Pitfalls #7 – Part-load operation: damper, inlet vanes or variable speed drive
A fan enquiry usually gives us an airflow and a pressure. It rarely tells us how long the installation actually runs at that point. On a central dust extraction system, a furnace with variable rate or a line with frequent changeovers, the fan spends a large part of its time at part load. The question is then how that airflow is reduced.
The energy comparison of the three control methods, with the annual balance and the payback time, is covered in the article « Setting the operating point with a variable speed drive ». This article complements it: it applies the same example to a real selection, then covers stability, tightness, response time and motor requirements.
The same example, on a real selection
A Ø1120 fan at 1487 rpm, fitted with inlet vane control and sized for 65,000 m³/h at 3,900 Pa and 20 °C, absorbs 93.6 kW. At 50,000 m³/h, the system only requires 2,300 Pa.
| Control method | Absorbed power at 50,000 m³/h |
|---|---|
| Damper | about 82 kW |
| Inlet vanes about 50 % open | about 59 kW |
| Variable speed drive, speed reduced to 1144 rpm | 42.5 kW |
If the installation runs 40 % of its 8,000 annual hours at part load, the difference is about 126,000 kWh per year between damper and drive, and about 52,000 kWh between inlet vanes and drive, to be multiplied by your price per kWh.
The orders of magnitude from the variable speed drive article are confirmed. The choice of control method also depends on the following criteria.
Stability: the point that is often overlooked
A fan with good efficiency often has a curve that first rises, then falls. On the rising part, operation is unstable: airflow oscillates, noise becomes pulsating, pressure gauge and ammeter needles swing, and ducts and supports take vibration. This is surge.
Closing a damper moves the operating point towards that zone. In our example it starts around 30,000 m³/h, less than half the nominal airflow: control at 50,000 m³/h stays stable, but an installation throttled further with a damper gets close to it. Inlet vanes, as they close, remove the rising part of the curve. A variable speed drive moves the operating point along the system curve, away from that zone.
Limits of the variable speed drive and of inlet vanes
- Impeller inertia limits the response time. For a process that needs very fast airflow changes, such as glass tempering, inlet vanes remain the better choice.
- The motor must be suitable: cooling at low speed, insulation, bearing protection against shaft currents.
- Even fully open, inlet vanes add a pressure drop, about 140 Pa here, paid permanently at full airflow. They are not tight either: fully closed, they still let about 13,000 m³/h through in our example. If the system must be isolated, a separate isolation damper is needed.
What we need from you to specify correctly
The maximum airflow, and also the operating profile: which airflows, for how long, how fast they change, and whether the system must be isolated. With this data, we specify the fan and its control device together, and check that every operating point stays in the stable zone.
Key point
For a fan that mostly runs at part load, the nominal point is not enough. The real operating profile determines the control method, and that choice is made together with the fan selection.
An installation running permanently with a half-closed damper? Send us the nominal point and the operating profile, and we will look at what it represents.