Salem Bouhairie, Group Lead Research

The performance of air-cooled heat exchangers (ACHE) is prone to the effects of wind. Wind has a greater effect on ACHEs operating in natural draft (buoyancy-driven air flow) than ACHEs operating in forced draft (fan-driven air flow). Subsequently, engineers follow API 661 guidelines that recognize the importance of wind in ACHE design. However, the Natural draft air-cooler mode in Xace® does not currently rate ACHE performance with wind.

To understand the effect of wind on a natural-draft API 661 ACHE, HTRI collected measurement and visualization data from the Air-cooled Unit (ACU) at the Research & Technology Center (RTC). The ACU data were used to correlate trends between crosswind velocity, ACHE plume velocity and plume height, and duty.

Crosswinds can impede airflow, reducing the effective flow rate of air passing over the ACHE tubes. In some cases, wind can push the hot air plume rising from the heat exchanger sideways, causing hot air recirculation (HAR) back into the air intake.

The plume behavior depends on chimney height, wind, and the stability of the ambient air temperature represented by its lapse gradient (air temperature change with elevation). The faster the air temperature decreases with height, the "steeper" the lapse gradient and the more unstable the plume air parcel becomes in the atmosphere. There are three air stability conditions:

  • Unstable air: The plume air parcel is warmer than its surroundings, so it rises and expands but occasionally drops down.
  • Neutral air stability: The plume air parcel is the same temperature as its surroundings, with no change in vertical position.
  • Stable air: The parcel is cooler than its surroundings, so it sinks and compresses.

In Video 1, unstable air is characterized by "plume looping" (mostly rising with occasional dipping).

The process conditions are as follows:

  • Airside temperature = 19 °C
  • Wind speed = 0.2 m/s
  • Inlet tubeside temperature = 88 °C
  • Tubeside flow = 0.02 m3/s
  • Chimney height = 1.2 m

In Video 2, neutral air stability is characterized by "plume coning."

The process conditions are as follows:

  • Airside temperature = 11 °C
  • Wind speed = 0.9 m/s
  • Inlet tubeside temperature = 88 °C
  • Tubeside flow = 0.02 m3/s
  • Chimney height = 2.4 m

In Video 3, stable air is characterized by lateral "plume fanning."

The process conditions are as follows:

  • Airside temperature = 28 °C
  • Wind speed = 2.2 m/s
  • Inlet tubeside temperature = 88 °C
  • Tubeside flow = 0.02 m3/s
  • Chimney height = 1.2 m

From the measurements and visualization, HTRI is developing a model that predicts Xace plume heights and subsequently predicts duty as a function of wind, for Natural draft air-cooler mode.

This content was taken from the technical presentation "Wind Effect on Natural-draft Air-cooler Performance", delivered at our 2025 Global Conference. This presentation and other conference materials can be purchased by members for $495. For information on purchasing the 2025 Global Conference materials, contact [email protected].

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