Vacuuming Device Comprising a Vacuum Cleaner and a Bag Filter

Abstract

The invention relates to a vacuum-ceaning apparatus comprising a vacuum cleaner and a filter bag made of non-woven fabric, wherein the vacuum cleaner has a nominal electric input power of less than 1200 W, preferably less than 1100 W, more preferably less than 900 W, wherein the vacuum cleaner comprises a motor-fan unit and a controlling device controlling the vacuum cleaner such that the airflow is kept substantially constant at a value of at least 34 l/s, preferably substantially constant at a value of at least 37 l/s, more preferably substantially constant at a value of at least 40 l/s when the filter bag is loaded with DMT8 test dust analogous to EN 60312, and wherein the filter bag is a disposable filter bag made of non-woven fabric which has an airflow drop of less than 15%, preferably less than 10%, more preferably less than 5% in tests demonstrating the reduction of the maximum airflow with a partially filled dust container analogous to EN 60312.

Description

FIELD OF THE INVENTION

The invention relates to a vacuum-cleaning apparatus comprising a vacuum cleaner and a filter bag made of non-woven fabric.

DEFINITIONS

The description of the prior art and the invention is based on the standards, definitions and measuring methods specified below:

EN 60312: EN 60312 denotes the standard in version EN 60312:1998 +A1:2000 A2:2004.

EN 60335: EN 60335 denotes the standard in version EN 60335-2-22010.

Determination of air data: The air data of a vacuum cleaner are determined according to EN 60312, chapter 2.8. The measuring device B according to chapter 5.2.8 is used. if motor-fan units sob are measured, viz. without a vacuum cleaner case, the measuring device B is applied equally.

The measurement of the reduction of the maximum airflow with a partially filled dust container according to chapter 2.9 is carried out with orifice 8 (40 mm).

Nominal electric input power of a vacuum cleaner: The input power of a vacuum cleaner is determined according to EN 60335. According to EN 60335 and EN 60312 the input power is denoted with P₁. According to EN 60335 the nominal input power is the arithmetic mean from the maximum input power and the minimum input power. The maximum input power is measured at the highest airflow (open airflow), and the minimum input power at an airflow of 0 l/s (sealed suction), Electromotively driven attachments such as brushes and the like are disregarded in the input power determination.

Airflow: According to EN 60312 the airflow is determined using the version B measuring chamber. In the prior art this airflow is often also referred to as volume flow or suction airflow.

Airflow drop, constant airflow: The airflow drop is determined in usability tests of vacuum cleaners following EN 60312 (chapter 2.9 of this standard) using the version B measuring chamber. Deviating from the standard the reduction of the airflow is tested by vacuuming 400 g of DMT8 test dust in 50 g portions, provided the highest usable volume of the filter bag (see chapter 2.7 of this standard) is above 2 l. The three conditions described in chapter 2.9.1.3 of the standard as leading to the discontinuation of the test are disregarded. Chapter 2.9.1.3 is relevant for volumes below 2 l. This method of measuring the airflow drop thus modified as against the EN 60312 standard will be referred to as “analogous to EN 60312” in the present description and the present patent clairns.

A constant airflow q is assumed if the airflow q_c is not lower after the vacuuming of the DMT8 test dust than the airflow q_max with an empty dust container (cyclone vacuum cleaner), respectively empty filter bag (bag vacuum cleaner). Typically, 400 g of DMT8 test dust are vacuumed in 50 g portions. The test is performed with orifice 8 (40 mm). With regard to the definition of the term orifice reference is made to EN 60312, chapter 5.2.8.2. This orifice corresponds to a relatively open floor nozzle. The airflow drop is calculated according to:

Airflow drop [%]=((q_max.q_c)/q_max)×100

• • q_max=maximum airflow with empty dust container
  • q_c=maximum airflow with partially filled dust container

However, in the present description of the prior art and the invention a substantially constant airflow does not mean that the airflow remains constant in different working situations, e.g. the vacuuming of carpeted floors, respectively hard floor surfaces, or the vacuuming with accessory nozzles. The different orifice areas of these nozzles and the differently strong reduction of this orifice area on different floor coverings result in different airflows, depending on the working situation. With respect to EN 60312 this would correspond to a test with different orifices, with orifice 0 corresponding to a state with a dogged nozzle, Orifice 9 (50 mm) corresponds to a nearly unobstructed inflow. Current floor nozzles typically have an operating point in the range of orifice 7 (30 mm) to 8 (40 mm).

Power increase of the fan motor: The power increase of a fan motor implies an increase of the input power [W]. In a universal motor the power is adjusted by a phase-angle control, In the SR motor (see below) the control voltage of the motor is controlled,

SR motor: An SR motor is a switched reluctance motor which is characterized by a simple and robust construction and high possible speeds (>100,000 rpm). The torque is generated by the reluctance force.

Flat bags: Rat bags as used in the present invention are filter bags whose filter bag wall comprised of two individual layers of a filter material with identical surface areas is formed such that the two individual layers are connected to each other only at their circumferential edges (the term identical surface area does not preclude, of course, that the two individual layers differ from each other by the fact that one of the layers includes an inlet opening).

The connection of the individual layers may be realized by a welding seam or adhesive seam along the total circumference of the two individual layers. However, it may also be realized such that one individual layer made of a filter material is folded about one of its axes of symmetry while the other, open circumferential edges of the so created two sub-layers are welded or bonded to each other (so-called tubular bag). Thus, this type of manufacture requires three welding or bonding seams. Two of those seams then form the filter bag edge. The third seam may equally form a filter bag edge or he on the filter bag surface.

Flat bags as used in the present invention may also comprise so-called gussets. These gussets may be fully unfoldable. A flat bag having such gussets is shown, for instance, in DE 20 2005 000 917 U1 (see FIG. 1 with folded gussets, and FIG. 3 with unfolded gussets). Alternatively, the gussets may be welded to sections of the circumferential edge. Such a flat bag is shown in DE 10 2008 006 769 A1 (see FIG. 1 thereof).

Surface folds: A filter bag whose filter bag wall comprises surface folds is known per se from the prior art, e.g. from the European patent application 10163483.2 (see in particular

FIG. 10a and FIG. 10b, respectively FIG. 11a and FIG. 11b thereof). If the filter bag wall comprises a plurality of surface folds this material is also called a pleated filter material. Such pleated filter bag wags are shown in the European patent application 10002964.4.

FIG. 1 and FIG. 2 show a cross-section of a filter bag comprising a wall with two surface folds. Such surface folds enlarge the filter surface of the filter bag so that a higher dust absorption capacity of the filter bag, along with a high collection efficiency and longer service life, is obtained (as compared with a filter bag having same outer dimensions and without surface folds).

FIG. 1 shows a filter bag 1 comprising a filter bag wall 10 with two surface folds 11 in the form of so-called dovetail folds. The figure shows a cross-section of the filter bag through the filter bag center. The longitudinal axes of the surface folds accordingly extend in one plane which, again, extends perpendicular to the plane of projection, and the surface folds extend at theft longitudinal ends into the welding seams of the filter bag which extend in parallel to the plane of projection and are positioned in front of and behind the plane of projection. Thus, the strongest unfolding of the surface folds is in the middle thereof. The filter bag is here shown in a state in which the surface folds are already unfolded to some extent.

FIG. 2 shows a filter bag 2 comprising a filter bag wall 20 with two surface folds 21 in the form of so-called triangular folds. The figure shows a cross-section of the filter bag through the filter bag center. The longitudinal axes of the surface folds accordingly extend in one plane which, again, extends perpendicular to the plane of projection, and the surface folds extend at theft longitudinal ends into the welding seams of the filter bag which extend in parallel to the plane of projection and are positioned in front of and behind the plane of projection. Thus, the strongest unfolding of the surface folds is in the middle thereof. The filter bag is here shown in a state in which the surface folds are already unfolded to some extent.

Apart from the surface folds illustrated in FIG. 1 and FIG. 2 surface folds having different shapes are feasible, too. It should not be regarded as a limitation that the surface folds in the embodiments of FIG. 1 and FIG. 2 extend perpendicular to a bag edge. Of course, the surface folds may also extend at an angle to the bag edges.

Suction power: The suction power is the product of negative pressure [kPa] and airflow [l/s]. According to EN 60312 the suction power is denoted with P₂.

Efficiency: The efficiency of a vacuum cleaner or a motor-fan unit is determined in accordance with EN 60312, chapter 2.8.3.

PRIOR ART

The requirements made on vacuum-cleaning apparatus have been subject to considerable change in recent years.

One essential point expected by the users of vacuum-cleaning apparatus is that the vacuum-cleaning apparatus produces a constant airflow even as the dust load increases or, in other words, that the vacuum-cleaning apparatus does not experience an airflow drop as the dust load increases.

The study by “AEA Energy & Environment Group” on behalf of the “European Commission Energy” for the definition of the requirements on an eco design for vacuum cleaners demonstrates that it would be desirable to limit the input power to below 1100 W in the future for energy policy aspects. The users of vacuum cleaners do expect, however, that the cleaning performance will not significantly deteriorate as compared to vacuum-cleaning devices with a substantially higher input power as are available nowadays.

The customers' hygiene requirements on a vacuum-cleaning apparatus relate no longer to a lowest possible dust emission of the apparatus only, but also to the hygienic disposal of the vacuumed dust.

In terms of the collection concept a difference is made between vacuum cleaners without filter bags and vacuum cleaners with filter bags. These apparatus each have typical advantages and disadvantages.

Vacuum cleaners with filter bags are characterized by a high airflow. However, as the filter bag is more and more loaded the airflow drops more or less strongly. Approximately up to the year 2000 filter bags made of paper were primarily used. In tests demonstrating the reduction of the maximum airflow with a partially filled dust container analogous to EN 60312 such paper filter bags show an airflow drop of about 80% (respectively 60% if an internal tissue is used). After that, filter bags having non-woven fabric inserts slowly began establishing themselves. Initially, filter bags with non-woven fabric layers of a low dust storage capacity were used (SMS filter bags). By introducing filter bags of non-woven fabrics with a capacity layer it was possible to clearly reduce this drop of the airflow (see EP 0 960 645). In tests demonstrating the reduction of the maximum airflow with a partially filled dust container analogous to EN 60312 such filter bags show an airflow drop of approximately 30%. Further enhancements were achieved by an advance filtration by loose fibers in the bag (DE 10 2007 060 747, DE 20 2007 010 692 and WO 2005/060807) or an advance separation by a bag in the bag (WO 2010/000453, DE 20 2009 002 970 U1 and DE 20 2006 016 303 U1). Flow deflections and flow distributions in the filter bag are proposed in EP 1 915 938, DE 20 2008 016 300, DE 20 2008 007 717 Ul (dust-storing liner), DE 20 2006 019 108 U1, DE 20 2006 016 304 Ul, EP 1 787 560 and EP 1 804 635. In tests demonstrating the reduction of the maximum airflow with a partially filled dust container analogous to EN 60312 an airflow drop of approximately 15% was achieved with such filter bags. Thus, a further improvement of the suction power stability is obtained. The European patent applications 10002964A, 10163463.2, and 10163462.2 disclose an improved dust storage capability by pleating the filter material or by providing so-called surface folds. The European patent application 10009351.7 shows how the suction power stability can be improved by an optimized positioning of the bag in the vacuum cleaner. Thus, in the tests demonstrating the reduction of the maximum airflow with a partially filled dust container analogous to EN 60312 filter bags of this type show an airflow drop of about 5%.

With regard to the hygienic disposal of the vacuumed dust, holding plates were developed by means of which the filter bag, prior to removing it from the vacuum cleaner, is tightly sealed manually, semi-automatically or automatically. (e.g. EP 2 012 640).

Vacuum cleaners without bags—in particular cyclone vacuum cleaners—are characterized by a substantially constant airflow as the dust collecting container is loaded with dust. At first sight, the constant airflow of a cyclone vacuum cleaner is an advantage, compared to vacuum cleaners with filter bags which get clogged more or less strongly as the load of the filter bag increases so that the airflow is reduced correspondingly. However, this is bought by a very high nominal electric input power of the cyclone vacuum cleaners. This high input power is necessary owing to the high losses brought about by the separating principle, namely the loss for the maintenance of the high rotational speed of the dust-laden air in the cyclone separator.

By combining a number of cyclone separators to multi-stage cyclones it was attempted to increase the efficiency and the separating efficiency (EP 0 042 723). With such vacuum-cleaning apparatus an aelow of 33 l/s can be achieved. However, this is opposed by a nominal electric input power of far more than 2000 W. Cyclone vacuum cleaners having an electric input power of approximately 1400 W allow the realization of an airflow of about 25 l/s.

With conventional vacuum-cleaning apparatus working with filter bags, freshly inserted and empty, it is nowadays possible to realize an airflow of approximately 40 l/s. Such vacuum cleaners have a nominal input power of about 1300 W.

However, the airflow strongly decreases with the dust loading, as can be seen in FIG. 3. FIG. 3 shows the reduction of the airflow in response to the vacuumed amount of DMT8 dust analogous to EN 60312 in known apparatus with filter bags (e.g. Miele S5210 with a nominal electric input power of 2200 W and different filter bags of a non-woven fabric) and without filter bags (Dyson DC23 alergy with a nominal electric input power of 1400 W).

In addition to improvements of the filter bags approaches have been made to realize a constant airflow in vacuum cleaners with filter bags by means of an electronic control.

A vacuum-cleaning apparatus is described, for instance, in U.S. Pat. No. 4,021,879, whose vacuum cleaner comprises a controlling device controlling the vacuum-cleaning apparatus in such a way that a substantially constant airflow is realized. However, in this apparatus filter bags made of paper are used. Owing to the great clogging tendency of filter bags made of paper (about 80% airflow drop with 400 g of DMT8; inner tissues were not used as yet at the publication date of U.S. Pat. No. 4,021,879) a very broad control range has to be provided for the nominal electric input power. Although a constant airflow is thus theoretically realizable, same is very low. For this reason, this concept was not pursued and, therefore, could not be implemented in a product successful on the market.

DESCRIPTION OF THE INVENTION

Given the aforementioned disadvantages of the prior art the invention is based on the object to provide a vacuum-cleaning apparatus in which a constantly high airflow is realized despite a low nominal electric input power.

This object is achieved by a vacuum-cleaning apparatus comprising the features of patent claim 1, viz. by a vacuum-cleaning apparatus comprising a vacuum cleaner and a filter bag made of non-woven fabric, wherein the vacuum cleaner has a nominal electric input power of less than 1200 W, preferably less than 1100 W, more preferably less than 900 W, the vacuum cleaner comprises a motor-fan unit and a controlling device controlling the vacuum cleaner such that the airflow is kept substantially constant at a value of at least 34 l/s, preferably substantially constant at a value of at least 37 l/s, more preferably substantially constant at a value of at least 40 l/s when the filter bag is loaded with DMT8 test dust analogous to EN 60312, and the filter bag is a disposable filter bag made of non-woven fabric which has an airflow drop of less than 15%, preferably less than 10%, more preferably less than 5% in tests demonstrating the reduction of the maximum airflow with a partially filled dust container analogous to EN 60312.

The present invention is based on the concept that a vacuum-cleaning apparatus with a filter bag, viz. an empty filter bag, is operated with an input power that is adjusted to be lower than the maximum power of the motor, so that the input power of the motor can be increased in correspondence with the increasing load of the filter bag. Surprisingly, it has shown that only with filter bags having a clogging tendency of less than 15%, preferably less than 10%, more preferably less than 5% a relatively small increase of the input power of the motor is necessary to keep the airflow constant on a level required for the efficient vacuum-cleaning, i.e. at least 34 l/s. Only thus had it been possible to realize a vacuum-cleaning apparatus that can provide a substantially constant volume flow as the filter bag is continuously loaded, while, at the same time, the maximum electric input power of the vacuum cleaner remains below a predetermined value of 1200 W which is acceptable from the viewpoint of power consumption.

According to a further development of the above-described invention the vacuum-cleaning apparatus comprises an electronic controlling device which is adapted to control the electric input power of the motor-fan unit.

Preferably, the apparatus is then adapted such that the increase of the input power of the motor-fan unit required to maintain the substantially constant airflow when the filter bag is loaded with DMT8 dust analogous to EN 60312 is not more than 35%, preferably not more than 20%, and more preferably not more than 15% in relation to the input power of the motor-fan unit when the filter bag is empty. According to this embodiment it is possible to realize vacuum-cleaning apparatus with a constant airflow, with a vacuuming behavior as is known from today's non-controllable apparatus, whereby the future energy policy standards can be satisfied without problems.

Particularly suited for such an apparatus is a motor-fan unit comprising a reluctance motor, preferably a switched reluctance motor. Such motors are characterized in particular by their robustness and durability.

Alternatively, according to another preferred further development of the invention an apparatus may be provided wherein the controlling device comprises a throttle valve which controls the airflow to be substantially constant.

In both alternative further developments of the controlling device the controlled variables may be the negative pressure downstream of the filter bag, the negative pressure upstream of the filter bag or the flow rate measured at an optional position in the flow path. Optional combinations of these three quantities are also feasible.

According to a preferred further development of all inventions described above the filter bag may be provided in the form of a flat bag. The flat bag shape is the most widely spread shape for non-woven bags as bags of this shape are very easy to manufacture. As opposed to the paper filter material used for paper filter bags the non-woven fabric material is very hard to fold permanently owing to the great resilience, so that the manufacture of more complex bag shapes, such as block bottom bags or other bag shapes having a bottom, is very complicated and expensive.

Particularly suited for use in the apparatus according to the invention are vacuum cleaner bags having a pleated filter material or surface folds. Such vacuum cleaner bags are characterized by a particularly low airflow drop.

According to a preferred further development of the invention the motor-fan unit is adapted such that the vacuum cleaner generates, with a filter bao being inserted, with an orifice 0 a negative pressure between 30 kPa and 6 kPa, preferably a negative pressure between 20 kPa and 8 kPa, and more preferably a negative pressure between 15 kPa and 8 kPa , and with an orifice 40 an airflow of more than 50 l/s, preferably more than 60 l/s, and more preferably more than 70 l/s. This special characteristic of the motor-fan unit differs from the characteristic of motor-fan units used in conventional vacuum-cleaning apparatus in that the latter generate an essentially higher negative pressure and an essentially lower maximum airflow. Surprisingly, it has shown that such motor-fan units are particularly energy-saving in use, yet fulfill the requirements on a constant airflow of sufficient power.

According to a particularly preferred further development of all inventions described above the vacuum cleaner may have, with an orifice 8 (40 mm), a rate of airflow of more than 250 W, preferably of more than 300 W, more preferably of more than 350 W. If the invention is constructed in this way a fully satisfying vacuuming operation is ensured during the complete filling of the filter bag.

Preferably, the motor-fan unit may have, with an orifice 8 (40mm), an efficiency according to EN 60335 of at least 20%, preferably of at least 25%, and more preferably of at least 30%. This further development of the invention results in a particularly energy-saving vacuum-cleaning apparatus.

According to another further development of all inventions described above the vacuum cleaner may comprise a filter bag change indicator indicating it during the vacuum-cleaning, the airflow drops under the substantially constant value for a predetermined period, To this end, in particular the sensors can be applied that are provided for measuring the controlled variables.

According to another preferred further development of the above-described inventions the filter bag has a volume in a range of 1.5 l to 8 l measured according to EN 60312. Filter bags of this type are primarily used in vacuum cleaners that are constructed as canister vacuum cleaners, hand-held vacuum cleaners, wetidry vacuum cleaners or uprights for domestic use.

BRIEF DESCRIPTION OF THE FIGURES

The figures serve to explain the prior art and the invention, in which

FIG. 1 and

FIG. 2 show filter bags according to the prior art with surface folds;

FIG. 3 shows the reduction of the airflow for vacuum-cleaning apparatus comprising vacuum cleaners and filter bags according to the prior art as well as for a vacuum-cleaning apparatus without filter bag according to the prior art;

FIG. 4 shows the air characteristics for a motor-fan unit used in vacuum-cleaning apparatus according to the prior art;

FIG. 5 shows the air characteristics for a motor-fan unit not used in vacuum-cleaning apparatus according to the prior art, which is particularly suited for implementation in the present invention; and

FIG. 6 shows the airflow and electric input power of a first and a second embodiment of the present invention.

EMBODIMENTS OF THE INVENTION

FIG. 5 shows the characteristic curve of the motor-fan unit according to an embodiment of the invention. Same is characterized by a comparatively low maximum negative pressure with an orifice 0, and a high volume flow with orifice 9 (50 mm). Especially with orifice 0 a negative pressure of 14.3 kPa is obtained. Orifice 9 (50 mm) results in an airflow of 86.5 dm3/s. Hence, the characteristic curve is very flat. With the maximum airflow the motor consumes a power of 1240 W. The airflow power (product of negative pressure and airflow) amounts to a maximum of 498 W with orifice 7 (30 mm).

FIG. 4, however, shows the characteristic data for a motor-fan unit as used according to the prior art in vacuum-cleaning apparatus. With orifice 0 the motor-fan unit generates a negative pressure of 35.8 kPa, orifice 9 (50 mm) results in an airflow of 53.5 dm³/s. Hence, the characteristic curve of the fan is very steep. With the maximum airflow the motor consumes a power of 1900 W. The airflow power is 614 W. In the case of greatly dogged paper filter bags such a design had been necessary and sensible.

In the particularly preferred embodiment of the present invention filter bags with surface folds are used as are described in the above chapter DEFINITIONS.

The motor-fan unit shown in FIG. 5, in combination with a filter bag having surface folds and an installation space adapted to the filter bag, allows with a corresponding automatic controlling of the airflow the realization of a vacuum cleaner that achieves a high, constant airflow with an input power of below 1000 W. FIG. 6 shows the results for two embodiments according to the present invention, both having in common that a very high, constant airflow is achieved with a low electric input power.

Claims

1. Vacuum-cleaning apparatus comprising a vacuum cleaner and a filter bag made of non-woven fabric, wherein

the vacuum cleaner has a nominal electric input power of less than 1200 W

the vacuum cleaner comprises a motor-fan unit and a controlling device controlling the vacuum cleaner such that the airflow is kept substantially constant at a value of at least 34 l/s, when the filter bag is loaded with DMT8 test dust analogous to EN 60312, and

the filter bag is a disposable filter bag made of non-woven fabric which has an airflow drop of less than 15 in tests demonstrating a reduction of a maximum airflow with a partially filled dust container analogous to EN 60312.

2. The apparatus according to claim 1, wherein the controlling device is an electronic controlling device adapted to control an electric input power of the motor-fan unit,

3. The apparatus according to claim 2, wherein an increase of the input power of the motor-fan unit required to maintain the substantially constant airflow when the filter bag is loaded with DMT8 dust analogous to EN 60312 is not more than 35% in relation to the input power of the motor-fan unit when the filter bag is empty.

4. The apparatus according to claim 1, wherein the motor-fan unit comprises a reluctance motor.

5. The apparatus according to claim 1, wherein the controlling device comprises a throttle valve provided to control the airflow to be substantially constant.

6. The apparatus according to claim 1, wherein the controlling device is adapted such that a negative pressure downstream of the filter bag is used as a controlled variable.

7. The apparatus according to claim 1, wherein the controlling device is adapted such that a negative pressure upstream of the filter bag is used as a controlled variable,

8. The apparatus according to claim 1, wherein the controlling device is provided such that the flow rate measured at an optional position in the flow path is used as a controlled variable.

9. The apparatus according to claim 1, wherein the filter bag is a flat bag.

10. The apparatus according to claim 1, wherein the filter bag comprises at least one surface fold.

11. The apparatus according to claim 1, wherein the motor-fan unit is adapted such that the vacuum cleaner generates, with a filter bag being inserted, with an orifice 0 a negative pressure between 30 kPa and 6 kPa, and

with an orifice 8 (40 mm) an airflow of more than 50 l/s.

12. The apparatus according to claim 1, wherein the vacuum cleaner has, with an orifice 8 (40 mm), a rate of airflow of more than 250 W.

13. The apparatus according to claim 1, wherein the motor-fan unit has, with an orifice 8 (40 mm), an efficiency according to EN 60312 of at least 20%.

14. The apparatus according to claim 1, wherein the vacuum cleaner comprises a filter bag change indicator indicating if, during the vacuum-cleaning, the airflow drops under a substantially constant value for a predetermined period.

15. The apparatus according to claim 1, wherein the filter bag has a volume in a range of 1.5 l to 8 l measured according to EN 60312.

16. The apparatus according to claim 1, wherein the vacuum cleaner has a nominal electric input power of less than 900 W.

17. The apparatus according to claim 1, wherein the airflow is kept substantially constant at a value of at least 40 l/s.

18. The apparatus according to claim 1, wherein the disposable filter bag made of non-woven fabric which has an airflow drop of less than 5% in tests demonstrating the a reduction of the a maximum airflow with a partially filled dust container analogous to EN 60312.

19. The apparatus according to claim 3, wherein the increase of the input power of the motor-fan unit required to maintain the substantially constant airflow when the filter bag is loaded with DIMT8 dust analogous to EN 60312 is not more than 15% in relation to the input power of the motor-fan unit when the filter bag is empty.

20. The apparatus according to claim 4, wherein the motor-fan unit comprises a switched reluctance motor.