In cooling systems, fouling is any material or deposit that collects on the heat exchange surfaces. This can be airborne such as dust and leaves, or water borne depositions such as scales, inorganic and organic particles.
Why does it happen?
Passing fluids (air and water) over a cooling surface can result in the accumulation of foreign matter on the surfaces of cooling tower fills, or on the fin-coils of an air-blast cooler. Fouling can lead to a reduction in the cooling efficiency of a system, as deposits interfere with air and/or water flow across the heat exchanger.
Any type of build-up in most settings is usually considered detrimental, but in cooling towers that is not always the case. Light fouling in cooling tower heat exchange media has little or no effect on cooling tower performance. In fact, small amounts of fouling can aid the system as light fouling on the heat exchange media increases the surface area of the water and the air to water contact time without increasing the airside pressure drop. This could result in a slight improvement in performance in some cases.
There is a fine line between build up that works alongside the tower and build up that can cause major problems. Advanced fouling can result in a weight gain of up to 20x, leading to collapse and expensive replacement costs, as well as dangerous levels of harmful bacteria forming. HSG274Pt1 offers guidance of acceptable fouling within cooling tower fill from the perspective of bacterial growth.
Fouling in dry cooling systems
In contrast, light fouling on an air cooler coil creates a thermal barrier between the air and the cooling fins, which affects heat transfer through the materials of finned coils, leading to much larger and faster performance degradation. This is known as thermal contact resistance.
Fin coil performance can be affected in three ways: fin fouling; fluid side fouling; and air flow reduction.
For wet cooling systems the solution is to monitor the weight of cooling tower heat exchange media over time to track fouling, all while actively seeking opportunities to reduce it. Alternatively, endoscopic photography of the internals of the media is an effective way to assess fouling.
With air-blast coolers the fin-coils will show evidence of fouling on the individual cooling fins, and this assessment can be conducted externally, whereas with an evaporative cooling system these will need to be offline, in most cases.
As with any system, cleanliness is key. In evaporative cooling systems, once an assessment has been made of the heat exchange media and it is deemed to be fouled, it should be cleaned as per the guidelines of HSG274 Part 1. This will involve the removal of the media from the system for cleaning externally, or if removal is not practical, cleaning in situ, with a thorough post examination. Although this can be time consuming, it is not a particularly difficult task to undertake and the media is very resilient to handling.
In air-blast coolers the fin-coils (which are the most expensive part of the system) are known for being difficult to clean and are often very delicate which can lead to damage during cleaning activities. Generally, fin-coils require much more frequent cleaning than wet cooling media. Even with regular cleaning of these fin-coils the long-term performance of many air-blast systems will continue to reduce over time.
In contrast, after cleaning, cooling tower media will have almost the same characteristics as when it was new, providing no damage occurs, resulting in little or no performance reduction.
Balance is key
Fouling is a phenomenon that is intrinsic to the use of cooling systems, as the large volumes of air drawn into the unit will almost always be contaminated with foreign material, and as water is evaporated, impurities can build-up. It should be assessed regularly, and corrective action taken to prevent severe fouling.
External filtration systems can be employed to help reduce the level of impurities drawn into any cooling system and can help increase cleaning intervals, but will not eliminate them.
Light fouling in wet systems has little or no detrimental effect on the performance of the unit, whereas in air-blast systems light fouling has a marked effect.
Balance is key when it comes to maintaining cooling systems, too much or too little maintenance can both cause harm, so if in doubt it's always better to call in an expert for some advice or support in keeping the cooling systems running as effectively and efficiently as possible and prevent any further complications.