Figure 2

Reverse flow is a common problem in older air-coolers.  This misdirected flow causes two problems for the heat exchanger:  1) the obvious one is a net loss in the amount of air that travels through the tube bundle to provide cooling; 2) the secondary effect is that when the air flow returns to the suction side of the fan, it is again sucked up and creates artificially high inlet air temperatures which ultimately lead to less heat transfer capacity.

There are two areas where reverse flow is most prevalent.  The most common one is at the tip of the fan blade where it meets the plenum housing.  Over time or with incorrect installation, a gap can be found which will increase the amount of air flow that loops around the blade and travels back to the fan suction side.  This gap should be approximately 3/8” but not greater than 3/4” as per API-660.  Reverse flow can be detected by measuring the fan tip gap but the recommended way to determine if reverse flow is present is to conduct an air velocity profile (shown above in Figure 2) and look for a negative air flow number.  Again, it is better to look at the air profile from an exchanger immediately after it was cleaned.  If the exchanger is not clean, the high pressure drop caused by the dirt could lead to reverse flow which could be eliminated with a simple cleaning.  If reverse flow exists after cleaning, the fix is to install tip a seal on the plenum which will eliminate the gap.

A less common form of reverse flow in ACHE occurs if there is a gap in the area above the motor or hub.  Air will loop back through the center of the fan blade and be caught in a recycle.  This problem will again be obvious if an air flow profile is taken.  The fix to this problem is more complicated and costly but in most cases installing a hub seal will eliminate this problem.

ACHE fans can have fixed or adjustable pitch blades.  Adjustable pitch blades are most often used and the adjustment can be either manual or automatic.  The blade angle on manually adjusted pitch fans can only be changed when the air cooler shutdown.  Automatically adjusted fan blades can be rotated to various angles while the air cooler is in operation.  Newer air-cooled heat exchangers are usually provided with manually adjusted fan blades and use variable speed motors to provide the required air flow variability.  Blades require an initial angle setting to achieve optimum performance. Quite often, automatically adjusted fan blades get stuck after some time and the air flow variability from the variable blade pitch angle is not longer available for process control.

A related problem that is often encountered is motor not running near their full load amps (FLA).  To optimize peak air flow and heat transfer, fan motors should operate near their full load amp (FLA) set point.  If a motor is running below 70% of FLA, adjusting either the motor or the blade pitch angle to increase the air flow will lead to better performance.  It is preferred to have the %FLA at or above 85.   

   
The overall condition of an ACHE can greatly reduce its ability to transfer heat.  The following list contains common areas where air-coolers can experience minor problems that are relatively easy to fix.

Air-coolers sometimes use louvers to control the outlet temperature by throttling the air flow.  Missing or inoperable louvers are a common problem.  Louvers should be inspected periodically to validate that the actuators are working properly.  At full open, the louvers should be at least 50% to 60% open to allow unimpeded air to travel through the tube bundle. 

Another structural component of every air cooled heat exchangers is the plenum housing.  The plenum should be inspected periodically to confirm that no panels are missing or that no large holes exist.  If there are gaps in the plenum, the air will have a path through which to escape without going through the tube bundle.  This reduces the overall heat transfer capability of the heat exchanger.

Although a fairly common practice, it is not recommended to spray water on tube bundles to provide temporary additional heat transfer capacity during hot summer days.  Operating plants which adopt this practice see a deterioration of the aluminum tube fins overtime from corrosion and fouling due to chloride formation in the heat transfer surface.  This practice leads to a reduction in performance over time and in time the tube bundle needs to be replaced.  Replacing tube bundles is expensive and time consuming and should be used as a last resort.  If the fins are corroded or have become detached, there may not be any other option than replacing the tube bundle. 

A common problem is bent or crushed tube fins.  In this case, a comb type device can be used to rake through and lift the fins back into a position perpendicular to the tubes.   This will help increase the heat transfer performance of the air-cooler.

This section covers a few of the potential control problems with air-coolers. 

Over time or periodically, the inlet process conditions can change.  The process flow rate, composition and inlet temperature may vary from design conditions.  Often air-coolers are thought of as under-performing when actually the total required heat duty has changed over time or suddenly increased. 

Another less common cause for air-cooler problems is improper control set points.  Often the use of heat exchangers changes over time but the set points tend to be “fixed and forgotten”.  A quick check of the current heat exchanger design documentation can provide insight into the expected process conditions and set points.  The plant data should be compared to the design data and any discrepancies investigated.

Many air-coolers are used as overhead condensers on distillation columns.  Similar to all other heat exchangers, air-coolers performance can suffer if non-condensable vapor gets trapped in the tubes reducing the effective heat transfer area.  The usual design technique to eliminate this effect is to provide a non-condensable purge line that will provide a way out of the system.  If an air-cooler is not performing well in an overhead condensing service, one cause could be that the non-condensable purge line has been closed or plugged.  This line should be checked periodically to assure a clear path and prevent gas build-up.

Process Side Issues

The easiest and most cost effective fixes to air-cooler problems usually occur on the air side of the exchanger.  Unfortunately this does not fix all problems and we also find  that heat transfer limitations can occur because of poor conditions on the process side.

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