The invention relates to a ventilation and filter module for clean room ventilation and with a housing to be mounted in a ceiling and having one end wall facing the ventilated room and formed by a high efficiency filter, while a centrifugal fan with comple¬ tely circumferential air outlet is disposed in connec¬ tion with an air intake in the opposite end wall, a space division being provided in the interior of the housing to accomplish a uniform distribution substan- tially free of turbulence of the amount of air radially emitted from the fan across the surface of the filter facing the fan, coverings of sound absorbing material being mounted on the side walls of the housing in a chamber surrounding the fan. To fulfil the extremely heavy requirements for purity of ventilation air in rooms for the production of microelectronics, certain high-precision mechanics and optical productions and within the pharmaceutical and biotchnological industry and in certain hospital premises, ventilation systems are known, in which the air exhausted from the room is recirculated through ventilation and filter modules, also designated tunnel modules, mounted in the ceiling, and is blown into the room as a laminar flow at a comparatively low velocity through a high-efficiency filter in each module.

The demands made on such ventilation systems and measuring and calculation methods for the concentration of particles are inter alia specified in a classifica¬ tion system published in US Federal Standard No. 209D of 15 June 1988.

Prom EP-A2-0 340 433 such a tunnel module is known in which the space above the filter is divided by substantially horizontal walls into three chambers with

the centrifugal fan disposed asymmetrically in the central chamber quite adjacent tp one side wall of the housing. The air is directed from the fan through a comparatively long channel under a reversal of direc- tion through 180° to the upper side of the filter. The asymmetrical positioning makes it difficult to obtain a uniform air distribution across the entire filter and the length of the slot together with said reversal of the flow direction results in a relatively low effi- ciency. Even though the partitions partially consist of sound dampening guideways no precautions have been taken to attenuate the low-frequency noise from the fan and, for some of the above applications the total noise level is unacceptably high. From US patent No. 4 560 395 a module is known having a structure of the above mentioned kind. The space division is here effected by means of a flat bowl-shaped wall of sheet material positioned imme¬ diately beneath the fan with a very low overhead clearance above the filter. The flow path of the air is reversed through 180° at the side walls of the housing. Even though sound absorbing coverings are provided along the side walls around the fan, the sound level of this design is also unacceptably high and the reversal through 180° of the air flow quite adjacent the lateral edges of the filter also applying to this module likewise results in reduction of the efficiency.

Compared to said prior art designs it is the object of the invention to provide a ventilation and filter module of the above mentioned type by which a substantial lowering of the noise level is obtained in unison with an improved efficiency and thus a lower energy consumption.

In order to obtain this, a ventilation and filter module according to the invention is charac¬ terized in that the space division is provided by means

of a partition arranged beneath the fan at a distance above the filter, substantially parallel thereto, said partition having a covering of sound absorbing material on the side facing the fan, and defining together with the coverings disposed on the side walls an air slot substantially linear in its longitudinal direction from the chamber around the fan to a chamber located above the filter.

By the geometry of the air slot resulting from this space division the air slot will extend mainly linearly from the fan chamber and have its opening positioned at a distance from the side walls of the housing corresponding to the thickness of the sound dampening coverings disposed thereon and thus, at a corresponding distance over the upper side of the filter reckoned from the edges of the filter at the side walls of the housing. With a suitable overhead clearance between the upper side of the filter and the upper side of the core wall a static pressure with an even air distribution free of turbulence may in this manner be established throughout the filter.

The slot geometry further entails that the discharge flow from the fan only has to change direc¬ tion through 90° in order to pass through the slot, thereby obtaining an improved efficiency.

By means of a suitable sound absorbing material, preferably mineral wool, for the core walls and the coverings, together with a suitable dimensioning of the thicknesses of the core wall and the covering and the width and length of the slot determined thereby, a very pronounced noise attenuation is obtained, not at least in the low-frequency range around the blade frequency of the fan from which the greatest contribution to the noise level originates. Said advantages of the invention may be further improved by means of the measures stated in the sub- claims.

The invention will now be explained in detail with reference to the schematical drawings, in which

Pig. 1 is an axial sectional view of an embodi¬ ment of a ventilation and filter module according to the invention.

Pig. 2 is a sectional view along the line II-II in Fig. 1, and

Fig. 3 shows a modified embodiment. In the embodiment illustrated in Fig. 1 the ven- tilation and filter module comprises a housing which may have a substantially square cross-section with side walls l. One end wall of the housing facing the ven¬ tilated room is formed by a high-efficiency filter 2, e.g. of the so-called HEPA type, generally known in connection with clean room ventilation.

In the middle of the opposite end wall 3 an air intake 4 is positioned beneath which a centri¬ fugal fan 5 with fully circumferential air outlet is mounted in the housing. The fan 5 is preferably of a low-noise design, as explained in Applicants' concurrently filed Patent Application No.

To obtain a uniform distribution mainly free of turbulence of the amount of air radially discharged by the fan 5 across the upper side of the filter 2 a space division is provided in the housing which accord¬ ing to the invention comprises a first chamber 6 around the fan 5 and a second chamber 7 immediately above the filter. The two chambers 6 and 7 are separated by a partition 8 positioned below the fan 5 and at a distance above the filter 2 and parallel thereto and having on the side facing the fan a covering 9 of a sound absorbing material that may be formed as a cut- to-shape member of compressed mineral wool which is a material with good sound attenuation properties in the

low-frequency range in which the blade frequency of the centrifugal fan 5 is located.

Coverings 10 and 11 of sound absorbing material of the same type as the covering 9 are placed on the side walls 1 of the chamber 6 as well as the end wall 3 to obtain a further noise dampen¬ ing. The lower edges of the coverings 10 on the side walls 1 facing the filter 2 are positioned on a level with the underside of the partition 8 so as to create in the illustrated embodiment between the partition 8 and its covering 9 of mineral wool and the coverings 10 a circumferential quadratic air slot 12 the longitudinal direction of which is parallel to the side walls 1. In order to prevent undesired emission of mineral wool fibers the sides of the coverings 9 and 10 facing the chamber 6 and the air slot 12 are provided with a surface treatment of an appropriate binding agent. In the illustrated embodiment the coverings 9 and 10 have a comparatively large thickness, e.g. 200 mm as regards the coverings 10, and 300 mm as regards the covering 9. The air slot 12 the length of which with the illustrated geometry is determined by the thickness of the covering 9, i.e. in the illus¬ trated embodiment about 300 mm, has a width dimensioned to attain a high sound dampening. With the above men¬ tioned dimensions the slot width may e.g. be 55 mm. On the face of it, it would have to be expected that the larger length of the air slot 12 would result in a deterioration of the air-technical efficiency but, on the contrary, it has suprisingly turned out that the efficiency has been improved. It has been found that the efficiency is improved when the length of the air slot 12 is substantially larger, e.g. at least three times larger than the width of the slot. The improved

efficiency is probably obtained because the larger slot length offers a more uniform velocity of the air flow¬ ing out through the outlet 14 of the air slot 12. In view of the fact that the discharge loss at the outlet 14 is determined by the square of the flow velocity the blow-off loss is diminished when the air flow velo¬ city becomes more uniform.

On the underside facing the filter 2 the partition 8 consists of a plate of a sound reflecting material, e.g. sheet steel, and also the lower edges of the coverings 10 facing the filter are covered by plates 13 of a sound reflecting material.

The covering 11 on the underside of the end wall 3 may like the coverings 9 and 10 be com- posed of mineral wool members cut-to-shape. The thick¬ ness of this covering is substantially determined by the distance from the end wall 3 to the air outlet of the fan 5.

The illustrated design of the ventilation and filter module entails that the air flow from the air outlet of the fan 5 through the air slot 12 that is linear in its longitudinal direction to the outlet 14 thereof in the chamber 7 is subjected to a change in direction not exceeding 90°. At the same time the circumferential slot 12 with the outlet 14 positioned at a distance from the side walls 1 of the housing determined by the thickness of the coverings 10 entails in combination with the location of the partition 8 at a suitable overhead clearance above the filter 2 that a mainly uniform air distribution free of turbulence is obtained with a velocity profile showing a faint maximum in the proximity of the peripheral lateral edges of the filter

2. The laminar character of the air flow from the fan 5 to the filter 2 may be further ensured by

rounding the edges of the covering 9 of the partition 8 facing the inlet 15 of the air slot 12 in the chamber 6.

It has turned out that a completely satisfactory air distribution may be obtained with a relatively low overhead clearance in chamber 7 by the above men¬ tioned dimensions, e.g. 200 mm, thereby allowing the total height of the module to be kept comparatively low, e.g. 850 mm. The dimensions stated here by way of example do in no manner restrict the scope of the invention. The thickness of the coverings 10 is thus mainly deter¬ mined by the desired noise dampening, whereas the thickness of the covering 9 must be determined also in consideration of the desired length of the air slot. In the embodiment illustrated in Fig. 3, the housing of the module has side walls 16 forming above the filter a frustum of a pyramid with a square cross- section. The coverings 18 on the side walls 16 extend quite up to the end walls 19 in which the air intake 20 is positioned. The partition 21 has inclined lateral edges 22 that are somewhat steeper than the walls 16 of the housing so that the air slot 26 from the chamber around the fan 24 diverging conically towards the chamber 25 above the filter 17 has an increasing width in the direction towards the chamber 25, thereby acting as a kind of diffuser which may further enhance the efficiency.

In a further development of this embodiment in which the sound dampening coverings are made of moulded material a further improvement of the efficiency may conceivingly be obtained through such a design of the partitions and the coverings applied to the walls of the housing that the air slot has a circular inlet in the chamber around the fan and an outlet towards the chamber above the filter matching the side walls of the housing, e.g. quadratic.

Clearly, the module according to the invention does not need to have a substantially square cross- section. The module may for instance have an oblong cross-section with the side ratios 1:2, provided the module fits into the supporting structure in the ceiling on site of use.