The technology is already widely used within process users of air, certain manufacturers and by those with an interest in a high quality indoor air supplies, such as call centres, banks and other high employee density locations. The technology is now being applied to reduce the incidence of nosocomial infection at healthcare providers worldwide.
The technology
Ultraviolet light is generated by the excitement of electrons in orbit around a mercury nucleus. The mercury is contained within a quartz tube, and varying voltages are applied to electrodes situated at either end of the quartz body. Lamp types are separated by gas pressure into two principal types: low pressure and medium pressure.
Low pressure uv lamps are generally one metre in length and have an input power of up to 200 W. The lamps are up to 30% efficient, so can emit up to 60 W of germicidal light per lamp. These type of lamps have a life of about 10 000 h and should be left switched on, as frequent cycling (switching on/off) will damage the filament and shorten the lamp life.
This type of lamp is best suited to upper air space disinfection, tank headspace disinfection, or for the disinfection of poor transmittance fluids. Low pressure lamps have a single line output at 254 nm, and are often called monochromatic lamps. High output lamps are now being used that have an amalgam of mercury and bismuth or indium. These lamps do not show variation in output caused by air temperature and have been measured to have an efficiency of 32-35%.
Medium pressure lamps are typically 300 mm in length and have an input power of up to 3500 W. This type of lamp is hot running, with a surface temperature of 600-900°C. The lamp needs to have a nominal flow of air across it to keep it cool. Lamp life is between 6000-8000 h and the lamps have an efficiency of up to 23%.
Typically a 3·5 kW lamp will output up to 800 W of germicidal uv light, so typically one medium pressure lamp has an equivalent output of up to 10-12 low pressure lamps. Medium pressure lamps are used to treat high flows of dynamic air contained with an air duct, or high fluid volumes. Medium pressure lamps have a broad spectral output, ranging from 170 nm up to 450 nm.
Kill mechanism
Uv light between 240 nm and 280 nm is germicidal. The principal kill mechanism is absorption of uv at 265 nm by the DNA, contained in the nucleus of bacteria yeast and moulds. RNA is found within the nucleus of a virus and the mechanism is similar. UV light centred at 265 nm will dimerize the thymine base and effectively prevents replication.
The damage is permanent and irreversible. The action of this part of the spectrum has been documented by Meuleman, who compared the effect of low pressure lamps (with a single output at 254 nm) with the polychromatic medium pressure lamps.
UV light centred on 240 nm causes a lethal insult to the outer cell membrane of airborne species. As the wavelength decreases, the frequency of the radiation increases, and so lower wavelength uv is more energetic than long wave uv.
UV centred on 240 nm behaves in a similar fashion to other oxidants, like chlorine or ozone. The damage to the outer cell membrane is permanent and occurs instantaneously.
The organism is more prone to mechanical damage and in water is unable to regulate osmotic pressure, and quickly becomes soluble. UV light centred on 280 nm causes damage to protein in the cytoplasm.
The damage to the outer cell membrane is permanent and occurs instantaneously.
Target organisms
The amount of uv required to achieve 90% kill is known as a D10 dose. Double the D10 dose achieves a 99% kill, and three times the D10 dose achieves a 99·9% kill. It is important to note that yeasts and mould species are proportionately harder to kill, and systems do need to be sized to accommodate the highest D10 species.
The D10 rating of the organism is also a key determinant of kill, and the D10 valves do vary considerably across species. Methycillin Resistant Staphylococcus Aureus and tuberculosis are relatively easy to achieve high log kill, however A Niger and other mould species are far more difficult to de-activate and do require a higher uv dose. The influenza virus has a low D10 rating and uv is highly effective at destroying it.
System sizing
The principal factors which affect system sizing are:
- air flow rate
- relative humidity of air
- D10 of target species
- duct dimensions and materials of construction
The relative humidity (rh) of the air has a pronounced effect on its ability to transmit uv light, and as shown in the graph, high rh leads to a marked reduction in the transmittance of the air.
Duct construction
The duct dimension and material of construction play important roles. The ideal profile for duct is rectangular with the uv lamp inserted horizontally.
The material of construction of the duct is also important and the correct choice is vital. Most plastics or pvc ducts will fail when exposed to germicidal uv light, as uv at 240-280 nm is absorbed by the pvc and the resultant damage is known as 'chalking', which is irreversible and leads to catastrophic failure. Certain metals have a reasonable reflectance of germicidal uv light and duct work should be fabricated from these materials.
It is important that operators are shielded from uv light as short term exposure can cause erythema (reddening of the skin) and conjunctivitis (inflammation of the mucous membranes around the eye). Electrical interlocks are fitted to the control panel and mechanical key retentive interlocks are fitted to the duct. Maintenance is limited to a lamp change every 6000-8000 h, a sleeve change every couple of years. In addition the sleeve should be wiped periodically with a soft clean cloth to remove any dust or other airborne deposits.
Pressure variations
Medium pressure uv is now widely accepted by a large number of process users of air in the UK and overseas. Dynamic air environments generally adopt a single medium pressure lamp due to economic considerations.
Examples of use include a call centre operator in Surrey which has observed a 40% reduction in absence due to sickness following installation of a uv system in the recirculating air duct at one of their offices. Also mushroom growers the world over have benefited from yield improvements of up to 60% following installation of 2·5 kW systems in the chilled air ducts. Low pressure technology is generally unsuitable due to the low operating temperature, and resultant output reduction
Low pressure uv equipment and low pressure systems are used in most bulk sugar syrup storage tanks to control airborne yeasts and mould that would effect the integrity of the syrup and is inserted into the headspace. Usually medium pressure uv is used to treat the syrup in line.
Source
Building Sustainable Design
Postscript
J C McClean BSc MBA is managing director at Hanovia. This paper was given at the BSRIA Ventilation Hygiene Conference in May. Copies of the proceedings are available for £75 by phoning BSRIA 01344 426 511.