Finding the Best Flow Control Method for Mechanical Draft Fans

flow control

When it comes to controlling flow to meet fluctuating load demand in a mechanical draft fan system, there are two general types: inlet damper controls and variable speed/frequency drives.

Each method has its own advantages and disadvantages. Here, we’ll talk about the two general categories of flow control methods for draft fans and why you would want to use either option.

Inlet Damper Controls

One method by which flow control is achieved is through inlet damper controls.

Inlet damper controls can be broken down into two categories:

  1. Inlet box multi-louver parallel blade (MLPB)
  2. Inlet variable guide vane (VIV)

One key difference is placement. VIVs are placed in the fan inlet cones themselves, while MLPBs are located in the fan inlet box; only the blades themselves come in contact with the gas flow.

There are several advantages that MLPB dampers have over VIV dampers. MLPB dampers are less expensive than VIV dampers, with a less expensive installation. They also require lower torque, resulting in lower costs for the actuator. Maintenance is also easier for MLPB dampers because VIV components are more difficult to get to. Additionally, you can inspect, maintain, and replace components on a MLPB damper without having to shut down the fan. That’s not the case with VIV dampers.

With all of these disadvantages, why are VIVs still preferred in many cases? Most operators believe that VIVs are more efficient, owing to their location (on the inlet cone near the eye of the wheel) versus the placement of the MLPB damper. Since VIVs are closer to the eye of the wheel, you can generate greater pre-spin, which increases the efficiency with which the airflow can enter the eye.

Variable Speed/Frequency Drives

Restricting-type flow control devices like dampers can waste energy and lower efficiency. To increase efficiency, there are additional options in the form of variable speed/frequency drives (VSDs/VFDs).

VSDs and VFDs match driver speed to load demands and improve operating efficiency to a significant degree. This is possible due to the relationship between driver speed and volume, pressure, and power. As load is reduced for an extended period of time, the operating speed can be reduced to save energy.

VSDs and VFDs are more cost-effective than damper controls because they have quicker payback due to higher energy savings. They also have greater value in the long run and result in a lower total cost of ownership because of higher efficiency.

If you’re only concerned about initial purchase price, dampers are the most cost-effective. They save space as well, and don’t require foundation structure. They do require more maintenance than VSDs/VFDs.