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Nevada Water Environment Association
2001 Annual Conference
Las Vegas, Nevada - March 21-23, 2000

Improving Aeration Blower Efficiency With Variable Speed Drives
Gregory P. Harris, Kurt A. Gardner, Steve Beck, Ken Black

INTRODUCTION

The application of variable speed drives (VSDs) to the aeration process in wastewater treatment plants is relatively new. VSDs have made their way into many municipal pumping applications and have shown significant advantages over traditional methods of flow control. These advantages include better flow and pressure control and significantly higher pumping efficiency. The major disadvantage has been that the addition of VSDs results in a higher capital cost that must be justified. Experience shows that, if the systems are designed correctly, the energy savings and the simplification of pumping system controls can more than justify the increased capital cost of the VSDs. As the use of VSDs in municipal pumping applications grew, it was found that VSD control is best suited for systems dominated by dynamic frictional losses and not by systems dominated by static losses.

Because most municipal aeration processes are dominated by the static head losses and because, historically, large horsepower VSDs were quite expensive, the application of VSDs to control the larger horsepower blowers required for municipal aeration has been much slower to develop. In recent years, the cost of large horsepower VSDs has decreased substantially and at the same time their controllability and the quality of the power produced has improved. In addition, newer technologies now give even more choices in the types of variable speed drives available. These include electric variable frequency drives and mechanical magnetic drive couplings. This paper presents a comparison of variable speed drives, including variable frequency drives and magnetic drive couplings, with the traditional method of inlet throttling as a method of air flow control for municipal aeration blowers.

BACKGROUND
The South Truckee Meadows Water Reclamation Facility (STMWRF) located in Reno Nevada is currently in the design phase of a capacity expansion from the current average day maximum month flow (ADMMF) of 1.0 mgd to an ADMMF of 3.0 mgd. The plant currently consists of a single oxidation ditch with course bubble diffusers and operates in full nitrification. The existing plant is seen in Figure 1.

To meet the new capacity requirements, a second oxidation ditch is being added and fine bubble diffusers installed. The average air demand is increasing from approximately 1,100 standard cubic feet per minute (scfm) to 2,400 scfm. Additional aeration blowers are needed to reliably supply the increased air requirement, and two new 200 hp, 2,400 scfm multi-stage centrifugal blowers were selected for this purpose. As part of the blower design, a detailed comparison was performed to compare inlet throttling and variable speed control of the blowers to adjust blower airflows. The results are discussed below.

BLOWER AIR FLOW CONTROL
The traditional method of airflow control for multi-stage centrifugal blowers has been inlet valve throttling. Inlet valve throttling of the blowers is less energy-intensive than discharge throttling and has historically been significantly less expensive than variable speed control. Figure 2 is a photo of an existing blower at STMWRF. Airflow enters on the motor end of the machine and discharges at high pressure at the opposite end. A manual inlet control valve for blower capacity adjustment can be seen in the photo. If a variable speed drive is used to adjust the speed of the blower, then no control of the blower inlet valve is needed.

There are several technologies available that can be used to adjust the blower speed. These include variable frequency drives (VFDs) and magnetic drive couplings. VFDs are commonly used in all types of variable speed applications and have been used in the municipal industry for variable speed pumping applications for years.

The newer of these two options is the magnetic drive coupling. This coupling is inserted between the motor and the blower shaft. Figure 3 shows a picture of a magnetic drive coupling mounted on an aeration blower with a motorized actuator.

The motorized actuator is used to adjust the distance between a permanent magnetic assembly and a copper conductor assembly that transfers the drive torque from the motor side of the coupling to the blower side of the coupling. By adjusting the distance between the two assemblies, the drive torque is varied and the output speed to the blower changes in concert. This technology has only recently been applied to aeration blowers.


Figure 4


Figure 4 shows inlet throttling curves for the new blowers at STMWRF at 80°F. Two pressure versus flow curves and the associated shaft horsepower versus flow curves are presented. Each curve has a unique inlet pressure to the blower that determines the amount of throttling taking place. The design atmospheric pressure in Reno is 12.5 psia. The lower the number, the more pressure drop is taken across the inlet valve and the more throttling is occurring. Also shown in the figure is the aeration system curve for STMWRF. It is at the intersection of one of the throttling curves and the system curve that the blower actually operates. Note that even the largest curve is still a throttled curve at 11.46 psia inlet pressure. This is because blowers are sized to deliver their maximum flow rate on the hottest design day. For the STMWRF this is 105°F. For airflows less than or equal to the blower design capacity on the hottest day, the blowers must still be throttled.

Figure 5 shows variable speed curves for the new blowers at STMWRF at 80°F. As with the throttling curves, two pressure versus flow and two shaft horsepower versus flow curves are shown. The aeration system curve is also shown. Note that the variable speed curves are much shallower and smoother than the inlet throttled curves. Note also that the entire range of blower speed is from 3256 rpm to 3570 rpm. This is less than 10%, and is a result of the fact that the aeration system is dominated by static head and not frictional pressure losses. The maximum blower speed is 3600 rpm. Similar to the throttling curves, these blowers operate at reduced speeds when not at the maximum design temperature. Again, it is where the variable speed curve and the system curve intersect that the blower actually operates. Please note that care must be taken when using variable speed control. In this example, speed changes of less than 10% control the entire airflow change for the blowers. Extreme care must be taken when designing the aeration control system to ensure that the mechanical and electrical system responses are accurate enough to allow very small changes in blower speed to minimize the resultant changes in air flow. Otherwise, significant system hunting and instability could result.


Figure 5


The actual shaft horsepower of the blower for 11.46 psia (depicted in Figure 4) may be figured as follows. A vertical line is drawn through the intersection of the 11.46 discharge pressure curve (blue curve) and the system curve (red curve). Where that vertical line intersects the horsepower curve is the shaft horsepower required for those conditions for the blower. This shaft horsepower must be divided by the motor efficiency to get the total machine horsepower. In this case, at 2,400 scfm and 80°F, the shaft horsepower is 138 hp. With a motor efficiency of 91%, the total machine horsepower is 151 hp.

For the variable speed case (depicted in Figure 5), a similar analysis shows that the shaft horsepower is 135 hp. However, to get the total system horsepower, a motor efficiency of 91% and a VFD efficiency of 95% were used to obtain the actual horsepower draw of 156 hp. While the variable frequency drive case at this point actually exceeds the inlet throttled condition, evaluation of Figures 4 and 5 shows the horsepower draw for the variable speed case is significantly lower than the inlet throttled case as the blower air flow goes from its maximum condition to the minimum air flow condition. It is the difference between the corresponding horsepowers for the inlet throttled case and the variable speed system case over the entire airflow range that constitutes the energy savings possible with VFD control.

For the magnetic drive coupling, the shaft horsepower is the same as shown in Figure 5 as this Figure applies to speed changes only for the blower, independent of the device that instituted the speed changes. For the magnetic coupling, the shaft horsepower is converted to motor horsepower by dividing the shaft horsepower by the motor efficiency of 91% and the coupling efficiency. The coupling efficiency is a function of the amount of differential slip across the coupling and ranges from 99% to 85% at a blower speed of 3000 rpm. 3000 rpm is the minimum blower speed for STMWRF. For the 3570 rpm case in Figure 5, the magnetic coupling efficiency is 99% and the motor horsepower is 150 hp.

A comparison between the total horsepower for the inlet throttled case and both variable speed systems for the new blowers at STMWRF are shown in Figure 6. Note that the horsepower differential varies from 20 hp to 2 hp over the operating range of the blower. Also note that because the efficiency of the magnetic coupling changes with blower speed, the VFD curve and the magnetic drive curve actually cross midway through the blower turndown. The VFD is more efficient at very low turndown. The magnetic coupling is the most efficient at little or no turn down. However, through much of the operating range of the blower, there is little difference in efficiency between the two variable speed technologies. The average energy savings shown in Figure 8 is 11 hp.

This analysis was repeated for the new blowers at STMWRF for 10°F, 35°F, 60°F and 80°F to determine the changes in the horsepower savings over the entire operating range of the blowers. The averages for each temperature ranged from 2 hp at 80°F to 34 hp at 10°F.


Figure 6: Horsepower Differential at 60 Degrees F


ECONOMIC ANALYSIS

Power Savings
To account for the variation in seasonal and diurnal temperatures, the year was divided into equal day and night periods for all 4 seasons. The average horsepower consumption for the inlet throttled case and the variable speed case for each of these conditions is shown in Figure 7. As can be seen, the horsepower savings for variable speed control (regardless of the variable speed technology used to obtain it) is significant and fairly uniform throughout the year.

Based on one new blower running, the yearly average horsepower savings for the variable frequency drive system is estimated at 12.7 hp. This is approximately a 10% savings over inlet throttled horsepower. This results in a total of 83,200 kW-hr savings over a year. At the STMWRF power cost of $.08/kW-hr, this yields $6,650 annual energy savings.


Figure 7: Seasonal Power Differential for one blower


For the magnetic coupling system, the yearly average horsepower savings is estimated to be 11.3 hp or approximately 9% of the inlet throttled horsepower. This results in a total of 73,997 kW-hr, which yields $5,920 in annual energy savings.

Equipment Costs
The installed costs for the inlet throttled case include motorizing the inlet valve and providing a solid state soft starter to limit inrush current at blower startup. The equipment cost for these items at a 200 hp rating is estimated at $4,000 and $8,500 per blower, respectively. For the two new blowers being provided, the cost is $25,000.

For the variable frequency drive case, the inlet valve is not motorized and no soft starter is required. There are two types of VFDs that can be used: the traditional 6 pulse VFD or the newer 18 pulse clean power VFDs. The primary difference between the two types is the amount of harmonic distortion introduced into the electrical system by the operation of the VFD. The 18 pulse VFD is almost distortion-free and is considered “clean” power. How much distortion is allowable for the electrical system takes careful analysis and is primarily dependent on the existing electrical system characteristics.

The installed cost for a 6 pulse VFD to match the blowers selected for STMWRF is approximately $34,000. The installed cost for similar size 18 pulse VFD is $68,000 each. The associated total VFD costs for STMWRF are therefore $68,000 and $136,000.

For the magnetic coupling case, the installed cost for the coupling and motorized operator is $22,000 per blower. In addition, since the coupling acts to limit in rush current at blower start up, a soft starter is not required. Therefore, the total cost for STMWRF for the magnetic couplings is $44,000.

Economic Results
The economic analysis is based on the installed equipment cost differences. This analysis is summarized in Table 1. As can be seen, when the total differential cost between the alternatives is compared to the annual savings realized, the simple payback for the cost of the VFD is approximately 6.5 years for 6 pulse VFDs and 16.7 years for 18 pulse VFDs. For the magnetic coupling, the simple payback is 3.2 years.

TABLE 1: STMWRF Return on Investment
Air Flow Control Option Base Cost Differential Cost Annual Savings Payback
Inlet Throttled $ 25,000 $ 0 $ 0 N/A
6 Pulse VFD $ 68,000 $ 43,000 $ 6,650 6.5 years
18 Pulse VFD $ 136,000 $ 111,000 $ 6,650 16.7 years
Magnetic Drive $ 44,000 $ 19,000 $ 5,920 3.2 years

CONCLUSIONS

The results of the economic analysis for the STMWRF indicate a significant power savings can be obtained when applying variable speed control technology to the blower operation. The relatively short pay back for the magnetic drives at the STMWRF indicates it is a clear choice, provided the control system is designed to accommodate the very small changes in blower speed.

In addition to the economic advantages of the magnetic drive, there are also several ancillary advantages to the device. Because it is a mechanical device, it is able to control the speed of the blowers without inducing any harmonic distortion on the electrical system. Also, since the coupling assemblies do not touch each other, the magnetic drive provides significant blower protection and maintenance advantages that affect the long-term life of the blowers.

In addition to the specific results for STMWRF, there are some general observations that can also be made based on the analysis.
  • The energy savings increase as the operating temperatures decrease from the maximum design temperature.
  • The energy savings will increase as the number of operating blowers increases.
  • The blower curve selection significantly affects the energy savings obtainable.
  • The aeration control system must be carefully designed to function properly with the small changes in blower speeds seen with systems dominated by static head.
  • The application of magnetic drive technology to the aeration process can result in significant cost savings and potential mechanical benefits.
Gregory P. Harris, P.E and Kurt A. Gardner P.E are Electrical Engineers with Herwit Engineering, Concord, CA. Steve Beck P.E. is an Environmental Engineer with ECO:LOGIC Engineers, Roseville, CA. Ken Black P.E. is Senior Applications Engineer with MagnaDrive Corporation, Seattle WA.

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