U.S. patent number 3,752,056 [Application Number 05/086,833] was granted by the patent office on 1973-08-14 for laboratory exhaust hood.
This patent grant is currently assigned to E. H. Sheldon and Company. Invention is credited to Richard I. Chamberlin, Joseph E. Leahy, Frederick J. Viles.
United States Patent |
3,752,056 |
Chamberlin , et al. |
August 14, 1973 |
LABORATORY EXHAUST HOOD
Abstract
A laboratory exhaust hood is described including in combination
a hood superstructure and a novel auxiliary air supply plenum. The
auxiliary air supply plenum is of modular design suitable for use
wherever an efficient conversion of a relatively high energy air
stream of a given cross sectional area to a much lower energy
laminar air stream of larger cross sectional area is desired. In
particular, the auxiliary air supply is adapted for incorporation
with a conventional laboratory exhaust hood, and has exterior
vertical front, rear and end walls and a cover and interior baffles
which define an air entry chamber, an air slot at the top of said
chamber, an expansion chamber extending downward from the slot, an
air balance chamber and an air supply outlet. Within the plenum, an
air vector controller is positioned between the expansion chamber
and the air balance chamber, and the final air balance means,
including a back pressure plate, an air jet entrainment eliminator,
and a shallow chamber therebetween, is positioned between the air
balance chamber and the air outlet. The combination of the hood
superstructure and the auxiliary air supply plenum includes a
movable vertical closure sash and immediately above it, when the
sash is closed, a sight-tight bypass. Beneath the closed sash is a
horizontal air foil for the provision of a flow of air across the
work surface of the hood, by the entrainment of room air in
auxiliary air jets.
Inventors: |
Chamberlin; Richard I.
(Hanover, MA), Leahy; Joseph E. (West Quincy, MA), Viles;
Frederick J. (Norwood, MA) |
Assignee: |
E. H. Sheldon and Company
(Muskegon, MI)
|
Family
ID: |
22201214 |
Appl.
No.: |
05/086,833 |
Filed: |
November 4, 1970 |
Current U.S.
Class: |
454/59 |
Current CPC
Class: |
B08B
15/023 (20130101) |
Current International
Class: |
B08B
15/00 (20060101); B08B 15/02 (20060101); F23j
011/00 () |
Field of
Search: |
;98/115LH,101,114,36,37,121 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Perlin; Meyer
Assistant Examiner: Capossela; Ronald C.
Claims
We claim:
1. Apparatus for converting a rapidly moving air stream to slower
air having substantially laminar flow free of localized non-uniform
energy comprising: receiving means comprising an entry chamber for
receiving said air stream; means including two diverging walls for
expanding the flow of said stream substantially uniformly across
the cross-section of its flow; and means for distributing the flow
of said stream substantially uniformly from said receiving means to
said expansion means; said distributing means comprising an
elongated slot having a cross-sectional area which is sufficiently
smaller than the cross-sectional area of said entry chamber
transverse to the flow of said air to promote a spreading of the
flow of said air into such slot along its length.
2. Apparatus for converting a rapidly moving air stream to slower
air having substantially laminar flow free of localized non-uniform
energy, comprising: receiving means comprising an entry chamber for
receiving said air streams; means including two diverging walls for
expanding the flow of said stream substantially uniformly across
the cross-section of its flow; and means comprising a passage
between said receiving means and said expansion means for
distributing the flow of said stream substantially uniformly from
said receiving means to said expansion means, wherein said entry
chamber and air expansion means are adjacent, share one of said
walls as a common wall, and are constructed and arranged to reverse
the direction of flow of said stream.
3. Apparatus for converting a rapidly moving air stream to slower
moving air having substantially laminar free flow of localized
non-uniform energy, comprising means for receiving said air stream;
means including two diverging walls for expanding flow of said
stream substantially uniformly across the cross section of its
flow; means comprising a passage between said receiving means and
said expansion means for distributing the flow of said stream
substantially uniformly from said receiving means to said expansion
means; and means for air jet entrainment being downstream of said
expansion means and including a multiplicity of closely spaced
widely distributed air flow openings, together with means
operatively associated with said openings upstream thereof for
effecting a slight back-pressure.
4. The apparatus of claim 3 further characterized by an air balance
chamber between said air expansion means and said air jet
entrainment elimination means.
5. An auxiliary air supply plenum of modular design suitable for
incorporation with a conventional laboratory exhaust hood, which
hood is provided with an air exhaust means, an interior work space,
a front face, and a movable closure sash in said face for supplying
a substantially uniform laminar flow of auxiliary air to said
exhaust hood, said auxiliary air supply comprising:
exterior front, rear and end walls, a cover and interior baffles
defining (1) an air entry chamber, (2) a slot along the upper edge
of one wall of said entry chamber, (3) an expansion chamber
extending vertically downwardly from said slot toward the interior
of the plenum and (4) an air supply outlet downstream of said
expansion chamber; wherein said chambers, slot and outlet extend
substantially the length of said plenum and said outlet extends
substantially across the width of said plenum.
6. The air supply plenum of claim 5 wherein the width of said slot
is no more than one-half the height of said entry chamber, and said
entry chamber and said expansion chamber share a common wall having
said slot at the top thereof.
7. The air supply plenum of claim 5, wherein an air balance chamber
and final air balance means are provided between said expansion
chamber and said outlet, said air balance means including (1) a
back pressure plate perforated with laminar flow holes
substantially uniformly across its width and length, (2) an air jet
entrainment eliminator, and (3) a shallow chamber between said
plate and said entrainment eliminator.
8. The air supply plenum of claim 7, wherein the dimensions and
orientation of the elements thereof are such that:
kA.sub.f w = s + 2(l+h) tan (.theta./2) ; wherein
k is at least 1.4,
A.sub.f is the free area of said plate,
w is the width of said plenum,
s, the width of said slot, is 1.4-1.8 inches,
l is the length of said expansion chamber,
h is the height of said balance chamber, and
.theta., the expansion angle of said expansion chamber, is
7.degree.-12.degree..
9. The air supply plenum of claim 7, wherein said air supply outlet
is provided on each of its end walls with curved profile vanes,
whereby air exiting said plenum tends to assume a flow rate profile
parabolic in shape along the length of the outlet, and the portion
of the front wall defining the front of the air supply outlet is an
adjustable vane hinged at the top and adjustable from the vertical
toward the rear wall of the outlet, whereby the auxiliary air
velocity may be varied.
10. The air supply plenum of claim 7 wherein said back pressure
plate is a fin-tube heat exchanger, whereby auxiliary air may be
heated between said air balance chamber and said shallow chamber
without affecting the substantially uniform laminar flow of air
from said air supply outlet.
11. A laboratory exhaust hood, having an open face, a bypass
incorporated in said hood above said open face, a movable closure
sash adapted to move from an open position, in which it closes said
bypass and opens said face, to a closed position in which it opens
said bypass and closes said open face, and means to maintain the
total volumetric flow of air into said hood substantially constant
during movement of said movable closure sash from its open position
to its closed position comprising a plurality of substantially
parallel walls arranged in said bypass sequentially in angular
relation to provide a sight-tight labyrinth, wherein the angular
relation of said walls forming said labyrinth to change direction
within the range of substantially 50.degree. to 70.degree. at any
one corner.
12. The laboratory exhaust hood of claim 11, wherein said hood has
a front wall in which said open face is located, and said hood is
also provided with an auxiliary air supply plenum located above
said open face, and wherein said bypass is constructed of a series
of angled louver vanes extending horizontally across the length of
said front wall from the bottom of said plenum to the top of said
open face.
13. The laboratory exhaust hood bypass of claim 12, wherein said
louver vanes are angled to form a vertical series of V-shaped
passages and adapted to induce an upward direction upon air
traversing said bypass.
14. A laboratory exhaust hood including in combination a hood
superstructure having a vertical, rectangular face and a movable
vertical closure sash, and an auxiliary air supply plenum,
comprising: means within said auxiliary air supply plenum to
provide auxiliary air substantially vertically downwardly from said
plenum toward said open face of said hood superstructure and in
substantially uniform laminar flow; bypass means operative upon the
closing of said movable, vertical closure sash to direct at least a
portion of said auxiliary air into said hood super-structure by
passage through a sight-tight by-pass above said sash; and means to
maintain the total volumetric flow of air into said hood
superstructure substantially constant during movement of said
movable closure between its open and closed positions.
15. The laboratory exhaust hood combination of claim 14, wherein
further means are provided to control the maximum face velocity of
the air entering the hood superstructure through said rectangular
open face during the movement of said movable closure from its open
position to its closed position between 2-3 times that of normal
operation, with a fully opened sash.
16. A laboratory exhaust hood including in combination a hood
superstructure and an auxiliary air supply plenum, comprising:
a. a horizontal work surface, a rear wall, side walls, and a front
wall including an open vertical face immediately in front of said
work surface; and
b. auxiliary air supply means including means to provide auxiliary
air from a point above said open vertical face downwardly to
substantially all parts of said open face in a substantially
uniform laminar flow:
wherein said auxiliary air supply means comprises an entry chamber,
a slot in one wall of said entry chamber having a width no more
than one-half the dimension of said entry chamber parallel to said
slot, and an expansion chamber extending from said slot.
17. A laboratory exhaust hood including in combination a hood
superstructure and an auxiliary air supply plenum, comprising:
a. a horizontal work surface, a rear wall, side walls, and a front
wall including an open vertical face immediately in front of said
work surface; and
b. auxiliary air supply means including means to provide auxiliary
air from a point above said open vertical face downwardly to
substantially all parts of said open face in a substantially
uniform laminar flow:
wherein said auxiliary air supply means comprises an entry chamber,
a slot in one wall of said entry chamber having a width no more
than one-half the dimension of said entry chamber parallel to said
slot, and an expansion chamber extending from said slot and having
an expansion angle of about 7.degree.-12.degree., an air balance
chamber, and means for air jet entrainment elimination.
18. A laboratory exhaust hood including in combination a hood
superstructure and an auxiliary air supply plenum, comprising:
a. a horizontal work surface, a rear wall, side walls, and a front
wall including an open vertical face immediately in front of said
work surface; and
b. auxiliary air supply means including means to provide auxiliary
air from a point above said open vertical face downwardly to
substantially all parts of said open face in a substantially
uniform laminar flow:
wherein a bottom air foil is provided in front of and immediately
above said horizontal work surface to define the bottom edge of
said open face and air conduit means are provided between said
auxiliary air supply means and said airfoil, including means to
entrain room air across said work surface under said airfoil.
19. A laboratory exhaust hood including in combination a hood
superstructure and an auxiliary air supply plenum, comprising:
a. a horizontal work surface, a rear wall, side walls, and a front
wall including an open vertical face immediately in front of said
work surface;
b. a sight-tight bypass within said front wall immediately above
said open face;
c. a movable, vertical closure sash located on tracks immediately
inside said front wall and operable to open such face and to close
said bypass when at the upper end of said tracks and to close said
face and to open said bypass when at the lower end; and
d. auxiliary air supply means including means to provide auxiliary
air substantially vertically downwardly at a point above said open
vertical face and in front of said bypass in substantially uniform
laminar flow;
wherein said auxiliary air supply means comprises an entry chamber,
a slot in one wall of said entry chamber having a width no more
than one-half the dimension of said entry chamber parallel to said
slot, and an expansion chamber extending from said slot.
20. A laboratory exhaust hood including in combination a hood
superstructure and an auxiliary air supply plenum, comprising:
a. a horizontal work surface, a rear wall, side walls, and a front
wall including an open vertical face immediately in front of said
work surface;
b. a sight-tight bypass within said front wall immediately above
said open face;
c. a movable, vertical closure sash located on tracks immediately
inside said front wall and operable to open such face and to close
said bypass when at the upper end of said tracks and to close said
face and to open said bypass when at the lower end; and
d. auxiliary air supply means including means to provide auxiliary
air substantially vertically downwardly at a point above said open
vertical face and in front of said bypass in substantially uniform
laminar flow:
wherein said auxiliary air supply means comprises:
exterior vertical front, rear and end walls, a cover and interior
baffles defining (1) an air entry chamber, (2) a slot at the top of
the plenum and said entry chamber, (3) an expansion chamber
extending vertically downwardly from said slot toward the interior
of the plenum, and (4) an air supply outlet; wherein said chambers,
slot and outlet extend substantially the length of said plenum, and
said outlet extends substantially across the width of said
plenum.
21. A laboratory exhaust hood including in combination a hood
superstructure and an auxiliary air supply plenum, comprising:
a. a horizontal work surface, a rear wall, side walls, and a front
wall including an open vertical face immediately in front of said
work surface;
b. a sight-tight bypass within said front wall immediately above
said open face;
c. a movable, vertical closure sash located on tracks immediately
inside said front wall and operable to open such face and to close
said bypass when at the upper end of said tracks and to close said
face and to open said bypass when at the lower end; and
d. auxiliary air supply means including means to provide auxiliary
air substantially vertically downwardly at a point above said open
vertical face and in front of said bypass in substantially uniform
laminar flow:
wherein a bottom air foil is provided in front of and immediately
above said horizontal work surface to define the bottom edge of
said open face, and air conduit means are provided between said
auxiliary air supply means and said airfoil, including means to
entrain room air across said work surface under said airfoil.
22. A laboratory exhaust hood including in combination a hood
superstructure and an air supply plenum, comprising:
a. a horizontal work surface, a rear wall, side walls, and a front
wall including an open vertical face immediately in front of said
work surface;
b. a sight-tight bypass within said front wall immediately above
said open face;
c. a movable, vertical closure sash located on tracks immediately
inside said front wall and operable to open such face and to close
said bypass when at the upper end of said tracks and to close said
face and to open said bypass when at the lower end; and
d. auxiliary air supply means including means to provide auxiliary
air substantially vertically downwardly through an outlet at a
point above said open vertical face and in front of said bypass in
substantially uniform laminar flow; said auxiliary air supply means
also including an air balance chamber provided with a perforated
back pressure plate and a shallow chamber between said air balance
chamber and said outlet;
wherein said back pressure plate is a fin-tube heat exchanger,
whereby auxiliary air may be heated between said air balance
chamber and said shallow chamber without affecting the
substantially uniform laminar flow of air from said air supply
outlet.
Description
BACKGROUND OF THE INVENTION
This invention relates to an improved laboratory hood for use in
air-conditioned laboratories in order to facilitate the handling of
radioactive isotopes or other hazardous chemicals, to prevent the
escape of dangerous toxic dust, vapors or gases into the
laboratory, and to provide safe, sanitary and healthy working
conditions for laboratory personnel. In particular, the present
laboratory exhaust hood represents a significant improvement in
operation and safety over conventional hoods and specially designed
hoods for hazardous use as described in the literature, for example
in U.S. Pat. Nos. 3,237,548 and 3,318,227, and in the references
referred to and cited in said patents.
The accepted method employed for controlling laboratory air
contamination and potential exposures of laboratory personnel to
toxic, hazardous or radioactive materials is the laboratory exhaust
hood. The main purpose of a laboratory hood is to confine air
contamination within the hood working area so that contaminant
concentrations in the air in the workers' breathing zone outside of
the hood face are well below the threshold limit values. This is
accomplished by exhausting air from the room and creating a flow of
a clean air past the worker into the hood. In order for an air flow
barrier to provide maximum effective worker protection from
hazardous hood contamination, the air velocity must be adequate and
uniform over the hood face with a velocity vector essentially
perpendicular to the plane of the hood face opening. These are the
fundamental concepts on which all acceptable laboratory hood design
and performance must be predicated. For conventional laboratory
hoods, therefore, our invention provides several features which
overcome interferences with the face control velocity. Of prime
importance is the reduction of interior hood turbulence, and
secondly, the reduction of or prevention of excessive face
velocities as the hood sash is lowered. This is achieved by
provision of a special bypass for air to enter the hood as the sash
is lowered with the bypass becoming operational before the velocity
of entering air reaches a value at which undesirable turbulence
occurs.
With the provision of greater numbers and increased sizes of
laboratory hoods, a marked increase in laboratory air changes has
resulted and has imposed serious additional costs for both heated
and in particular conditioned air. The need for a supplied or
auxiliary air hood to reduce the cost has been recognized for many
years. However, prior to our invention, the designs have been such
that the safety features which the hood was meant to provide have
been seriously reduced or impaired. A basic criterion for a supply
air design is that under normal operating conditions there be no
compromise with the requirements mentioned for good conventional
laboratory hoods.
In order to meet this criterion the auxiliary supply air must be
provided entirely outside of the hood face, and in no way interfere
with the control velocity required for containment of the
contaminant within the hood. It should also be designed so that if
the exhaust fan fails or exhaust air flow is reduced it would be at
least "no less safe" than a conventional hood experiencing the same
difficulty. Of course it should operate such that a sufficient
quantity of air could be supplied, entrained, and exhausted to make
its use economically feasible. In the course of a complete review
of the existing commercially available equipment, including those
described in the above patents, it was found that the designs were
such that potentially hazardous conditions could be expected with
their use.
A general objective of the present invention is to convert an air
supply stream to a uniform laminar flow pattern efficiently and
within a minimum space. More specifically, an objective is to
obtain an air bypass unit for laboratory hoods that assures
completely adequate hood face air velocities under all conditions
of hood face openings without excessive hood air velocities and
hood air turbulence. A further objective is an air supply device
(auxiliary air or makeup air unit for hoods or other needs) which
can reduce the amount of air removed from laboratories or other
areas (particularly, conditioned air).
Another objective is an auxiliary supply air device which can be
attached to most if not all good laboratory exhaust hoods, to
provide a uniform laminar (nonturbulent) flow of air downward and
initially parallel to the front of the laboratory hood. If the
amount of auxiliary air is 75 percent or less of the total hood
exhaust air quantity and the exhaust hood provides an essentially
uniform exhaust at its open face, another objective is that at
least 95 percent of the auxiliary air be captured by the hood and
enter the laboratory hood smoothly and without significant
turbulence. In accordance with the present invention, the
laboratory worker at the hood face is provided with smooth, clean,
low velocity, noncontaminated air past his head and down in front
of his work area, thus assuring a clean noncontaminated
environment. Contamination originating within the hood is not only
prevented from escaping the hood into the room by the hood exhaust
but it is also forced to remain in the hood by the positive supply
air barrier. This double protection and clean air supply provide
significantly improved protection from hood contamination losses
caused by high room air convection and poor hood location. The
remaining 25 percent, more or less, of air exhausted by the hood is
taken from the room in which the hood is located. This room air is
blended smoothly at the hood face resulting in a uniform air front
into the hood producing minimum air turbulence with no reverse air
flows.
A further object of the invention is to provide an air bypass
device which both provides safety for the operator and permits air
to bypass the hood face without either a significant pressure drop
or excessive air velocities and turbulence in the hood. If a bypass
of some type is not provided for hoods with vertical sliding
sashes, the air velocity at the open face of the hood increases
inversely proportionally to the size of the hood face opening as
the sash is lowered. High face velocities and excessive turbulence
are particularly undesirable for several reasons, including
contamination of chemicals, equipment, and samples being analyzed,
interference with burners and chemical reactions, and
uncontrollable loss of toxic or radioactive materials. Safety of
the operator is, of course, of paramount consideration. Therefore,
as it relates to the bypass, the objective of this invention is to
provide a bypass which offers protection for the operator without
introducing any substantial, undesirable resistance to the free
flow of air into the hood.
BRIEF DESCRIPTION AND ADVANTAGES OF THE INVENTION
The present invention provides an auxiliary air supply plenum
capable of providing air substantially vertically downwardly from
its outlet in substantially uniform laminar flow, towards the face
of a laboratory exhaust hood. Preferably, the auxiliary plenum is
mounted above the hood face, with a bypass located therebetween.
The auxiliary air supply plenum is operative to direct auxiliary
air into the hood face, beneath a movable closure sash, when it is
open, and above the sash through the bypass, as it is lowered and
closed. It is a feature of the present invention, that the total
volumetric flow of air into the hood superstructure, both room air
and auxiliary air, remains substantially constant during movement
of the hood sash between its open and closed positions. A further
feature of importance in the air supply means of this invention is
that the air supply is totally without prior contact with the
interior of the hood.
The auxiliary air supply plenum is of modular design suitable for
incorporation with conventional laboratory exhaust hoods, and in a
preferred embodiment includes walls and baffles defining, in
sequence following the flow of air from a supply conduit through
the plenum, an air entry chamber, a vertical slot at the top of the
entry chamber, an expansion chamber which extends downwardly and at
right angles to the slot, an air balance chamber, and an air supply
outlet. The device also includes an air vector controller between
the expansion chamber and the air balance chamber and a back
pressure plate and air jet entrainment eliminator between the air
balance chamber and the outlet.
Among the features of the auxiliary air supply is its adaptation
for efficient conversion of the relatively rapidly moving air
stream, in the supply conduit, to a slower moving stream of
substantially greater cross-sectional area having a substantially
uniform laminar flow free of non-uniform energy points within the
stream, at the outlet. Another feature is the reversal of general
flow direction between the entry chamber and expansion chamber
which permits accomplishment of the objective with a minimum of
height for the plenum. Still another feature relates to the
vertical slot between the entry and expansion chambers. The slot is
dimensioned relative to the cross sectional area of the entry
chamber to cause a general distribution of the air to the inlet end
of the expansion chamber. A feature relating to the expansion
chamber itself is that its walls are gauged to provide a generally
uniform adiabatic expansion for the air. In addition, a feature of
the combined elements downstream of the expansion chamber is the
provision of elements which in combination provide a slight but
important back-pressure, coupled with a greatly multiplied, highly
dispersed closely spaced substantially uniformly resistant air
paths. The end result is the virtual elimination of air jets and
air jet entrainment (air picked up by the jets and entrained with
them), such that the air flow from the auxiliary supply is
substantially laminar and free of non-uniform energy spots.
The laboratory exhaust hood combination of the present invention
includes a hood superstructure having a horizontal work surface,
rear, side and front walls, and an open vertical face together with
conventional air balancing baffles. A movable, vertical closure
sash is located on suitable tracks immediately inside the front
wall. A sight-tight bypass is provided immediately above the open
face, in such a position that as the movable sash is lowered to
close the face the bypass becomes open, and when the hood is
completely closed the bypass is fully open and receives
substantially all of the auxiliary air. In either case the
auxiliary air exits the air supply plenum substantially vertically
downwardly in substantially uniform laminar flow, whether it is
directed to the open face, to the bypass, or partly to both.
Among the advantages of the present bypass, as the sash is lowered
to close the hood, the maximum velocity provided to the face is
50-80 percent of that created by conventional laboratory hood
bypass arrangements, such as those referred to in the above
patents. Moreover, in the present device the operator is protected
since the bypass is sight proof and accordingly offers a
significant safety barrier between the inside of the hood and the
external work areas with sImultaneously impairing the uniformity of
air flow.
The use of auxiliary air supply plenum and bypass in combination of
this invention in a laboratory exhaust hood provides excellent
performance. The auxiliary air is substantially completely captured
by the hood and the air velocity profile at the hood face is
substantially uniform for all sash positions. The sash never
extends into the air supply, and consequently the auxiliary air
cannot be contaminated by either the interior of the superstructure
or the sash.
Beneath the closed sash, above the front edge of the hood work
surface, is a horizontal air foil of novel construction. The air
foil is provided with auxiliary air from the supply plenum by way
of a by-pass conduit. The air is blown under the air foil toward
the work surface where it entrains room air, by means of a
horizontal line of air jet orifices. Preferably at least 70-80
percent of the air passing over the work surface from under the air
foil is entrained room air, the remainder being the auxiliary
air.
The air inlet to the auxiliary supply plenum may be at the top,
side, back or front, making it readily adaptable to any supply duct
configuration. As the air passes through the plenum it is
efficiently and with a minimum of height reduced in energy and
distributed in a manner which provides uniform, nonturbulent
nonpulsating air to the hood.
Among the additional features of the auxiliary air supply plenum
are: (a) all of the air is provided outside the hood; (b) at least
95 percent of the air supply is efficiently captured by the hood,
due to the uniform laminar flow of air; (c) a steady supply of low
velocity, clean air in the zone of the operator is provided,
assuring clean air free from hazardous materials; (d) heating units
may be readily incorporated into the plenum without changing the
size or the shape or affecting uniform air delivery; (e) the plenum
may be modular and capable of ready installation on any good
laboratory hood without disturbance to exhaust ducts or other
possibly contaminated areas of the hood; and (f) the unit may be
readily adapted to receive supply air from the rear, front or
either side, as well as from the top as specifically described
herein.
The specific bypass employed in the combination of this invention
also obtains several combined advantages: (a) there is less air
turbulence in the hood than with comparable bypass systems; (b) the
bypass controls the increase in face velocity; (c) the pressure
loss through the bypass is extremely low, insuring substantially
constant air flow through the hood, even with the sash completely
closed; and (d) because the preferred bypass is contructed of
V-shaped louver vanes providing a deflected air path, an effective
safety barrier against the direct flight of material is obtained
.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front elevation in perspective of a laboratory exhaust
hood combination employing the auxiliary air supply plenum and
preferred bypass of the present invention shown in a typical fume
hood construction;
FIG. 2 is an enlarged cross sectional view in elevation of the
preferred auxiliary air supply plenum;
FIG. 3 is a cross section view of the plenum illustrated in FIG. 2,
taken along line 3--3 thereof;
FIG. 4 is an enlarged elevation view in cross section of the
V-shaped louver vanes which comprise a preferred bypass;
FIG. 5 is an enlarged cross-sectional view of the bottom air foil,
showing the entrainment of room air by the auxiliary air jets;
FIG. 6 is a view similar to FIG. 2, showing the air flow through
the air supply plenum during operation;
FIG. 7 is a view similar to FIG. 3, showing the air flow through
the air supply plenum during operation;
FIG. 8 is a cross sectional view in side elevation of an exhaust
hood employing the plenum of FIGS. 2-3 and the bypass of FIG. 4 in
combination with a hood superstructure, showing the direction of
air flow with the sash in fully open position, through the open
face and the bottom air foil, taken in section at the face
center;
FIG. 9 is a view similar to that of FIG. 8, showing the air flow
through both the bypass and the bottom air foil, with the sash in
fully closed position; and
FIG. 10 is a view similar to that of FIGS. 8 and 9, showing the air
flow through the bypass, the partially opened face and the bottom
air foil with the sash partly closed.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is broadly directed to apparatus for
converting rapidly moving air to slower moving air, including in
combination means for receiving the air stream, means for expanding
the air uniformly across its cross-section and means for
distributing the air uniformly from the receiving means to the
expansion means. The distributing means preferably is a elongated
slot of a sufficiently small cross-section to promote a spreading
of the air along the slot length.
The preferred embodiment of the present invention includes an
auxiliary air supply means, suitably of modular form, of novel
construction and operating capabilities. The auxiliary air supply
means is preferably employed in combination with a laboratory
exhaust hood superstructure, and particularly adapted to supply
auxiliary air to the hood. The invention also includes in that
combination a novel bypass construction, which permits both the
auxiliary and the room air to pass through the bypass without risk
of contamination because of prior contact with the hood interior,
and substantially uniform exhaust flow even as the hood sash is
lowered into closed position, together with shielding against the
direct flight of particles from within the hood.
The auxiliary air supply plenum is substantially rectangular in
construction, having an air inlet at or near the top and an
auxiliary air outlet at the bottom. The auxiliary air supplied from
the outlet is substantially uniform and laminar, and flows
substantially vertically downwardly from the plenum.
At the top of the plenum is an air entry chamber; along the top of
one side of which is a horizontally extending vertical slot.
Extending downwardly from the slot, and at substantially right
angles to it is an expansion chamber. Preferably, the relationship
between the air inlet, entry chamber, slot and expansion chamber is
such that: the inlet is spaced away from the slot and expansion
chamber; the vertical, cross-sectional area of the entry chamber is
at least twice the vertical cross-sectional area of the slot; said
slot area is less than about 1.5 times the horizontal
cross-sectional area of the inlet to the entry chamber; and the air
changes or reverses direction when passing through the entry
chamber and expansion chamber. The expansion chamber is preferably
at an expansion angle of about 9.degree., which serves to expand
the air received from the vertical slot adiabatically into an air
balance chamber at about the middle of the plenum. An angle of
between 7.degree. and 12.degree. will provide adiabatic expansion.
An air vector controller, suitably of "egg-crate" type
construction, is situated at the end of the expansion chamber, and,
together with a slight back pressure, assists in directing the
expanded air into vertical and uniform flow. Preferably, the
"pockets" of the air vector controller are rectangular or square,
and at least as deep as wide.
The air balance chamber extends substantially across the entire
width and length of the air supply plenum, and is hence
considerably larger in cross-section than the expansion chamber,
which in cross-section occupies between about 10 and 25 percent of
the plenum. Consequently, the air flow rate is considerably reduced
upon arrival at the air balance chamber, thus assisting attainment
of uniform, laminar flow.
Beneath the air balance chamber is a final air balance means, which
includes a perforated pressure plate, an air jet entrainment
eliminator, with a shallow chamber therebetween. Air flow through
the pressure plate is substantially laminar, which further assists
in the attainment of uniform laminar flow in a vertical direction
downwardly.
Preferably, the orientation and dimension of the expansion chamber
are such that the cross-sectional area of the expansion chamber
projected upon the perforated plate is at least 1.4 times the free
area of the plate perforations.
A unique feature of this combination is its ability to eliminate
localized non-uniformity of air energy across the auxiliary air
supply in a minimum of vertical space. This is assisted first by
changing or reversing the air flow direction between the air entry
chamber and the expansion chamber through the vertical slot.
Thereafter, the vector controller, balance chamber, pressure plate,
shallow chamber and air jet entrainment eliminator cumulatively
contribute to the desired result.
Preferably the air supply plenum includes immediately beneath the
air outlet, parabolic profile vanes, one on each end wall, which
serve to force the air somewhat inwardly from the two edges.
Preferably the air flow rate as it enters the open face of the hood
superstructure, is such that a plot of the air rate from edge to
edge across the open face would assume the curvature of a shallow
parabola, having a maximum at or near the center of the face, and
substantially equal minimums at each end. Such a profile is
desirable in order to avoid turbulence at the sides of the face and
work area, and to avoid loss of auxiliary air into the laboratory.
In the auxiliary air supply plenum, with the parabolic profile
vanes, it is possible under normal operating conditions for the
hood face to recover at least 95 percent of the air exiting the
plenum outlet.
The present auxiliary air supply plenum in modular form is readily
attached to laboratory exhaust hood superstructures, above and in
front of the open face. Thus it is possible to obtain the benefits
of the present invention, at least in part, without the expense of
an entirely new laboratory exhaust hood, including superstructure.
Preferably, however, the present auxiliary supply plenum is used in
combination with the hood superstructure herein described and the
novel bypass construction of the present invention. While the
preferred mode of operation includes directing the air vertically
downward and special benefits are derived therefrom, certain
advantages of the auxiliary supply of laminar air flow can be
obtained regardless of the direction of air travel. Therefore, as
it relates to the auxiliary air supply per se, it is a feature of
the auxiliary air supply that it can be employed without limitation
to any special orientation of the air flow direction.
The present bypass is located immediately above and in front of the
open face, in place of a portion of the front wall which exists at
that location in conventional exhaust hoods. As the sash is lowered
from its fully open position to its closed position, the bypass
becomes open to the interior of the hood superstructure and part of
the auxiliary air is bypassed in that direction above the sash. As
the sash is lowered even further and almost closed, more and more
of the auxiliary air passes through the bypass. The construction of
the auxiliary air supply plenum and bypass, and their location in
the present invention is such as to minimize the pressure drop of
the air when the sash is completely closed from the auxiliary
supply to the interior of the hood superstructure. The result,
therefore, is to maximize the flow of air to the exhaust when the
sash is closed, such that the rate of air flow is substantially
constant whether the sash is open, partly open or closed. One
feature of the bypass is that although it provides a sight-tight
labryinth, it does not require the air flow to change direction at
any one point sharply enough to impair the kaminar air flow or to
introduce local points of uneven energy in the air stream. Still
another feature is that the bypass directs the air upwardly into
the same path as that followed by air coming under the sash.
The invention will be better understood by reference to the
attached drawings, wherein like reference numerals indicate like or
corresponding supply 12. Located immediately belowthe auxiary r
supply 12 illustrate preferred embodiments of the invention, and
which demonstrate its advantages.
In FIG. 1 there is shown a laboratory exhaust hood 10, including a
hood superstructure 11 and an auxiliary air supply 12. Located
immediately below the auxiliary air supply 12 is bypass 13. Within
the superstructure 11 is work surface 14. The superstructure 11
includes rear wall 15, side walls 17 and 16, front wall 18, open
face 19, closure sash 20 and sash window 21, rear baffle 22 and
exhaust outlet 23. The auxiliary air supply 12 includes auxiliary
air inlet 24, triangular side baffles 25 and 26, and adjustable
vane 27.
FIGS. 2 and 3 show the details of auxiliary air supply 12,
including the above mentioned auxiliary air inlet 24, triangular
baffles 25 and 26, and adjustable vane 27. In addition, the
auxiliary air supply 12 includes a front wall 28, rear wall 29, end
walls 30 and 31, and cover 32. Immediately within the air inlet 24
is air entry chamber 33, bounded by rear wall 29, and baffles 34
and 35, baffle 35 including baffle reinforcement 35a. Air entering
inlet 24 surges into chamber 33 and reverses direction moving
upwardly towards slot 36, a substantially rectangular slot oriented
in a vertical plane bounded by the top of baffle 35, cover 32, and
end walls 30 and 31. Preferably, the air inlet 24 is as large as
practical, and it may be as long as the chamber 33, although it
should not extend over the top edge of baffle 35. In order to
maintain a low sound level, the minimum area of the inlet 24 is
one-third the width of plenum 12, times 0.24 times its length. The
orientation shown in the figures, with the air inlet 24 and entry
chamber 33 adjacent rear wall 29, may of course be reversed to
locate these elements in the front of plenum 12, adjacent front
wall 28 (as shown in the alternative embodiment shown in FIG. 9).
Also, air inlet 24 may be located in any position desired in the
top 32, or rear wall 29 (as shown in the alternative embodiment
shown in FIG. 10), to provide for air entry into chamber 33,
provided that the inlet 24 does not extend over the edge of baffle
35 forming slot 36. In this respect, the present air supply
provides the unique advantage of being able to be connected with an
air conduit at the top, side, front, or rear. Preferably, air inlet
24 is centered in the top of the air supply at either the rear
edge, as shown in FIGS. 2 and 3, or front edge thereof. The
cross-sectional area of the slot 36 has an important relationship
to the vertical cross section of entry chamber 33, such that the
air tends to spread along the full length of the slot rather than
channelling through the slot in one place. The chamber 33 is at
least twice as high as slot 36, and preferably at least three times
as high. The air passes through slot 36 substantially horizontally
and rapidly across the length of the air supply 12 into baffle 37
which, in combination with baffle 35, forms expansion chamber 38,
extending downwardly from and substantially perpendicular to slot
36. Each of the baffles 35 and 37 are angled at about 4.5.degree.,
to give expansion chamber 38 an expansion angle of about 9.degree.,
which is best for a slot 36 width of about 1.7 inches. It has been
determined that a total angle between about 7.degree. and
12.degree. is adequate for achieving the desired adiabatic
expansion. Preferably, a dividing baffle 39 is centered in
expansion chamber 38 in order to form two compartments, which is
particularly advantageous for exceptionally long expansion
chambers. Preferably the horizontal opening of expansion chamber
38, at the top of baffle 35, is the same length and width as
vertical slot 36, or in any event the area thereof is 0.8-1.2 that
of slot 36. In the embodiment shown, the expansion chamber 38 is as
long as the height of entry chamber 33, less the height of slot 36.
This arrangement permits the reversal of air flow from the entry
chamber 33 to expansion chamber 38 within a minimum height. FIG. 2
shows this arrangement with the elements to scale, in which the
entry chamber is 8-inches high, slot 36 is 15/8 inches high, and
expansion chamber 38 is 63/8 inches high, 15/8 inches wide at the
top and 23/4 inches wide at the bottom. At these dimensions, the
length of chamber 38 is about at its minimum, although it may be
longer if desired, with appropriate rearrangement or lengthening of
the elements. At the end of expansion chamber 38 is air vector
controller 40, preferably constructed of one-half inch cubed
plastic louvers, sometimes described as "egg crates." The depth of
the louver units is desirably at least equal to their width.
Air vector controller 40 is attached to the bottom of baffle 34 and
baffle 41, which combination forms the top of air balance chamber
42. Since air balance chamber 42 extends substantially across the
entire cross section of the air supply plenum 12, the air which is
rapidly passed through expansion chamber 38 quickly slows down upon
exiting through air vector controller 40, thereby achieving
considerable lowering in velocity, and a laminar air flow is
achieved within a minimum of height. Preferably, a desirable
relationship between the expansion chamber angle and dimensions and
the free area and location of the perforated plate 44 is
maintained, such that the following relationship exists:
kA.sub.f w = s + 2 (l + h) tan (.theta./2); wherein
k is a constant,
A.sub.f is the fraction of free area in perforated plate 44,
w is the width of plenum 12, or plate 44,
s is the width of the entry slot of chamber 38,
l is the length of expansion chamber 38,
h is the height of balance chamber 42, and
.theta. is the expansion angle of chamber 38.
Preferably, the constant k is at least 1:4; A.sub.f is about 0.19 -
0.25; w is about 12 inches, standard; s is about 1.4 - 1.8 inches;
l is about 6-8 inches; h is at least 0.5 times w; and .theta. is
7.degree.-12.degree., being inversely related to s. In the
illustrated embodiment; A.sub.f = 0.2, w = 12 inches s = 1.625
inches, 1 = 6.375 inches, h = 6 inches, .theta. = 9.degree., and k
= 1.5. The value of preferably should be such that the width of the
outlet of expansion chamber 38 is at least 0.9 (A.sub.f) (w). While
h is preferably at least 0.5 the value w), it may be less if
reduction in height of plenum 12 is a major consideration. The
value of s may be less than 1.5 inches, at a sacrifice of increased
plenum pressure and noise; if s is greater than 1.8, poor air
spreading may result, requring an increase to 1 or a decrease in
.theta. below 7.degree..
Beneath air balance chamber 42 is a final air balance means 43,
including back pressure plate 44 and air jet entrainment eliminator
45, defining a shallow chamber 46 therebetween. Pressure plate 44
may suitably be constructed of a perforated 1/8-inch board having
1/4-inch holes on 1/2-inch centers. Passage of the air through the
holes of pressure plate 44 is substantially uniform and laminar. In
the illustrated plenum, for 1/4-inch holes in plate 44 a Reynold's
number of 2140 is obtained, and for 9/32-inch holes, the value is
1,900, and hence laminar flow through plate 44 is achieved. The
depth of shallow chamber 46 is controlled by the diameter of the
perforations in plate 44, such that the air jets leaving the
perforations preferably entrain the air adjacent the closed
portions of plate 44 before arrival at the top of air jet
entrainment eliminator 45. Air exiting the holes of pressure plate
44 flows uniformly and without pulsation and expands and fills
shallow chamber 46, bounded downstream by air jet entrainment
eliminator 45, suitably of low resistant, all glass fiber
construction. This air jet entrainment eliminator blends any minor
non-uniformities or pulsating flow into a smooth air flow which is
discharged from the unit truly laminar, moving in a substantially,
non-pulsating, vertically, downward direction. The final air
balance means 43 may be constructed of radiator type, honeycomb
fins of high open area and very low closed area, of sufficient
depth to provide the same back pressure as the device shown. In
such an embodiment the need for an air jet entrainment eliminator
is obviated. Such an embodiment may be preferred where, for
example, it is desired to heat the air stream by means of such a
radiator.
The many steps involved in the present air supply plenum in
balancing the air and recovering air energies not only provide
better air balance and uniformity than possible with conventional
units, but this is also achieved at a low air noise level.
Beneath air supply outlet 47 are curved profile vanes 48 and 49,
which serve to reduce air velocity at the ends of the outlet and
increase the velocity at the center. As a result the flow-rate
profile into the open face of the exhaust hood superstructure
assumes a shallow parabola, having a maximum at or near the center,
and substantially equal minimums at each side. This type of air
profile is desirable since it minimizes losses of air supply to the
laboratory, and reduces the likelihood of turbulence at the sides
of the hood face. The profile vanes are suitably constructed from
50 percent free-area screens. At the bottom of the frontwall 28 of
the air supply 12, there is an adjustable vane 27 hinged to front
wall 28, and secured by adjustable engagement means 50. In general,
the adjustable vane 27 will be pointed at the lower closure point
of the sash, but will be adjusted inwardly or outwardly, by
engagement means 50, depending upon whether the auxiliary air is
respectively hotter or colder than the room air. It will also be
adjusted inwardly if there is excessive turbulence in the room
air.
The bypass device useful in the combination of the present
invention is shown in detail in FIG. 4 and in the present
combination in FIGS. 1, 8, 9 and 10. Bypass 13 is constructed of
V-shaped angled louver vanes 51, which define V-shaped labyrinth
passages 52. As air passes through bypass 13, an upward direction
upon the air is induced by the vanes 51, but with minimum pressure
drop. Bypass 13 also provides a sight-tight closure above the open
face 19. Our tests have shown that the vanes may define an angle of
120.degree. without introducing a significant pressure drop or
disturbing the laminar flow of the air, and that a sight-tight
labyrinth is provided by employing 2 inches wide, 20-gauge metal
strips space one-half inch on centers having 7/16 inch clearance
therebetween. This angle only re-directs the air flow through a
60.degree. angle. An angle of 50.degree.-70.degree. would be
feasible, with corresponding increase or decrease to the number of
vanes 51. It is possible to achieve virtually equivalent bypass air
flow by turning bypass 13 upside down, although such an orientation
would be incapable of holding any possible liquid splashes, as with
the illustrated bypass 13, and hence is less preferable.
FIG. 5 illustrates the details of a preferred horizontal air foil
providing auxiliary and entrained room air to the work surface 14.
The foil is comprised of horizontal plate 59, slanted plate 60 and
end plate 61. Air jets, shown as solid arrows in FIG. 5, emit from
entrainment air conduit 62 through air orifices 63. Conduit 62 is
connected via air inlet 64 and suitable hosing, not shown, to
outlet 56 (FIG. 2). The orifices 63 are positioned 4 inches in
front of edge 65 of work surface 14, and, at a flow rate of about
1.33 cfm per linear foot of conduit 62, are capable of entraining
up to 9.5 times as much room air. In order that at least 70 - 80
percent of the bottom air flow be room air, orifices 63 are
positioned at least 3.5 inches in front of front edge 65 of work
surface 14. Preferably, orifices 63 are 5/64-inch holes on 1/2-inch
centers across the length of conduit 62, and conduit 62 has a
cross-sectional area of at least 0.8 square inches. The initial
velocity of the air jets in the illustrated embodiment is about
2,000 fpm. The residual velocity of the air jets at front edge 65
is about 180 fpm in the illustrated foil 55, which offers a decided
improvement over prior foil jets, having a residual velocity of
about 1000 fpm, reflected in considerably decreased turbulence
above the work surface 14. The shape of conduit 62 is not critical,
as it may be round, trapezoidal, square or rectangular; similarly,
the distance of orifices 63 from slanted plate 60 is not critical,
but preferably they are centered. As can be seen by the schematic
arrows showing air flow, the air jets (solid) entrain room air
(dashed), and direct the resulting stream of air up to the opening
between plate 59 and work surface 14. Because the orifices 63 are
spaced a substantial distance from the front edge 65, the room air
is completely entrained and constitutes 90 percent or more of the
total air flow to the work surface 14.
FIGS. 6 and 7 show air flow through plenum 12 during operation. Air
enters inlet 24 very rapidly and is dispersed in chamber 33 across
its width and length (see dashed arrows, showing air movement
behind baffle 35, in FIG. 7). The air reverses direction and passes
up over baffle 35 through slot 36, and enters expansion chamber 38.
Upon leaving chamber 38, through air vector controller 40, the air
surges into balance chamber 42, with some entrainment and
turbulence, but with considerable decrease in velocity. Finally,
the air passes through plate 44 and air jet entrainment eliminator
45, and emerges from outlet 47 in uniform, laminar flow, and passes
by profile vanes 48 and 49, and adjustable vane 27, where the
uniform stream is directed towards the hood face.
FIGS. 8, 9, and 10 illustrate air flow of both the auxiliary air
and room air into the hood superstructure with the sash in various
positions. Closure sash 20 moves vertically up and down on closure
tracks 53. In these figures, the flow of supply air is shown by
solid arrows and the flow of room air is shown by dashed
arrows.
In normal operation as shown in FIG. 8, sash 20 is fully opened up
to the top of tracks 53. Auxiliary air exits supply outlet 47 in a
substantially vertical downward direction, and passes through open
face 19 of superstructure 11 into the hood interior above work
surface 14. Similarly, room air passes through open face 19. The
air passes into the hood uniformly and with minimum turbulence
guided by side edges 54 and bottom air foil 55. To a lesser extent,
air, both from the room and from auxiliary air provided through
bypass conduit 56 through inlet 64 and air jet orifices 63, enters
under air foil 55, with the jets serving the function of entraining
and assisting the movement of air into the hood at that point. The
air within superstructure 11 passes around rear baffle 22 and
baffles 57 and 58, leaving through exhaust outlet 23. When the sash
20 is completely closed, as shown in FIG. 9, air flow is
principally through bypass 13, with a small amount of air still
passing under the closed sash 20 and bottom air foil 55. As shown
by the arrows both auxiliary air and room air pass through bypass
13 into the interior of superstructure 11, being given an upward
thrust by vanes 51 during passage through V-shaped passages 52.
FIG. 10 illustrates the air paths during the process of closing
sash 20, while the sash is partly closed to an extent sufficient to
open bypass 13. It can be seen that a portion of the auxiliary air
passes through the upper part of bypass 13 into the hood
superstructure interior, while the bulk of the auxiliary air still
passes through the face 19 under sash 20. At this point the room
air mixes with the auxiliary air and enters the hood in increasing
proportion toward the lower end of the face. At the base, air from
both the auxiliary supply and from the room passes under air foil
55 into the interior.
In the embodiment illustrated in the figures, the auxiliary supply
is capable of delivering up to 75 percent of the exhaust air, at
face velocities of 50-150 fpm, or more. The flow is uniform and
laminar, and meets the most exacting design specifications, and the
objectives stated herein.
It will now be apparent that various modifications can be made to
the preferred embodiment without departing from the spirit of the
invention in its various aspects. For instance, certain of the
advantages of laminar flow provided by the auxiliary air supply can
also be employed in other uses. Therefore, in its broadest usage
the auxiliary air supply per se should not be limited to direction
of air flow. The same applies to the bypass which provides certain
advantages whether positioned above, below or to the sides.
In addition the combined air jet entrainment elimination function
of the elements downstream of the expansion chamber does not
require the precise components shown, although they accomplish the
objective in the most efficient manner. For instance, in an
apparatus designed to regulate the temperature of the auxiliary
air, a fin tube heat exchanger with extended honey-comb air
passages can be employed, and provided the walls of the passages
are sufficiently long to provide a significant back-pressure and
the air outlet openings on the downstream side are not separated by
large closed areas, such a heat exchanger can, by itself, provide
air jet entrainment elimination, thereby embodying the functions of
both the pressure plate and entrainment eliminator. Moreover, it is
possible to insert heating elements, such as calrod units, in the
air balance chamber, or in the air entry chamber, in the supply
plenum.
Thus, it is not our intention to confine the invention to the
precise form herein shown but to limit it only in terms of the
invention, as particularly and distinctly pointed out in the
appended claims.
* * * * *