Technical
Disadvantages
• Installation and maintenance costs
• Potential occupational exposure from improper installation
• Requires a minimum room height of 2.4 m ( 8 ft )
• Possible degradation to exposed paint , plants , surfaces , and filters
• Requires expertise to design , commission , install , operate , and maintain .
IN-AHU UVGI ( COIL , FILTER AND DRAIN IRRADIATION ) The principal design objectives for installing UV-C lighting systems in air handling unit ’ s ( AHUs ) are to prevent the growth of bacteria and mould on system components like coils , filters and drain pans .
A large dose of UV-C light can be delivered with a low UV-C irradiance because of the constant exposure time to the mentioned AHU components . Coil surface irradiance levels in the order of 1 μW / cm ² are effective , though 50 to 100 μW / cm ² is more typical xx .
Cooling coil treatment systems have the two-fold objectives of maintaining coil performance and minimising energy use by reducing air-side flow resistance and increasing the overall heat transfer coefficient relative to a conventionally maintained , mechanically and chemically cleaned coil .
Figure 3b : Section view of typical HVAC surface treatment in an
AHU xxi
irradiate cooling coils and condensate pans , increasing overall benefits of the system .
At an air velocity of 500 fpm ( 2.54 m / s ) a typical irradiance zone 7.8ft ( 2.4m ) in length achieves a 1 second exposure . Generally , in-duct or in AHU systems should be installed in a location that can provide a minimum of 0.25 second of UV exposure ; otherwise , system cost and power consumption will be excessive xxiii . Proposed ASHRAE Standard 185.1 provides a testing method for UV-C lights in-AHU and in-duct applications to inactivate airborne microorganisms xxiv .
Advantages
• Suitable for all climates
• Enclosed system ( exposure to humans reduced )
• Treats re-circulated air from a centralised location
• Prevents mould and biofilm on cooling coils
Disadvantages
• Potential occupational exposure when safety procedures are not followed
• Expensive to install
• Requires UV-save filters
• Possible material degradation ( example filters )
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Figure 2b : UV-C lamps inside an AHU
A field study in a hot environment experienced a 22 % reduction in pressure drop and 15 % increase in air-side heat transfer coefficient after less than two months of surface treatment system operation xxii .
In most cases , the lowest maximum velocity in a system occurs inside an AHU , increasing the effectiveness of in-AHU UV-C lights due to increased exposure time . In-AHU UV-C additionally treat air from many spaces and simultaneously
RACA Journal I November 2021
IN-DUCT AND IN-AHU UVGI ( AIRSTREAM DISINFECTION ) The principal design objective for UV-C in duct or AHU is to distribute UV energy uniformly in all directions throughout the length of the duct or AHU , and to deliver the appropriate UV-C dose to air moving through the irradiated zone with minimum system power .
It should be noted that it is not very effective to prevent transmission of an airborne virus from an infected person to a non-infected person in the same room . www . hvacronline . co . za