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Contractor Bulletin: Important Questions to Ask Before
Deciding
Which Negative
Air Machine Brand to Buy!
(Questions with links to answers below)
The performance, safety and reliability of negative air machines can vary
greatly from brand-to-brand. The following questions should be answered by
the vendor before purchasing a negative air machine:
- Does the machine actually produce the
airflow you think you are getting?
- True airflow of negative air machines can vary substantially
from brand-to-brand because different suppliers use different rating methods
and components. One of the most confusing, inaccurate and potentially misleading
methods is ‘free air’, which is nothing more than the test bench
rating of the blower. The airflow produced will be 35% to 50% lower when
the blower is enclosed in a cabinet and must pull air through the resistance
of the filters.
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In addition, the machine must be equipped with a powerful enough
motor to enable the squirrel cage blower to pull sufficient volumes
of air through the filters. Typically, a 1 3/4 horsepower motor is required
to produce peak airflows of 2,000 cfm. A machine equipped with a 1-horsepower
or smaller motor doesn’t have enough power to produce 1,500 cfm
with clean filters, let alone 2,000 cfm. The likely result: you will
need up to twice as many machines for a given size job.
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to see a larger image. |
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- Does the HEPA filter truly provide 99.97%
HEPA filtration?
- A filter made with HEPA media may or may not provide
true HEPA efficiency. Serious leakage can go undetected if filters are not
individually tested and certified at the end of the manufacturing process
in accordance with IEST-RP-CC001.3 guidelines for Type A HEPA filters. Even
the tiniest pinhole leaks in the media or breach of the seal between the
media pack and the filter frame can cause the filter to fail efficiency
requirements. If this happens, contaminated air can pass through the filter
and be exhausted into ‘clean’ areas of the facility.
HEPA testing requires very specific procedures using a thermally generated
mono-dispersed aerosol and a laser particle counter. Test results, including
test airflow, percent penetration, date of manufacture and other detailed
information must be included on a label affixed to the filter. Testing should
be conducted at the rated airflow of the machine. A filter designed for
and tested at 1,000 cfm or 1,100 cfm may or may not pass at 2,000 cfm.If
the filter label simply states “99.97% HEPA Filter” or “2,000
CFM HEPA Filter”, but does not include all of the detailed testing
information, it’s probable that the filter has not been individually
tested.
- Is the machine designed to prevent bypass
leakage around the HEPA filter?
- Even the best HEPA filter can’t prevent contamination
problems if the machine is not designed to prevent contaminated air from
bypassing the filter. Look for features such as:
- Flat, rigid HEPA sealing frames with no welds or other surface
irregularities that can disrupt the seal
- Closed-cell neoprene gasket material rather than open cell
foam or silicone gel to seal critical seams
- Solid rivet fasteners versus weaker and potentially leaky hollow
pop-rivets or metal screws
- Sealing enclosures behind control panels to prevent air leakage
around and through components
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- Is the machine safe?
- Why take the chance on an untested and potentially unsafe
machine? OSHA (U.S.) and CSA (Canada) require electrical safety testing
and certification by an approved Nationally Recognized Testing Laboratory
(NRTL), such as UL, CSA or ETL. This overall testing is required even if
the individual electrical components are all NRTL listed. Contractors may
be surprised to learn that the HEPA-AIRE negative air machines manufactured
by Abatement Technologies are currently the only major brand that meets
these requirements. Our machines have been tested by Intertek Testing Services
NA, Inc. and carry their ETL/ETLC mark for safe operation on 115V/15A electrical
supplies.
- What is it that makes HEPA filters so
efficient?
- The ultra-fine glass-fiber medium captures microscopic
particles that can easily pass through other filters by a combination of
diffusion, interception and inertial impaction. To qualify as a Type A HEPA
filter, the filter must capture at least 99.97% (9,997 out of 10,000) of
particles 0.3 microns in size–about 300 times smaller than the diameter
of a human hair, and 25 to 50 times smaller than we can see. To a HEPA filter,
catching a one-micron particle (1/1,000,000 of a meter) is like stopping
a cotton ball with a door screen.
- Why is the testing done with a 0.3-micron
particle size test aerosol?
- Filter efficiency studies have shown that 0.3-microns
is the "Most Penetrating Particle Size (MPPS)" for HEPA filter
media. Efficiency is typically greater than 99.97% against larger or smaller
particle sizes. Particles larger than 0.3 microns are typically more easily
trapped, or intercepted, by the media. Smaller particles often lack sufficient
mass to penetrate the media.
- Does HEPA filter efficiency decrease
as the filter gets dirty?
- No. The dirtier a HEPA filter gets, the more efficient
it typically becomes.
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