The ASME PTC-13 test code is here
By Tom McCurdy
The power required for aeration is 50–60 percent of the electric bill for a municipal wastewater plant, sometimes accounting for up to four percent of the associated community’s total power. Energy-efficiency is a critical factor in evaluating blower performance. After years in development, a useful tool — ASME PTC-13 — has been introduced that can measure performance, regardless of blower technology.
When the high-speed turbo blower entered the market in the mid-2000s, it was unlike any other machine. It had a high-speed motor directly connected to an impeller, requiring it to rotate at speeds upward of 40,000 RPM. In order to reach these speeds, the motor required a special inverter (or VFD) that would convert the 60 Hz power to 600 Hz or more. To ensure the machine’s speed and power could satisfy the system’s air needs while ensuring the blower would continue to operate safely, a control panel was required. As a result, the high-speed turbo blower was a completely integrated package (see Fig. 1).
The turbo promised high efficiency, as much as 30 percent greater efficiency than positive displacement blowers. The question was how to prove it. Until then, the only available performance test methods were ASME PTC-10 and ISO 5389. These methods were “stage only,” meaning they only accounted for the performance of the air end itself. This was more common for multi-stage centrifugal blowers, where the blower was a separate entity driven by a motor and commonly controlled by throttling the air inlet valve. For the highly integrated turbo blower, it would be difficult to apply these methods and account for the additional energy required inside the package.
The existing methods also had additional challenges that would lead to inaccuracies in determining performance:
Air flow was measured on the inlet rather than the outlet. Depending on the design, there was either some inlet flow that was lost in the process or unaccounted-for air from other sources that was added to the process flow.
On test day, it was difficult to account for environmental differences between the specified ambient conditions and the test bay ambient conditions, given centrifugal machines are dynamic and the density of the air has a significant effect on performance.
Wire-to-air performance needed to account for not only the efficiencies of the individual components but also the power required for ventilation and cooling.
Tolerances on performance varied by the protocol that was used.
While protocols were being developed over the past seven years, engineers designed work-arounds. Some would use ASME PTC-10 and add “wire to air.” Some would assign restrictive tolerances on power and flow (as low as 0 percent). Others would levy performance penalties for non-compliance on individual points, charging as much as $30,000 per kilowatt. All of these had a level of inherent ambiguity and didn’t always provide the desired results.
ASME PTC-13 is a welcome answer for both engineers and manufacturers. Under this protocol:
• Flow is measured on the discharge in accordance with ASME PTC-19.
• Test results are corrected for the difference in air density between the test bay and the site, using a set of standard calculations.
• The “envelope” of the components to be included must be specified by the engineer.
• Tolerances must be specified. With every manufacturer subject to the same standards, there’s less “wiggle room.”
Although the turbo blower brought about the need for performance measurement improvements, ASME PTC-13 is not limited to turbo blowers; it can be applied to any blower project, whether it’s centrifugal or positive displacement. For several manufacturers, the low-pressure screw compressor packages are proving to be as efficient, if not more efficient, than turbo blowers. This test protocol will provide that proof. WW
About the Author: Tom McCurdy is national sales manager with the environmental division of Aerzen USA Corporation located in Coatesville, Pa. He is a Board Member of the Water and Wastewater Equipment Manufacturers Association (WWEMA). For more information about WWEMA, visit www.wwema.org.
