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Extraction cleaning - design and specification
The grease filters in a kitchen canopy are the system's first line of defence against grease reaching the ductwork - and how well they do that job is largely decided at the design stage, by the filter type chosen and the air velocity across them. Get the specification right and the filters catch grease efficiently for years; get it wrong and grease loads the ductwork faster than it should. Here is what specifying grease filters properly involves.
The short answer
Specifying grease filters at the design stage comes down to two connected things: the type of filter, and the face velocity - the speed of air across the filter. The DW172 kitchen ventilation guidance recommends baffle grease filters as the primary filtration in the canopy, for their easy-clean form, high grease-removal efficiency and fire and flame resistance, with a mesh filter as an optional secondary stage. But the filter type only delivers its rated efficiency at the correct face velocity: a baffle filter reaches its high grease-removal efficiency at its design air speed, and a mesh filter has a maximum recommended face velocity of around 2.5 metres per second. So the filters and the airflow have to be specified together, so the air moves across the filters at the speed that gives efficient grease capture - which is why filter specification belongs at the design stage, when the canopy and airflow are being sized.
The filters are the first defence
In a kitchen extraction system, the grease filters in the canopy are the first thing the greasy air meets, and their job is to catch as much of the grease as possible before the air carries it on into the ductwork. Every bit of grease the filters capture is grease that does not reach the ducts to accumulate there - so the efficiency of the filters directly affects how fast the ductwork loads with grease. Efficient filters mean less grease reaching the ducts, slower ductwork build-up, and a system that stays safer for longer between cleans. Inefficient filters mean more grease getting through, faster ductwork loading, and more frequent cleaning needed to keep the fire risk in check.
This is why getting the filters right matters so much, and why it is worth doing at the design stage. The filter specification set at design determines the system's first-line grease capture for its whole life - it is not something easily changed later without reworking the canopy and airflow. Filters chosen and sized well at design will protect the ductwork efficiently for years; filters chosen or sized poorly will let grease through faster, loading the ducts and raising the cleaning burden and fire risk, for as long as the system runs. So the design-stage filter specification is a decision with long consequences, worth getting right up front rather than living with the results of getting it wrong.
Baffle filters as primary
On filter type, the established kitchen ventilation guidance - DW172, the specification for kitchen ventilation systems - recommends baffle grease filters as the primary grease filtration in the canopy. Baffle filters work by forcing the air through a series of baffles that change its direction sharply; the grease, heavier than the air, cannot follow the sharp changes and is thrown out onto the baffle surfaces, where it drains away. They are recommended for several reasons: they capture grease efficiently, they are easy to clean - typically robust stainless steel units that can be removed and washed - and, importantly, they offer fire and flame resistance, helping to check flame passing through the filter into the system. That combination of efficiency, cleanability and fire resistance is why baffle filters are the recommended primary choice.
A mesh filter can be specified as a secondary stage, after the baffle, to catch further grease - the baffle doing the main work and the mesh refining it. But mesh filters have limitations that make them less suitable as the primary filter, including a lower maximum face velocity and, in some forms, less favourable fire behaviour, which is why the guidance puts the baffle first. So a well-specified canopy typically has baffle filters as the primary grease filtration, chosen for their efficiency, cleanability and fire resistance, with mesh as an optional secondary refinement rather than the main defence. Specifying the right filter type at design - baffle primary - sets the system up with an efficient, cleanable, fire-resistant first line.
Face velocity is the catch
The point that is easy to miss - and that makes filter specification a design-stage job rather than just a shopping choice - is that a filter only delivers its rated grease-removal efficiency at the correct face velocity, the speed of the air passing across the filter face. A baffle filter reaches its high efficiency, around ninety per cent grease removal, at its design air speed; too slow and the grease is not thrown out effectively, too fast and grease is carried through rather than captured. A mesh filter, similarly, has a maximum recommended face velocity - around 2.5 metres per second - above which its efficiency falls off. So the efficiency figures that make a filter attractive only hold if the air moves across it at the right speed.
This means the filters and the airflow have to be specified together, so the face velocity across the filters lands in the range that gives efficient capture. That depends on the filter area, the number of filters and the total airflow the system moves - all of which are set at design. A common failure is retrofitting filters like-for-like without checking the velocity: swapping mesh for baffle, or changing the filter arrangement, without redesigning the airflow can leave the face velocity wrong, so the new filters do not achieve their rated efficiency and grease gets through despite the better filter type. That is exactly why filter specification belongs at the design stage, alongside the canopy sizing and airflow design - so the filter type and the face velocity are matched, and the filters actually deliver the efficiency they are capable of.
Getting it wrong loads the duct
When grease filters are specified poorly - the wrong type, or the right type at the wrong face velocity - the consequence is not immediately obvious, but it plays out over the whole life of the system. Filters that do not capture grease efficiently let more grease through into the ductwork, so the ducts load with grease faster than they would with efficient filters. That means the ductwork reaches the point of needing cleaning sooner, so cleaning is needed more often to keep the fire risk in check; and between cleans, the ductwork carries more grease and more fire risk than it would with better filtration. So a filter specification error at design translates into a heavier grease burden, more frequent cleaning and higher fire risk for years.
Getting the specification right avoids all of that. Efficient filters, of the right type and at the right face velocity, capture grease well, so less reaches the ducts, the ductwork loads more slowly, cleaning intervals can be longer, and the fire risk between cleans is lower. The difference between a good and a poor filter specification is, over the life of the system, the difference between a manageable and an excessive grease burden on the ductwork. That is what makes the design-stage filter decision worth careful attention - and why, if you are involved in designing or refurbishing a kitchen ventilation system, the grease filters deserve to be specified properly, with the type and face velocity considered together, rather than treated as an afterthought.
The takeaway
The grease filters are the extraction system's first defence against grease reaching the ductwork, and how well they perform is largely decided at the design stage. Specifying them properly means getting two connected things right: the filter type - with baffle filters recommended as the primary filtration for their efficiency, cleanability and fire resistance, and mesh as an optional secondary stage - and the face velocity, the air speed across the filters, which must land in the range that gives the filters their rated efficiency. A baffle achieves its high efficiency at its design velocity; a mesh has a maximum recommended velocity of around 2.5 metres per second. The type and the velocity have to be specified together.
That is why filter specification belongs at design, alongside the canopy and airflow sizing, rather than being a retrofit choice - a common error is swapping filters like-for-like without checking the velocity, leaving good filters under-performing. Get it right and the filters catch grease efficiently for years, loading the ductwork slowly and keeping cleaning intervals and fire risk manageable. Get it wrong and grease reaches the ducts faster, raising the cleaning burden and fire risk for the life of the system. So specify the grease filters properly at the design stage - type and face velocity together - and give the ductwork the efficient first-line protection it needs.
Questions
Two connected things: the filter type, and the face velocity - the air speed across the filter. The type determines the filter's capability, but it only delivers its rated grease-removal efficiency at the correct face velocity. So the filters and the airflow have to be specified together, which is why filter specification belongs at the design stage.
The DW172 kitchen ventilation guidance recommends baffle grease filters as the primary filtration in the canopy, for their high grease-removal efficiency, easy-clean form and fire and flame resistance. A mesh filter can be specified as an optional secondary stage after the baffle, refining the capture, but the baffle is the recommended primary filter.
Face velocity is the speed of the air passing across the filter face. A filter only achieves its rated efficiency at the right velocity - a baffle reaches around 90% grease removal at its design speed; a mesh has a maximum recommended velocity of about 2.5 metres per second. Too fast or too slow and efficiency falls, so the velocity has to be designed to suit the filters.
Because the filter efficiency depends on the face velocity, which depends on the filter area, number and total airflow - all set at design. Swapping filters like-for-like without redesigning the airflow can leave the velocity wrong, so the new filters do not reach their rated efficiency. Getting filters and velocity matched really needs to happen at design stage.
The filters are the first defence - every bit of grease they catch is grease that does not reach the ducts. Efficient filters mean less grease reaching the ductwork, slower build-up and longer cleaning intervals. Poorly specified filters let more grease through, loading the ducts faster and raising the cleaning burden and fire risk for the life of the system.
No. Even the best filters capture only a large share of the grease, not all of it - a portion, especially grease vapour, passes through into the ductwork and accumulates over time. Good filters slow that build-up but do not stop it, so the ductwork still needs professional cleaning; the filters just make the intervals more manageable.
Well-specified filters slow how fast grease reaches the ducts - but grease still gets through, so the ductwork still needs cleaning. Our extraction cleaning removes what the filters do not catch.
