Diagnosing Performance Problems of Air-Cooled Exchangers

Air-cooled exchangers generally operate for a longer period of time between maintenance

intervals than do shell and tube exchangers. However, performance problems do arise.

Below are listed, in order of frequency, common causes of impairment, ways to detect them, and

their remedies:

I:

Fouling of Extended Surface: Airborne contaminants accumulate on and between the fins

of air-cooled exchangers. Contaminants include, but are not limited to dirt, lint, insects, and

overspray from nearby painting / sandblasting operations.

Impairment occurs due to reduced airflow across the tube bundle.

Note: The practice of spraying water on air-cooled exchanger extended surface to increase

capacity during maximum ambient conditions, in addition to accelerating corrosion, generally

leads to deposition of external foulants. These foulants cannot be removed without damaging

extended surface.

DETECTION:

Inspect bundles from underneath. Finned tubes of induced draft units are

in plain view. View finned tubes of forced draft units through the fan ring.

REMEDY:

Most foulants can be removed by water wash of extended surface when unit is off

line. Detergent may be required to removed corrosion from aluminum fins. Notes of caution on

washing:

Water wash should generally be avoided while the unit is in service. Most Air-cooled exchangers

are hot enough that thermal shock, and consequent roll leaks could result.

Water blasting should not be used. Aluminum fins are easily bent.

Apply water spray in a plane parallel to the fins only, to avoid bending. Exercise extreme caution

in washing serrated-type fins. Serrated fins are formed, and thus it is not possible to maintain

spray in a parallel plane.

II:

Drive-belt slippage: Most air-cooled exchangers utilize belts as fan drives. Multiple “V”–

belts are the most common. Slippage of these belts will lower fan RPM, and severely limit

airflow across the tube bundle.

DETECTION / REMEDY:

Most slipping belts will emit a...