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Proper Design of a HVAC Bacterial Control System
Using UV Technology
By James A. Hart, P.E.There is much discussion today about the quality of home air. It seems everyone now is touting the latest UV technology to provide clean air throughout a household. UV technology is nothing new, it has been around for years, and properly applied, does work. Lately, there has been a number of companies making claims that their particular device will purify the air for marginal cost. The literature provided by these companies show purified air circulating through the house, providing air "…as clean as the great outdoors", and killing germs throughout the house. While these claims sound good to the uneducated person, in fact, they are bogus. UV Technology is very simple: take a known quantity of UV light, and expose a surface or substance to the light for a specified period of time, and a percentage of the bacteria are killed. The product of the UV light intensity, multiplied by the time at that intensity, is termed the "CT Value". Tables of CT Values have been published, and are well known. To properly determine what values to use, look in one of these tables and pick a microbe, then read the corresponding CT Value, and design the system around that CT Value. Sounds simple, doesn’t it. Well…….yes and no. A proper understanding of the CT Value is essential in designing a HVAC cleaning system. High UV light concentrations will result in short exposure times; low UV light concentrations result in long exposure times. The proper balance of these two variables is what is at the heart of good design. High UV light concentrations are accomplished simply: provide more lamps per volume of space. The downside of this is the cost. The other way to meet the CT value is provide a long "residence time". Residence time is the time a particular volume of air is in the presence of UV light; the "T" in CT. Residence time is almost always a fixed value in most home HVAC systems. Typical HVAC home systems operate around 2000 ft3/min for a 2,000 sq. ft. home. Typical ducting in such a home is on the order of 6" x 8" or 8" x 10". The plenum attached to the air handling unit is typically on the order of 2 ft by 2ft in cross section. With these values, it is easy to determine the velocity leaving the unit. For our example, 2,000 ft3/min will have a exit velocity of 500 ft/min, or 8.33 ft/sec. From this value, the proper residence time can be evaluated for specific CT values. Selection of the proper HVAC treatment will depend next on the type of mold. Typical molds found in households are in the Aspergillius family. Aspergillius has CT values for 90% kill ranging from 44,000 to 132,000 m W-sec/cm2. Bacillus subtilis spores, a common bacteria, has a CT value if 11,600 m W-sec/cm2. For our example, we will use the Bacillus number, because as we will see, that relatively low number will mean a relatively high residence time. CT values higher than Bacillus will require an even longer residence time. Designing the Proper HVAC System  For our example, we will use the HVAC system previously described, a 2,000 ft3/min unit, with a 24" x 24" outlet plenum. These parameters will provide a velocity of 8.33 ft/sec. We will use low-pressure UV lamps, which typically provide about 800 m W-sec/cm2 at a 12 inch radial distance, a lamp typically sold by HVAC contractors. The lamp placed at the outlet of the coils. However, at this flow rate, you will only have a residence time of 0.24 seconds. That is only 1.7% of the required 14.5 seconds for 90% kill. This is not even close to the required CT Value to kill Bacillus. In fact, this value is so small, the UV will have no effect on any airborne pathogen.
To properly design a system to provide a 90% kill, your system will need to be designed accordingly. For a 12 inch UV radius, a duct no more than 12 inches high will be used. Standard 30 inch lamps will be placed every two feet (an estimate for this example only) for the desired residence time of 14.5 seconds. The duct cross sectional dimensions are 12" x 30", for a cross sectional area of 5 ft2. To maintain 14.5 seconds residence time, a length of 95 feet will be required. This is beyond the normal HVAC unit in most houses. This simple example shows the absurdity of any claims that a single UV bulb can clean, much less sanitize, the typical household air system.
 Proper Duct Length for 90% Kill What to Do The only way to effectively remove, and prevent from returning, household pathogens is using broad spectrum, high efficiency UV bulb (HE/UVTM) to work with a hydrated
five-metal target. The target acts as a catalyst for a hydroxyl radical reaction of the broad spectrum 100-300 nm HE/UVTM energy with water vapor on the hydrated target. The results are hydro peroxides, super oxide ions, passive negative ions hydroxides and ozonide ions. The cell creates a very aggressive advanced oxidation atmosphere that has the ability to reduce airborne bacteria, mold and viruses up to 99% also odors and VOC's by up to 97%. Air Oasis has designed such a unit to safely install into existing duct systems based on their patented AHPCO Cell. The units use the air provided by the HVAC system to produce and maintain friendly oxidizers, which diffuse through the house, providing safe levels of bacteria and mold killing ozidizers. Air Oasis’ patented AHPCO Cell is unique because it’s not a filter or an ozone generator. It is a cell that radiates friendly oxidizers. These oxidizers travel through a room or home by Brownian Motion (natural air movement). One of the five oxidizers is hydro peroxides. In layman's terms, treating a room with hydro peroxides is like misting a room with a weak hydrogen peroxide (H2O2) mist. Each time a hydrogen peroxide particle finds an airborne organic contaminant it will oxidize or neutralize the contaminant, and in the process kill itself. The hydrogen peroxide particle (H2O2) will revert to water vapor (H2O). This will permit the next H2O2 particle to move a little further into the room until the entire area is purified. This is the reason one small AHPCO cell can work on large areas up to a 3,000 sq. ft. house. The factor is time. The more pollutants or contaminants, the longer it may take to reach a 90%+ reduction level.
Air Oasis
UV Induct Models
 For Whole House
and Whole Building Air Purification
The Air Oasis Induct models were were created to reduce mold, bacteria, viruses, yeast, VOCs and odors
in a whole house or building. While most competitors use simple germicidal UV lamps or high ozone producing UV lamps, the
Air Oasis Induct utilizes seven advanced technologies in one. UV germicidal lamps have a proven track record against mold, bacteria, yeast and viruses, but do not help to reduce VOCs or odors. The problem
with ozone is that it can cause health side effects when used in levels capable of providing the desired results
and, over time, breaks down rubber and plastic used in HVAC systems. Therefore, the
Air Oasis Induct units combine the use of a germicidal UV lamp, ozone and
their patented quad-nano Xtreme™ catalyst to effectively provide the desired results while converting ozone to friendly redundant oxidizers. This combination will provide all the desired benefits without the potentially harmful side effects. There is no catalyst on the market today that can compete with the quad-nano Xtreme™ coating. This coating is the first to combine the benefits of utilizing five catalyst metals along with nano technology.
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