COLUMBUS, Ohio -- Devices that contain air filters may have to run at slower speeds if they want to use new, high-efficiency filter media to their full potential, according to a study at Ohio State University.
The study revealed that at high airspeeds, dust particles or other contaminants are likely to build up on the front of a filter that utilized the new materials, instead of spreading through the whole filter. Also, smaller particles will have a higher chance of blowing through the filter material and escaping through the exhaust air.
Kambiz Vafai, professor of mechanical engineering, said that manufacturers of vacuum cleaners and other air-filtering devices can sidestep this effect by running their products at lower speeds or increasing the size of the filter. Either change would help contaminants spread more evenly throughout a filter.
With former graduate student James Giuliani, Vafai published the results of the study in a recent issue of the Journal of Fluids Engineering.
"Originally, these high-efficiency filter materials were designed for lower airflow, but today many manufacturers are trying to retrofit or design new devices that operate at higher airflow," said Vafai.
He explained that manufacturers were limited to less efficient filter materials in the past, and had to design their devices to run at higher speeds to compensate.
"In general, more air through a filter means more contaminants removed," Vafai said, meaning more particles are pulled through the filter and get trapped inside. "But our research suggests that high-efficiency filter materials won't perform better at high speeds."
In their research paper, Vafai and Giuliani noted that manufacturers may be tempted to upgrade their products simply by adding the new filter materials to their existing hardware, but doing so may actually lower the efficiency and shorten the life cycle of the filter.
Vafai and Giuliani ran computer simulations of contaminant particles catching on porous, non-woven glass microfiber material typical of today's high-efficiency filter media. Such media resemble tangles of Fiberglas, although the filter fibers are much smaller.
The researchers modeled the flow of particles around a single tiny fiber and then expanded their calculations to include the effect of surrounding fibers. They calculated where contaminant particles -- similar to typical dust particles --would stick to individual fibers at different airspeeds.
The simulations revealed that at high airspeeds, contaminant particles were more likely to contact the fibers at an angle instead of covering them in a more uniform manner. The particles' angle of impact increased with airspeed, so that particles tended to pile up in specific locations instead of depositing uniformly around the fiber.
The particles formed long, narrow tendrils, which grew longer and narrower as air velocity increased. As airspeed increases, such tendrils could break off. Although the researchers did not calculate a specific airspeed at which this would happen for the purposes of this research project, Vafai estimates that such an effect would begin to happen when airspeed through a filter reached approximately 8 feet per minute.
Vafai said the tendrils would form a thick layer of contaminant in the front of a filter which would not only block air flow, but also shorten the lifetime of the filter. He thinks manufacturers of devices that contain air filters could lengthen that lifetime by lowering the velocity of air through the filter media. The results hold implications for manufacturers of vacuum cleaners who want to add high-efficiency filters to their products. Vafai estimated the typical airspeed through a vacuum cleaner filter at approximately 15 feet per minute, much too fast for the new filter material to operate efficiently.
While this study examined the spread of contaminants similar to dust particles, Vafai said the principles hold true for other contaminants, such as soot or waste particles from chemical or manufacturing processes.
Traditionally, nuclear facilities and chemical and metallurgical plants have all used air filters to capture particles of waste products suspended in gas.
Contact:
Kambiz Vafai, 614-292-6560; Vafai.1@osu.edu
Written by Pam Frost 614-292-9475; Frost.18@osu.edu
Journal
Journal of Fluids Engineering