Draft:Air cooled heat exchanger

An air-cooled heat exchanger (ACHE), also known as an air cooler, fin-fan cooler, or air fin cooler, is a type of heat exchanger that rejects heat from a process fluid directly to ambient air, without the use of an intermediate cooling-water circuit. The hot process fluid flows through finned tubes; ambient air, driven by one or more axial fans, passes in crossflow over the tube exterior, removing heat by convection and conduction through the tube wall and fins.

ACHEs are widely used in oil refineries, petrochemical plants, natural gas processing facilities, and power stations. Water scarcity, rising water costs, environmental regulations on cooling-tower plumes, and the elimination of water-treatment systems have driven the broad industrial adoption of air-cooled heat exchangers as an alternative to water-cooled equipment.

ACHEs are sometimes called aircoolers in process-plant contexts, but should not be confused with devices designed to cool air (correctly described as air chillers).

Operating principle

In an air-cooled heat exchanger the hot process fluid flows inside finned tubes, while ambient air flows perpendicularly across the outside of the tube bundle in a single crossflow pass. Heat passes by conduction through the tube wall and attached fins, and by convection into the airstream. Because air has a much lower thermal conductivity than water, external fins are required to provide sufficient surface area to compensate.

ACHEs are generally suitable when the required outlet temperature of the process fluid is at least approximately 11 °C (52 °F) above the maximum expected ambient dry-bulb temperature. For smaller temperature approaches, the combination of a cooling tower and a water-cooled exchanger is often more economical.

Components

Tube bundle

The tube bundle is the primary heat-transfer element of an ACHE, consisting of finned tubes whose ends are connected to rectangular header boxes.

Core tubes

Core tubes are typically of 25.4 mm (1 in) outside diameter; tubes up to 50.8 mm (2 in) outside diameter are used for low-pressure or highly viscous services. Tube lengths are selected to suit the installation — commonly matched to a pipe rack span — and generally do not exceed 15 m. Materials include carbon steel, stainless steel, and various alloy steels depending on the process fluid.

Fins

External fins increase the air-side surface area by a factor of 5 to 12 relative to a bare tube, largely offsetting the poor heat-transfer properties of air. Fins are almost universally made of aluminium because of its high thermal conductivity, light weight, and ease of fabrication. The principal fin types used in process ACHEs are as follows.

Comparison of common fin types for air-cooled heat exchangers

Fin type	Construction	Typical temperature limit	Notes
L-footed (single L)	Aluminium strip helically wound under tension; the foot of the L lies against the tube OD, protecting the tube surface	≤ ~175 °C (350 °F)	Most common type; good atmospheric-corrosion protection but contact resistance increases with temperature as the fin loosens
Overlapped L-footed (double L)	As above, but feet of adjacent fins overlap completely, covering the entire tube OD	≤ ~175 °C (350 °F)	Superior tube-surface coverage compared with single L
Embedded (grooved, G-fin)	A helical groove is machined into the tube OD; the fin material is wrapped into the groove under tension, then the displaced metal is peened back to lock the fin mechanically	≤ ~400 °C	Excellent bond; tolerates cyclic service; requires greater tube-wall thickness; tube OD exposed, so unsuitable for aggressive marine atmospheres
Extruded (integral)	An aluminium sleeve is swaged over the core tube, then extruded outward to form integral fins	≤ ~300 °C	No interface between fin and tube; eliminates crevice corrosion; preferred for coastal and offshore installations
Galvanised steel	Steel strip fins; hot-dip galvanised to bond fin to tube	High	Used in aggressive environments requiring long service life; often paired with elliptical tubes to improve airflow characteristics

Fin density typically ranges from 7 to 16 fins per inch (275–433 fins/m), and fin height from 9.5 to 15.9 millimetres (0.37 to 0.63 in). API 661 requires cast zinc bands at tube ends to prevent L-fin unwinding.

Headers

Headers are the rectangular-section pressure vessels at each end of the tube bundle that distribute the process fluid to and from the tubes. Unlike the cylindrical shells of shell-and-tube heat exchangers, ACHE headers are rectangular in cross-section. Common header types include:

• Plug-type box header — the most widely used type; a threaded plug is provided directly opposite each tube end, allowing individual tubes to be accessed, inspected, or plugged without disturbing adjacent piping.
• Bolted-cover header — a removable flat cover plate gives full access to all tube ends simultaneously; suited to lower-pressure services.
• Bonnet header — a rounded or dished cover that can be removed with minimum dismantling of header piping; used in moderate-pressure services.
• Manifold header — a cylindrical header fabricated from pipe; used where the rectangular header design cannot achieve the required pressure rating.
• Pipe (high-pressure) header — a thick-walled pipe or forged billet construction for very high-pressure services above approximately 200 bar.

When a multipass tube bundle experiences a large temperature drop across its length, split headers may be required to accommodate differential thermal expansion between passes.

Fans

Air is driven across the tube bundle by axial-flow fans. At least two fans are provided per exchanger bay so that cooling is maintained in the event of a single fan failure; fans should cover at least 40% of the total bundle face area. A typical face velocity across the tube bundle is approximately 3 m/s. Higher face velocities increase both the air-side heat-transfer coefficient and the mean temperature difference, reducing the required surface area, but at the cost of greater power consumption and fan noise.

Forced draught and induced draught

The two principal fan arrangements are:

Forced draught

Fans are located below the tube bundle and push ambient air upward through it. This is by far the more common arrangement. The fan shaft is short, the tube bundle is accessible for maintenance from below, and fans are not exposed to the hot exit airstream. However, air distribution across the bundle face is less uniform, and the risk of hot-air recirculation into the fan inlet is greater.

Induced draught

Fans are located above the tube bundle and draw air upward through it. This gives more uniform air distribution and lower risk of hot-air recirculation, and provides better process stability because approximately 60% of the bundle face is covered by the plenum, sheltering the tubes from sun, rain, and hail. However, the fans operate in the hot exit airstream; API 661 therefore limits the exit air temperature to 220 °F (104 °C) to protect fan blades and bearings. Motor and gearbox are usually mounted below the tube bundle with an extended drive shaft when fan diameter exceeds approximately 2.4 m.

Forced draught vs. induced draught: key differences

Parameter	Forced draught	Induced draught
Fan location	Below tube bundle	Above tube bundle
Air distribution	Less uniform	More uniform
Hot-air recirculation risk	Higher	Lower
Fan exposure to hot exit air	No	Yes (temperature limit applies)
Motor/gearbox access	Easy	More difficult
Process control stability	Lower	Higher
Typical power requirement	Slightly lower	Slightly higher if exit air is very hot

Plenum chamber

The plenum is the enclosed transitional volume between the fan ring and the tube bundle. In forced-draught units the plenum is below the bundle; in induced-draught units it is above. The plenum distributes air uniformly across the full face area of the bundle.

Supporting structure

ACHEs are among the largest individual items of process equipment and require a robust supporting structure. In refineries and chemical plants they are routinely mounted over pipe racks, saving grade-level plot space and ensuring unobstructed airflow. An elevated position also shortens overhead piping runs from distillation columns, reducing both capital cost and pressure drop. Where no elevated structure is available — or where plot space is ample — ground mounting is used.

Louvres

Optional adjustable louvre blades across the inlet or outlet face of the tube bundle allow manual or pneumatic regulation of airflow. Louvres are particularly important where controlled recirculation is needed to prevent process-fluid freezing, waxing, or hydrate formation during cold-weather operation.

Tube bundle orientation

Horizontal

The most common orientation. Air flows vertically upward through a horizontal tube bundle. Suitable for the majority of process and condensing applications.

Vertical

The tube bundle is mounted vertically with horizontal airflow. This configuration significantly reduces the plan area occupied by the exchanger, but performance is strongly affected by ambient wind speed and direction; airflow opposing the fan can substantially reduce cooling duty.

A-frame (V-frame)

Two tube bundles are inclined at 45° to 60° and joined at their headers, forming a shape resembling an inverted V or the letter A. This arrangement balances plot-area economy with thermal performance, and is the standard configuration for large air-cooled steam condensers in power stations. Forced-draught fans are located at the apex or beneath the frame.

Thermal design

Tube-side heat transfer and pressure drop

Tube-side heat transfer coefficients and pressure drops are calculated using the same methods as for shell-and-tube heat exchangers, following the Sieder–Tate or Gnielinski correlations for turbulent flow, corrected for entrance effects and fluid-property variation.

Air-side heat transfer

Established correlations for the air-side heat-transfer coefficient of banks of finned tubes include those of Briggs and Young (1963), the Process Heat Transfer Reference (PFR Engineering, 1976), and the Engineering Sciences Data Unit (ESDU, 1986).

Fin efficiency

Because there is a temperature gradient along the fin from root to tip, not all of the fin surface is as effective as the root. The concept of fin efficiency (η_f) accounts for this: it is the ratio of the actual heat transferred from the fin to the heat that would be transferred if the entire fin surface were at the fin-root temperature. For the fin types and dimensions typical of process ACHEs, fin efficiency lies in the range 0.80–0.90. The overall surface efficiency is then used to derive the air-side overall heat-transfer coefficient.

Air-side pressure drop

Correlations for air-side pressure drop across finned tube banks include those of Robinson and Briggs (1966), PFR (1976), and ESDU (1986).

Design ambient temperature

The design dry-bulb temperature is typically chosen as the value not exceeded for approximately 95% of hours in the year. Accepting a modest cooling shortfall on the hottest days reduces exchanger size appreciably. Where plant throughput is reduced in summer, a lower design temperature may be appropriate. Most ACHEs are designed with 4 to 6 tube rows in the airflow direction; beyond 6 rows, successive rows contribute diminishing additional heat transfer while cost continues to rise.

Process control

Several methods are used to regulate ACHE duty in response to ambient temperature changes, process load variations, or winterisation requirements:

Fan switching

Fans are switched on or off as required. With a large number of fans per bay, this can give reasonably close control.

Louvre adjustment

Manual or pneumatic louvre blades modulate the effective airflow area.

Two-speed motors

Allow the fan to run at full or reduced speed, giving an intermediate control step between full-on and off.

Auto-variable pitch fans

The fan-blade pitch angle is adjusted automatically while the fan rotates, allowing continuous modulation of airflow. This provides precise control and significant energy savings during cooler periods.

Variable-frequency drives (VFDs)

Electronic speed controllers vary fan rotational speed smoothly and continuously. VFDs have become the preferred control method in modern installations because they minimise power consumption and fan noise at all operating conditions.

The flood-to-condenser control strategy commonly used in shell-and-tube condensers is not applicable to ACHEs; reduction of effective surface area can only be achieved by valving off complete tube bundles.

Controlled recirculation

Where the process fluid is susceptible to freezing, waxing, or hydrate formation, a controlled fraction of the hot exit air is recirculated beneath the tube bundle and mixed with incoming cold ambient air to maintain the tube-wall temperature above a safe minimum. Recirculation is regulated by louvres, adjustable baffles, or partial closure of the fan intake.

Noise

The principal noise source in an ACHE is the fan. Occupational noise limits in process plants are typically around 85 dB(A); community noise limits are considerably lower, and in many jurisdictions sound-power emission limits now govern the ACHE design rather than local sound-pressure levels.

Moderate noise reductions can be achieved by reducing fan speed, increasing the number of blades, or using wide-chord blades. Very-low-noise designs require low face velocities — and consequently larger bundle areas — so that fans can operate at very low rotational speeds. Special low-noise fan designs have been developed for sites with stringent community noise requirements.

Standards

The principal standards governing the design, fabrication, inspection, testing, and shipping of air-cooled heat exchangers are:

• API Standard 661 (Petroleum, Petrochemical, and Natural Gas Industries — Air-Cooled Heat Exchangers), 7th edition, 2013, published by the American Petroleum Institute. This is the primary industry standard for process-plant ACHEs; it is also published as ISO 13706. It covers both forced-draught and induced-draught configurations and contains requirements for tube bundles, headers, fans, drive systems, noise, and testing. API 661 requires the vendor to provide a certified noise data sheet with the fans operating at rated speed and design conditions.
• ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 — governs the pressure design of headers and other pressure-containing components.
• IOGP JIP33 Supplementary Specification S-710 — a supplementary specification to API 661 developed by the International Association of Oil and Gas Producers for standardised procurement in the oil and gas sector.

Applications

Oil, gas, and petrochemical industries

ACHEs are the dominant process cooling technology in oil refineries and gas-processing plants. Their ability to operate in remote locations without a water supply makes them indispensable in arid environments. Typical services include:

• Overhead condenser duty on atmospheric and vacuum distillation columns
• Cooling of compressed gas after each stage of a multi-stage compressor
• Product coolers for refined petroleum streams
• Lean-amine coolers in gas sweetening units
• Cooling of pumped liquids and separator overheads in natural gas liquids recovery

Power generation

Air-cooled condensers (ACCs) are used in steam-turbine power stations where water availability or water-discharge regulations preclude the use of wet cooling towers or once-through cooling. An A-frame forced-draught configuration is standard. Large ACCs are among the biggest single items of process equipment: the condensers for a 4,000 MW power station in South Africa incorporate more than 2,300 tube bundles, 288 fans each 9.1 m in diameter, and cover a total plot area of approximately 500 m × 70 m. The 330 MW Wyodak power station near Gillette, Wyoming, opened in 1978, was one of the first large coal-fired stations to use an air-cooled condenser, enabling siting directly above coal seams in an arid region without concern for a large plant water supply.

Chemical processing

Chemical plants use ACHEs to cool reactor effluents, control exothermic reaction temperatures, and condense overhead vapours. The robust design is suited to corrosive and hazardous fluids when appropriate tube materials are selected.

HVAC and refrigeration

In large commercial and industrial HVAC systems, air-cooled condensers reject heat from the refrigerant circuit in chillers and rooftop units. These units follow the same operating principles as process ACHEs but are generally smaller and more standardised.

Other applications

• Hydraulic fluid and lubricating-oil cooling on rotating machinery
• Charge-air cooling (intercoolers) on turbocharged engines
• Swimming-pool water heating and cooling
• Cooling of high-power power-electronic equipment

Advantages

• Elimination of cooling water — no cooling-tower make-up water, drift loss, blowdown, or water-treatment chemicals are required, reducing both operating cost and environmental impact.
• Suitability for remote and arid locations — can be installed wherever there is open land and airflow, irrespective of water availability.
• No risk of process-fluid contamination by cooling water — the process fluid is entirely contained within the tubes; there is no contact with, or risk of leakage into, the cooling medium.
• Lower fouling on air side — the air-side surface is accessible and can be cleaned by water washing or air blowing; air does not introduce scaling minerals.
• Reduced corrosion — the cooling medium (air) is non-corrosive to the tube material for most applications, eliminating the shell-side corrosion mechanisms common in water-cooled heat exchangers.
• Simplified operation — no cooling tower, circulating water pumps, water-treatment plant, or associated instrumentation and chemistry control are needed.

Disadvantages

• Dependence on ambient temperature — cooling duty decreases on hot days; significant overdesign for cold climates creates control difficulties and may require winterisation measures.
• Large plot area — ACHEs require considerable open space around and above them for unobstructed airflow; this can conflict with the dense layouts of modern process plants.
• Higher capital cost per unit of heat duty — for small stand-alone duties, ACHEs can be more expensive than water-cooled alternatives.
• Limited close-approach temperature — when the required process-outlet temperature is only a few degrees above the ambient air temperature, the exchanger becomes very large and the VFD or variable-pitch control system complex.
• Fan noise — compliance with community noise regulations may require costly low-noise fan designs or acoustic enclosures.
• Winterisation requirement — in cold climates, louvres, heating coils, or controlled recirculation are needed to prevent freezing or waxing of the process fluid inside the tubes. API 661 Appendix G and winterisation heating coils (which must be in a separate bundle and cover the full width of the process bundle) address this requirement.

See also

• Heat exchanger
• Shell-and-tube heat exchanger
• Plate heat exchanger
• Cooling tower
• Fin (extended surface)
• Log mean temperature difference
• ASME Boiler and Pressure Vessel Code

Further reading

• Kakaç, S.; Liu, H.; Pramuanjaroenkij, A. (2002). Heat Exchangers: Selection, Rating, and Thermal Design (2nd ed.). CRC Press. ISBN 978-0-8493-0902-1.
• Briggs, D. E.; Young, E. H. (1963). "Convection heat transfer and pressure drop of air flowing across triangular pitch banks of finned tubes". Chemical Engineering Progress Symposium Series. 59 (41): 1–10.
• Robinson, K. K.; Briggs, D. E. (1966). "Pressure drop of air flowing across triangular pitch banks of finned tubes". Chemical Engineering Progress Symposium Series. 62 (64): 177–184.
• ESDU International (1986). Convective heat transfer during crossflow of fluids over plain tube banks (ESDU Data Item 73031). ESDU.
• Serth, R. W. (2007). Process Heat Transfer: Principles and Applications. Academic Press/Elsevier. ISBN 978-0-12-373588-1.
• United Heat Exchangers (2026). "What Is an Air Cooled Heat Exchanger — Working & Benefits". United Heat Exchangers. Retrieved 10 June 2026.

External links

• Thermopedia: Air Cooled Heat Exchangers — C. Summers, Begell House Inc.
• Inspectioneering: API 661 overview
• What Is an Air Cooled Heat Exchanger
• American Petroleum Institute — publishers of API Standard 661

Category:Heat exchangers Category:Cooling technology Category:Industrial equipment Category:Power station technology