Energy is the second largest expense in most water resource recovery facilities. In an activated sludge plant, for example, more than half of total energy consumption goes to the aeration blowers.

Operators looking to reduce energy costs naturally look at the blowers. Many blower-related energy conservation measures are available, including:

• Advanced process control to match airflow to demand
• Supplementing existing blowers with new right-sized blowers
• Replacing existing blowers with new high-efficiency models

These measures are often supported by electric utility incentive programs that pay customers to reduce their bills. A typical incentive covers up to half of project costs, including equipment, installation and engineering. The programs are generally financed by conservation charges on customers’ utility bills.

An essential part of such projects is to document to the utility that the estimated energy saving will actually be realized. Here, measurement and verification is important, and a new test code for blower systems from the American Society of Mechanical Engineers enables this to be done reliably and accurately.

Evaluating the system

Installing new blowers as replacements or supplemental units is a common energy-saving practice. The first step in M&V is to test the existing blower system and establish the baseline energy consumption. For blower replacement projects, before-and-after measurements of kilowatts/100 scfm provide a useful baseline. Plant operators are usually involved in the testing and data analysis of the existing system.

Once the baseline is established, the replacement blower requirements must be defined. Matching the design pressure and flow rate of the existing blowers is a simple approach, but it may not optimize performance under actual operating conditions.

Operators can provide right-sizing requirements based on their current and past operating experience. The new blowers may be rated for a lower design airflow to provide the turndown needed to meet current process demand.

Actual operating pressure is often lower than the original specified discharge pressure. In some cases, a large high-efficiency blower is provided for the base load and smaller, more flexible units are installed to trim the total air supplied to the process and match load variations.

Testing challenges

Once the blowers are selected and purchased, the second phase of M&V involves verifying the new blowers’ performance. To minimize disruption to plant operations, new blowers are typically tested before they are shipped from the supplier. Operators have an interest in specifying the test method and often demand to witness it.

In the past, selection of the appropriate test method was straightforward: The ASME Performance Test Code PTC 10 was used for centrifugal blowers and ASME PTC 9 for positive-displacement blowers. However, recent developments in blowers have complicated the picture.

One factor was the entry of foreign blower suppliers to the United States market. These suppliers wanted to use test methods identified by ISO – similar to the traditional ASME tests and yet with important differences.

A more significant problem was the introduction of packaged blowers. These complete plug-and- play systems generally include the blower, motor, variable-frequency drive, controls, filters, silencers and isolation valves in a factory-assembled, compact, sound-attenuated enclosure. Packaged units offer convenience and cost advantages over custom-engineered, field-assembled systems, but they don’t match the test configurations identified by the commonly used ASME codes.

Whether blowers are factory packaged or field assembled, the operator and the utility are concerned with total system power. The total power demand measured should include losses from motors and VFDs as well as filters and silencers — essentially any component between the blower inlet and the process connection. This is referred to as wire-to-air performance. The traditional test codes, developed to test bare blowers but not complete systems, were not suitable for establishing total system power demand.

Operators and utility staff were forced to accept the procedures and interpret the results of testing based on a supplier’s modifications to traditional methods. This led to a lack of confidence that the results were comparable to the baseline system testing; doubts often remained that the test represented actual operating energy demands.

Developing a new code

The Consortium for Energy Efficiency, created in 1991 to assist utility energy efficiency program administrators, recognized the deficiencies in the blower testing methods and understood that it compromised M&V procedures. The organization approached ASME and requested a single test code that could determine wire-to-air power for any blower technology and system configuration.

The result is ASME PTC 13, the Wire-to-Air Performance Test Code for Blower Systems. The code provides true wire-to-air power measurement to accurately reflect all the operating energy blower systems require. It accommodates configurations from bare blowers to complete factory packages with all accessories.

Test configurations include the blower motor, control panel, and power conditioning equipment such as harmonics filters. Mechanical components that increase power demand, such as silencers, inlet filters, check valves and belt drives, are addressed. The code is extremely adaptable: any blower technology can be tested, and the procedures are nearly identical. Blower types covered include:

• High-speed gearless single-stage centrifugal (turbo)
• Geared single-stage centrifugal
• Multistage centrifugal
• Lobe-type (Roots) PD
• Screw-type PD

Simple calculation

Modern blower systems use a variety of control methods including throttling, guide vanes and VFDs. PTC 13 provides testing methods for all. Most test stand equipment for PTC 13 is identical to the equipment required for traditional testing. The principal exception is the need for high-quality electric power analyzers.

Ambient conditions during the blower test are unlikely to match those selected by the operator in defining required performance. In the older codes, the thermodynamic calculations used to predict site performance from test data were extremely cumbersome. On the other hand, the calculation methodology in PTC 13 is greatly simplified and was expressly developed to accommodate automated data collection and analysis. Most calculations are identical for all blower types.

A stack of raw data is of little use to operators comparing the blower system performance to either existing equipment or promised performance. PTC 13 stipulates that a report must be provided that describes the tested equipment, the specified performance parameters, and the expected performance parameters adjusted from test data to specified conditions. The operator may also require the report to include any special performance parameters needed to compare the new equipment to the baseline performance, such as kW/100 scfm.

Operators face many challenges in upgrading aeration blowers to reduce energy demand. One is verifying that the new blowers meet performance and efficiency expectations. The ASME PTC 13 Wire-to-Air Performance Test Code for Blower Systems was developed specifically for testing both conventional and state-of-the-art systems. Operators and electric utilities can use this code with confidence to meet their M&V requirements.

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About the author: Thomas E. Jenkins, P.E., (tom.jenkins.pe@gmail.com) is president of JenTech, providing consulting services to the wastewater treatment industry.