The need for absolute filtration arose during World War II and was developed by the Atomic Energy Commission. There was a pressing need to address the health and safety issues raised by the handling of radioactive dust. Research and development produced the first High Efficiency Particulate Air (HEPA) filters for the ventilation systems used to deliver ultra-clean air to clean rooms.
In order for a filter to be certified HEPA, it must be tested and proven to filter particles as small as 0.3 microns to 99.97% efficiency at its designed air flow. To understand what HEPA filters do, it helps to know the definition of a micron. A micron is one millionth of a meter. A human hair is approximately 100 microns wide. Particles smaller than 10 microns are not visible to the human eye. A particle of tobacco smoke averages 0.01 to one micron. Most bacteria range from 0.35 to 10 microns. Almost all viruses, however, are smaller than 0.03 microns, and HEPA filters are not effective.
HEPA filtration for clean-room applications soon found its way into other industries such as microelectronics, pharmaceuticals, chemicals and the nuclear power industries. Soon, other uses were realized for HEPA technology central and portable vacuum collection systems. One of the earliest successful uses of portable HEPA vacuums was their employment in the asbestos abatement industry from the late 50s to the present day. Portable HEPA vacuums also proved useful for asbestos brake removal at auto repair shops. Next came lead paint abatement projects in public housing and government buildings. Today, HEPA vacuums are employed in a wide variety of industries to address the safety and health of workers and residents in any environment where the inhalation of microscopic particles can pose a health risk. One of the most widespread uses of HEPA vacuums today is in the duct cleaning service industry where the recovery of mold spores and other dangerous particles is critical. Our latest challenge is the threat of biological or chemical attacks against unsuspecting populations.
What Makes a High-quality HEPA Vacuum
Not all HEPA vacuums are of equal quality. Starting from the ground up, look for high-quality ball bearing wheels and casters that are of sufficient size and can be easily moved over irregular floors and up and down stairs. Next, examine the construction of the collection devices for ease of cleaning and disposal of contaminants. Look for smooth, easily wiped surfaces like stainless steel and aluminum. Plastic surfaces should be smooth and free of texturing.
Next, examine the power source the motors. Better quality electric vacuums utilize bypass, or two stage motors. Lesser quality vacuums use single stage motors. Here's the difference: A bypass motor is made so the motor chamber is isolated from the impeller chamber. The impeller is the fan that creates the lift and suction that the vacuum produces. The motor chamber has its own cooling fan that uses clean air from outside the vacuum head to keep the motor and its carbon brushes cool. A single stage motor uses the airflow generated by the impeller to cool the motor. In a single stage motor, any particles that escape the filter media are actually blown into the motor chamber, reducing the life of the motor. ·
Finally, check the pre-filters used on the vacuum there should be multiple layers of pre-filtration to remove as much contaminant as possible before they reach the HEPA filter. Effective pre-filters extend the life of the filter and help reduce operating costs. Good quality HEPA vacuums utilize three and even four pre-filters in some cases.
Comparing Performance Ratings
When comparing vacuums, two performance ratings should be evaluated static lift and air displacement. Many buyers make the mistake of comparing horsepower or wattage when shopping for a good vacuum. The speed of the motor, number of motors and the design of the impeller have more to do with the performance of a vacuum than horsepower or wattage.
Some vacuums are designed to produce high static lift, which is the ability of the machine to get heavier materials like liquids or metals in motion. Static lift is measured in inches of water or mercury. Others are designed to displace large amounts of air, measured in cubic feet per minute (CFM). These machines are best for recovery of light particles such as bacteria, pharmaceutical dust, lead paint sanding dust and asbestos fibers. Electric vacuums are usually designed for either high lift or high flow. Pneumatically powered vacuums, however, can be made to produce a high level of both performance characteristics.
The Proof is in the Filters
Filters are the area where, perhaps, the clearest distinctions can be made between HEPA vacuums. Early in the incorporation of the use of HEPA filters in portable vacuums, the HEPA filter was simply attached to the exhaust port of a conventional vacuum. In many cases, the HEPA filter was necessarily undersized for its intended purpose. As the vacuum industry matured in its use of HEPA filtration, some manufacturers chose to position the filter on the inlet side of the power source, while some chose to stay with the filter positioned at the exhaust side.
Which is the best? The debate on this is inconclusive. Positioning the HEPA filter on the exhaust of the power source on an electric vacuum does provide the ability to filter the carbon dust expelled by the motor brushes, but it also allows contaminated air to come into contact with the motors. In addition, the issue of filter leaks is more critical when the filter is positioned on the exhaust as the filter is always in a positive pressure mode. When the HEPA filter is installed on the intake side of the motors, the motors are better protected from contamination, and the issue of filter leaks is lessened due to the filter being in a negative pressure mode during operation.
It is also important to compare the size of the HEPA filter used in each machine. It stands to reason that the larger the filter, the longer it will perform between filter changes. Make sure the HEPA filter is certified at the air displacement that the vacuum generates. If the filter is certified at a lower CFM air displacement than the vacuum generates, it will not perform at 99.97% efficiency.
Interestingly, HEPA filters actually become more efficient during their life span because the passages between the glass fiber filter medium reduce in size as the filter captures particles.
HEPA Versus HEPA-type Filters
HEPA filters must be individually tested and certified to meet the 99.97% efficiency at 0.3 micron performance level. Each filter is subjected to a DOP test in accordance with Military Standard. MIL-STD-282.DOP (dioctyl phthalate) is oil that is used to create a smoke with a very narrow particle-sized distribution. The particle size selected is 0.3 microns in diameter. Following the test, each filter that has passed is assigned a serial number, and the test results are recorded on the filter itself. If you are looking at a vacuum without this information on the filter, it is not a true HEPA vacuum.
HEPA-type filters are not subjected to the same rigorous testing as true HEPA filters, and range in efficiency from 85% to 95%. Obviously, these units are less expensive than true HEPA units, but their filtering capability is not known exactly. Spending a few dollars more is easily justifiable when factoring in health and safety concerns.
Lower-tier vacuums do not provide for a means to determine when it is time to replace the HEPA filter, causing the user to rely on guesswork. Unfortunately, this too often leads to too frequent or too infrequent filter replacement. Too frequent replacement is just a waste of money, while too infrequent replacement results in a period of time where the vacuum is used at less than ideal efficiency.
Look for machines that incorporate a device that tells the operator when to change the filter. Some models use a manometer to monitor the HEPA filter's performance, while others use pressure differential devices that measure the pressure drop across the filter and give the operator a signal such as a light when it is time to replace the filter. These features may add a little to the initial cost of the machine but usually result in lower long-term cost of ownership.
Purchasing a HEPA vacuum is not a decision to be taken lightly. Many factors must be weighed and features compared, but armed with the right knowledge, it is a decision that can be made with confidence.
Steve Spielmann is quality assurance/technical manager for Goodway Technologies Corporation, Stamford, Connecticut. For additional information, contact him at 800-370-2855 or visit www.goodway.com.