Clean Air Delivery Rate
From Wikipedia, the free encyclopedia
The AHAM-certified seal lists a rating for tobacco smoke, pollen, and dust removal, and a rating for room size.
Clean Air Delivery Rate (CADR) is a
figure of merit that is the
cubic feet per minute (CFM) of air that has had all the particles of a given size distribution removed. For
air filters
that have air flowing through them, it is the fraction of particles (of
a particular size distribution) that have been removed times the CFM
air flow rate through the device. More precisely, it is the CFM of air
in a 1008 cubic foot room that has had all the particles of a given size
distribution removed, over and above the rate at which the particles
are naturally falling out of the air. Different filters have different
abilities to remove different particle distributions, so three CADR's
for a given device are typically measured:
smoke,
pollen, and
dust. By combining the amount of
airflow
and particle removal efficiency, consumers are less likely to be misled
by a high efficiency filter that is filtering a small amount of air, or
by a high volume of air that is not being filtered very well.
Applicability
The CADR ratings were developed by
Association of Home Appliance Manufacturers (
AHAM)
and are measured according to a procedure specified by ANSI/AHAM AC-1.
The ratings are recognized by retailers, manufacturers, standards
organizations, and government bodies such as the
EPA[1]and the
Federal Trade Commission.
Whole house air cleaners are not covered by the CADR specification
because the measurement is performed in a standard 1008 cubic foot room,
the size of a typical house room, which has different airflow patterns
than whole-house filters. Measurements are made with the filter running
and not running, so particles that naturally fall out of the air are not
being counted as part of filter's operation. The measurement only
applies to particulate matter, not to gases.
Any device or technology that removes particulate matter from the air
can be tested for CADR numbers. Anyone with the necessary equipment can
perform the ANSI/AHAM AC-1 measurements. The AHAM performs the tests
for manufacturers who choose to use their service which allows the
manufacturer to display a seal that certifies AHAM performed the test.
[2]
The rating is only valid for a given filter used in a specific
equipment design, and only at the beginning of its service life.
Choosing a higher or lower efficiency filter than the unit was designed
for may decrease its ability to filter air. An exception is when a high
efficiency filter does not decrease the fan's airflow rate. This is
usually achievable only with physically larger or thicker filters, which
usually can't be used in a unit designed for smaller filters. Filters
with efficiencies higher than the original may slow the fan's air flow
rate down which may result in a lower CADR rating.
Due to the measurement process, the CADR rating is intended for use
only with equipment designed for residential spaces. Clean rooms,
hospitals, and airplanes use high-efficiency
HEPA filters and do not use a CADR rating, but instead may use
MERV ratings.
Understanding the Rating
The AHAM seal (usually found on the back of an air cleaner's box)
lists three CADR numbers, one each for smoke, pollen, and dust. This
order is from the smallest to largest particles and corresponds to the
most dangerous to the least dangerous particles. The higher the CADR
number, the more air it filters per minute for that particle size range.
Consumers can use these ratings to compare air cleaners from the
various manufacturers.
[3]
The defined particle size ranges are 0.09–1.0
µm for smoke, 0.5–3 µm for pollen, and 5–11 µm for dust.
AHAM recommends that filters be sized for rooms so that the smoke
CADR times 12 is equal to or greater than the cubic feet volume of the
room. This recommendation is based on the assumption that the room will
have air exchanged with other rooms at a rate of less than 1 room volume
per hour, and that the customer desires at least 80% of the smoke
particles removed from the air. For 8 foot ceilings, this means the area
of the room (in square feet) should be less than 1.5 times the CADR
rating for smoke. Much larger rooms can be effectively filtered if there
is no air coming from the outside, and if there is no significant
continuing source of particulates in the room.
[4]
MERV
14 filters are capable of reducing smoke particles by approximately 80%
when operating at the filter's design velocity, so a CADR smoke rating
on a simple filtering unit that uses a MERV 14 filter will be
approximately 0.80 times the fan flow rate in CFM. If the filtering unit
does not mix the test room's air very well, it may receive a lower CADR
measurement because it does not operate as efficiently as it should. If
a filtering unit uses a MERV 12 filter that removes roughly 40% of the
smoke particles, then it may still obtain a smoke CADR of 80 by
filtering 200 cubic feet per minute instead of 100 CFM. Conversely, a
99.97% HEPA filter (MERV 17) that removes over 99.9% of the smoke
particles needs to filter 80 cubic feet per minute to get a CADR of 80.
This shows the CFM airflow of a unit is always equal to or greater than
the CADR rating.
Large particles naturally fall out of the air faster than small
particles, but the CADR rating is based on how well the filter works
over and above this effect. So CADR ratings for dust and pollen come out
lower than would be expected by looking only at the filter's efficiency
at removing large particles. This "bias" against the filter's
efficiency at removing large particles is a relative bias in favor of
the filter's ability to remove small (smoke) articles. Since smoke
particles are the most difficult to filter (lower filter efficiency
relative to large particles), the two effects largely cancel so that
CADR ratings are usually similar for both small and large particles. A
filter that is very good at removing smoke particles by using a slow fan
or electrostatic effects will not get as good CADR numbers for pollen
and dust because those particles will fall down and deposit on room
surfaces during the test, before the filter has had a chance to collect
them.
[5]
For smoke-sized particles, a MERV 12 filter may function as well as a
MERV 14 filter at half its rated air velocity (for smoke particles),
and a MERV 14 may function like a MERV 12 at double its rated velocity.
This is because smoke-sized particles depend on diffusion (
Brownian motion)
onto fibers as much as impaction, rather than completely on impaction
like dust. A slower air speed gives diffusion more time for the particle
to stick to the fiber or previously attached particles. Conversely, a
higher filter speed may increase the collection of larger particles
because impaction depends on the inertia of the particles. As a filter
gets clogged from use, the fan air speed drops so that the effective
CADR for smoke may actually rise rather than decrease, while the CADR
for dust will be lower from the decrease in fan speed, especially
because the particles fall out before they are filtered.
If half the room's volume of air is exchanged with other rooms every
hour, then HEPA filters are not more effective than dust spot 85%
efficiency filters (roughly, MERV 13 or 3M's MPR 1900). The unit's fan
speed will be the dominant factor if air from outside the room is coming
in too quickly.
[6] [7]
History
In the early 1980s, AHAM developed a method for measuring the clean
air delivery rate for portable household electric room air cleaners. The
resulting standard became an
American National Standard
in 1988 and was last revised in 2006. Known as ANSI/AHAM AC-1, it
measures the air cleaner's ability to reduce tobacco smoke, dust and
pollen particles in a room. It also includes a method for calculating
the suggested room size.
See also
References
- Residential Air Cleaners (2 ed.). United States Environmental Protection Agency. 2009.
- "Identifying AHAM Verifide Products". AHAM Verifide. Retrieved 20 November 2013.
- Saar, Ramona J. "Response to Public Comments on Proposed Test Procedure for Air Cleaner Energy Star Program". Association of Home Appliance Manufacturers. Retrieved 20 November 2013.
- "About the Program". AHAM Verifide. Retrieved 20 November 2013.
- Shaugnessy, R.J.; R. G. Sextro (2006). "What Is an Effective Portable Air Cleaning Device? A Review". Journal of Occupational and Environmental Hygiene 3: 169–181. doi:10.1080/15459620600580129. Retrieved 20 November 2013.
- Fisk, William J.; David Faulkner; Jari Palonen; Olli Seppanen (2001). "Performance and Costs of Particle Air Filtration Technologies". Lawrence Berkeley National Laboratory. Retrieved 20 November 2013.
- "Comparing 3M Filtrete Filters". iaqsource.com. Retrieved 2012-06-23.
