Tube bundle

01

Choose tube layout and pitch for cleaning

A lecture on shell-and-tube exchangers by Abhishek Kumar Chandra defines the tube bundle as the assembly of tubes, baffles, tubesheets, spacers and tie rods, and explains that triangular and rotated-triangular layouts (30° and 60°) are compact, give better shell-side heat transfer and fit about 15% more tubes in a given shell for the same pitch ratio, but cannot be cleaned mechanically on the shell side, only chemically or by water jet; square and rotated-square layouts have lower pressure drop and heat transfer. [1]

A second lecture gives the recommended pitch as 1.25 times the tube outside diameter unless the process dictates otherwise, and a minimum clearance between tubes of 6.4 mm (0.25 in) where a square pattern is used for cleaning; square layouts are used for heavily fouling shell-side fluids. [2]

Editorial calculation: for 20 mm tubes, 1.25 × 20 = 25 mm pitch leaves 25 − 20 = 5 mm between tubes, less than the 6.4 mm cleaning clearance, so a mechanically cleaned square layout would need at least 20 + 6.4 = 26.4 mm pitch. [2]

Give the supplier the exchanger type and shell size, tube OD, wall, length, material and count, layout and pitch, number of passes, baffle type, cut and spacing, tubesheet material and thickness, tube-to-tubesheet joint, design pressure and temperature, design code and, for replacements, the existing drawing.

02

Set baffles, supports and tubesheet joints

The lecture notes that segmental baffle cuts range from 20% to 49%; cuts below 20% raise pressure drop and larger cuts create stagnant zones. Baffle spacing ranges from one-fifth to one shell diameter, and the lecture allows larger inlet and outlet spacings to clear the nozzles, and TEMA sets minimum baffle thickness. [1]

Tie rods are screwed into the stationary tubesheet and run to the last baffle, where locknuts hold them, and with spacers they keep baffles in place and can block bypass lanes. Tubes are expanded into grooved tubesheet holes by rolling, which must seal without over-thinning the tube; fixed-tubesheet designs may need an expansion joint in the shell, and the lecture lists TEMA classes R, C and B. [1]

Editorial calculation: heat transfer area is tube count × π × tube OD × effective length. For 738 tubes of 20 mm OD and 6 m length, area ≈ 738 × π × 0.020 × 6 ≈ 278 m², matching the worked example in the second lecture. [2]

A 2024 Scientific Reports paper from Northeast Petroleum University reports that in one Daqing Petrochemical exchanger, 15 of 128 tubes failed and were plugged, the causes being touching abrasion against baffle plates and fatigue cracking from bundle vibration. In that five-baffle exchanger the authors group tubes by support: type B passes through all five baffles (high stiffness), type A through three baffles and two baffle-gap areas, and type C through two baffles and three baffle-notched areas (low stiffness); the failed tubes sat at the interface between tube types. Their simulation of a square bundle at pitch ratio 1.28 found the target tube's amplitude smallest when neighbouring tubes had equal stiffness, and that lower stiffness upstream or downstream can reduce the critical flow velocity, "making it more prone to experiencing fluid-elastic instability". [4]

Editorial calculation: 15 ÷ 128 = 11.7% of the tubes in that exchanger were lost to vibration damage; ask the supplier for a flow-induced vibration check covering the tubes in the baffle windows and at the inlet and outlet spans. [4]

03

Where the listed bundles go: air coolers, refrigeration, boilers and fired heaters

Sinton and Ever list tube-bundle and shell-and-tube heat exchangers; Datang lists tube-bundle exchangers, air-cooler tube bundles and U-bend tubes; Shenglin lists air-cooler tube bundles and shell-and-tube exchangers; VR Cooler lists tube bundles for refrigeration; Xuyi lists titanium U-tube and shell-and-tube exchangers; ORL lists convection tube bundles for boilers; Tuopu lists tube bundles for fired heaters and DRI plants.

04

Name the tube specification, wall basis and unit system

ASTM International's abstract for A179/A179M-19 states that it covers minimum-wall-thickness, seamless cold-drawn low-carbon steel tubes for tubular heat exchangers, condensers and similar heat transfer apparatus, and that the tubes shall be made by the seamless process and cold drawn. Its scope covers tubes from 1/8 to 3 in. [3.2 to 76.2 mm], inclusive, in outside diameter. [3] Heat and product analysis must show conformance on carbon, manganese, phosphorus and sulfur, and the steel must pass hardness, flattening, flaring, flange and hydrostatic tests. Mechanical property requirements do not apply to tubing smaller than 1/8 in. [3.2 mm] in outside diameter or with a wall thickness under 0.015 in. [0.4 mm]. [3]

The scope also states that values in inch-pound and SI units are not exact equivalents, that each system must be used independently of the other, and that combining values from the two systems may result in nonconformance with the specification; the inch-pound units apply unless the order specifies the "M" designation. [3]

Editorial note: when ordering a carbon-steel bundle to this specification, state the wall as a minimum (not nominal) value, give the designation with or without "M" to fix the unit system, and ask for certificates showing the chemical analysis and each listed mechanical and hydrostatic test. [3]

05

What holds a bundle together: tubesheets, baffles, tie rods and seals

The baffle row carries the vibration finding from the 2024 paper as well as the lecture, because tubes in the baffle windows have fewer supports.

06

Tube count, pitch and joint details a bundle drawing must fix

Pitch, layout angle and area follow the lecture rules worked through above; design pressure, temperature, tube material and weight have no published value here and must come from your duty or the supplier. For a replacement bundle, take the values from the existing drawing.

07

Send the exchanger, tube and cleaning data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Exchanger: [type; TEMA designation; shell ID; new or replacement]
Tubes: [OD; wall; length; material; count; straight or U]
Layout: [triangular or square; pitch; passes; cleaning method]
Baffles: [type; cut; spacing; thickness]
Tubesheets: [material; thickness; joint type]
Design: [pressures; temperatures; code]
Documents: [drawing; material certificates; hydrotest]

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