Heat Exchangers: Types, Working Principle, TEMA Design and Maintenance Guide

Heat exchangers are the most common piece of equipment in refineries, petrochemical complexes, fertilizer plants, power stations, HVAC systems and food plants. A typical refinery crude unit alone can contain more than a hundred of them, and their fouling and leaks quietly drive fuel cost, throughput limits and unplanned shutdowns.

This guide explains how heat exchangers work, every major type and when to use it, the TEMA designation system, thermal design with a fully worked LMTD example, the current codes and standards, materials, damage mechanisms, inspection techniques and a field-proven maintenance procedure. It is written from 26+ years of hands-on project, shutdown and turnaround work on exchangers in oil & gas, petrochemical, fertilizer, cement and power plants.

How a heat exchanger works

Heat always flows from the hotter fluid to the colder fluid, and a heat exchanger simply provides enough surface, turbulence and temperature difference for that transfer to happen at the required rate. In a recuperative (wall-type) exchanger, heat passes in series through the hot-side film, any hot-side fouling, the metal wall, any cold-side fouling and the cold-side film. The sum of these resistances gives the overall heat transfer coefficient U.

Q is the duty (W), U the overall coefficient (W/m²K), A the heat transfer area (m²), F the LMTD correction factor for multipass or crossflow (F = 1 for pure counterflow), ΔT₁ and ΔT₂ the terminal temperature differences, R_f the fouling resistances and C = m·c_p the heat capacity rate of each stream. The LMTD method is convenient when all four terminal temperatures are known (design); the ε–NTU method is better when outlet temperatures are unknown (rating an existing exchanger).

Heat exchanger flow arrangements compared

The flow arrangement decides how much of the available temperature difference can actually be used. For the same U and area, counterflow transfers the most heat, parallel flow the least, and crossflow or multipass designs fall in between. Industrial shell-and-tube exchangers are mostly multipass (one shell pass, two or more tube passes), so their LMTD is corrected with the factor F.

Classification of heat exchangers by construction

At the top level, heat exchangers are either recuperative or regenerative. In a recuperative exchanger both fluids flow at the same time in separate passages and heat crosses the wall between them. In a regenerative exchanger the hot and cold streams take turns flowing through the same matrix: the matrix stores heat during the hot period and releases it during the cold period. Recuperators are then divided into indirect-contact (wall-separated), direct-contact (fluids touch) and special designs.

Types of heat exchangers: selection table

Use the table below as a first screening tool. Pressure and temperature limits are typical commercial ranges and depend strongly on size, material and manufacturer; always confirm with vendor data and the governing code.

Shell-and-tube heat exchangers and TEMA types

The shell-and-tube heat exchanger is the workhorse of the process industries because it can be built for almost any pressure, temperature, size and fluid. One fluid flows inside the tubes (tube side) and the other flows over the outside of the tubes inside the shell (shell side). Its four main assemblies are:

• Front (stationary) head – the channel or bonnet where the tube-side fluid enters; pass partition plates divide it for multiple tube passes.
• Shell – the pressure envelope around the bundle with inlet and outlet nozzles, impingement protection and sometimes a longitudinal baffle.
• Tube bundle – tubes, tubesheets, baffles or support plates, tie rods and spacers; it may be fixed or removable.
• Rear head – returns or discharges the tube-side fluid and, in floating-head designs, allows the bundle to expand freely.

TEMA designation system (front head – shell – rear head)

The Tubular Exchanger Manufacturers Association (TEMA) describes every shell-and-tube exchanger with three letters followed by size: for example, a 24-192 AES has a 24 in nominal shell inside diameter, 192 in nominal tube length, a removable-cover channel (A), a one-pass shell (E) and a floating head with backing device (S). The current TEMA Standards (11th edition) also define three mechanical classes: R for severe petroleum and process service, C for moderate commercial service and B for chemical process service.

Which fluid goes on the tube side?

Fluid allocation is one of the most important decisions in shell-and-tube design because it fixes cleanability, cost and safety. The usual rules are summarised below; when rules conflict, safety and maintainability normally win over thermal performance.

Tube and baffle geometry: typical design values

Plate, plate-fin and spiral heat exchangers

Plate heat exchangers pack a large area into a small volume by stacking thin corrugated plates, so each fluid flows in alternate narrow channels. The corrugations (chevron pattern) create strong turbulence even at low velocity, which gives overall coefficients typically three to five times higher than a shell-and-tube unit in water service and allows temperature approaches as close as 1–2 °C.

• Gasketed plate-and-frame: plates held between a fixed and a movable frame plate by tie bolts. Gaskets seal each channel and route the fluids through corner ports. Easy to open, clean and extend; limited by gasket pressure and temperature. Double-wall plates or leak-detection gaskets are used where cross-contamination is unacceptable.
• Brazed plate: plates copper- or nickel-brazed into a sealed block. Compact and inexpensive, but cannot be opened, so only clean fluids or chemical cleaning are acceptable.
• Welded plate: laser-welded plate packs (block, shell-and-plate or semi-welded cassettes) for higher pressure, temperature or aggressive fluids where gaskets are not acceptable.
• Plate-fin: corrugated fins brazed between flat parting sheets, usually aluminium. Up to about a dozen streams in one core with very close approach – the standard choice for LNG and air separation cold boxes, designed to ALPEMA.
• Spiral plate: two long plates rolled into a spiral to form a single curved channel for each fluid. The single channel scours itself, so it tolerates slurries, fibres and sludge that would block other designs.
• Lamella: flattened tubes (lamellae) inside a shell; a hybrid of plate and shell-and-tube used in pulp and paper where wide flow passages are needed.

Air-cooled heat exchangers (fin-fan coolers)

An air-cooled heat exchanger rejects process heat directly to ambient air, removing the need for cooling water, cooling towers and water treatment. It consists of finned-tube bundles with headers, one or more axial fans, drives and a supporting structure. Design and supply in the oil and gas industry follow API 661 / ISO 13706, and thermal performance testing follows ASME PTC 30.

Air coolers are selected when cooling water is scarce or expensive, the process outlet temperature can be about 10–15 °C above the design dry-bulb temperature, and plot space is available. Key design checks include design ambient temperature (summer extreme), header type (plug, cover-plate or manifold) for the process pressure, fin type (embedded, extruded or wrapped) for temperature and corrosion, winterisation for viscous or freezing fluids and fan noise limits.

Direct-contact exchangers and regenerators

Direct-contact exchangers transfer heat by mixing or intimately contacting the two streams, so there is no wall to foul and no wall resistance. They are only suitable when the fluids are immiscible, can be separated afterwards, or when one of them is meant to change phase. Examples are cooling towers (where most heat leaves by evaporation, so make-up water is needed continuously), spray and barometric condensers, steam injection heaters, desuperheaters, quench towers and submerged-combustion evaporators.

Regenerators store heat in a solid matrix. Fixed-bed (static) regenerators need at least two beds switched by valves so that one heats while the other cools; rotary regenerators such as the Ljungström boiler air preheater rotate a packed wheel continuously between the hot flue gas and the cold combustion air. Their main issues are carry-over and leakage between streams through the seals, which is why they are used mostly for gas-to-gas heat recovery where small cross-contamination is acceptable.

Thermal design: LMTD, overall U and fouling

Thermal design means finding the area that delivers the required duty within the allowable pressure drops, with enough margin for fouling, while staying mechanically sound and free of flow-induced vibration. A practical sequence is:

1. Fix the process data: flows, inlet and outlet temperatures, allowable pressure drops, fluid properties and fouling resistances for both streams.
2. Close the heat balance so that the hot-side and cold-side duties agree.
3. Calculate LMTD for counterflow and the correction factor F for the chosen shell and pass arrangement; add shells in series if F < 0.75–0.80.
4. Estimate U from typical values, then calculate the required area A = Q / (U · F · LMTD).
5. Select geometry (TEMA type, tube size, length, passes, baffles) and calculate film coefficients and pressure drops with a rigorous method (Bell–Delaware or stream analysis).
6. Iterate until calculated U, area margin and pressure drops are acceptable; check vibration, nozzle ρv² and tube-rupture relief per API 521.
7. Complete mechanical design to ASME Section VIII (tubesheets per Part UHX), TEMA class and API 660, then issue the datasheet.

Fouling resistances

Fouling is the build-up of scale, corrosion products, biological growth, coke, polymer or particulate on heat transfer surfaces. It adds a thermal resistance R_f to each side. Designers include it as extra area (typically 10–30% over the clean area), but excessive fouling margins can backfire: lower velocities in an oversized unit can make fouling worse. Typical design values based on TEMA are:

Worked example: sizing a shell-and-tube cooler

A light hydrocarbon stream of 10 kg/s (c_p = 2.5 kJ/kgK) must be cooled from 120 °C to 60 °C with cooling water entering at 30 °C and leaving at 40 °C (c_p = 4.18 kJ/kgK). The exchanger is a one-shell-pass, two-tube-pass (1-2) unit, cooling water on the tube side, with an assumed service U of 450 W/m²K.

With F close to 1, one shell is adequate. In practice the designer would add about 10–20% area margin, check that the tube-side water velocity stays above roughly 1 m/s (95 tubes per pass in 19.05 mm × 2.11 mm tubes gives about 2.2 m/s, which is acceptable for carbon steel or stainless tubes), and then verify pressure drops and vibration in thermal design software.

LMTD, area and fouling calculators

Quick checks for design reviews and plant troubleshooting. Calculations run in your browser; nothing is sent to a server. For full shell-and-tube sizing use the Aienginear heat exchanger sizing calculator.

Heat exchanger codes and standards (latest editions)

Heat exchanger design is governed by a pressure-equipment code for the pressure parts plus an equipment standard for the exchanger type. Always work to the edition named in the project specification; the list below reflects the documents in common use in 2026.

Heat exchanger material selection

Heat exchanger failure and damage mechanisms

Most heat exchanger problems are caused by a small number of damage mechanisms, and each leaves a recognisable signature in operating data or inspection results.

Inspection and NDT of heat exchanger tubes

Tube NDT should be matched to the tube material and the expected damage. Results are used to decide whether to plug, sleeve, retube or replace the bundle.

Heat exchanger maintenance and turnaround procedure

Heat exchangers rarely fail without warning. In the field, rising pressure drop, a growing approach temperature, a falling overall U or traces of process fluid in the cooling water almost always appear weeks or months before a forced shutdown. Good maintenance turns these signals into planned work during a turnaround rather than emergency repairs.

Step-by-step exchanger maintenance in a turnaround

1. Plan from data: review U and ΔP trends, previous inspection reports, RBI plan and leak history; decide scope (clean, inspect, repair, retube) before shutdown.
2. Isolate safely: drain, depressurise, steam-out or flush, then positively isolate with blinds and lock-out/tag-out; gas test before opening.
3. Open and record as-found: photograph deposits, take deposit samples for analysis, measure fouling thickness and note gasket condition.
4. Pull the bundle: use a bundle extractor or approved lifting plan; protect tubes and baffles from damage; check lifting lugs and slings.
5. Clean: select the method from the table below; avoid damaging tube surfaces or enhanced tubes.
6. Inspect: visual, ECT / RFT / IRIS as specified, tubesheet ligaments, gasket faces, shell and channel thickness (UT), nozzle welds.
7. Repair: plug leaking or thin tubes at both ends, re-roll joints, sleeve tube ends or retube; record every plugged tube on the tubesheet map.
8. Re-assemble: new gaskets of the specified type, bolt-up to ASME PCC-1 with calibrated tools and a bolting record.
9. Pressure test: shell-side test to check tube and joint leaks, then tube-side test; ASME VIII Div. 1 hydrotest is normally 1.3 × MAWP × the stress ratio. Drain and dry stainless and copper-alloy units promptly.
10. Hand back and baseline: record post-start-up U, ΔP and approach temperatures as the new clean baseline.

Tube plugging, retubing and bundle replacement

Plugging takes leaking or thin tubes out of service quickly, but every plugged tube reduces area and raises velocity in the remaining tubes. Many operators set a review point at about 5–10% of tubes plugged; beyond that a thermal-hydraulic re-rating decides whether to sleeve, retube or replace the bundle. Plugs must be compatible with the tube material and pressure, both ends of the tube must be plugged, and a tube that has failed from vibration should trigger a check of the neighbouring tubes, which are usually next.

Frequently asked questions about heat exchangers

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Related Aienginear guides and tools

References and standards

• Tubular Exchanger Manufacturers Association – Standards of the Tubular Exchanger Manufacturers Association, 11th edition.
• ASME Boiler and Pressure Vessel Code, Section VIII Divisions 1 and 2 (including Part UHX), and Section V (NDE).
• API Std 660 / ISO 16812 – Shell-and-tube Heat Exchangers; API Std 661 / ISO 13706 – Air-cooled Heat Exchangers; API Std 662 / ISO 15547 – Plate Heat Exchangers; API Std 664 – Spiral Plate Heat Exchangers.
• API Std 521 / ISO 23251 – Pressure-relieving and Depressuring Systems (tube rupture).
• API 510, API RP 571, API RP 572, API 579-1/ASME FFS-1, API RP 580 and API RP 581 – inspection, damage mechanisms, fitness-for-service and RBI.
• ASME PCC-1 – Guidelines for Pressure Boundary Bolted Flange Joint Assembly; ASME PCC-2 – Repair of Pressure Equipment and Piping.
• ASME PTC 12.5 and PTC 30; AHRI Standard 400; HEI Standards for Steam Surface Condensers and Closed Feedwater Heaters; ALPEMA Standards.
• Kern, D. Q., Process Heat Transfer; Incropera et al., Fundamentals of Heat and Mass Transfer; Shah and Sekulić, Fundamentals of Heat Exchanger Design – general heat transfer theory.

Written and technically reviewed by Muhammad Raza, founder of Aienginear, with 26+ years managing projects, plant shutdowns and turnarounds in oil & gas, petrochemical, cement, fertilizer and power generation. First published 6 December 2025; updated 11 October 2026. All diagrams are original Aienginear drawings. Values in the tables are typical figures for education and preliminary design; the latest editions of the referenced codes, project specifications and vendor data always govern.