Industrial Utility Efficiency

Aeration Blower Control Strategies

Real world blower applications rarely operate at steady state design conditions. There are a variety of reasons for this. Designs usually include a margin of safety to accommodate unforeseen conditions. Typically, the process demand itself is variable, requiring a corresponding ability to modulate the blower flowrate.

Blower modulation can be provided by a variety of controls. The various blower designs have unique operating characteristics that must be considered in control selection. Some of these designs and control systems have a long history and some have been available for a comparatively short time. This article will describe the most frequent applications.


Basis of Process Demand

The process demand for air establishes the required performance. In almost all cases process performance depends on the blower supplying the appropriate airflow rate. Processes that depend directly on maintaining a specific pressure are rare. Most control systems that use a pressure setpoint use pressure indirectly to regulate airflow.

Airflow demand can be categorized as either volumetric or mass flow. Examples of processes that require specific volumetric flow rates include pneumatic conveying, air knives and agitation. In wastewater treatment, for example, the need to aerate channels and equalization tanks is satisfied by volumetric flow rate.

Processes based on mass flow rate are common. In most of these processes the mass flow rate of oxygen is critical. The predominant constituent of air, nitrogen, is important in cooling applications, but in combustion and biological processes it is oxygen content delivered to the process that determines performance.


Control Strategies

There are three blower performance parameters of interest. Airflow rate, volumetric or mass, is the controlled parameter. Pressure is required to overcome the resistance to airflow through distribution piping and into the process. The pressure consists of constant static pressure and variable friction losses. [See Figure 1.] The pressure and flow, in turn, establish the power needed to produce the desired airflow rate at the pressure necessitated by the system.

Figure 1: Examples of System Curves

The blower performance curve identifies the pressure capability of a blower as a function of its flow rate. The system curve identifies the pressure created by the system as a function of the air flow through it. The intersection of the two curves will identify the actual operating point.

Control strategies are employed to modulate the airflow in response to changes in process demand or system resistance. This is usually accomplished by a feedback loop. The error between set and desired performance initiates a change in the airflow modulating device.

It is often necessary to predict the impact of control changes on system operation. An example would be pre-implementation analysis of Energy Conservation Measure (ECM) power cost.
There are multiple blower types and multiple control devices available for most applications. The most common combinations are shown in Table 1.

Table 1: Blower and Control Types

Blower Type


Variable Speed (VFD)

Guide Vanes

Lobe Type PD


Only Practical Method


Screw Type PD


Only Practical Method


Multistage Centrifugal

Very Common

Very Common


Geared Single Stage Centrifugal



Very Common

Gearless Single Stage Centrifugal (Turbo)


Always Provided



Least Efficient

Most Efficient

Intermediate Efficiency


Positive Displacement (PD) Blowers

Both lobe and screw PD blowers are modulated by varying their speed. The PD blower curve is theoretically a vertical straight line, but actually internal leakage (slip) increases with higher pressure ratios. [See Figure 2.] Speed reduction reduces the flowrate, with the discharge pressure changing to match the system pressure at that flow. To avoid over pressure and damage to the system or blower a pressure relief valve should always be provided for PD blowers.

Figure 2: Example PD Variable Speed Performance

At constant discharge pressure blower flowrate change is linear with speed and blower power is linear with flowrate. [See Figure 3.] This simplifies calculating the response to speed changes. The slope and intercept can be calculated from tabular or graphical data.

Figure 3: Examples of PD Blower Performance. Click to enlarge.

If the expected operating discharge pressure differs from the available data the performance can be evaluated at pressures above and below the anticipated value. Linear interpolation is then used to estimate performance at the anticipated pressure.

There are limitations in applying Variable Frequency Drives (VFDs) for controlling PD blowers. At constant discharge pressure the blower demands constant torque from the motor, which in turn demands constant output current from the VFD. The horsepower rating of most VFDs is based on variable torque loading, so the VFD for a PD blower must be oversized to accommodate the high current load at reduced speed.

At reduced speed blower efficiency decreases, which increases discharge air temperature. Excessive temperature causes distortion and failure of mechanical components. This limits minimum blower speed. For fan cooled motors reduced cooling at low speed is also a concern. Either temperature sensing or the manufacturer’s suggested minimum speed should be included in the control strategy to prevent damage.

Aeration Blower Turndown Strategies – Webinar Recording - By Tom Jenkins

Download the slides and watch the recording of the FREE webcast to learn:

  • How to determine turndown needs and actual blower system turndown
  • Identify the factors that limit turndown for each of the blower technologies employed in wastewater treatment
  • Techniques for improving turndown and staying within the safe operating range
  • Proper initial design and blower sizing, process optimization and comprehensive integrated control strategy can reduce energy consumption
  • Control strategies (time based, DO control, volume control, pH & DO control such as in Deammonification, etc.), best practices and lessons learned

Take me to the webinar


Centrifugal (Dynamic) Blowers

Centrifugal blowers have variations in design, but all share common operating characteristics. All centrifugal blowers use impellers to transfer kinetic energy to air. The volute or diffuser section at the periphery of the case converts some of the kinetic energy in the airflow to potential energy, i.e. pressure. The flow vs. discharge pressure and flow vs. power characteristics for a given set of inlet conditions are shown in the blower curves.

There are a variety of control methods available. [See Table 1.] All of them modify the blower curve to control the airflow.

Throttling at the blower inlet shifts the curve downward and makes it steeper. Throttling is the least efficient control method, but it has also the lowest equipment cost. It functions by creating a pressure drop at the blower inlet. The total head and the volumetric flow at the impeller eye are unchanged, but the available pressure and mass flow at the discharge are reduced. The intersection of the reduced blower curve and the system curve identifies the actual operating airflow.

Creating throttled performance curves is a two-step process. The pressure drop through the inlet valve is calculated in the first step.

If the Δp at a given flow is known a simplified analysis may be used:

In the second step the impact of reduced density and inlet pressure for multiple points is calculated to create a new performance curve. [See Figure 4.]

Figure 4: Example of Inlet Throttling Control.

The most efficient method of controlling airflow for all types of centrifugal blowers is variable speed, usually using a VFD. In the past the high cost of medium voltage VFDs (>600 Volts) made their use on large blowers uneconomical. Increased competitiveness in the VFD market and high energy costs now make them attractive for medium voltage applications.

Creating the new performance curve for variable speed uses the affinity laws. They define the relationship between the performance curve at the original and new speeds. The calculations are performed on several points to construct the new curves. [See Figure 5.]

Figure 5: Example of Variable Speed Control. Click to enlarge.

A common error in evaluating performance with variable speed is to take an existing operating point, apply the affinity laws to it, and assume the result will be the new operating point. [See Figure 6.] This is not correct! The proper procedure is to construct a new performance curve and determine its intersection with the system curve.

Figure 6: Incorrect Application of Affinity Laws. Click to enlarge.

Most existing applications of geared single stage centrifugal blowers use guide vanes for control. These may be inlet guide vanes (IGV) or variable discharge diffuser vanes (VDV). In many cases a combination of both is used to optimize performance. The efficiency of controlling blower airflow with vanes is better than throttling, but worse than using a VFD. The design and control algorithms for guide vanes are manufacturer specific and usually proprietary. Projecting performance for control changes should be made using manufacturer supplied data. [See Figure 7.]

Figure 7: Example of IGV Control. Click to enlarge.

Providing effective surge control is a concern for all centrifugal blowers. Surge is a pulsating flow condition occurring at low flow and high pressure. It can cause blower failure in a short time. Surge control consists of monitoring flow and taking corrective action. That may mean modulating the blower control device to increase flow to a safe operating point or opening a blow-off valve to increase flow and reduce discharge pressure. In other cases the blower is simply stopped to prevent damage.

Common Protection Features

There are many protection methods commonly employed to prevent premature failure of blowers and motors. These may be implemented with simple switches or may employ sophisticated analog transmitters connected to advanced programmable controls. The complexity varies with blower type and size. Employing advanced methods may not be economically justified on small blowers.

Table 2: Blower Protection Systems

Protection Method

Positive Displacement Blower

Centrifugal Blower

Surge Protection

Not applicable


Motor Overload Protection1



Phase Loss / Phase Imbalance1

Larger Units

Larger Units and All Turbos

Blower and Motor Bearing Temperature

Larger Units


Blower Case Vibration

Larger Units

Larger Units Without Bearing Vibration Protection

Blower and Motor Bearing Vibration


Larger Multistage and Most Geared Single Stage Units

Mag Bearing Turbos

Discharge Air Temperature

Larger Units (May Use Differential Temperature)


Discharge Air Pressure

Larger Units

Usually (May Use Differential Pressure)

Lube Oil Temperature and Pressure

Larger Units

Geared Single Stage Only

Lube Oil Level

Larger Units

Geared Single Stage and Larger Multistage2


  1. Overload and phase monitoring is generally a standard feature of VFDs and solid-state starters.
  2. Not applicable to grease lubricated bearings on multistage blowers.


Trends In Control

Blower control is rapidly changing as new technology in instrumentation and control is combined with new blower configurations. Better economics and the rapid implementation of complete blower packages have accelerated the rate of change.

Blower controls have developed increased communications capabilities to integrate their functions and operating data into SCADA systems. Many packages include advanced capabilities such as employing virtual machines, cloud data storage, and the Internet of Things (IoT).

Energy optimization is an increasingly important goal for blower control systems. This includes incorporation of advanced VFD designs. Initially applied exclusively to turbo blowers, permanent magnet synchronous motors are being applied to other blower types and at higher power.  Advanced control algorithms like floating control and direct process flow control are becoming more common. These trends will continue to shape and improve blower control technology in the future.

About the Author

Tom Jenkins has over forty years’ experience in blowers and blower applications. As an inventor and entrepreneur he has pioneered many innovations in aeration and blower control. He is an Adjunct Professor at the University of Wisconsin, Madison. For more information, visit

To read similar articles on Aeration Blower Technology, please visit