Overview of Air Cooled Heat Exchangers

Air Cooled Heat Exchangers are one of the heat exchanger types frequently used in Process, Power, Steel, and several other Industries where a process system generates heat that must be removed, for which there is no local use. The main function of the Air Cooled Heat exchanger is the direct cooling of various process mediums by atmospheric air. These heat exchangers are also known as Air Fin Fan Coolers or Air Fin Coolers or Air Coolers or Fin-tube heat exchangers.

Advantages of Air Cooled Heat Exchanger

The main advantage of Air-Cooled Heat Exchangers (ACHE) is its very low maintenance and operating cost.

As compared to cooling towers and shell and tube heat exchangers, Air Coolers are a “green” solution. They do not require an auxiliary water supply because of the lost water due to drift and evaporation. Where there is no utility such as water available as a cooling medium.

They are usually used when the outlet temperature is more than about 20 deg. F above the maximum expected ambient air temperature. They can be used with closer approach temperatures, but often become expensive compared to a combination of a cooling tower and a water-cooled exchanger. Fig. 1 shows the operating principle of a typical Air Cooled Heat exchanger.

Operating principle of Air Cooled Heat Exchanger
Fig. 1: Operating principle of Air Cooled Heat Exchanger

Air Cooled Heat Exchanger Construction

The construction of Air-Cooled Heat Exchanger is fairly simple. Fig. 2 and Fig. 3 provides the component parts of an air fin fan cooler. Refer the animated video at the end of the article for proper understanding.

  • Typically air-cooled exchangers consist of a finned tube bundle with rectangular box Headers on both ends of the tubes.
  • Cooling air is provided by one or more fans.
  • Usually, air blows upwards through a horizontal tube bundle.
  • The fans can be either forced or induced draft depending on whether the air is pushed or pulled through the tube bundle.
  • The space between the fans and the tube bundle is enclosed by a plenum chamber that directs the air. The whole assembly is usually mounted on legs or a pipe rack.
Construction of Air Cooled Heat Exchangers
Fig. 2: Construction of Air Cooled Heat Exchangers
Construction of Air Fin Fan coolers
Fig. 3: Construction of Air Fin Fan coolers

Components of an Air Cooled Heat Exchanger

An Air Cooled Heat Exchanger consists of the following primary components (Refer to Fig. 11):

  • One or more bundles of the heat transfer surface.
  • A fan or blower that moves the air.
  • A mechanical driver and power transmission for running the fan or blower.
  • A plenum
  • A supporting structure
  • Header and Fan maintenance platforms.
  • Optional louvers and recirculation ducts
  • Optional variable-pitch fan hub or variable-frequency drive for temperature control and power savings.

Bay Arrangement in Air Cooler

One or more tube bundles, serviced by two or more fans, including the structure, plenum, and other attendant equipment is called a bay in an air-cooled heat exchanger. Refer to Fig. 4 for typical bay arrangements.

Bay arrangements in Air Coolers
Fig. 4: Bay arrangements in Air Coolers

Air Cooled Heat Exchanger Tubes

Wall thickness for tubes with an OD of 1 inch (25.4 mm) to 11/2 (38.1 mm) shall not be less than specified in the table below: (Fig. 5)

Minimum wall thickness requirements for Tube Material
Fig. 5: Minimum wall thickness requirements for Tube Material

What kind of finned tubes are used for Air Coolers?

  • Fin types
    • Embedded
    • Extruded
    • Footed
    • Externally bonded
    • Elliptical
  • Fin density (FPI) – 7 to 16
  • Fin height – 3/8 inch to 5/8 inch
  • Fin thickness – 0.012 to 0.02 inch

Fin Types (Fig. 6):

Figure showing various types of fins
Fig. 6: Figure showing various types of fins

Fin Selection (Fig. 7):

Refer to Fig. 7 for limiting temperatures of various types of fins.

Limiting temperatures for various types of Fins.
Fig. 7: Limiting temperatures for various types of Fins.

Fin Material

Typical materials used for air cooled heat exchanger Fin construction are

  • Aluminum
  • Carbon steel
  • Carbon steel finned tubes can be hot-dip galvanized to prevent corrosion & to provide a metallic bond between the fin and the bare tube.
  • Copper

Air-cooled heat exchanger tube bundle orientation:

Three types of ACHE tube bundle orientation are possible; horizontal, vertical, and sloped orientation.

Horizontal orientation is the most preferred and common type and widely used in process industries.

Orienting the tube bundles in a vertical direction (Fig. 8) can save a considerable area but the air cooler performance is greatly dependent on prevailing wind speed and direction. So if the prevailing wind is in the opposite direction, it will greatly reduce the performance of the air-cooled heat exchanger.

Tube Bundle Orientation of air cooled heat exchanger
Fig. 8: Tube Bundle Orientation of air cooled heat exchanger

A-frame or V-frame air-cooled heat exchanger units are a compromise between the ground area requirement and exchanger performance. In this type of design, two bundles are sloped at 45°-60° and joined by their headers at the top or bottom. Thus it takes the shape of A or V. In steam condensing applications, the A-frame type (Fig. 8) with forced draft fans is used.

What are Air Cooler Headers?

Headers of air coolers are the boxes at the ends of the tubes which distribute the fluid from the piping to the tubes. The cover plate header design shall permit the removal of the cover without disturbing header piping connections. Fig. 9 shows a typical construction of tube bundles with removable cover plate headers.

a typical header for air fin fan cooler
Fig. 9: Figure showing a typical header for air fin fan cooler

The bonnet header design shall permit the removal of the bonnet with the minimum dismantling of header piping connections. Threaded plug holes shall be provided opposite the ends of each tube for access. Holes shall be threaded to the full depth of the plug sheet or 50 mm (2 inches), whichever is less.

Common header types (Fig. 10) used in air-cooled heat exchanger construction are

  • Plug Box Header: Widely used
  • Bolted Cover Header
  • Bonnet Header
  • Manifold Header
  • Pipe Header: Cylindrical header for very high-pressure services (>200 bar)
common air cooled heat exchanger header types
Fig. 10: common air cooled heat exchanger header types

Why some coolers are forced-draft & some induced drafts (Fig. 11) and which one is better?

  • Mostly air-cooled heat exchangers are of forced draft construction. Forced draft units are easier to manufacture and maintain. The tube bundle is mounted on top of the plenum, so it can be easily removed & replaced. The fan shaft is short since it does not have to extend from the drive unit through the tube bundle and plenum to the fan as in an induced draft design. Forced draft units require slightly less horsepower.
  • Since the fan is moving a lower volume of air at the inlet than it would at the outlet. If the process fluid is very hot, the cooling air is hot at the outlet, this could cause problems with some fans or fan actuators. If the fan is exposed to very hot exhaust air since forced draft coolers do not have the fans exposed to hot exhaust air.

In addition to the above, The major differences between an Induced Draft and a Forced Draft Air Cooled Heat exchanger are listed below with respect to some design parameters:

Sr. NoParameterInduced Draft Air CoolerForced Draft Air Cooler
1Air DistributionBetter distribution of air across the bundle.Less uniform distribution of air over the bundle.
2Air Re-circulationLess possibility of hot effluent air recirculating into the intakeIncreased possibility of hot air recirculation
3Process Control and StabilityBetter process control and stability. The plenum covers 60% of the bundle face area which reduces the effects of sun, rain, and hail.Less Process Control 
4Fan FailureIncreased capacity in the fan failureLow natural draft capability of fan failure
5Horsepower RequirementPossibly higher horsepower requirements if the effluent air is very hot.Possibly lower horsepower requirements if the effluent air is very hot
6Temperature LimitationEffluent air temperature should be limited to 220°F to prevent damage to fan blades, bearings, or other mechanical equipment in the hot air streamAccommodates higher process inlet temperatures
7Fan Maintenance AccessFans are less accessible for maintenanceBetter accessibility of fans and upper bearings
for maintenance.
8Bundle Maintenance AccessPlenums must be removed to replace bundlesBetter accessibility of bundles for replacement
9Structural CostIncreased structural cost and lower mechanical life.Reduced Structural costs and improved mechanical life.
Table: Difference between Induced Draft and Forced draft Air Coolers
Types of Air Fin Fan Cooler
Fig. 11: Types of Air Fin Fan Cooler

What kinds of Air Cooler controls are used?

As one might expect the best kind of control scheme depends on the application. Does the process require a very tight control on the process outlet temperature or is it better to allow the process temperature to go down with the ambient air temp. Following is a list of some of the commonly used control devices for air coolers, but in no particular order.

  • Manually operated louvers
  • Pneumatically activated automatic variable pitch fans.
  • Variable frequency fan drive (VFD).

VFD if used shall suit the motor nameplate rating and similarly for the Belt Pulley transmission it shall suit the motor nameplate rating.

What types of Air Cooled Heat Exchangers are used for Cold Climates?

For extremely cold regions like Canada, Siberia, etc an air-cooled heat exchanger with an internal recirculation system is used. Such systems can control the cooling air temperature regardless of ambient temperature.

Positive or Negative Step Auto variable fans are used for such systems. By using one fixed-pitch fan blowing upward and one Auto-variable pitch fan, which is capable of negative pitch and thus of blowing air downward, it is possible to temper the air to the coldest portion of the tubes and thus prevents freezing. Normally, forced draft units have the negative pitch fan at the outlet end, while induced draft units have the positive pitch fan at the outlet end.

Air Cooled Heat Exchanger with internal recirculation
Fig. 12: Air Cooled Heat Exchanger with internal recirculation

Codes and Standards for air-cooled exchanger Design

The following codes and standards govern the design of air fin fan coolers.

  • API 661 / ISO 13076 ( Air-cooled heat exchangers for general refinery service )
  • ASME SECT.VIII Div.1 or Div.2
  • Shell DEP 31.21.70.31

SHELL DEP special requirement for air-cooled exchangers:

  • The removable Bonnet type Header Shall not be used.
  • Plug-type headers shall be used for all air coolers in hydrocarbon gas or liquid service.
  • Removable cover plate type headers shall be utilized in auxiliary utility services such as lube oil hot oil, cooling circuit.
  • Bolted joints (stud construction or flanged construction) shall be designed with confined gaskets
  • Tubes shall be provided with aluminum fins. The fins for all the air cooler tubes shall be of the extruded type with 394 fins per meter

Maintenance and Inspection of Air-Cooled Heat exchangers

The performance and reliability of ACHE depend on proper inspection and maintenance of components. Regular maintenance and inspection of Air Cooler systems include

  • Lubrication of motor and fan bearings (once a month if coolers work continuously)
  • Checking of belts for wear and ensure their tension.
  • Inspect fans, blades, bolting, etc at regular intervals.
  • Maintaining proper oil levels at gearboxes.
  • Checking the condition of tube bundles.

Few more useful resources for you..

Basic Considerations for Equipment and Piping Layout of Air Cooled Heat Exchanger Piping
Considerations for development of Plant Layout: A brief presentation
Few articles related to heat exchangers

Animated video tutorial of Assembly Sequence of an Air Cooled Heat Exchanger

Refer to the below attached animated video for proper understanding and visualization of each part of a forced draft type air fin fan cooler. The assembling sequence of one single bay is shown in the video.

Air Cooler Assembly Sequence

References and Further Reading for Air Cooled Heat Exchanger (pdf)

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

13 thoughts on “Overview of Air Cooled Heat Exchangers

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  2. Very nice article! (i suppose. i have never read) anyway, i would like to ask about your profile photo. Where did you take this photo? And what are you hugging there? This is a strange photo Mr. Anup Dey.

    Regards,

    Blank

    https://whatispiping.com/wp-content/uploads/2020/01/Anup-Dey-author-whatispiping.com_.png

  3. Do you consider that the system of piping need a spool removal?? when we have a removal header that include the header nozzles??

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