How to Choose a UV Lamp – Length, Wattage and Type 

In choosing a UV lamp for a given application, the first selection criterion is typically the lamp length due to the physical constraints of the system i.e., in-line or cross flow reactor geometry plays an important role in determining lamp length. Coupled with this first criterion of length, the lamp diameter is also important to consider especially when a lamp matrix or bundle arrangement is involved. If for example, the lamp is inserted within a quartz sleeve in a drinking water/wastewater treatment application and a pressure drop/head loss becomes critical then the overall diameter or cross sectional area of the reactor along with the diameter of the lamp and subsequently quartz sleeve need to be optimized accordingly. 

The lamp wattage or electrical power consumed by the lamp is another important criteria to determine in choosing a lamp. However, knowing the UV dose delivery and retention time requirements for the given application ahead of time will facilitate in determining the total power delivered by the reactor and subsequently the individual lamp power required. 

For example, if the dose delivery and retention time requirements are such that a high reactor power is required and the pressure drop/head losses are such that a smaller overall diameter or cross sectional area of the reactor is dictated then a larger diameter medium pressure lamp (as opposed to a low pressure one) may be the preferred or logical choice. 

In essence, there are two (2) categories of ‘Types’ for mercury (Hg) based UV lamps ie. the first being low pressure vs medium pressure and the second being instant start vs non-instant start or preheat. Focus in this article will be on the instant start vs preheat type UV lamps. An instant start lamp typically requires a high enough starting or ignition voltage to be applied across the cathodes or electrodes at either end of the lamp (without the need to first heat the electrodes) to break down the gas and Hg column and create an instantaneous arc. The lamp length typically dictates the magnitude of the ignition voltage required to perform this function. A preheat lamp requires the filaments of the lamp to be heated during or at the beginning of the ignition or starting sequence prior to the lamp arc being struck. The voltage required to strike the arc doesn’t need to be as high as that of the instant start lamp and therefore does not experience as much sputtering as that of the instant start lamp. The preheat lamp can sustain more frequent on/off cycles than an instant start lamp and can have a longer lamp life. 

by Bill Himaras

Aug 4th, 2021