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Kitchen extraction cleaning - airflow basics
Two terms describe how an extraction system moves air: volume flow (how much air it moves) and static pressure (the resistance the fan has to work against). They matter because grease build-up changes both - raising the resistance and cutting the flow - which is exactly why a grease-choked system extracts poorly, and why cleaning restores its performance. Here is what these terms mean and why they matter for cleaning. This is general commentary on airflow.
The short answer
You'll hear extraction and ventilation people talk about 'static pressure' and 'volume flow' - and while they sound technical, they explain something very practical: why grease chokes a system, and why cleaning fixes it. Here's what these two terms mean and why they matter for your extraction cleaning. This is general commentary on airflow. Volume flow: how much air moves. Volume flow (or volume flow rate) is how much air the extraction system moves - the volume of air per unit time (for instance, cubic metres per second). It's the actual extraction the system is achieving: the air being drawn through the canopy, filters and ductwork and out. Volume flow is what does the work - it's the moving air that captures the smoke, steam, grease and heat at the canopy and carries it away. So more volume flow means stronger extraction (better capture); less means weaker extraction. It's essentially 'how much is the system actually extracting?' Static pressure: the resistance to overcome. Static pressure is the resistance the fan has to work against to move the air - the pressure the fan must develop to push and pull the air through the filters, ductwork, bends and fittings (all of which resist the airflow). A system with more resistance (more or tighter ductwork, more restrictive filters, or - crucially - grease narrowing the path) requires more static pressure to move a given amount of air. So static pressure is essentially 'how hard does the fan have to work against the system's resistance?' How they're linked. The two are connected through the fan. A fan can move a certain volume of air against a certain resistance - and there's a trade-off: if the resistance (the static pressure required) rises, the volume flow the fan achieves falls (the fan can't move as much air against the greater resistance). So higher resistance means lower flow, for a given fan. This is the key relationship. Why grease matters. Here's why this matters for cleaning: grease build-up directly worsens both. As grease accumulates in the filters, ductwork and fan, it narrows the airflow path and roughens the surfaces - which increases the resistance (so the static pressure needed to move the air rises). And because the fan is now working against more resistance, the volume flow it achieves drops (less air moved). So a grease-choked system has higher resistance and lower flow. This is exactly, in airflow terms, why a grease-laden system extracts poorly: the volume flow has dropped (weaker extraction), because grease has raised the resistance. The weak capture, the smoke and heat escaping the canopy - that's the reduced volume flow. Why cleaning restores it. And this is why cleaning works. Removing the grease clears the airflow path and smooths the surfaces, lowering the resistance (the static pressure needed drops back). With less resistance, the fan can move air more freely again, so the volume flow recovers - restoring the extraction performance (capture restored, smoke and heat drawn away again). So cleaning literally reduces the resistance and restores the flow. That's the mechanism behind 'cleaning restores the airflow'. Measuring them. These aren't just concepts - they're measurable, and measuring them is part of how systems are assessed and (in LEV extraction) tested: static pressure readings and airflow measurement are used to check a system's performance and can reveal problems like grease build-up (as covered in the static-pressure-readings page). A rising static pressure or falling flow over time signals increasing resistance - often grease. The takeaway. So volume flow is how much air the system moves (the extraction it's achieving), static pressure is the resistance the fan works against, and they're linked (higher resistance, lower flow). Grease build-up raises the resistance and cuts the flow - which is why a choked system extracts poorly - and cleaning removes the grease, lowering the resistance and restoring the flow. So these two terms explain, in airflow terms, why cleaning your extraction restores its performance. So grease raises pressure and cuts flow; cleaning reverses it. This is general commentary on airflow.
Key points
Volume flow
Volume flow (or volume flow rate) is how much air the extraction system moves - the volume of air per unit time (measured, for instance, in cubic metres per second). It's the actual extraction the system is achieving: the air being drawn through the canopy, filters and ductwork and out of the building. Volume flow is what does the work - it's the moving air that captures the smoke, steam, grease and heat at the canopy and carries it away.
So volume flow is essentially the answer to 'how much is the system actually extracting?' More volume flow means stronger extraction (more air moved, better capture at the canopy); less volume flow means weaker extraction (less air moved, poorer capture - smoke and heat escaping). So when an extraction is performing well, it's moving a good volume of air; when it's performing poorly (a choked system), the volume flow has dropped. Volume flow is therefore the measure of extraction performance that matters most practically - it's the extraction. Understanding it is half the picture; the other half is the resistance the fan works against to achieve it (static pressure), covered next. So volume flow - how much air the system moves - is the extraction the system delivers. So it's the amount of air actually being extracted. This is general commentary on airflow.
Static pressure
Static pressure is the resistance the fan has to work against to move the air - the pressure the fan must develop to push and pull the air through the system's filters, ductwork, bends and fittings, all of which resist the airflow. Every part of the system offers some resistance to the air passing through it, and the fan has to overcome that resistance to move air; the static pressure is a measure of that resistance (and the fan's effort against it).
A system with more resistance requires more static pressure to move a given amount of air. More or tighter ductwork, more bends, more restrictive filters - and, crucially for cleaning, grease narrowing the airflow path - all increase the resistance, so the fan has to work against a higher static pressure to move the air. So static pressure is essentially 'how hard does the fan have to work against the system's resistance?' A clean, open system has lower resistance (lower static pressure needed); a choked, restricted system has higher resistance (higher static pressure needed). Static pressure and volume flow are linked through the fan (covered next), and it's that link - and how grease affects it - that explains why cleaning matters. So static pressure - the resistance the fan works against - is the counterpart to volume flow: the effort needed to achieve the flow. So it's the resistance the fan pushes against. This is general commentary on airflow.
How they're linked
Volume flow and static pressure are connected through the fan, and the key relationship is a trade-off: for a given fan, if the resistance (the static pressure required) rises, the volume flow the fan achieves falls. A fan can move a certain volume of air against a certain resistance - but as the resistance increases, the fan can't move as much air against it, so the flow it delivers drops. Conversely, lower resistance lets the fan move more air (higher flow).
So the two are inversely related through the fan: higher resistance means lower flow; lower resistance means higher flow. This is the crucial relationship for understanding extraction performance and cleaning. It means the volume flow a system achieves (its extraction) depends not just on the fan, but on the resistance the fan is working against - and anything that raises the resistance cuts the flow. This is exactly where grease comes in (covered next): grease raises the resistance, so it cuts the flow. So how they're linked - higher resistance, lower flow - is the mechanism that connects the state of the system (its resistance) to its performance (its flow): choke the system (raise resistance) and the extraction (flow) drops. So resistance and flow trade off through the fan. So more resistance means less air moved. This is general commentary on airflow.
Why grease matters
Here's why static pressure and volume flow matter for cleaning: grease build-up directly worsens both. As grease accumulates in the filters, ductwork and fan, it narrows the airflow path and roughens the surfaces - which increases the resistance (so the static pressure needed to move the air rises). And because the fan is now working against more resistance, the volume flow it achieves drops (less air moved). So a grease-choked system has higher resistance and lower flow.
This is exactly, in airflow terms, why a grease-laden system extracts poorly: the volume flow has dropped (weaker extraction), because grease has raised the resistance the fan works against. The symptoms you see - weak capture at the canopy, smoke and heat escaping into the kitchen, the extraction 'not pulling' - are the reduced volume flow, caused by the grease-raised resistance. So grease doesn't just sit there; it actively chokes the airflow by raising the resistance and cutting the flow. This is the airflow explanation of the grease problem (matching the practical symptoms covered in the airflow-dropping-off and reduced-suction pages). So why grease matters - grease raises resistance and cuts flow - is the heart of it: the build-up increases the static pressure and reduces the volume flow, which is why a choked system performs badly. So grease chokes the system by raising resistance and cutting flow. This is general commentary on airflow.
Why cleaning restores it
And this is why cleaning works, in airflow terms: removing the grease clears the airflow path and smooths the surfaces, lowering the resistance (the static pressure needed drops back). With less resistance, the fan can move air more freely again, so the volume flow recovers - restoring the extraction performance (capture restored, smoke and heat drawn away again). So cleaning literally reduces the resistance and restores the flow - that's the mechanism behind 'cleaning restores the airflow'.
This is worth understanding because it shows cleaning isn't just cosmetic grease removal - it directly restores the system's airflow performance by removing the resistance grease had added. A clean system has low resistance and full flow (good extraction); a choked one has high resistance and reduced flow (poor extraction); and cleaning moves you from the second back to the first. It's also why these quantities are measured to assess systems: static pressure readings and airflow measurement can reveal grease build-up (a rising static pressure or falling flow over time signals increasing resistance - often grease), and are part of how systems are checked and, in LEV extraction, tested (as covered in the static-pressure-readings page). So why cleaning restores it - clear the grease, recover the flow - completes the picture: grease raises resistance and cuts flow, and cleaning reverses both, restoring performance. So cleaning lowers the resistance and brings the extraction back. This is general commentary on airflow.
Questions
It's how much air the system moves - the volume per unit time, the actual extraction being achieved; more volume flow means stronger extraction and better capture, less means weaker. Volume flow (or volume flow rate) is how much air the extraction system moves - the volume of air per unit time (for instance, cubic metres per second). It's the actual extraction the system is achieving: the air drawn through the canopy, filters and ductwork and out. Volume flow is what does the work - the moving air that captures the smoke, steam, grease and heat at the canopy and carries it away. So more volume flow means stronger extraction (better capture at the canopy); less means weaker extraction (poorer capture, smoke and heat escaping). It's essentially the answer to 'how much is the system actually extracting?' When an extraction performs well it's moving a good volume of air; when it performs poorly (a choked system) the volume flow has dropped. So it's the amount of air the system extracts. So how much air the system is moving. This is general commentary on airflow.
It's the resistance the fan has to work against to move the air - the pressure needed to push and pull the air through the filters, ductwork and fittings; more resistance (including from grease) needs more static pressure. Static pressure is the resistance the fan has to work against to move the air - the pressure the fan must develop to push and pull the air through the system's filters, ductwork, bends and fittings, all of which resist the airflow. Every part offers some resistance, and the fan has to overcome it to move air; static pressure measures that resistance and the fan's effort against it. A system with more resistance (more or tighter ductwork, more bends, restrictive filters, or grease narrowing the path) requires more static pressure to move a given amount of air. So it's essentially 'how hard does the fan have to work against the system's resistance?' A clean, open system has lower resistance (lower static pressure); a choked, grease-restricted system has higher resistance (higher static pressure). So it's the resistance the fan pushes against to move the air. So the resistance the fan works against. This is general commentary on airflow.
Through the fan, as a trade-off: for a given fan, if the resistance (static pressure required) rises, the volume flow it achieves falls - higher resistance means less air moved, and lower resistance means more. Volume flow and static pressure are linked through the fan, and the key relationship is a trade-off. A fan can move a certain volume of air against a certain resistance - but if the resistance (the static pressure required) rises, the volume flow the fan achieves falls (it can't move as much air against the greater resistance); and if the resistance drops, the flow rises. So they're inversely related through the fan: higher resistance means lower flow, lower resistance means higher flow. This matters because it means the volume flow a system achieves (its extraction) depends on the resistance the fan is working against - so anything that raises the resistance (like grease) cuts the flow (the extraction). This is the mechanism connecting the state of the system to its performance. So higher resistance means lower flow, through the fan. So they trade off - more resistance, less flow. This is general commentary on airflow.
Grease build-up narrows the airflow path and roughens surfaces, raising the resistance (so static pressure rises), and because the fan works against more resistance, the volume flow drops - so a choked system has higher pressure and lower flow, extracting poorly. Grease build-up directly worsens both. As grease accumulates in the filters, ductwork and fan, it narrows the airflow path and roughens the surfaces - increasing the resistance, so the static pressure needed to move the air rises. And because the fan is now working against more resistance, the volume flow it achieves drops (less air moved). So a grease-choked system has higher resistance (static pressure) and lower flow. This is exactly, in airflow terms, why a grease-laden system extracts poorly: the volume flow has dropped (weaker extraction) because grease raised the resistance. The symptoms - weak capture, smoke and heat escaping the canopy - are that reduced flow. So grease actively chokes the airflow by raising the resistance and cutting the flow. This is the airflow explanation of the grease problem. So grease raises the resistance and cuts the flow. So it raises pressure and lowers flow. This is general commentary on airflow.
Because removing the grease clears the airflow path and lowers the resistance, so the fan can move air freely again and the volume flow recovers - cleaning literally reduces the resistance grease had added and restores the flow. Cleaning restores extraction performance by reversing what grease did to the airflow. Grease build-up raises the resistance (static pressure) and cuts the volume flow, so a choked system extracts poorly. Removing the grease clears the airflow path and smooths the surfaces, lowering the resistance (the static pressure needed drops back); with less resistance, the fan can move air more freely again, so the volume flow recovers - restoring the extraction (capture restored, smoke and heat drawn away). So cleaning literally reduces the resistance and restores the flow - that's the mechanism behind 'cleaning restores the airflow'. It shows cleaning isn't just cosmetic grease removal but a direct restoration of the system's airflow performance: from high resistance and low flow (choked) back to low resistance and full flow (clean). So cleaning removes the resistance grease added, and the flow comes back. So because it lowers the resistance and recovers the flow. This is general commentary on airflow.
Yes - a rising static pressure or falling volume flow over time signals increasing resistance, often from grease; these are measured to assess systems and (in LEV extraction) as part of testing, and can reveal grease build-up. Yes - because grease raises the static pressure (resistance) and cuts the volume flow, measuring these can reveal grease build-up. A rising static pressure or a falling volume flow over time signals increasing resistance in the system - and a common cause of increasing resistance is grease accumulating and narrowing the airflow path. So these measurements act as indicators of the system's condition: a system whose static pressure has risen or whose flow has dropped is likely choking up with grease and due a clean. Measuring static pressure (static pressure readings) and airflow is part of how systems are assessed, and in LEV (local exhaust ventilation) extraction it's part of the required testing (as covered in the static-pressure-readings page). So these quantities aren't just concepts - they're measurable indicators that can flag when a system needs cleaning. So yes - rising pressure or falling flow signals grease and a needed clean. So yes; they can indicate grease build-up. This is general commentary on airflow.
Grease raises the resistance in your system and cuts the airflow; we clean the whole system to TR19 Grease, clearing the grease that chokes the flow - restoring the volume of air the system moves and the extraction performance. Ask us to clean your system and bring its airflow back. This is general commentary on airflow.
