Directional air diffuser panel for clean room ventilation system

Abstract: A clean room diffuser panel (36) for positioning below a ceiling (32) grid mounted filter element (34). The diffuser panel (36) is perforated throughout its area with increased size and density of perforations in a peripheral region to provide increased airflow beneath the ceiling grid (32). The peripheral regions may further be angled or provided with directional vanes (66,66a) to create a lateral airflow beneath the ceiling grid (32).

Description: This invention relates to diffusers for reducing airflow turbulence, and more particularly for diffuser panels for use with clean room ceiling filter panels. In addition to the filtering effect provided by clean room ventilation systems, such systems are generally designed to reduce turbulence by providing a generally laminar airflow pattern. In a laminar flow clean room, air typically flows downward from the ceiling to the floor so that a contaminant particle, such as might be shed by a worker, would be drawn directly downward to be removed by a filter. Thus, such a particle would spend no more than a few seconds in the clean room. Where the airflow is turbulent, there are eddies that create localized upward airflow currents. A contaminant particle might be trapped in such an eddy for up to several minutes. Consequently, turbulent airflow generally results in a higher concentration of contaminants at any given time and creates a greater likelihood that such contaminants might come to rest on sensitive equipment or materials. Perforated diffuser panels are commonly used in clean room ceilings to reduce turbulence and to provide a generally laminar airflow. Such diffuser panels are typically positioned below ceiling-mounted filter elements, preferably with at least a small space in between. Although air may exit the filter element at uneven pressures across the element, the diffuser panel creates a small backpressure which equalizes the air pressure in the space above the panel. The air then passes through perforations in the diffuser panel, with the perforations acting as point sources having generally equal flow rates. Although there is turbulence immediately below the diffuser panel due to the "nozzle effect" of the perforations, such turbulence is quickly dissipated. Typically, such turbulence becomes negligible at a distance below the diffuser panel equal to several times the diameter of the perforations. However, while a single diffuser panel can provide an effectively laminar airflow away from boundaries or obstructions, clean room ventilation systems are typically constructed with multiple filter elements mounted in a grid system to permit modular manufacturing and assembly. In such a system, each filter element is separated from adjacent filter elements by a "dead zone" occupied by a grid structural element creating an obstruction to air flow with a corresponding airflow dead zone immediately below. As a result, air flowing downward from the periphery of a filter element tends to be drawn into the airflow dead zone and form a turbulent vortex having an upward airflow beneath the grid element. A rule of thumb predicts that a turbulence zone extends downstream of an obstacle by a distance of about four times the obstacle's width. Thus, in typical systems having a frame or grid element with a width of two to three inches, the turbulence zone may extend 12 inches below the ceiling surface, substantially impairing the clean room function. An existing approach to reduce the turbulence between filter elements is to extend a V-shaped shroud beneath the grid element. Such a shroud is usually in the form of a "tear drop" fluorescent light fixture cover, and functions like the trailing edge of an airplane wing, allowing the laminar flow regions from adjacent filter elements to rejoin more smoothly with reduced turbulence. This approach typically reduces the turbulence zone to within about 7 inches of the ceiling surface, an improvement, but still problematic. In addition, tear drop light fixtures have the disadvantage of further reducing ceiling height, which is typically at a premium due to the substantial ducting and equipment required above the ceiling. Also, such light fixtures create an obstacle to modular sub-dividing walls that preferably hand immediately below the ceiling surface without a substantial gap. Further arrangements aimed at minimising or eliminating the dead space below filter support members in clean rooms are described in JP-A-62 138 635 and JP-A-60 241 212. From the foregoing it will be recognized that there is a need for an air diffuser panel that overcomes these drawbacks of the prior art. Accordingly, one aspect of the invention provides an air diffuser panel having the features of Claim 1. In another aspect, the invention provides a clean room air diffusion system having the features of Claim 8. In a further aspect, the invention provides a clean room air diffusion system having the features of Claim 16. The present invention offers an increased or directional airflow at the periphery of the diffuser panels to fill in the dead space below the structural elements and creates a balanced net airflow that becomes generally laminar within close proximity to the ceiling surface. In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section side view of a clean room system embodying the present invention; Figure 2 is an enlarged sectional side view of a grid element junction between adjacent filter elements in the system of Figure 1, showing alternative filter and diffuser configurations; Figure 2a is an enlarged, fragmentary view of a diffuser panel of Figure 2; Figure 2b is an enlarged, fragmentary view of an alternative diffuser panel of Figure 2; Figure 3 is an enlarged sectional side view of a grid element junction between adjacent filter elements in the system of Figure 1, showing additional alternative filter and diffuser configurations; Figure 4 is an enlarged sectional side view of a grid element junction between adjacent filter elements in the system of Figure 1, showing further alternative filter and diffuser configurations; and Figure 5 is an enlarged sectional side view of a junction between a filter element and a diffuser panel according to an embodiment of the present invention. FIG. 1 shows a clean room system 10 for providing an uncontaminated downward laminar airflow in an enclosed chamber 12. The chamber includes a perforated floor 14 with a subspace 16 below the floor 14. A blower 20 draws air through a duct 22 in communication with the subspace 16 and expels it at a higher pressure into a plenum 26 above the chamber 12. A ceiling 30 separates the plenum 26 from the chamber 12. The ceiling 30 generally includes a rigid grid structure formed of grid elements 32, with the grid elements arranged orthogonally to define a matrix of rectangular spaces. A set of rectangular filter elements 34 is carried by the grid elements 32 with each filter element 34 occupying a rectangular space formed by the grid elements. Each filter element 34 is sealed to the grid to prevent airflow between the plenum 26 and clean room chamber 12 from circumventing the filters at its edges. A set of diffuser panels 36 is attached to the ceiling 30, with each diffuser panel 36 being positioned below and generally coextensive with a filter element 34. Each diffuser panel 36 is preferably a rigid perforated sheet for providing a non-turbulent laminar airflow when air is forced through it from a volume of higher pressure above the sheet. Alternatively, the diffuser panel may be a rigidly supported air-permeable membrane (not shown) formed of flat filter paper media such as a long-strand polyester fiber sheet. Such a sheet may be installed in conjunction with a perforated panel as discussed above, and may be folded and formed by bending the peripheral portions at an angle so that air passing perpendicularly through the peripheral portions of the sheet is directed outwardly. The desired higher volume peripheral flow rate may be provided by manufacturing a composite membrane with a more highly permeable material in the peripheral portions than in the central portions. Alternatively, a single membrane may be perforated at its periphery to provide increased airflow. As shown in FIG. 2, the grid element 32 has a constant cross-section and is preferably formed of extruded aluminum. The grid element 32 defines a pair of upwardly facing troughs 40, each trough being filled with a gel-like sealant 44. The grid element 32 further defines a large downwardly facing channel 42 defined by opposed, spaced apart grid sidewalls 46. The channel 42 is generally sized to fit a fluorescent light fixture 48 entirely therein. A ledge 49 may extend laterally from the sidewall 46 at its lower edge as shown on the left sidewall in FIG. 2. The ledge 49 provides a resting or attachment point for the filter element 34a, and prevents airflow in the space between the filter element and the sidewall 46. FIG. 2 shows the filter elements 34a, 34b in two alternative configurations. Preferably, only one of these configurations would be selected and exclusively used throughout each clean room installation. A hanging-type filter 34a includes a peripheral flange 50a depending downwardly from the top of the filter 34a so that a substantial portion of the filter hangs below the flange. A standing-type filter 34b includes a peripheral flange 50b extending directly downward from the lower edges of the filter, with the entire filter element standing above the flange. Because a flush ceiling surface is desired for functional and aesthetic reasons, a hanging filter 34a is used when a diffuser panel 36a is to be attached directly to the filter; a standing filter element 34b is used when a diffuser panel 36b may be attached directly to the lower edge of the grid sidewall 46. To prevent airflow from circumventing the filter elements 34 and contaminating the clean room chamber 12, each element is installed with its peripheral flange 50 received in the gel sealant 44 contained in the troughs 40. Generally speaking, each diffuser panel 36 is preferably a perforated metal sheet having equally sized and spaced-apart perforations 54 distributed over the majority of the panel's area in its central region. In the preferred embodiment, the total area of the perforations accounts for about 20% of the panel area. This percentage is known as the free area of the panel. The panel 36 may be useful with a free area anywhere within a wide range. A free area of less than 10% is suitable when a high backpressure and flow resistance is desired; a free area of greater than 90%, such as provided by a honeycomb panel, is useful when substantial backpressure is not necessary. The panel 36 must provide sufficient backpressure to create a lateral airflow that equalizes differences between high pressure zones and low pressure zones below the filter 34. To permit this lateral flow, the diffuser 36 is preferably spaced below the filter element 34, although the diffuser may be mounted adjacent the lower surface of a filter that permits lateral air flow within itself. FIG. 2a shows a first alternative perforated panel 36a suitable for attachment to the filter element 34a. The panel includes a central portion 56a populated by a plurality of evenly spaced, equally sized central perforations 54a. A peripheral portion 58a surrounds the central portion 56a on all sides and defines an array of peripheral perforations 60a. The peripheral perforations 60a are larger and more closely spaced than the central perforations 54a. Consequently, the peripheral portion has a substantially higher percentage free area than the central portion 56a and will permit a greater airflow rate per unit area than the central portion. The peripheral portion 58a is angled upward from the central portion 56a so that air passing perpendicularly therethrough will be directed peripherally away from the central portion. This deflection angle is preferably about 30 degrees, although the angle may range up to 90 degrees, or be eliminated altogether. The diffuser panel 36a is preferably terminated by an edge flange 62a that is parallel to the center portion 56a and attachable to the edge of the filter element 34a. Alternatively, the edge flange 62a may rest on the grid ledge 49. FIG. 2b shows an alternative diffuser panel 36b suitable for attachment directly to the grid sidewall 46 in conjunction with the standing-type filter element 34b. The panel 36b is bent perpendicularly at its periphery to create a radiused peripheral portion 58b populated by peripheral perforations 60b. A plurality of central perforations 54b populate the panel and are sized substantially smaller than the peripheral perforations 60b. This permits the diffuser panel 36b to achieve the same increased peripheral airflow provided by the diffuser panel 36a. Similarly, the peripheral airflow is directed away from the center of the diffuser panel 36b, because the peripheral perforations 60b are centered approximately at the midpoint of the radiused bend of the panel so that they face outward. FIG. 3 shows an alternative diffuser panel 36c that is uniformly populated by central perforations 54c throughout its area. The panel 36c is spaced below the filter element 34a so that air may escape laterally beyond the edge of the diffuser panel 36c, and below or beside the lower edge of the grid sidewall 46. The space is sized to create a higher flow volume in the lateral airstream than through the central perforations 54c. The panel may be directly attached to the lower edge of the filter element 34a by spacer posts 64 or other suitable means. The same diffuser panel 36c may be used in conjunction with a standing filter element 34b, as shown on the right side of FIG. 3, with the panel being attached to and spaced below the lower edge of the grid sidewall 46. In either embodiment of FIG. 3, each panel 36c and corresponding grid sidewall define a side gap 65 having an effective free area of 100%. FIG. 4 shows additional alternative diffuser panel configurations. A diffuser panel 36d includes vertically oriented vanes 66 in the central portion of the panel and angled vanes 66a in the peripheral portion of the panel for directing air peripherally away from the center of the panel. The angled vanes 66a are angled from the vertical in correspondence with their proximity to the edge of the panel. In this embodiment, the airflow volume is not substantially increased at the periphery, but the directional effect alone adequately fills in the area underneath the grid element to substantially reduce turbulence. A further alternative diffuser panel 36e is shown on the right side of FIG. 4. In this embodiment, a diffuser panel 36e has a peripheral flange 68e that is bent upwardly and inwardly at an acute angle to leave a peripheral gap 70 between the panel 36e and the grid sidewall 46, thus permitting increased airflow at the periphery. The bent angle of the flange also serves to direct air into the area beneath the grid element 32, acting as a vane and a funnel. FIG. 5 illustrates a filter configuration that permits the removable attachment of the diffuser panel 36a from below and without requiring removal of the filter element 34a. The filter element 34a has an extruded aluminum filter frame 74 that peripherally surrounds a mass of filter material 76. The frame 74 includes a frame sidewall 78 with a lower frame ledge 80 attached at a lower edge thereof and extending perpendicularly inward toward the center of the filter panel. A slanted wall 82 is attached to the inner edge of the lower frame ledge 80 with the slanted wall extending downward and peripherally outward to permit airflow to spread outward therebelow. An attachment flange 84 is attached to the lower end of the slanted wall 82 and extends peripherally outward therefrom to be spaced below the lower frame ledge 80. The attachment flange 84 is integral with the filter frame 74 so that it extends around the entire periphery of the filter element 34a. The attachment flange 84 is positioned entirely below the filter frame 74 so that it does not extend peripherally outward past the frame element to interfere with the grid sidewall 46 of the grid element 32. The diffuser panel 36a may be positioned for attachment to the filter element 34a by positioning the diffuser panel edge flange 62a against the attachment flange 84. A U-shaped clip 88 defines a gap for receiving the attachment flange 84 and edge flange 62a to bias the flanges together to secure the diffuser panel 36a to the filter element 34a. The clip 88 may be formed of spring steel or extruded aluminum with a clip extending the full length of each side of the diffuser panel. As a result of the attachment system shown in FIG. 5, the diffuser panel may be removed and re-attached from below. It is not necessary to remove or disturb the filter element, to enter the plenum 26 or to shut down the clean room system to effect such an operation. The clip 88 has the added aesthetic advantage over conventional screws and rivets in that it does not require drilled holes or visible fasteners. By providing increased and directional airflow to fill in the area beneath the grid element 32, the diffuser panel 36 effectively provides a net downward airflow beneath the grid element more nearly equal to that which would be provided by replacing the grid elements with an infinite uniform filter element and perforated panel. The airflow below the grid element 32 has sufficient pressure and volume to permit the vertical laminar flow layers flowing from the central portions of the panels to remain vertical and laminar. These layers are thus not drawn upwardly into vortexes beneath the grid element 32. The initial turbulence beneath the grid element 32 is further minimized by providing a symmetrical airflow from the opposite sides of the grid element 32. Thus, the laterally directed peripheral airstreams will impinge on one another, substantially cancelling their lateral flow and resulting in a generally less turbulent, downward flow. The increased airflow provided by the diffuser panel peripheral regions should create a net airflow below the grid element that is generally equal to the net airflow per unit area in the central regions of the diffuser panels. The velocity of the airflow from the peripheral regions need not be greater than the velocity through the central perforations. only the net airflow rate (e.g. the air volume flow per unit time through unit horizontal area) of the peripheral region need exceed that of the central region. Also, as noted above, this airflow rate differential may act alone to achieve the goals of the invention, or the directional effect alone may provide the same advantages. In the preferred embodiment, both principles act cooperatively.

Claims: An air diffuser panel (36) for a clean room air diffusion system comprising a central region (56) and a peripheral region (58), the central region having perforations (54) the aggregate area of which is a first fraction of the total area of the central region, the peripheral region having perforations (60) the aggregate area of which is a second fraction of the total area of the peripheral region, the second fraction being greater than the first fraction. A panel according to Claim 1, wherein the peripheral region (58) is adapted to direct air flowing therethrough in a direction angled peripherally from the perpendicular to the central region (56). A panel according to Claim 2, wherein the peripheral region (58) is sloped at an angle to the central region (56). A panel according to any preceding claim formed from a rigid sheet formed with spaced-apart perforations. A panel according to any one of Claims 1 to 3, comprising an air permeable membrane. A panel according to Claim 5, wherein the membrane is a long strand fiber sheet. A panel according to Claim 5, wherein the membrane is formed of flat filter paper media. A clean room air diffusion system for controlling an airflow, the system comprising: a support grid consisting of grid elements (32) defining a panel space for transmitting the airflow and defining a dead space below the grid elements; and a diffuser panel (36) according to any preceding claim attached to the grid in registration with the panel space, the central region (56) of the panel transmitting a first portion of the airflow at a first airflow rate per unit area and the peripheral region (58) of the panel transmitting a second portion of the airflow at a second airflow rate per unit area greater than the first airflow rate per unit area, thereby to provide airflow to fill in the dead space below the grid. A system according to Claim 8, wherein the diffuser panel (36) comprises a plurality of edge portions angularly disposed with respect to a plane in which the central region is positioned, and the diffuser panel is horizontally oriented, with the central region generally lower than the edge portions. A system according to Claim 8 or 9, wherein the peripheral region includes a terminal edge flange (62a) adapted to be attached to the grid, with the central region being spaced generally below the edge flange (62a). A system according to any one of Claims 8 to 9, comprising an air filter panel (34) adjacent the grid for transmitting the airflow. A system according to Claim 11 wherein the air filter element (34) has a frame (74) with a peripherally extending attachment flange (84), the diffuser panel (36) has an edge flange (62a) corresponding to the attachment flange (84) and positionable in sandwiched relation therewith and the edge flange (62a) is secured to the attachment flange (84) by an edge clamp (88). A system according to Claim 12, wherein the edge clamp (88) defines a gap for receiving and biasing together the sandwiched edge flange (62a) and the attachment flange (84). A system according to Claim 12 or 13, wherein the attachment flange (84) is positioned below the frame (74) such that the attachment flange (84) does not extend beyond the periphery of the frame. A system according to any one of Claims 12 to 14, wherein the attachment flange (84) is positioned in a generally horizontal plane when the filter element (34) is horizontally oriented. A clean room air diffusion system for controlling an airflow, the system comprising: a support grid consisting of grid elements (32) defining a panel space for transmitting the airflow and defining a dead space below the grid elements; and a diffuser panel (36) attached to the grid in registration with the panel space, the diffuser panel having perforations and being spaced below the panel space to define a peripheral region comprising a side gap (65) for transmitting air through the panel space toward the dead space below the grid elements, the panel transmitting a first portion of the airflow at a first airflow rate per unit area and the peripheral region transmitting a second portion of the airflow at a second airflow rate per unit area greater than the first airflow rate, thereby to provide airflow to fill in the dead space below the grid.

Inventors: LUDWIG CRAIG S, LUDWIG, CRAIG S.
Applicants: BROD & MCCLUNG PACE CO, BROD & MCCLUNG - PACE CO.
Application year: 1992
Main classification: F24F3
IPC codes: E04B9, F24F13, F24F3, F24F7