In industrial cooling sites, rows of operating air coolers can be commonly seen. Upon close observation, it can be found that the fans of some air coolers are installed below the tube bundles, while others are mounted on the top. This difference corresponds to two basic ventilation modes of air coolers: forced draft and induced draft.

Forced Draft: Blowing Air from Bottom to Top

Forced draft air coolers adopt fan installation beneath the tube bundles. During operation, ambient cold air is drawn in from the lower part and forced to pass through the finned tube bundles. The heated hot air is then discharged from the top of the bundles.

In this configuration, fan motors are placed on the cold air side with low ambient temperature, delivering a relatively long service life during long-term operation. Meanwhile, the exposed fan structure enables convenient inspection and replacement. Thanks to these advantages, forced draft air coolers are widely applied in oil refineries and chemical plants.

Nevertheless, forced draft design has obvious limitations. Air is blown upward at a low outlet velocity. In windless or breezy weather, the exhausted hot air tends to accumulate above the equipment and be re-ingested by the fan inlet, causing hot air recirculation and a notable decline in cooling efficiency. In addition, the fully exposed upper tube bundles are vulnerable to direct sunlight, rain and snow erosion.

Induced Draft: Extracting Air from Top to Bottom

For induced draft air coolers, fans are arranged above the tube bundles. When running, negative pressure is formed at the top of the bundles, guiding ambient air to enter from the bottom or side sections. The air flows upward through the heat exchange bundles and is finally extracted outward by the fans.

The greatest advantage of induced draft is the uniform airflow distribution. Since air is drawn through the tube bundles rather than forcefully blown, the wind speed across the entire bundle section remains consistent, achieving higher heat transfer efficiency. Meanwhile, the hot air outlet is located at the fan top with high exhaust velocity and a safe distance from the air intake, which effectively avoids hot air recirculation. For stringent process cooling conditions, induced draft is usually the preferred solution.

On the downside, fan motors of induced draft units are situated above the tube bundles, working continuously in high-temperature exhaust air. This places higher requirements on motor insulation and heat dissipation performance. Moreover, the top-mounted transmission mechanism is less convenient for daily maintenance, and scaffolding or cranes are often required during overhaul.

Core Principles for Model Selection

The key factors to distinguish and select between the two modes are as follows:

Climatic conditions: In hot and windless regions in summer, induced draft shows stronger resistance to hot air recirculation.

Footprint and space: Induced draft equipment features an overall higher height and has stricter requirements on installation space.

Maintenance capacity: For plants with sufficient maintenance resources and high efficiency demands, induced draft performs better.

Noise control: With fans installed on the top, induced draft effectively reduces noise interference in ground operating areas.

In essence, both forced draft and induced draft are designed to realize efficient heat exchange between air and process media. Clarifying their structural and operational differences helps select the most suitable solution for actual working conditions.

Shanghai Rivbay Chemical Equipment Co., Ltd. specializes in the R&D and production of non-standard equipment, including reaction vessels, heat exchangers, tower equipment, modular units, freeze dryers and cold traps. The company holds ASME U-stamp certification, EU PED, Korea Kosha and EAC certifications. Its products are widely used in chemical, petrochemical, pharmaceutical, food, environmental protection and other industries, providing customized high-matching equipment solutions. For more information, please feel free to contact us.