There is some confusion regarding the similarities and differences of kitchen exhaust hoods using ultraviolet light bulbs (UV-C) in the exhaust plenum and systems that are referred to as “corona discharge” that generate ozone.
Ultraviolet Light or UV-C is a type of bulb that has a very high intensity that allows it to break down organic compounds. UV has been applied in a variety of sterilization and water treatment applications for decades. The wavelength varies by application. In the case kitchen exhaust systems, UV-C light is used to break down grease particles through two processes: photolysis and ozonolysis. The photolysis process (through direct UV exposure) contributes to grease destruction in two means: first it can break the molecular bond of grease molocules and secondly this light results in the creation of ozone and other free radicals (such as hydroxyl) around the perimeter of the UV lamps Ozonolysis is the process where these radicals oxidize (add oxygen molocules) to grease molecules.
Ultraviolet Light or UV-C
A technical explanation for UV-C light operation is when energized (excited) it creates a glowing plasma of electrons The dominant emission (>90%) from these lamps is UV-C energy. The “C” frequency of the electromagnetic UV family has, amongst other things, germicidal effects as well as helping to break down organic compounds such as cooking effluent.
Note: The blue light is a representation of what a UV-C light looks like in operation. An operating system could not operate in this manner for safety reasons. Personnel should not be exposed directly to UV-C light
A major difference with UV-C lamps that they are in direct contact with grease-laden vapors. The level of conversion is related to the exposure time to the light.
Corona discharge devices generate ozone which is “injected” into the affected ductwork. These systems sit outside of the exhaust hood. The ozone acts on the VOCs and reduces odors. The amount of time the VOCs are exposed to ozone relates to its effectiveness.
Corona discharge occurs when the current flow from and the electrode is large enough air ionizes This generate gases such as ozone (O3) and nitric oxide (NO), and in turn, nitrogen dioxide (NO2), and thus nitric acid (HNO3) if water vapor is present.
Unlike a self-contained UV-C equipped hood system, ozone generators sit outside of the hood system. Corona Gas Cells produce ozone that is introduced into the ductwork.
The grease extraction efficiency of the filters within the hood system will play a role in the corona discharge system’s effectiveness. Less efficient grease filters allow more grease to enter the duct system.
The ozone output from an Ozone system shown from one representative manufacturer is at the outlet of their generator, not in the duct.
A medium/heavy-duty hood with UV-C has the same or greater ozone output than a typical ozone generator for the same airflow.
Additionally, the UV-C lamps have the added benefit of photolysis:
- The light reacts with ozone (O3) and water (H2O) to create hydroxyl (OH) which is an even better oxidizer than O3
- Photolysis can break single carbon bonds which further helps break down the grease molecules
Corona discharge systems are easier to retrofit to existing hood systems. Caution must be taken regarding the expectation of what can be achieved with Corona discharge. In conventional commercial hood systems with gas and electric appliances, one cannot realistically expect exhaust ducts treated with ozone alone to be free from all grease exhausted by the kitchen hood. Since oxidation is a slow process, there will always be some grease residue remaining.
UV-C hood systems can be very effective on a wide variety of menus on both grease and odor reductions. It is imperative routine maintenance is done to maximize system performance.
It is important to qualify the expectation of each system and its maintenance requirements.
Read more on UV technology and kitchen exhaust hoods by clicking on the link below:
What are UltraViolet lights in kitchen hoods used for and what outcome can I expect when using them?
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