Vacuum accumulation inductive extraction hoods installed at the required height, with sufficient overhangs and adequate accumulation space, are now a necessary standard equipment for large-scale kitchen facilities. In light of recent developments in induction hoods, where the pursuit of the lowest cost leads to the obfuscation of the true principle of vacuum induction with cheaper pseudo-replacements, it is necessary to communicate the following:

The vacuum induction hood was first introduced into operation and patented in 1987 in Switzerland by Ing. Langer under the name INDUCTair®. Since then, it has undergone a long development process, including function verification and fulfilling required parameters. The final design of the induction hood with a regulated nozzle system emerged as the only technically viable solution that met the requirements (reducing the amount of extracted air by about 30% while increasing the efficiency of extracting harmful substances).

A) Description of the Function of the INDUCTair® Inductive Hood with Injected Air

As the only technical solution, the design of INDUCTair® hoods is based on the concept of extracting air as a carrier of harmful substances, heat, steam, grease particles, etc., in the smallest quantity and with the highest possible concentration in the extracted air.

1) The built-in unidirectional suction and bidirectional exhaust radial fan in the hood draws a small amount of air (2 to 3% of the extracted air) from the vicinity of the hood. This air is then ejected as primary air from small adjustable and lockable nozzles directed at the relevant extraction elements, precisely at the locations of the warm ascending currents of harmful substances.

2) The decrease in the velocity of the incoming air stream is accompanied by significant induction of warm ascending currents, leading to the formation of an overall flow with an increased concentration of harmful substances. It is essential to achieve maximum induction of ascending currents in areas with a higher temperature of the air flow.

3) The grease particles and aromatic aerosols from the accumulation space of the hood are extracted, which would otherwise not reach the extraction elements without the support of injected inductive air, and would escape into the kitchen space.

4) Especially today, as the volume of fried and grilled foods increases—ranging from beef tenderloin to steaks—in restaurant kitchens, even in luxury hotels, the ventilation system becomes a significant problem requiring a perfect solution.

5) Therefore, improving efficiency by up to 30% and significantly reducing operational costs in large kitchen facilities' ventilation can only be achieved with professionally adjusted INDUCTair® hoods.

The effectiveness of the hoods using inductive properties of injected air currents depends primarily on the following:

- Directionally adjustable and lockable air nozzles: These nozzles ensure the directed flow of harmful, saturated air to the "right place," as mentioned above. The warm ascending flow has a certain velocity—varying according to the current temperature of the heating device beneath the hood—which affects the induction of the air currents based on the relative speed of the flows and the distance of the nozzles from the extraction elements.

- Precise adjustment of the injected inductive air flow speed: The amount of induced air is dependent on the flow speed, nozzle diameter, and its distance from the extraction element.

- Project-specific air flow quantities: These often differ in reality, and it is necessary to carry out "harmonization" of the induction system with the actual amount of extracted air for proper functioning. Only INDUCTair® products can be correctly and precisely adjusted thanks to the adjustable nozzles in the induction system. Harmonization is included in the cost of INDUCTair® hoods

During development, cheaper and less demanding designs (without nozzles, but with longitudinal slits, punched holes, snap-out panels, etc.) were rejected.

All such units delivered and labeled as inductive are pseudo-inductive for the following reasons:

Pseudo-Induction No. 1

Pseudo-inductive hoods without nozzles, with only circular or rectangular holes, cannot be adjusted or set to the required parameters. This means that the required air flow cannot be directed or quantified, which negates one of the main benefits—reducing the amount of extracted air by about 30%. Therefore, this cannot be defined, and no calculations or predictions can be made!

Even with INDUCTair® hoods delivered without on-site harmonization, it is impossible to guarantee proper functioning, and the maximum effectiveness of inductive currents cannot be achieved.

Pseudo-Induction No. 2

Pseudo-inductive hoods with only a continuous horizontal slit, while not adjustable in terms of air flow direction, often act more like a shield that induces thermal currents only from below, leaving the top side practically cut off.

Pseudo-Induction No. 3

Pseudo-inductive hoods using holes instead of nozzles and with outlets on the front of the hood, which use external air directly from the supply ventilation system, cannot adjust the direction or speed of injected air currents. The speed depends on the restriction of the outlets, and if a perforated front panel is used as a large outlet, the air speed becomes negligible, making it unsuitable for induction.

Pseudo-Induction No. 4

In some cases, the Reven System is mistakenly considered an inductive hood. Here, untreated external air is blown into the accumulation space via a slit across the entire length of the hood, replacing 30–80% of the total extracted air. This results in the air mixing in the accumulation space, which eventually leads to the formation of a "gas cylinder" where the two air currents mix.

However, if the temperature and humidity within this "gas cylinder" reach certain values, the dew point will be exceeded, causing condensation and dripping water.

Conclusion

Kitchen odors pervading not only "open restaurants" but also the entire building—such as hotels or shopping centers—are becoming increasingly common. This is unpleasant and, especially for foreign visitors, unacceptable. Visitors will leave with the smell lingering on their clothes and the thought that they will never return to such a restaurant or hotel.

However, kitchen staff, working in this environment for 10-12 hours a day, often leave with headaches, aerosols from kitchen oils covering their respiratory system and accumulating in their lungs, causing health issues that worsen with each workday. There is a risk that, in the future, kitchen workers may claim compensation for work-related injuries or occupational diseases due to oil in their lungs.

Nevertheless, some owners remain unteachable, operating under the motto: "COSTS MUST BE SAVED, NO MATTER THE PRICE."

Therefore, it is essential to highlight that even high-quality inductive hoods with correctly designed airflow may not function properly for several reasons:

Lack of harmonization: If the direction and speed of the inductive air flows were not correctly adjusted according to the thermal air flow and the actual amount of extracted air, it is impossible to guarantee that the thermal air flow will be directed to the grease traps and extracted fully.

Insufficient overhangs: Often, when extraction hoods are designed by kitchen technology designers, the overhangs do not meet the necessary requirements, sometimes none at all. This results in significantly reduced extraction efficiency.

Unfortunately, it also happens that the ventilation designer succumbs to the pressure from the investor and, in order to save a few thousand CZK, reduces the size of the proposed hoods well below the permissible limits. The installation company then orders the hoods according to the equipment list and does not check if they are correctly designed (in many cases, they do not even check whether the hoods will actually fit into the kitchen). The result is clear: once the air escapes outside the range of the hood, no hood can save it.

Side air currents: These often occur due to incorrect placement of air supply elements or the staff leaving doors or windows open, introducing air currents that carry harmful substances far from the hoods.

This phenomenon is also often observed in covers above kitchen appliances located directly in sales areas or those with an open area adjacent to them, and preventing it is very difficult, if not impossible.

For kitchen appliances in sales areas (island kitchens), where it is necessary to capture the maximum amount of grease particles and aromatic gases, we recommend increasing the ventilation flow rate by at least 30% and using all available means to stabilize the rising warm air currents.

Mixing of the rising warm air current with the supplied air. This phenomenon reduces the effectiveness of ventilation systems by mixing the rising warm air current in the edge zones with the supplied air, i.e., with air that is adjusted to a quality suitable for human presence. This mixing

cannot be prevented in small or complex-shaped kitchens (for example, the VDI guidelines take this fact into account with appropriate coefficients that adjust the ventilation flow rate based on the method of air supply). However, with an appropriate ventilation system design, mainly by placing supply elements correctly, it is possible to prevent the conditioned air from being removed directly from the vents, i.e., "short-circuited", before it has warmed up in the kitchen. If this phenomenon occurs, and the nearest intake elements are partially or even fully overloaded with supply air, the exhaust system's ability to remove the warm air flow containing pollutants in the required volume will be proportionally reduced. This fact is identified in the vast majority of cases only when the ventilation system malfunctions, and the solution is not simple.

A typical example of mixing is the aforementioned covers with perforated front panels as large-area outlets.

The correction factor according to Table 4 of VDI should not focus on the direction of the supply air flow but rather on how close this flow is to the thermal air current being exhausted, i.e., how they influence each other. When the air is supplied by a radial anemostat downward on the opposite side of the room, this will likely be different than when the same anemostat is placed right next to the cover.

The supply air is intended to ventilate the kitchen and replace the exhausted air; therefore, it should pass through as large an area of the kitchen as possible before it becomes polluted air to be exhausted.

Regarding: Ventilated ceilings

are installed at a higher height than covers (approximately 2500 - 2800 mm above the floor) and do not have an accumulation space with exhaust elements above the thermal appliances, which leads to massive mixing of rising warm air currents with the conditioned supply air and also to the separation of a significant percentage of grease particles from these currents before they are removed through grease traps. Exhaust elements – grease traps – are mostly placed regardless of the kitchen technology layout, even in areas without thermal load. This leads to a significant reduction in temperature and the concentration of pollutants in the exhaust stream. It is recommended to increase the total ventilation flow rate, calculated for standard exhaust covers (without injected inductive flows), by at least 30%.

In Pelhřimov on July 21, 2016

INDUCTair s.r.o.