Abstract: A vent cover is configured for attachment to a downwardly-facing surface, such as a soffit panel of a building roof overhang. The vent cover includes a housing coupled to a base portion, the housing defining a fluid discharge outlet and fluid passageway, with the outlet being selectively blocked by at least one movable door. The door is hinged at its upper end portion and is biased to a closed position by gravity. A sufficiently high air pressure will raise the door and permit an outflow of air through the discharge outlet. Where separate inner and outer doors are provided, the outer door may be manually movable to its open position while the inner door remains closed, so that the inner door continues to restrict access to an interior of the housing.

Abstract: An electronic device includes an air dam module. The air dam module includes a frame defining a vent, a first rotating member, a second rotating member, and two third rotating members. The first rotating member is rotatably connected to a surface bounding the vent, to vertically block a lower half of the vent by its gravity. The second rotating member is rotatably connected to the surface bounding the vent. The third rotating members are rotatably connected to opposite ends of the second rotating member. The second and third rotating members vertically block an upper half of the vent by their gravities.

Abstract: An extraction fan has a housing mounted in a wall with an outlet duct extending through the wall to an outlet opening outside the wall. A damper flap is mounted on the fan suspended by a hinge at an upper edge so as to extend in a closed position across the opening and is movable to an open position under forward air flow from the fan, and tightly closes against a resilient deformable seal when the fan is shut off. A spring biasing cam applies inwards force to the damper flap to bias the damper flap into the closed position, and an outwards force also to bias the damper flap into the fully open position. The mounting plate includes a hood having a top wall extending over the top of the damper flap and a down-turned front flange extending parallel to the damper flap to a bottom edge.

Abstract: An extraction fan has a housing mounted in a wall with an outlet duct extending through the wall to an outlet opening outside the wall. A damper flap is mounted on the fan suspended by a hinge at an upper edge so as to extend in a closed position across the opening and is movable to an open position under forward air flow from the fan, and tightly closes against a resilient deformable seal when the fan is shut off. A spring biasing cam applies inwards force to the damper flap to bias the damper flap into the closed position, and an outwards force also to bias the damper flap into the fully open position. The mounting plate includes a hood having a top wall extending over the top of the damper flap and a down-turned front flange extending parallel to the damper flap to a bottom edge.

Abstract: An extraction fan has a housing mounted in a wall with an outlet duct extending through the wall to an outlet opening outside the wall. A damper flap is mounted on the fan suspended by a hinge at an upper edge so as to extend in a closed position across the opening and is movable to an open position under forward air flow from the fan, and tightly closes against a resilient deformable seal when the fan is shut off. A spring biasing cam applies inwards force to the damper flap to bias the damper flap into the closed position, and an outwards force also to bias the damper flap into the fully open position. The mounting plate includes a hood having a top wall extending over the top of the damper flap and a down-turned front flange extending parallel to the damper flap to a bottom edge.

Abstract: A damper assembly for use with an axial fan to prevent air from entering the discharge side of the fan when the fan is at idle. The damper assembly includes a damper door having multiple sections. The sections are able to pivot independently in response to the air flow created by the fan to minimize airflow resistance when the fan is operating.

Abstract: An animal barn includes air inlets which are filtered to extract pathogens and a plurality of extraction fans for generating an air stream so as to draw replacement air into the containment area through the air inlets. Each fan has a housing mounted in a wall with an outlet duct extending through the wall to an outlet opening outside the wall. A damper flap is mounted on the fan suspended by a hinge at an upper edge so as to extend in a closed position across the opening in a mounting plate. The damper flap is movable to an open position under forward air flow from the fan and closes against a resilient deformable seal to seal the damper flap to prevent reverse flow of pathogen contaminated air through the fan housing when the fan is shut off. An adjustable abutment member can act to hold the flap partly open against opposing wind forces during low fan operation.

Abstract: An extraction fan has a housing mounted in a wall with an outlet duct extending through the wall to an outlet opening outside the wall. A damper flap is mounted on the fan suspended by a hinge at an upper edge so as to extend in a closed position across the opening and is movable to an open position under forward air flow from the fan, and tightly closes against a resilient deformable seal when the fan is shut off. A spring biasing cam applies inwards force to the damper flap to bias the damper flap into the closed position, and an outwards force also to bias the damper flap into the fully open position. The mounting plate includes a hood having a top wall extending over the top of the damper flap and a down-turned front flange extending parallel to the damper flap to a bottom edge.

Abstract: A heat dissipating structure includes a frame. An accommodating space is formed inside the frame for accommodating a fan, and an opening is formed on a lateral side of the frame. The heat dissipating structure further includes at least one shutter connected to the frame in a rotatable manner. The fan is disposed on a side of the shutter. The shutter prevents airflow driven by the fan from flowing back to the other side of the shutter through the opening when the shutter rotates to a close position. The heat dissipating structure further includes a resilient component for providing a resilient force to the shutter so as to drive the shutter to the close position.

Abstract: A refrigerator is provided that includes a cold air duct that receives cold air circulating inside of a refrigerating chamber and a freezing chamber, an evaporator in the cold air duct, at least one defrosting heater in the cold air duct that selectively emits heat, a fan in the cold air duct, that selectively directs the cold air in an upward or downward direction, a motor that drives the fan, and an open/close device that closes a space having the evaporator, the defrosting heater, and the fan positioned therein selectively, thereby providing an improved defroster for an evaporator.

Abstract: A method and apparatus for providing ventilation to a selected structure. The apparatus may include various features such as flexible portions, rigid portions, and assembly portions. Further, various steps may be used to form the structure to achieve selected results, such as monolithic formation, inclusion of various positioning members, and the like.

Abstract: An air moving device assembly in a computer system has an air moving device such as a fan or blower within a housing for moving air from an air inlet to an air outlet. A mesh with a plurality of openings covers the air outlet. A back flow damper assembly is positioned over the air outlet. The back flow damper assembly further comprises at least one damper door movable in an open position and a closed position. The damper door has at least one tooth such that when the damper door is in the closed position, the tooth interacts with at least one of the plurality of openings in the mesh. By periodically cycling the air moving device controlled by device management software, the teeth will close and open, loosening debris from the mesh, allowing the debris to be blown out the rear of the housing once the device is turned back on.

Abstract: A damper assembly for use with an axial fan to prevent air from entering the discharge side of the fan when the fan is at idle. The damper assembly includes a damper door having multiple sections. The sections are able to pivot independently in response to the air flow created by the fan to minimize airflow resistance when the fan is operating.

Abstract: An air moving device assembly in a computer system has an air moving device such as a fan or blower within a housing for moving air from an air inlet to an air outlet. A mesh with a plurality of openings covers the air outlet. A back flow damper assembly is positioned over the air outlet. The back flow damper assembly further comprises at least one damper door movable in an open position and a closed position. The damper door has at least one tooth such that when the damper door is in the closed position, the tooth interacts with at least one of the plurality of openings in the mesh. By periodically cycling the air moving device controlled by device management software, the teeth will close and open, loosening debris from the mesh, allowing the debris to be blown out the rear of the housing once the device is turned back on.

Abstract: A system, method, and apparatus for preventing exhaust backflow into a blower are disclosed. Embodiments may include a blower system with an invertible blower chassis having a blower exhaust to direct airflow from the blower chassis at an airflow angle. The system may also include a backflow damper frame attached to the blower chassis and positioned to receive airflow from the blower chassis and one or more vertical damper vanes rotatably attached to the backflow damper frame. Each damper vane may freely rotate between a first, closed position and a second, open position. The damper vanes may block airflow into the blower exhaust when in the closed position and may freely rotate to a position where the damper vanes are substantially parallel to the airflow from the blower exhaust. The damper vanes may each include a vane pin to rotatably attach to frame holes of the backflow damper frame.

Abstract: The invention relates to a wall sleeve with a ventilation opening at a guide tube (2), and a cover plate (5), which is mounted at the end of the guide tube and can be moved between a closing position and an open position, said cover plate (5) automatically opening and closing against a reset force, propelled by the pressure difference between the air pressure in the guide tube (2) and the external ambient air.

Abstract: A reverse flow preventing device arranged in a flow path of fluid formed by a fan and an electronic apparatus equipped with the same. The reverse flow preventing device has a frame, a plurality of support shafts arranged about the center of the frame and mounted on the frame, and a plurality of pivotable plate members supported by the support shafts. The plate members can move between the first position (reverse flow preventing position) in which they lie in a substantially single plane and the second position (normal operating position) in which they move. The support shafts and the plate members are arranged substantially concentrically or radially.

Abstract: A hooded exhaust vent having walls bounding an exhaust opening covered by a guard, in the assembly of which the walls and guard respectively have laterally extending slides urged in sliding movement into slide tracks in the path of the movement to establish a sliding interconnection therebetween.

Abstract: A pressure relief valve (10) includes a housing (12) with a peripheral side wall defining at least one central airflow passage and at least one flap valve element (14). The housing (12) and the flap valve element (14) have a molded connection for enabling rotation of the flap valve element relative to the housing between a closed condition overlying the at least one central airflow passage (24) to prevent airflow from atmosphere (21) into the passenger compartment (19) and an open condition permitting airflow out of the passenger compartment of the vehicle to atmosphere. The at least one flap valve element (14) moves from the closed condition to the open condition in response to differential pressure between the passenger compartment (19) and atmosphere (21).

Abstract: An efficient axial fan has a housing enclosure with a square inlet opening and a round outlet opening, a fan assembly with a drive motor and a propeller secured to a shaft and coupled thereto. The entrance of the fan housing is provided with a shutter assembly which is disposed horizontally or vertically across the square inlet opening and comprises a plurality of vanes including a central vane operated by a centrifugal shutter mechanism secured to one end of the propeller shaft, opposite to the propeller and adjacent to the central vane of the shutter assembly. The centrifugal shutter mechanism is normally provided with a hermetic housing comprising a pair of radial masses operatively connected via a pair of pivoting arms to an axial sliding reciprocating actuator which is disposed adjacent to the shutter central vane.

Abstract: A reverse flow preventing device arranged in a flow path of fluid formed by a fan and an electronic apparatus equipped with the same. The reverse flow preventing device has a frame, a plurality of support shafts arranged about the center of the frame and mounted on the frame, and a plurality of pivotable plate members supported by the support shafts. The plate members can move between the first position (reverse flow preventing position) in which they lie in a substantially single plane and the second position (normal operating position) in which they move. The support shafts and the plate members are arranged substantially concentrically or radially.

Abstract: A ventilation fan assembly (10) having a fan (22) mounted in a housing (12) and a shutter assembly (60). The housing has a square inlet portion (12C), a circular center portion (12E) and a conical outlet portion (12D) having rifling grooves (20). The shoulder (16) formed between the inlet portion and the center portion is radiused to reduce disruption of air flowing from the inlet portion to the center portion. The shape of the blades (32) of the propeller (28) is such that the axial velocity produced as the propeller rotates is essentially constant along the length of the blades. The shutter assembly has a large, radiused entrance and airfoil shaped blades which reduce air disruption. The blades have a flexible flap (68) along the trailing edges (64D) which form a seal between adjacent blades to prevent air from leaking from the housing through the shutter assembly.

Abstract: A radiator, adopted on electronic devices with radiator fans installed in series, includes a case, two or more radiator fans and a membrane check valve. The radiator fans are located on the inner wall surface of the case. The membrane check valve has a frame, at least one bracing bracket and membrane petals. The frame is located between the radiator fan and the inner wall surface of the case. The bracing bracket is located in the frame to form a plurality of cells with the frame. The membrane petals' one end attaches to one side of the frame and is more directed towards air outlets of the radiator fans than the bracing bracket. When any radiator fans stops operation, air inside the electronic devices flow outwards in the positive direction to balance the pressure and prevent heat dissipation efficiency of the electronic devices from dropping excessively.

Abstract: A blower includes a base having a transverse passage for receiving a motor therein and a fan device driven by the motor extends from a hole in an end of the base. A casing including an outlet is connected to the base and mounted to the fan device. An anti-back device is engaged in the outlet and includes a frame to which a plate is pivotably connected. The plate is opened by the air sent via the fan device in one direction and seals the outlet when air goes in opposite direction.

Abstract: A replacement dryer vent for remodelers includes a typical exterior vent portion and an interior tube portion. The tube portion extends into the building and has an internal diameter that increases towards its innermost edge, i.e., toward the interior of the building. The extension is designed to slip over an existing cylindrical duct already in a wall. This allows one to replace an existing dryer vent by simply removing the exterior vent portion and leaving the duct work in position. The new vent is simply inserted in the existing hole with the tube slipping over the duct making a tight connection.

Abstract: A power assisted ventilation system for a conservatory of the type having a roof extending away from a wall or other supporting structure that is mountable where the roof meets the wall of other supporting structure has a back plate mountable to the wall or to a glazing panel support means already on the wall and an apertured cover securable to the back plate.

Abstract: A ventilation fan assembly (10) having a fan (22) mounted in a housing (12) and a shutter assembly (60). The housing has a square inlet portion (12C), a circular center portion (12E) and a conical outlet portion (12D) having rifling grooves (20). The shoulder (16) formed between the inlet portion and the center portion is radiused to reduce disruption of air flowing from the inlet portion to the center portion. The shape of the blades (32) of the propeller (28) is such that the axial velocity produced as the propeller rotates is essentially constant along the length of the blades. The shutter assembly has a large, radiused entrance and airfoil shaped blades which reduce air disruption. The blades have a flexible flap (68) along the trailing edges (64D) which form a seal between adjacent blades to prevent air from leaking from the housing through the shutter assembly.

Abstract: A shutter assembly for an intake opening in an exhaust fan for improving heating and cooling efficiency of a structure by preventing air in the structure from flowing through the exhaust fan and out of the structure. The shutter assembly for an intake opening in an exhaust fan includes a door for selectively positioning over an air intake opening in an exhaust fan. The door is mountable on the surface round the exhaust fan by at least one hinge member. The hinge member may be mounted on the door for pivotally coupling the door to the surface around the exhaust fan such that the door is pivotable between an open position and a closed position. A locking assembly is mounted on the surface and releasably couplable to the door for selectively securing the door in the closed position.

Abstract: A valve for a vent. The valve includes an inner leaf having a closed state which covers the vent and blocks gas flow through the vent in a first direction, and an open state which opens the vent and allows gas flow through the vent in a second direction. The valve includes an outer leaf opposing the inner leaf. A method for controlling gas flow through a vent. The method includes the steps of moving an inner leaf and an outer leaf opposing the inner leaf on the vent from a closed state which covers the vent and blocks gas flow through the vent in a first direction to an open state which opens the vent and allows gas flow through the vent in a second direction with gas flow moving in the second direction through the vent. Then there is the step of stopping the gas flow in the second direction allowing the inner leaf and the outer leaf to each move to the closed state.

Abstract: Apparatus for removing excessive heat which has built up in a garage, including vents, movable louvers connected to each vent, exhaust fans, and a fan motor. The fan motor may be controlled by a manually-operated switch or a thermostat.

Abstract: An apparatus and method for cooling a heat-generating component according to which a body member is placed over a portion of a heat sink so that flanges on the body member engage the component. The body member is forced away from the heat sink so that the flanges force the component and the heat sink into intimate contact.

Abstract: A simplified installation construction for a venting device which requires from a select one of several sizes of an input diameter a correlation to a matching diameter of a flexible conduit, the simplification resulting from using an interconnection of the diameters which once made, is difficult to disconnect but is known not to require a disconnecting mode, and wherein the sizes of input diameters not used are merely discarded. This is a noteworthy alternative to attempting to correlate the diameter of a flexible conduit to all input diameter sizes of the venting device.

Abstract: A ventilation fan assembly (10) having a fan (22) mounted in a housing (12) and a shutter assembly (60). The housing has a square inlet portion (12C), a circular center portion (12E) and a conical outlet portion (12D) having rifling grooves (20). The shoulder (16) formed between the inlet portion and the center portion is radiused to reduce disruption of air flowing from the inlet portion to the center portion. The shape of the blades (32) of the propeller (28) is such that the axial velocity produced as the propeller rotates is essentially constant along the length of the blades. The shutter assembly has a large, radiused entrance and airfoil shaped blades which reduce air disruption. The blades have a flexible flap (68) along the trailing edges (64D) which form a seal between adjacent blades to prevent air from leaking from the housing through the shutter assembly.

Abstract: A power assisted ventilation system for a conservatory of the type having a roof extending away from a wall or other supporting structure that is mountable where the roof meets the wall of other supporting structure has a back plate mountable to the wall or to a glazing panel support means already on the wall and an apertured cover securable to the back plate.

Abstract: An intake and exhaust damper assembly has a motor and fan assembly pivotally mounted for movement toward and away from a louvered rotatable blade damper.

Abstract: An intake and exhaust fan assembly is provided having a motor and fan assembly mounted for movement on rails toward and away from a rotatable blade damper mounted on an angle to the motor/fan assembly.

Abstract: An apparatus, for installing a ventilation fan, highly effective for preventing dust, gas or the like from coming in a room has been provided by the present invention. An apparatus 1 for installing a ventilation fan comprises a L-shaped housing 5, for fixing a ventilation fan 3 therein, having an inside opening 7 positioned inside a wall W, and an outside opening which looks down and is positioned outside the wall W. The inside opening 7 is provided with a filter 11 for exhaust able to be slide vertically, and able to be taken in and out from the housing 5. Further, the inside opening 7 is covered with a metallic lid 13 able to be open or shut. A filter 15 for absorbed air is provided near the outside opening 9. The filter 15 is able to slide horizontally. The ventilation fan F is rotatable clockwise and counterclockwise. The filter 15 for absorbed is installed in absorbing air. The filter 11 for exhaust is installed in exhausting air.

Abstract: A computer system includes a housing that is divided into three compartments; two fan compartments, each containing a fan, and a third compartment containing heat generating electronic components. The air discharged from the fans flows through outlets in the fan compartments and into the third compartment to cool the electronic components. A single baffle is rotatably coupled between the two fan compartments using a vertically oriented hinge. When one of the fans fails, the air pressure from the working fan rotates the baffle until it covers the outlet of the fan compartment containing the failed fan. This enhances the flow of air from the working fan into the third compartment, and restricts the reverse flow of air through the dead fan compartment. When both fans are functioning, air pressure from both fans moves the baffle to a "midway" or "neutral" position, such that the air from both fans flows into the third compartment.

Abstract: An aerodynamic-shaped fan blade for use in a fan apparatus. The blade has a cross-section which is essentially an inverted pan-shape with an intermediate section, a leading edge section, and a trailing edge section which form concave and convex surfaces. The trailing edge has a flange doubled back toward the leading edge within the concave envelope.

Abstract: A plurality of parallel fans are mounted for moving side-by-side columns of air in a chassis. A louver member is provided adjacent each fan. Each louver member is bendable from a closed position to an open position by a flow of air generated by its respective fan, and is restrictedly bendable to a closed position, in engagement with a portion of an air passageway, in response to failure of its respective fan.

Abstract: An aerodynamic-shaped propeller blade. The blade has a cross-section which is essentially an inverted pan-shape with an intermediate section, a leading edge section, and a trailing edge section which form concave and convex surfaces. The trailing edge has a flange doubled back toward the leading edge within the concave envelope.

Abstract: Disclosed is a system for assisting in preventing over heating of the internal electrical components of a computer in the event of a failure of one of the adjacent fans of an array of fans employed to circulate cooling air over the components. The fans include openings through which air is forced from their inlet sides to their outlet sides. The system includes a vent for each fan arranged adjacent to and aligned with the fan opening at its outlet side. The vents are open and closed by vanes, having hinged slats construction to allow them to fall downwardly by their own weight or additional assistance. The vanes are kept open by the airflow created by the fans. Upon the failure of a fan, the absence of airflow through the opening thereof allows the slats of the vane to drop to close off the fan opening and prevent airflow into the fan opening from its outlet side.

Abstract: An air flow reversal prevention door is mounted to a discharge opening of a ventilation fan. The door includes a frame member with a void in it to permit air flow to pass through the frame and is attached fixedly to a shaft which supports the door and is located between the door and a discharge opening of the ventilation fan. While the door is held open by air from the ventilation fan, the frame member is moved from the edge of the discharge opening toward the door and back and this enables the shaft to rotate because it is fixedly attached to the frame member and it causes the shaft to rotate with movement of the frame member. This causes the shaft to rotate through the entire range of motion of the door between its open position and its collapsed position. This tests the operation of the door without turning off the fan. Whenever the fan fails or is turned off the air flow ceases and the door collapses by gravity toward the discharge opening to prevent air reversal flow.

Abstract: In a multiblower system for cooling electronic components, each blower has a backflow preventor. The backflow preventor has two pieces of flexible material and a screen attached to the intake side of the blower assembly. Each of the pieces of flexible material is scored through to form a petaled opening through which air may pass. The scores on one of the pieces of flexible material are out of alignment with the scores on the other piece when assembled together in the blower assembly. When the blower is running, the petals in the pieces of flexible material open to allow air to pass through the blower assembly. When the blower is stopped, the petals are closed against the screen due to the pressure differential between the inside of the enclosure and the outside of the enclosure caused by the effect of the draw of the other blowers in the system.

Abstract: The present invention provides a cooling fan system for a chassis configured to contain heat generating electrical components and to assume horizontal and vertical alternative operating orientations. The cooling fan system includes a support member securable within the chassis and having an air flow opening formed therein. Additionally, the cooling fan system includes a louver member hingedly attached to the support member to rotate between an open position wherein a substantial air flow is allowed through the air flow opening and a closed position wherein the louver member covers the air flow opening. The louver member may be made from plastic, however, other types of rigid materials that can withstanding the chassis' operating conditions could also be used, such as metal.

Abstract: An outdoor exhaust's end of part of an indoor building system requiring an exhaust duct to evacuate air or other kinds of smoke or gases to the building's outside. The exhaust's end comprising a double enclosure that prevents the outdoor air from comming into the building by the exhaust duct and captures an air chamber in a rotary inner cylinder, which act like a thermal insulation between the inlet and the outlet of the exhaust's end. A trap located on the rotary inner cylinder is activated by the exhaust flow, that comes upward and rotate the inner cylinder, permitting to the exhaust flow to past through an opening located under the trap, to come in the cylinder and flow out by a second opening. The exhaust flow out by the outlet downward duct, that protect the exhaust's end against rain or snow without restricting the flow.

Abstract: A new Soffit Mounted Dryer Vent for offering a non-maintenance exhaust vent system for a clothes dryer vent that terminates in a soffit of a roof overhang. The inventive device includes a flange, a dryer hose adapter ring, an outlet fitting, and a vent flapper. The flange has an air passage, an upper surface, and a lower surface. The air passage allows exhaust air to pass through the flange from the dryer hose adapter ring to the outlet fitting. The dryer hose adapter ring is symmetrically mated to the upper surface of the flange. The dryer hose adapter ring has an outer wall diameter of substantially the same dimensions as the clothes dryer vent hose thereby allowing the clothes dry vent hose to matingly slip over the dryer hose adapter ring. The outlet fitting, having an outlet opening, is adjoiningly mated to the lower surface of the flange. The vent flapper is pivotally attached to the outlet fitting at the outlet opening.In use, the clothes dryer vent hose is slipped over the dryer hose adapter ring.

Abstract: A forced air vent for allowing the unidirectional outflow of exhaust air from the interior of a structure into the atmosphere by way of an eave opening includes a generally rectangular main body, arranged with a perforate bottom wall and a pair of oppositely disposed side walls and two hingedly connected movable internal baffle plates. An interior space is defined by the bottom wall and the two oppositely disposed side walls. The perforate bottom wall is arranged with a solid central portion and two oppositely-disposed distal portions. A corresponding abutment ledge is disposed between each distal portion and the central portion. Each distal portion includes a grate panel through which air can freely pass. From a closed position, the baffle plates are movable outwardly and upwardly into an opened position in response to positive air pressure from within the attic interior. The bottom wall has a convex surface protruding into the central portion to focus the air flow onto the baffle plates.

Abstract: A ventilator to be mounted over a square opening in a roof of an establishment producing smoke below. The ventilator has a lower transitional section transforming a square configuration into a round configuration and an upper section in the shape of a cylinder attached to the lower section. The cylinder has an axial force fan mounted therein to create an upwardly moving stream of smoke-laden air therein. The lower section has symmetrically spaced lateral openings in its wall to cause an aspiration of ambient air into the air stream to thereby mix ambient air into the air stream and to thereby reduce the concentration of smoke micro-particles being emitted into the ambient air surrounding the establishment. On top of the cylinder there is provided a self-operating cover to close the ventilator when not in use.

Abstract: A decontamination chamber entranceway to an asbestos removal work area includes rigid swinging doors for a rapid escape from the contaminated work area due to an emergency such as a fire or smoke. Also during a power failure it is easy for the workers to leave the work area through the rigid swinging doors of the invention. Each of the doors may include an air inlet opening having at least one plastic flap covering the inlet opening which allows suitable amounts of air to flow through the inlet to maintain a negative air pressure in the work area while having air flow in the work area sufficient to change the air at least every 10 to 15 minutes. The plastic flaps of the doors are preferably pivotally-mounted rigid clear plastic which seal automatically upon loss of negative air pressure in the work area. The rigid flaps can be electro-mechanically operated to positively control the size and functioning of air inlet openings and close off the openings upon emergency conditions.