Ventilation – Workstation ventilator – Covered workbench chamber
Reexamination Certificate
2000-05-18
2002-08-06
Lu, Jiping (Department: 3749)
Ventilation
Workstation ventilator
Covered workbench chamber
C454S057000, C454S059000, C454S062000
Reexamination Certificate
active
06428408
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to fume hoods, and in particular to energy-efficient laboratory fume hoods. More specifically, the invention relates to laboratory fume hoods which use low flow rates and further relates to structural features which facilitate containment of contaminants in a fume hood.
A fume hood may be generally described as a ventilated enclosed workspace intended to capture, contain, and exhaust fumes, vapors, and particulate matter generated inside the enclosure. The purpose of a fume hood is to draw fumes and other airborne matter generated within a work chamber away from a worker, so that inhalation of contaminants is minimized. The concentration of contaminants to which a worker is exposed should be kept as low as possible and should never exceed a safety threshold limit value. Such safety thresholds and other factors relating to testing and performance of laboratory fume hoods are prescribed by government and industry standards by organizations, such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) of Atlanta, Ga., for example, ANSI/ASHRAE 110-1995. ASHAAE Standard, “Method of Testing Performance of Laboratory Fume Hoods.” This and all other documents cited in this application are incorporated herein by reference for all purposes.
FIG. 1
shows a cross-sectional side view of a conventional fume hood. The hood
100
includes a work chamber
102
, bounded by walls
103
and a front open face
105
which may be covered partially or completely by a moveable sash
114
. The hood may be supported by a base
104
. In many designs, the base contains cabinets for storage of solvents and other materials used in the hood's work chamber
102
.
While hood sizes vary considerably, a typical conventional fume hood is about 4 to 10 feet wide with a sash opening of between about 26 and 34 inches, and a standard interior vertical size of about 52 inches. The hood's sidewalls
103
typically have considerable thickness because they contain mechanical and electrical services for the hood. Again, while dimensions of fume hoods greatly vary, the depth of a typical fume hood ranges from about 32 to about 37 inches. A typical conventional hood design includes an air foil
106
at the bottom front of the work chamber
102
and a baffle
108
at the rear of the work chamber
102
. The depth of the work chamber
102
between these two features
106
and
108
is typically approximately 21 inches.
The opening in the front of the fume hood
100
which provides access to the work chamber
102
by a worker, is referred to as the face of the fume hood. In some conventional fume hood designs, referred to as open-faced hoods, the face area of the hood is fixed. In other designs, such as that depicted in
FIG. 1
, the moveable sash
114
provides the ability to alter the face area of the hood
100
. Sashes come in either vertical or horizontal arrangements, with the vertical design typically being preferred since it can provide a full open face area.
Other elements of conventional fume hoods illustrated in
FIG. 1
include an air bypass area
116
above the sash in the top front of the fume hood
100
which provides an additional path for ambient air to enter the work chamber
102
. The bypass
116
provides sufficient air flow to dilute contaminants in the hood, and to avoid air whistling when the sash
114
is closed. Air is exhausted from the fume hood through an exhaust system equipped with a fan (not shown) which draws air into the fume hood's work chamber
102
, through the baffle
108
, and into ducting
118
outside the work chamber
102
of the fume hood
100
for exhaustion from the building. The top wall of the fume hood is also typically equipped with a light fixture
120
to illuminate the work chamber
102
. The back baffle
108
typically includes two or three horizontally disposed slots to direct air flow within the work chamber
102
. Further details regarding the design and construction of conventional laboratory fume hoods may be found in Sanders G. T., 1993. Laboratory Fume Hoods, A User's Manual. John Wiley & Sons, Inc.
Containment of contaminants in many conventional fume hoods is based on the principal of supplying an abundant amount of air into the face of the hood and withdrawing this air, along with the contaminants, from the work chamber. As noted above, the face corresponds to the area below the sash (in the case of a vertical sash arrangement) at the front of the hood through which air enters the work chamber. This abundant amount of air is supplied at a high enough rate such that contaminants within the hood are prevented from moving against the incoming air entering the face of the hood. Under conventional principles, air flow is typically increased to improve containment of contaminants within the work chamber.
An important factor in a conventional fume hood's ability to contain contaminants is its face velocity. The face velocity of a fume hood is determined by its exhaust and its open face area. Recommendations for face velocity of conventional fume hoods range from 75 feet per minute (fpm) for materials of low toxicity (Class C: TLV>500 ppm) to 130 fpm for extremely toxic or hazardous materials (Class A: TLV<10 ppm). Cooper, E. C., 1994
. Laboratory Design Handbook
, CRC Press. In general, industrial hygienists recommend face velocities in the range of 100 fpm plus or minus 10 fpm for containment of contaminants by conventional hoods with open sashes.
Face velocities at these speeds typically produce turbulent air flow conditions within the hood. As a result, unpredictable and inconsistent air flow patterns, such as vortices near exhaust outlets and near the face of the hood, often occur. The unpredictability of turbulent air flow conditions within the hood may result in reversal of flow near the face of the hood despite the high velocity of incoming air, causing contaminants to spill from the hood's work chamber into the surrounding laboratory space. Turbulent air flow within the hood also increases mixing between the fresh air and other airborne contaminants generated within the work chamber.
The abundant amount of air supply provided to the hood and turbulent air flow conditions formed therein are often compounded by conventional fume hood design.
FIG. 2
shows a cross-sectional side view of a conventional fume hood design, such as that illustrated in
FIG. 1
, further illustrating ideal air flow through such a conventional hood. Air is shown entering the hood
200
from the surrounding laboratory space
201
by arrows
202
. The air flows through the open face
203
of the hood
200
defined by the fully open sash
206
and the air foil
208
into the work chamber
205
. Inside the work chamber
205
the air is drawn towards slots
204
in the baffle
207
at the rear of the work chamber
205
. In the particular design depicted in
FIG. 2
, the air flow generated by the slots establishes a vortex
210
in the upper region of the work chamber. If this vortex extends to or below the upper limit of the open face
203
, the risk of spillage of airborne contaminants from the hood
200
is increased. Having passed through the baffle
207
, the air is then exhausted through the exhaust system
212
.
In addition to the hood design, the position of the worker with respect to the air flow direction may have a significant influence on the air flow patterns in the hood, and particularly in the face of the hood. Air flows surrounding a body standing in front of the hood create a region of low pressure downstream of the body. This region, which is deficient in momentum, is called the wake. It disturbs the directed air flow in the face of the hood, adding to any turbulence and may further result in reversal of flow causing contaminants to spill from the hood's work chamber into the surrounding laboratory space.
As described above, the air source for conventional fume hoods is the ambient air in a laboratory in which the fume hood is located. The
Bell Geoffrey C.
Dickerhoff Darryl J.
Feustel Helmut E.
Beyer Weaver & Thomas LLP
Lu Jiping
The Regents of the University of California
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