UV-C TECHNOLOGY ELIMINATES COIL CLEANING, IMPROVES HVAC PERFORMANCE

by Thomas J. Kelly

Product Manager, Indoor Air Quality Products Carrier Corporation, Syracuse, N.Y.

Nearly every service technician has had the experience of starting an air handler and being greeted by an odor one could only describe as a "locker room." This experience, also known as the "dirty socks syndrome," is first-hand evidence of what happens when mold grows on the coil and in the drain pan of an air-conditioning unit. Every service technician has also had the experience of opening a unit to find the drain pan and coil covered with a slimy residue.

These conditions can be as unhealthy for building occupants as they are unpleasant. While the smell associated with mold growth is a bad situation, mold growth on coils also has a detrimental effect on system performance. This degradation of performance ultimately leads to higher energy costs and poor cooling performance.

Numerous coil-cleaning methods have been used to control this problem. Some of those techniques involve the use of detergents or even solvents, which can pose safety issues - health and flammability, for example - and diminish the life of the coil, because sometimes acids are involved. Often coil cleaning isn't done with regularity and even when it is done on schedule, the mold growth can return in a very short time.

The application of C-band ultraviolet light (UV-C) technology in air-handling systems now allows for a proactive method of keeping the coil clean and operating in "as new" performance all the time. UV-C lights can be added to air handlers and other pieces of equipment through a relatively simple retrofit kit (Fig. 1), and manufacturers like Carrier also offer them as a factory-installed option in HVAC equipment.

Interestingly, while UV-C light has been promoted for its positive impact on indoor air quality (IAQ), the "bottom line" impact - its contribution to system efficiency and lower maintenance costs - might ultimately be considered to be its greatest asset.

What Causes the Build-Up?

Four conditions will result in mold and fungus growth:

1. A source of mold spores. Sufficient mold spores are found in nearly every environment and brought into the building through door openings and outdoor air supplies.

2. Organic material on which the mold can grow. Dust and particles of organic material are also readily available in every system, even with the best filtration systems.

3. The right temperature range. Temperatures from 50° F to more than 100° F provide the right incubation range.

4. Moisture, which is in more than adequate supply on cooling coils and drain pans of all air conditioning units.

Even when filtration is provided, a large part of the build-up on the cooling coil is the result of biological growth.

The odors associated with this problem stem from the natural growth and decay of mold. As the molds break down they decay into toxins and volatile organic compounds (VOCs), which cause the characteristic odor. In addition, some people have allergic reactions to these toxins and VOCs. These problems certainly contribute to poor building IAQ, but mold growth can impact energy use through lost performance as well.

Effects of Bio-film Growth on a Cooling Coil

Bio-film growth can have a tremendous impact on coil velocity, putting it considerably off design specification. A good rule of thumb for evaporator coil size is about one sq ft. of face area per ton and 400 cfm per ton. If we consider that a 10-ton unit coil has 10 sq ft. of surface and 4,000 cfm (400cfm/ton x 10 ton), our gross face velocity is 400 ft per minute. But this is not the actual velocity through the coil. The fin and tube material takes up a very large portion of the coil.

Let's consider that our coil is a typical 14 fin/in. three-row coil with 3/8-in. tubes. The material in the face of the coil is considerable. With a 0.0055 in. fin thickness, the fin material alone reduces the free area by 8 percent (14 fins/inch x 12 inches/ft x 0.0055 fin thickness divided by the 144 inches/sq ft.). If we also consider that there are 12 tubes in a one-ft. high section, we reduce the free area by 37 percent more. The actual free area is now only 5.78 sq ft. and the resultant net face velocity is 692 ft/min.

How does a bio-film build-up affect the performance? If the bio-film thickness is .002 in. on the fin and tube surfaces, this will reduce the free area to 5.38 sq ft. and a velocity of 744 ft/min. What will an increase of 50 fpm mean to static? From a typical unit coil performance this can result in an increase of static of 16.5 percent.

However, unless the supply fan is on a variable frequency drive (VFD), the result is not an increase in motor horsepower, but instead results in a decrease in supply air. In our case, the supply airflow decreased by 9 percent to 3,650 cfm to regain system balance. Performance of a cooling coil is a function of the amount of coil surface, the amount of time the air is in contact with the surface and the heat transfer coefficient between the fin, tube and the air.

From a performance side, what does it mean to decrease the airflow by 9 percent? If we look at performance for a 10-ton coil we lose about 3 percent in total capacity. This analysis looks only at the affects of increased velocity. If we consider the effects on the heat transfer coefficient, the performance loss will be even more dramatic.

Using UV Light to Clean Coils

Coil cleaning can bring performance back to the original operating conditions.

Typical coil cleaning methods include chemical treatments and steam cleaning.

However, recent evidence suggests that both methods can be ineffective. Chemical cleaning may only remove surface growth while leaving material still embedded in the center of the fin pack. Some reports indicate steam cleaning can actually force the surface growth deeper in the fin pack compressing the growth material so tightly that the only solution may be a new coil. Both methods can also be detrimental to some of today's enhanced coil surfaces.

Coil cleaning is certainly necessary, but cannot be done economically with the frequency and level that will keep the coil operating at design conditions on a daily basis. In essence, with UV-C lights, coil cleaning becomes a continuous, automatic and labor-free alternative. The UV-C light works by attacking the DNA of the mold and rendering it sterile so that it can not reproduce.

Contrasting physical cleaning methods to the use UV-C light is analogous to the difference between treating the symptoms and curing the disease.

UV-C technology is not new, as it has proven itself for years as a way to provide sterilization in medical and food processing applications. What is new, is the development of high-energy output UV-C lights that can achieve very high mold kill rates under the cold temperature and rapid air velocity conditions of an HVAC system.

The effectiveness of the UV-C light is a function of the light intensity, the reflectivity of the space and the distance from the light. In general, if the mold is exposed to the light for a brief period of time, it will be destroyed. Aluminum coil fins are a good reflective surface and, as a result, the UV-C energy is capable of penetrating three- and four-row coils with excellent results.

Given continuous exposure, UV-C lights can clean up a coil already contaminated by mold growth and keep the coil cleaner than other methods.

The Proof is in the Performance

While UV-C light is an emerging technology for the HVAC business, it is already starting to prove its effectiveness. For example, an air handler in an office building in Los Angeles exhibited the classic problems of bioaerosol film growth. The air handler shown in Fig. 2 demonstrates the degree of the problem. Mold growth is evident both on the coil and in the drain pan.

The unit was equipped with a bank of UV-C lights, as shown in Fig. 3, and its operation was monitored for 30 days. The results are shown in Fig. 4. Within 30 days, the unit was operating at near design conditions. The coil pressure drop decreased by 28 percent, resulting in 8.6 percent improvement in airflow. The coil dry bulb depression had increased by nearly 4° F, providing more than five tons of sensible cooling.

Wet bulb depression also increased by 1.8° F, enabling a total capacity increase of seven tons. Continued use of the light bank kept the air handler operating at the design conditions.

Other buildings have experienced similar results. A southwestern utility company applied UV-C lights to the air handlers in its headquarters office building. The city went through one of its hottest summers ever. In previous years the standby chiller was required just to keep up with the load. However, with the now-clean coils, the extra chiller was never required during the extremely hot summer.

In most office buildings, the demand-charge savings alone from this improvement would result in significant savings. Reduced maintenance costs and improved IAQ are a welcome bonus.

How the UV-C Light is Applied

The ideal location for the UV-C light is on the discharge side of the cooling coil and mounted so as to expose both the coil surface and drain pan to as much light as possible. In general, the light should be positioned about a foot from the coil surface. As a rule of thumb, the number of lights can normally be based on one light for each five sq ft. of coil.

The light used for UV-C is not dangerous, but some common sense installation safety precautions should be followed when the light is installed. As the UV-C light is similar to the UV light used for tanning beds, but it's wave band is of a level such that it does not pose an exposure risk to humans. Selecting the right light, however, is important. In an HVAC environment, a traditional UV-C light may lose much of its effectiveness because the light is located in a cold area with air movement over the coil.

Care must be taken to assure that materials exposed to the UV light - particularly wiring and plastic materials - are UV stabilized so they will not degrade over time. While the concept is simple, reliable performance depends on applying lights designed for this application and applying them in units where the safety and compatibility issues have been evaluated.

Typical operation for the UV-C light is to run the light whenever the fan is operating. Under normal operation, the light will provide the required output for about one year of service, at which time a simple change-out is required.

Payback

Technology can be a great thing, but to the user, the real question is: does this investment pay off? Assume that the first cost of installing two typical lights for our 10-ton example unit is $400. Also assume that it takes one hour to install them, so the installed cost is approximately $500. Contrast this with the cost of manually cleaning the coils. Based on coil cleaning costs from one service company, our 10-ton coil would cost $250 to clean. If we could actually get the coil cleaned three times in a year, which is probably a minimum, the annual coil cleaning expense would be $750. Since the bulb would be replaced on an annual basis at a cost of about $100, then our investment should repay itself in less than one year - just based on maintenance costs. If you consider the energy savings, the payback can be reduced to a fraction of that.