What is a recirculator and when does it work eectively?

A UV recirculator (also known as a closed irradiator) is a device intended to disinfect indoor air in occupied rooms.

The recirculator intakes air from a room, passes it through an irradiation chamber (ultraviolet reactor) and discharges it back into the same room. In the chamber, the air is exposed to ultraviolet radiation and disinfected.

Rate and UV dose are the key characteristics of the irradiator performance.

1. RATE. The effective operation of a recirculator relates to the volume of a treated room. A recirculator capacity should be 4-6 times higher than this volume. In this case the whole air in the room can recirculates every 10-15 minutes through the device. This rate is essential since the people in the room are potential sources of contamination and the expired air shall be disinfected as soon as possible. For example, for a room of 36 m2 with 3-meter ceilings, the total capacity of a recirculator or recirculators should be about 500 m3 /hr.

2. UV DOSE. A certain UV dose is required for the disinfection of each microorganism type (for example, by 99.9%). UV dose is the product of the exposure time by UV intensity.

A. For a given UV output and air flow rate (capacity), the larger UV disinfection chamber, the more time the air is exposed to UV and the greater the UV dose is delivered to it.

B. It is clear that the higher the intensity, the higher the UV dose. The intensity can be increased with enlarged number of UV lamps (then the power consumption is higher). Also special coatings can be applied on chamber walls. These coatings do not absorb, but reflect UV light back into the volume of the reactor and the intensity will grows. There are many UV reflecting materials to coat reactor walls. In simple terms: the higher the reflection factor/coefficient, the more times the UV light is reflected from the walls and penetrates through the treated air. For example, steel - with a reflection factor/coefficient of about 40% - reflects a UV ray only 6 times. Aluminum - with a reflection factor/coefficient 80-85% - 21 times. Contrary to the previous generation, a modern AEROLIT recirculator utilizes special materials with a reflection coefficient of more than 95-98% (patent-protected solution), which significantly increases UV intensity in the chamber.

Therefore, the design of modern energy-efficient recirculators is focused on the maximum volume of the UV chamber and the maximum reflectivity of its walls. It is important to note that from all of the above, we can conclude that an effective recirculator with a large flow rate cannot be small in size. And we come to the conclusion that an effective recirculator with a large flow rate cannot be small.

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Ultraviolet technology for water, air and surface disinfection is based on germicidal effect of UV-C radiation. 

UV radiation is electromagnetic radiation between x-rays and visible light. UV wavelengths range from 100 to 400 nanometer.

The UV wavelengths are divided in 4 groups, each with a different germicidal effect – UV-A (315–400 nm), UV-B (280–315 nm), UV-C (200–280 nm) and Vacuum UV (100–200 nm).

Ultraviolet in

Within the UV spectrum, UV-C range is considered the strongest UV radiation, which is easily absorbed by DNA, RNA and proteins. This range is often called germicidal due to its high disinfection efficiency against bacteria and viruses. The highest germicidal effect occurs at 205-280 nm and the maximum germicidal sensitivity of microorganisms at 265 nm. 

The germicidal effect is based on photon absorption by DNA and RNA molecules. Photochemical reaction provokes dimerization of DNA and RNA bonds, which inhibits the ability of microorganisms to replicate. This process is known as inactivation of microorganisms.

Mechanism of
UV disinfection

UV disinfection technology can be applied for potable water supply, wastewater treatment as well as for air and surface disinfection applications.

The major advantages of this technology:

  • high efficiency against a wide range of microorganisms including chlorine resistant ones (viruses and protozoa oocysts);
  • no impact on physical, chemical and organoleptic properties of water and air; no by-products; no dangerous overdosing;
  • low capital costs, power consumption and operational costs;
  • UV systems are compact and easy to operate; no need for special operational safety precautions.

Main industrial available sources of UV radiation are low pressure amalgam lamps and mercury medium pressure lamps. Medium pressure lamp technology have higher power per lamp (several kW) but significant lower efficiency (9-12%) in comparison to low pressure lamp technology with efficiencies between of 35-40% and power per lamp up to 1000 watt.

UV systems equipped with amalgam lamp technology generally have a little larger physical footprint but they are significantly more energy efficient.

The design of UV application depends on the required UV dose, flow rate and physical and chemical parameters of media to be disinfected. The facility design criteria, flexible, economical and sustainable operation are also the decisive design parameters.