CA1176997A - Apparatus for the leak testing of filters - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE Apparatus for the leak testing of one or more filters, which requires minimum space, and which comprises a housing (12) adapted to sealably mount the filters (30), means (40, 41) for introducing DOP particles within the housing and immediately up-stream of each of the filters, and a restriction member (54) mounted in the housing upstream of each filter and downstream of the point at which the DOP is introduced. The member (54) may be selectively positioned to assume an operative position transverse to the airstream wherein it acts to condense and mix the airstream and DOP and then disperse the mixture over the full frontal area of the adjacent filter, or a non-operative position wherein the member is substantially withdrawn from the airstream. Means (70) are also mounted downstream of each filter for monitoring the airstream to detect DOP particles, and thereby permit a determin-ation as to whether any of the particles are leaking through or around the filter.

Description

11~76997 This is a division of our co-pending Canadian Patent Application No. 347,061 filed March 5th, 1980.
The present invention relates to a housing for mounting one or more banks of high efficiency air filters in an air duct system, and which requires minimum space, permits the leak testing of the filters from outside the housing, and which permits the identification of a leaking filter in the bank.
In the manufacture of high efficiency air filtration systems, the filters, housings, and related components are usually initially tested as the final manufacturing step at the factory to locate any leaks and ensure compliance with the desired efficiency rating. In addition, it is recognized that in-place testing of the completed installation is essential to ensure leak tightness of the housing and filters in their assembled form. Also, in-place tests are usually conducted after each change of a filter in the system to ensure proper installation of the new filter, as well as at regular intervals after installation to detect any deterioration of the gaskets of clamping devices, or the develop-ment of weld cracks or the like in the housing which result in leaks. Such periodic in-place testing is particularly important where harmful materials, such as potentially radioactive substan-ces, dangerous viable organisms, or carcinogenic or toxic mater-ials, are removed from the airstream by the filtration system.
Both factory and in-place efficiency testing of filters and filter installations conventionally utilizes either a mono-dispersed or a polydispersed aerosol of ~., 11'7f~997 ¦ dioctyl phthalate (DOP) droplets or particles which have J a light-scattering mean diameter of 0.3 or 0.7 microns, respectively. The DOP is introduced into he airstream at a point in the duct far enough upstream of the filter or filter bank to assure complete dispersion by the time lt reaches the filter or bank of filters. An upstream ~ampling device is usually pro~ided immediately upstream of the filter to determine the concentration of the DOP
~n the duct, and a downstream sampling device is provided to detect leakage. A portion of the airstream is with-drawn from the duct through the sampling devices and conveyed to an external photometric light-scattering - instrument or the like which is designed to determine - the DOP concentration in both the upstream and downstream samples. : --In order to insure that the air-DOP mixture charged to the filters is thoroughly mixed to obtain a substantially uniform dispersion, and that the upstream 8ample is representative of the DOP concentration at that point, it has been reco~mended that the DOP be lntroduced at least ten duct diameters upstream of the filters, or introduced upstream of baffles or turning .
vanes in the duct. Also, the use of a Stairmand d~sk positioned three to five duct diameters upstream of the 2S ~ilters has been recommended for this purpose, see for example Chapter 7 of "Design, Construction, and Testing of High ~fficiency Air Filtration Systems for Nuclear Applications" By Burchsted and Fuller, published January ~970 by Oak Ridge National Laboratory. Thus in order to properly introduce the DOP into the airstream utilizing these recommended procedures, extra equipment and a great deal of space is required.
It is also presently recommended that the down-~tream sampling port be located a considerable distance 3S downstream of the filter, and preferably downstream of the exhaust fan, to insure that any DOP which leaks through the filter becomes sufficiently mixed with the filtered ~ir so that the leak will be-detected no matter where ; .

_3_ ln the airstream the sample is withdrawn. This procedure not only serves to further increase the space requirements ~f the test apparatus, but also, it reduces the assurance that a leak wili be detected by reason of the dilution effect of the leaking DOP in the great volume of filtered air. As will be apparent, this dilution effect is parti-cularly pronounced where a large bank of filters is employed.
In installations requiring muitiple banks of filters employed in series, the space requirements for introducing the DOP, and withdrawing the samples, are multiplied since the banks must be separated a distance sufficient to accomplish the above objectives. In an attempt to alleviate this problem, it has been proposed ~5 to employ temporary ducts between the various banks with ~uitable valves therein, and with the temporary ducts serving to selectively isolate each bank in the apparatus during the test procedure and introduce the bOP and ~ith- ~
1 draw the samples at an appropriate distance from each I . 20 bank. However, this proposed system is not entirely catisfactory since it requires a great deal of extra - equipment which is only used during the test procedure, . and the use of temporary ducts adds addi~ional problems of possible leakage through the various required connections 1- Z5 and valves.
¦ Where the above tests indicate the presence , of an unacceptable leak, it is standard practice for ¦ a workman to physically enter the duct at the downstream 8ide of the filter bank to conduct leak probing of individual filters, and thereby permit identification ; and replacement of the faulty filter. In some cases, it is also necessary for the workman to enter the upstream ~ide to locate the source of the leakage problem. Such direct personnel access is often impractical and~or dangerous however, either because of the system con-iguration, or because of high radioactivity levels or the presence of other dangerous materials in the air-~tream. In such cases, it has been previously proposed ! -.. . :

'7~ 99 7-4-that the housing incorporate remotely operated long.
handles probes which are able to cover the entire downstream area of the filter bank, or alternatively, that the workman don protective clothing prior to entering the duct. As will be apparent, neither of these alter-natives is satisfactory, since the design and fabrication of suitable remotely operated probes significantly adds to the cost of the housing, and the physical entry of a workman into the duct greatly increases the risk of contamination to himself as well as other workers.
Still another problem associated with the above described conventional testing procedure for a bank of ilters is the fact that a great deal of DO~ must be fed into the airstream to assure an adequate dispersion in front of each filter. Since the area of the bank ~ay be quite extensive, it is often difficult to achieve 'an even distribution o the DOP over the face of the entire bank, even when the DOP is introduced'ten duct ~
diameters upstream as recommended, and in addition, it . 20 is difficult to generate a sufficient quantity of the DOP for such large banks. Also, the DOP tends to become deposited on all of the filters while each individual filter in the system is being probed, thereby unduly loading the filters and increasing their resistance to the passage of the air.
Applicants' prior U.S. Patent No. 4,055,07S
discloses a method and apparatus for the leak testing ''~'~''''' of ilters wherein the above noted disadvantaqes of the earlier techniques and equipment are effectively overcome.
More particularly, the applicants' prior patent discloses a test housing wherein the DOP is introd~ced and dispersed - ~nto the duct at a location closely adjacent the upstream face of the filter to thereby minimize 1Oor space, and wherein each filter is charged with DOP and tested indivi-dually to permit the ready identificat'ion of a leakingilter without manually entering the duct and probing the f~lters. Further, in one embodiment of the patented housinq, there is provision for selectively condensing , 11'~tj997 and imparting turbulence to the airstream immediately downstream of the filter so that the sample may be withdrawn at a location immediately downstream of the filter to thereby further reduce the space requirements.
The present invention provides an apparatus for filter-ing an airstream through a bank of filters and having provision for testing for leakage of particles of predetermined size through or around any of the filters and characterized by substantially balanced flow among the filters and by the ability to permit the immediate identification of a leaking filter in the bank, and comprising: a housing adapted to form a part of an air duct system having an airstream passing therethrough; means for seal-ably mounting a bank of filters in parallel arrangement in said housing such that the airstream passing through the housing passes through the bank of filters; panel means mounted within said housing and immediately upstream of the bank of filters for dividing the housing into a separate air channel for each of the filters, and such that the airstream is divided into an isolated portion within each of the separate channels which passes through a single filter; means mounted within each of said separate chan-nels for selectively introducing particles of predetermined size into the portion of the aristream passing therethrough; means mounted within each of said separate channels and upstream of the filter and downstream of the particle introducing means for seIectively partially restricting and imparting turbulence to the flow through all of the isolated portions, and so as to obtain turbulence throughout substantially the full cross sectional area 11769~7 of each isolated portion and thereby mix the airstream and particles, and also achieve a substantially balanced flow among the isolated portions, and means mounted within said housing and downstream of said filter bank for detecting at least a portion of any particles in the airstream to thereby permit a determinina-tion as to whether any of the particles are leaking through or around any of the filters in the filter bank.
A single structure is able to selectively function to mix and condense the air passing through an upstream filter to thereby permit a representative sample to be readily withdrawn and analyzed during testing of the upstream filter, or to mix and disperse the particles in the airstream during the testing of a downstream filter. The structure preferably may be select-ively moved into an operative position transverse to the airstream during a test procedure, and then moved to a withdrawn position so as to provide little interference with the normal air flow through the housing.
Preferably, the restriction means comprises a funnel-shaped member mounted within the housing and having a base with a peripheral outline closely corresponding in size and configuration to the outline of the air passageway and an apex having an opening therethrough. The funnel-shaped member is movable between an operative position wherein the member is disposed transversely acxoss the housing and the airstream is thereby condensed and passes through the opening in the funnel-shaped member, and a non-operative position wherein at least a substantial portion of the member is disposed along the side of the housing and is sub-; stantially withdrawn from interference with the airstream. Means .

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1:~'7t;~97 are also provided for detecting at least a portion of any particles in the airstream at a location downstream of the filter to thereby permit a determination as to whether any of the particles are leaking through or around the filter.
In the preferred embodiment, the housing is rectangular in cross-section, and the funnel-shaped member is in the form of a truncated four sided pyramid, and further comprises a tube affixed about the periphery of the opening and extending in a direction toward and perpenidcular to the base of the pyramid.
Also, the base of the pyramid is preferably disposed downstream of the apex thereof, so that the member acts to condense the airstream while imparting turbulence thereto, and the airstream exiting from the tube expa~s so that the airstream and particles are substantially uniformly dispersed over the full frontal area of the adjacent, downstream filter.
The filters may be individually tested by initially moving each of the funnel-shaped members to their tra~suerse opera-tive position, sequentially introducing DOP into each air channel upstream of the filter, and monitoring the airstream downstream of the filter bank to detect for the presence of leaking DOP. By this procedure, the funnel-shaped members function to partially restrict the flow thorugh each of the channels, resulting in a substantially balanced flow among the isolated portions. This in turn facilitates the uniform dispersion of the DOP within each channel.

1176~97 Where the housing mounts a number of filters in series, a funnel-shaped member as described above may be mounted inter-mediate the filters, where it functions to mix and condense the air passing through the upstream filter during testing thereof to permit a representative sample to be readily withdrawn and analyzed, and to mix and disperse the particles in the airstream during the testing of the downstream filter.
Some of the objects having been stated, other objects will appear as the description proceeds, when taken in connection with the accompanying drawings in which --~j - 7a -1:1'7~?97 Figure 1 is a perspective view of a filter test housing which embodies the features of the present invention;
. Figure 2 is a perspective view, partly broken S .away, of the upper half of the filter housing shown in Figure l;
' Figure 3 is a sectional eleva'tion view taken . substantially along the line 3-3 of Figure 2, but with the final funnel-shaped member being shown in its non-operative position;
Figure 4 is a fragmentary perspective view . illustrating the apparatus ~or restricting and distributinq an airstream in accordance with the present invention, Figure S is a front.elevation view, on a reduced . . 15 scale, of the upstream side of the apparatus shown in ' ' Figure 2; . ' .
. . Figure 6 is a fragmentary elevation view, i ~llustrating the DOP sampling device and taken sub-. .stantially along the line 6-6 of Figure 3;
' . Figure 7 is a sectional plan view o the upstream portion of the apparatus shown in Figure 3, but illustrating only one of the air channels; and Figbre 8 is a perspective view of the funnel-~haped member of the present invention, and illustrated 25 ~n its non-operative position 'in the housing. :
Referring more specifically to the drawings, . one'embodiment of the present invention is illustrated which includes a housing 12 which is adapted to form a part of an air duct system, such as an exhaust air 30 cleaning system in a nuclear fuel manufacturing facility.
More particularly, the'housing 12 comprises a generally box-like enclosed structure having two side walls 14 ~and 15, a top wall 16, and a bottom wall 1~. Also! the housing defines a forward open end which is joined to an air inlet opening of the inlet duct 21 via the transi-tion member 22, and a rear open end which is similarly ~oined to the outlet duct 24 'via the transition member 25. ~he housing is typically fabricated from a sheet metal material, such as 14 gauge stainless steel. ' . . . :

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In the illustrated embodiment, the housing 1~ mounts two banks of filters in series, with each bank comprising four filters 30 in parallel. Typically, each ~ltcr 30 is about 24 x 24 x 12 inches in size, and comprises a wooden or chipboard frame 31 surrounding and supporting a pleated filtering media 32, such as a non-woven fiberglass or cellulose-asb~stos sheet material.
~ilters of this type may have extremely high dust retention characteristics, usually greater than about 9S% efficiency, - ard typically 99.97% eficiency, as measured by the con-ventional DOP test, and are known as "absolute" or HEPA
filters in the art.
- The filters 30 of each bank are adapted to be inserted and removed from the housing 12 through the two associated side access doors 34, 35 with the two upper filters in each bank being inserted and removed through the upper door 34, and the two lower filters be~ng inserted and removed through the lower door 35.
As is well known in the art, clamping means as shown ~omewhat schematically in Figure 7 at 29, and sealing 20 means are provided in the housing 12 for sealably mounting each of the ilters therein. This sealing means may ~nclude a resilient gasket on the ront peripheral face of the filter frame 31, or a fluid type seal such as ~hown schematically at 33 and as described in the U.S.
Patent No. Re. 27,701 to Allan. Where a dangerous sub-stance, such as radioactive material or viable organisms ~ay be present on the filters or in the airstream, the housing may further include a conventional "bag-out"
r~ng (not shown) about each of the access doors 34, 35 30 to permit the filters 30 to be enclosed in a plastic ; bag while being removed from the housing and thereby avoid possible contamination.
The portion of the housing 12 immediately upstream o each bank of filters is divided into four separate 35 and rectangul~r air channels by means of the four per-pendicularly disposed panels 36, 37, 38 and 39. Each channel is aligncd with one of the filters, and generally . .

11769~7 --10-- .
con~orms in cross-sectior.al ou~line and sizo to that of the associated filters. Thus for example, the panels 36 and 37 cooperate with the side wall 14 and top wall 16 to define a separate air channel upstream of the S filter located in the upper left hand corner of the bank as seen in Figure 1. By this arrangemént! the airstream entering the housing from the transition member 22 is divided by the panels into four equal portions, with each portion being isolated from the other portions and directed to pass through a single filter. Where the housing mounts a number' of banks of filters in series, ' the panels 36-39 may extend the entire distance between the banks as best seen in Figure 2 so as to similarly define air ch'annels for the downstream filters. In this regard, it will be understood that the panels 36 and 38 are discontinuous at points immediately adjacent the ~ilters to accommodate the sliding receipt of the inner ' filter, as well as the clamping mechanism 29, note Figure '' 7.
In accordance with the present invention, means are provided for selectively introducing particles of ' predetermined size (typically DOP particles having a mean diameter of between about 0.3 to 0.7 microns) in ' the portion of the airstream passing 'through each one 25 of the channels.' As illustrated, this particle intro-' ducing means comprises two pairs of suppiy pipes ; ' 40 and 41 which extend horizontally through the housing .upstream o the first bank of filters. One pair of supply ` pipes is operatively associated with the two upper filters '' 30 in each bank, and another pair is operatively associated with the two lower filters. As best seen in Figure 5, ' the pipe 40 terminates at a p~int centrally disposed ~n the adjacent air channel, and the pipe 41 extends to a central location in the far air channel. Both pipes 35 ~nclude an exteriorly positioned cap i2, to permit the plpes to be selectively coupled to a DOP supply source.

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11'76~37 The apparatus o the present invention also includes means mounted within each of the separate channels for selectively mixing the airstream and DOP particles, and then substantially uniformly dispersing the same over the full frontal' area of the adjacent filter. In the illustrated embodiment, this mixing and dispersing means comprises restrict'ion means in the form of a funnel-shaped member 54 mounted upstream of the filter and down-stream of the associated DOP supply pipe. The funnel-shaped member S4 comprises a trunca~ed, fobr sided pyramidwhich includes a base 56 which has a rectangular peripheral ' outline corresponding in size and configuration to the cross-sectional outline of the interior of the channel, and which defines a plane disposed transversely across ' 15 the channel. A flange 57 is affixed to the base 56 to' facilitate the mounting of the member in a rectan~ular duct. The member 54 also has an apex having a rectangular ' '' opening 58 therethrough, and a tube 59 having a rectangulàr ; cross-sectional outline corresponding to that of the opening 58. The tu~e is affixed to the periphery o~
the opening 58 and extends a substantial distance into the interior of the member and in a direction parallel to the direction of air fiow, i.e., toward and perpendicular to the plane defined by the base 56. Preferably, the ' tube extends'at least about one-half the distance t'o the plane of the base 56.
A transverse deflector 80 overlies in spaced relation the ree end of the tube 59, and as best seen ~n Figure 7, the deflector acts to laterally deflect : 30 the airstream moving through the tube, and thereby achieve a 5ubstantially uniform dispersion to the downstream filter. In the illustrated embodiment, the deflector 80 comprises a four sided pyramid which is disposed with ~ts base overlying the do'wnstream end of the tube 59.
35 The deflector is attached to the tube by any suitable means,' and as illustrated, the attachment means comprises a longitudinally directed threaded membcr 81 fixed to the inside of the deflector, and a cooperating transverse 1~7~99~7 strap 82 mounted across the opening o~ the apex of the member 54. By this arrangement, longitudinal adjustment of ~he de~lector with respect to the tube is perrnitted ~o that the spacing therebetween can be readily adjusted ' to best achieve the desired uniform dispersion.
The funnel-shaped member 54 also is composed ' o two separate components, namely, the lower base portion 61 which is adjacent the base 56, and a separate apex portion 62 adjacent the apex and including the tube 59 and deflector 80. The base portion 61 of the member 54 is fixedly mounted in the channel of the housing by welding or the like, and it may be described as being truncated to define a relatively large rectangular opening (not numbered) disposed centrally in the housing. The ~5 apex portion 62 is hingedly mounted to the base portion by a hing'e 63'which extends transversely across the housing and adjacent the opening of the base portion. In addition, 'the two portions 61, 62 include mating flanges 64 to 'facilitate the interface thereof.
~o more particularly describe the size and ;' coniguration of an exemplary embodiment of the me~ber ; '-' 54, the base 56 and opening 58 are each square in cross- ' ~ection, with the base being about 24 inches along each 8ide and the opening 58 being about 6 in&hes along each side. The flange 57 is about 1 inch wide, the longitudinal distance from the base 56 to the apex is about 5 1~4 ~nches, and the tube 59 has a longitudinal length of '-'~'''~' about 3 inches. ' '' The apex portion 62 may'be pivoted between an operative position wherein the member is in the form of a truncated pyramid which is disposed transversely across the air passageway, and a non-operative position (note the position of the member S4c in Yigure 3) wherein the apex portion 62 is disposed along the side of the air passageway so as to be substanti'ally withdrawn from ~nterference with the airstream. The means for operatively controlling the positioning of the apex 11769~7 , portion 62 o~ the member 54 comprises a control rod 65 which extends through the wall 14 of the housing and in a direction parallel to the hinged interconnection 63. A linkage 66, 67 interconnects the rod 65 and apex portion 62 ~note Figure 4), and a crank 68 is positioned ; on the outside of the housing for rotating the rod to pivot1the apex portion 62 between its operative and non-operative positions. In this regard, it will be noted that the rod 65 extends through the two laterally adjacent cbannels, such that the two members 54 are concurrently operated by rotation of the rod.
Viewing Figure 3, it will ~e seen that the - housing includes a first funnel-shaped member 54a posi-tioned upstream of the first filter, a second like funnel-shaped member 54b mounted immediately downstream of the first filter and upstream o the second filter, and a third like funnel-shaped member 54c wpich is mounted ~mmediately downstream of the second filter.
. . The housing 12 also includes means for with-. 20 drawing a sample of the airstream at each of three separate locations along its length. A first sampling means 70a provided immediately downstream of the first funnel-~haped member 54a, a second sampling means 70b immediately downstream of the second member 54b, and a third sampling means 70c immediately downstream of the third mem~er 54c. Each of these three sampling means comprises a irst pipe 71 extending laterally into the first adjacent channel and having an apertured sampler 72 positioned immediately downstream of and in alignment with the tube - 59 of the adjacent funnel-shaped member, and a second p~pe 73 extending into the laterally remote channel and -~ncluding a similarly positioned sampler 74, note Figure 6. Each pipe extends through the wall 14 of the housing, and includes a removable end cap 75 for permitting the sampler to be selectively connected to a conventional DOP detecting apparatus.

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During normal opera~ion o~ the ~ilter housing 12, the apex portions 62 of all of the funnel-shaped members 54 are lowered into their non-operative position, 80 as to minimize air resistance. When leakage tests are to be conducted on the upstream bank, the apex portions 62 of the four members 54a which are upstream of the initial bank of filters, as well as the four members 54b immediately downstream of this bank, are all raised into their operative positions. The third members 54c preerably remain open, such that the various members ' are oriented in the manner shown in Figure 3.
The above disposition of the members results in the air flow being partially restricted as it moves through each of the four channels, and this restriction ~roduces a back pressure upstream of the housing, which in'turn serves to substantially balance the flow ~mong the four channels. Thus an uneven flow pattern resultin~ ~' from the design of the transition 22, plenum connections, or other upstream duct components, is avoi'ded.
'The DOP particles are then selectively and lndependently introduced into each of the four channels by means of their associated pipes 40 and 41.' In each ' lnstance, the airstream and entrained particles approach the first member 54a, and substantial turbulence is ' 25 imparted thereto to mix the particles'and air as illus-trated schematically in the right hand portion of Figure 3, and the airstream is condensed into the ~ube 59 of the member. As the airstream exits the tube 59 and deflector 80, it is dispersed over the full cross-sectional area of the channel. At relatively low flow rates, such as where testing is conducted for example at 20~ of the designed flow rate of the housing, substantially uniform dlspersion over the full frontal area of the filter is achieved. The first sampling means 70a may at this point ln the test be connected to the DOP detecting apparatus to determine the upstream concentration of the DOP.

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The airstream continues through the first filter and strikes the downstream second member S4b, which again imparts turbulence to the airstream and condenses it to pass through the tube S9 of the member. Thus the air and any leaking DOP is thoroughly mixed by the time it passes through the tube 59, and such that the sampler 72 (or 74), which is positioned immediately downstream of the tube 59 will be assured of picking up a repre-sentative portion o any leaking DOP when it is connected to the detecting apparatus. By this testing procedure, - each filter in the first bank may be individually tested, - so that the identification of a leaking filter is readily apparent.
To test the filters o the second bank, the four members 54a are preferably moved to their non-operative lowered position, and the two downstream sets o members 54b and 54c are each moved to their operative positions. DOP is then selectively introduced from the associated pipes 40, 41 into each of the four channels,-and samples are withdrawn through the samplers of thethird sampling means 70c to detect any leaking DOP in the manner described above. In this instance, it will be no~ed that the members 54b now serve the dual functions of ~alancing the flow to the second bank of filters, and mixing the air and DOP particles and dispersing the mixture to the second ilters. Thus a single component in the housing, namely the members 54b; perform distinctly di~erent functions during the testing of the two filter banks.
In the drawings and specification, there has been set orth a preferred embodiment of the invention, and although specific terms are employed, they are used in a ~eneric and descriptive sense only and not for purposes of limitation.
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Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for filtering an airstream through a bank of filters and having provision for testing for leakage of particles of predetermined size through or around any of the filters and characterized by substantially balanced flow among the filters and by the ability to permit the intermediate identification of a leaking filter in the bank, and comprising:
a housing adapted to form a part of an air duct system having an airstream passing therethrough, means for sealably mounting a bank of filters in parallel arrangement in said housing such that the airstream passing through the housing passes through the bank of filters, panel means mounted within said housing and immediately upstream of the bank of filters for dividing the housing into a separate air channel for each of the filters, and such that the airstream is divided into an isolated portion within each of the separate channels which passes through a single filter, means mounted within each of said separate channels for selectively introducing particles of predetermined size into the portion of the airstream passing therethrough, means mounted within each of said separate channels and upstream of the filter and downstream of the particle intro-ducing means for selectively partially restricting and imparting turbulence to the flow through all of the isolated portions, and so as to obtain turbulence throughout substantially the full cross sectional area of each isolated portion and thereby mix the airstream and particles, and also achieve a substantially balanced flow among the isolated portions, and means mounted within said housing and downstream of said filter bank for detecting at least a portion of any particles in the airstream to thereby permit a determination as to whether any of the particles are leaking through or around any of the filters in the filter bank.

2. The apparatus as defined in Claim 1 wherein said detect-ing means comprises means positioned immediately downstream of each filter for selectively condensing and imparting turbulence to the portion of the airstream passing therethrough, and means for withdrawing a sample of the airstream downstream thereof to permit an analysis of the same.