Evaluating Different Blower Technologies on a Wire-to-Air Basis
In the absence of official third party specifications on energy efficiency, it is difficult to evaluate and compare blower technologies fairly and effectively. The lack of readily available evaluation tools leads to misinformation and unfair comparisons between technologies. Further, the performance verification process is difficult to prove.
Blower manufacturers and their related industries are researching ways to develop fair evaluation criteria and specifications that can be used to determine which blower technology offers the best energy efficiency and performance for a particular application. The evaluation would measure the total energy consumption used by the entire blower system in real world conditions, while taking into account all potential energy losses.
Identifying the Problem:
Five basic blower technologies serve the water and wastewater markets: Positive Displacement Lobe Type, High Speed Screw, Multistage Centrifugal, Integrally Geared Single Stage and Gearless Single Stage “Turbo” Technologies that incorporate air or magnetic bearings. Each have all the same basic components, a compressor, motor, starter and inlet filter, and some technologies also have cooling systems, an oil pump, gears, belts, couplings and control systems. Whether the components are shipped separately and assembled on location or pre-assembled with a controller, variable frequency drive and sound enclosure, the evaluation should include all relevant pieces that consume or affect performance.
Each of the five different technologies offers a potential solution to water and wastewater aeration applications; however, common evaluation criteria are necessary to offer a fair comparison. Unfortunately, there is no rule of thumb that would reveal one technology as better than the other when considering a 20-year life cycle cost (with power representing at least 75 percent of the total cost analysis) that includes power consumption, capital costs, ongoing maintenance and repairs. Because blowers consume 50 percent of the total power consumed by a typical wastewater plant, energy efficiency is a primary criterion when selecting new technology, but not the sole criterion.
The term “wire-to-air” describes the total energy needed to produce the required flow and pressure for any particular application. The relationship between total input power and the amount of flow and pressure produced is replacing a straight efficiency percentage as a comparison criterion. Efficiency as a percentage can be defined as isentropic or polytropic, along with many other definitions; however, defining efficiency this way does not necessarily correlate to useful energy. For instance, higher discharge heat can increase efficiency as a percentage, but higher discharge heat is not a useful state of energy for the process and therefore is a misleading indicator of true energy usage. Rather, the factors that matter the most for biological processes are input power and pounds of oxygen on the discharge (mass flow rate).
With the introduction of high speed “Turbo” and other prepackaged blowers, where the motor and compressor are combined into one unit that comes in a prepackaged system with drive, controller and cooler, the only way to evaluate the technology’s efficiency is through a wire-to-air approach. Traditionally, evaluations have only taken into account the shaft brake horsepower, which is the power to rotate the compressor, excluding the motor and all other drive losses. But to make an equal comparison between technologies, the drive losses of the complete system that includes the motor, starter, variable frequency drive, inlet filters, gears, belts, guide vanes, valves and cooling system need to be measured.
The average user will not fully understand all the different technologies available when it comes to their inner-workings and design, so a wire-to-air specification will require that the entire blower system be fully assembled for testing. The use of nominal or nameplate efficiencies of the various components will not be allowed. The wire-to-air specification puts the responsibility on the manufacturer to design and build a complete blower system where all losses have been accounted for due to real life testing within the application.
Instead of relying on just one particular flow and pressure to compare energy usage, dynamic performance should also be noted when measuring the evaluated power usage over a range of operations. See the table for an example of a simple evaluation that lists four performance points and gives each a weighted average for real process conditions. The basic premise is to evaluate how the blower will work across the entire range of its expected operation. The variable flow or pressure and the power that different blower technologies consume over a range of performances will also have a large impact. Also note that air densities are accounted for, given that temperature, humidity and elevation all impact performance.
The definition of wire-to-air will include all the actual components required and supplied on the particular application and will be tested as a working system over a range of operating points. Verification will involve measuring kilowatts (kW) at the system’s entrance and measuring mass flow rate at the system’s discharge. These two points can be independently verified by using a three phase kW meter that measures power from the wall and a fixed orifice plate flow meter where the delta pressure and temperature can be measured on either side to determine the mass flow rate produced. To produce the exact design points of flow and pressure during the verification testing, the blower system can be regulated by its own control system. Then, the corresponding kW reading can be recorded to prove any guaranteed kW points offered by the manufacturer. Such a test can only be done in a qualified testing facility under controlled conditions and not in the field.
Existing Testing Codes:
Performance test codes in use today are not suited for a wire-to-air scenario across all technologies. Current test codes do not address power measurement adequately; nor do they define the scope of supply components or address specifically where to measure mass flow.
ASME PTC-10 is a commonly prescribed test code for centrifugal blowers. This code only covers the power that is based on the shaft power to the element and does not cover losses across the motor or any other part of the blower system. The code allows for many deviations and does not indicate which processes should be used to measure flow as it allows inlet or discharge flow measurements. The translation from test conditions to site conditions is also not very clear and subject to interpretation. Translations from test conditions to site conditions are not linear and assuming that they are linear results in an inaccurate representation of the actual performance of the blower. These allowable deviances have been used to overstate a blower’s true performance by as much as 10%.
While a common practice today, using nameplate efficiency data on motors, valves or other accessories reveals loose data sets that vary greatly between manufacturers. These efficiency changes are based on loading; therefore, the deviance is exacerbated the further you are from nameplate. Even a 1% efficiency loss can equal be larger than the cost of the product over 20 years.
ASME PTC-9 is a now an inactive specification originally written for positive displacement blowers in much the same way as PTC-10 does for shaft power. This specification is obsolete and no longer active.
ISO 1217 is an active specification covering positive displacement blowers and was originally written for shaft power like the other specifications. This specification has been amended to include a wire-to-air annex as listed below.
A test code should take into account the entire blower system to make sure every item is maximized to its energy use. This would give a much more realistic estimate of how much power savings will occur in an application with variable loads that simulate the field.
The industry is already aware of the need for a new power evaluation and there are several efforts underway to present a wire-to-air specification to meet this need. The American Society of Mechanical Engineers (ASME) formed a new committee in 2008 to write a completely new power test code for low pressure compressors. The committee largely consists of environmental consultants, manufacturers and end customers. ASME PTC-13 encompasses a strict procedure that can be used for “all” blower technologies on a simple wire-to-air principle. The procedure would test the entire blower system including motor, drive, gears, belts, filters, cooling systems, etc. and measure the flow and pressure on the discharge of the system proposed. This is a lengthy and detailed test procedure that can be used to verify the actual performance of the equipment before it is shipped to the field.
ISO 1217 Annex C
The ISO 1217 specification and subsequent Annex C revision is currently a published specification and can be used today. It was written for positive displacement lobe type blowers. This specification encourages routine tests that report base performance on a wire-to-air format of positive displacement blower packages. This specification is currently published and available for use when considering positive displacement type blowers.
ISO 5389 Annex G
The ISO 5389 specification is similar to PTC 10 because it covers centrifugal type blowers on a shaft break horsepower only; however, there is an effort underway to add a wire-to-air Annex G section to encourage a wire-to-air evaluation. This specification is not currently available as a published specification by ISO, but will be in a draft form this year with an official publication date several years later. Annex G can also be used like the predecessor ISO 1217 Annex C as a routine test when it becomes available.
Power consumption, while important, is just one criterion in the selection of blower technology. Other factors such as noise levels, environmental impact, access to service, or even the performance characteristic of the application itself should be evaluated when selecting a blower technology. When comparing power usage, a wire-to-air approach, backed by a published standard, is the only fair and unbiased way to evaluate between technologies. Beyond evaluation, the manufacturer should have the facilities to perform testing in accordance with the specified code. Please consider this approach in your next blower application.
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