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Technical · Air cooler performance

Hot air recirculation (HAR)

Hot exhaust loops back down into the cooler's own fans. Inlet air runs warmer, and you lose capacity on the hottest days.

Reviewed by Nick Agius, Founding Partner Russell De Jesus, P.Eng., VP of Engineering

See it happen

Exhaust that comes back for a second lap.

Forced draft cooler, side view
Hot air leaving the top of a forced draft cooler, with some looping down the side into the fan inlet Tube bundle E-Bell™ Fan HAR
Ambient air inHot exhaust

Bare fan ring: hot exhaust curls down the side and back into the fan.

Why it happens

Fast inlet air pulls the exhaust back down.

  • Speed sets pressure

    Fast inlet air means low pressure at the sides, and that drags exhaust back down.

  • Fans below the bundle

    Forced draft exhaust drifts out slowly, so more falls back. Induced draft fires it out much faster.

  • Wind

    A crosswind bends the plume into your cooler, or the one next door.

Forced vs induced draft
Forced draft cooler with slow, wide exhaust beside an induced draft cooler with a fast, tall plume Forced Slow, wide Induced Fast, tall
Same job. The induced draft plume leaves faster and clears the unit.
Is your cooler at risk?

Four things that stack the odds.

  • Forced draft fans the most exposed design
  • Middle of a bank air gets in from two sides only
  • Tight approach outlet within about 10°C of peak summer air
  • Summer wind pushes exhaust where it shouldn't go
Top view: where the air gets in
A stand alone cooler takes air from four sides; a cooler in the middle of a bank takes air from only two Stand alone In a bank
Fewer open sides, faster inlet air, more HAR.

Common tell: one cooler in a bank runs hotter than its neighbours on hot, windy afternoons.

What helps

Fixes, from design stage to retrofit.

  • Design stage

    More ground clearance

    More room under the cooler slows the inlet air.

  • Design stage

    Smarter plant layout

    Line coolers up with the summer wind and keep engine exhausts away from fan inlets.

  • Retrofit

    Wind screens

    Side mesh screens calm crosswinds at the fan.

  • Retrofit

    Deep inlet bells

    An E-Bell™ elliptical inlet bell adds depth below the fan ring, which can help steady the air in and reduce how much exhaust short cycles back.

Blue E-Bell elliptical inlet bell installed below an air cooler fan, seen from the scaffold underneath
An E-Bell™ installed on the fan ring, seen from below.
Book a HAR assessment

Think HAR is costing you capacity?

Send us what you've got. We'll tell you if recirculation is part of the problem and what it takes to fix it.

Book a HAR assessment See E-Bell™ sizes
  • What to send
  • Plot plan or cooler layout
  • A few site photos, including the fan inlets
  • Summer outlet temps and any trips
  • Prevailing summer wind, if you know it