GB2098317A

GB2098317A - Method and device for control of airborne particles

Info

Publication number: GB2098317A
Application number: GB8111175A
Authority: GB
Original assignee: Klenzaids Engineers Private Ltd
Current assignee: Klenzaids Engineers Private Ltd
Priority date: 1980-11-24
Filing date: 1981-04-09
Publication date: 1982-11-17
Also published as: IN152016B; GB2092296B; GB2098317B; GB2092296A

Abstract

A method and apparatus for controlling airborne particulates in enclosed spaces, wherein air is made to flow in parallel airstreams, with velocities of the streams graded by means of a filtering device 5 having a plurality of filtering media pleats 4 in tapering formation, each successive pleat having an area incrementally different from adjacent pleats to produce a graded velocity profile on a projected exit face of the said filtering device. <IMAGE>

Description

1 GB 2 098 317 A 1

SPECIFICATION

Method and device for control of airborne particles The present invention relates to a method of controlling airborne particles, in particular, the migration and deposition of contaminating airborne particulates. It further relates to a device having instru mentalities for achieving such control which may be used, for example, to obtain a work space substantially void of airborne particulate contaminants, both biological and non-biological, such as is necessary for the manufacture of sterile pharmaceutical products and micro-eiectronic components. Another use is in hospitals, where isolation of patients and prevention of contamination during surgical procedures is intended.

Previously proposed airborne contamination control methods include the socalled---Laminar Air Flow- system for equipment like work stations, 85 and sterilizing tunnels; and also for clean or sterile rooms.

These rely on conveying previously filtered and conditioned air unidirectionally along a clean to dirty axis at a single non-turbulent velocity of approximately 27.45 metres/min across the undisturbed cross-section of the work space or room or sterilizer. This laminar flow stratifies the air so that minimum cross- stream particular migration occurs with the result that activity generated particulates that become airborne are carried linearly along a predicted path.

These previously proposed methods are deficient in that when any object, for example, the work piece or manufacturing equipment or operator, the presence of which is unavoidable in practical work situations, is introduced into the air- stream, the laminar airflow is altered such that it is not possible to regulate the particulate content within the work space, as would be possible without the object altering the airflow pattern.

It is known that when an object is introduced into a moving stream of air, a higher pressure exists on the upstream side than on the downstream side of the object. The pressure gradient around the object creates turbulence, which in turn leads to eddy currents around the object, with consequent back-mix effects, which entrap in the proximity of an obstructing object particles shed by the object into the air mass around the object. The particles are thus retained within the work space and not transported by the single velocity moving air stream. This reduces the cleanliness levels otherwise obtainable.

Single velocity Laminar Air Flow systems are also liable to be affected by particular migration caused by different energy gradient forces. Free particulates travel to the region where the energy level is lowest. For example, particulates settle under the influence of gravity; and, where a thermal gradient exists, particuiates tend to move towards the cooler zone; and statically charged particulates move towards a neutralising field. In practical work situations, a single uniform velocity does not effectively counter all these energy differentials.

In single velocity horizontal Laminar Air Flow rooms, for example, where equipment and other heat-generating devices are placed, a stratified hot zone results along the entire room axis. Besides being a heat-loading problem, this thermal energy gradient potentiates the particulates to a variable deposition probability. Likewise, in such rooms, considering the matter from an ergonomic viewpoint, contaminating particulates are generated in a work zone from 0.75 metres to 1.2 metres above the floor level. Streamlines caused by the single velocity profile carry these 'contaminants en masse and compromise the cleanliness levels of areas downstream of such work zone rendering these areas unacceptable for critical operations.

It is an object of this invention to provide a method for filtering and conveying air at multiple velocities that are selectively graded across the cross-section of the air path, such that airborne particulates, which migrate due to aerodynamic drag and other kinetic forces and are potentiated by thermal or electrostatic energy differentials, are regulated more efficaciously.

It is another object of the invention to provide a device having instru mentalities for generating a graded multiple velocity airflow, such that particulate migration can be better controlled.

It is a further object of this invention to increase the number of airborne particulates that will stay in linear orientations further along a clean to dirty axis, when such particulates are released into a forward or reverse multiple velocity gradient cross flow clean or sterile room, or clean air work station.

It is another object of this invention to decrease the particulate contamination during sterilization and depyrogenation of objects, for example, glass containers for sterile pharmaceutical products by previously heated air, while at the same time to utilise multiple velocity airflow oriented to transfer heat a thermodynamically dictated gradients.

In keeping with these objectives and with others which will become apparent hereafter, one feature of the invention resides in the novel configuration of a final filter which has a tapering formation and comprises a plurality of pleats with each successive pleat run being progressively and incrementally greater, and thereby with a larger surface area; which, with a constant media traverse air velocity gives rise to a graded pressure differential at the projected face; thus yielding a correspondingly graded exit air velocity profile.

The features of novelty which are considered to be characteristic of the invention are set forth in particular in the appended claims.

The invention itself, however, both as to the construction and method of operation of embodiments thereof together with additional objects and advantages thereof, will be better understood from the following description of

2 GB 2 098 317 A 2 specific embodiments when read in connection with the accompanying drawings, oresented by way of illustration but not limitation, in ihich:

Fig. 1 is a cross-sectional side view, with basic elements in position, of one embodiment according to the present invention, arranged in a horizontal airflow configuration; Fig. 2 is a fragmentary schematic view illustrating the formation of eddy currents and entrapment of particulates when the air velocity across the work space has a graded profile; Fig. 3 is a view similar to Fig. 2, but with air velocity having a uniform profile; Fig. 4 is a plan-view of another embodiment, also in horizontal air flow configuration but with the graded velocity profile oriented differently; Fig. 5 corresponds to Fig. 2, but applies to Fig. 4; Fig. 6 likewise, corresponds to Fig. 3, but applies to Fig. 4; Fig. 7 is a view similar to Fig. 1, but illustrates one embodiment in a vertical airflow arrangement; Fig. 8 corresponds to Fig. 2, but applies to Fig. 7; Fig. 9 likewise corresponds to Fig. 3, but applies to Fig. 7; Fig. 10 is a perspective view of one form of a filtering device having the instrumentality for generating graded velocity profiles; Fig. 11 is a plan-view of another form of a filtering device as illustrated by Fig. 10; Fig. 12 is a velocity curve of the filtering device illustrated by Fig. 10; Fig. 13 shows the trajectory path of particulates in a single airstream; Fig. 14 shows the corresponding path of particulates in multiple and graded airstreams; Fig. 15 is a cross sectional end-view of one form of apparatus for sterilizing objects; and Fig. 16 is a chart showing curves exemplifying the sterilizing parameters.

In each of the embodiments now to be described, a blower-pressurised housing is a source of air, incident on, and traversing without bypassing the filtering device; which device, besides performing its function as a filter, delivers air in multiple airstreams which are at velocities that are graded progressively at predetermined rates and orientations.

Referring first to Fig. 1 and Fig. 2 which schematically illustrate one embodiment of the invention, a work space, having a particulate density of less than three particles of 0.5 micron per litre of air is achieved by the filtering device, comprising a blower 1, having an air intake through another filter 2, and arranged to deliver air to a plenum 3 under sufficient pressure to get a single uniform traverse velocity of 2 to 3 metres/min across the media 4 which offers 125 uniform resistance to airflow of between 25 and mm water gauge pressure. At the projected exit face of the filtering device 5, the rate of flow is determined by the air of each pleat which is successively and progressively increasing from one pleat to the next.

When an object 6 (Fig. 2) is placed in such a multiple and graded velocity airstream, a cone of turbulence 7 is disposed along an ascending vector and the eddies are reduced and have shorter orbits. For comparison the same object 6 also shown in Fig. 3 in a single velocity airstream, where the corresponding cone of turbulence 7 is disposed normally and further along the work area with larger orbits.

Fig. 4 graphically indicates the influence of gravity and skin-friction drag of the containment surfaces like such as a table 9, panels 10 or a canopy 11, which determine the critical edge 12.

The re-entrainment of the particulates in the airstream is sharply affected in the single velocity airflow system.

Fig. 5 exemplifies the same characteristic for another embodiment of this invention. Here, the multiple velocity gradient achieved by the filtering device 5, is oriented to reduce the aerodynamic drag caused by the operator 0. Fig. 5 shows that the linearity L of the airstreams is regained in a shorter length beyond the containment surfaces 9, 10 and 11; and also shows that the shadow of operator 0, indicated as a turbulent zone X, is smaller than for a single velocity airstream, as represented in Fig. 6.

Fig. 7 shows a vertical configuration, comprising a blower 13, intake filter 14, plenum 15 and a final filtering device 16, the work space being bounded by containment surfaces such as a perforated table 17, back panel 18 and side panels 19, which access to a work space W. In this configuration the gradient flow is oriented to retain airstream linearity across the open working access W.

Fig. 8 elucidates the airflow characteristics that would affect particulate trajectories. The presence of an object 20 gives rise to a turbulent zone Z and a non-linear velocity zone Y. For a single velocity airstream per Fig. 9, the corresponding turbulent zone by object 20 is of greater extent than Z and the non-linear velocity zone greater than Y.

Fig. 10 shows a first embodiment of filtering device which comprises a continuous filter media 4 pleated around separators 21, each plate of which is supported by a wider separator with a progressive but fixed increment ranging from 0.3 mm to 1.0 mm, such that they support and separate the pleats 22 formed around them and channel the air.

The assembly is housed in a frame 23 and sealed to prevent bypass around the media. The increase in surface area of successive pleats correspondingly increases the air quantity that passes through each pleat as the pressure differential and traverse air velocity across the media 4 is constant. This air is channelled by the separators to the projected f i Iter face, where this variable volume yields graded velocities in overlapping steps and these velocities increase from 100 mm to 250 mm per minute per pleat. This is graphically represented by Fig. 12, where the exit air velocity is plotted against the pleat run.

i 3 GB 2 098 317 A 3 Fig. 11 is another embodiment of the filtering device as utilised in a different configuration, for example, as shown in Fig. 4 oriented to reduce the aerodynamic drag caused by an operator.

The utility of multiple velocity airstreams is highlighted by Fig. 13 and Fig. 14 which plot typical curves for different trajectories followed by spherical particulates having a specific gravity of 2.0. Particulates are subjected to velocity vectors; the first vector is the single horizontal airstream velocity Vh, and the second, the vertical settling velocity Vs conforming to STOKES' LAW. The resultant velocity vector Vp, is the one that causes the trajectory path followed by the particulate. In actual. practice, particulates are widely distributed in shape, size and density. This is advantageous to 80 multiple velocity airstream gradient flow, as particulates will find their energy levels better in their respective streamlines. This applies particularly to microbial contaminants. With multiple airstreams, at graded velocities increasing across the particulates' settling path, the altered trajectories are attributable to aerodynamic forces.

Fig. 15 illustrates another embodiment of the invention where an arrangement continuously to sterilize and depyrogenate glass containers 24 for sterile pharmaceutical formulations, by mearrs-of pre-heated air through a filtering device 26 located in a housing 27 of a sterilizing zone 28.

Containers 24 are transported on a conveyor 25 under graded airflow achieved by means of the filtering device 26 and a blower 29. They are intended to reach a temperature of approx.

3500C. The containers 24, are further transported on the same conveyor 25 to a cooling zone 30. In this zone they are cooled by graded airstreams from filtering device 31 and blowers 32, located in a housing 34 which is segregated from the sterilizing zone housing 27.

Fig. 16 shows comparative curves for t6e temperature cycle that the containers undergo during sterilization. The axis represents the residence time within each zone, which is a function of the rate of travel of the conveyor. The ordinate represents the air temperature. Since heat transfer would in addition be a function of the air velocity and thereby the air volume striking the containers, the ordinate also represents enthalpy.

This explains the reduction in heating-up time as shown in Fig. 16, yielding faster container 115 throughputs, when multiple velocity are thermodynamically graded for precise energy transfer and are also correspondingly graded for prevention of particulate deposition.

Fig. 16 the hot airstream gradient is so oriented that the highest velocity impacts on the coolest containers at the start of the heating cycle. Again at the start of the cooling cycle the hottest containers receive higher velocity, but at the end of the cooling cycle, consistent with Fig. 8, a higher velocity is oriented to ensure better control of airborne particulates, as the containers at this stage 33 are vulnerable to microbial contamination. The broken line represents the temperature cycle the same containers undergo with single velocity airstream at identical air temperatures.

Although illustrative embodiments of the present invention has been described herein with reference to drawings, it is to be understood that the invention is not limited to those particular embodiments and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of this invention.

Claims

1. A method for control of airborne particulates in enclosed workspaces; which method comprises filtering and conveying air at predetermined, multiple, and selectively graded velocities across the said workspaces; by means of a filtering device, having a plurality of filtering media pleats in tapering formation, each successive pleat having an area incrementally different from adjacent pleats and contained in a frame to produce a graded velocity profile on a projected exit face of the said filtering device; a housing having an air inlet surface and an air discharge surface; first means of mounting the filtering device; and second means for providing a source of air under pressure by a powered blower; the said air being forced through the said filtering device and thereafter conveyed at multiple velocities by appended containment surfaces such that the said workspaces are suitable for clean and sterile manipulations and wherein said workspaces contaminating particulate migration and deposition is regulated.

2. A method for control of airborne particulates as claimed in claim 1, suitable for work stations or rooms for clean and sterile conditions, by multiple airstreams of graded velocities ranging from 25 metres/min to 40 metres/min for air between 20c1C and 400C.

3. A method for control of airborne particulates as claimed in claim 1 suitable for sterilizing objects which are to be kept free from contaminating particulates by multiple streams of pre-heated air between 3000C and 4000C with graded velocities ranging from 40 metres/min to 70 metres/min.

4. A method for control of airborne particulates as claimed in claim 1 suitable for solid state diffusion of impurities into exposed silicon wafers at elevated temperatures from 7501C to 12501C in a furnace required to be free from contaminating particulates, using graded multiple velocity streams of pre-heated gases ranging from 30 metres/min to 70 metres/min.

5. A device for filtering and transposing air at predetermined multiple and selectively graded velocities across its projected air exit face; comprising of a plurality of pleats made from continuous media arranged in a tapering formation around separators such that each successive pleat has a longer run and progressively larger surface area, mounted in a frame, having instrumentalities to orient the said 4 GB 2 098 317 A 4 multiple velocity airstream.

6. A device as claimed in claim 5, comprising a plurality of pleats made up from a media, having a pressure drop of 30 mm 5 mm water gauge pressure for air at 201C and a penetration between 0.1 % to 10% for Di Octyl Phthalate aerosol of 0.3 micron or larger; both the pressure drop and penetration being at traverse velocity across the middle of 2.0 metres/min to 3.0 55 metres/min.

7. A device as claimed in claim 5 or 6, having an air velocity and pressure differential gradients varying from ratios of 1A.3 and 1A.7 between its tapering extremities.

8. A device, as claimed in claim 5, 6 or 7, comprising a plurality of pleats having a shortest pleat run starting at and between 100 mm to 250 mm and a pleat length increment ranging from 0.3 mm to 1.0 mm up to a longest pleat run of 350 mm.

9. A filtering device, as claimed in claim 5, 6, 7 or 8, suitable for continuous sterilization and depyrogenation of objects where heated air from 3000C to 4000C can be filtered and transposed in multiple airstreams at graded velocities between metres/min and 70 metres/min.

10. A filtering device, as claimed in claim 5, 6, 7 or 8, suitable for continuous sterilization and depyrogenation of objects where cooled air below 200C can be filtered and transposed in multiple airstreams at graded velocities between 25 metres/min and 40 metres/min.

11. A filtering device as claimed in claim 5, 6, 7 or 8, suitable for solid state diffusion of impurities into exposed silicon wafers where heated gases from 7501C to 12500C can be filtered and transposed in graded velocity gas streams from 30 metres/min to 70 metres/min.

12. An apparatus for control of airborne 85 particulates, suitable for horizontal airflow clean benches comprising a filtering device as claimed in claim 5; a housing having an outlet for air filtered by the said filter device conveyed across the work area having a substantially impervious and horizontal working surface; an air blower means; a plenum for ducting air, and appended Printed for Her Majesty's Stationery Office by the Courier Prest, Leamington Spa. 1982. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained containment surfaces such that a work space for clean and sterile manipulations and where contaminating particulate migration and deposition is regulated by multiple and graded air velocities selected and oriented against energy forces affecting such particulates.

13. An apparatus for control of airborne particulates suitable for vertical airflow clean benches comprising a filtering device as claimed in claim 5; a housing having an outlet for air filtered by the said filter conveyed across the work area having a perforated and horizontal working surface in the vertical plane; air blower means; a plenum for ducting air; and appended containment surfaces such that a work space for clean and sterile manipulations and where contaminating particulate migration and deposition is regulated by multiple graded air velocities selected and oriented against energy forces affecting such particulates.

14. An arrangement of more than one filtering device as claimed in claim 5, suitable for horizontal cross flow clean or sterile rooms comprising of an array of such filtering devices in tapering formation, and a blower means, such that de-stratification of streamlines across the crosssection of the room is effected to regulate unwanted contaminating particulate migration and deposition.

15. An arrangement of more than one filtering device, as claimed in claim 5, suitable for reverse flow cubicles, booths and rooms comprising of an array of such filtering devices in tapering formations, and a blower means to regulate scavenging and retention of contaminating particulates, by multiple graded air velocities selected and oriented against energy forces affecting such particulates.

16. A filtering device substantially as hereinbefore described with reference to and as shown in any one or more of the accompanying drawings.

17. A method of controlling airborne particulates substantially as hereinbefore described with reference to any one or more of the accompanying drawings.

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