Technology

Generation of UV rays

The technical generation of UV rays is essentially possible in various ways. Nowadays, generation via a mercury-vapour discharge has established itself as the most effective method. In this process, mercury (Hg) is evaporated in an inert-gas-filled reaction chamber by means of active heat input. In this mercury-vapour-saturated inert-gas atmosphere, the electrons of the mercury atoms are excited through the supply of electrical energy. Once these atoms subsequently fall back to their stable energy level, they re-emit the previously supplied energy primarily in the form of UV radiation.

Here, the radiation level or measurable spectral-energy distribution emitted by the plasma is first and foremost dependent upon the quantity of free mercury atoms, the gas-pressure ratio, and the level of supplied energy.

High, medium and low pressure lamps

A core problem with UV emitters is the high dependency of the reaction mass on the internal-pressure ratio of the reaction body. The higher the ratio between mass and pressure, the more the resonance lines shift out of the 250-to-280-nm range necessary for disinfection.

For this reason, we rely almost exclusively on Hg low-pressure emitters for disinfection. At a vapour pressure of around 0.006 torr (which corresponds to about 8*10^-7 bar), they have a sharp peak at 254 nm. Moreover, they have the best cost-benefit ratio on account of their efficiency factor of around 40%.

On a purely technical level, these UV emitters hardly differ from sources known as fluorescent tubes or energy-saving lamps, although in such sources the UV radiation generated is used to transform the luminescent materials applied to the inner wall of the tubes into visible light. Furthermore, the glass bulb of fluorescent tubes is not made of quartz, and is thus not UV-permeable.

Temperature dependency

Since the degree to which gases expand is largely temperature-dependent, the Achilles heel of the technology immediately becomes apparent. Thus, it is not only the energy input, the quantity of vaporisable mercury and the choice of inert gas which is relevant for the output of the emitter, but also in particular the cooling load or heating adjacent to the reaction body.

This effect can be illustrated especially clearly using the example of a fluorescent tube: If a light source of this sort is installed outdoors, its luminosity is significantly lower in winter than in summer. This effect becomes even more apparent in the case of a mercury-vapour lamp optimised for disinfection purposes, although the effect can only be determined here by the use of suitable photometers.

Consequently, it is important to bear this fact in mind and to counter an anticipated excess- or negative pressure of the plasma through technical measures. sterilAir therefore distinguishes between H-, N- and K tubes, all of which differ in terms of their physical properties and purposes. A bakery, for example, most certainly requires a different radiation source than a cold store.

: Screenshot: UVGI

: Screenshot: UVGI

Calculation

In addition to the purely temperature-related efficiency rating of a radiation source, however, there are two further parameters in UV disinfection which have an effect: owing to solarisation effects, a mercury-vapour lamp ages continuously, i.e. the emitted power decreases; and the calculation of the irradiance at an increasing distance from the source follows a non-linear function.

Without evaluating lethality and exposition time of the recipient – UV disinfection follows the dose principle – the complexity of the method is shown here:
What aging-related, temperature-dependent real-power value is to be expected for the emitter? Are there absorbing factors such as pollution or high atmospheric humidity? What positive influences are exerted by turbulence, and secondary radiation by reflection?

For maximum predictive reliability, therefore, sterilAir employees and partners use a scientifically-sound simulation program.