Proper filtration

The single most important factor in keeping coil surfaces clean is proper filtration of the airstream. Particles in the air not only clog coils and reduce their efficiency, but also enter the occupied spaces of a building and cause IAQ problems.

The first step in keeping the coils clean is careful filter maintenance. Filters need to be inspected and replaced frequently, following the manufacturer’s recommendations.

A dirty, clogged filter will not keep the air clean and can impede the passage of the airstream. In fact, the fan may actually begin to pull on the filter media, even tearing it out of the frame. This leaves gaps that enable air to pass through the hvac system without any filtration.

Standard 2-in. filters that many commercial buildings use have an average efficiency of less than 20%. They eliminate larger particles that can build up on coils, but they do not screen out and reduce spores and other contaminants that may affect the health of building occupants.

To address IAQ concerns, filter manufacturers have developed higher-efficiency filters that can help keep both the airstream and the coils cleaner. These extended-surface, pleated filters offer efficiencies of 35% to 40%, and while they cost more, are more effective at removing smaller particles from the air.

We now understand that filtering the smaller microbial particles is even more important than removing nuisance dust. A study by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) on the effects of microbes on IAQ, stressed that controlling airborne microbes is a major challenge, because they contain organisms such as bacteria, fungi, algae, pollen, and viruses.

Many of these particles cause allergic reactions and even various infectious diseases. Other studies indicate that fungi and bacteria account for nearly twice the number of causes of IAQ problems as dust.

In fact, it has been determined that dust accounts for only 25% of the particulate that causes IAQ problems in a building.

Cooling coils and drain pans, the most likely areas to trap microbes and dirt in the hvac system, can act as reservoirs for microbial growth. As water vapor condenses in the air passing over the coils, particulate matter is washed out of the airstream and can collect on the coils and in the drain pan.

This wet, dark, temperature-controlled environment is perfect for the growth of molds and fungi, which can then be circulated by the fan into occupied spaces.

Controlling microbes

For this reason, newer standards require double-sloped rather than flat drain pans. Coil cleaning and sanitizing products with biocidal characteristics can help control the microbial growth on these surfaces.

It is important to clean both the coils and the drain pans at the same time. All surfaces should be thoroughly rinsed when the cleaning is completed.

When considering cleaning products, look for those that are easy to apply. Many are caustic and acidic, relying on strong chemical interactions to remove microbial growth (and often the top layers of the metal).

Fortunately, new products employ surfactant technology (as opposed to brute chemical strength) to clean coils. Not only are these products better for the coils, they are also more environmentally friendly.

Other cleaning methods include a pressure pump sprayer with hot water and an environmentally friendly detergent applied to all sides of the coil.

Cleaning surfaces within the hvac system is only the beginning, however. Controlling growth once the surface is clean is equally important. A variety of antimicrobial products continue to control the growth of microbes for three to six months after application. These can easily be incorporated into regular maintenance.

It also is important to be aware of worker safety. EPA registration is the assurance to the contractor that the product is safe for the intended use. Read product labels carefully, to ensure that the product is registered; and follow the directions.

Design issues

Considerations for cleaning actually begin with the application and specifications of the coils.

Consider the number of rows deep, fin corrugation, fin style, fin spacing, fin material, and thickness. All of these factors will influence the ease of cleaning (which usually has a large bearing on whether or not it is actually done).

The depth of the coils makes a difference as to how easy it is to clean them. The depth is often determined by the arrangement of the tubes (staggered or inline) within the coil. In general, a six-row inline tube pattern coil is the same depth as an eight-row staggered tube pattern coil. For ease of cleaning, the tubes in the coil should not exceed four rows.

This means that in eight-row applications, the tubes should be split into two four-row sections, with space to clean each coil from both sides. The coils could be piped in series or parallel; selections should be made accordingly.

There are several types and configurations of coil fins to select. All of them have differing characteristics that influence the coil’s efficiency and cleanability. These fin styles include plate, spiral, and lanced.

Plate fins feature alternating ridges and grooves called corrugation. Highly corrugated fins provide maximum performance and air pressure drop (APD). Mildly corrugated fins require two more fins/in. to achieve the performance of highly corrugated fins, but the APD is 30% to 35% lower even with the extra two fins.

Flat fins have no corrugation on the face, but the edges may have corrugation to add strength. They require two additional fins/in. to achieve mildly corrugated performance, but their APD will be 30% to 40% lower than mildly corrugated fins.

In addition, the lack of corrugation on flat fins reduces fin strength so, depending on the method of cleaning, a heavier-than-normal fin material would be recommended to avoid damage. While this improves cleanability, it also increases the first cost.

Spiral fins are usually tapered and held in place by notches around the inner diameter of the fin. These notches may catch lint and dirt, and are extremely difficult to clean. This fin is best described as a stretched out “slinky.” Damage to the fin may cause it to snap, exposing bare tube to the airstream.

Lanced fins, the result of technological advances in tooling and manufacturing, have slots extruded off each side of the fins’ face. This exposes more secondary surfaces to the airstream, resulting in greater performance per row. However, the lanced fin is extremely thin. Efforts to clean it may result in damage, even to the extent of requiring a new coil.

The number of fins/in. will also make a difference in cleaning. Normally, wider fin spacings are desirable for easier coil cleaning. Eight fins/in. is usually requested to create the most easily serviced arrangement.

However, this means that selections requiring more fins/in. than specified will result in more rows and deeper fin depth. This makes the coil more difficult to clean and again drives up the first cost.

Condenser coils are probably the worst from this standpoint, because they usually have a high number of fins/in. (14-plus). In addition, they have no filters to protect the coils from dust and airborne particles, such as cottonwood seeds, which can clog the coils.

Regular schedule

As with most building maintenance programs, coil cleaning is more likely to be done if it is part of a regular schedule. While a maintenance schedule must be customized to the specific application to be effective, there are some ground rules to follow in establishing one.

Any cleaning-maintenance program should be based on hours of usage, not months. This ensures that hvac systems with heavier use get adequate maintenance attention. It’s a good idea to clean the coils and drain pans at the time the filters are changed. Moving clean air over dirty coils will not provide cleaner air for occupants.

Because all surfaces in an indoor environment can contribute to poor IAQ, it’s important to take your efforts beyond the air filters and cooling coils. For example, applying an antimicrobial treatment to the interior housing when the coils and filters are treated will further curb microbial growth.

IAQ continues to be a major issue that building managers and hvac contractors must address. Among the areas that will certainly come under additional scrutiny are coil design and maintenance.

As coil cleaning becomes more of an issue, it may well increase the cost of future applications. However, designing for it now will keep cleaning costs down, equipment efficiency up, and occupants’ health and productivity optimal.