"Can You Slow Down the Fan and Still Move the Dust?" — How Ecogate Maintains Minimum Transport Velocity While Cutting Energy Costs by 53%
- Mar 11
- 14 min read
A common concern we often encounter is whether Ecogate can reduce the main duct air velocity while still upholding the minimum required transport air velocity. Some people believe this to be impossible. We would like to explain how it works.
The Ecogate on-demand dust collection system operates using gates at individual workstations. Sensors continually monitor machine activity and automatically close the corresponding gate (stopping dust collection) when a machine is idle. To guarantee that minimum transport air velocities are maintained throughout the system, the greenBOX control system can open gates at other, non-active machines if necessary. The system's Power Master VFD adjusts the fan speed based on commands received from the greenBOX control unit.
Design Drop Air Velocities Versus Duct System Air Velocities
This explanation focuses on air velocities in main ducts and branch lines, not the air velocity in drops. Drop air velocities are non-negotiable; they must always remain at the "design air velocity" and thus at the "design air volume," meaning they cannot be modulated.
Design and Minimum Transport Air Velocity In Duct System
Consider a typical dust collection system where the main duct is designed for an air velocity of 4,500 FPM. The minimum transport air velocity required to convey the material in the duct system is for example 3,500 FPM ( based on guidelines in "Industrial Ventilation: A Manual of Recommended Practice for Design"). Since the minimum transport air velocity is 3,500 FPM and the main duct is sized for 4,500 FPM, Ecogate can effectively modulate the air volume within this range. These air velocity figures are common for most dust collection systems.
Ecogate operates in the Safe Operating Range
How High is the Average Active Cutting Utilization of Workstations?
Typical dust collection systems commonly see an average active (cutting) utilization of drops at approximately 50%. This percentage can fluctuate based on factors like shift timing, differences between shifts, and the specific industry. While some highly automated facilities—such as those operated by Ikea, Sauder Woodworking, or Andersen Windows—may achieve higher utilization rates, the general average for active cutting utilization remains around 50%. It is not necessary to run dust collection to the workstations that are not producing dust, so these drops can be closed by Ecogate gates.
Report of real factory data reveals typical workstation utilization: the most-used machines operate about 75% of the time, while the least-used machines are as low as 5%. With an average utilization of approximately 50% per shift, an Ecogate system could eliminate 50% of unnecessary dust collection.
How Many Gates Can We Close And Still Maintain Minimum Transport Air Velocity in The Duct System?
Closing 50% of the dust gates on non-operational machines is not feasible because it would reduce the main duct air velocity by half (e.g., from 4,500 FPM to 2,250 FPM), which is below the minimum transport air velocity required to move dust.
To determine how many gates can be closed while still maintaining the minimum transport air velocity (e.g., 3,500 FPM), we calculate the ratio: 3,500 FPM / 4,500 FPM = 0.77 or 77%. This means 77% of the gates must remain open, allowing us to close a maximum of 33% of the gates*1.
Therefore, if the system is operating with an average active (cutting) utilization of 50%, the Ecogate control system (called greenBOX) must open additional gates. Specifically, since 77% of gates must be open, and only 50% are active, the system must open an extra 27% (77% - 50%) of gates at inactive workstations*2. This ensures the required air volume is maintained to achieve the minimum transport air velocity. This process of opening additional gates at non-active workstations until the minimum transport air velocity is reached is what the Ecogate system accomplishes.
What Electricity Saving Can We Achieve At Minimum Transport Air Velocity In the Duct System?
Can electricity savings be achieved by reducing the fan speed to ventilate 77% of the air volume instead of 100%?
The answer is yes, as demonstrated by the Fan Laws (Affinity Law). The power consumed by the fan changes with the cube of the air volume. Therefore:
77% Air Volume Power = 100% Air Volume Power * (0.77)^3 ≈ 47%
This means that reducing the fan air volume to 77% of the original volume cuts power consumption to 47%. This translates to 53% electricity savings - all while still maintaining the minimum transport air velocity as required.
Considering today's escalating electricity costs, this 53% reduction in fan operating cost is a significant result. In fact, the average electricity saving on installed Ecogate systems is typically even higher: 66% savings on fan operating costs*3.
What About If an Existing System Has Duct System Air Velocities Below 4,500 FPM?
If air velocities in the duct system are measured to be below 4,500 FPM, we will still calculate electricity savings based on our standard principle. However, the resulting savings will be lower, potentially extending the return on investment (ROI) to a point where the Ecogate system may not be justified.
In these situations, we recommend modifying parts of the duct system, often just replacing the main duct with a smaller diameter. This simple modification can significantly increase achievable savings and shorten the ROI period. Furthermore, power company incentives may cover the cost of this modification, further reducing the return on investment time.
How Are You Designing New Duct Diameters for Dust Collecting With an Ecogate System?
Duct sizing depends on the expected workstation utilization. If 100% utilization is anticipated, the duct diameters will be the same as in traditional dust collection systems.
However, if utilization will not reach 100% (with a typical average around 50%), the ducts can be slightly smaller*4. The system should be designed to achieve ideal duct air velocities at the average operating point (i.e., average utilization).
This ensures:
Air velocities are safely above minimum transport air velocities.
High electricity savings are achieved.
While the system will occasionally run at higher utilization—resulting in higher duct air velocities and increased pressure losses—this is acceptable because the percentage of time spent at high utilization is low, minimizing the impact on overall electricity consumption. It is crucial, however, to select a fan capable of generating sufficient static pressure and air volume to handle these high-utilization operating points.
Benefits of Ecogate On-Demand Ventilation Systems
Ecogate's on-demand system offers numerous benefits beyond just electricity savings on fan operation:
Flexibility and Adjustability: Unlike legacy systems that require redesign when workstations are added, removed, or relocated, the Ecogate system's performance can be easily adjusted to accommodate changes in the system set up (all done in minutes).
Noise Reduction: By reducing the air rushing to all open outlets, the system significantly lowers noise levels. This is a key benefit for environments like schools and contributes to a more pleasant working environment everywhere. Noise is also reduced at fans - often important for your neighborhood relationship.
Fully Automated Operation: The system eliminates the need for manual starting and stopping via a central switch before and after shifts. Ecogate automatically starts and stops based on workstation use. It reduces wear & tear, and further reduces electricity consumption.
Comprehensive Data Collection: Ecogate provides comprehensive data collection for the entire ventilation system, which goes beyond the limited dust collector data tracked by others. It allows factory management to make better-informed decisions, for example gathering workstation utilization data is crucial. Additionally, reporting drop air velocities verifies that the system is operating correctly.
The fundamentals above explain what the Ecogate system does. What follows is how it does it — the fan curve mechanics, pressure compensation physics, and real-time measurement and regulation that make this possible. If you're evaluating this system technically or fielding engineering objections, this is the section that answers them.
How Can an Ecogate System Maintain Fan Static Pressure While Slowing the Fan Down?
For some industrial ventilation systems, a basic explanation might suffice, but engineers with advanced knowledge often require a deeper understanding. A common counter-argument to reducing fan speed is based on the relationship between fan RPM, power consumption, and static pressure. While decreasing fan RPM significantly reduces power consumption (by the cube function), it also lowers the fan's static pressure (by quadratic function). The fan's static pressure must be sufficient to overcome all system pressure losses, including those from hoods, the duct system, abort gates, filters, and return air.
New System Curve With Some Gates Closed, and New Fan Curve with Reduced Fan RPM
When gates are closed in a ventilation system, the "system curve" (which represents the pressure the fan must generate for a given system air volume) shifts. This change is illustrated in the simplified chart below, showing the system curve moving to the left as gates close. Concurrently, the fan curves move down as the fan's RPM is reduced.
Conventional and Ecogate Operating Points:
Conventional System (#1): The fan runs at 100% speed with no gates installed (or all gates in open position). This is the typical operating point of conventional ventilation systems.
Gated System with Reduced Flow (#2): With gates installed and, for example, 30% of them closed, the new system curve intersects the fan curve at point #2 if the fan speed is not reduced.
Ecogate System with Reduced Fan Speed (#3): The fan's speed is reduced at the same time, establishing a new operating point at #3.
A key principle of fan operation is that it generates higher pressure at lower air volumes, because the total fan power is distributed between generating pressure and moving air. As a result, at the new operating point #3, the fan pressure is approximately the same as it was at the conventional operating point #1. This first mechanism that compensates for reduced fan pressure at lower speed.
Fan Curves and System Curves
Reduced System Air Volume Also Reduces System Static Pressure Losses
The system incorporates a second "Pressure Compensating Mechanism." This mechanism is based on the principle that a 33% reduction in air volume also lowers all pressure losses in the duct system, at the abort gates, at the dust collector filters, and in the return air (approximately by a quadratic function).
The combined effect of these two static pressure compensations is that, given the new system and fan curves and the reduced static pressure losses from the lower air volume, the fan generates the correct static pressure needed to move the reduced system air volume.
What About Real Fan Air Curves - Is It Same As on the Simplified Chart Above?
The following figure shows the actual fan curve for a Twin City fan:
As the graph illustrates, the fan pressure at the conventional operating point (#1) is approximately the same as the pressure at the reduced-RPM operating point (#3).
While individual fan curves can be fine-tuned for each application (for example by adjusting the fan's width-to-diameter ratio), the fundamental principle remains the same: fan pressure decreases as air volume increases, and, conversely, fan pressure increases as air volume decreases*5.
As illustrated by the chart for the Twin City fan, the red, blue, and green operating areas (multiplication of air volume and static pressure) are approximately similar (with differences primarily due to lower fan efficiency when operating away from point #2). Consequently, the core relationship holds true: higher air volume necessitates lower fan pressure, and lower air volume necessitates higher fan pressure.
What Is Not Measured Cannot Be Regulated
Ecogate Systems fundamentally differ from traditional industrial ventilation in a key way: Ecogate actively measures air velocities within the duct system. Legacy systems, on the other hand, are designed for a fixed air velocity—for instance, 4,500 FPM.
However, this designed air velocity becomes incorrect as soon as factories implement changes. Adding, removing, or relocating workstations (common, often weekly or monthly changes in industries like woodworking to boost factory efficiency) alters the system velocities without necessary modifications to the ductwork or fans.
The core, unanswered question remains: What are the current air velocities within the duct system? Although such modifications are common, standards like NFPA 660 prohibit them without corresponding fan and duct system alterations. Yet, these changes are made frequently despite the standard, and the resulting actual duct air velocities are typically unknown afterward.
In the chart below are main duct air velocities measured in 137 US woodworking factories. As you see in real life, the main duct air velocities vary from 1,500 FPM to 8,300 FPM.
Many of these systems often fail to maintain the necessary minimum transport air velocities, and some are consuming excessive electricity due to operating at unnecessarily high air velocities.
This is exactly why Ecogate's on-demand system measures air velocities in the duct system and uses it for system regulation and for maintaining minimum transport air velocities.
How System Air Volume and Main Duct Air Velocity is Measured
The Ecogate on-demand system measures air velocity and volume using three different methods*6:
Averaging Air Velocity Sensor (AAVS): This is the most precise method.
Sum of Gate Air Volumes: Calculates volume based on the total flow through active gates (and based on main duct diameter air volume is recalculated to air velocity)..
Fan Power, Total Fan Static Pressure, and Measured Fan Efficiency: A calculation based on fan measured performance data.
The most precise measurement comes from the Ecogate Averaging Air Velocity Sensor (AAVS). The AAVS operates on the same principle as standard HVAC industry sensors, reading both total and static pressure across the entire duct diameter to calculate an average velocity, similar to taking multiple point measurements across the duct cross-section.
The Airflow and Air Volume Sensor (AAVS) must be situated in the clean air section, typically positioned between the dust collector's outlet and the filter's inlet. Though the duct diameter in this area is frequently larger than the main duct, the total air volume passing through remains constant. The AAVS measures the air velocity at its installation diameter, and this data is then used to accurately calculate the air velocity in the main duct. This two-step calculation ensures highly precise readings for both air velocity and air volume for the entire system.
Ecogate Averaging Air Velocity Sensor (AAVS)
How Drop and Branches Air Velocities Are Measured
The Ecogate system is designed to comply with NFPA 660, which states “Where dumpers or gates are used for individual equipment, the volume and velocity resulting from operation of such dampers or gates must not reduce the system velocity below the “design minimum”. Therefore, the Ecogate system must continuously measure and maintain the minimum design and transport air velocities throughout the entire ductwork.
Ecogate's patented gates are equipped to measure pressure, air velocity, and air volume, transmitting these values to the greenBOX control unit via Modbus communication.
For the branch ducts, the system utilizes the air velocity and air volume communicated by each gate. By entering the gate-to-branch connections and the branch diameters, the Ecogate system calculates the air volume and resulting air velocity in the branches.
How Ecogate System Air Volume Is Regulated
As described in previous paragraphs, the Ecogate system utilizes measured air velocity and volume at each gate, calculates these values for branch ducts, and measures them on system level. These data points are then used for optimal regulation of industrial ventilation. Specifically, the system aims to reduce fan power consumption while still guaranteeing the minimum required transport air velocity.
Design Air Volume of Every Workstation Is Entered To The Control System
For every gate within the Ecogate system, the technician inputs the "Design Drop Air Velocity." Based on this velocity and the gate diameter, the software then calculates the "Design Air Volume."
The greenBOX control system displays live gate readings and allows technicians to configure design parameters for each workstation.
The Ecogate control system determines the "Required Air Volume" by summarizing the design air volume of all currently active gates, which are identified through data received from the Workstation Activity Sensors. This total air volume represents the system's immediate ventilation requirement and is both displayed on the dashboard and utilized for system regulation.
Standard And Closed-Loop Air Volume Regulation
The greenBOX control system utilizes two fan regulation methods: a Standard method and a Closed-Loop Regulation - PID method, which is based on comparing the required and measured air volumes.
The Standard method relies on a fan's mathematical model (based on Affinity Laws) and is calibrated by an Ecogate technician during system setup. This calibration establishes two key fan speeds:
Maximum Speed: The speed necessary to properly ventilate all workstations when the system is at maximum utilization.
Minimum Speed: The lowest speed required to achieve the necessary minimal transport air velocities, with only one workstation active and additional gates open at inactive workstations.
The fan regulation between the minimum and maximum speeds is determined by the required air volume (as discussed in a prior section) and the fan's mathematical model.
Closed Loop Regulation
The Closed-Loop method employs a PID control loop to regulate fan speed, ensuring the measured air volume precisely matches the required air volume. Since duct systems are not perfectly balanced—meaning air velocities will vary across different drops—a slight air volume reserve is added beyond the exact requirement. System performance is easily and reliably fine-tuned, as drop, branch, and overall system air velocities are continuously displayed on the greenBOX control system screen.
The greenBOX 12, designed for smaller dust collection systems, utilizes Standard air volume regulation.
For the greenBOX Master system, the preferable and advanced method is Closed-Loop air volume regulation, which will also be available for new Adelie greenBOX systems in the next software release.
The control system includes the Standard regulation method as a backup for the advanced Closed-Loop regulation. For example, the system will automatically switch to the Standard method to ensure continuous operation if the air velocity meter fails.
But What About All Duct System Branches - How Can You Maintain Minimum Transport Air Velocities In Every Part of the Duct System?
Minimum and Maximum Airflow settings in greenBOX User Interface
The greenBOX control system uses the same principle across the entire system as it does for individual duct zones. When only a few machines are active, the system opens additional gates in a branch on non-active machines until the minimum required airflow velocity is met.
The duct system can be segmented into up to ten distinct duct zones (branches), with gates assigned to their respective zones. The duct diameter for each zone is also input into the control system. Using the reported air volume from the assigned gates, the greenBOX calculates the branch (zone) air velocity. The greenBOX then regulates the number of open gates and displays the resulting branch air velocity and air volume on screen.
Summary
The Ecogate on-demand dust collection system is designed to optimize energy consumption by safely reducing the main duct air velocity, all while maintaining the minimum air speed essential for effective dust transport. This optimization is managed by a greenBOX unit, which uses a Variable Frequency Drive (VFD) to dynamically adjust fan speed based on real-time air volume requirements.
When a workstation becomes idle, Ecogate closes its corresponding gate. To prevent the main duct air velocity from falling below the required safe transport level, the system automatically compensates it by opening additional gates at non-active workstations, thereby ensuring the air volume maintains the necessary minimum velocity.
For instance, reducing the air volume to 77% of the design capacity allows the fan speed to be significantly lowered. According to the Fan Laws, this translates into substantial electricity savings—approximately 53%. Crucially, the system not only ensures reliable dust conveyance by sustaining the required air pressure and flow, but also delivers secondary benefits, such as reduced noise levels and enhanced operational data collection.
The key benefits of this approach are:
Significant Energy Savings: Substantial reduction in electricity consumption for fan operation.
Adaptability: The system easily adjusts to changes, such as adding, removing, or relocating workstations.
Increased Capacity: More workstations can be connected to the same dust collection system based on average and peak utilization of workstations.
Monitoring: Measured air velocity values are displayed locally and can be accessed remotely.
Reduced Noise: The system operates more quietly.
These capabilities showcase the progress in dust collection technology beyond legacy systems, providing significant advantages for customers.
Footnotes
For simplicity, we are using the percentage of gates in this example. However, in a real factory setting, the workstations have varying duct diameters, so the Ecogate control system uses the design air volume for these calculations.
In certain dust collection systems, such as those in pharmaceutical plants or laboratories, it is prohibited to use minimal airflow gates at inactive workstations. To address this, Ecogate incorporates "adjustable bleed gates," which are preferably "Ecogate Precision Airflow Gates." These gates are installed at the end of each branch and are adjusted by the system to introduce the required minimal air volume into the ductwork. They are typically placed above the ceiling to avoid drawing expensive conditioned air from the rooms.
The higher the initial air velocity (e.g., 5,000 FPM, based on new system design or existing system measurements), the wider the regulating range becomes. This greater operating range leads to increased savings while maintaining the minimal transport air velocity.
Ecogate provides a training program and templates for its dealers and customers. These resources are designed to help calculate the optimal duct diameters for Ecogate's on-demand ventilation systems, based on the specific workstation utilization figures.
However, the fan cannot be used within its "stall region." In this range, the fan's behavior is erratic, leading to unstable or oscillating air volume.
There are three different methods to measure air velocity because the optimal approach depends on the dust collecting system configuration. For instance, if a smaller dust collector has a fan mounted directly on top, the Averaging Air Velocity Sensor (AAVS) method is not possible. In that scenario, we can use the "Sum of Air volumes from gates" method. Furthermore, if not all drops in the system utilize Ecogate gates, we can still rely on the "Fan performance data" method.
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