The catalog says 1,200 m³/h, why is it different in the field? Fan Curve Reading Guide.

Flow rate isn't a single number, it's a curve, and those who don't know how to read that curve will buy the wrong unit.

It's a scene we often encounter: The customer installs the unit, holds an anemometer to the suction inlet, does the calculations, and comes back to us: "The catalog said 1,200 m³/h, but I'm measuring just over 700. Is the machine faulty?"

The machine isn't faulty. The catalog isn't lying either. The problem is the lack of understanding of what that 1,200 figure means (or rather, what it doesn't mean). Think of it like a car catalog stating "maximum speed 210 km/h." That speed is achieved on a flat, empty test track. If you can't reach 210 km/h going uphill with the same car, towing a trailer, the car isn't faulty; the conditions are different.

Similarly, the highest flow rate figure in the fan catalog is usually the "empty track" value: the flow rate achieved in free blowing with no resistance in front of or behind the fan. In a real unit, however, the fan never goes on an empty track.

The Anatomy of the Curve

Every fan has a characteristic curve.

The horizontal axis represents the flow rate, and the vertical axis represents the pressure the fan can generate. The two ends of the curve show two extreme conditions.

• The bottom right shows the free blowing point. Pressure is zero, flow rate is highest.

• The top left shows the opposite. The highest pressure the fan can generate as the flow rate approaches zero.

While operating, the fan is somewhere on this curve. The fan alone cannot decide where it will be.

The decision-making partner is the system itself: the suction arm, hose, elbows, hood, and most importantly, the filter. Each of these resists the passage of air, and the sum of these resistances forms a curve. As the flow rate increases, the resistance increases proportionally to the square of the flow rate.

The point where the fan curve intersects the system resistance curve is the unit's actual operating point. The unit will not give the maximum flow rate listed in the catalog, nor a random value; it will give exactly the flow rate indicated by that intersection. Therefore, there is no single answer to the question, "How many cubic meters does this fan draw?" The correct question is: "How many cubic meters does this fan draw in the total resistance of my system?"

Example fan curve. Flow rate of 3500 m³/h at 500 Pascal at sea level.

The part that changes over time: Filter clogging

Now let's make things a step more difficult. System resistance is not constant either. When the filter is clean, the resistance is low, and the operating point is on the right side of the curve, at high flow rates. As the filter gets loaded, the resistance increases, the system curve steepens, and the operating point shifts to the left (i.e., to lower flow rates) on the curve. If the unit performs perfectly on the first day but feels "weakened" after six months, this is often what happens.

A good unit design takes this shift into account from the start.

The fan is selected not according to the clean state of the filter, but according to its dirtiest acceptable state. This is also why ISO 21904 requires a minimum flow rate warning. When the flow rate falls below the critical threshold, the unit still appears to be working from the outside, but it is no longer capturing the smoke. The warning system makes this invisible shift visible.

We manufacture in Ankara: Why is altitude important?

There's also a detail often glossed over in small print in most catalogs: Fan curves are given according to standard air density at sea level.

However, air becomes less dense as altitude increases. Ankara is at approximately 900 meters altitude, and the air density here is roughly ten percent lower than at sea level. The fan continues to pump the same volumetric amount of air; but with less dense air, the pressure it produces and the motor power it draws decrease proportionally with the density.

In practice, this means that a fan selected as "perfectly adequate" according to the sea-level catalog may fall below the limit in high-altitude cities.

In our calculations, we apply altitude correction as a standard step. It's a seemingly small detail; but a ten percent pressure loss can significantly shift the operating point on the curve.

Three questions to ask before buying

To reduce all this to a single practical recommendation: ask your supplier for the complete fan curve, not just a single flow rate figure. Ask at what static pressure the flow rate mentioned in the catalog is valid. If the answer is "free blowing," know that you won't see that figure in your own system. And confirm in writing whether the unit still provides the minimum flow rate with your arm and hose configuration under dirty filter conditions. These three questions will eliminate a large portion of the disappointments you might experience in the field, right at the bidding stage.

So where does that "minimum flow rate" threshold come from? How many cubic meters of air must pass through the suction nozzle for the suction arm to actually capture the smoke, and how does the arm's position disrupt this calculation? That will be the subject of our next article.