Field of invention

The present invention relates to an apparatus for silent air cleaning and a method for reducing the sound pressure of a portable air purifier according to the preambles of the independent claims 1 and 3 and according to the preambles of the dependent claims.

Background of the invention

Portable air cleaners (PACs) are small hand transportable air cleaning units used in occupied spaces (classrooms, offices, homes) to reduce the concentration of airborne particles and sometimes vapours and gases. They typically weigh 5-100 kg, are freestanding, use local electrical current, and can be placed anywhere in a room.

The best ways to improve indoor air are to remove the pollutant sources and ventilate with clean outdoor air. Portable air purifiers can help when those methods are insufficient or not possible. Also known as air sanitizers, or air cleaners, portable air purifiers are designed to filter the air in a single or a few rooms, not the entire house, like a HVAC system does.

There are two basic types of technology that portable air cleaners use to remove particulate matter from the air:

• Some portable air cleaners use fibrous media air filters to capture particles in the same way that MERV-rated filters capture particles in a central HVAC system. In these units, there is often a prefilter, which would have a lower MERV (Minimum Efficiency Removing Value) rating, and a HEPA (High Efficiency Particulate Air) filter, which would have a much higher MERV rating. This is a simple technology, and it has the advantage that little can go wrong with it.

• Some portable air cleaners use electricity to make particles in the air adhere to plates or other • surfaces. Cleaners in this category include electrostatic precipitators (ESPs) and ionizers. Some of these devices may unintentionally produce ozone, which can be harmful.

Additionally, some portable air purifiers offer ultraviolet germicidal irradiation (UVGI), a technology that uses ultraviolet (UV) light to damage microorganisms such as bacteria and viruses, making them non-infectious.

Portable air purifiers with pleated filters use fans to force air through a dense web of fine fibers that traps particles. Filters with very fine mesh are High Efficiency Particulate Air (HEPA) filters— those certified to collect typically more than 99.5 percent (E12) or 99.95 percent (H13) of particles of a certain size (0.3 microns in diameter— smoke and paint pigments, for example). HEPA filters can remove larger particles, too, including dust, pollen, and some mold spores while they're suspended in the air. HEPA filters can also very effectively reduce the number of smaller particles, 0.3 microns typically being the particle size which is most penetrable through the filter.

The 0.3 miti mentioned above is a typical Most Penetrable Particle Size (MPPS) for fibrous filters manufactured from microglass fibers or other fibers having an average diameter of 10- 20 miti. If the diameter of the filter fibers is reduced - as in the case of using e.g. polyvinylidene fluoride (PVDF) or expanded polytetrafluoroethylene (ePTFE) fibers - the MPPS decreases even to less than 100 nm (0.1 miti) which must be taken into account in filter measurements as evaluation in the ISO 29463 standard is based on particle arrestance at MPPS.

There is a wide variation in the performance of air cleaners that is dependent on the specific air cleaner design and various indoor factors. The most widely used method in the United States to assess the performance of new air cleaners is the procedure described in the American National Standards Institute (ANSI)/Association of Home Appliance Manufacturers (AHAM) AC-1-2002. This method describes both the test conditions and the testing protocol. The protocol yields a performance metric that is based on the measured decay rate of contaminant concentrations with the air cleaner operating compared with the measured decay rate with the air cleaner turned off.

The resulting metric, the clean air delivery rate (CADR), permits both an intercomparison of performance among various air cleaners and a comparison of air cleaner operation to other contaminant removal processes.

US 2012085231 Al, Kristensson Dan Allan Robert et al., 12 April 2012 (12.04.2012) discloses a portable air cleaner, a method for cleaning air, and use of a HEPA filter. The portable air cleaner is intended for purifying air passing through said cleaner at least from particulate matter. The portable air cleaner comprises a fan and a high efficiency particulate air (HEPA) filter. A suitable HEPA class, according to the document, is Hll or higher. The filter media of the HEPA filter mainly consists of polymer fibers and/or fiberglass. The document states that comfort for a sleeping user may be increased by reducing the noise of the portable air cleaner and that most of the unwanted noise is generated by the fan: increased pressure drop across the HEPA filter increases the fan speed which, in turn, generates unwanted noise. It is thus preferable to minimize the pressure drop of the HEPA filter.

The document leads the reader away from the current invention as it states that "any suitable filter media or device adapted to filter particles or gases unwanted at the point of care may be used. Including for example any combinations of fiberglass and/or polymer fiber filters or electrostatic filters, or hybrid filters (i.e. charging incoming particles and/or the filter media), or radiation methods (i.e. UV-light), or chemical and/or fluid methods, or activated carbon filters or other filter types." (paragraph [0036]). In fact, this is not the way to reduce pressure drop of the HEPA filter as the pressure drop is minimized by reducing the fiber diameter of the filter fibers. This can be achieved by using expanded polytetrafluoroethylene (ePTFE) fibers, which document US 20120085231 Al is totally silent of.

US 5507847 A, George Nelson A et al., 16 April 1996 (16.04.1996), discloses an ultra-low penetration air (ULPA) filter, a method of leaning air, a use of an ultra-low penetration air filter and a filter testing arrangement. First independent claim of the document claims "a filter for use in air conditioning systems in clean facilities which comprises a primary filtration layer of microporous polytetrafluoroethylene (PTFE); at least one upstream layer of permeable support material; at least one down stream layer of permeable support material, the filtration layer being loosely sandwiched between the upstream layer and downstream layer of support material; wherein the primary filtration layer and the support materials combine to provide a filtration efficiency of at least 99.97% at 0.3 miti and less that 1 inch water pressure drop at lOO ft/min air velocity." (Claim 1).

The document speaks on filtration in clean rooms, like semiconductor manufacturing or pharmaceuticals and is silent on portable air cleaners (room air purifiers). Also, the document is not consistent as the description speaks about 10.5 ft/min or 100 ft/min air flow rate (column 4, rows 31-62) but claim 1 speaks only about 100 ft/min air flow rate. All these examples are again given for clean room environment.

These two documents have existed for almost ten years (and there are similar documents which have been public for even longer time). There has been motivation for reducing noise of a portable air cleaner for decades as well. However, we have not found any single document or device which would use ePTFE material for the filter in a portable air cleaner and thus we claim that combining such documents has not been obvious for a person skilled in the art.

A problem with the current portable air purifiers comprising a HEPA filter is their noise. The sound pressure of a portable air purifier depends on the power level where it is used and with effective CADR (around 450 m ³ /h), the sound pressure can be like the one generated by a dishwasher. Such noisy equipment cannot be used e.g. in bedrooms and thus the portable air purifier power may need to be turned considerably lower for the night-time which obviously reduces the air purification result.

Brief description of the invention

The object of the present invention is to provide a method so as to overcome or at least alleviate at least some of the prior art disadvantages. The objects of the present invention are achieved with method according to the characterizing portions of independent claims 1 and 3 and according to the characterizing portions of the dependent claims.

After the accidental discovery of expanded polytetrafluoroethylene (ePTFE) in 1969, fibrous filters manufactured from it have been used to capture contaminants and other particles (since 1973). First HEPA filters based on ePTFE were introduced in 1994.

A unique feature of ePTFE-based HEPA filters is their low operating resistance. This is assumed to arise from the "slip flow" principle caused by the much thinner fibers in an ePTFE HEPA compared to conventional glass fiber HEPA. Nanofibers bend air less and thus air sees lower resistance. The lower operating resistance means that less power is required to operate the fans pushing air through the filter which generates savings in power consumption.

There are other advantages of using ePTFE HEPA filters in centralized ventilation systems, too and thus, despite of their higher cost, ePTFE HEPA filters are used in some special ventilation systems. Within almost 30 years of existence of ePTFE HEPA filters, their market share is still low, mostly because of their higher cost.

The low operating resistance of ePTFE HEPA filters produces, however, another advantage which according to our understanding has not been used as an advantage despite of the long existence of ePTFE HEPAs: the lower power requirement also means lower operating noise.

It has to be noted that in centralized ventilation the machine rooms where fans are situated, are typically placed far away from the terminals pushing air into the points where clean air is needed. Placement in distance does not generate a need to considerably reduce the sound pressure. Situation is very different with portable air purifiers, where the fan is situated in the apparatus (air purifier) itself. The dominant source of noise in a portable air purifier is the fan and reducing fan's power level (and rotation speed) reduces the noise level. Changing a glass- fiber based HEPA filter to an ePTFE filter allows fan power reduction because of the lower flow resistance (lower pressure drop across the filter) of the ePTFE filter. Sound is the term for the air pressure waves emitted by a vibrating object (such as an air purifier fan) which are picked up by the ear and perceived by the brain through our hearing sense. Noise is merely unwanted sound. Some people's sounds are other people's noise - but there are certain sounds that most people perceive as unpleasant and would call noise. Sound pressure is the actual change in air pressure caused by the vibrating source.

A portable air cleaner, like all other sound-emitting objects, has its sound/noise levels measured in decibels [dB], usually multiplied by an 'A' factor which accounts for the effect of human hearing on the sound. The dB(A) scale is logarithmic. That is, it goes up in powers of ten, rather than in single units. Thus, an air purifier marked as 70dB(A) is actually ten times noisier than one marked as 60dB(A).

Table I Sounds of different sources

Noise levels greater than 50dB(A) can make normal conversation difficult and long-term exposure to levels greater than 70db(A) can damage hearing.

The unique feature of the current invention is a portable air purifier which comprises a HEPA filter and characteristic to the invention is that the HEPA filter mainly comprises fibers of ePTFE for filtration. The invented method describes air purification with a portable air purifier and the characteristic feature is that the method for cleaning air with a fibrous filter is based on the use of ePTFE as the main filtration media.

The current invention is also focused on the use of an ePTFE-based HEPA filter in a portable air filter.

"HEPA filter" in this document means all classes of high-efficiency air filter in the E-, H- and U classes as shown in the table below. Table II HEPA classes

Brief description of the figures

In the following the invention will be described in greater detail, in connection with preferred embodiments, with reference to the attached drawing, in which:

Fig. 1 shows a schematic view of the invented apparatus and method for measuring sound pressure. Detailed description of the invention

Figure 1 shows a portable air cleaner/purifier 101 for purifying air passing through the cleaner 101 at least from particulate matter (PM). The air cleaner 101 comprises a fan 103 and a high efficiency particulate air (HEPA) filter 105. The air cleaner 101 may also comprise a prefilter 107 which may be placed either upstream or downstream the fan 103. Characteristic to the present invention is that the filter media of the HEPA filter 105 mainly comprises expanded polytetrafluoroethylene (ePTFE).

The portable air cleaner 101 comprises a frame 109 where fan and filters are placed. The frame 109 may also be connected to wheels 111 for easy movement of the portable air cleaner 101.

The portable air cleaner 101 of the present invention preferably removes at least 99.5 % of the particles with 0.3 miti diameter passing through the air cleaner 101 when the volumetric flow of air through the air cleaner 101 is at least 300 m ³ /h.

Example 1

Essential for the present invention is to create a portable air purifier and a method for purifying air with a portable air purifier 101 based on the use of a HEPA filter 105 where the noise generated by air purification is essentially lower than with prior-art technology.

The invention was tested with a SmAIRT ^® 600 air purifier 101 manufactured by AirO Oy, Finland. SmAIRt600 uses an electrical charger for generating ions which charge the particles passing through the purifier and thus smAIRt600 conventionally uses electrical enhancement to improve particle filtration. In the experiment of Example 1, the electrical charger was set to be OFF, i.e. there was no electrical enhancement. The air purifier 101 was equipped with four (4) glass-fiber based E12 HEPA filters 105. With 450 m3/h air flow through the purifier 101, the sound pressure of air cleaner 101 was 56 dB(A), when the sound pressure was measured at the front of the air cleaner 101, from the height of the centre line of the air cleaner 101, at 1.5m distance from the front surface of the air cleaner 101. The measurement set-up is schematically shown in Figure 1, showing means for sound pressure measurement 151 which are connected to a measurement microphone 153.

The E12 glass-fiber HEPA filters 105 where then changed to ePTFE-based E12 HEPA filters 105. with the same volumetric flow (450 m ³ /h) the sound pressure measured according to the set up of Figure 1 was 50 dB(A), i.e., using ePTFE based HEPA filters 105 reduced the sound pressure by 6 dB(A),

Example 2

In another embodiment of the present invention, example 1 was repeated with 300 m ³ /h air flow through the purifier lOl.The difference in sound pressure in using glass fiber -based HEPA and ePTFE HEPA was at least 3 dB(A).

Example 3

In yet another embodiment of the present invention, example 1 was repeated with 150 m ³ /h air flow through the purifier lOl.The difference in sound pressure in using glass fiber -based HEPA and ePTFE HEPA was at least 1 dB(A).

The invented method can be applied to any air purifier using fibrous filter for air cleaning. The filter may be pure mechanical filter, it may be an electret filter, or it may be an electrically enhanced filter. What is characteristic of the present invention is the use of an ePTFE-based HEPA filter in a portable air cleaning apparatus.

It is apparent to a person skilled in the art that as technology advanced, the basic idea of the invention can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above examples, but they may vary within the scope of the claims.