Abstract: An improved gas scrubber system is disclosed in which air from a paint spray booth is mixed with a liquid to remove paint particles and other contaminants in the air. A wall is positioned immediately downstream of an outlet of the gas scrubber and a jet of intermixed water, entrained contaminants and air impinges off of that wall, to separate the majority of water and contaminants. The gas scrubber member has a unique structure such that the air and water are directed through a first portion generally aligned with an inlet and into a second portion which extends at a 90.degree. angle with respect to the first portion. A restriction is formed on the first portion to provide a noise barrier. The gas scrubber member is formed by planar walls to ensure that the water continues to adhere to them under surface tension, and in essentially laminar flow, while in the first portion. A top wall is spaced from a base wall to define the second portion.

Abstract: A method for detackifying oversprayed paint solids in a paint spray booth utilizes an oil-in-water emulsion as the hydrophilic fluid for capturing and detackifying oversprayed paint, which emulsion contains a certain nonionic surfactant, which surfactant greatly improves detackification performance when the paint formulation being sprayed is a low VOC formulation, and particularly a two-component formulation. The nonionic surfactant is an ethoxylated C.sub.8-10 alkyl phenol having an HLB value of from about 4.5 to 15.5.

Abstract: An improvement for a production line, downdraft type paint spray booth having a conveyor for carrying the parts to be painted, having a recessed portion for containing the conveyor and dual slots, on one or either side of the conveyor for air flow is disclosed.The improvement provides the advantage of improved air flow around the parts being painted and around the conveyor to keep it clean. Additionally, the provision of the dual slots permits the conveyor to be at least partially recessed into the booths floor to provide a lower profile and a more compact arrangement.

Abstract: The invention relates to a cabin for carrying out treatments in it, comprising four walls which are arranged in pairs in spaced substantially parallel relation, the two pairs of walls being mutually substantially perpendicular and at least one or a part of one of the walls being constructed as a door for access to the cabin, and a roof connecting at its lower side as a ceiling onto the four walls and in which the roof comprises means for displacing streams of air in a substantially vertical direction from the ceiling through the cabin and at the bottom of the cabin means are provided for discharging air supplied from the ceiling, the roof comprising a more or less box-shaped construction, at least a part of a large area at the bottom of this construction forming the ceiling and said means comprising fans, valves, filters and air ducts mounted in the box-shaped construction of the roof, while a large portion of the ceiling is in the shape of a grid through which an air stream can be passed in a substantially d

Abstract: A cabinet (1) for electrostatic painting which includes a framework (2) supporting flat walls (3) whose surfaces (5) bound a volume inside of which at least pneumatic distributor (6) is installed to distribute powder onto a workpiece (7) which is to be painted thereby. The powder which is not applied to the workpiece settles on the interior surfaces of the walls and is removed therefrom by a combination of air blown from a blowing mouth (20) against the inner surface (5) of each wall in opposition to an air stream generated by a suction mouth (19). During the opposition of the suction and blowing mouths (19, 20, respectively), each wall is moved past the mouths whereby each wall is thereby progressively cleaned of the powder.

Abstract: A spray cabin for powder spray having a spray opening on one side and an let opening on an opposite side connected to a powder separator applying suction to the outlet opening has at least a baffle directing the outflow upwardly along the wall provided with the outlet opening to entrain particles which have deposited on the floor upwardly and provide a quiescent or calming chamber immediately upstream of the outlet opening.

Abstract: Procedure for the denaturation of paints collected in painting booths and comprising a cleansing apparatus in which the air laden with paint droplets is placed in contact with a fluid to which is added a powder selected from phenolic or polyisocyanate powders.Applications to the denaturation of paints in painting booth installations.

Abstract: A painting structure for use as, by way of example, a paint spray booth or paint baking or drying oven, utilizes a trapezoidal cross-section, instead of the conventional rectangular/square or rectangular-gabled cross-section. The painting structure of the present invention provides inherent rigidity without the need for special framing, and provides more uniform air flow than a gabled roof structure, as air swirls in air pockets beneath the gables are eliminated. Due to the simplified construction, the trapezoidal cross-sectional booth of the present invention is less expensive to make and install and more easily erected.

Abstract: A paint spray booth for painting articles is disclosed having a liquid wash scrubber beneath an upper chamber for removing the overspray paint particles from the air by intermixing the paint latent air with a liquid such as water. The scrubber forms a substantially horizontal water and air flow path to reduce the vertical height of the scrubber and therefore reduce the vertical height of the overall spray booth. The scrubber forms a trough extending longitudinally through the booth having a liquid flow exit from the bottom of the trough laterally from the bottom of the trough. Downstream from the trough, a baffle blocks the flow of the liquid forming a liquid trap to create turbulence in the liquid flowing through the scrubber to intermix the liquid with the paint latent air.

Abstract: The exhaust air from a paint-spray or lacquer spray booth is cleaned after passing through the grate bottom thereof directly to a wet electrostatic filter. The air which is sprayed with atomized water immediately before entering the wet electrostatic filter is partly recirculated after passing through the latter while the water containing the paint and lacquer particles is cleaned and recycled as well.

Abstract: A structure and a filter suitable for a paint spray booth, oven, or a combination booth and oven is described and includes outer walls enclosing the structure, only a portion of which provides the structural support for the booth's walls and roof. The interior of the booth has a plurality of filters on the walls and ceilings for supplying air from essentially the walls and ceiling to provide a more uniform air flow so that the painter is less exposed to vaporizing solvents, and when used as an oven, the heating is more uniform. Each of the filters made of a cloth material held in place by filter frames and may be used with either battens fixed to the frames or removable and held in place in pockets provided in the filter cloth.

Abstract: A wet scrubber is provided for use in combination with a work station in which airborne particulates are generated. The wet scrubber includes a generally horizontal partition which defines a lower boundary of the work station and a discharge structure depending from the partition. A flow of liquid is supplied into the discharge structure, and a flow of air is supplied which will carry the particulate from the work station into and through the discharge structure. A scrubber chamber is also provided into which the discharge structure projects. This chamber includes a receptacle for containing a pool of liquid. The discharge structure itself is constructed to include an elongated inlet channel and a plurality of discrete discharge tubes extending down into the scrubber chamber. The inlet channel has a top, a bottom and a pair of converging sidwalls which depend from the partition at the top of the channel.

Abstract: A method and apparatus for the removal of overspray in coating booths. The systems include a circulating solvent which dissolves the overspray carrier and transports the non-soluble components of the overspray. The systems also include a method and apparatus to remove undesirable elements from the circulation working solvent and to recapture the reusable circulation working solvent to replenish the system.

Abstract: A spray booth is provided with inexpensive, disposable pre-baffles which collect overspray thereon prior to the overspray reaching the filtering stages. The preferred disposable pre-baffles are made of paperboard and after being coated with overspray for a short period, e.g., one week, are easily detached and replaced. The paint on the disposable pre-baffles can be baked to convert the paint and the baffle from a hazardous to a non-hazardous waste. The pre-baffles may collect as much as 80% of the overspray, some of which may be collected and re-used, and reduces the amount of sludge, filter cleaning, chemicals used in the reservoir water, and the emissions. Thus, the overall savings easily overcome the expense of replacing the disposable pre-baffles.

Abstract: A centrifugal separator type paint mist containing gas cleaning apparatus having a concave guide face extending downwardly from a downstream side end of a cleaning liquid flow-down face in a flowing down direction of cleaning liquid, the concave guide face being formed as a curved arc, a discharge guide face disposed at a downstream side end of the concave guide face for laterally discharging the cleaning liquid, a gas orienting face for orienting a paint mist containing gas fed from the upperside towards the concave guide face, whereby the paint mist containing gas is discharge through a discharge opening formed between a downstream side end of the gas orienting face and a portion of the concave guide face most adjacent the downstream side end of the gas orienting face, the discharge opening having a width smaller than an average radius of the concave guide face, the paint mist containing gas being discharge in a direction substantially along a tangential line of the concave guide face, the discharge opening

Abstract: A painting machine unit including a painting booth having a side wall with a substantially vertical wall portion insertable in an opening in the booth side wall and having a viewing window therethrough. The unit also includes a painting machine attached to an inner side of the wall portion and has a device for automatic painting of objects within the booth, and a control unit for controlling the device for automatic painting of objects within the booth. The control unit attaches to an outer side of the wall portion and is surrounded by a control chamber housing for accommodating painting booth operator.

Abstract: Sheet material is fed from a sheet supply station to a sheet receiving station and is guided by guiding means which define the floor as well as the side walls of a spray chamber. The sheet material can be replaced when soiled by advancing a new length from the supply station to the receiving station.

Abstract: A moveable enclosure for use in applying a sprayed material on the surface over which the enclosure is moved includes a frame assembly defining a plurality of side walls with rollers for movably supporting the frame. A mesh skirt is supported by the frame adjacent the plurality of side walls and is positioned with its lower edge adjacent the surface. A mesh top canopy is positioned on the top of the frame with its lower edge overlying the upper portion of the skirt to complete the enclosure.

Abstract: A painting installation with several treatment zones and a system for washing-out paint mist by means of circulated water, in which for each treatment zone or group of treatment zones with similar solvent concentration in the exhaust air, a closed-cycle water circulation of its own is provided. The gasing out of solvents out of the water in zones with low solvent concentration is avoided thereby, and the solvent emission of the painting installation is markedly reduced.

Abstract: The filter is formed by a wall (6) separating a vacuum chamber (3a, 3b) from a space (1) where a paint spray gun (2) is located. The lower part (5) of the wall (6) dips into the water of a collecting tank (7). The wall (6) is pierced by a multiplicity of holes, in each of which is fastened a tube (8) projecting on its two faces. On the same side as the vacuum chamber (3a, 3b), each tube is equipped with a deflector (10) at its outflow. Water from a reservoir (11) flows off via an overflow (12) and streams (15) over the entire upstream face of the wall (6). This water rebounds on the projecting parts of the tubes (8), and the shape and size of the pent roofs (9) guarantee that the tubes (8) suck up the best possible mixture of air and water. This assists the capture of paint droplets by the water. The deflectors, by compelling the mixture of air and water laden with paint droplets or other impurities to change direction abruptly, assist the capture of these droplets by the water.

Abstract: A paint spray booth with a filter and means within the spray booth to incinerate paint from the filter. A filter particularly adapted for collecting paint overspray in a paint spray booth. The filter has a plurality of members with a first portion parallel to the path of the air and paint particle flow which is to be filtered, each of the members having a second portion extending normally in a first direction from the downstream side of the first portion. The second portions of the members are spaced from each other and form openings therebetween on a plane normal to the flow of air and paint particles. Each of the members has a third portion extending from the other side of first portion and in the opposite direction from the second portion, and the third portions of the members form slots between members with areas less than the area of the downstream openings.

Abstract: A method is described for spraying toxic chemicals onto agricultural plants that produces only a small amount of contained drift and then removes this drift from the air polluted.The environment is not badly damaged, less toxic chemical is used, the probability that a user will injure anyone with toxic spray drift is reduced and spraying work is made more tolerable.

Abstract: A generally rectangularly shaped powder coating booth of a powder spraying apparatus is fomed of two main sections, each of which is constituted of wall and roof elements which are swingably interconnected to one another. The walls and roof of each half of the booth may be straightened out to form a flat, continuous and rectangularly shaped surface from which residues of powder may be removed easily and rapidly. Thus when the two halves of the booth are flattened out the booth is transformed into two parallel and spaced surfaces which can be cleaned by being vacuumed by a respective suction nozzle that is disposed to travel over and vacuum the surfaces automatically.

Abstract: An apparatus for removing spray mist from a spraying booth. The apparatus comprises an exhauster for forcibly discharging a mixture of air and a mist of superfluous paint from the spray booth, and a plurality of constricted flow passages arranged in an exhaust route extending from the spraying booth to the exhaust for causing the air/mist mixture to strike against purifying water. This mist removing mechanism utilizing a difference in inertial mass between the air and the spray mist at times of passing through the constricted flow passages, whereby the spray mist is trapped in the purifying water upon collision therebetween.

Abstract: An electrostatic dry powder coating spraying system has a multi-color powder coating recovery system comprising a plurality of vertically extending cyclonic filter barrels laterally spaced about a central vertical axis and having vertically aligned upper ends. A single filter cleaning apparatus, including a filter backflushing system, is adapted to rest upon and operatively engage any selected one of such upper barrel ends, and is operative to backflush the filter within the barrel upon which it rests. Oversprayed powder from the spraying system is reclaimed by drawing it through the "active" filter barrel to which the filter cleaning apparatus is connected.

Abstract: A paint waste separating and collecting apparatus comprising a separator tank for receiving cleaning water laden with paint waste and discharged from a cleaning device, and a circulating passage for supplying cleaning water stripped of the paint waste from the separator tank to the cleaning device.

Abstract: A paint waste separator-collector apparatus having a separator tank for receiving water containing paint waste and for causing the contained paint waste to separate and float, a weir for causing the floating paint waste together with overflowing water from the separator tank into a collector container and a water level adjusting mechanism for switchably providing a paint waste accumulating condition in which a water level in the separator tank is lowered for the water not to flow over the weir thereby causing the floating paint waste to accumulate inside the separator tank and a paint waste collecting condition in which the water level in the separator tank is raised for the excess water to flow over the weir into the collector container.

Abstract: A moveable enclosure for use in applying a sprayed material on the surface over which the enclosure is moved includes a frame assembly defining a plurality of side walls with roller means for movably supporting the frame. A mesh skirt is supported by the frame adjacent the plurality of side walls and is positioned with its lower edge adjacent the surface. A mesh top canopy is positioned on the top of the frame with its lower edge overlying the upper portion of the skirt to complete the enclosure.

Abstract: A booth for applying a powdered coating to the surface of workpieces in which a clean gas zone (47) is joined to the coating zone (18) via the rear wall (14), which is connected to an extractor fan. Located in a recess (21) of the rear wall (14) is a frame-like structure (22), the outer frame (17) of which is split up into individual areas (24) by means of frame cross-members (23), each area receiving a filter element (25), which is housed in an interchangeable cassette, and consisting of a continuous zig-zig shaped or wave-shaped surface-membrane filter strip. Each area of the frame is covered in a leakproof manner by a protective hood (69) in whose hood space (25) in an oblique manner towards the coating zone (18) and detach the excess powder attached to the membrane applied thereon, which is diverted through a slat-type screen (55) down to the floor of the booth (67) and into a powder container (80).

Abstract: A painting apparatus having a booth with a painting room for defining a path having a U-shaped configuration wherein a front painting zone and a rear painting zone are adjacent each other in the legs of the path. An intermediate zone is located at the bight of the U-shaped path. An air supply and exhaust system is provided for the zones. A duct to the front and rear zones does not interfere with a duct to the intermediate zone. An exhaust system re-circulates the air in the intermediate zone and includes a solvent adsorber and a total heat exchanger upstream in the direction of air flow for adsorbing solvent and recovering sensible and latent heat.

Abstract: A process is provided for denaturing paints which are collected in paint rooms of the type comprising a scrubber in which air charged with droplets of paint is brought into contact with a scrubbing liquid. An additive in the form of an expanded polyurethane powder is included in the scrubbing liquid. The apparatus includes an arrangement for injecting the expanded polyurethane powder into the scrubbing liquid at the top of the scrubber.

Abstract: A device is provided for scrubbing a polluted gas in which this gas passes through at least one sheet of a scrubbing liquid. The device comprises trickle walls (7) which together define a first opening (8), a first transverse wall (12) disposed beneath the first opening, and a trickle passage with a vertical axis, surrounding the first transverse wall, and limited by two confining walls (10) attached to the underside of the trickle walls on either side of the first opening. Inside the trickle passage, under the first transverse wall (12), further walls (15) are arranged defining a second opening (16), with a second transverse wall (17) being disposed beneath the second opening.

Abstract: A method of operating an air-supplied type coating booth adapted to enforce conditioned air by an air supply blower through a filter downwardly to the inside of a tunnel-shaped coating booth at a predetermined flow velocity and then drawn to discharge together with coating mists, etc. to the beneath of the floor comprises the step of detecting the flow velocity of conditioned air flowing through the filter to the inside of continuously, applying primary and secondary averaging processings for the detected signals containing variations fluctuating at relatively shorter and longer periods respectively, comparing the value obtained from the signals applied with the secondary averaging processing with a predetermined flow velocity value and variably controlling the flow rate of the air supplied from the air supply blower depending on the deviation and thereby maintaining the flow velocity of the conditioned air flowing to the inside of the coating booth always constant.

Abstract: A method and apparatus for treating wastewater from a cleaning tank or the like is provided. A settling tank provides for separating the fluid and sludge, the sludge being passed to a filtration system having a plurality of filter trays with the filtrate being recycled to the settling tank. The decontaminated water from the settling tank is recycled to the cleaning tank.

Abstract: A paint spray booth in which articles such as automobiles are sprayed while moving along a conveyor line passing through the working area of the booth. Air is moved from an area above the articles and downwardly around the articles for discharge through a plurality of longitudinally spaced vertical passages positioned along the center line of the booth and water supplied to a pan positioned beneath the articles is caused to cascade down the vertical passages for mixture with the spray-laden air so that the spray is transferred from the air to the water. As the air leaves the lower ends of the vertical passages, it is directed laterally to impact one side of the booth, thence abruptly upwardly to separate entrained water, and thence in the other lateral direction toward the other side of the booth for passage through the spaces defined between the vertical passages.

Abstract: A spray booth includes an air cleaning apparatus providing an air intake beneath its floor, and provides air curtains along the interior surfaces of its side walls. The preferred air cleaning apparatus provides a first slot and water curtain through which air flows with the air velocity increased in the slot and then slowed down in a larger volume separation chamber. The direction of air and water flow are reversed several additional times with the air flowing through several additional water curtains. The water and air are discharged into a large precipitation chamber in which the water is collected and the air velocity is reduced very substantially before discharge through an air exhaust duct.

Abstract: A fluid wash spray paint system which includes a plurality of paint spray booths is provided. Each paint spray booth includes a fluid wash device to entrap paint overspray in a moving fluid to remove the overspray from the booth. The system also includes a sludge pit for collecting the fluid and entrapped paint overspray from the plurality of spray booths. An automatic feeder is provided to introduce a deflocculant powder into the fluid and entrapped paint overspray to cause the paint to settle as sludge to the bottom of the sludge pit. A centrifugal separator is provided to separate and remove the settled sludge from the fluid and to return the fluid to the sludge pit. The system further includes a pump to circulate the fluid from the sludge pit through the plurality of fluid wash spray devices to form a closed system.

Abstract: In accordance with the various aspects of the invention, a spray booth media support module (1) includes a rectangular frame (5) which is of generally channel section. The frame forms opposing spaced end members (7) and connecting side members (8). One frame end has an inwardly bent lip (11) having spaced notches (13) therein. The opposed frame side members have slots (14). A generally rectangular grid (6) is adapted to be removably mounted to the frame. The grid includes spaced elements (15, 16) arrayed generally parallel to the frame end and side members. A plurality of wide prongs (17) extend outwardly from the grid for impaling an overspray-catching pad (4) thereon. At least several prongs are aligned in a vertical tier arrangement.

Abstract: An apparatus and method is provided for regulating the climate in a spray booth. The spray booth comprises a chamber for housing articles during spray application of a coating material to the articles and an air circulation system for supplying either ambient air or recirculation air to the chamber. The chamber has an outlet for exhausting a contaminated mixture of chamber air and coating material overspray from the chamber. The spray booth further includes a liquid filter for washing the contaminated mixture exhausted through the chamber outlet to wash the coating material overspray out of the mixture. The air circulation system of the improved spray booth includes a system for controlling the temperature of the air supplied to the chamber to regulate the relative humidity of the air in the chamber.

Abstract: A refinishing unit comprises an air handling module and a spray booth, both of which may be used separately. An air handling module suitable for use with a spray booth comprises a housing; fresh air inlet means to said housing; first fan means in said housing receiving fresh air from said fresh air inlet means; a heat generator in said housing; a heat exchanger in said housing, associated with said heat generator and through which air drawn in by said first fan means is passed; an outlet for heated air, heated in said heat exchanger, from said housing; an exhaust gas intake into said housing; second fan means in said housing and connected to said exhaust gas intake and outlet means exhausting gas, drawn in through said exhaust gas intake by said second fan means, to atmosphere. The spray booth comprises a roof structure unit and a floor structure unit, the roof structure unit being supported on the floor structure unit at suitable places by means of support columns.

Abstract: A method and apparatus are disclosed for removing paint overspray suspended in air exhausted from a paint spray booth, in which the exhausted air is drawn through a conveyor-carried bed of small diameter beads or granules of plastic or other material allowing only a mild adherence of the paint to their surfaces upon drying. The paint overspray is removed from the air by being deposited on the surfaces of the beads as the air passes through the beads and paint is subsequently rapidly dried on the bead surfaces in a drying chamber. The paint carrying beads are then mechanically agitated to dislodge the dried paint to clean the beads for reuse, and the dislodged paint particles are separated from the cleaned beads for collection and disposal.

Abstract: An inexpensive, disposable liner is shaped and dimensioned to conform to the hole spacings of a grid-like grating used as the floor of a paint spray booth. The liner shields the grating surfaces from airborne or spilled paint, while having holes to permit the necessary airflow through the grating. When the paint build-up on the liner significantly occludes the holes, the liner is removed and disposed of, and replaced by a clean one.

Abstract: A spraying booth comprising a spraying area defined by a ceiling and side walls. The ceiling includes top feed openings connected with a feed duct extending from a conditioner for controlling a fresh air temperature. The side walls include side feed openings connected with a blast duct for supplying air from which paint mist has been removed at mist removing devices. Fresh air supplied through the top feed openings and the mistless air supplied through the side feed openings combine to produce an advantageous atmospheric condition for a paint spraying operation.

Abstract: A self-contained, compact and portable powder spray booth and powder recovery system includes a base unit, and a separate booth removably mounted upon the base unit which can be easily customized to accommodate articles of varying shape and size without changing the design of the base unit. When assembled, the booth and base unit form a powder coating chamber in which spray guns apply powder to moving articles. The articles enter and exit the powder coating chamber through openings which are shaped to accommodate the particular configuration of the articles to be coated. The systems for feeding powder to the spray guns, collecting oversprayed powder and filtering oversprayed powder are all contained within the base unit so that no exterior lines are required in the operation of the system except for a power line and an air line.

Abstract: The invention relates to coagulating compositions for coagulating lacquers, waxes and coating compositions, more especially two-component polyurethane lacquers, which contain a mixture of cyanamide and/or dicyanodiamide and/or calcium cyanamide in an amount of from 1% to 80% by weight in conjunction with substances normally present in coagulating compositions.

Abstract: A painting booth in which an appropriate temperature and humidity control is effectively performed for that zone in the painting chamber in which the object to be painted is conveyed, for enabling a drastic reduction in the amount of energy required for such temperature and humidity control, as compared with a conventional painting booth within which such control is performed for the whole painting chamber.

Abstract: The invention concerns a process to recover paint from overspray whereby one collects paint which misses the target object by means of a collector device which is flushed on all side and floor surfaces with circulating water, concentrates the overspray extracted in the overspray-circulating water mixture from the collector device to an overspray content of approx 20%, then feeds the mixture to a filtration chamber in which the water is separated from the reclaimed raw paint, then measures the physical characteristics of the reclaimed paint, compares these with the physical characteristics of the fresh paint, then adjusts the characteristics of the reclaimed paint to the characteristics of the fresh paint material and mixes the reclaimed paint into the fresh paint. The invention also concerns as device to carry out the process.

Abstract: A wet pump-less spray painting booth is provided that more efficiently removes overspray paint particles from the air and uses substantially less make-up water than prior designs of wet pump-less booths. In a rear air cleaning section of the booth, blower-generated air flow sucks water from a lower reservoir up through ducts into an upper plenum where the air and water impact against a splash plate, creating a turbulent mix of air and water. Water from the upper plenum drains through tubes to an upper reservoir and overflows the upper reservoir along a sheet, the bottom edge of which is spaced a substantial distance from the lower reservoir, creating a water curtain.

Abstract: A spray booth has an intermediate grating type floor (20) through which air is circulated. The air passes through a cleaning liquid nozzle (A) and entrains and atomises cleaning liquid derived from an overflow trough (30) with weirs (46,48). A spent air channel (24) is enclosed by a cover (28) located at least 20 cm from a flow off edge (28a) of the cover. The cover may itself form part of the washing-liquid feed device. A guide channel leads from weir (40) to the upper edge of a slot (34) forming the orifice of liquid nozzle (A). The cover (28) shields the working area of the booth from the noise generated by the nozzle (A).

Abstract: Apparatus for washing paint mist from exhaust air from a spray-painting operation draws the exhaust air, paint mist, and a washing liquid down a funnel inlet into an open ended cylindrical mixing chamber. In the mixing chamber, they turbulently spiral outwardly to exit said chamber through the open ends thereof. An elongated blocking structure is centrally disposed in said mixing chamber to block access of exhaust air to the central portion of the mixing chamber which has low spiral turbulence and thus low mixing efficiency. The inlet to the mixing chamber and the open end outlets therefrom preferably form venturies to further increase the mixing efficiency.