Phoenix Journal · Ductwork
Static pressure and volume flow are the two numbers that decide whether your kitchen extract actually captures grease and heat - or just looks busy. Here's what they mean and why they matter.
Ventilation fundamentals
Every commercial kitchen extract system stands or falls on two numbers: how hard the fan is working, and how much air is actually moving through the ductwork.
Those two numbers are static pressure and volume flow, and they are not the same thing - though they are often confused. A fan can be roaring away, drawing plenty of current, and still be shifting far less air than the canopy needs. When that happens, grease-laden vapour spills out over the cooking line instead of being drawn up and away, the kitchen turns hot and hazy, and the fire risk in the ductwork quietly climbs. Understanding how pressure and flow relate to each other is the difference between a system that protects your kitchen and one that only looks the part.
This is not abstract theory. The way your extract behaves day to day - whether the canopy captures cleanly, whether the fan is fighting itself, whether a clean makes a visible difference - is governed by these fundamentals. Here is what they mean and why they matter to anyone running a busy catering operation.
Volume flow is the amount of air moving past a point in a given time, normally quoted in cubic metres per second or litres per second. It is what actually removes heat, steam and grease vapour from over your appliances. When a ventilation engineer sizes a canopy to a heat load, the output of that calculation is a volume flow target - the air the system must physically move to capture what your cooking line produces.
Static pressure is different. It is the resistance the fan has to overcome to move that air, measured in pascals (Pa). Every metre of duct, every bend, every grease filter, every damper and every dirty section adds resistance. The fan does not care about distance directly - it cares about the total pressure it has to fight against to keep air moving. A well-designed low-pressure extract system typically sits below around 250 Pa; long, convoluted or neglected systems climb well above that.
The link between the two is captured in a single relationship engineers use constantly: volume flow equals air velocity multiplied by the cross-sectional area of the duct. Fix the duct size, and the only way to move more air is to move it faster. That is why velocity matters so much, and why the industry specification for kitchen ventilation in the UK - BESA's DW172, Specification for Kitchen Ventilation Systems - sets design velocities rather than leaving them to chance.
Under DW172, extract main duct runs are typically designed to around 6 to 8 m/s, with branches and spigots a little lower at roughly 5 to 7 m/s, and supply air slower still at 3 to 5 m/s. Those figures are a balance. Too slow, and grease vapour stops staying airborne - it drops out and coats the inside of the duct. Too fast, and the system generates excess noise, wastes fan energy and drives static pressure through the roof. There is a floor, too: below roughly 2.5 m/s, air simply cannot keep grease entrained, and it settles out to form the exact fuel load that TR19 cleaning exists to remove.
The numbers that matter
These are the design benchmarks a compliant commercial kitchen extract system is built around. Miss them and you compromise capture, safety or both.
The make-up air figure is easy to overlook and quietly critical. DW172 calls for replacement air of no less than 75% and no more than 95% of the total extracted volume. Take air out of a sealed kitchen faster than you let it back in, and you create negative pressure - the room fights the fan, doors become hard to open, volume flow collapses and combustion appliances can be starved of air. Extraction is only ever half of a balanced system.
Here is where the two numbers meet the real world. A fan has a fixed character, described by its fan curve - the graph of how much air it will move at any given static pressure. Your ductwork has its own character too, the system resistance curve. The point where those two lines cross is the operating point, and it decides how much air your kitchen actually gets. It is not chosen; it is where physics settles.
Now let grease build up. Every deposit narrows the duct and roughens its walls, and both push the system resistance curve upward. The fan responds the only way it can - it rides up its own curve to a new operating point at higher pressure and, crucially, lower volume flow. In plain terms: the dirtier the duct, the less air moves, even though the fan is working harder. Capture over the canopy weakens, the kitchen gets hotter, and the grease that is choking the flow is the same grease that TR19 identifies as a fire risk once it exceeds an average thickness of 500 µm. It is a vicious circle, and it is why a thorough clean so often produces an immediate, noticeable improvement in draw - you are physically lowering the resistance the fan has to fight.
The same logic applies to anything you add into the airstream. Grease filters, fire dampers and secondary treatment such as a UV or ozone unit or an electrostatic pollution control unit all add resistance and therefore raise static pressure. They can be well worth it for odour and grease control at source, but they have to be accounted for in the design and kept clean, or they steadily erode the volume flow the canopy depends on. If you are weighing up that kind of kit, it is worth reading our take on whether UV and ozone canopy systems are worth it before you commit.
This is also why extraction performance sits squarely within the remit of the people who inspect kitchens. Environmental health officers and insurers treat DW172 as the benchmark for a properly specified system, and a system that cannot demonstrate adequate airflow or a maintained cleaning regime is a compliance problem as much as an engineering one. If you are unsure what an inspector can actually require of you, our guide to what an EHO is and what they can do sets out where you stand. The practical upshot is simple: measured airflow and regular, verified cleaning are not optional extras - they are what keep the two numbers where they belong.
Questions
Volume flow is the amount of air actually moving through the system, usually in cubic metres or litres per second - it is what removes heat, steam and grease from over your appliances. Static pressure is the resistance, in pascals, that the fan has to overcome to move that air. A fan can generate high pressure while still delivering low flow, which is why the two must always be considered together rather than judged by how loud the fan sounds.
BESA's DW172 specification typically calls for around 6 to 8 m/s in extract main duct runs, with branches and spigots a little lower at roughly 5 to 7 m/s, and supply air slower at 3 to 5 m/s. Below about 2.5 m/s, air can no longer keep grease vapour entrained and it settles out inside the duct. Too fast, and you waste fan energy, create noise and drive static pressure up unnecessarily.
This is usually grease build-up raising the system resistance. As deposits narrow and roughen the ductwork, the fan is forced to a new operating point at higher static pressure and lower volume flow - so it works harder but shifts less air. A thorough, TR19-compliant duct clean lowers that resistance and typically restores a noticeable amount of airflow straight away.
Yes, significantly. DW172 requires replacement air of no less than 75% and no more than 95% of the volume you extract. If you pull air out faster than you let it back in, the kitchen goes into negative pressure - doors become hard to open, volume flow collapses and combustion appliances can be starved of air. Extraction only works properly when it is balanced by adequate make-up air.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.