Extraction alone is not enough
Ventilation systems in commercial kitchens are intended to protect the “kitchen brigade” in compliance with industrial health and safety standards. For this purpose, an extractor hood or a ventilation ceiling with grease separators is installed above the cooking block. A particularly efficient extraction technique is based on induction ventilation.
To make sure that the kitchen staff is not left in the fumes, the kitchen ventilation system has to extract and filter the greasy fumes. However, this only works efficiently if an important task is performed beforehand – the capturing of all fumes and all pollutants they contain. And this is the crux of the matter: Most extractor hoods and exhaust ceilings cannot do this.
Capturing – this is how experts call the collection of the kitchen fumes – is the first task of a kitchen ventilation system. After all, what good is a high separating efficiency of the grease traps if only two thirds of the fume volume reach the exhaust air duct, while one third circulates in the dead zone of the hood or leaks back into the kitchen. Recent investigations using CFD numerical flow simulation reveal that such dead zones and leakage air volumes occur in most canopy designs. The cause: The sucking in of fumes is only effective up to a distance of 30 cm from the separator (Fig. 1). Further away, the vapours vagabond in an uncontrolled way in the hood space and partially escape from it, even if the hood protrudes correctly. The required air hygiene in the kitchen cannot be ensured this way.
The Swabian manufacturer of extractor hoods and ventilation ceilings Rentschler Reven has meticulously examined the flow conditions in many commercial kitchens and came to the following conclusion: The flow conditions in the canopy are highly complex. They are much more complicated than the VDI Code of Practice 2052 presents. The result of the flow simulation reveals that extracting alone is not enough. Additional impulses are necessary to ensure the capture of all fumes. With the help of the CFD software mentioned above (see Box 1), a new flow principle was developed under the name of (air-)induction technology. It was initially discussed controversially in expert circles. How it works:
A supporting jet is blown horizontally into the extractor hood (Fig. 2). This attracts (induces) the fumes rising from below. The necessary air volume is drawn from the indoor air at the front of the hood (Fig. 3). The temperature of the supporting jet of about 20 to 25 °C is distinctly lower than that of the fumes with about 80 °C. Consequently, the cooking fumes condensate in the separator and not just in the exhaust air duct. This keeps the air duct dry and germ-free and increases preventive fire protection.
As there are no more dead zones in the new hoods, the cooking fumes are captured completely and driven to the separators. An infinitely variable blower allows continuous operation between 0 and 100 %. This prevents draughts in the chefs’ working area, even at peak times.
Reven did not invent the induction principle. Tens of thousands of induction hoods are in operation in the United States. They are typical “light hoods”, namely coarsely cobbled tin boxes with insufficient effect and high noise level. The first air-induction hoods in Germany also had their teething troubles and often did not achieve the desired effect: Cold outside air was blown in for the supporting jet; this required an additional air supply device. The extra costs compared to conventional hoods were considerable. Therefore, specialist planners were often critical of the principle.With Swabian thoroughness, Reven has evolved this principle using CFD to meet European requirements, however, and brought it to market as a premium product. In practice, it is important that a clean flow leaves the induction nozzle in the direction of the separator. Therefore, the most important component is the induction nozzle. Reven has optimized it in view of the flow technology and had it internationally protected by a patent specification (Fig. 4). The design improvements, especially the omission of the introduction of cold outside air, have reduced the price difference to former induction hoods to zero. This is how induction technology became a story of success, which is demonstrated by many reference ventilation systems.
Ideal for narrow kitchens
Induction ventilation reveals its true strength in narrow kitchens with high power density. In this case, large air volumes must be introduced without draughts and at low noise levels. Restaurant kitchens are becoming increasingly smaller because the tilting pan, kettle, cooker and deep fryer are often combined into a single cooking block as the cost of space is becoming more and more expensive. The use of convenience food also contributes to the diminishing of kitchen sizes. In the near future, 3D food printing will also support this trend. Hence, a connected load of 100 kW is not uncommon in a kitchen with 20 m². Meanwhile, in the catering sector, there are production kitchens that prepare 1,000 (!) meals a day on a floor space of 15 m². Fumes, stale air and heat must be removed, however, and this without draughts, so that the kitchen staff does not catch a cold. This is a challenge for the ventilation contractor. With induction technology, the specialist planner has the appropriate solution at hand. Here are some case studies:
The “Herrenküferei” in the Swabian town of Markgröningen is a 600-year-old, plastered half-timbered building that houses a hotel and restaurant. The old hotel kitchen has recently been renovated with high effort in craftmanship. The listed building required a great deal of imagination and technical finesse from the ventilation contractor because many of the walls are crooked and not at right angles. Furthermore, the protection of historical monuments prohibits changes not only to the façade but also to the stud framework and the timberwork inside.
The narrow kitchen has a low ceiling height and is packed with appliances. It measures around 80 m2. The air volume calculation resulted in a supply air volume of about 10.000 m3/hr at peak times. How can this high supply airflow rate be achieved without pulling the hats off the chefs’ heads? Rentschler Reven solved the problem with several custom-made induction hoods made of stainless steel (Fig. 5).
A three-storey residential building with the restaurant “Da Antonio” on the ground floor was recently built in Kelkheim, Hesse. The “glass” kitchen with about 30 m2 floor space offers Indian and Italian specialities. The guests can watch the preparation. Pizza ovens, combined steamers, gas woks and powerful tandoori ovens, however, cause a high power density in a small space. The planner and the promoter opted for a ventilation system based on Reven’s induction principle. The tailor-made extraction hood was installed by the Mainz-based HVAC company Dornhöfer.
The high heat and humidity loads in Kelkheim made a high air exchange rate necessary. However, only 3,000 m3 of the supply air volume of 5,000 m3/hr flows through the cooking zone, the rest extracts efficiently kitchen fumes as induction airflow. This ensures the necessary air hygiene without drafts. Since the supply air volume to be preheated is less than 4,000 m3/hr, the heat recovery required by the German Energy Saving Ordinance (EnEV) could be dispensed with.
Meiko also favours the induction solution. The leading dishwasher manufacturer in Offenburg/Baden recently moved into a new training building with two conveyor belt dishwashers. Among other things, different operating states and load cases are simulated for the new generation of kitchen staff. For the ventilation of the scullery, the promoter selected four induction hoods of the latest generation from Reven. The stainless steel hoods have an air-handling capacity of 2,000 m3/hr each and were installed by the company Büchele Lufttechnik from Karlsruhe (Figure 6). They condition the kitchen air without draughts.
Last but not least: The Chinese smartphone manufacturer Huawei has its European headquarters in the Globalworth Tower in Bucharest (Fig. 7). The central kitchen and the canteen are located on the 25th floor of the 120-m-high office building. In the kitchen, the large woks alone, with a diameter of approximately one metre each, generate a lot of fumes and high heat and moisture loads. The ventilation system went into operation when the building was completed in 2015. It proved to be inefficient and the kitchen staff found themselves in a draughty and hazy kitchen. The renewal was urgently required.
When designing the new system, the changing pressure conditions (wind pressure) at an elevation of approximately 70 m above the ground had to be taken into account. Furthermore, the soiling of the façade by greasy outgoing air had to be avoided under all circumstances.
Reven agreed with the promoter on a system concept with several induction hoods and an aluminium ventilation ceiling. Integrated are X-cyclone grease separators, LED lighting and a UV-system to eliminate odours from the outgoing air. Total exhaust air volume: 30,000 m3/hr. The ventilation system was installed within four weeks in the framework of the total renovation of the kitchen. All induction hoods are equipped with highly effective X-cyclone grease separators, which have a constant separation efficiency that does not drop to its knees when the ventilation is throttled (Fig. 8 and Box 2).
Delivered just in time
When designing an induction ventilation system, Reven uses the most modern methods: The system is virtually designed on screen using the BIM method (Fig. 9). BIM (Building Information Modelling) is currently the most advanced working method for the design and construction of building service systems. All parties involved in the execution are networked and can inform themselves at any time about the current status of the building and about changes (see Box 3).
The hood components are prefabricated in the factory to the nearest millimetre (Fig. 10) and shipped to the construction site just in time. This saves the promoter expensive wage hours on site and the installation company achieves a higher hourly turnover per man. This also guarantees cost and schedule security. In the case of retrofitting, the installation is sometimes carried out without interrupting the operation of the kitchen. During the day, the kitchen staff is at work, at night the ventilation technicians come.
Some remarks on the introduction of the supply air. It is introduced from above through low-velocity air outlets in a low-pulse and laminar manner and seeps almost vertically to the ground (Fig. 11). This way, the occupied area is supplied with fresh outside air without draughts (by a so-called laminar flow). The ceiling remains clean because no greasy air is stirred up in the kitchen. According to Reven, horizontal injection has not proven to be effective, however, because the horizontal supply airflow can deflect the rising thermal airflow. If the supply air is blown in horizontally, the minimum distances to the extraction points recommended by the manufacturer must be observed in any case.
Induction also in ventilation ceilings
The positive experience with induction hoods prompted Reven to also equip ventilation ceilings with induction technology. The ceiling spans the entire room; its smooth, condensate-tight welded surfaces can be cleaned with the steam jet and the aerosol separators can be displaced when the kitchen is extended or reorganized (Fig. 12). Changes to the layout of the kitchen are therefore no problem from the ventilation point of view. Like the hood, the induction ceiling is free of dead space, so that the fumes are completely captured
The exhaust air leaves the room through linear suspended exhaust modules. The aerosol separators are inserted in them. The slot-shaped induction modules are suspended opposite them (Fig. 13).
Exhaust modules and the exhaust air space behind them form a closed, condensate-tight welded structure. Therefore, the exhaust air does not come into contact with the building. To dispense with the need to use aggressive cleaning agents and avoid pitting corrosion, the supply and exhaust modules are preferably made of stainless steel. Thanks to this closed air conduction, the promoter saves the periodic cleaning of the ceiling cavity and prevents sooting of the masonry by fatty acids. This also improves structural fire protection. And some more examples of best practice:
The 80-m-high Main-Forum in Frankfurt/Main is a mixed residential and office building (Fig. 14). Among other tenants, it houses the headquarters of the German metal trade union IG Metall. The central kitchen of the building, which was erected in 2003, was fitted with a conventional ventilation ceiling, which caused considerable energy costs. It was recently demolished and replaced by a modern induction ceiling made of V2A stainless steel with a surface area of 60 m2 and an exhaust air volume of 11,000 m3/hr.
An induction ceiling of 200 m² made of stainless steel and powder-coated aluminium was recently installed in the kitchen of the Bundeswehr hospital in Ulm.
Another case: In Frankfurt/Main, Caritas operates a new kitchen for homeless people. Due to the tight budget, the administration decided to use the extractor hood of the cheapest provider. The complaints of the kitchen staff were not long in coming: The draught was intolerable and comfortable temperature conditions could not be achieved. In addition, condensate dripped into the soup at the low-speed stage. Already after the first winter, the hood was replaced by an induction ventilation ceiling. The company RUF Gebäudetechnik from Kleinheubach installed the white-coated ceiling, designed for an exhaust air volume of 5,400 m3/hr. Since then, the staff is quite satisfied with the air conditions and does not risk a cold anymore.
Reven realised a gigantic project at Frankenberg Food in Würselen near Aachen. The family-owned company produces high-quality frozen meals for international airlines (in-flight meals) and other bulk consumers. The ventilation ceiling with the induction modules made of chrome-nickel steel measures around 1,000 m² and is designed for a total airflow rate of 80,000 m³/hr (Fig. 15). In order to save energy, the flow rate is matched to the amount of fumes released; humidity and heat sensors measure the density of the fumes and automatically control the fan speeds. Each cooking zone has its own sensor system for air volume control. This saves energy, extends the service life of the grease separators, and avoids draught.
The Frankenberg ceiling is likely to be the largest induction ventilation ceiling ever built and is one of the most complex projects in Reven’s history. The company used 12 tons (!) of stainless steel for the ceiling in Würselen alone.
The hybrid ceiling – combining an extractor or induction hood with a flat ceiling – is becoming increasingly popular among planners and interior designers. Its benefits: The flat structure is also suitable for low rooms with heights starting at about 2.50 m. The displacement air diffusers for the air supply are integrated. The supply air trickles into the room without draughts and with low impulse. The hybrid ceilings are designed by the Reven headquarters together with the design office.
The elegant design of the hybrid ceiling improves the “vista” of the kitchen. The ceiling is easy to clean; there are no visually disturbing inspection openings because the ceiling can be opened easily at any point. It is water-repellent, flame-retardant and complies with hygiene regulations. Moreover, it is delivered from a single source including assembly and cabling of the LED lamps. This creates planning reliability and the recurring interface problems with other trades are off the table.
In Frankfurt/Main, the new restaurant in the basement of the Messeturm received a stainless steel hybrid ceiling with a surface of 100 m2, designed for an exhaust air volume of 15,000 m3/hr.
The ceiling solution pays off for rooms of 50 m² and more; the extractor hood or the hybrid ceiling mentioned above are the favourites. Induction technology is also increasingly used in the food and pharmaceutical industries to ensure compliance with hygiene regulations in production. Digital interfaces allow the integration into a building management system. The Reven induction technology is thus prepared for the smart kitchen and for production 4.0.
Explanation of terms
CFD flow simulation
CFD (Computational Fluid Dynamics) is an imaging method to simulate complex airflows. It supplements the empirical experiments in the flow laboratory. Aircraft and racing car designers, for example, use it to optimize wings and spoilers. Rentschler Reven uses the CFD method for the realistic representation of particle trajectories during grease separation in commercial kitchens. This is because not all particle sizes are separated equally well; they move on their own trajectories and do not necessarily follow the airflow. It was found, for example, that pollutant aerosols with a size of 3 to 5 µm move on a different trajectory than particles as small as 0.1 µm due to their inertia. The latter are relatively easy to capture whereas the particles of 3 to 5 µm in size require an agglomerator which combines (agglomerates) the small particles into larger droplets.
The CFD software allows the optimization of the geometry of ventilation components such as displacement diffusers and induction air outlets on a standard PC. This helps to control turbulence-causing components and tear-off edges. The advantage of CFD over experimental procedures in the laboratory: The measured values are not only determined at specific points, but all physical quantities are recorded and displayed at once. Therefore, CFD is a cost-effective alternative to smoke tests and endless wind tunnel tests.
X-Cyclone grease separator
The patented X-Cyclone separators do not store the grease in a filter medium but eject it by repeated air deflection. The exhaust airflow is set into rapid rotation in the cyclone separator elements. Due to centrifugal forces, even ultrafine aerosols are ejected without residues. They flow along the stainless steel blades into a collecting channel. The separator practically cleans itself. This stabilizes the separation process – in contrast to air filters, whose efficiency decreases with increasing saturation and drives up fan costs due to the higher air resistance. If maintenance is neglected, the filter medium can become overgrown and detachments may occur on the clean air side. This means that viruses that adhere to the aerosols and dust particles can also be transmitted. Regular filter replacement is indispensable. In contrast to these filters, X-Cyclone separators can be operated up to one year without any downtimes and be cleaned with a steam jet.
Holistic design with BIM
Building Information Modelling, or BIM for short, is a holistic working method for designing, executing and managing a building. All parties involved in the construction process are networked and can inform themselves at any time about the current status of the building and about changes. Experts also speak of building site optimization. Planning errors and collisions of the trades are avoided, reworking is minimized.
In the HVAC sector, Rentschler Reven is one of the first vendors to plan and execute their projects using the BIM method. The manufacturer of commercial kitchen ventilation systems designs the system using CAD and then depicts the project as a 3D model. The viewer can virtually enter the kitchen and view it from all perspectives.
Market researchers from BauinfoConsult found out that in 2019 around 60 % of architects use BIM, while HVAC companies are still reluctant. This is partly due to the fact that the trades have not yet been able to agree on uniform standards. Reven supports this trend by providing architects and specialist planners with technical data sets for its products.