Drawing No. EH–TH–030 // Thermal Engineering & HVAC
Reviewed August 2026
A fan mounted above the fill pulls air upward through the tower, counter to the falling water — the most common cooling tower configuration in industrial and power plant service.
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The fan draws air in through louvers at the base and up through the fill counter to the falling water, discharging warm, saturated air above the fan deck.
An induced draft tower uses one or more axial fans mounted on a deck above the fill to actively pull air through the tower, rather than relying on buoyancy alone. Air enters through louvers around the base, travels upward through the fill in direct counterflow to the falling water — water moving down, air moving up, meeting head-on — and is discharged well above the fill by the fan, which both reduces the chance of the humid exhaust being drawn straight back into the air inlets (recirculation) and gives the plume enough exit velocity to disperse away from the structure.
Counterflow contact is thermodynamically the most favourable geometry available: at every point in the fill, the air meeting the coldest water is itself the coldest, driest air just entering, and the air meeting the hottest water at the top is already the warmest, most humid air about to leave. That arrangement maximises the average driving force between water and air across the full height of the fill compared with a crossflow arrangement, which is the physical reason induced draft counterflow towers can achieve a tighter approach for a given fill volume than a comparable crossflow design.
Because a fan actively sets the airflow, the water-to-air mass flow ratio (L/G) — the single parameter that most determines a tower's thermal characteristic — becomes a genuine design and operating choice rather than something fixed by the structure's geometry alone. Variable-speed or multi-speed fans let the tower track a varying heat load or ambient condition directly, trading fan electricity for approach in a way a natural draft tower simply cannot.
The fan and its motor sit directly in the warm, saturated exhaust air stream leaving the tower — the defining maintenance trade-off of this configuration. That environment is corrosive and humid, so induced draft fan motors, drive shafts, and gearboxes need to be specified for continuous duty in a wet, warm atmosphere, and routine maintenance means working at height inside that same exhaust stream.
Want the numbers behind these trade-offs — approach, water consumption, fan power, sizing, and operating cost for your own heat load and weather? Use the Cooling Tower Calculator.
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