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.Summers, C. (2011). "Air Cooled Heat Exchangers". Thermopedia. . Begell House Inc.

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 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 . 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 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.