Shell and Tube Heat Exchanger Design: TEMA Types (AES, BEM, AEU), Baffles & Tube Layout

Anatomy of a Shell and Tube Heat Exchanger

A shell and tube heat exchanger consists of a bundle of parallel tubes enclosed in a cylindrical shell. One fluid flows inside the tubes and the other flows through the shell around them. The tubes are held at each end in tubesheets, and baffles support the tubes and force the shell-side fluid to zig-zag across the bundle. Channels or bonnets at the ends distribute the tube-side fluid and, with partition plates, divide it into multiple passes. For the fundamentals, see our introduction what is a heat exchanger.

Reading TEMA Type Designations

TEMA names each exchanger with three letters. The first letter describes the front-end head, the second the shell, and the third the rear-end head. The most common letters are:

Examples: AES is a removable channel, one-pass shell, floating head with backing device, and is a classic choice for fouling services where the bundle must be pulled for cleaning. BEM has a bonnet, one-pass shell and a fixed tubesheet with bonnet at the rear, and is simple and economical for clean services. AEU has a U-tube bundle, which expands freely. AKT or AKU are kettle reboilers (see reboiler guide).

Fixed Tubesheet vs Floating Head vs U-Tube

Tube Layout, Pitch and Diameter

Tubes are arranged on a triangular (30 degrees), rotated triangular (60 degrees), square (90 degrees) or rotated square (45 degrees) pitch. A triangular layout packs more tubes in a given shell and gives higher shell-side heat transfer, but the lanes between tubes are too narrow for mechanical cleaning. A square layout leaves straight cleaning lanes and is preferred for fouling shell-side fluids. The tube pitch is usually about 1.25 times the tube outside diameter. Common tube outside diameters range from about 19 mm (3/4 inch) to 25 mm (1 inch), with wall thickness selected for pressure, corrosion allowance and tube material. Longer tubes give more area at lower cost, within limits of bundle handling and plant space.

Baffles: Type, Cut and Spacing

• Single segmental baffles: the most common; the cut (opening) is typically around 20 to 25 percent of shell diameter.
• Double segmental baffles: split the flow into two parallel streams, lowering shell-side pressure drop.
• No-tubes-in-window design: removes tubes from the baffle window to avoid unsupported spans and reduce vibration.
• Rod baffles: grids of rods support tubes with very low pressure drop and high vibration resistance.

Baffle spacing typically falls between about 0.2 and 1.0 times the shell inside diameter. Closer spacing raises velocity and heat transfer but also pressure drop and vibration risk. The right choice balances heat duty, allowable pressure drop and flow-induced vibration.

Velocities, Fouling and Pressure Drop

Higher fluid velocity raises the heat transfer coefficient and reduces fouling, but it also increases pressure drop and the risk of erosion and vibration. For water in tubes a typical design target is in the range of roughly 1 to 2.5 metres per second, though materials, fluids and allowable pressure drop will change this. Low velocities invite deposits of sludge and scale, so a design that meets duty on paper may foul quickly in service. The fouling factor in the design adds surface to allow for deposits, and the data sheet should state it for each fluid.

Pressure drop is part of the specification, not only the result. A pump or compressor limits how much you can spend on the exchanger, so state the allowable pressure drop on each side.

Thermal Expansion and Mechanical Design

Tubes and shell expand differently when they are at different temperatures or made of different metals. In a fixed-tubesheet exchanger this difference stresses the tubesheet and tube joints, so large temperature differences call for an expansion joint, a floating head or a U-tube design. Mechanical design follows ASME Section VIII Div 1 for pressure parts and TEMA for tubesheet thickness, tolerances and construction. Tube-to-tubesheet joints may be expanded, seal-welded or both, depending on pressure, temperature and leak tolerance. Hydrostatic tests are performed on both shell and tube sides, with the bundle assembled and inspected.

Worked Example: Estimating Required Area

Heat transfer area follows from the basic equation Q = U × A × LMTD, where Q is the duty, U the overall heat-transfer coefficient, A the area and LMTD the log-mean temperature difference. Suppose a process liquid is cooled from 100 °C to 60 °C by cooling water heated from 30 °C to 50 °C in counter-current flow.

1. Temperature differences at the two ends: 100 − 50 = 50 K and 60 − 30 = 30 K.
2. LMTD = (50 − 30) / ln(50/30) = 20 / 0.511 = about 39 K.
3. If the duty is 500 kW and an assumed U is 800 W/m²K, then A = 500,000 / (800 × 39) = about 16 m².
4. After adding allowances for fouling, uncertainty in U and a design margin, the final area is selected, often 10 to 25 percent larger.

This is a simplified illustration: real designs use a correction factor for multi-pass flow, detailed film coefficients on each side and a vendor design program. It shows why inlet and outlet temperatures, duty and an honest estimate of U drive the size of the exchanger.

Testing, Inspection and Maintenance

• Hydrostatic test: shell and tube sides are tested to code to prove strength and tube-joint tightness.
• NDT of welds: radiography or other methods on shell and channel welds as specified, with dye-penetrant checks for tube-to-tubesheet welds.
• Eddy-current testing of tubes may be specified for critical services.
• In service: monitor outlet temperatures and pressure drop for fouling; clean on a schedule; replace tubes or retube when pitting appears.
• Vibration check: unsupported spans and high shell-side velocity can cause tube failure; redesign baffles if tubes show wear at baffle holes.

Our quality system covers NDT by RT, UT, PT, MT and VT with full material traceability, and we supply a complete documentation dossier with each exchanger.

TEMA Class R, C and B

Class R is for the severe requirements of petroleum and related processing, Class C for moderate commercial and general process applications, and Class B for chemical process service. The classes differ in details such as minimum thickness, corrosion allowance and construction. Specify the class on your data sheet. Creative Engineering builds to TEMA classes B, C and R and supports IBR and IS 4503 requirements where applicable.

Data Sheet Checklist for Ordering

1. TEMA type and class, orientation and number of shells in series or parallel.
2. Fluids, flow rates, inlet and outlet temperatures and duty for shell and tube sides.
3. Design pressure and temperature for each side, vacuum conditions.
4. Allowable pressure drop and fouling factors.
5. Material of tubes, shell, tubesheet, channels and baffles.
6. Tube size, length, pitch and number of passes if specified.
7. Nozzle sizes, orientation and flange ratings.
8. Code, third-party inspection and documentation.

Send the data sheet through the contact page. For related equipment see box heat exchangers, the heat exchanger manufacturing page and reboilers.

Frequently Asked Questions