Ventilation – Workstation ventilator – Covered workbench chamber
Reexamination Certificate
2001-08-17
2002-10-08
Boles, Derek (Department: 3743)
Ventilation
Workstation ventilator
Covered workbench chamber
C454S056000
Reexamination Certificate
active
06461233
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
BACKGROUND OF THE INVENTION
This invention relates to laboratory work cabinets. More specifically, this invention relates to laboratory work cabinets which provide improved exhaust containment such that the cabinets may be operated at lower inflow air velocities than traditional work cabinets, thereby reducing the energy required to condition the exhausted air.
Laboratory work cabinets, or “fume hoods”, are ventilated enclosures where undesirable and dangerous fumes or vapors are captured, contained and removed. These fumes or vapors are prevented from escaping into the external laboratory environment and, instead, are diluted with room air and exhausted through the hood's exhaust system where they can be adequately dispersed. This capture and containment is accomplished by controlling gas and vapor contaminants present in the hood work area and directing them away from the user.
Fume hoods known in the prior art generally comprise a cabinet which defines an enclosed work area. The cabinet has an opening in the front face thereof for providing access to the work area and typically includes a panel or sash which is movable between open and closed positions to provide selective access to the work area. The sash is movable between various heights to accommodate positioning of hands and arms in the work area. Prior art fume hoods typically include an exhaust system, often connected to the top of the cabinet, for venting fumes that collect in the work area of the cabinet.
Air flow into prior art fume hoods, or “exhaust hoods”, typically is achieved by an exhaust blower which “pulls” air from the external laboratory environment into and through the hood and its associated exhaust system. Thus, contaminants are drawn away from the operator. This pull of air into the hood is measured as face velocity. It is, of course, essential to provide an adequate face velocity for any laboratory fume hood so as to ensure containment of the fumes or other contaminants and to ensure that these contaminants will ultimately be removed through the exhaust system. The face velocity, however, must not be so high that it creates turbulent conditions within the hood which can lead to the escape of contaminants. Accordingly, it is desirable that the face velocity be maintained nearly constant, not varying appreciably throughout the normal working range of the sash. Since raising of the sash increases the effective size of the exhaust hood opening, the volume of air pulled through the face opening of the hood must be increased in order to maintain a constant face velocity as the sash is raised. For most materials that are handled in fume hoods, a face velocity of approximately 100 feet per minute (fpm) is satisfactory.
Fume hoods known in the prior art often incorporate a bypass opening located in the front face of the cabinet above the opening into the work area, or located in the top face of the cabinet in the area at or near the front face. Bypass hoods are designed so that as the sash is moved toward a closed position, air which normally enters the hood through the sash opening is redirected through the bypass opening thus reducing fluctuations in face velocity as the position of the sash is varied. Therefore, the possibility that the velocity will reach a level that would be detrimental to the procedures being performed in the work area, or to those persons in the vicinity of the work area, is reduced.
Bypass hoods are not without weaknesses, however. For instance, the angle at which bypass air enters the work area changes with sash position and the face velocity may increase to as much as three times the normal face velocity as the sash moves toward the closed position even though air is being directed through the bypass opening.
Additionally, prior art fume hoods, including bypass hoods, exhibit characteristic internal vortex air flows known as “a roll” in which a portion of the incoming air flow rolls up the interior side of the rear face and down the interior side of the sash. Fumes generated within the hood from laboratory procedures often are entrained into the roll resulting in an increase in the concentration of contaminants throughout the work area. Specifically, this tendency for contaminated air to roll forward produces high concentrations of contaminants in the area directly behind the sash increasing the opportunity for leakage at the sash handle.
A primary factor in creating this undesirable air flow is that traditional fume hoods are unable to remove contaminants from the work area as quickly as they are generated, i.e., contaminants are not removed on the “first pass”. Rather, fumes generated by laboratory procedures freely mix with incoming air, circulate in the vortex and come back down the interior surface of the sash. This raises the parts per million concentration of the contaminant throughout the work area.
A further weakness of prior art fume hoods is that air flow within the hood interior varies with the position of the sash making it difficult to idealize conditions for optimum containment. Still further, bypass air, while beneficial to maintaining a constant face velocity, continuously varies with sash position, both in volume and direction, as influenced by the changing internal conditions. This makes the hood less robust in its ability to contain contaminants. Additionally, while traditional and bypass fume hoods effectively contain contamination behind the vertical plane of the sash, they are susceptible to external conditions, traffic patterns near the sash opening and work procedures, as contaminants are concentrated directly behind the plane of the sash and under the sash handle. Another weakness of prior art fume hoods is their susceptibility to lead around the front sash foil. Existing air foils are not designed to provide uniform velocities both with and without a person standing in front of the hood.
Because of the afore-described deficiencies, typical prior art fume hoods are designed to provide a certain excess air flow so that even during more optimal operating conditions there will be adequate air flow to satisfy safe operating conditions. This design criteria results in significant energy loss both through exhaust system power requirements and by removing conditioned air from the building which must be replaced by other conditioned air.
Accordingly, there remains a need for a laboratory work cabinet which more effectively contains contaminated air and is less susceptible to external conditions such as traffic patterns near the sash opening or air fluctuations caused by work procedures. Further, there remains a need for a laboratory work cabinet which substantially removes contaminants from the work area as quickly as they are generated. Still further, there remains a need in the fume hood industry for a laboratory work cabinet which operates at lower inflow air velocities than traditional hoods thereby reducing the energy required to operate the exhaust system and reducing the demand on the building HVAC system.
SUMMARY OF THE INVENTION
Accordingly, in one of its aspects, the present invention provides a laboratory fume hood which more effectively contains contaminated air and is less susceptible to external conditions.
In another of its aspects, the present invention provides an exhaust hood in which fumes are contained deeper into the interior of the hood reducing contaminant concentrations directly behind the plane of the sash.
In yet another aspects, the present invention provides an exhaust hood which operates at lower inflow air velocities than traditional hoods thereby reducing the energy required to exhaust air at acceptably low contaminant concentrations and reducing the demand on the HVAC system.
In another of its aspects, the present invention provides a laboratory fume hood which substantially removes contaminants from the work area as quickly as they are generated.
In an additional aspect, the present inventio
Dyer Kermit W.
Gilkison Kevin C.
Hambleton Larry G.
Ma Yu Rich
Roepke Gary P.
Boles Derek
Labconco Corporation
Stinson Morrison & Hecker LLP
Wharton J. David
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