One of the unavoidable facts of life is that all systems require maintenance to perform optimally. Air handling units and fin tube heat exchange coils are no exception.
From the moment an air handling unit is put into use, the need for maintenance increases. Coil fouling is a common occurrence and requires regular care, but with proper maintenance can be all but avoided. Without adequate maintenance, coil fouling has the potential to result in all sorts of performance and efficiency problems.
What exactly is coil fouling?
Coil fouling is the increased thermal resistance due to the accumulation of contaminants on heat transfer surfaces. Coil fouling occurs when indoor dust and other particulate matter are deposited on evaporator heat exchangers.
In layman’s terms, coiling fouling is the technical term for a dirty coil. It might seem obvious that dirty coils won’t operate as well as clean coils, but let’s explore why this is and what if anything can be done to avoid coil fouling.
Why does fouling occur?
All fin tube heat exchangers, regardless of design, are prone to fouling, which occurs if there is airflow or waterflow in the unit. That being said, some designs are less prone to fouling than others, which must be considered when selecting a coil for your particular application.
There are four main reasons coiling fouling occurs: debris and particulate fouling, biological fouling, chemical reaction fouling, and freezing and frosting fouling.
Debris and particulate fouling occur when dust and biological materials find their way into the system. Mixed in with the air stream, this slowly begins sticking to coil fins and other surfaces, degrading performance. Not surprisingly, coils used for cooling pick up much more debris than their dryer heating coil cousins.
Moisture condensing on a coil not only attracts dust and debris, but also encourages the growth of microorganisms. Between the biological debris floating around the system, moisture, and temperature, a perfect breeding ground can be created. As gross as this may sound when the bacteria and fungi accumulate enough, they can form a film on fin surfaces that negatively impacts coil performance.
Chemical reaction fouling is another common type of coil fouling that is primarily the result of corrosion accumulation. Over time, metal coils in a dark, damp environment will corrode, and depending on composition, the introduction of debris in the airstream can accelerate this process.
During winter in outdoor air installations, frosting and freezing fouling can be a concern. Particularly a problem for low-temperature HVAC and refrigeration systems, ice accumulation negatively impacts coil performance, airflow, and pressure.
If you have been in the HVAC industry for any amount of time, you have most certainly seen coils that exhibit signs of fouling from all of these factors.
What are the negative implications of a coil with fouling?
Since the purpose of a coil is as a heat transfer device, and coil fouling is essentially insulating the coil, the result of fouling is reduced heat exchanger efficiency. This coupled with the decrease in airflow and pressure drop can create a situation where you now have a significant lack of cooling capacity.
What does this mean for your system as a whole? Decreased performance, increased operating costs, and reduced life.
What are some signs of coil fouling?
In addition to visually inspecting coils for fouling, your equipment will show byproduct signs of fouling if regular maintenance and cleanings are not performed. Some of the signs of fouling include equipment running longer to satisfy requirements, having to reset system controls to accommodate for thermal efficiency loss, and other equipment in the system, including fans, pumps, compressors or valves, failing more frequently than standard.
How does design and selection impact the coil fouling rate?
At its most basic level, coil fouling is the accumulation of debris on a coil. There are different types of debris, and ways it can accumulate, but the concept is pretty straightforward.
With that in mind, it is easy to understand why fin spacing and coil fouling would be related to each other. Fins spaced very close together are more likely to trap debris on the coil face and in between fins. The more fins per inch (FPI), the higher the fouling risk. Additionally, fin design can play a large part in fouling risk, with no fin and flat designs bearing the least amount of fouling risk, and more complicated designs like corrugated, sine, louvered and lanced fins bearing the most fouling risk.
Similarly to how more fins per inch increases fouling rate, the more rows of tubes in a coil, the higher the fouling risk. If you are putting any part of a coil in the way of airflow, whatever debris is in that airflow will eventually stick to the coil, whether that be to its tubes or fins.
Optimizing fin design, number of tube rows, and fin spacing may create a perfect coil under ideal operating conditions. Still, both heat transfer and fouling risk have to be considered when designing and selecting a coil. A design too optimized for heat transfer will have increased fouling risk, and require additional maintenance to actual deliver that performance. Meanwhile, a less aggressive design will be more resistant to fouling and more likely to consistently deliver overtime, even if maintenance can’t be done as frequently as should be required.
How to avoid coil fouling?
In a perfect world, all prevention techniques for fouling should be considered at the time of system design. These methods include filtration, screening, automatic cleaning processes, UV lighting, protective coatings, defrost systems and more. If your system was built without these prevention techniques implemented, retrofit is possible. Else, more diligent cleaning and maintenance are required.
Even with the best filtration systems and fouling mitigation systems in place, coils will develop some fouling over time, making regular cleaning necessary.
What do I do if I can’t clean my fouled coil?
It is not unheard of to have to replace a coil due to extreme fouling. Whether as a result of corrosion, or just a seemingly impossible to remove substance, coil fouling can become so bad that replacement is the most economical option.
If you are replacing a coil due to fouling, consider coil and system design modifications that will reduce fouling in the future.
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