FIELD OF THE INVENTION

The present invention relates in general to air treatment devices such as air purifiers, possible also capable of humidifying and/or de-humidifying. Particularly, the present invention relates to a method for measuring and controlling the purification performance of such devices.

BACKGROUND OF THE INVENTION

Air, both indoors and outdoors, can be polluted with several types of substances such as gases, vapours, solid particles, micro-organisms, typically indoors more so than outdoors. Poor air quality indoors can be harmful to the health of the inhabitants. Therefore, air purifiers have been and still are being developed for removing pollutants from the air, as a consumer device for use in a user's home. Figure 1 schematically illustrates the basic design of an air purifier 1, comprising a housing 2 having an inner chamber 5, with an inlet port 3 and an exit port 4. An air flow passes through the chamber 5 from the inlet 3 to the exit 4, forced by a fan 6. A filter 7 is arranged between inlet 3 and exit 4, such that all air must pass the filter 7. The fan 6 may be positioned upstream or downstream of the filter 7. The fan 6 may be part of the purifier 1, and typically is, although it is also possible that the fan 6 is an external device. In this respect it is noted that the purifier may be of a type integrated in air conduits, for instance in office buildings, but the purifier may also be of a stand alone or mobile type, placed in the room of which the air is to be purified. In such case, the inlet 3 will typically be implemented as an opening in the housing, provided with a grating. Likewise, the exit may be implemented as an opening in the housing, provided with a grating, typically opposite the inlet.

There are different types of filters possible, but generally they have a tendency of getting polluted so that the filter efficiency reduces. For instance, in the case of non-woven structures made from electrostatically charged fibres to collect particles, also indicated as Electret filter media, the filter surface may get polluted by non-conducting particles. Surface pollution by non-conducting particles similarly reduces the performance of Electrostatic Precipitation (ESP) modules in which particles are first charged by a corona wire or other means and afterwards are collected when passing electrostatically charged plates. It is also possible that the filter 7 is of an adsorption type, typically containing activated carbon, in order to remove volatile and hazardous organic components and gasses from the air. The activated carbon can have many different shapes (e.g. granules, sponge structures, deposited on corrugated surfaces, monolithic etc.). The activated carbon filter can also be impregnated with additional species (like KMn0 ₄ ) to increase its activity towards specific gasses. It is to be noted that the adsorption filter may also be made of other materials like zeolites, clay, polymers, or the filter may contain mixtures of any of the above- mentioned adsorbing materials. The adsorbing materials typically have active spots

(adsorption sites or micro-pores) where the molecules to be trapped are engaged and held. However, such active spot can only hold a certain maximum number of certain types of molecules. Once such active spot has trapped the maximum number of molecules, it cannot trap another molecule: thus, this spot is no longer an active spot trapping molecules. During use of the filter, more and more active spots become occupied and the removal efficiency of the filter medium decreases with time.

There are also filters having an operation principle based on oxidation, for instance using UV light or photochemical or corona discharge techniques. Also in these cases, reduction of the filter efficiency can occur, for instance caused by surface pollution (UV lamps, photocatalytic surfaces, corona wires).

The decay rate of the filter efficiency depends on many factors such as filter structure, filter material, filter material properties such as pore size distribution, and operating conditions. It is desirable to be able to measure the actual filter efficiency and take appropriate measures.

The air purification performance of air purifiers is often expressed as the Clean Air Delivery Rate (CADR), defined as CADR = Φ * η, in which:

Φ = flow of air that leaves the purifier through exit port 4

η = 1-pass removal efficiency of the purifier for a specific pollutant, defined as

in which

C = the fraction of the air-flow that passes the purification modules

Cm = the pollutant concentration in the incoming air- flow

Cout = the pollutant concentration in the exit air-flow

With respect to filter modules in air purifiers, a break-through β is defined according to the formula:

β = C ₀ ut,f/Cin,f in which

c ₀ ut,f indicates the pollutant concentration downstream of the filter, and

Cm,f indicates the pollutant concentration upstream of the filter.

Using a filter efficiency r|f defined similarly as the purifier efficiency η, it can be seen that β = 1 - r\f applies.

The break-through β indicates the amount of pollutant passing the filter: the higher β, the lower is the quality of the filter.

The CADR value of air-treatment devices can change dramatically over time. This change can result from a change of Φ as well as from a change of η. In most cases, both Φ and η decrease over time.

As an example, figure 2 is a graph showing test results in accordance with DIN 71460 of an activated carbon filter when placed in a 250 m ³ /hr air-flow of 50% relative humidity containing 20 ppm of toluene. The horizontal axis represents test time, the vertical axis represents the break-through β (expressed as a percentage p = β· 100%). It can clearly be seen that the break-through increases with time, hence the CADR decreases.

It is obvious that the removal performance of activated carbon filters can drop significantly during the life-time of an indoor air purifier. Hence, if no preventive measures are taken, users of air purifiers have to replace their activated carbon filter regularly. Many producers of air purifiers mention that their filters must be replaced annually or even more frequently.

Figure 3 is a graph showing test results of the same filter type as figure 2, when placed in a 250 m ³ /hr air- flow of 90% relative humidity containing 20 ppm of toluene. As is typical for most activated carbon filters, figures 2 and 3 show that the filter breakthrough is substantially influenced by humidity, increasing faster at high relative humidity. High relative humidity as high as 90% and more can exist within residential homes in humid tropical regions and during rainy summer periods in moderate climates.

If no preventive measures are taken, in humid environments, the removal efficiency of activated carbon filters will rapidly decrease to low values. As a consequence, the air purifier must operate for longer periods of time before the indoor air quality has improved to acceptable levels. This causes annoyance to the user and also increases the daily power consumption and electricity costs of the air purifier.

A decrease of η causes a decrease of CADR, but CADR can also decrease as a result of a decrease in Φ, which often occurs because of an increasing pressure drops over the purification module. To illustrate this, figure 4A shows pressure drop values of a HEPA filter of360*280 mm ² in four different situations:

- new (curve 41)

- loaded with 2.5 gram of diesel soot particles (curve 42), the particles having a size in the range of 30 - 300 nm;

- loaded with smoke of 100 cigarettes(curve 43);

- loaded with 10 gram of micron-sized Isofine A2 particles (curve 44).

The three loaded filters simulate filters that have been polluted during use, and the results show that the pressure drop of the particle filters in air purifiers can change over time. Typically, particle filters in air purifiers collect, depending on the environment and application:

- Inorganic micron-sized particles that are generated by construction activities, other physical processes or can be emitted by organisms

- Nano-sized organic particles that can be emitted by incineration processes like the engines of automotive cars, candles or burning processes for domestic

heating/cooking purposes

In addition, one of the filters is loaded with cigarette smoke which is an important pollution type in many air purification applications. This particle type consists of organic nano- and micronparticles.

The horizontal axis represents flow rate in m ³ /h, the vertical axis represents pressure drop in Pa. It can clearly be seen that ageing of the filter results in a higher pressure drop, and that the measure of increase depends on the type of particles that are collected on the filter. In the results of figure 4A, it seems that smoke does not play an important role.

However, figure 4B shows the results of the same test but for a different filter type: while in figure 4A the filter was of type Coway AP- 1008BH/AP- 1008DH, the filter of figure 4B was of type YADU HJH2801. It can clearly be seen that in this filter type smoke particles do increase the pressure drop.

Figure 4C is a graph illustrating that aging also results in a decrease of the filtering efficiency. The figure shows the 1-pass efficiency for soot particles (obtained in the exhaust gas of a diesel motor, and having a particle distribution in the size range of

30-300 nm) for the same filter type as used in figure 4 A, and shows the results obtained for a new filter (curve 47), a filter loaded with 2.5 gram of diesel soot particles (curve 48), and a filter loaded with smoke of 100 cigarettes (curve 49). The figure clearly shows the reduced efficiency, depending on particle size (horizontal axis). So, it can be seen that ageing (pollution) of a filter can reduce the efficiency and/or can reduce the air flow. It is to be noted that an increase in filter pressure drop as such does not necessarily reduce the air- flow: for instance, it may be that the purifier is provided with a fan control that measures the air flow and keeps the air flow constant by increasing the motor power.

SUMMARY OF THE INVENTION

The problem of reduced performance of air treatment apparatus has already been recognized in the prior art. However, methods disclosed in prior art for monitoring the performance of an air treatment apparatus are either directed to measuring pressure drop (see for instance US 7261762 or 7186290) or directed to detect a reduction of r\f (see for instance US 6454834). This means that they are not really suitable for reliably predicting end-of-live of an air treatment apparatus, or in any case the filter thereof. In fact, prior art apparatus do not control CADR values, and the CADR values of these apparatus are not known and are also not guaranteed. As a result, based on inadequate information, an air treatment apparatus may suggest the user to change filters while this is actually not needed yet, leading to unnecessarily high costs, or conversely the user continues too long with an air treatment apparatus that is actually not operating properly any more, thus leading to the user being exposed to unhealthy air.

It is an objective of the present invention to overcome or at least reduce the above-mentioned problems. Particularly, the present invention aims to provide an air purifier capable of providing a reliable indication of its performance and a reliable indication as to when a filter unit must be replaced or regenerated. To this end, the present invention proposes to measure both Φ and η and to multiply the measurement results in order to obtain a more reliable value of the CADR. Specifically, the present invention provides a purifier device as mentioned in claim 1.

Further advantageous elaborations are mentioned in the dependent claims. It is noted US-2010/0058927 does recognize that both Φ and η can change over time, but even this document does not propose a method wherein measurements of Φ and η are combined to provide a measurement of CADR. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:

Fig. 1 is a diagram schematically illustrating the basic design of an air

purifier;

Fig. 2 is a graph showing test results of an activated carbon filter;

Fig. 3 is a graph showing test results of the same filter type as figure 2, in different circumstances;

Fig. 4A-B are graphs showing test results of pressure drop measurements on new and aged filters;

Fig. 4C is a graph showing test results of efficiency measurements on new and aged filters;

Fig. 5A is a diagram schematically illustrating an embodiment of an air

purifier according to the present invention during normal operation;

Fig. 5B is a diagram schematically illustrating the air purifier of figure 5A when operating in monitor mode.

DETAILED DESCRIPTION OF THE INVENTION

An air purifier 100 according to the present invention will be discussed with reference to figures 5A-5B. The air purifier 100 has a housing 2 with an inlet opening 3 and an outlet opening 4. Within the housing 2, a filter support 9 holds a filter 7 in place. Parallel to the filter 7, a bypass 10 is arranged in the housing, provided with a controllable closure device 11 such as a valve. The interior 5 of the housing 2 has a flow path from inlet 3 to outlet 4 configured as an inlet chamber 53 communicating with the inlet opening 3 at the upstream side of the filter 7, and an outlet chamber 54 communicating with the outlet opening 4 at the downstream side of the filter 7. In extreme embodiments, the filter 7 may be placed immediately behind the inlet 3 or immediately before the outlet4, in which case the inlet chamber 53 or the outlet chamber 54 has only a small volume or is actually absent.

A controllable propelling device 6 such as a fan (compare 6 in figure 1) is shown upstream of the inlet opening 3, but may alternatively be arranged in the inlet chamber 53, in the outlet chamber 54 or downstream of the outlet opening 4. In a stand alone or mobile purifier unit, the fan will typically be located within the housing. Since the present invention can be implemented with prior art propelling devices, a further explanation of design and operation of the propelling device, which will hereinafter simply be indicated as "fan", is omitted here.

The filter support 9 forms a sealing separation wall between the inlet chamber 53 and the outlet chamber 54, so that air flow from the inlet chamber 53 to the outlet chamber 54 is only possible via the filter 7 and, if the valve 1 1 is open, via the bypass 10.

The filter may be identical to prior art filters, specifically filters that remove volatile and gaseous air pollutants or particle pollutants. Filters for volatile and gaseous pollutants can be made of many different materials. Most of these filters are made of activated carbon (which is the preferred embodiment, having various morphologies such as granules, sponge structure, monolith, activated carbon on corrugated structures, woven or non-woven carbon fiber structures), zeolite and/or clay materials. However, these filters can also be made of polymeric materials, metal oxides, metals, glass and other adsorbing substrates.

Particle pollutant filters are typically non- woven fiber structures made from polymeric or glass fibers but can also consist of an ESP unit that contains charged metal surfaces.

The air purifier 100 further comprises a control device 20, for instance a suitably programmed microcontroller or microprocessor or the like, controlling the valve 1 1. For sake of simplicity, the control device 20 is shown outside the housing 2.

The air purifier 100 can operate in a normal operating mode, in which, under control of the control device 20, the valve 1 1 is closed. In this mode, ambient air will pass from the inlet 3 through only the filter 7 and will leave the outlet 4 as purified air, as illustrated in figure 5 A. For monitoring the performance of the filter 7, the air purifier 100 comprises a first pollutant measuring device 21 arranged upstream of the filter 7, preferably in the inlet chamber 53, preferably close to the filter 7, and a second pollutant measuring device 22 arranged downstream of the filter 7, preferably in the outlet chamber 54, preferably close to the filter 7. Examples of suitable pollutant measuring devices include nano-particle sensor, micron- sized particle sensor, or sensor for volatile components like TVOC, formaldehyde, C0 ₂ . The control device 20 is connected to receive measurement output signals of the pollutant measuring devices, and is designed to process these signals and to calculate the 1-pass removal efficiency r\f of the filter 7, or a value proportional thereto.

If desired, the air purifier 100 may also comprise a humidity sensor 23 for sensing the relative humidity, also providing its measurement output signal to the control device 20. It is noted that parameters that are correlated with r| _f can be provided in many ways, depending among other things on the type of filter. If the filter is designed for removing particles, the pollutant measuring devices should typically include particle sensors. If the filter is designed for removing one or more gases, the pollutant measuring devices should typically include gas sensors. It is also possible that the filter is designed for removing gases and particles. In case a filter is designed for removing two or more gases and/or particles, it is possible that such filter has different efficiency values η corresponding to the different pollutants to be removed, and typically there will be multiple sensors, each designed for sensing a specific pollutant.

The air purifier 100 can operate in a flow monitoring mode, in which the control device 20 has opened the valve 11. In this mode, a small part of the air will flow through the bypass 10, bypassing the filter 7. A flow sensor 24, arranged in the outlet chamber 54 close to the bypass 10, senses the bypass flow and is capable of generating a measurement signal indicative of, preferably proportional to, the bypass flow. The control device 20 is connected to receive the measurement output signal of the flow sensor 24, and is designed to process this signal and to calculate the flow, as volume per unit time, or a value proportional thereto.

Suitable flow sensors are known and can be used here, such as for instance thermal sensors or acoustical sensors or rotary speed sensors, therefore a more detailed description of design and operation of such sensor is omitted here.

Alternatively or additionally, the air purifier 100 may comprise pressure sensors arranged at opposite sides of the filter 7 and providing their measurement signals to the controller 20 in order to be able to calculate flow speed. In such case, the bypass 10 and controllable valve 11 may be omitted, and the flow speed can be calculated continuously during normal operating mode (in other words: the flow monitoring mode can be

simultaneous with the normal operating mode).

In case of the first embodiment, switching between normal operating mode and flow monitoring mode can be done in different ways. For instance, it is possible that the switching is done on the basis of time, i.e. the control device 20 would be responsive to an input timer signal triggering the control device 20 to switch at regular intervals.

In any case, based on the measurements described, the control device 20 calculates the filter efficiency r| _f and the air flow Φ through the filter, and calculates the CADR by multiplying these values. Based on the thus calculated CADR (or a value proportional thereto), the control device 20 adapts the power of the fan 6 to keep the CADR substantially constant. Thus, a reduction in filter efficiency r| _f can be compensated by an increase of the air flow Φ. If the CADR becomes too low while it is no longer desirable or even impossible to compensate by increasing the air flow Φ, the control device 20 produces a warning for the user to replace the filter.

Summarizing, the present invention provides an air purifier 100 which comprises:

a housing 2 with an inlet chamber 53 communicating with an inlet opening and an outlet chamber 54 communicating with an outlet opening;

a filter unit 7 for purifying air;

- a controllable fan 6 for causing air flow from the inlet chamber to the outlet chamber through the filter;

a control device 20 for controlling the fan;

first measuring means 21, 22 for measuring a 1-pass removal efficiency η of the purifier;

- second measuring means 24 for measuring an air flow Φ through the purifier.

The control device 20 processes the measuring signals received from the first and second measuring means, calculates a Clean Air Delivery Rate CADR as CADR = Φ * η and, on the basis of the calculated CADR, controls the fan such as to keep CADR

substantially constant or higher than a predetermined level.

While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims.

For instance, the air flow Φ can be measured by other means, and the invention includes different methods for measuring air flow in a purifier device. It is for instance possible to use an air speed measuring device arranged in an inlet duct leading to the inlet opening 3 or in an outlet duct leading away from the outlet opening 4; an example of such measuring device is an anemometer. In such case, it is not necessary to operate a valve in a bypass opening.

A measuring signal correlated to the air flow Φ can also be generated on the basis of air pressure drop over the filter 7. This can be measured by arranging pressure sensors in the chamber 53 and 54, but it is also possible to sense the force exerted on the filter as caused by the pressure difference, for instance by measuring the reaction force exerted by the filter on the housing, for instance by using deformation sensors such as piezo-elements or strain gauges.

When using a bypass opening, it is possible to measure directly the flow speed of air passing this opening, but it is also possible to measure a parameter correlated to such flow speed, for instance the frequency and/or intensity of (acoustic) vibrations caused by such flow.

With respect to measuring sensors for measuring the pollution in the incoming air, depending on the implementation of the purifier, it is not specifically necessary that the sensor is located inside the housing. Particularly in the case of stand alone or mobile devices, which are to be placed inside a room in for instance an office building, the quality of the air in the room corresponds to the quality of the incoming air, so an external sensor, or a sensor mounted in the inlet opening, can be used to monitor the quality of the ambient air before entering the purifier. In cases where the controller is designed to switch the purifier on or off on the basis of the quality of the incoming air, such external sensor has the added advantage, when the purifier is switched off and is not sucking in air, that the air pollution outside the purifier can be sensed so that the device can respond quicker.

Likewise, although the above describes that the quality of the exit air-flow is measured with sensor(s) placed in the outlet chamber 54, suitable sensor(s) can also be located in the outlet opening 4 or mounted at the outside of the housing 2 if the sensor(s) are exposed to an air-flow that has the same quality as the air in outlet chamber 54.

Further, the purifier may be provided with other air treatment devices. For instance, a dehumidifier may be arranged upstream of the filter 7.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the words "comprising" and

"containing" do not exclude other elements, components or steps, a signal containing some information may also contain further information, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. The control device 20 may be implemented in hardware, where its function is performed by individual hardware components, but it is also possible that the control device 20 is implemented in software, so that its function is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.