October 6, 2017 - HVAC
By Daniel Jones
Facilities install UV-C lamps to disinfect HVAC component such as cooling coils and drains. But in many of those facilities, the performance of the UV-C system may be inadvertently compromised because the UV-C lamps are not replaced when the lamps “burn out.”
Only 20 to 30 percent of building engineers actually replace their lamps. There are two main reasons for this. One is that UV-C lamps are often installed out of sight in hard-to-reach areas, making maintenance easily forgettable. The other is that engineers see a blue hue coming from the lamp and assume that the lamp is functioning properly.
Unfortunately, in the case of UV-C, the lamp’s blue hue continues long after its effective germicidal output has decreased. This is not a lamp defect, but rather a function of the useful life of a lamp. The UV-C wavelength (253.7 nm) is invisible to the eye; the visible blue light that one sees represents only five percent of the total output of the lamps. (The blue color actually comes from the inert argon gas within the lamp.)
As a result, the blue light is not an indicator of lamp performance. After roughly 9,000 working hours, the lamp may continue to emit blue light even after UV-C output has decreased by 20 to 50 percent, thereby jeopardizing the AHU’s heat exchange efficiency and energy use and a building’s air quality. Meanwhile, as performance worsens, the building owner or facility manager is none the wiser.
The bottom line: Building engineers cannot assume that HVAC/R components are being properly disinfected based on a visual inspection because the blue light is not a determinant of UV-C output.
The issue of UV-C lamps not being replaced when they should be is one of growing significance. The global growth rate for microbial ultraviolet (UV) disinfection equipment has been increasing annually as more facility managers and building engineers recognize its energy savings and decontamination value. By 2020, the total UV market is estimated to be worth nearly $3 billion.
When it comes to indoor air applications, the premise of ultraviolet-C (UV-C) technology is simple. It uses light energy in the UV-C spectrum to kill, or prevent the growth of, virtually all known microorganisms living in upper air streams or HVAC/R components such as cooling coils and drain pans, thereby boosting energy efficiency and indoor environmental quality (IEQ) and comfort.
By disinfecting HVAC/R components or air streams, UV-C maximizes system efficiencies, reduces the need for manual maintenance, and improves indoor environmental quality. In fact, researchers have found that exposing a fouled cooling coil to UV-C resulted in a 10 percent decrease in pressure drop and a 14.55 percent increase in heat transfer coefficient at reference conditions.
These long-term benefits more than pay for the UV-C, which has an average installed cost of $0.15 per CFM, with many users reporting that their cost for an installed UV-C system featuring high output lamps was even less.
By destroying microorganisms that accumulate on HVAC/R components, retrofitted UV-C lamps can help return A/C units to initial capacity and reduce or eliminate costly coil and plenum cleanings in as little as 90 days, while new system installations maintain “as-built” conditions. The UV-C energy reaches deep into the coil to eliminate build-up that cannot be reached by either mechanical (pressure washing) or chemical treatment, and the overall improved heat transfer efficiency and reduced coil pressure drop can slash energy use by 10 to 25 percent on average.
That energy savings makes UV-C cost-effective. Using a 10,000 cfm system as the example, the UV-C fixtures would cost $1,500, with an annual operating cost of $188 at $0.10/kW — operating 24/7/365. That is less than one percent of the average 18 percent power savings gained through a more efficient (better heat transfer and lower pressure drop) air conditioning system.
Furthermore, field reports indicate that the initial cost of a UV-C system is about the same as one properly performed coil-cleaning procedure, and less when system shutdowns, off-hours work with the associated overtime, or contractor labor costs are considered. Indeed, without UV-C, cleanings are recommended at least once per year to prevent mold growth and capacity loss and to keep contaminants from compacting deep within the coil. A one-time investment in a UV-C system will keep cooling coils at as-built conditions.
Of course, proper performance depends on right-sizing the lamps for the particular application. To do so, engineers must take such factors into account as temperature, reflectance, and the minimum amount of energy at the coil’s farthest points. This will ensure that UV-C lamps provide the required level of irradiance.
In some applications, especially in the healthcare industry, UV-C lamps are installed via wall-mounted units that irradiate the upper air region of a given space, killing viruses, bacteria, and other disease-causing microorganisms. A 2013 CDC-funded study conducted in two hospitals found that UV-C reduced the total number of colony-forming units of any pathogen in a room by 91 percent. ASHRAE, too, has recognized that the UV-C wavelength inactivates virtually all microorganisms living on HVAC/R surfaces with a kill ratio of 90 percent or higher, depending on UV-C intensity and length of exposure.
Regular replacement of UV-C lamps is therefore necessary. When UV-C lamps are not replaced, their output falls below effective operating levels.
Historically, replacing UV-C lamps had a significant impact on a facility’s budget, but this has changed in recent years. As the use of UV-C fixtures in HVAC/R systems has increased, lamp costs have been significantly reduced. Typically, high output lamps can be replaced at less than $0.03 per CFM.
All UV-C lamps will lose output over time, regardless of their make or model. But how often should they be replaced?
Most lamp manufacturers recommend replacing UV lamps every 9,000 hours; since there are 8,760 hours in a year, most facility managers employ an annual re-lamping schedule. Quality lamps will still emit at least 80 percent of their original UV-C germicidal output rating at the end of one year, but it will only be a matter of time before they lose effectiveness.
Lamp monitoring can take different forms. A portable radiometer can measure the level of UV-C output in the 253.7 nm wavelength. However, it raises safety concerns, as the measurement process requires someone to enter the plenum where the UV-C is located, which risks exposure to the harmful effects of the germicidal light. Stationary radiometers have also been used; they are mounted on the outside of the plenum, with their UV sensors being mounted inside the plenum, and offer a relative indication of UV-C output. (Some even provide an hour meter.) In “remote” systems that are out of sight — mechanical rooms, interstitial spaces, etc. — there are accessories available for continuous monitoring of lamp-ballast functionality that communicate with building management systems. Lamp-on hour meters can provide BMS feedback as well.
It’s a mistake to replace individual UV-C lamps only as they stop functioning. This method is highly labor intensive and inefficient, requiring routine inspections of the individual lamps to determine the need for replacement. Furthermore, as that 9,000-hour mark passes, the lamps’ germicidal effectiveness begins to diminish, potentially causing HVAC/R system performance to be compromised.
A better solution is to establish an annual replacement schedule whereby all lamps are methodically swapped out at a designated interval. Such a strategy eliminates the need to carry a large inventory of replacement lamps (with the exception of a few spares in case of breakage). Instead, it offers plenty of lead time and a more logical routine or schedule.
Annual replacement is also more cost-effective. Individual replacement means higher lamp and labor costs and downtime during each procedure — not to mention frequent monitoring by facility staff. An annual schedule consolidates change-outs into one service interval, allowing the facility to purchase lamps in larger quantities to save money and time. This also reduces the need, with the exception of critical environments, to invest in an expensive radiometer or other equipment to monitor individual UV-C output. Knowledge that lamp replacement will simply be “taken care of” each year will allow peace of mind for facility managers.
Most important of all, annual replacement guarantees that UV-C output will never fall below effective levels and HVAC/R efficiency and IEQ will not be compromised. By implementing such a schedule, you are doing your duty to protect both your building occupants (and your pocketbook), as well as your equipment from the efficiency-robbing effects of microbial buildup.
Daniel Jones is the president and co-founder of UV Resources, a provider of UV solutions and replacement lamps for HVAC systems. He is a corresponding member of the ASHRAE Technical Committee 2.9 and ASHRAE SPC-185.2, devoted to Ultraviolet Air and Surface Treatment. He may be reached at dan.jones@uvresources.com.
