Getting Technical
THE EFFECTIVENESS OF UV
By Charles Nicolson
CHARLES NICOLSON
Charles Nicolson has a physics and chemistry degree from Natal University which he subsequently put to good use by applying speciality chemicals in mining and industrial processes where water is a major factor . This created an enduring interest in water technology , a passion that expanded to the HVAC industry in 1984 when he joined BHT Water Treatment . Since then , water technology in HVAC water circuits has continued to be an abiding interest .
The term ‘ ultraviolet ’ refers to radiations beyond violet radiation in the radiation wavelength spectrum , starting where the visible section ends , continuing to the beginning of X-rays at wavelengths of around 100 nm .
Supplied
Figure 1 : Section of the Electromagnetic Spectrum which includes UV radiation between 100 and 400 nm [ nanometres ].
Beyond ultraviolet , as wavelengths continue to diminish , radiation becomes X-rays , then gamma rays and finally cosmic rays . Figure 1 shows how UV between 400 and 200 nm
wavelengths is divided into three bands denoted UV-A , UV-B and UV-C . ( Note – ‘ nm ’ denotes nanometre – one thousand-millionth of a metre ).
The ability of ultraviolet radiation to control or kill micro and macro-organisms is contained mostly in the UV-C band ( with some overlapping into UV-B ) which from now on will be referred to simply as ‘ UV ’ or ‘ ultraviolet ’. UV is effective at sufficient dosing levels against all micro-organisms including bacteria , fungi , moulds , viruses , yeasts , and algae present in water , air and on surfaces .
All UV wavelengths between 200 nm and 320 nm are effective biocides and a specific wavelength of 265 nm has the greatest killing effect on the micro-organisms most commonly occurring in water circuits used in HVAC installations . In terms of frequencies rather than wavelengths , the frequency range of ultraviolet radiation goes from just below to just above 10 17 Hertz .
UV works as a fast-acting biocide by penetrating directly into the cells of organisms and destroying their DNA which terminates the ability of organisms to propagate . However , the fact that UV has to penetrate right into the cells of organisms to accomplish this means that there must be minimum dosages , measured by combinations of UV strength and exposure time .
UV radiation is generated by lamps containing small amounts of mercury . Wavelengths and overall emission strengths depend on gas pressures inside the lamps which are either low ( 0.001 to 0.01 bar ) or high ( 1.0 to 2.0 bar ) with corresponding differences in electrical power demands .
Water circuits in HVAC installations are either ‘ once through ’ such as used in humidifiers and hot water supply sections , or ‘ re-circulating ’ as chilled water and condenser cooling circuits . UV dosing is sized on water flow rates and therefore may have to be installed as side-stream configurations in re-circulating water circuits .
Earlier , it was stated that UV is capable of killing all microorganisms and for air conditioning considerations this includes legionella bacteria . Also , corrosive anaerobic bacteria such as Sulphate-reducing bacteria ( SRB ) is controlled indirectly by the UV exterminating all the other aerobic bacteria which use up any dissolved oxygen in circulating water thereby creating favourable conditions for anaerobic bacterial growth .
The intensity of UV is the amount of energy per unit area which needs to be present at the required maximum penetration distance through the water flow and is normally measured in mW / cm 2 . Assuming that the intensity is sufficient , the next consideration is whether the micro-organisms in the water flow are subjected to this UV intensity for sufficient time to receive lethal doses of the UV .
Doses are measured as : intensity x exposure time – so the units used are mJ / cm 2 . Intensity reductions as the UV travels through water flow depend on the quality of the water according to :
• Colour Discolouration of water is normally measured in Hazen units where less than 10 Hazen units is clear , and 100 Hazen units is a distinctive brown / yellow colour .
Iron ( and sometimes manganese ): Rapid corrosion in a water circuit can be produced by sufficient iron in corrosion products to reduce UV transmission .
www . hvacronline . co . za RACA Journal I August 2021 39