ENERGY SAVING
Ionisation cleans coils
reduction in a cooing coils thermodynamic performance can occur when its surface accumulates anything that insulates heat transfer, such as bacteria, mold, and biofilm. This can result in system controls, and often building engineers, compensating for the loss in HVAC system capacity by speeding up fans, lowering chilled water temperatures and increasing chilled water flow, all at the expense of reduced system capacity and increased energy use. This may be a leading contributor of what is known as 'low delta-T syndrome'.
A
When cooling coils can be restored to their design operating performance, or maintained clean from the start, some of these energy inefficiencies may be reversed or avoided.
Methods for consideration A recognised method to remove accumulated bio-growth from cooling coils is with steam or chemical cleaning treatment, a physical process that may be both interruptive (system downtime) and labor-intensive. While effective if done correctly, it must be repeated on a regular basis – typically annually – as bio-contaminants can proliferate again shortly after treatment. Beyond physical methods, proven technologies may be employed that can work around the clock to help effectively keep coil surfaces continuously clean, without disruption. The two technologies being compared here
are ultraviolet disinfection (UV-C) lamps and needlepoint bipolar ionisation (NPBI), both having a long history of satisfactory results. While either of these technologies may offer a
30 February 2022 •
www.acr-news.com
and saves energy
David Schurk, director of healthcare and applied engineering markets for Global Plasma Solutions contrasts ultraviolet disinfection lamps and needlepoint bipolar ionisation.
number of additional benefits for most HVAC systems, it is important to note that neither, on its own, saves energy. Rather it is as a result of their ability to restore and maintain cooling coil performance, returning coil pressure drop and heat transfer efficiency to original design conditions, that it has the potential for savings of between 10 and 25%.
For UV-C lamps, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends irradiance levels of 50 µW/cm2
to 100 µW/cm2 . A simpler
way to achieve a desired dosage is to convert 100 µW/cm2
to a more easily understood and specifiable 7.5 lamp watts (as printed on the lamp surface) per 0.1 m2 (ft2).
(ft2 ) of coil surface area
Lamps are typically installed inside an air handling unit, on the air-leaving (downstream) side of the cooling coil, approximately 300 mm (12”) horizontally off its surface and spaced every 750 to 1000 mm (30” to 40”) of vertical coil height. There are many site-specific conditions that may negatively impact the effectiveness of UV-C if not accounted for in design. Sizing will typically require derating lamps based on actual discharge air temperature, humidity, velocity, and wind chill, as well as the distance to or from the lights, lamp orientation, lamp encapsulation, run-time depreciation, ballast selection, and duct reflectance. Each of these has a potential capacity reduction effect on UVC lamp output, which must be aggregated to determine proper UVC fluence requirements in order to ensure optimum results are achieved.
Manufactures of NPBI recommend the placement of the ionisation device at the top of the cooling coil, with the carbon fiber needlepoints directed downward in order to 'blanket' the air-entering (upstream) side of the coil with ions, from top to bottom. Simply applied, bars are spaced no more than 1500 mm (60”) vertically, with one located above the other should multiple bars be required considering coil height. Ionization is drawn-through the coil by system airflow, treating internal surfaces (from back to front). The inherent benefit of positioning NPBI on the air-entering (upstream) side of the cooling coil is that ionization is conveyed to the entirety of its internal surface area, keeping it free of microbial growth from back-to-front. UV-C lamps are typically mounted on the air-leaving (discharge) side of the coil where arguably most of the bio-contamination may accumulate, but depending on how deep the coil is (number of rows) and how tightly the fins are spaced (fins per-inch) the UV-C light may only be able to reflect and refract off internal surfaces a short distance, falling short of its entire depth and only partially cleaning the coil. Only internal coil surfaces that are exposed directly to UV-C light will be sanitized, those that are unexposed will not.
Energy consumption and life-cycle costs The installed first cost of either technology may be viewed as very affordable when considering the advantages that a cleaner cooling coil can provide to the building owner and its occupants. Beyond that of cost savings in coil maintenance
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