Service. A cooperative research

organization, with headquarters at Harwell, UK, involved in research into the fundamentals of heat transfer and two phase flow and the production of design guides and computer programs for the design of industrial heat exchange equipment

HTRI Heat Transfer Research Incorporated. A cooperative research

organization, based in the USA, involved in research into heat transfer in industrial sized equipment, and the production of design guides and computer programs for the design of such equipment.

TEMA Tubular Exchanger Manufacturers Association. An organization of

(US) heat exchanger manufacturers. Their publication "Standards of the Tubular Exchanger Manufacturers Association" is a widely accepted industry standard.



4 STANDARD DIMENSIONS In the past, it has been the practice to have a range of standard shell diameters and tube lengths, based on imperial units. These standards can now be regarded as obsolete in most cases, and the Engineer can in general specify what is really wanted. Some Projects or Works may still require the use of standard dimensions, for reasons such as to reduce the range of spares required. These should be agreed before commencement of the design. 4.1 Shell Diameters There is no need to use the Tubular Exchanger Manufacturers Association (TEMA) standard shell internal diameters, except for carbon steel shells of diameter less than or equal to 18", where shells can be made from standard pipework. For all other materials, and for larger diameter carbon steel shells, the shells are formed by rolling and welding, so any diameter can be made. However, for small diameter shells (less than 8") in other materials, it may be worth investigating the availability of standard pipe. 4.2 Tube Lengths There is no need to use standard tube lengths in multiples of feet. If a tube length of 3 m is required, then that length should be specified rather than 3.048 m (10'). Above 12 m, tubing becomes more expensive, but is available; exchangers with tube lengths up to 75' (22.86 m) have been built for special situations. However, such tubing may not be available in stock. For U-tube exchangers in particular, the requirement for standard tube lengths makes little sense, as before bending the various tubes will be of different lengths, depending on whether they are for the outside or inside of the bundle. 4.3 Tube Diameters Heat exchanger tubing is specified on an outside diameter (O.D.) basis. Tubes are available in either Imperial or Metric sizes, although Metric sizes in the UK generally command a premium.



Imperial sizes are typically: - 0.625”, 0.75", 1.0", 1.25" and 1.5" O.D. Of these 0.75" and 1.0” are

preferred, as they are most commonly available. Typical metric sizes are: - 16, 20 and 25 mm O.D. But intermediate sizes are available. The availability of tubing in some exotic materials (e.g. Hastelloy) is limited, and sometimes only unusual sizes may be readily obtained (e.g. 26.2 mm O.D.). Vessels Section should be consulted before the thermal design is finalized. 4.4 Tube Wall Thicknesses Tube wall thicknesses for imperial sized tubes are normally expressed in terms of Standard Wire Gauge (swg) or Birmingham Wire Gauge (bwg). Usually the even numbered gauges are preferred. For metric tubing, the equivalent metric size will be used. It is exceedingly unlikely that the tube thickness will be determined by the simple pressure containment criteria (i.e. hoop stress); because of the small diameters used, even very thin walled tube would stand very high pressures. The decision on what thickness to use is generally determined more by weld details and longitudinal stresses. Beware of unthinkingly specifying corrosion allowances of 2-3 mm for the tubes, as these can result in excessive tube thicknesses. It is normal practice to have no corrosion allowance for tubes (but see below). If in doubt, Metallurgical specialist should be consulted. For carbon steel, the minimum thickness used is normally 14 swg (0.080”, 2.00 mm); in the North West, particularly on cooling water duties where water corrosion may be expected, 12 swg (0.104”, 2.65 mm) or even 10 swg (0.128”, 3.15 mm) are commonly specified. For stainless steels, typical thicknesses range from 12 swg (0.104”, 2.65 mm) to 16 swg (0.064”, 1.6 mm). For the more exotic materials, which will be specified for their low corrosion, thinner tubing is often used to reduce the cost. Thicknesses down to 22 swg (0.028”, 0.71 mm) have been used for Hastelloy and tantalum. Note: The metric dimensions given above are the 150 metric preferred series equivalents of the swg sizes, not direct conversions).



5 CLEARANCES Minimum clearances between the components of the exchanger have to be maintained either to allow for appropriate weld details, or to accommodate manufacturing tolerances. 5.1 Tube Pitch The minimum tube pitch, defined as the distance between the centers of neighboring tubes, depends on the method of tube to tubesheet attachment. TEMA recommends a minimum pitch of 1.25 times the tube outside diameter. However, for tube sizes of less than 1” this would in general only allow an expanded tube-tubesheet joint. The GBHE standard for all process exchangers is to use welded tube-tubesheet joints. The minimum thickness of metal in the tubesheet between adjacent tubes, known as the 'ligament' (equal to the tube pitch minus the tube O.D.) The minimum ligament is necessary in part to avoid overlap of the welds for neighboring tubes (Figure 1). A 6 mm ligament allows a medium integrity weld. This is suitable for most cases, including exchangers handling toxic or flammable materials.



FIGURE 1 DEFINITION OF TUBE PITCH, LIGAMENT THICKNESS & PASS PARTITION LANE WIDTH

For critical duties, where leakage may be catastrophic due to interaction of the hot and cold streams (e.g. chlorine/water), a 10 mm or 0.375" ligament is necessary for a high integrity weld, to avoid overlap of the welds from neighboring tubes. (Some manufacturers may be able to produce high integrity welds with a 6 mm ligament, using an orbital welding machine, but this should not be assumed without consulting Vessels Section). Explosion welding usually also requires a 10 mm ligament; the stresses induced in the tubesheet during the welding process may cause excessive distortion with smaller ligaments.



5.2 Pass Partition Lane Widths A greater ligament is necessary between the outer tubes in neighboring passes than between the tubes in a pass, to allow for the pass partition plate. This ligament is known as the pass partition lane width (Figure 1). The default values assumed by the thermal design programs are too small to allow for reasonable plate thickness and a welded tube-tubesheet joint (HTFS use 0.625" in OPTU3, 0.625" - 0.75" in TASC3; HTRI use between 0.625” and 0.875” depending on shell diameter). A reasonable clearance is 1" or 25 mm. 5.3 Minimum 'U' Bend Clearance The pass partition lane width for U-tube bundles is determined by the tightness to which the tubing can be bent. The minimum radius achievable for carbon steel is generally 1.5 times the tube diameter, giving a centre-centre distance of 3D, and a pass partition lane width of 2D (Figure 2). Note that this only applies to the pass partition lane normal to the plane of the Ubend; the other lanes will be governed by the need for weld clearance as above. For some materials, for example nickel alloys, it may not be possible to achieve such tight bends. Further information is available in the standard B163 of the American Society for the Testing of Materials (ASTM). Vessels section should be consulted for advice. 5.4 Tube-to-Baffle Clearance For a maximum unsupported tube length of 36", TEMA recommends a clearance of 1/32" (0.8 mm). However, this may lead to an excessive tube-to-baffle leakage, resulting in a low cross flow fraction. A clearance of 0.4 mm, the TEMA standard for unsupported lengths above 36", is the normal GBHE standard for all baffle pitches. 5.5 Baffle-to-Shell Clearance The size of this clearance is governed by the allowable tolerance on circularity of the shell, particularly if rolled. The values in TEMA, which are the default values used by the standard exchanger programs, are reasonable.



Note that the quoted tube-to-baffle and baffle-to-shell clearances are those for a clean exchanger. If the shell side suffers from fouling in service, these clearances may become blocked by the dirt deposits. When checking designs, calculations should also be done with zero clearance, as this will give the worst case for vibration and pressure drop. In some cases, the pressure drop may rise by a factor of 2 to 4 times the clean value. Some of the computer programs used for thermal rating interpret a zero clearance in the input as a request for the default value. In these cases, it is necessary to input a small value, e.g. 0.1 mm. FIGURE 2 DEFINITION OF PASS PARTITION LANE WIDTH FOR U-TUBES



5.6 Bundle-to-Shell Clearance The value used for this has an effect on the tube count (see below) and also on the 'C' stream leakage (bypassing round the outside of the bundle). A high clearance, with a consequent high leakage, is undesirable as it will in general seriously affect the exchanger performance. The minimum value obtainable depends on the type of exchanger. The minimum clearance is obtained with a fixed tubesheet or U-tube bundle, and the maximum with a pull through floating head. The values used by the HTRI program ST-4, which are shown in Figure 3, are reasonable for most purposes. FIGURE 3 BUNDLE TO SHELL CLEARANCES FOR DIFFERENT BUNDLE

TYPES



6 TUBESHEET THICKNESSES An estimate of the tubesheet thickness is necessary to determine the length of tube available for heat transfer, and to calculate the baffle pitch and end zone lengths. The precise calculation of the necessary tube sheet thickness is a complex procedure beyond the scope of this Guide. Consideration should be given to both bending and shear forces, and a knowledge is required of the mean metal temperatures for both shell and tube, as well as the pressures, under normal operation and upset conditions. The tubesheet thickness for fixed tubesheets can be estimated using Figure 4. These data were calculated using BS 5500 (1991), assuming all carbon steel construction, with the tubeplate welded to both shell and channel and the tube metal temperature 20°C hotter than the shell. Departure from these assumptions will introduce error. For U-tube exchangers, the formula for bending is usually the dominant consideration. BS 5500 gives the following equation:

where: t is the tubesheet thickness C is a constant (take as 0.66) Do is the outer tube limit EP is the differential pressure across the tubesheet ] is a constant (take as 2) o is the ligament efficiency = ligament/tube pitch f is the design stress (take as yield).



Note:

For carbon steel: f will normally range from 150 N/mm2 at ambient to 120 N/mm2 at 250° C. Figure 5 is based on this formula. (The figures are for guidance only)



FIGURE 4 ESTIMATED TUBESHEET THICKNESS FOR FIXED TUBE CONSTRUCTION



FIGURE 5 ESTIMATED TUBESHEET THICKNESS FOR U-TUBE CONSTRUCTION



7 END ZONE LENGTHS The central baffle pitch can usually be specified simply from hydraulic considerations to give an acceptable velocity and pressure drop. This does not apply to the inlet and outlet end spaces, which are influenced not only by the diameter of the nozzles, but also by the mechanical details of the exchanger. Tables 1 and 2 give values of minimum end space for a range of nozzle sizes. These data are intended as a guide for use in thermal design, and as approximations for branch positioning where no exact dimensions have been established by detailed mechanical design. Where exact dimensions do exist, they should be used in preference to these Tables. The assumptions made in deriving the Tables were: (a) The branch is a pipe. (b) Pipe wall thickness is 10 mm in all cases, with dimensions being calculated on pipe nominal bore. (c) The tubesheet thickness, including tube standout, is estimated at 50 mm for Class 150 pressure rating and 75 mm for Class 300 rating. A 15 mm deep spigot is assumed on the tubesheet back face for the tubesheet to shell weld. (d) The pipe flange edge is aligned with the back face of the tubeplate flange extension, as illustrated in Figure 6. The data are principally for tubeplates which are welded directly onto the exchanger shell. Where the tubeplate is clamped between a pair of flanges, an additional allowance is required for the flange. Typical flange thicknesses are between 50!mm and 150 mm for design pressures between 10 bar and 100 bar. Notes: 1 Tables 1 and 2 are based on PIPE nozzle walls. 2 The data shown in Tables 1 and 2 will normally be adequate for most thermal design/approximate orientation purposes, but should be used with caution if: (a) tubesheet thickness is likely to differ significantly from given values, (b) detailed dimensional data (e.g. exact end baffle pitch for a construction drawing) is required.



FIGURE 6 END ZONE



TABLE 1 HEAT EXCHANGER SHELLS - GEOMETRICAL DATA FOR

INLET & OUTLET BRANCHES: PIPE WITH ANSI 150 FLANGE (50 mm Tubesheet assumed)



TABLE 2 HEAT EXCHANGER SHELLS - GEOMETRICAL DATA FOR

INLET & OUTLET BRANCHES: PIPE WITH ANSI 300 FLANGE (75 mm Tubesheet assumed)



8 TUBE COUNTS The number of tubes which can be fitted into a given shell size is determined by many factors, based on required clearances, as described above. Relevant factors are: (a) Tube diameter. (b) Tube pitch. (c) Layout angle. (d) Bundle-shell clearance. (e) Number of tube-side passes and pass arrangement. (f) Minimum pass partition lane width. (g) Whether or not the exchanger layout is symmetrical about the axes. (h) Whether or not the tubes in different passes are to be aligned on the same overall grid. (j) The presence or absence of an impingement plate. (k) The diameters of the shell side nozzles. (l) The requirements for tie rods. 8.1 Program Correlations Most shell and tube thermal programs have built-in correlations for tube count. These can be useful guides in the early stages of a design. However, while these may take account of some of the above factors, they can only be regarded as approximations, and may predict tube counts significantly different from what can be achieved. This is particularly the case for small diameter exchangers with many tube passes.



8.2 Use of Tubecount Tables Standard tube count tables are available in the literature. They do represent tubecounts which can be achieved, based on assumed, (not always stated) values of the above variables, but may not be applicable to the GBHE standards for tube pitch etc. 8.3 Graphical Layout The traditional way to determine the tubecount for an exchanger is to draw out the tubesheet with the correct clearances. While this does give a tubecount which can be achieved, it is a time consuming process, especially for a multi-pass exchanger. Moreover, some skill is needed to produce the optimum arrangement. 8.4 Use of computer programs Computer programs are available for estimating the tubecounts in a shell and tube exchanger. Typically the programs allow for different pass layouts for multi-pass exchangers, and the user can input the various clearances desired. Note that the default values for tube pitch, clearances etc. may differ from those given in Clause 5. Clause 5 values should be used in the program input. The programs attempt to fit the maximum number of tubes into a shell within the constraints of the clearances, while keeping the numbers of tubes in different passes to near the same value. The program can either be run to estimate the number of tubes in a given shell diameter, or, using 'WRAP' options, the programs will provide the shell diameters and estimate the tubecounts for the two 'standard' shells which contain more and fewer tubes than the desired tubecount. There are graphical output options, which will produce a scale drawing of the tubesheet. An example of a graphical output is given in Figure 7. Programs do not cover every possibility. For instance, one program offers a maximum of six alternative pass layouts for multi-pass arrangements, and certain combinations of baffle orientation and pass arrangement are excluded. With a certain amount of ingenuity, most of these constraints can be overcome, but there will be cases where there is no substitute for hand layout.



8.5 Tie Rods The transverse baffles or support plates in a shell and tube exchanger are located and supported by a set of tie rods and spacers. The rods are fastened to the inside face of the stationary tubesheet. Where possible, some of the rods may be located in the pass-partition lanes, where they perform the additional duty of seal rods, reducing flow bypassing in the lanes. The other rods will have to be located in the main parts of the bundle, occupying positions which would otherwise contain tubes, and reducing the total tube-count. Some programs do not make allowance for tie rods. The user will have to decide whether tie rods can be located in the free areas or whether it will be necessary to remove tubes to allow for them. Table 3 gives the TEMA recommendations for the number and diameter of tie rods. TABLE 3 TEMA TIE ROD STANDARDS



FIGURE 7 EXAMPLE OF OPTU3 GRAPHICAL OUTPUT



DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: BRITISH STANDARDS

BS 5500 Specification for unfired fusion welded pressure vessels (referred to in 5.1 and Clause 6)

AMERICAN STANDARDS

ASTM B163 Specification for seamless nickel and nickel-alloy condenser and heat-exchanger tubes (referred to in 5.3)

ENGINEERING SPECIFICATIONS

E488 Specification for Welded Tube/Tubeplate Joints in Ferritic and Austenitic Steel Heat Exchangers (referred to in 5.1)

EDS.VES.01.06 Requirements for Shell and Tube Heat Exchangers (referred to in Clause 1 and Clause 6)

OTHER DOCUMENTS

TEMA Standards of the Tubular Exchanger Manufacturers Association (referred to in Clause 3, 4.1, 5.1, 5.4, 5.5 and 8.5)

