An Air Cooled Heat Exchanger is a heat transfer device for rejecting heat from a hot fluid directly to fan-blowing ambient air.
 The obvious advantage of an Air Cooled Heat Exchanger is that it does not require water, which means that plants requiring large cooling capacities need not be located near a supply of cooling water. An Air Cooled Heat Exchanger may be as small as an automobile radiator or large enough to reject the heat of turbine exhaust steam condensation.

Components of Air cooled heat exchanger 

 An Air Cooled Heat Exchanger may consists of the following components
     One or more tube bundles.
     Air moving parts such as a fan, blower, or stack.
     Fan driver such as Electric Motor.
     Plenum between air moving parts and the tube bundle.
     Steel structure to support and allow space for Inlet ambient air to enter.
     Optional header and Motor maintenance walkways with ladders to grade.
     Optional louvers for process outlet temperature control.
     Optional recirculation ducts and chambers for protection against freezing or solidification of high          pour point fluids in cold weather.
     Optional variable pitch fan hub for temperature control and power savings.

 Temperature Control

When one needs to control the outlet temperature of the fluid with the changes of ambient temperature or the flow rate, or to prevent the fluid from excessive cooling, freezing or solidification during the cold season, one of the following contorol method can be used.

Control by Fan 
With the changes of ambient temperature or the flow rate of fluid, the air flow rate can be controlled by means of automatically adjustable pitch fans. Manually adjustable pitch fans may be partially switched off according to the operating conditions. It is recommended to use 50% auto fans and 50% manual fans to control the operating conditions and to save running cost.

Control by Louver
Automatic or manual louvers are equipped over the tube bundles to control the air flow rate in accordance with the ambient temperature.

Control by steam coil
In case it is difficult to prevent excessive subcooling fans or louvers during cold season particularly at start-up, the air can be heated by steam coils located underneath the tube bundles. In many case steam coils are supplied with louvers.

Control by hot air recirculation system

In case it is difficult to prevent subcooling, solidification or freezing of the process fluid during operation, a hot air recirculation system can be used. This will also save steam costs.

Performance Control of Air Cooled Heat Exchanger.

In addition to the fact that the process flow rate, composition, and inlet temperature of the fluid may vary from the design conditions, the ambient air temperature varies throughout a 24-hour day and from day to day. Since air coolers are designed for maximum conditions, some form of control is necessary when overcooling of the process fluid is detrimental, or when saving fan power is desired. Although control could be accomplished using by-passing of process fluid, this is rarely done, and the usual method is air flow control.

The Method of Air Flow Control:
    Adjustable louvers on top of the bundles.
    Two-speed fan motors.
    Fan shut-off in sequence for multifan units.
    Auto Variable pitch fans.
    Variable frequency fan motor control