Comparing Different Flow Control Devices for Mechanical Draft Fans
Mechanical draft fans are integral parts of a plant’s operations. They are essential to moving large amounts of air and gases through a system duct and components, but they use a large amount of power and can result in serious inefficiencies if not properly designed and managed.
One factor that plays a role in the efficiency of mechanical draft fans is the type of flow control device used in the system. Flow control devices are crucial because they have a direct effect on power savings in terms of how effectively these devices can control flow and pressure through the fan.
Here, we’ll compare several common flow control devices used in air handling solutions in order to better understand their effects on mechanical draft fan performance and efficiency.
Inlet Damper Controls
Inlet damper controls are used to adjust flow to meet fluctuating load demand. This involves using damper controls for throttling or restricting flow at the fan inlet and outlet with inlet and outlet dampers. While common, these throttling devices can be inefficient and waste energy.
Inlet damper controls can be categorized into two types:
Inlet box multi-louver parallel blade (MLPB) damper controls
Inlet variable guide vane (VIV) controls
VIVs – also known as vortex dampers – are typically located in the fan inlet cones. They reduce net effective flow area in the cone with a defined profile that increases the gas flow velocity, resulting in additional resistance.
By contrast, MLPB dampers are located in the fan inlet box with only their blades coming in direct contact with the gas flow. Minimum resistance is experienced when the MLPB blades are positioned in the full open position.
A MLPB damper is typically less expensive than a VIV counterpart for the same size fan. Installation of an MLPB also costs less since some VIVs have to be placed around the shaft and lowered into place.
The required torque for an MLPB damper is usually lower than a comparable VIV, which makes the actuator less expensive.
Moreover, all of the bearings and linkage assemblies for the MLPB dampers are outside of the duct so they can be easily accessed for maintenance and replacement. Inspection, maintenance, and replacement of these components can be performed while the fan is operating.
VIV dampers, on the other hand, are often placed inside the fan inlet cones. Access to linkages and bearings is through the fan casing only. This means that VIV components are exposed to the dirty gases, elevated operating temperatures, and high gas velocities, and that the inspection, maintenance and replacement of VIV components require the fan to be shut down, causing expensive downtime.
Many consider VIVs worth all this extra hassle because they are generally understood to be more efficient than the MLPB dampers. The main reason for this is because a VIV damper is placed in the inlet cone closer to the eye of the wheel, whereas an MLPB damper is further away.
By placing a damper in the eye, greater pre-spin can be induced, thus allowing airflow to enter the eye of the wheel more efficiently.
However, on a lot of large industrial fans where there are large volumes and pressures, secondary vortex shedding may occur. To offset this, dorsal fins are added to the vortex dampers to break up the pre-spin effect, reducing the efficiency factor of the pre-spin.
Inlet box dampers may be subject to stall conditions at maximum turndown. To offset this, anti-spin baffles are placed in the inlet box.
Variable Speed/Frequency Drives
All restricting type flow control devices waste energy to varying degrees. Fortunately, there is a more efficient option: variable speed/frequency drives (VSDs/VFDs).
VSDs and VFDs match driver speed to load demands and thus improve operating efficiency dramatically.
Because many centrifugal machines operate at a reduced load for an extended period of time, significant energy savings can be realized by reducing their operating speeds.
Speed control can be accomplished by utilizing a variable speed, adjustable frequency drive and steam turbine drive, with a hydraulic coupling and a two speed motor. The selection of each speed control option will depend on project specifics.
Often, the speed control has limitations in meeting load demands that are not within desired specifications. Also, with speed control, instability is a concern. Instability can cause serious aerodynamic issues.
In order to mitigate these issues and to establish the highest degree of control, a combination of VSD/VFD and dampers is often recommended.
In most applications, variable speed/frequency drive controls are the most efficient method to control process flow.
Advantages and Disadvantages of Each System
Each system – dampers or speed/frequency controls – have advantages and disadvantages.
Advantages of Dampers
- Very cost effective when you only care about purchase price
- No impact on the floor space
- Do not require foundation structure
Disadvantages of Dampers
- Very inefficient method of controlling air and gas flow
- Electric motor power factor will vary with the load when the damper is used
- Require more mechanical maintenance because they have more moving parts
Advantages of VSD/VFD Controls
- Quicker payback due to significant energy savings
- Will result in the reduction of the motor purchase price because of reduced inertia
- Terminal power factor will be near unity regardless of fan load
- Have better total cost of ownership
Disadvantages of VSD/VFD Controls
- Purchase price is higher
- Take up more floor space in electrical control rooms
- Steam turbine drive and hydraulic coupling require foundation structure
Conclusion
Understanding flow control concepts for mechanical draft fans will undoubtedly help plants and engineers operate their fans at the maximum efficiency rating attainable for their process needs. In doing so, significant energy savings can be realized.
A draft fan operating at its best efficiency point will exhibit superior fluid, acoustic and structural performance, reduce housekeeping costs and maximize return on investment.
ProcessBarron has decades of experience constructing mechanical draft fan systems with both types of controls, in addition to engineering custom air handling solutions for facilities. Contact the team for more information or to consult with our professionals about your next project.