Why Doesn't My Air-Cooler Work?

(And what can I do to fix it?)

Jim Stone

Stone Process Equipment Co.

Lack of performance seems to be a common problem in certain sectors of industry. We would like to offer a few ideas about this and possibly how to prevent it in the future.

Here's a short list of possibilities for lack of performance. I will elaborate on them below.

1. The process flow may have been changed requiring a higher duty or more surface that was originally needed.

2. The design ambient air temperature was not high enough to allow for full performance during the hottest days of the year.

3. The finned tubes have become fouled, reducing air flow through the tube bundle.

4. Accidental warm air recirculation is increasing the effective incoming air temperature.

5. Loss of thermal contact between the fins and the tubes.

6. The cooler was improperly designed in the first place. This is probably more common than you might expect and can take many forms.

Let's take a look at these items one at a time.

1. Change in process.

Sometimes the cooling load put on an exchanger changes over time due to increased throughput of the process or perhaps some change on the heat generating end of the process. If this change is small, it may be possible to get more performance out of the cooler by making a few changes.

The least expensive is by simply increasing the pitch of the fans to deliver a little more air flow. Care should be taken so as not to exceed the amperage limits of the motor. If the fan is a manually adjustable pitch type, and not on a variable frequency drive, remember that the fan horsepower will increase as the ambient air temperature goes down. You don't want to exceed the limitations of the motor!

The next steps would probably be (in order of cost):

A change in the drive ratio between the motor and the fan. This is pretty easy to do with V-belts, but would be expensive with gear drives. Caution!! Remember that the fan noise is directly related to the tip speed. An increase in RPM can greatly increase noise.

Changing the drive motor to the next available size.

Changing the fan to one with more blades or a different series (wider blade chord, larger diameter hub, etc.)

Change to tube bundle to add more rows of tubes, different tube diameter, etc. This is usually a very expensive option, but less than a complete new cooler.

Replace the entire cooler. Sometimes this is the only practical solution. Older coolers often were designed to be very noisy, or undesirable for other reasons.

I would suggest that you discuss any of these changes with the cooler manufacturer before spending a lot of money. The manufacturer may be able to offer a cost-effective fix that will exactly meet your needs. It would probably be some combination of fan and drive change that would be most cost effective. However, for some processes, a change in the amount of air flow will do practically nothing to increase performance. In other applications, such as simple water coolers, it may be possible to get an additional 5-10% heat rejection by making simple changes.

2. Too low an ambient air temperature in the design

Sometimes this is a big deal. Other times it is not at all significant. What's the difference? Primarily, it's the approach temperature. The approach temperature is the difference between the process outlet temperature and the ambient air temperature. If this difference is low, say 15 deg. F, a cooler designed for 95 F on a day 100 F day will likely not achieve the required cooling. However, the the process outlet temperature is, say 200 F, then the difference is not very significant. Also, a low LMTD (log mean temperature difference) makes matters worse. A low LMTD occurs when both the approach and the cooling temperature range of the process are low. Example: Cooling from 125F to 115F with a 100 F ambient temperature.

What's the fix? It depends on what you can live with and how critical your process temperature is. If the process outlet temperature is very critical, you are probably going to need an expensive fix like replacing the tube bundle or the complete cooler.

3. Fouled Fins

This is probably one of the easier things to find and fix. Air-side fouling is often caused by fibrous air-born matter such as the stuff that comes from cottonwood trees. In extreme cases, this can plug the spaces between the tubes and fins, almost completely blocking air flow. The usual fix is to carefully wash the finned tubes down from the opposite direction of the air flow. Some fins, such as the extruded ones, are pretty sturdy, and can take a lot of water pressure. On the other hand, If you have a cooler with paper thin (like .006" to .010") fins, you must be very careful to spray them parallel to the fins. Otherwise the fins can bend over and cause more blockage. This is much easier to do on a forced-draft cooler than on an induced-draft one, by the way. On an induced-draft cooler, you may have to crawl around inside the plenum to clean the fins.

Be sure to do this when the cooler is shut down.

4. Accidental warm air recirculation

This is one area where an induced-draft cooler has an advantage. Accidental recirculation occurs when some of the warm exhaust air from a cooler is drawn back into the intake side. On an induced-draft cooler, the exhaust air velocity is much higher than on a forced-draft one since it is blowing directly out of the fan ring. This tends to get the hot air away from the cooler, preventing it from coming back down into the intake.

In a bank of coolers, sometimes the bays may be spaced with a few feet distance between them. This creates an area of low pressure between the adjacent bays, and can easily result in accidental recirculation. In this case, the space should be simply closed off. In a large bank of coolers, they should be sufficiently elevated to keep the intake velocity around the perimiter to a reasonable level.

On a stand-alone cooler with this problem, it may be possible to erect a wind-skirt on the leeward side of the cooler from the prevailing wind to minimize recirculation.

4. Loss of fin attachment

If the fins lose their attachment to the tubes, the cooler's performance will be drastically reduced. This seems to be especially common at installations located in coastal areas or on islands or offshore platforms. The most common type of fin attachment is the L-footed fin. It is fabricated by finning machine which wraps a continuous strip of fin stock around the tube, forming an L-shaped foot in the process. The foot of the fin acts like a spring to help maintain a good contact. This type of fin is commonly used where the process temperatures is below about 300-350 F and there is little likely hood of corrosive attack on the fin material. Note: I would recommend avoiding edge-wrapped (sometimes called edge-tension) fins which do not have L-foot.

For most applications, the L-footed fin is ideal. However, it has some drawbacks. At high process temperatures, these fins can become loose on the tube, creating a loose "slinky" effect. When this happens the fins do essentially nothing, and you are stuck with effectively a bare tube. Also, these fins can sometimes break or unwrap at a splice in the fin. There are two ways around this problem. 1. Avoid any splices of fins in the middle of a tube. 2. Use cast zinc collars at the tube ends and the tube supports. The collars act as a stop, and prevent any possible unwrapping beyond the six feet or so between collars. The collars at the tube ends are a requirement of the API-661 specification. A few "good" manufacturers also use collars at the tube supports as a standard.

In coastal, island, or offshore applications, extruded aluminum fins are often preferred. Extruded fins are formed by placing an aluminum sleeve (sometimes called a "muff") over the tube and running it through an extruding machine which squishes the aluminum sleeve into fins. It works kind of like a thread-rolling machine. The idea here is to prevent any corrosion between the fin and the tube, since there are no cracks or spaces to allow salt water to get between the tube and fin except at the tube ends. The most typical extruded fin has an OD of 2.0" or 2.25" on a 1" diameter tube. They are available in other sizes, too. But, I digress....

One would expect this to result in an absolutely tight bond between the fin and the tube. However, this is not necessarily so. I have seen cases where extruded fins became loose, causing the heat transfer to drop by over 60%. Some manufacturers simply fin the tubes and hope for the best. Others have procedures in place to test the tube to ensure a complete thermal contact (Cooling Products has an extensive procedure for this). If you are buying coolers with extruded fins, it might be important to ask about how your cooler manufacturer does this.

6. Improper Cooler Design

If you are a regular user of coolers, you have probably experienced some form of bad cooler design.

Here's a typical example. You visit a small natural gas transmission station. They have a cooler used for diesel engine jacket water. It's a hot summer day, and the plant guys are spraying water on the cooler tube bundle. Why? Because the engine automatically shuts down when the jacket water reaches a certain temperature, and this has been happening a lot lately. After some time, the finned tubes become encrusted with calcium. More and more water is needed. Eventually, the whole tube bundle has to be replaced.

What's going on here? Probably several things. Here are some examples of what is typically wrong with certain coolers.

A. Intentional Underdesign

That's right. Intentional. Some manufacturers will intentionally shave their rating by as much as 20%. This is done for competitive reasons. For some manufacturers, this seems to be a standard practice. If they are questioned by their customers about this, there seem to be a few standard answers:

1. Don't worry. We guarantee it!

2. We have these magic fins. They improve our performance tremendously.

3. Sorry, but it's out of warranty. It's your problem now.

4. What do you expect? It was cheap!

You get the idea. If you build enough safety factor into your cooler specification, you just might get one that works from one of these guys. I will leave it to the reader to figure out which manufacturers do this. However, the customer shouldn't have to do that to get a cooler that actually works.

B. Poor Air Distribution

The axial flow fan used on air-cooled exchangers have an interesting phenomenon. The produce much more air flow in the area of the blade tip than anywhere else. In cooler designs, this is accounted for in several ways. First, by making the plenum sufficiently deep, this localized maldistribution is minimized. The fact is, the deeper the plenum, the more evenly the air will be distributed across the cooler bundle. The second way is by providing a fan which is sufficiently large (40% per API-661) compared to the bundle face area so that the internal fan losses are minimized. The third way (as specified in API-661) is to provide a maximum 45 degree dispersion angle from the fan ring to the sides and ends of the tube bundle.

One additional measure that can be taken is to use rounded-and-eased fan rings. They act similar to a velocity stack on a race car engine. These reduce internal fan losses and also improve air distribution. (This is a Cooling Products standard feature on forced-draft coolers.)

There are a few more common problems with the design of some coolers:

1. Using very small or no fouling factors. Fouling factors are actually safety factors applied to the heat transfer calculation. They are supposed to account for fouling and/or uncertainties in the heat transfer correlations. They typically provide about a 10-20% safety factor in the calculations compared to a theoretically "clean" exchanger.

If the fouling factors are eliminated in the design, the cooler may work through the warranty period, but as some fouling builds up over time, the performance may degenerate enough to be a problem.

2. Use of "optimistic" air-side static correlations. This takes two forms. One form results in cooler designs with high air-flows and usually large motors, since designers are sometimes shooting for static pressures within some maximum limit. The second form is to rate the fans as though they have rounded-eased fan rings, even though the cooler is not built that way.

If you are experiencing problems with your cooler, we will be happy to help analyze the problem. Please send us an email. Jim@stoneprocess.com