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 their very low maintenance and operating costs.
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.
Air Cooled Heat Exchanger Construction
The construction of an Air-Cooled Heat Exchanger is fairly simple. Fig. 2 and Fig. 3 provide the component parts of an air fin fan cooler. Refer to 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.
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.
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)
What kind of finned tubes are used for Air Coolers?
- Fin types
- Externally bonded
- 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):
Fin Selection (Fig. 7):
Refer to Fig. 7 for limiting temperatures of various types of fins.
Typical materials used for air-cooled heat exchanger Fin construction are
- 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.
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 is 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.
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.
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)
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:
|Induced Draft Air Cooler
|Forced Draft Air Cooler
|Better distribution of air across the bundle.
|Less uniform distribution of air over the bundle.
|Less possibility of hot effluent air recirculating into the intake
|Increased possibility of hot air recirculation
|Process Control and Stability
|Better 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
|Increased capacity in the fan failure
|The low natural draft capability of fan failure
|Possibly higher horsepower requirements if the effluent air is very hot.
|Possibly lower horsepower requirements if the effluent air is very hot
|Effluent air temperature should be limited to 220°F to prevent damage to fan blades, bearings, or other mechanical equipment in the hot air stream
|Accommodates higher process inlet temperatures
|Fan Maintenance Access
|Fans are less accessible for maintenance
|Better accessibility of fans and upper bearings
|Bundle Maintenance Access
|Plenums must be removed to replace bundles
|Better accessibility of bundles for replacement
|Increased structural cost and lower mechanical life.
|Reduced Structural costs and improved mechanical life.
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 very tight control of the process outlet temperature or is it better to allow the process temperature to go down with the ambient air temperature? 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 prevent freezing. Normally, forced draft units have a negative pitch fan at the outlet end, while induced draft units have a positive pitch fan at the outlet end.
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 188.8.131.52
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, and 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 belts for wear and ensuring 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..
Animated video tutorial of the 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.