US3721067A

US3721067A - Clean air system for hospital operating rooms

Info

Publication number: US3721067A
Application number: US00176909A
Authority: US
Inventors: B Agnew
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-11-12
Filing date: 1971-09-01
Publication date: 1973-03-20
Anticipated expiration: 1990-03-20

Abstract

Two relatively shallow floor to ceiling cabinets are located against one end wall of a hospital operating room and extend side by side along the wall for a major portion of the room width. Each cabinet is provided with a bank of filters encompassing substantially its entire frontal extent and includes a pressurized plenum chamber behind the filters, coextensive therewith. Blowers mounted within the cabinets pressurize the plenum chambers to direct a laminar horizontal flow of clean air through the filters and thence over an operating table positioned near the end wall. A pair of full height but narrow deflecting vanes extend from opposite outboard ends of the cabinets across the face of the filters at an acute angle therewith. The vanes narrow the outflowing stream, increase its velocity, improve its cross-sectional configuration, and help to separate and isolate the cabinet air intakes. The clean air stream extends from the operating room floor substantially to the ceiling and has a width sufficient to envelope not only the operating table but also persons standing and working alongside the table. Air is returned to the cabinets along relatively narrow portions of the room at opposite sides thereof and withdrawn from the room into the blowers through prefilters mounted on oppositely disposed outboard end walls of the cabinets that are positioned to face opposite side walls. Outflowing clean air is balanced against return air streams.

Description

United. States Patent 1191 Agnew CLEAN AIR SYSTEM FOR HOSPITAL OPERATING ROOMS [76] Inventor: Boyd F. Agnew, 111 Via Lido Nord,

Newport Beach, Calif. 92660 [22] Filed: Sept. 1, 1971 21 Appl. No.2 176,909

Related US. Application Data [63] Continuation-in-part of Ser. No. 88,667, Nov. 12,

1970, abandoned.

52 US. en. ..55/97, 21/53, 21/74 R, 55/350, 55/385, 55/415, 55/473, 55/483,

55/484, SS/DIG. 18, 55/010. 29, 98/36,

51 1111. 1:1. ..B01d 46/00 [58] Field of Search ..55/DlG. 29, DIG. 18,279, 1, 55/97, 410, 413, 414, 415, 418, 385, 467,

36, 40 R, 115 R; 21/53, 74 R;128/1, 191,

Kranz, Peter Jet Stream Ventilation for Extreme Air Cleanliness, ASHRE Journal, August 1962, pages 37 14 1Ma1re11 20, 1973 Primary Examiner-Dennis E. Talbert, Jr. Attorney-Allan Rothenberg [57] ABSTRACT Two relatively shallow floor to ceiling cabinets are located against one end wall of a hospital operating room and extend side by side along the wall for a major portion of the room width. Each cabinet is provided with a bank of filters encompassing substantially its entire frontal extent and includes a pressurized plenum chamber behind the filters, coextensive therewith. Blowers mounted within the cabinets pressurize the plenum chambers to direct a laminar horizontal flow of clean air through the filters and thence over an operating table positioned near the end wall. A pair of full height but narrow deflecting vanes extend from opposite outboard ends of the cabinets across the face of the filters at an acute angle therewith. The vanes narrow the outflowing stream, increase its velocity, improve its cross-sectional configuration, and help to separate and isolate the cabinet air intakes. The clean air stream extends from the operating room floor substantially to the ceiling and has a width sufficient to envelope not only the operating table but also persons standing and working alongside the table. Air is returned to the cabinets along relatively narrow portions of the room at op osite s1des thereof and withdrawn from the room 1n 0 the blowers through prefilters mounted on oppositely 26 Claims, 10 Drawing Figures PATENTEU HARZO I575 SHEET 10F 4 INVENTOR. 5070 F flG/VFW 4 TTOF/VE/E'.

PATENTED AR IBB v 3,721,067

SHEET 30F 4 4 770 FA/[Ki CLEAN AIR SYSTEM FOR HOSPITAL OPERATING ROOMS REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to air treatment and purifying systems and more particularly concerns a circulating system for providing a continuous flow of clean air over and around the entire area of and adjacent to an operating table of a hospital operating room.

2. Description of Prior Art The relation between infectious organisms and contamination as discovered by Pasteur and initially implemented by Lister is one of the outstanding advances in recent medical history. Following these early concepts, great care is normally taken in sterilization of instruments and equipment, cleanliness of wearing apparel is insured, and the use of surgical masks in or about hospital operating Nevertheless, other than the use of conventional airconditioning systems having temperature control as a primary goal, relatively little attention has been paid, in actual practice, to airborne micro-organisms and other contaminating particles.

Normally, the hospital air-conditioning system itself will accomplish some six to eight full changes of air each hour in the conventional 2,500 to 3,000 cubic feet hospital room. With such a relatively low rate of air change, it is not uncommon for several million particles to be present in each cubic foot of hospital air. Bacteria counts in conventional hospital surgeries may be anywhere from to 60 bacteria per cubic foot. It is known that hospital atmospheres contain types and strains of bacteria that are more likely to be resistant to control. Such bacteria may have developed from having been exposed to less than lethal doses of antibiotics and other means of bacterial control.

Recent studies have'demonstrated that the use of sterilized air within an operating room can reduce the rate of infection to one-tenth the level existing in rooms where air is not so sterilized. Particularly in lengthy operations such as'hip joint replacement or open heart surgery, the rate of infection can be as high as 50 percent. Hospital acquired infection, the so'called nosocomial infections, often are far more serious in terms of patient recovery or non-recovery than the cause for which the original operation was performed.

Recognizing the need for a system to isolate a patient from the infectious organism laden atmosphere that permeates the hospital, a number of suggestions have been made to provide air treatment systems. For example, U. S. Pat. Nos. 3,294,480 and 3,239,305, each invented by G. Potapenko, and U. S. Pat. No. 3,267,955 to H. H. Logan et al., illustrate systems for delivering aseptic air to and over the patient during or subsequent to a surgical procedure in the hospital-operating room. U. S. Pat. No. 3,367,257 to G. K. Raider et al. and U. S. Pat. No. 3,465,666 to J. V. Knab, illustrate arrangements off plural filters for providing a supply of air to a clean room that is generally directed vertically from the ceiling toward the floor of the room. None of these arrangements are useful and practical systems that are compatible with existing operating room structures and requirements.

An improvement in hospital room arrangements stems from the teaching of Whitfield as described in U. S. Pat. No. 3,158,457. Whitfield suggests the circulation of large amounts of air. Nevertheless, even with larger numbers of air changes as taught by Whitfield,

the danger of airborne infection may not be fully or completely reduced. It will not be reduced as much as possible without a proper control of the flow of clean A air. Thus, the arrangement of Whitfield and also the arrangements of several of the above-mentioned patents in which cleansing air is directed downwardly from the ceiling, may not be of practical application without substantial modification and rearrangement of equipment and appurtenances that have been developed and positioned for optimum use over periods of many years.

rooms is standard practice...

Of significance in this regard is the conventional overhead lighting system of the standard operating room. Such a lighting system is of a substantial horizontal extent and, for optimum application is generally positioned immediately above the operating table andpatient. Consequently, a vertically directed air stream, directed downwardly from the room ceiling and from a point above the lights, would first impinge upon the upper surface of these depending lighting systems whereby all or part of the operating table and the patient thereon would, in effect, be in the shadow of the lighting system insofar as the vertically directed air stream is concerned.

Accordingly, if systems such as that suggested by Whitfield are to be employed, the operating room must be substantially modified or rebuilt. In fact, for use of the Whitfield system, specially built rooms are required. Various partitions and chambers are built .within the operating room itself. The floor, through which air is exhausted from the room, is formed of a metal grating capable of supporting relatively large weight.

A major portion of the operating room space of the Whitfield arrangement is taken up by air chambers, passage ways, equipment compartments, and the like. Arrangements of other air-cleaning systems as described in some of the patents identified above do not depend upon vertical air flow. Nevertheless, they require partitions, side walls, or curtains that are positioned to continuously direct the air flow. All of these systems, accordingly suffer from a severe limitation, particularly in those locations where the law requires that major and minor surgical rooms each have specified minimum dimensions. Thus one could follow the teaching of many of the patentees and employ cleansed working areas of dimensions that are necessarily minimized due to the use of movable walls and partitions. In such case, either the-rooms would'fail to meet legal codes, or-walls and partitions must be made readily portable. However, even if in compliance with codes, such flow directing walls or partitions would interfere with normal room traffic. Further, they make cleansing and washing of the room difficult. Again, movable or portable walls and partitions would seriously interfere wit the use of the-increasingly complex and extensive stands, equipment, testing and monitoring apparatus that is commonly employed. Even so, the prior arrangements fail to suggest optimum patterns of air flow.

SUMMARY OF THE INVENTION In carrying out principles of the present invention in accordance with a preferred embodiment thereof, cleansing aseptic air is circulated through and within an operating room by forcing a stream of air from an end wall of the room in a pattern that extends from a point near the floor to a point near the ceiling or at least well above the heads of operating personnel, over the entire width of the operating table, and for a distance on either side beyond the position at which surgeons, nurses, and other attending personnel normally would be stationed. Instead of employing side walls, curtains or room partitions to direct the air flow, a pair of narrow deflecting vanes at the outer edges of the air stream produce inwardly directed air flow components as the filtered air enters the room. The flow pattern is thus shaped, increased in velocity and continues into the room without further confinement or guidance. The air stream is retro-directed by impingement upon the far end of the room and returned along opposite side walls to intakes at opposite corners of the room at the firstmentioned end wall. Outflowing and return air streams are mutually balanced for optimum results. The air flow is provided by portable filter module units positioned against the one end wall. Including their deflecting vanes, the modules project but a short distance into the room from such wall, but extend substantially entirely from floor to ceiling within the conventional room and from side to side for a major portion of the width of the room. The entire surface of the portable unit that faces toward the room is provided with a bank of filters so that the large cross-section air stream is fully filtered. By providing the portable units with air intakes on their oppositely facing and outwardly directed ends, the desired air flow pattern is enhanced. The deflecting vanes are preferably adjustably mounted at outboard ends of module units to control air flowing out of the units and isolate the air intakes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevational view of a pair of filter units constructed in accordance with the principles of the present invention;

FIG. 2 is a plan view of a hospital room embodying filter units of the invention showing a general pattern of air flow provided by the filter modules;

FIGS. 3 and 4 are vertical cross-sectional views of a single filter unit;

FIG. 5 is a horizontal cross-sectional view of the unit of FIGS. 3 and 4;

FIG. 6 illustrates details of the mounting and securement of one of the front loading filters and a deflecting vane, and

FIGS. 70, 7b, 7c and 7d are observed velocity profiles of air flow patterns at various distances from the filter units.

DETAILED DESCRIPTION As illustrated in FIG. 2, a conventional hospital room has

end walls

10 and 12, side walls l4, 16, a floor l8, and a ceiling (not shown in this figure) of about 9% to 10 feet in height. Typically, the room may be square, being 18 20 feet by 18 20 feet in horizontal dimensions. An operating table 20 is positioned substantially centrally of the room. The area adjoining the table is normally occupied by the surgical team, including one or more surgeons and one or more surgical assistants and nurses. This table and adjoining area are to be decontaminated by a steady laminar flow of cleansing air.

In accordance with principles of the present invention, a flow of cleansing air indicated by arrows 21 pointing to the left in FIG. 2 is provided from substantially the entire vertical and lateral extent of a pair of

filter module units

22, 24. The cleansing air stream flows outwardly from the modules providing an air flow that has the indicated straight waisted coke bottle configuration (a substantially horizontal flow pattern as further described below) for substantially the full vertical height of the room. The outwardly flowing air stream impinges upon the far end wall 10 and is retrodirected, returning to the first end wall 12 along

side walls

14 and 16 as indicated by arrows 23. The outwardly facing or outboard end walls 26, 28 of the

filter modules

22, 24 are positioned substantially at the corners of the room along the end wall 12 and near the

opposite side walls

14, 16. Each of these module end walls 26, 28 comprises a floor to ceiling return air intake for blowers mounted within the filter module units as will be described in detail below. Thus, air is forced from the filter module units through a bank of forward facing filters that is coextensive with the full extent of both of the modules and is withdrawn into the modules through the oppositely disposed outboard end walls thereof at respectively opposite corners of end wall 12.

In accordance with teachings of the present invention, a wall-less cleansing air stream is desired in order to avoid the use of confining side walls, partitions, or air stream curtains that seriously interfere with the use of the floor space in and about the operating table. However, such a wall-less stream of air raises several problems. For the large cross-sectional area of stream that is desired, high-powered blowers are required. Since only relatively small increases in air stream velocity are available with relatively large increases in power of the blower motors, desirable velocities may well be obtainable in a wall-less tunnel only at the expense of unacceptably large, noisy and heat generating motors. Further, provision of a wall-less tunnel in a recirculating system may well result in an unacceptable short cut or short circuited path between the air flowing out of the filter units and the air flowing back directly into the filter intake.

These problems are substantially eliminated by two features of this invention. The first of these features, more particularly described below, is the balancing of outflowing and returning air streams. The second of these features is control of the air stream configuration.

The cleansing air stream is provided with the desired constricted or inwardly tapering flow configuration (the coke bottle shape illustrated in FIG. 2) by means of a pair of deflecting

vanes

27, 29 that extend for substantially the full height of the

filter module units

22,, 24, respectively. Each module has a single deflecting vane movably mounted thereto at its outboard front corner and extending for but a short distance and at an acute angle across the filter face. The deflecting vanes, in effect, extend substantially or nearly transverse to the direction of flow of the main air stream and thus provide, at the outer edges of the main air stream, inwardly directed flow components that combine with the main body of the outflowing air to provide the illustrated air flow pattern. As seen in FIG.

2, the air stream is considerably narrowed or constricted at and about the portion of its path that extends along the length of the operating table whereby this portion of the air stream is of increased velocity. Accordingly, the area of the table, which is of primary concern in the functioning of the present invention, receives a concentrated higher velocity portion of the air flow. Operation of the deflecting vanes contributes to the air flow balancing described below.

In addition to the improved flow pattern provided by

vanes

27, 29, it will be seen that the inwardly directed air flow components at the outer sides of the outflowing air stream enhance the isolation of the intake provided in outboard end panels 26, 28. In other words, instead of flowing directly out of the

filter modules

22, 24, into the intake of the filter modules without traversing any significant portion of the room, the air stream now is forced to flow in the indicated pattern.

Still another advantage of the deflecting

vanes

27, 29

resides in the diminished size of the opening through which the cleansing air flows from the

filter modules

22, 24. Thus velocity of the outflowing air is increased by virtue of the diminished opening without a required increase in blower power.

Each deflecting

vane

27, 29, is mounted to .the modules for limited pivotal motion about a substantially vertically extendingaxis as will be ore particularly described below. In a preferred arrangement, the vanes extend at an acute angle across the face of the filter modules. For example, where a single module has a width of 62 inches in a direction extending between

side walls

14 and 16, each vane may be about 14 inches in width and positioned so that its free end is spaced at about 4% inches from the face of the filter units. The deflecting vanes extend but a very short distance into the room andthus enable the provision of a balanced, controlled and shaped cleansing air stream without interfering walls, partitions, curtains, or the like.

The drawing illustrates a more or less idealized flow pattern, which will be somewhat disturbed by the various objects in the pattern path such as the table, operating personnel and equipment. Nevertheless, the pattern will generally remain in the indicated configuration with a large cross-section of cleansing air concentrated to sweep both the table and personnel in a floor to ceiling pattern, and returning for withdrawal into the modules at the room corners.

It will be recalled that the above described system provides a circulation of air in a closed or substantially closed room and, further provides an outflowing cleansing air stream over the work table with optimum pattern and efficiency, but without the use of extensive partitions or guide walls (other than the deflecting vanes). A significant feature of the method and apparatus of producing the desired circulation pattern.

without side walls or partitions comprises the balancing of outflowing and returning air streams. To achieve such balance and to achieve optimum flow pattern, the area of the outflowing cleansing air stream must not be significantly different than the area of the two returning air streams. The significance of this balancing of out flowing air against return air is due to an effect that may be termed a venturi effect, or an aspiration effect. Consider a lateral edge of the outflowing air stream and an adjoining lateral edge of one of the return air streams, that is, the boundary area between the two oppositely directed air streams. Such a boundary, of course, is not a sharp line. Rather, the boundary extends over a considerable area. Consider the crosssecton of such a boundary, as taken in a plane normal to flow direction. As the central portion of such a theoretical boundary is approached, for example, from the-center of the outflowing air stream, flow velocity decreases until it reaches zero. Continuing further into the returning air stream, the velocity is in the opposite direction and the speed in such direction increases as distance into the returned stream increases. If the 'two oppositely directed streams areflowing steadily but one is moving in considerably greater flow volume and velocity than the other at or closely adjacent the boundary, the faster moving stream will tend to draw into it air from the other stream. Accordingly, if the outflowing air stream is of considerably greater flow than the adjoining return air streams, the patterns of the latter are disrupted, and quantities of the return air (which have previously passed over the work table and are thus now contaminated) would be mixed with the outflowing cleansing air stream. The intake to the blower units would tend to be drawn from above, behind,,or some other diverse pattern rather than from a smooth freeflowing return flow along the sides of the room.

On the other hand, if the outflowing cleansing air stream flow is too low, or is significantly less than the flow of either adjoining return stream, clean air from the outflowing stream would be drawn into the return stream before such clean air had traversed all or a significant part of the work table. Thus the outflowing air stream would be considerably slowed as it flows across the room.

The'above described balanced air flow is achieved in the apparatus and the method described herein by specifically selecting a width of outflowing air stream that has a desired relation to the width of the room in which the work table is located. Preferably the ratio of width of outflowing air stream as measured at a point adjacent the end of the work table that is closer to the filter modules is between 0.4 and 0.7 of the width of the room.

In the light of this significant range of ratios of width of filter modules to room width, and in the light of the use of standard operating rooms of between 16 and 20 feet in width, each filter module is made 5 feet wide so that the total outflowing air stream width at or near the I closer end of the table is about 10 feet. With a 10 foot wide outflowing stream, each return stream is approximately 3 feet wide in a 16 foot wide room. If this 10 foot wide outflowing stream is employed in a narrower room, such as for example, a 14 foot wide room, it is found that the width of the return air stream is insufficient and the velocity of return air stream is too high. The outflowing air stream is diminished and the entire system slows up. Using a 10 foot wide outflowing air stream in a 14 foot wide room, the outflowing clean air, instead of projecting outward to the far wall of the room, slows considerably just past the end of the work table and commences a major portion of its return flow there.

With such 10 foot wide outflowing air stream in an 18 or 20 foot wide room, no such slowing effect is noticeable. The outflowing air stream actually accelerates and retains most of its initial velocity until it reaches a point close to the opposite wall. This is so because in the 18 and 20 foot wide rooms, the return air streams on either side of outflowing air streams are each about feet wide or 6 feet wide, respectively. In these situations, it will be seen that the total cross-sectional area of return air flow is very nearly equal to the total cross-sectional area of the outflowing cleansing air flow, wherefore return and outflowing flows are substantially balanced.

Although some detrimental effect of increased return air velocity and flow exists to some extent with a foot wide pair of filter modules in a 16 foot wide room, the effect is so small as to be negligible for practical purposes.

In a wider room, such as a room of a 22 foot width, for example, each return air stream would be more than 6 feet wide with a 10 foot wide outflowing air stream. Although such an arrangement would cause some detrimental effect, such that some of the contaminated more slowly returning air would be drawn into the outflowing clean air stream, the magnitude of this effect would be of little practical effect.

It will be noted that the deflecting

vanes

27, 29, operate so as to decrease the width of the.outflowing air stream (as compared to the total width of the filter modules) at a point close to the forward face of the modules. Nevertheless, the outflowing stream has a width for the major portion of its travel over the work table that is significantly greater than the width of the operating table.

A significant effect of the deflecting vanes is the improvement of the flow pattern and the improvement of what may be termed the throw of the outflow stream. This throw may be defined as the effective distance from the filer face over which the outflowing stream retains its desired velocity. Thus, using the above described ten foot wide pair of filter modules in a fourteen by fourteen foot room, it is possible to maintain a higher velocity of the outflowing air stream by moving the deflector vanes closer to the face of the filters. In such an arrangement, however, even though the outflowing air stream velocity is increased, the overall rate of air circulation may be somewhat decreased because of the relative lack of balance of outflowing and return air streams. Even though the deflector vanes may be moved closer to the face of the filter in a narrow room so as to improve the balance of outflow and return air stream, the relatively narrow return streams are considerably less efficient and operate against increased restriction to the entire system flow. The total flow of air to the intake of the blowers is restricted to thereby limit the total air that can be handled by the system.

In sixteen by sixteen or eighteen by eighteen foot rooms, the 14 inch wide deflector vanes are preferably spaced with their free ends at about 4% inches from the outer edge of the vanes to the face of the filter modules. Optimum balance and optimum system operation are achieved with this adjustment. For a somewhat wider room, balancing of outflowing and return air stream rates may be achieved with the 10 foot wide filter modules by positioning the free end of the deflector vanes somewhat further from the face of the filter modules to thereby in effect widen the outflowing air stream and thus balance the width of the outflowing stream against the combined width of the two return streams.

Illustrated in FIGS. 7a, 7b, 7c, and 7d are velocity profiles plotted from measurements made in a 20 foot square room having a pair of 5 foot filter modules of the present invention positioned therein as illustrated in FIG. 2. The flow pattern illustrated by the arrows of FIG. 2 is based upon the velocity measurements shown in FIGS. 7a through 7d. For these measurements velocity indicating instruments were positioned at 1 foot intervals across the full width of the 20 foot room along four different lines spaced at distances of 1 foot, 3 feet, 6 feet, and 9 feet, respectively from the outward face of the filter unit. The instruments were located 4 feet above the room floor. In these profiles outflow is positive and return flow negative.

From analysis of these velocity profiles and the flow pattern plotted therefrom as shown in FIG. 2, it may be observed that there is in effect, a corridor extending along the operating table approximately 2 to 3 feet from he operating table on each side thereof. On one side of each corridor air is flowing outwardly, and on the other side of the corridor it is flowing in a return direction. The velocity profiles of FIGS. 7a through 7d reveal that the zero velocity boundary between outward and return air flow streams is located adjacent the lines indicated at 25 in FIG. 2. This boundary extends with very slight curvature along either side of the operating table from the filter unit out to at least nine feet from the filter unit.

The relatively straight-waisted coke bottle" shape is evident from the review of these velocity profiles.

Another feature, as set forth above, that is evident upon the study of the velocity profiles of FIGS. 7a through 711, is the acceleration that is imparted to the air over the operating table as it flows outwardly from the filter unit. Thus the profile of FIG. 7a shows a velocity at the center of the filter unit and therefore, at the center of the operating table, to be less than 100 feet per minute at a distance of 1 foot from the filter wall. At a distance of three feet from the filter wall, the velocity of the central portion of the outward flowing stream is above 200 feet per minute as shown in FIG. 7b and increases somewhat at

points

6 and 9 feet from the filter wall as shown in FIGS. and 7d.

This centrally accelerating air flow pattern is highly desirable because the surgeons and other persons of the operating group will generally stand fairly close together, adjacent the table. The higher flow rate has a greater ability to penetrate the group of people and thereby rinse particles shed by them.

The illustrated filter modules are readily portable and easily installed in and removed from the convenfilter modules are substantially identical to each other but are of opposite hand. Thus, filter module 22 has its return intake facing toward side wall 14, whereas filter module 24 has its return air intake toward side wall 16. Arrangements of internal structure and components (to be described below) are likewise substantially identical but of opposite hand. I

Illustrated in FIGS. 3, 4, 5, and 6 are details of construction of one of the filter modules, module 24. The other of the pair of modules, module 22, is constructed and arranged similarly except that it is of opposite hand with its outboard end wall on the right side, and other parts and construction arranged accordingly. The filter banksextend across substantially the entire front area of both modules, and being symmetrical, require no opposite hand arrangement.

Each filter module comprises a substantially rectangular cabinet having a rigid and'self-supporting skeletal structural frame covered with a suitable outer surfacing material such as an appropriately decorated or painted sheet steel of 16 gauge for example. Thus, the cabinet has a solid sheet steel top and bottom and similarly a solid sheet steel back wall 40 and inboard end wall 42. The bottom of the cabinet is spaced approximately 4 inches above the floor by means of legs 44 and the space between the frontlegs and the floor is closed by means of a a hinged skirt 46 (FIG. 1) that may be lifted to facilitate cleaning under the cabinet. The height of the cabinet is preferably such that it extends to a distance of several inches below the ceiling of the room so as to facilitate its portability. A detachable decorative trim strip 48 is secured to the top of the cabinet, extending into snug engagement with the room ceiling.

As best illustrated in the horizontal cross-section of FIG. 5, the interior of the cabinet is divided into three major portions. The first portion, at the front of the cabinet, extends for the full width and full height and comprises a plurality of horizontally extending struc- I tural members 88, 90, 92, between which are mounted the primary filters as will be described more particularly below. Horizontal structural members 88, 90, 92, are rigidly secured to a peripheral rectangular frame having upper and lower horizontal members 84, 86,

and vertical members 83, 85. Other angle iron stiffeners such as those shown at 87, 89, may be provided as deemed necessary or desirable. Theremaining portion of the cabinet interior is divided into two compartments: a pressurized plenum chamber and a blower chamber or equipment compartment. The plenum chamber 50 is of substantially L-shaped horizontal cross-section that is formed by an interior continuous L-shaped

compartment wall

52, 54. The chamber has a long narrow leg 56 formed between plenum wall 54 and the inwardly facing side of the filter bank. This leg of the chamber extends for the full width and height of the module to thereby provide a pressurized chamber that is coextensive with the filter bank.'The short leg of the plenum chamber is formed between the plenum wall 52 and the inboard cabinet wall 42 and is connected to receive air under pressure discharged by the module blowers.

.Rigidly mounted within the equipment compartment of the cabinet, as by attachment to

structural members

62, 64, is a pair of blower assemblies. The blower as semblies comprise centrifugal blowers 70,.72, respectively, mounted one above the other to the

structural members

62, 64. Blower is belt driven from a motor 76 and blower 72 is driven by a belt 78 from blower 70. The motor is mounted in an enclosed housing or exhaust duct 80 at the upper end of the equipment compartment. As illustrated in FIGS. 3 and 5, the blowers both discharge into the short inboard leg off the plenum chamber to effect pressurization of the entire plenum. Motor housing or exhaust duct 80 has an opening 81 communicating with the pressurized plenum chamber 56 and has its uppermost end' extending through the top of the cabinet for connection to an external exhaust system such as the airconditioning exhaust of the conventional hospital room. If deemed necessary or desirable, an adjustable closure or damper (not shown) may be provided for opening 81.

In this manner, where thehospital room airconditioning system has only marginal capacity, or will not have the additional capacity required to handle heat load added by the fan motor 76, this additional heat may be exhausted whereby-the portable aircleaning system may be installed without raising or adversely affecting room temperature. Opening 81 ensures flow of heated air from the vicinity of the motor to the room exhaust system.

The arrangement of the rigid

rectangular frame members

83, 84, 85, 86, and fixed intermediate horizontal members 88, 90, 92, permits each cabinet to receive four

separate filters

94, 96, 98, 100, that collectively extend across the entire front face of the filter peripheral rectangular gasket (FIG. 6) that extends completely around the rear edge of the frame. Each of the filters is conveniently inserted into the cabinet module from the front of the cabinet, individually. The lowermost filter rests on frame member 86 and each other filter rests upon the next lower filter,

separated therefrom by a

spacer

93, 97,99, interposed therebetween.

Upon insertion into the cabinet each filter is sealed by the engagement of its peripheral gasket 95, with the rectangular frames formed within the cabinet by the several

structural members

83, 84, 85, 86, 88, 90, 92, I

as best seen in the detail of FIG. 6.

A vertically extending retaining strip such as strip 101 (FIG. 6) is detachably secured at each side of the cabinet to hold the filters in place. Decorative trim channels such as channel 105 are removably fastened to the retaining strips as by a snap-on frictional fit.

For protection of the forward face of the filters and to enhance repair and assembly, each cabinet has secured to the forward face thereof a pair of flat sheet metal perforated screens as indicated at 102, 103, in FIG. 1. This arrangement provides a protection for the several filter units as separate elements having a smooth readily cleanable exterior surface that enhances the overall cleansing operation of the system. Horizontal retaining strips are fixed to the cabinet and extend thereacross to hold the horizontal edges of

screens

102, 103.

Details of vane mounting are shown in FIG. 6. The vane comprises a solid sheet 107 of transparent plastic, sheet metal, or the like, fixed to and carried by a group, such as three, hinge straps 108, 109, 1 10, each of which is pivoted to the cabinet for motion about a vertical axis as indicated at 111 in FIG. 6. An adjustable stop member such as bolt 1 12 is threaded in a short leg 113 of one or more of the angulated hinge straps to limit counterclockwise pivotal motion of the deflecting vane (as viewed in FIG. 6). Air forced out through the filter tends to urge the vane to pivot against its stop to thus hold it in proper position as it constricts the air flow and produces the inward flow components.

As is best seen in FIG. 5, the outboard end 28 of the filter module includes a solid panel portion extending from top to bottom and rearwardly for a portion of the module depth. A plurality of

prefilters

104, 106, are vertically aligned and extend collectively from top to bottom of the module at the rearward portion of the outboard end. The prefilters are detachably secured to the cabinet from the exterior thereof by suitable fastening means and trim, and are provided with a protective cover substantially similar to the perforated sheet metal screen employed with the

primary filters

94, 96, 98, 100. Since the area of the prefilters is considerably less than that of the primary filters and since all of the air discharged into the plenum chamber and through the primary filters is drawn into the cabinet and into the fans through the prefilters, the air flow through the latter is at a higher velocity. Partly for this reason and partly because of the purpose of the prefiltering function itself, these filters are of a coarser type than the primary filters and will entrap relatively larger particles, particularly lint from clothing and the like.

Described in a co-pending application for Respiratory Mask and Ducting, Ser. No. 47,414, filed June 18, 1970, now U.S. Pat. No. 3,625,207 Boyd F. Agnew, inventor, is a system for employing a vacuum aspiration with a surgical mask to minimize danger of infection from surgeons and other members of the surgical team. The disclosure of such co-pending application is fully incorporated herein by this reference. Such system employs a vacuum source to withdraw and remove from the room exhalations of the personnel therein. In accordance with principles of the present invention, the vacuum source for a system such as is described in the copending application conveniently may be mounted within one of the

modules

22, 24, and exhausted into the suction compartment. Alternatively, the vacuum exhaust tubes of the surgical mask aspirators may be connected with the low pressure or suction compartment of the

blowers

70, 72, whereby particles and bacteria contained in such exhalations are drawn into the blowers, thence forced into the plenum and trapped in the

primary filters

94, 96, 98, 100.

In an exemplary arrangement for use in an 18 foot square hospital room having a 9 foot 6 inch ceiling, each of the cabinets of the

filter modules

22, 24 is 9 foot 2 inches high having a 4 inch detachable top trim and having a bottom panel approximately 4 inches from the floor. Each cabinet is 3 feet'O inches deep and 5 feet 2 inches wide, whereby the entire system requires a floor space of only 3 feet deep by 10 feet 4 inches wide. The 14 inch full length deflecting vanes protrude little more than 4 inches further into the room. In this typical embodiment, the total blower fan output is 8,000 cubic feet per minute. Motor waste heat is readily exhausted from the room as previously indicated so that no additional heat load is imposed by the cleansing system of this invention.

With the indicated blower capacity, average air speed of clean air exiting from the front of the filter modules is as indicated in FIG. 7. For the indicated exemplary dimensions, about 140 complete changes of air within the operating room are provided during each hour. This, of course, is independent of the roughly six to 10 air changes per hour accomplished by the standard airconditioning system.

Normal surgeries contain something on the order of 400,000 particles per cubic foot, including both dust particles and bacteria. As indicated above, bacteria counts per cubic foot in conventional hospital surgeries, not employing a clean air system of the type described in this invention, may contain from 15 to 60 bacteria per cubic foot. In a hospital operating room employing a clean air system of the type described in this invention, dust count per cubic foot will vary from less than particles per cubic foot in an area adjacent the front of the filter units to a maximum 'of 1,500 to 2,000 particles per cubic foot in other parts of the room. Bacterial count is correspondingly reduced, varying from zero up to a maximum of about 5 per cubic foot.

This greatly improved air cleansing operation is achieved without modification of existing room structures, using a minimum of floor space, and with a minimum affect upon room temperature. Specially constructed walls, partitions, or movable curtains for confining the air throughout its flow are not required since the deflecting vanes guide the air in the balanced pattern of flow that is substantially the same everywhere from floor to ceiling. The air moves outwardly from the filter modules, is laterally constricted by inward flow components as it passes over the operating table and the circumjacent equipment and personnel, and returns along the side walls for withdrawal into the recirculating filter modules at the room corners, at flow rates substantially equal to outflow rates. Thus, the patient on the table is placed in an area of maximum protection against potential airborne infection.

The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.

lclaim:

l. A method of cleaning air in a work room wherein a work table is positioned adjacent to but spaced-from an end wall of the room and between two adjoining side walls of the room, said method comprising the steps of forcing an inwardly tapering outflowing cleansing stream of air from said end wall toward and along said work table and along both ides thereof through an area immediately adjoining the table and extending from a substantial distance laterally thereof, said stream of air having a cross-section extending from near the floor to near the ceiling of said work room, filtering said stream of air throughout its entire cross-section at a point between said work table and said end wall, and withdrawing air from said room in and via return streams respectively directed toward said end wall, and adjacent both said side walls.

2. The method of claim 1 including the step of introducing inwardly directed air flow components into said outflowing stream at outer sides thereof adjacent said end wall to thereby achieve said tapering.

3. The method of claim 2 including the step of prefiltering said return streams throughout the entire cross-section thereof, and

recirculating said returned air as said cleansing air stream that is filtered between said work table and said end wall.

4. The method of claim 2, including the step of balancing said outflowing cleansing air stream against the return streams adjacent said side walls, said step of balancing comprising controlling the width of said outflowing cleansing air stream and said return stream so that the ratio of the width of said outflowing air stream to the width of said workroom between said sidewalls is substantially within the range of 0.4 to 0.7

5. The method of claim 1 wherein step of forcing includes the step of inwardly baffling outer edges of said outflowing stream of air at a substantial angle to the flow direction of the major portion of said stream to obtain sad tapering.

6. The method of claim 5 wherein said baffling occurs over at'least a major portion of the vertical extent of said stream and across only a minor portion of the horizontal extent of said stream at the outer edges thereof.

7. The method of claim 1 including the step of adjusting the width of said outflowing cleansing stream of air in accordance with the width of said room so that the outward flow of said cleansing air stream is substantially equal to the flow of the return stream at said side walls.

8. The method of claim 7 wherein said step of adjusting comprises adjusting inwardly directed airflow components into said outflowing cleansing stream of air at outer side edges thereof adjacent said end wall.

9. The method of claim 7 wherein said step of adjusting includes selecting the width of said outflowing cleansing air stream to be in the range of from 0.4 to 0.7 ofthe width of said room.

10. A system for'cleaning air on and about a table in a room having side and end walls, floor and ceiling, said system comprising first and second air cleaning modules,

said modules being positioned end-to-end and each extending along one of said end walls toward a different side wall, each module comprising a cabinet extending vertically from the floor of said room substantially to the ceiling thereof and extending horizontally along said one end wall from a point substantially aligned with the center of said table to a point adjacent to but spaced from a side wall of said room,

a plenum chamber formed in the module extending for substantially the full height thereof,

a plurality of filters mounted in mutually sealed relation and collectively extending for the full height of the module and from an inboard end to an outboard end thereof, the inner side of said plurality of filters forming one wall of said plenum chamber,

a blower carried in the module and connected to force air into said plenum chamber, said blower having an intake in fluid communication with the outboard end of said module, whereby air is forced from said plenum through the filters of the module in a stream extending substantially from floor to ceiling of the room and extending horizontally from the center of said table toward either side of the room for a distance significantly greater than the extent of said table in a like direction, and whereby air is returned along a side wall of said room and drawn into said blower by means of an intake air stream flowing into said module outboard end from a side wall of the room substantially along said end wall.

11. The system of claim 10 wherein each module further comprises a deflecting vane carried by said cabinet at an outboard end thereof and extending at an acute angle for a relatively short distance across the outer face of said filters.

12. The system of claim 11 including prefilter means mounted in said module outboard end walls.

13. The system of claim 12 wherein said blowers include at least one motor mounted within said module and displaced from the blower, and wherein a motor heat exhaust duct is mounted within each module enclosing said motor and adapted to be connected to a flow of air that is being discharged from said room, whereby heat of said motor is vented externally of said room.

14. The system of claim 13 wherein said exhaust duct has a fluid communication with said plenum chamber whereby to assist flow of air from the vicinity of said motor into said flow of air that is being discharged from said room.

15. The system of claim 10 wherein the air forced from said plenum through the filters of the module is substantially balanced against the air returning along side walls of said room as intake air streams flowing into module outboard ends,

said balancing being achieved by a total width of said first and second air cleaning modules that is substantially equal to about one half the total width of said room between said side walls.

16. An air cleaning and recirculating system for a room having a pair of end walls, a pair of side walls, a ceiling, a floor, and a table positioned on the floor between side walls and adjacent one of said end walls, said system comprising a bank of air filters extending substantially from said floor to said ceiling between said table and said one end wall, said bank of filters being substantially aligned with the centerline of said table and extending laterally therefrom toward both said side walls for a distance between about 0.4 and about 0.6 of the width of said room,

a pair of deflecting vanes connected with said bank of filters and extending inwardly for a relatively short distance from outer edges thereof at an acute angle across the face of said bank of filters,

blower means mounted between said filter and said end wall for propelling a horizontally flowing stream of air through said bank of filters over the entire extent thereof, said stream of air through and from said bank of filters comprising a substantially laminar flow having a cross-section extending vertically from said floor to said ceiling and extending laterally across said table and on either side thereof a distance sufficient to envelop persons standing at or working in connection with said table, said blower means including means for withdrawing air from said room in return air streams extending substantially from said floor to said ceiling adjacent respectively opposite corners of the room formed by the junction of said side walls with said one end wall.

17. The system of claim 16 wherein said room is between 16 and feet wide and wherein said bank of filters is approximately 10 feet wide.

18. The system of claim 17 wherein said deflecting veins are approximately 14 inches wide and have a free edge positioned about 4% inches from the face of said bank of filters.

19. The apparatus of claim 16 including a pair of cabinets each extending from floor to ceiling of said room, both positioned in end-to-end abutment along said one end wall and each extending respectively toward the corresponding one of said side walls, each said cabinet comprising a substantially L-shaped plenum chamber having first and second legs extending for the full height thereof, said first leg of said plenum chamber comprising one wall formed by said bank of filters and extending for the full width and height of said cabinet, said second leg of said plenum chamber extending from the back of said cabinet at the outer end thereof to the inner surface of said filter bank,

a bank of prefilters mounted in the outer end of said cabinet, said blower means comprising a blower mounted within said cabinet to discharge air into said second leg of said plenum chamber and arranged to pull return air into said cabinet through said bank of prefilters, whereby return air is withdrawn from the room and prefiltered by suction at the outer end of the cabinet and the air is thereafter discharged into said plenum chamber so that air in said chamber under pressure from said blower is evenly forced through said filters, and is narrowed by inwardly directed flow components from said vanes to flow in a horizontally directed laterally tapered stream that extends transversely of the room and vertically from near the floor of said room to a distance at least above the heads of said personnel. 20. The apparatus of claim 19 wherein each said deflecting vane comprises a narrow elongated panel connected to the forward outboard end of an assocrated cabinet for pivotal motion about a substantially vertical axis, and adjustable means for limiting pivotal motion of said vanes as urged by air flowing from said bank of filters.

21. The apparatus of claim 19 wherein said bank of filters comprises a vertical array of filter units, each said units being individually and separately detachably secured to said cabinet from the front of said cabinet.

22. The apparatus of claim 19 including a flat perforated sheet of protective screen detachably connected to the front of each cabinet and removable therefrom.

23. The apparatus of claim 19 wherein said blower means comprises first and second substantially vertically aligned fans,

a motor having a driving connection to at least one of said fans, and a heat exhausting duct enclosing said motor,

said duct having an intake connected with said plenum chamber and being adapted for connection to a venting system that exhausts air from said hospital room. 24. The apparatus set forth in claim 19 including a bank of prefilters extending substantially from floor to ceiling of said room between said blower means and said respective side walls so as to prefilter all of said return air being withdrawn from said room by said blower means.

25. A method of cleaning air in a workroom having side and end walls and wherein a work table is positioned adjacent to, but spaced from one of the end walls, and midway between the side walls of the room, said method comprising the steps of forcing a cleansing stream of air from said end wall toward and along said work table along both sides thereof through an area immediately adjoining the table and extending for a substantial distance laterally thereof, filtering said outflowing cleansing stream of air throughout its entire cross-section at a point between said work table and said end wall,

withdrawing air from said room via return streams respectively directed toward said end wall and adjacent both said side walls, and

controlling the width of said outflowing cleansing stream of air so that outflowing and return air streams are substantially balanced and said outflowing cleansing air stream has a width, as measured in a direction substantially between said side walls, that is not less than 0.4 and not more than 0.7 of the distance between said side walls.

26. The method of claim 25 including the step of inwardly baffling outer edges of said outflowing stream of cleansing stream of air at a substantial angle to the flow direction of the major portion of said stream so as to increase velocity of said outflowing cleansing air stream.

Claims

2. The method of claim 1 including the step of introducing inwardly directed air flow components into said outflowing stream at outer sides thereof adjacent said end wall to thereby achieve said tapering.
3. The method of claim 2 including the step of prefiltering said return streams throughout the entire cross-section thereof, and recirculating said returned air as said cleansing air stream that is filtered between said work table and said end wall.
4. The method of claim 2, including the step of balancing said outflowing cleansing air stream against the return streams adjacent said side walls, said step of balancing comprising controlling the width of said outflowing cleansing air stream and said return stream so that the ratio of the width of said outflowing air stream to the width of said workroom between said sidewalls is substantially within the range of 0.4 to 0.7
5. The method of claim 1 wherein step of forcing includes the step of inwardly baffling outer edges of said outflowing stream of air at a substantial angle to the flow direction of the major portion of said stream to obtain sad tapering.
6. The method of claim 5 wherein said baffling occurs over at least a major portion of the vertical extent of said stream and across only a minor portion of the horizontal extent of said stream at the outer edges thereof.
7. The method of claim 1 including the step of adjusting the width of said outflowing cleansing stream of air in accordance with the width of said room so that the outward flow of said cleansing air stream is substantially equal to the flow of the return stream at said side walls.
8. The method of claim 7 wherein said step of adjusting comprises adjusting inwardly directed airflow components into said outflowing cleansing stream of air at outer side edges thereof adjacent said end wall.
9. The method of claim 7 wherein said step of adjusting includes selecting the width of said outflowing cleansing air stream to be in the range of from 0.4 to 0.7 of the width of said room.
10. A system for cleaning air on and about a table in a room having side and end walls, floor and ceiling, said system comprising first and second air cleaning modules, said modules being positioned end-to-end and each extending along one of said end walls toward a different side wall, each module comprising a cabinet extending vertically from the floor of said room substantially to the ceiling thereof and extending horizontally along said one end wall from a point substantially aligned with the center of said table to a point adjacent to but spaced from a side wall of said room, a plenum chamber formed in the module extending for substantially the full height thereof, a plurality of filters mounted in mutually sealed relation and collectively extending for the full height of the module and from an inboard end to an outboard end thereof, the inner side of said plurality of filters forming one wall of said plenum chamber, a blower carried in the module and connected to force air into said plenum chamber, said blower having an intake in fluid communication with the outboard end of said module, whereby air is forced from said plenum through the filters of the module in a stream extending substantially from floor to ceiling of the room and extending horizontally from the center of said table toward either side of the room for a distance significantly greater than the extent of said table in a like direction, and whereby air is returned along a side wall of said room and drawn into said blower by means of an intake air stream flowing into said module outboard end from a side wall of the room substantially along said end wall.
11. The system of claim 10 wherein each module further comprises a deflecting vane carried by said cabinet at an outboard end thereof and extending at an acute angle for a relatively short distance across the outer face of said filters.
12. The system of claim 11 including prefilter means mounted in said module outboard end walls.
13. The system of claim 12 wherein said blowers include at least one motor mounted within said module and displaced from the blower, and wherein a motor heat exhaust duct is mounted within each module enclosing said motor and adapted to be connected to a flow of air that is being discharged from said room, whereby heat of said motor is vented externally of said room.
14. The system of claim 13 wherein said exhaust duct has a fluid communication with said plenum chamber whereby to assist flow of air from the vicinity of said motor into said flow of air that is being discharged from said room.
15. The system of claim 10 wherein the air forced from said plenum through the filters of the module is substantially balanced against the air returning along side walls of said room as intake air streams flowing into module outboard ends, said balancing being achieved by a total width of said first and second air cleaning modules that is substantially equal to about one half the total width of said room between said side walls.
16. An air cleaning and recirculating system for a room having a pair of end walls, a pair of side walls, a ceiling, a floor, and a table positioned on the floor between side walls and adjacent one of said end walls, said system comprising a bank of air filters extending substantially from said floor to said ceiling between said table and said one end wall, said bank of filters being substantially aligned with the centerline of said table and extending laterally therefrom toward both said side walls for a distance between about 0.4 and about 0.6 of the width of said room, a pair of deflecting vanes connected with said bank of filters and extending inwardly for a relatively short distance from outer edges thereof at an acute angle across the face of said bank of filters, blower means mounted between said filter and said end wall for propelling a horizontally flowing stream of air through said bank of filters over the entire extent thereof, said stream of air through and from said bank of filters comprising a substantially laminar flow having a cross-section extending vertically from said floor to said ceiling and extending laterally across said table and on either side thereof a distance sufficient to envelop persons standing at or working in connection with said table, said blower means including means for withdrawing air from said room in return air streams extending substantially from said floor to said ceiling adjacent respectively opposite corners of the room formed by the junction of said side walls with said one end wall.
17. The system of claim 16 wherein said room is between 16 and 20 feet wide and wherein said bank of filters is approximately 10 feet wide.
18. The system of claim 17 wherein said deflecting veins are approximately 14 inches wide and have a free edge positioned about 4 1/4 inches from the face of said bank of filters.
19. The apparatus of claim 16 including a pair of cabinets each extending from floor to ceiling of said room, both positioned in end-to-end abutment along said one end wall and each extending respectively toward the corresponding one of said side walls, each said cabinet comprising a substantially L-shaped plenum chamber having first and second legs extending for the full height thereof, said first leg of said plenum chamber comprising one wall formed by said bank of filters and extending for the full width and height of said cabinet, said second leg of said plenum chamber extending from the back of said cabinet at the outer end thereof to the inner surface of said filter bank, a bank of prefilters mounted in the outer end oF said cabinet, said blower means comprising a blower mounted within said cabinet to discharge air into said second leg of said plenum chamber and arranged to pull return air into said cabinet through said bank of prefilters, whereby return air is withdrawn from the room and prefiltered by suction at the outer end of the cabinet and the air is thereafter discharged into said plenum chamber so that air in said chamber under pressure from said blower is evenly forced through said filters, and is narrowed by inwardly directed flow components from said vanes to flow in a horizontally directed laterally tapered stream that extends transversely of the room and vertically from near the floor of said room to a distance at least above the heads of said personnel.
20. The apparatus of claim 19 wherein each said deflecting vane comprises a narrow elongated panel connected to the forward outboard end of an associated cabinet for pivotal motion about a substantially vertical axis, and adjustable means for limiting pivotal motion of said vanes as urged by air flowing from said bank of filters.
21. The apparatus of claim 19 wherein said bank of filters comprises a vertical array of filter units, each said units being individually and separately detachably secured to said cabinet from the front of said cabinet.
22. The apparatus of claim 19 including a flat perforated sheet of protective screen detachably connected to the front of each cabinet and removable therefrom.
23. The apparatus of claim 19 wherein said blower means comprises first and second substantially vertically aligned fans, a motor having a driving connection to at least one of said fans, and a heat exhausting duct enclosing said motor, said duct having an intake connected with said plenum chamber and being adapted for connection to a venting system that exhausts air from said hospital room.
24. The apparatus set forth in claim 19 including a bank of prefilters extending substantially from floor to ceiling of said room between said blower means and said respective side walls so as to prefilter all of said return air being withdrawn from said room by said blower means.
25. A method of cleaning air in a workroom having side and end walls and wherein a work table is positioned adjacent to, but spaced from one of the end walls, and midway between the side walls of the room, said method comprising the steps of forcing a cleansing stream of air from said end wall toward and along said work table along both sides thereof through an area immediately adjoining the table and extending for a substantial distance laterally thereof, filtering said outflowing cleansing stream of air throughout its entire cross-section at a point between said work table and said end wall, withdrawing air from said room via return streams respectively directed toward said end wall and adjacent both said side walls, and controlling the width of said outflowing cleansing stream of air so that outflowing and return air streams are substantially balanced and said outflowing cleansing air stream has a width, as measured in a direction substantially between said side walls, that is not less than 0.4 and not more than 0.7 of the distance between said side walls.
26. The method of claim 25 including the step of inwardly baffling outer edges of said outflowing stream of cleansing stream of air at a substantial angle to the flow direction of the major portion of said stream so as to increase velocity of said outflowing cleansing air stream.