Air cooled heat exchanger

What is the difference between Induced draft and Forced draft in Air cooled heat exchangers

Induced draft vs forced draft in Air cooled heat exchangers. This article will look at the difference between them.

Air cooled heat exchangers are widely employed in industry due to their advantage of being able to utilize free air in cooling a fluid stream eliminating the need for utilities or refrigerants for cooling. They can have large footprint as units can vary in size depending of process conditions.

Air cooled heat exchangers can be mounted vertically or horizontally as per need and available space. This short article will only highlight the difference between Forced draft and Induced draft configurations.

What is Forced draft configuration?

Forced Draft unit configuration is the most common style of air cooled heat exchanger, the design positions the fans beneath the process tube bundle allowing easy access to all mechanical components.

Forced draft air cooled heat exchanger
Air cooled heat exchanger – forced draft configuration
Lower power requirements if the effluent air is very hot. For the same duty the forced draught unit requires less horsepower because it moves air at the lowest available temperature and highest density.Possibility of hot air recirculation, resulting from low discharge velocity from the bundles, high intake velocity to the fan ring, and no stack
Better access for bundles replacementLess uniform distribution of air over the bundle
Better access for fans and upper bearings for maintenanceLow natural draft capability on fan failure
Can accommodates higher process inlet temperaturesComplete exposure of the finned tubes to sun, rain, and hail, which results in poor process control and stability

In most cases, the advantages of induced draft design outweigh the disadvantages.

What is Induced draft configuration?

Induced Draft unit configuration offers greater control of the process fluid through more efficient airflow distribution, the induced draft design also protects the pressure vessel by positioning the plenum chamber above the bundle. Locating the mechanicals components below the bundle, as configured in the forced draft design, maintains accessibility.

Other benefits include lower noise levels at grade and reduced potential for hot air recirculation.

Induced draft air cooled heat exchanger
Air cooled heat exchanger – Induced draft configuration
Good Air distribution across the bundle since the air velocity approaching the bundle is relatively
Could have higher power requirements to forced draft units if the effluent air is very hot.
Good protection of tube bundle against sudden
temperature changes due to the weather
Effluent air temperature should be limited to 200F to prevent damage to fan blades, bearings, or other mechanical equipment in the hot air stream.
Increased capacity in the fan-off or fan failure condition, since the natural draft stack effect is much greater.Fans are less accessible for maintenance
Low possibility of hot effluent air recirculating into the intake. The hot air is discharged upward at approximately 2.5 times the intake velocity, or about 25 ft/sPlenums must be removed to replace bundles

When the process inlet temperature exceeds 350F, forced draft design should be considered because high effluent air temperatures may occur during fan-off or low air flow operation.

Induced or Forced Draught

A forced draught unit pushes air across the tube surface with the fan located below the tube bundle. An induced-draught design has the fan located above the bundle and the air is pulled across the tube surface.

The two systems are compared below. The selection of induced or forced draught type is generally made by the vendor who makes a selection based on the advantages/disadvantages of each system related to the process, engineering requirements and economics. However, occasions do arise when the process itself may be a significant factor in this selection.

Induced draft vs forced draft

Comparison of forced draft and induced draft air-cooled heat exchangers

AttributeForced DraftInduced Draft
Air distribution across sectionPoorBetter
Effluent air recirculation to intakeGreatly increased possibility of
hot air recirculation due to low
discharge velocity and absence of stack
Lower possibility because fan
discharges air upward, away from the tubes, at about 2½ times the intake velocity, or about 450 m/min (25 ft/s)
Influence of weather conditionsTotal exposure of tubes to sun,
rain, and hail
Less effect from sun, rain, and
hail because 60% of face is
Freezing conditionsEasily adaptable for warm air
recirculation during freezing
Warm discharge air not
Result of fan failureLow natural draft capability on
fan failure due to small stack
Natural draft stack effect is
greater than forced draft type
Power requirementSlightly lower fan power because the fan is located in the cold air stream (air has higher density)Slightly higher fan power because the fan is located in the hot air stream (air has lower density)
Temperature limit – discharge air
No limitLimited to about 95°C (200°F) to
prevent potential damage to fan
blades, bearings, belts, and other components in the air stream
Temperature limit – tube side
process fluid
Limited by tube componentsLimited to 175°C (350°F)
because fan failure could subject
fan blades and bearings to
excessive temperatures
MaintenanceBetter access to mechanical
Mechanical components are more difficult to access because they are above the tubes
Comparison of forced draft and induced draft air-cooled heat exchangers


Rules of Thumb for Chemical Engineers, Fifth Edition, by Stephen Hall (Butterworth-Heinemann, 2012).

Useful software

Air cooled heat exchanger design by Webbusterz engineering software


Induced draft vs forced draft, induced draft, forced draft