Note: Descriptions are shown in the official language in which they were submitted.
2030~79
.
FIELD OF THE lNV ~:N'l'ION
This invention relates to a fume cabinet and to a
method of operating a fume cabinet.
BACRGROUND OF THE INVENTION
Fume cabinets are usually used to isolate
experiments or tests from the environment and from the
experimenter. In particular, they are usually used to
protect the experimenter from emissions produced by the
test process, to protect the experiment or test from
contamination by unwanted gases, particulates or bacteria,
and to protect the environment from the products of the
test process.
Conventional fume cabinets currently in use are
generally based on the "counterflow" principle. In such
cabinets the test or experiment is usually located in a
space which is enclosed except for a large front opening
to allow the experimenter access to the test or experiment.
Air is drawn into the cabinet through the front opening,
and the air flow into the cabinet is supposed to prevent
contaminants in the cabinet from travelling outwardly
through the front opening.
In such counterflow fume cabinets, the physical
mechanisms available for transport of cont~min~nt gasses
outwardly through the front opening are molecular and
turbulent diffusion. When the air flow into the front
opening is strictly l~min~r, only molecular diffusion
occurs, and calculations of molecular concentration sho~w~
~'
4 2030579
A that it falls off rapidly with upstream distance. With a
typical value of the binary diffusion coefficient, and an
airflow into the cabinet front opening of about one metre
per second, the cont~in~nt concentration may typically
5 decrease as much as six orders of magnitude in an upstream
distance of only one millimetre. Thus, it is easy in an
ideal l~in~r flow situation to ensure a negligible
concentration upstream of the plane of the cabinet front
opening (usually called the "face"). The net result is
10 similar for particulates, although the physical mechanism
for transport of particulates is quite different.
However the actual realization of the counterflow
principle in practical fume cabinets is far from ideal.
Typically there is a moveable sash at the top of the face
15 which partially obstructs the entry; the exhaust from
within the fume cabinet is from the top instead of from the
back; the air exterior to the cabinet is not quiescent but
normally is in motion; and the presence of an operator
near the face, and of apparatus inside the working space,
20 generate turbulent wakes which destroy the uniformity and
l~mi ~rity of the flow.
In the design of the best fume cabinets, great care
is taken, with a variety of flow control devices, to
achieve a uniform inlet velocity at the face in the absence
25 of an operator. The face velocity is the central feature
in most fume cabinet specifications and is typically about
0.5 metres per second. With such fume cabinets very low
cont~m;n~nt concentrations are achieved in practice outside
2030~79
the face under ideal conditions. However when conditions
become non-ideal, e.g. in the presence of a turbulent wake
produced by a manikin, the distance required between source
and measurement point to achieve a reduction in
concentration of six orders of magnitude is about 20
centimetres, as compared with 1 millimetre for ideal
laminar flow.
An even more serious non-ideal condition is external
air movement, which, if it exceeds 50 per cent of the face
velocity, can drastically reduce the containment of the
fume cabinet. Thus, cross flows at the face of the order
of about .25 metres per second are too large to be
tolerated by most conventional fume cabinets. However such
speeds can commonly be produced by personnel traffic,
ventilating flows, open doors and windows, and the like.
An entirely different approach to containment is the
air curtain principle. In this concept, "face velocity"
becomes irrelevant since cont~inment is based on the
property of the air curtain as a barrier to mass transport.
So far as is known, there are currently no fume cabinets
marketed using the air curtain principle. However a form
of such fume cabinet was described in German
Offenlegungrschrift 29 17 853 published November 6, 1980.
In this cabinet, a curtain of air is directed upwardly at
the face opening, to prevent cont~ in~nts inside the
cabinet from reaching the outside. As will be explained
later in this description, the applicant has determined
that the air flows used in the German document are
insufficient to prevent spill-back of cont~min~ted curtain
air into the room at the top of the face opening.
- 203057q
As will be explained, certain minimum exhaust air flows are
needed to provide reasonable assurance that the curtain
will not spill back such contA~inAted air. The minimum
flow needed is found, surprisingly, to be considerably more
than that which might have been expected. However it is
still less than that of many conventional counterflow fume
cabinets, and it provides better resistance to crosswinds.
The use of an air curtain to protect an operator
from harmful fumes while permitting the operator to have
access to a working space was also described in British
patent 1,582,438 published January 7, 1981 to Imperial
Chemical Industries Ltd. However in that patent, the air
Curtain together with noxious gases from the process are
removed via a flue, and there is no indication of the flows
required to prevent or reduce the likelihood of migration
of contaminants through the curtain. As will be discussed,
the ratio of exhaust to jet flows for a given range of
curtain jet height to thickness ratio is important in order
to improve the barrier properties of the curtain.
SUNMARY OF THE INVENTION
Accordingly, it is an object of the present
invention to provide an air curtain fume cabinet having a
working space, for isolating the air in a room in which
said fume cabinet is located from the air in said working
space, said fume cabinet comprising:
(a) a set of walls including upper and lower
walls, defining said working space,
A
203057~
~_ 7
(b) said walls further defining a face opening
which allows access to said working space from said
room,
(c) air jet supply means associated with said
lower wall for supplying an air curtain jet
extending across said face opening, said jet having
an outside interface between said jet and said room,
said outside interface separating said jet from said
room, said jet further having an inside interface
between said jet and said working space, said inside
interface separating said jet from said working
space, said jet entraining air into said outside and
inside interfaces from said room and said working
space respectively,
(d) exhaust means associated with the top of
said face opening for receiving said air curtain jet
and producing an exhaust flow,
(e) said exhaust means including means for
exhausting in said exhaust flow substantially (i)
the entire flow of said air curtain jet, plus (ii)
all of the air which said air curtain jet entrains
at least from said room into said outside interface,
plus (iii) a substantial quantity of additional air
from said room from a location in said room outside
said outside interface and adjacent the top of said
face opening, for causing air from said room, from
outside said outside interface, to flow into said
face opening and into said jet adjacent the top of
A
2030579
said face opening at a velocity which is greater
than the entrainment velocity of air from said room
into said outside interface that would normally be
produced by the action of said jet alone, for
reducing the likelihood of spillback of air from
said jet into said room from the top of said jet,
thereby to improve the resistance of said air
curtain jet to mass transfer thereacross in the
presence of disturbing cross winds.
In another aspect the invention provides a method of
providing an air curtain barrier across the face openlng of
a fume cabinet having a working space accessed through said
face opening, for isolating the air in a room in which said
fume cabinet is located from the air in said working space,
said method comprising directing an air curtain jet from
one side of said face opening across said face opening to
an opposing side thereof, said jet having an outside
interface between said jet and said room, said outside
interface separating said jet from said room, said jet
further having an inside interface between said jet and
said working space, said inside interface separating said
jet from said working space, said jet entraining air into
said outside and inside interfaces from said room and said
working space respectively, and providing an exhaust flow
at said opposing side to exhaust in said exhaust flow
substantially (i) the entire flow of said air curtain jet,
plus (ii) all of the air which said jet entrains at least
from said room into said outside interface, plus (iii) a
A:
t. ~'
~3~
8A
substantial quantity of additional air from said room from
a location in said room outside said outside interface and
adjacent said opposing side of said face opening, and
thereby causing air from said room, from outside said
outside interface, to flow into said face opening into said
jet adjacent said opposing side of said face opening at a
velocity greater than the entrainment velocity of air from
said room into said outside interface that would normally
be produced by the action of said jet alone, for reducing
the likelihood of spillback of air from said jet to outside
said face opening, thereby to improve the resistance of
said air curtain jet to mass transfer thereacross in the
presence of disturbing cross winds.
Further objects and advantages of the invention will
appear from the following description, taken together with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Fig. 1 is a side sectional view of a fume cabinet
according to the invention;
Fig. 2 is a front view of the cabinet of Fig. 1;
Fig. 3 is a side sectional view of a fume cabinet
similar to that of Fig. 1 but with the rear of the cabinet
not ventilated;
Fig. 4 is a diagram illustrating the air curtain
principle;
A
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-
Fig.5 is a diagram illustrating the structure of a
jet sheet;
Fig.6 is a diagram showing concentration profiles;
Fig.7 is a graph showing ratios of minimum exhaust
flow to curtain flow for attached flow;
Fig.8 shows air velocity in front of the air curtain,
plotted against height;
Fig.9 is a graph showing profiles of a specific test
gas concentration measured against horizontal distance from
the source, at two exhaust flows;
Fig.10 is a graph of test gas concentration versus
horizontal position;
Fig.11 is a graph showing the variation of test gas
(contaminant) concentration variation with side wind speed
for the Fig.l fume cabinet; and
Fig.12 is a graph similar to that of Fig.ll but for
the fume cabinet of Figs.2 and 3.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Reference is first made to Figs.l and 2, which show
a fume cabinet 8 according to the invention. As shown, the
fume cabinet includes a working space 10 defined by a lower
surface 12, side walls 14, a top 16 and a back 18. At the
front of the working space 10 there is a "face" or access
opening 20.
The lower surface 12 is defined by the top of a base
generally indicated at 22. The base 22 includes an air
inlet duct 24 which extends to the back of the base 22 (so
that the front portion of the base 22 can be used for
2030579
storage). The duct 24 then bends upwardly and then extends
forwardly and upwardly to an exit slot 26 which extends
across substantially the entire width of the face 20 at the
front of the lower surface 12. A secondary and smaller
duct 28 branches from the duct 24 and is directed to the
rear of the cabinet where it joins a smaller slot 30
extending across the rear of the lower surface 12.
Air is drawn into the duct 24 through air filters 31
by several (e.g. three) conventional fans 32, passes
through cleaning and flow smoothing screens 34, 36, and
exits through slots 26, 30. One or more plates 38 may be
placed parallel to the flow in slot 26 to smooth and direct
the flow.
A sash 40 extends downwardly from the front of the
top surface 16 to control the size of the face or opening
20. The sash 40 is moveable up and down in conventional
fashion (by means not shown) to allow adjustment to the
height of opening 20. The sash 40 has an outwardly and
upwardly turned lip 42 for a purpose to be described.
Just inside the sash 40, at the front of the opening
10, is a wide exhaust duct 44. Duct 44 has an intake slot
46 which extends across substantially the entire width of
the working space 10 and which has a substantial front to
rear dimension. The rear wall 48 of the duct 44 is formed
as a double wall having sheets 48a, 48b joined by a smooth
curve 48c, for a purpose to be described. Exhaust air is
drawn from the exhaust duct 44 by an exhaust fan 50.
If desired, the rear ventilation of the cabinet can
2030~79
11
be omitted by eliminating secondary duct 28 and slot 30.
This arrangement is shown in Fig.3, in which primed
reference numerals indicate parts corresponding to those of
Figs.l and 2.
It will be seen that slot 26 slants rearwardly. This
is because the air curtain issuing from slot 46 is wider at
its top than at its bottom, and the arrangement shown is
convenient to have exhaust duct 44 swallow the entire
curtain, including all the air which it entrains at least
at its front, as will be explained. However the rearward
slant is not necessary since the curtain will bend to
accomodate itself to the flows Q~ and Qll (which flows will
be described).
The operation of the Figs. 1 to 3 fume cabinets will
best be understood from the following description.
Reference is first made to Fig.4, which illustrates the air
curtain principle. Fig.4 diagrammatically depicts duct 24,
slot 26, and duct 44 with its intake slot 46. In Fig.2 the
following symbols are used:
Qj represents the air curtain jet flow supplied
through slot 26 by fan 32.
Qex represents the exhaust flow drawn by exhaust fan
50.
Q5 represents the flow from a contaminant source S.
Qen1 represents the air flow entrained into the jet
from outside the space 10.
Qen2 represents the air flow entrained into the jet
from inside the space 10.
20~057~
12
Q ' and Q 'l represent air flows drawn into the exhaust
at the top of the opening 20, from inside and outside the
space 10 respectively, for the situation where the flow
exhausted Qex is greater than that required simply to
swallow the jet Q and its entrained air.
The above flows may be expressed in any appropriate
units, e.g. cubic feet per minute (cfm) or liters per
minute (l/m) or cubic meters per hour (m3/h)
The exhaust flow is then
Qex Qi + Qex1 + Qex2 + Q + Q + Qs ( l)
As indicated, equation (1) allows for more air (Q l
and Qll ) to be exhausted than is required simply to swallow
the jet Q; and its entrained air. As will be explained, Qex
must be large enough so that Ql is greater than zero, if no
15 curtain air is to be spilled back into the face 20.
Some of the properties of an ideal jet sheet are
illustrated in Fig.5, which shows the jet of Fig.4 in more
detail. Fig.5 shows a lArin~r jet sheet 52 of thickness t
issuing from slot 26 into still air with a uniform initial
20 velocity v;. AB and A'B' are the dividing streamlines, i.e.
the average streamlines that contain the original jet flow.
Since the original jet flow is Qj, thus the flow contained
between the two lines AB and A'B' is Qj at all distances x
measured above the bottom surface 12. Thus:
Qj = Vj t w (2)
where w is the width of the jet sheet 52. The dividing
stream lines AB and A'B' have a precise mathematical
definition and can be identified experimentally.
-- 2030S79
13
The lines AC and A'C' are the edges of the overall
jet 54 and are not as well defined. The spaces between
lines AB and AC, and between A'B' and A'C', contain the air
entrained into the jet from each side of the jet. The
entrainment process is primarily turbulent in nature. From
some distance away, the jet can be perceived as a sheet
sink, drawing air inwardly, the inwardly drawn air having
a velocity vector approximately perpendicular to the jet
axis (as shown in Fig.4). The jet edge (i.e. lines AC and
A'C') can be defined as the location at which the x-
component of velocity becomes appreciable. The jet edge
can be approximately located with smoke or tufts.
If the entrainment velocity is ven, and the entrained
incremental flow is qen (volume/unit time/unit x) from each
side, and if Q(x) denotes the total jet flow at station x,
then:
dQ(x) = 2q (3)
and qen = ven N (4)
As shown in Fig.5, when the issuing jet 52 is
l~mi n~r and uniform, there is a transition zone 56,
typically about 3t in length, during which the uniform
velocity v; is eroded from both sides, as shown at 58 in
Fig.5. Beyond the transition zone 56 a cosine-squared sort
of profile, indicated at 60, is reached in the fully
developed flow.
An estimate of the amount of air entrained can be
obtained from data given in a text entitled "The Theory of
2030579
14
Turbulent Jets" by G.N. Abramovich, MIT Press, 1963,
Library of Congress CAT. No.63-21743. If Qen is the total
entrainment from one side between the exit of the jet from
slot 26 and station x, then from the information given in
the above Abramovich reference it can be deduced that:
for x/t ~ 4-5l Qen/Qi = 036(x/t) ( )
for x/t > 7 ~ Qen/Q; 5( 53 ~ - 1) (6)
and Q(x) = Qj + 2 Qen ( )
where Q(x) is the total flow (jet plus entrained air) at
station x.
It will be seen from equations (6) and (7) that
at x/t = 15, Q(x)/Qj = 2. Thus, it will be seen that
entrainment generates a large increase in the total flow in
the jet. The actual entrainment velocity can be estimated
as follows.
From equations (7) and (3)
dQen
dx = qen (8)
and from equations (5) (6) and (2)
for x/t < 4.5, dQen/dx = .036 Qj/t = .036vjw (9)
for x/t > 7, dQen/dx = .133 Qj/t (x/t)-~ = .133vjw (x/t)-~ (10)
By equating (8) to (9) and (10) in turn, and using
5 (4) we get
for x/t < 4.5 ven/vj = .036 (~)
for x/t > 7 ven/vj = .133 (x/t) (~2)
Thus, the entrainment velocity is estimated as being
about one thirtieth of the original jet velocity near the
203~579
jet exit, and diminishing with distance from the exit.
The mass transfer characteristic of the described air
curtain is illustrated in Fig.6. Assume that on the right
hand of the jet 54 the concentration of a species S is
maintained at CO~ that the region to the far left has
concentration C = 0, and that the air in the jet issuing
from the slot is also free of species S. The concentration
profile will then be qualitatively as shown as 62 in Fig.6,
falling from concentration C0 on the right to essentially
zero at a line AP. At locations above P, the concentration
to the left hand side of the jet is greater than zero and
is governed there by the entrainment velocity ven and by the
counterflow principle. For example if the original jet
velocity Vj is about three metres per second, and x/t at P
is 10, then the entrainment velocity ven is about 0.05
metres per second, about 1/10 of the usual face velocity.
The fall off of concentration in upstream diffusion is
proportional to the stream velocity, so the distance for a
decrease of six orders of magnitude in a 0.05 metre per
second stream may typically be 2 centimeters instead of 1
millimeter. While this appears to be a deterioration in
performance, it will be realized that in actual use,
laminar diffusion results are not representative. In
regions such as the wake of an operator, an increase in the
mean flow velocity external to the wake would result in an
increase in the turbulent velocities and an expected
increase in forward diffusion of the contaminant.
The performance of the fume cabinet shown in Figs.1
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16
and 2 will now be discussed in more detail. In the Figs.
1 and 2 cabinet, the jet 52 will issue from the slot 26,
travel up the face 20, and will with its entrained air
enter the exhaust slot 46 from which it is removed by
exhaust fan 50. The air entrained into the jet 52 from
inside the working space 10 is replaced by the auxiliary
air flow issuing from duct 28 through slot 30. Assume that
this auxiliary air flow is Qa Also assume that the flow of
contAminAnt into the working space 10 from a contAminAnt
source S is Q5-
Then the average concentration Co of contaminant in
the working space is
(13)
C = Q
-~
Equation 13 will be valid provided that there is no
recirculation of the curtain air into the cavity, i.e.
provided that there is no spill back of air from the
curtain into the cavity. This requires that Q~ be greater
than or equal to zero or that the auxiliary flow
Qa 2 Qen2 (14)
For the example x/t=15, equation (14) yields:
Qa 2 .5Qj.
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17
With the minimum value of the auxiliary flow Qa/ the
average concentration in the working space is then
2Q5
Qj
With a jet flow of, for example, 200 cfm (5660 1/m),
and a contaminant flow Q5 = 4 l/m (a typical representative
test condition), then the concentration of contaminant from
source S in the working space 10 is
CO = 2x 4 = .00141 = 1410 ppm.
5660
The calculation of 1410 ppm applies when just
sufficient air is supplied in the auxiliary jet from slot
30 to replace the air entrained in the jet from the working
space 10, so as to avoid spill back.
In the Fig.3 arrangement, where no auxiliary air is
supplied to the working space 10, the flow Q" of Fig.4 is
zero and the inner dividing streamline attaches at its
upper end to the inner lip 48C of the exhaust duct 44. All
the air entrained by the lower portion of the curtain is
then spilled back at the top of the curtain into the
working space 10 (since the air removed from the working
space 10 must be replaced). This sets up a vigorous
recirculating flow or vortex in the working space 10, in
which the concentration of species or cont~in~nt S builds
up to relatively high values. An equilibrium value is
attained when the rate of diffusion of species S past the
dividing streamline is equal to Q5 (i.e. the flow of species
S out of space 10 equals the flow of species S into space
2030S79
18
10). However despite the relatively high internal
concentration, this arrangement was shown by experiment to
provide satisfactory containment, although not as good as
that achieved by the Figs.l and 2 arrangement.
The resistance of the curtain to disturbing air cross
currents of speed vd in the room will now be discussed. In
such consideration, the governing parameter is the
disturbance velocity vd divided by the jet velocity, i.e.
vd/vj. One would expect serious interference with the
containment to occur at or above a critical value of this
ratio. Since the jet velocity v; diminishes with height
above the exit slot 26, and this reduction itself depends
on x/t, i.e. on the jet slot width, then the critical ratio
vd/vj will also depend on the jet width. The applicant's
experiments have shown that both the height of the face
opening 20, and the exhaust flow Qex~ are important
parameters in fixing the critical ratio vd/vj at which
containment disruption occurs. Thus, once the design value
of the disturbance velocity vd is chosen, the design value
of the jet velocity v; will follow, and so in turn will jet
flow Qj r the auxiliary flow Qa ~ and the exhaust flow Qex .
Experiments were carried out to establish the general
character of the flow field and to determine the ratio QexiQi
that would ensure smooth continuous inflow at the lip 42 at
the top of face opening 20 in the absence of any disturbing
cross flows. In other words, the objective was to see
whether observations agreed with the previously described
theory concerning what ratio of exhaust flow Qex to jet flow
20305~9
19
Q; was needed to prevent spillback to the outside at the
top of the air curtain. In the experiments lip 42 formed
part of a vertically movable sash (as is conventional for
fume cabinets) so that the height of the face opening 20
can be adjusted. The jet thickness (i.e. the front to back
dimension of slot 26) was varied, and the ratio of Qex/Qj
needed to prevent spillback to the outside of lip 42 was
observed, using tufts of fibre attached to the bottom of
lip 42. The results are shown in Fig.7 for a face opening
of 26 inches. The measured results are indicated by curve
72 and are much higher than the estimates of Q(x)/Qj
obtained from equations (5), (6) and (7), which are
indicated by curve 74 for comparison in Fig.7.
The reason why the actual exhaust flow needed to
prevent spillback to the outside is much higher than the
theoretical exhaust flow needed, is believed to be as
follows. The theoretical or calculated flow is simply the
exhaust flow needed to swallow the jet, plus the air
entrained into the jet from outside the working space 10,
all on a time averaged basis. However in fact the jet
produces some turbulence, and the turbulence produces
momentary localized flow reversals. To prevent these
reversals, a substantially higher exhaust flow is needed
than that necessary simply to swallow the jet and the air
entrained into the jet from outside the working space 10.
Thus, a substantially higher exhaust flow than would
otherwise be necessary, is required to ensure smooth
continuous inflow at lip 42 from outside the face opening
2030~79
20. This was in the absence of disturbing cross-flows. As
will be shown, if there are disturbing cross-flows, then an
even higher exhaust flow Qex will be helpful in preventing
spill back in the presence of such cross-flows.
Fig.8 illustrates the impact on velocity distribution
when an exhaust flow Qex f the magnitude indicated by curve
72 of Fig.7 is used. To produce Fig.8, the velocity of the
air inflow into the curtain or jet 54 from outside, was
measured at the centre of the face opening 20, just in
front of the curtain, and at varying heights above the
lower surface 12. The resulting curve is shown at 80 in
Fig.8 and is plotted for a three inch thick air curtain
- (i.e. slot 26 was 3 inches thick). A jet flow of 230 cfm
was used, and the average value of v; was 4.97 feet per
second at the exit slot 26. The exhaust flow Qex was 550
cfm so Qex/Qi = 2-4-
From equation 11 one would expect an entrainment
velocity of about .18 feet per second (fps) near the bottom
of the jet, and this velocity is shown in dotted lines at
82 in Fig.8. The actual measured velocity is indeed of
this order of magnitude at the lower portion of the
curtain, but increases to much larger values as the top of
the opening 20 is approached even though equation (12)
shows that the entrainment decreases with height. The
higher flow velocities near the top of the curtain are
produced because the exhaust flow Qex in the example given
is substantially larger than that needed merely to swallow
the jet flow Q; and to swallow the air outside face 20 which
would normally be entrained by the jet flow. In effect,
there is substantial extra flow Q ' (Fig.4) at the top of
2030579
21
the face opening 20. The extra flow Q~ ~ which may in a
sense be considered to be a "line sink" (since it is
relatively small in vertical dimension) is responsible for
the higher velocities there, and is highly beneficial in
5 controlling both the concentration of contaminants at the
outside of the face opening 20, and the resistance of the
air curtain to cross drafts.
The beneficial effect of the extra flow Ql on
concentration distribution is illustrated in Fig.9. For
10 Fig.9 a "cont~min~nt" source of helium was provided with a
flow of 1 cfm. The jet velocity Q; was 230 cfm and the jet
thickness was 2 inches. The helium source was located
approximately 12 inches inside the working space 10 as
measured from the left side of the slot 26, and was 1/2
15 inch above lower surface 12. In Fig.9 horizontal distance
is plotted on the horizontal axis, with the origin or zero
being at the left side of slot 26. Positive distances are
measured inside the work space 10, and negative distances
are distances to the left of the working space (as drawn),
20 i.e. outside the face 20. The vertical axis shows the
height in inches above the lower surface 12.
In Fig.9, curve 90 shows the shape of a low
concentration contour (14 ppm of helium) when
Qex was 440 cfm and Qe/Qi has a value of 1.9. Curve 92
25 shows the 14 ppm helium concentration contour when Qex was
550 cfm and Qex/Qi has a value of 2-4-
It will be noted that curve 90 (Qe/Qi = 1. 9 ) is atabout the lower limit for acceptable flow, and that any
lower ratio would result in too much cont~min~nt migrating
30 past the face. However when the ratio Qex/Qj is 2.4, the 14
2030579
22
ppm helium concentration profile 92 stays well inside the
face or opening 20. Thus the effect of increasing the
exhaust flow Qex in reducing concentration at the face is
seen from Fig.9 to be quite dramatic.
Fig.10 is a plot made by moving a helium
concentration measuring probe through the curtain at a
height 13 inches above the lower surface 12, for the air
curtain used for Fig.9 and with the exhaust flow Qex = 550
cfm. In Fig.10, again horizontal distance from the left
side of slot 26 is shown on the horizontal axis, as in
Fig.9. Helium concentration in parts per million is shown
on the y axis. It will be seen from curve 96 that as
expected, the helium concentration near the face was very
low. This indicated that with the ratio Qex/Qi = 2.4, little
or no helium was migrating across the curtain.
Figs. 11 and 12 illustrate the benefits on resistance
to cross flows of having the ratio Qex/Qi substantially
greater than the theoretically calculated ratio (based on
average flows needed to ensure no spillback to the outside
of the curtain). To produce Figs. 11 and 12, SF6 was used
as a test or cont~in~nt source gas. In both Figs. 11 and
12 the cross wind speed is shown in feet per minute on the
horizontal axis, and the cont~in~nt concentration in ppm
on the y axis. Fig.11 shows results for the Fig.3 version
of the invention (no auxiliary ventilation of the working
space 10), with an exhaust flow Qex = 600 cfm and a jet flow
Q; = 230 cfm (Qex/Qi = 2.6). Curve 100 shows the result with
a face opening of height 27 inches, and curve 102 shows the
2030~79
23
result when the face opening was 21 inches. The
concentration was measured where the face of a person would
be, using the ASHRAE standard for reporting. For Fig. 11
the measurements were taken without a manikin, but where
5 the manikin's face would be located, i.e. about 2 inches
outside the curtain and at the height of the manikin's
face.
It will be seen that with zero cross wind, the
cont~m;n~nt concentration at the manikin's face was
10 measured as being .018 ppm. This level can be achieved by
a conventional fume cabinet under ideal conditions. As the
velocity of the cross wind increased, the cont~rin~nt level
increased only very slightly, until the cross wind velocity
reached 110 fpm. Then, at a face opening height of 27
15 inches, a very large increase in cont~min~nt concentration
at the manikin's face occurred, as indicated by curve 100.
However, when the face height was reduced to 21 inches
(curve 102), a cross wind of 120 fpm (the limit of the test
equipment used) was unable to produce any breakdown in the
20 curtain. The cont~min~nt concentration at the manikin's
face remained very low.
An even better result appears from Fig. 12. The Fig.
12 measurements were taken using a manikin, and using the
Figs. 1 and 2 arrangement, i.e. the working space was
25 ventilated with auxiliary air from duct 28. In Fig. 12,
two curves 110, 112 were plotted, both for a face opening
height of 27 inches. For curve 110 the exhaust flow Qex was
500 cfm, and for curve 112 Qex was 700 cfm. In both cases,
2030S79
24
the jet flow was Q; = 230 cfm, so Qex/Qi was 2.2 for curve
110 and was 3 for curve 112. The auxiliary flow Qa was
sufficient to replace air entrained into the jet from
inside space 10 and was approximately 110 cfm.
In the absence of crossflow, an exhaust flow Qex of
500 cfm produced a cont~m;n~nt level at the face of the
manikin of .012 ppm. When the exhaust flow Qex was increased
to 700 cfm, the contaminant level at the face of the
manikin fell to .005 ppm, which is very low.
When the cross wind velocity increased to 90 fpm, the
contAmin~nt level increased substantially for curve 100
(i.e. for Qex = 500 cfm). However, for Qex = 700 cfm (curve
112), a cross wind velocity of more than 120 fpm (the limit
of the apparatus used) failed to produce any increase in
the contArin~nt concentration at the location of a
manikin's face. It will be seen that with sufficient
exhaust flow, Qex the device is extraordinarily resistant to
disruption by cross winds.
Thus in summary, it is important that the exhaust
flow Qex be sufficient to swallow not only the jet and the
air which would normally be entrained by it, but also to
swallow some additional air, to produce higher entrainment
velocities at the top of the face that would normally occur
by reason of the jet alone. The ratio Qex/Qi ~ for the ratio
curtain height to jet thickness x/t up to approximately 30,
is preferably between 2 and 3, and preferably between 2.4
and 3. Where the curtain is higher (x/t > 30) or where
cross winds may be particularly severe, the ratio Qex/Qi can
2030~79
be greater than 3, but if it is too high, more air will be
exhausted (which must be cleared and which carries room
heat) than is needed. However it is noted that an exhaust
flow of 700 cfm is relatively low as compared with that
used in a conventional counterflow fume cabinet, where the
exhaust flows are typically in the region 1000 to 1200 cfm.
The invention will particularly be appreciated by
comparison with that shown in German Offenlegungschrift 29
17 853 (supra), and particularly Fig. 6 thereof. The German
document shows an air curtain fume hood having an air
curtain jet of flow Qj = 100 m3/h. There is also direct air
and gas injection of 100 m3/h, of which 6 m3/H is air for a
burner which is supplied with a flammable gas at the rate
of 1 m3/h. The air curtain is shown as entraining 100 m3/h
from outside the working space and 50 m3/h from inside the
working space. An additional boosting flow of 80 m3/h is
added at the top of the air curtain and total exhaust flow
from the top of the air curtain is shown as 330 m3/h. From
the rear of the working space, 50 m3/h is separately
exhausted.
By scaling Figs. 2 and 12 of the drawings (which are
dimensioned), it was determined that the width of the jet
exit slit (corresponding to slit 26 in the applicant's
disclosure) is about 4 mm. Since the face opening is given
(Fig. 7) as .9 m, thus the ratio of the curtain is
x/t = 900/4 = 225
By contrast, the applicant's ratio x/t is typically
about 15.
2030579
26
Using data from the Abramovich reference (supra), the
entrainment into each side of a jet having x/t equal to 225
is :
_ = 1/2 (.530~ x/t - 1)
Qj
For a jet flow of Qj = 100 m3h and x/t = 225, this
yields:
Qen = 348 m3/h.
In other words, an air curtain of the height shown
would try to ingest or entrain 348 m3h of air from each
side. The air (100 m3/h) shown as being entrained in the jet
from outside is far less than that needed to provide the
air curtain with the air it needs, and the exhaust flow is
also far less than that required to exhaust this volume of
air. The consequence is a spillback of cont~r;n~ted curtain
air into the room at the top of the opening.
By contrast, the applicant's arrangement ingests
significantly more air through the face than the above
theoretically calculated entrainment, in order to help
ensure smooth continuous inflow at the lip 42 despite
momentary localized flow reversals caused by occasional
intermittent bursts of turbulence.
It is important that the exhaust fan 50 always be on
when the inlet fan 32 is on. Therefore, if desired a
conventional interlock can be provided, to ensure that if
the exhaust fan 50 is not on, then the inlet fan 32 cannot
be on.
Normally the flow provided by the exhaust fan 50
2030S7~
should be between 2 and 3 times that provided by the inlet
fan 32 for flow Qj (as discussed). If desired, and to
ensure that failure of the exhaust system cannot create an
unsafe operating condition, monitoring devices (not shown)
can be provided in conventional manner to monitor the flows
and to shut off the curtain fans 32 if the exhaust fan 50
is unable to provide the required ratio of flows.
Alternatively, both fans can be on a single shaft operated
by a single motor, as shown in the German document,
although additional duct work would be required in such an
arrangement. In addition, such an arrangement would not
deal with the possibility that the exhaust duct may become
partly obstructed.
Additionally, it is within the state of the art to
provide a sensor attached to the moveable sash, which can
be used to control either or both of the exhaust and
curtain flows, in order to maintain them at the magnitudes
and in the ratio appropriate to the sash opening.
It will be realized that the fume cabinet of the
invention may be supplied without its own exhaust fan and
may instead be connected to the building or laboratory
exhaust fan. In that case, the air flow required for the
fume cabinet exhaust will of course be specified so that
the necessary exhaust flow is achieved.
While a preferred embodiment of the invention has
been described, it will be appreciated that modifications
and other embodiments may be used, and all are within the
scope of the appended claims.
~1
