Abstrict An air curtain fume cabinet, in which an air curtain jet is directed
across the face opening to an exhaust duct. Sufficient flow is exhausted
at the exhaust duct to swallow (i) the entire air curtain jet, plus
(ii) all of the air which the jet entrains from outside the face
opening, plus (iii) a substantial additional amount of air. This
greatly increases the velocity of air moving into the curtain at
its top, beyond the normal entrainment velocity, and prevents spill-back
of jet air to the outside even with substantial crosswinds. Preferably
the ratio of exhaust flow to jet flow is between 2 and 3 for a jet
height to jet thickness ratio of up to about 15. Preferably auxiliary
air is supplied to the working space interior to replace air entrained
into the jet from inside the working space.
Claims I claim:
1. An air curtain fume cabinet comprising:
(a) a set of walls including upper and lower walls, defining a
working space,
(b) said walls further defining a face opening which allows access
to said working space,
(c) air jet supply means associated with said lower wall for supplying
an air curtain jet extending across said face opening and lengthwise
to the top of said face opening,
(d) exhaust means associated with the top of said face opening
for receiving said air curtain jet,
(e) said exhaust means including means for exhausting 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 outside said
face opening, plus (iii) a substantial quantity of additional air
from outside said face opening, thus to increase the velocity of
air from outside said face opening moving into said jet adjacent
the top of said face opening beyond the entrainment velocity that
would normally be produced by the action of said jet alone, thereby
to reduce the likelihood of spillback of air from said jet into
the space outside said working space from the top of said jet, and
thereby to improve the resistance of said air curtain jet to mass
transfer thereacross in the presence of disturbing cross winds.
2. Apparatus according to claim 1 and including auxiliary air flow
means for supplying auxiliary air into said working space to replace
air entrained into said jet from inside said working space.
3. Apparatus according to claim 2 wherein said walls include a
rear wall at the rear of said working space and said auxiliary air
flow means includes a slot adjacent said rear wall and lower wall
to introduce auxiliary air into said working space at the lower
rear corner thereof.
4. Apparatus according to claim 2 wherein the ratio of the flow
exhausted by said exhaust means to the flow of said jet (Q.sub.ex
/Q.sub.j) is at least 2 where the ratio of the height of said face
opening to the thickness of said jet is not greater than about 30.
5. Apparatus according to claim 4 wherein said ratio Q.sub.ex /Q.sub.j
is between 2 and 3.
6. Apparatus according to claim 4 wherein said ratio Q.sub.ex /Q.sub.j
is between 2.4 and 3.
7. Apparatus according to claims 1, 2 or 3 and including a smoothly
outwardly and upwardly turned lip at the top of said face opening.
8. Apparatus according to claims 1, 2 or 3 and including a smoothly
outwardly and upwardly turned lip at the top of said face opening,
and wherein said exhaust means includes an exhaust duct extending
downwardly into said working space from said upper wall, said exhaust
duct having a rear duct wall, said rear duct wall having a smoothly
curved lip at its lower end to guide air smoothly into said exhaust
duct from inside said working space.
9. A method of providing an air curtain barrier across the face
opening of a fume cabinet having a working space accessed through
said face opening, said method comprising directing an air curtain
jet from one side of said face opening across said face opening
to an opposing side thereof, and providing an exhaust flow at said
opposing side to exhaust substantially (i) the entire flow of said
air curtain jet, plus (ii) all of the air which said jet entrains
from outside said face opening, plus (iii) a substantial quantity
of additional air from outside said face opening, thus to increase
the velocity of air from outside said face opening moving into said
jet from outside said face opening adjacent said opposing side of
said face opening beyond the entrainment velocity that would normally
be produced by the action of said jet alone, thereby to reduce the
likelihood of spillback of air from said jet to outside said face
opening.
10. A method according to claim 9 and including the step of providing
a flow of auxiliary air into said working space to replace air entrained
into said jet from inside said working space.
11. A method according to claim 10 wherein, when the ratio of jet
length to jet thickness is not greater than about 30, the ratio
of said exhaust flow to the flow of said jet (Q.sub.ex /Q.sub.j
is at least 2.
12. A method according to claim 11 wherein said ratio Q.sub.ex
/Q.sub.j is between 2 and 3.
13. A method according to claim 11 wherein said ratio Q.sub.ex
/Q.sub.j is between 2.4 and 3.
14. A method according to any of claims 9 to 13 wherein said one
side of said face opening is the bottom of said face opening and
said opposing side of said face opening is the upper side of said
face opening. Description FIELD OF THE INVENTION
This invention relates to a fume cabinet and to a method of operating
a fume cabinet.
BACKGROUND 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 contaminant gasses outwardly through the front
opening are molecular and turbulent diffusion. When the air flow
into the front opening is strictly laminar, only molecular diffusion
occurs, and calculations of molecular concentration show 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 contaminant concentration
may typically decrease as much as six orders of magnitude in an
upstream distance of only one millimeter. Thus, it is easy in an
ideal laminar flow situation to ensure a negligible concentration
upstream of the plane of the cabinet front opening (usually called
the "face"). The net result is 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 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, generate
turbulent wakes which destroy the uniformity and laminarity 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 of an operator. The face velocity is
the central feature in most fume cabinet specifications and is typically
about 0.5 meters per second. With such fume cabinets very low contaminant
concentrations are achieved in practice outside 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 centimeters,
as compared with 1 millimeter 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 0.25 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 containment 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 Nov. 6, 1980. In this cabinet, a curtain of
air is directed upwardly at the face opening, to prevent contaminants
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 contaminated curtain air into the room at the top
of the face opening.
As will be explained, certain minimum exhaust air flows are needed
to provide reasonable assurance that the curtain will not spill
back such contaminated 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 Jan. 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.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide
a fume cabinet having an air curtain arranged to provide improved
isolation between its working space and outside. In one of its aspects
the present invention provides an air curtain fume cabinet comprising:
(a) a set of walls including upper and lower walls, defining a
working space,
b) said walls further defining a face opening which allows access
to said working space,
(c) air jet supply means associated with said lower wall for supplying
an air curtain jet extending across said face opening and lengthwise
to the top of said face opening,
(d) exhaust means associated with the top of said face opening
for receiving said air curtain jet,
(e) said exhaust means including means for exhausting 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 outside said
face opening, plus (iii) a substantial quantity of additional air
from outside said face opening, thus to increase the velocity of
air from outside said face opening moving into said jet adjacent
the top of said face opening beyond the entrainment velocity that
would normally be produced by the action of said jet alone, thereby
to reduce the likelihood of spillback of air from said jet into
the space outside said working space from the top of said jet, and
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 opening of a fume cabinet
having a working space accessed through said face opening, said
method comprising directing an air curtain jet from one side of
said face opening across said face opening to an opposing side thereof,
and providing an exhaust flow at said opposing side to exhaust substantially
(i) the entire flow of said air curtain jet, plus (ii) all of the
air which said jet entrains from outside said face opening, plus
(iii) a substantial quantity of additional air from outside said
face opening, thus to increase the velocity of air from outside
said face opening moving into said jet from outside said face opening
adjacent said opposing side of said face opening beyond the entrainment
velocity that would normally be produced by the action of said jet
alone, thereby to reduce the likelihood of spillback of air from
said jet to outside said face opening.
Further objects and advantages of the invention will appear from
the following description, taken together with the accompanying
drawings.
BRIEF DESCRIPTION OF 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;
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. 1 fume
cabinet; and
FIG. 12 is a graph similar to that of FIG. 11 but for the fume
cabinet of FIG. 3.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Reference is first made to FIGS. 1 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 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 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. 1 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 Q" (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:
Q.sub.j represents the air curtain jet flow supplied through slot
26 by fan 32.
Q.sub.ex represents the exhaust flow drawn by exhaust fan 50.
Q.sub.S represents the flow from a contaminant source S.
Q.sub.en1 represents the air flow entrained into the jet from outside
the space 10.
Q.sub.en2 represents the air flow entrained into the jet from inside
the space 10.
Q' and Q" 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 Q.sub.ex 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 (m.sup.3 /h)
The exhaust flow is then
As indicated, equation (1) allows for more air (Q' and Q")
to be exhausted than is required simply to swallow the jet Q.sub.j
and its entrained air. As will be explained, Q.sub.ex must be large
enough so that Q' is greater than zero, if no 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 laminar jet sheet 52 of thickness t issuing from slot 26 into
still air with a uniform initial velocity v.sub.j. 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 Q.sub.j, thus
the flow contained between the two lines AB and A'B' is Q.sub.j
at all distances x measured above the bottom surface 12. Thus:
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.
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 v.sub.en, and the entrained incremental
flow is q.sub.en, (volume/unit time/unit x) from each side, and
if Q(x) denotes the total jet flow at station x, then: ##EQU1##
As shown in FIG. 5, when the issuing jet 52 is laminar and uniform,
there is a transition zone 56, typically about 3t in length, during
which the uniform velocity v.sub.j 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 Turbulent Jets"
by G. N. Abramovich, MIT Press, 1963, Library of Congress CAT. No.63-21743.
If Q.sub.en 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:
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)/Q.sub.j
=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) ##EQU3## and from equations (5) (6)
and (2)
By equating (8) to (9) and (10) in turn, and using (4) we get
Thus, the entrainment velocity is estimated as being about one
thirtieth of the original jet velocity near the 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 C.sub.O, 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 C.sub.O 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 v.sub.en and by the counterflow principle.
For example if the original jet velocity v.sub.j is about three
metres per second, and x/t at P is 10, then the entrainment velocity
v.sub.en 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 meter 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 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 Q.sub.a. Also assume that the flow of contaminant into
the working space 10 from a contaminant source S is Q.sub.s.
Then the average concentration Co of contaminant in the working
space is ##EQU4##
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
For the example x/t=15, equation (14) yields:
With the minimum value of the auxiliary flow Q.sub.a, the average
concentration in the working space is then ##EQU5##
With a jet flow of, for example, 200 cfm (5660 l/m), and a contaminant
flow Q.sub.s =4 l/m (a typical representative test condition), then
the concentration of contaminant from source S in the working space
10 is ##EQU6##
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 contaminant 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 Q.sub.s (i.e. the flow
of species S out of space 10 equals the flow of species S into space
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.
1 and 2 arrangement.
The resistance of the curtain to disturbing air cross currents
of speed v.sub.d in the room will now be discussed. In such consideration,
the governing parameter is the disturbance velocity v.sub.d divided
by the jet velocity, i.e. v.sub.d /v.sub.j. One would expect serious
interference with the containment to occur at or above a critical
value of this ratio. Since the jet velocity v.sub.j 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 v.sub.d
/v.sub.j 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 Q.sub.ex, are important parameters in fixing the critical
ratio v.sub.d /v.sub.j at which containment disruption occurs. Thus,
once the design value of the disturbance velocity v.sub.d is chosen,
the design value of the jet velocity v.sub.j will follow, and so
in turn will jet flow Q.sub.j, the auxiliary flow Q.sub.a, and the
exhaust flow Q.sub.ex.
Experiments were carried out to establish the general character
of the flow field and to determine the ratio Q.sub.ex /Q.sub.j 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 Q.sub.ex to jet flow Q.sub.j 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 Q.sub.ex /Q.sub.j 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)/Q.sub.j 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 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 Q.sub.ex 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 Q.sub.ex of 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.sub.j was 4.97 feet per second at the
exit slot 26. The exhaust flow Q.sub.ex was 550 cfm so Q.sub.ex
/Q.sub.j =2.4.
From equation 11 one would expect an entrainment velocity of about
0.18 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
Q.sub.ex in the example given is substantially larger than that
needed merely to swallow the jet flow Q.sub.j 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 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 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 Q' on concentration distribution
is illustrated in FIG. 9. For FIG. 9 a "contaminant" source
of helium was provided with a flow of 1 cfm. The jet velocity Q.sub.j
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 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), 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 Q.sub.ex was 440 cfm and Q.sub.e /Q.sub.j
has a value of 1.9. Curve 92 shows the 14 ppm helium concentration
contour when Q.sub.ex was 550 cfm and Q.sub.ex /Q.sub.j has a value
of 2.4.
It will be noted that curve 90 (Q.sub.e /Q.sub.j =1.9) is at about
the lower limit for acceptable flow, and that any lower ratio would
result in too much contaminant migrating past the face. However
when the ratio Q.sub.ex /Q.sub.j is 2.4, the 14 ppm helium concentration
profile 92 stays well inside the face or opening 20. Thus the effect
of increasing the exhaust flow Q.sub.ex 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 Q.sub.ex =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 Q.sub.ex /Q.sub.j =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 Q.sub.ex /Q.sub.j 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 contaminant source
gas. In both FIGS. 11 and 12 the cross wind speed is shown in feet
per minute on the horizontal axis, and the contaminant 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 Q.sub.ex =600 cfm and a jet flow Q.sub.j
=230 cfm (Q.sub.ex /Q.sub.j =2.6). Curve 100 shows the result with
a face opening of height 27 inches, and curve 102 shows the 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 the manikin's face would be located, i.e. about 2 inches
outside the curtain and a the height of the manikin's face.
It will be seen that with zero cross wind, the contaminant concentration
at the manikin's face was measured as being 0.018 ppm. This level
can be achieved by a conventional fume cabinet under ideal conditions.
As the velocity of the cross wind increased, the contaminant level
increased only very slightly, until the cross wind velocity reached
110 fpm. Then, at a face opening height of 27 inches, a very large
increase in contaminant 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 curtain. The contaminant 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 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 Q.sub.ex
was 500 cfm, and for curve 112 Q.sub.ex was 700 cfm. In both cases,
the jet flow was Q.sub.j =230 cfm, so Q.sub.ex /Q.sub.j was 2.2
for curve 110 and was 3 for curve 112. The auxiliary flow Q.sub.a
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 Q.sub.ex of 500 cfm
produced a contaminant level at the face of the manikin of 0.012
ppm. When the exhaust flow Q.sub.ex was increased to 700 cfm, the
contaminant level at the face of the manikin fell to 0.005 ppm,
which is very low.
When the cross wind velocity increased to 90 fpm, the contaminant
level increased substantially for curve 100 (i.e. for Q.sub.ex =500
cfm). However, for Q.sub.ex =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 contaminant concentration at the location
of a manikin's face. It will be .seen that with sufficient exhaust
flow, Q.sub.ex the device is extraordinarily resistant to disruption
by cross winds.
Thus in summary, it is important that the exhaust flow Q.sub.ex
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
Q.sub.ex /Q.sub.j, 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 Q.sub.ex
/Q.sub.j can 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 Q.sub.j =100 m.sup.3 /h. There
is also direct air and gas injection of 100 m.sup.3 /h, of which
6 m.sup.3 /H is air for a burner which is supplied with a flammable
gas at the rate of 1 m.sup.3 /h. The air curtain is shown as entraining
100 m.sup.3 /h from outside the working space and 50 m.sup.3 /h
from inside the working space. An additional .boosting flow of 80
m.sup.3 /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 m.sup.3 /h.
From the rear of the working space, 50 m.sup.3 /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 0.9 m, thus the ratio of the curtain
is
By contrast, the applicant's ratio x/t is typically about 15.
Using data from the Abramovich reference (supra), the entrainment
into each side of a jet having x/t equal to 225 is: ##EQU7##
For a jet flow of Q.sub.j =100 m.sup.3 h and x/t=225, this yields:
In other words, an air curtain of the height shown would try to
ingest or entrain 348 m.sup.3 h of air from each side. The air (100
m.sup.3 /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
contaminated 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 should be between
2 and 3 times that provided by the inlet fan 32 for flow Q.sub.j
(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.
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