Induced draft vs forced draft in air cooled heat exchangers

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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

Advantages	Disadvantages
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 replacement	Less uniform distribution of air over the bundle
Better access for fans and upper bearings for maintenance	Low natural draft capability on fan failure
Can accommodates higher process inlet temperatures	Complete 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

Advantages	Disadvantages
Good Air distribution across the bundle since the air velocity approaching the bundle is relatively low.	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/s	Plenums 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

Attribute	Forced Draft	Induced Draft
Air distribution across section	Poor	Better
Effluent air recirculation to intake	Greatly 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 conditions	Total exposure of tubes to sun, rain, and hail	Less effect from sun, rain, and hail because 60% of face is covered
Freezing conditions	Easily adaptable for warm air recirculation during freezing conditions	Warm discharge air not recirculated
Result of fan failure	Low natural draft capability on fan failure due to small stack effect	Natural draft stack effect is greater than forced draft type
Power requirement	Slightly 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 stream	No limit	Limited 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 components	Limited to 175°C (350°F) because fan failure could subject fan blades and bearings to excessive temperatures
Maintenance	Better access to mechanical components	Mechanical components are more difficult to access because they are above the tubes