A St. Louis plant team, consultant and manufacturer team up with an innovative solution to extract better efficiency and process control from an older aeration blower.


Variable-frequency drives (VFDs) have been used for decades to control multistage centrifugal blowers. The increasingly popular turbo blowers also use VFDs for flow control and energy optimization.

VFDs have not been applied to geared, single-stage centrifugal blowers — economic rather than technical obstacles have limited such applications to smaller blowers. Recently, however, the staff at the Lemay Wastewater Treatment Plant in St. Louis has shown that changing economics can enable a cost-effective, energy-efficient application of VFDs to large blowers.

Background

Two types of single-stage centrifugal blowers are commonly used in wastewater aeration. They include large blowers that use a conventional AC motor and a speed-increasing gear set to obtain high impeller speeds and achieve required discharge pressures, and turbo blowers that use direct-coupled impellers and synchronous motors. In turbo blowers, a VFD provides very high-frequency power to rotate the motor at several thousand rpm. They are generally limited by bearing considerations to less than 400 hp.

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The 167 mgd (design) Lemay plant is the second-largest wastewater facility operated by the St. Louis Metropolitan Sewer District. The original process design dates to the mid-1980s and consists of conventional activated sludge with plug flow reactors. Six of the eight aeration basins — those in continuous operation — are equipped with fine-pore diffusers. The other two, used during high-flow loading periods, are being converted to fine-pore diffusers.

Four Howden Roots single-stage blowers provide the air. Originally, all four blowers were identical, rated for 57,000 scfm at 8.0 psig discharge pressure, with 3,000 hp/4,160 V electric motors. Design inlet conditions were 100 degrees F and 80 percent relative humidity, with an inlet pressure of 14.2 psia.

The plant’s aeration control strategy was fairly standard. Dissolved oxygen (DO) was the primary control variable. The DO loop cascaded to basin airflow control logic with 32 electrically operated flow-control valves. The blowers were controlled to maintain a constant 7.0 psig discharge pressure. Inlet guide vanes (IGVs) modulated blower airflow in response to pressure variations.

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Toward optimization

After 10 years of operation, the fine-pore diffusers were installed and two blowers were modified with new impellers to match the air supply to reduced demand. Capacity was reduced to 32,000 scfm, but rated discharge pressure and design inlet conditions were unchanged. IGVs were retained for modulating airflow.

The Lemay staff knew the blower characteristics and controls were not optimized for process or energy efficiency. Matching air supply to process demand required turndown beyond the available blower capabilities. The actual required discharge pressure was 6.5 to 7.0 psig, lower than the typical operating setpoint. Although the IGV control conformed to standard practice, the staff knew variable-speed control was more efficient.

Throttling a blower’s inlet is both the least expensive and the least efficient method of airflow control. The inlet valve creates a pressure drop, reducing flow by taking some of the blower pressure rise at the inlet, increasing total pressure drop and shifting the flow to the left on the blower curve. The inlet throttling also reduces inlet density, decreasing the pressure capability of the blower for a given flow.

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IGVs cause a spin, or prerotation, of the air at the eye of the blower impeller, reducing the pressure rise through the blower. The IGVs also obstruct the inlet flow stream, resulting in some throttling of the blower. Variable-diffuser vanes (VDVs) on the discharge side of the impeller are available for some blowers and are used to modulate the blower by changing the flow/pressure characteristics. The blowers at the Lemay plant had IGV control only.

Looking to VFDs

Variable-speed control is the most efficient method of modulating blower output, as the flow can be directly modulated by changing blower speed. Energy is saved because there is no parasitic pressure drop from throttling. Instead, the variable-speed blower is modulated so that only the pressure required to overcome the system resistance to airflow is created.

Most single-stage blowers are supplied with medium-voltage motors (600-6,000 V). Since these are often larger than 1,000 hp, medium voltage is more cost-effective for constant-speed operation. The cost of medium-voltage VFDs rated for the required power was historically very high, making IGV and VDV control the most cost-effective for single-stage blowers.

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However, advances in technology, increased competition and more demand have significantly reduced the cost of medium-voltage VFDs in recent years. The Lemay plant staff recognized that they could significantly improve process performance and energy savings by using VFDs to control the blowers.

A 2009 evaluation of blower performance and system demand found that the discharge pressure and operating airflow range exceeded the actual aeration requirements. Based on the projected cost of modifying the blowers and controls and a current power cost of $0.05/kWh, the engineering evaluation indicated a simple payback on energy efficiency of about six years.

Systems approach

The existing blowers were about 30 years old, but inspection showed that they were in excellent shape mechanically, and experience indicated they could easily last 20 more years.

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A system approach to blower modifications is necessary to make sure process concerns, equipment operation and control considerations are properly coordinated. In 2011, the staff made modifications to Blower 2 that included:

  • New impeller to adapt the blower airflow and discharge pressure to actual operating conditions, which differed from the original design requirements. The new impellers were custom-engineered by Howden Roots.
  • A new high-efficiency 2,000 hp 4160 V motor, sized to match the revised blower performance.
  • A new 2,000 hp medium-voltage, liquid-cooled VFD.
  • A new blower control panel to provide proper control of the variable-speed blower, including sequencing of start and stop functions. Howden Roots provided a proprietary control algorithm, based on extensive on-site testing, to optimize the coordination of the IGV and VFD controls for improved efficiency and turndown.
  • SCADA and supervisory control modifications, including most-open-valve (MOV) control and blow-off during low-flow operation. This is provided for both the constant-speed and the variable-speed blowers. The plant staff has been able to reduce the discharge pressure to 6.1 psig and lower the minimum speed from 52 Hz to 50 Hz.

Meeting challenges

There were many challenges to overcome during implementation. The electric utility offered a rebate for the upgrade, but to get it the plant had to complete the project on a tight schedule.

Heat dissipation from the VFD, motor and blower represented a challenge: Even a few percent inefficiency at 2,000 hp creates significant unwanted thermal energy. The selection of a water-cooled VFD required an air-to-water heat exchanger outside the blower building. The existing overhead crane and structures within the blower building created challenges in locating equipment and ductwork.

Despite these challenges, the project has been a success. Because electric rates have increased almost 50 percent since the 2009 evaluation, payback has been excellent. Improved process flexibility has allowed the plant to more closely match the air supply to aeration system demand. The MOV logic has meant discharge pressures from 6.1 to 7.0 psig, a significant improvement over the previous constant-pressure system.

Improved blower system efficiency was a primary objective. To verify the savings, the blower power demand for one year of operation was recorded by the SCADA system. The aeration system operates across a range of flows, discharge pressures and inlet temperatures, and each parameter affects blower power. To minimize variations, the isentropic efficiency was calculated and plotted for each blower.

The average “wire-to-air” isentropic efficiency of the variable-speed Blower 2 was higher than the average efficiency of the other two blowers operated during the same period. Even more important, the unmodified blowers had their best efficiency point (BEP) near the highest airflow. Because of the backward-curved impeller designed by Howden Roots for the modified Blower 2, peak efficiency occurs at the midrange of airflow, where the blower most often operates. This further improved the variable-speed blower’s efficiency.

Better payback

The installed cost was within a few percent of the estimate, totaling just over $1 million, including the SCADA changes and control wiring that were provided by plant personnel. Because of higher electricity cost and a financial incentive provided by the electric utility, the payback was reduced to about four years, clearly demonstrating the impact of changing economics on the feasibility of VFD control for single-stage blowers. As with any new application, lessons were learned:

Before replacing existing aeration blowers, it is important to find out if it is more cost-effective to optimize current equipment.

A system approach to evaluation and design is beneficial. It is especially important for the engineering team to consider and integrate process, blower characteristics and control systems.

The solution to heat dissipation from the VFD and motor required out-of-the-box thinking. The staff has developed solutions that would enable the use of air-cooled VFDs.

As often happens when energy conservation becomes a priority, the Lemay plant team identified other projects and modifications to the aeration system. The blower motor is being ducted to the outside of the blower room to reduce the building heat load. Howden Roots is examining the control strategy to further optimize turndown and energy consumption. Separate low-pressure blowers are being investigated for the aerating influent and effluent channels, reducing energy consumption and lowering the air demand on the aeration blowers.

The variable-speed operation of the Lemay single-stage aeration blowers is an innovative application of established technology. The project was implemented because the Lemay staff was willing to examine changing economics and take a system approach to the aeration process. The results can provide guidance for other municipalities seeking to improve their energy and process performance.

About the authors: Tom Jenkins, P.E., (tom.jenkins.pe@gmailcom) is a consultant specializing in aeration system energy and controls. Karl Nowak, P.E. (knowak@stlmsd.com), is plant engineer for the Lemay Wastewater Treatment Plant. Tim Hilgart (timothy.hilgart@howden.com) is wastewater vertical sales leader for Howden Roots Blowers and Compressors.


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