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An extractor hood (U.K.) or range hood (U.S.), also known as a kitchen hood, stove hood, exhaust hood, cooker hood, extraction hood, cooking canopy, ventilation hood, extractor fan or electric chimney, is a device containing a fan that hangs above the stove or cook top in the kitchen. It is used to remove airborne grease, combustion products, smoke, odours, heat, and steam from the air by a combination of filtration and evacuation of the air. An extractor hood consists of three main components: a skirt or capture panel to contain the rising gases (also known as the "effluent plume"), one or more grease filters, and a fan for forced ventilation.
Grease removal from the extracted air stream is facilitated by filters mounted in the extractor hood. Many different types of filters are commercially available but most use the same mechanical action to capture the airborne grease and other pollutants. Activated carbon filters are sometimes used as the final filter stage. All the grease particulates must be removed from the extracted fumes before this stage.
Carbon filters are used to remove odours, not grease. The activated carbon is rendered useless if it becomes coated in grease deposits. Once impregnated it cannot be effectively cleaned and replacements are expensive. For these reasons, carbon filters are best avoided in grease applications if possible Filter efficiency is usually a trade off against system static pressure.
The finer the filter medium, the more resistance it presents to the air flowing through it. This then means larger and more powerful fans must be used to maintain sufficient air extraction volume. Some extractor systems use two or three stages of filters to improve grease removal efficiency. The ultimate goal of the filter is to cause the grease particles to combine together to form larger particles and to come into contact with the surfaces of the filter and stick to them. The system used by the filter to cause this to happen are a combination of causing the air to change direction, causing the air to change velocity and causing the air to rotate. This causes grease particles to collide, combine and adhere to the baffles or course filter medium contained within the filter frame. As the grease builds up, it runs down the filter by gravity and usually into the extractor hood frame that the filters are mounted in. This act as a trough which holds the grease, or, depending on the hood design, diverts it into a collection tray. Mechanical baffle type filters vary greatly in their efficiency at grease capture and removal. All mechanical filters are more efficient at removing larger grease particles. The grease that escapes through the filters is prone to collecting in the ducting and the fan unit where it is a fire hazard and can also cause mechanical failure. Filters should also provide a measure of flame retardation to prevent a fire under the hood spreading into the plenum area of the hood and onward into the ducting and fan. The most common material for filter construction is stainless steel which is resilient to high temperatures, chemical action and is hygienic and easy to clean.
One of the single biggest problems service engineers find with cooker hood installations today where they are vented to the outside air, is the incorrect size of vent used most commonly, or just general poor ventilation. It’s worth noting the direction of the outside wall is facing. Most prevailing winds are south/south west. Best to avoid these walls as the extractor needs to work a lot harder to work against a strong incoming breeze from the direction. These will both lead to the appliance seeming faulty but neither is an actual fault with the appliance. It’s an installation fault & isn’t covered by manufacturer’s warranty. |
