Getting Technical
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• Organic matter In HVAC water circuits , organic matter is usually the water treatment chemical package used . Therefore , with UV doing microorganism control no organic biocides need to be added to the water .
• Suspended solids Suspended solids affect UV transmission by blocking , scattering and absorption . Therefore , water flowing through UV units may have to be filtered to reduce solids concentration to below 20 mg / litre .
• Turbidity Turbidity is due to suspension of finely divided matter which is not removed by normal filtration and is seen as lack of clarity or brilliance . Turbidity should not be confused with colour since water may have a dark colour but still be clear and not turbid .
These are the five main causes of reduction of UV transmissibility in water . The actual transmissibility of water is measured by a UV intensity meter as a percentage of the UV intensity emitted by a standard 254 nm UV lamp after passing through a 10 mm quartz cuvette holding a sample of the water as shown in Figure 2 .
Figure 2 : Measurement of UV transmissibility of water
Pathogenic bacteria are generally susceptible to relatively low UV dosing levels . For example , the lethal dosage for one of the most commonly occurring organisms , e-coli , is 19 mJ / cm 2 . Other familiar pathogens such as the bacteria which causes cholera , vibrio cholera , and the bacteria pneumophilia , the cause of Legionnaires ' disease , require similarly small lethal UV doses of 21 mJ / cm 2 and 15 mJ / cm 2 respectively .
The types of bacteria and other micro-organisms which can coagulate and form biofilms , however , can only be killed by far more powerful UV emitters which are available but usually more costly both to install and operate than established biocide dosing programs .
UV units have regained their former popularity for sanitising water but only if the water is sufficiently clean and clear as described above . In new and upgraded HVAC installations UV units are being increasingly seen in both cold and hot potable quality water flows and systems as well as make-up to adiabatic and heated water humidifiers .
Closed cooling and heating water circuits are still treated with initial doses of bio-dispersants and subsequently re-dosed at six- or 12- month periods which is more economical than installing UV units . RACA
“ UV is capable of killing all microorganisms and for air conditioning considerations this includes legionella bacteria .”
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RACA Journal I August 2021 www . hvacronline . co . za