Note: Descriptions are shown in the official language in which they were submitted.
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VACUUM PRESSURE SEAL
The present invention relates to gas exhaust
fans and is primarily concerned with vacuum pressure
seals adapted to prevent gas leakage from such fans.
It is common for centrifugal gas exhaust fans
to allow gas to escape through the space between the
casing of the fan and the impeller shaft. This is due in
part to the fact that it is difficult to create an
airtight seal between the stationary fan casing and the
rotating shaft.
While the leakage of gas from a centrifugal gas
exhaust fan is not always a concern, such fans are often
used for the extraction from a certain area of noxious
gases. Where such a fan is used to move noxious gases,
it is important that leakage from the fan be minimized.
A number of devices have been designed to draw
gas which has leaked out of the rear of a fan back to the
inside of the fan. U. S. Patents 3,927,889 and
3,927,890, both issued to Maurice L. Adams, Jr. on
December 23, 1975, disclose such a method. In these
patents, the leaked gas is returned to the interior of
the fan by means of an oil pump driven by a motor
separate from the fan motor. Clearly, this design
involves a certain expense as well as some design
challenges involved in incorporating a pump and motor in
the fan for such a task.
U. S. Patent 5,156,522, issued to Lynn P.
Tessier on October 20, l992, discloses another method of
impeding the escape of gases through the rear of a
centrifugal gas exhaust fan at the shaft. This method
does not require the incorporation of any additional
mechanical equipment as in the Adams patents. A tube is
used to create a fluid connection between the outlet of
the fan and the chamber at the rear of the fan into which
gas leaks. When the fan is operating normally, the
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pressure of the gas at the outlet is higher than the
pressure at the rear of the fan. The high-pressure
outlet gas is, therefore, drawn into the tube towards the
leaked gas in the chamber at the rear of the fan. The
movement of the high-pressure gas encourages the leaked
gas to return to the interior of the fan by retracing the
path through which it leaked in the first place.
Unfortunately, according to the Tessier patent, such a
method of encouraging gases into the interior of a fan
can cause problems of gas turbulence when the fan is run
at low speeds. The solution proposed by the Tessier
patent involves the addition of a cup shaped shroud over
the area through which the leaked gas is returned. This
shroud deflects the gas returning to the interior of the
fan along the radial direction of the impeller. The
shroud is fixed to the impeller by a retaining nut.
Though less elaborate than the additions required by the
Adams patents, this patent does call for the addition of
parts to the interior of the fan.
It would be difficult to modify an existing
centrifugal gas exhaust fan to accept the addition of the
parts necessary to incorporate the improvements of either
the Adams patents or the Tessier patent unless the fan
had been originally designed with such a modification in
mind.
Accordingly, there is a need for an improved
vacuum pressure seal which avoids the disadvantages
described above.
The invention comprises a vacuum pressure seal
for attachment to the exterior of the rear of a fan
casing through which passes the shaft of the fan. The
vacuum pressure seal defines a vacuum pressure chamber
which is placed so as to cover an existing conventional
seal between the rear of the fan casing and the shaft.
The vacuum pressure chamber is placed in fluid
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communication with the inlet of the fan. The gas
pressure in the vacuum pressure chamber is approximately
equal to the gas pressure at the inlet of the fan.
Because of the action of the fan, which moves gas from
the inlet through the interior near the impeller and then
to the outlet, the gas pressure at the inlet is lower
than that surrounding the impeller and the atmospheric
pressure. The pressure difference between the vacuum
pressure chamber at the rear of the fan and the inlet of
the fan is such that exhaust gas which leaks from the fan
casing into the vacuum pressure chamber is drawn from the
chamber through the fluid communication means and into
the inlet for re-circulation through the fan.
In accordance with the invention there is
provided, in a gas exhaust fan comprising (i) an
impeller, (ii) a shaft adapted to be attached to and to
rotate the impeller, (iii) a casing adapted to surround
the impeller and allow it to rotate, the casing having
openings for fluid communication with an inlet and an
outlet and to allow the impeller or its shaft to extend
to the exterior of the casing, (iv) a conventional seal
adapted for attachment to the casing and for rotating
attachment to the impeller or the shaft, and (v) the
inlet; the improvement comprising a vacuum pressure seal
defining a vacuum pressure chamber and adapted for
attachment to the conventional seal and for surrounding
the impeller or the shaft, and means for fluid
communication between the vacuum pressure chamber and the
inlet.
An advantage of the present invention is that
gas leakage from the fan is minimized.
Another advantage of the present invention is
that it can be readily incorporated into an existing fan.
Another advantage of the present invention is
that it can be incorporated without complex redesign of
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the fan and without the introduction of mechanical
devices.
The invention will be better understood by way
of the following detailed description of a preferred
embodiment with reference to the appended drawings, in
which:
Fig. 1 is a rear elevation of a centrifugal gas
exhaust fan mounted on a base and having incorporated
therewith a vacuum pressure seal and tubes in accordance
with the preferred embodiment of the present invention;
Fig. 2 is an axial cross-sectional view of a
centrifugal gas exhaust fan mounted on a base and having
incorporated therewith a vacuum pressure seal and tubes
in accordance with the preferred embodiment of the
present invention taken along line 2-2 of Fig. 1;
Fig. 3 is an enlarged rear elevation of a
vacuum pressure seal in accordance with the preferred
embodiment of the present invention; and
Fig. 4 is an axial, enlarged, fragmentary
cross-sectional view of a vacuum pressure seal in
accordance with the present invention taken along line 4
4 of Fig. 3, shown adjacent a centrifugal gas exhaust fan
impeller having attached thereto a shaft.
Referring to Figs. 1 and 2, there is shown a
centrifugal gas exhaust fan 10 having a casing 12 and
defining an outlet 14 and an inlet 16. A rear fan
support plate 18 is attached to the casing 12 by means of
bolts 20.
Within the casing 12, the fan 10 comprises an
impeller 22 having fixed thereto an impeller hub 24 which
in turn is fixed to a shaft 26. The shaft 26, the
impeller hub 24, and the impeller 22 are fixed to one
another by means of a bolt 28.
The frame supporting the casing 12 of the fan
10 comprises a rear frame base element 34, a front frame
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base element 36, and side frame base elements 38. The
basE~ elements support front frame supports 40 and rear
frame supports 42a and 42b. A bearing support bracket 44
is mounted horizontally to the rear frame supports 42a
and 42b. Bearing 32 is bolted to the bearing support
brar_ket 44.
The fan casing 12 is further supported with the
inlet 16 to the front frame supports 40 by means of bolts
46.
A vacuum pressure seal 50, in accordance with
an embodiment of the present invention, is shown attached
to the rear fan support plate 18 by means of bolts 52. A
vacuum pressure chamber 60 is shown surrounding the shaft
26. The vacuum pressure seal 50 comprises a rectangular
plate 54, a circular plate 56, and a circumferential
plate 58, a11 of which collectively define a vacuum
pressure chamber 60. The rectangular plate 54 is shown
mounted to the rear fan support plate 18 and has an
opening for the impeller hub 24. An outer seal 61 is
placed in between the rear fan support plate 18 and the
rectangular plate 54 of the vacuum pressure seal 50. The
circular plate 56 has an opening to accommodate the shaft
26. The openings in plates 54 and 56 have very close
tolerances with the shaft 26 and hub 24 in order to
minimize the air leakage into the pressure seal 50. The
circumferential plate 58 seals the vacuum pressure
chamber 60 by joining the rectangular plate 54 and the
circular plate 56.
A pair of tubes 62a and 62b are connected at
one end to spigots 64a and 64b on circumferential plate
58. The other end of each tube 62a and 62b is fixed to
the inlet 16 by means of a spigot 66. The tubes 62a, 62b
are preferably made of PVC but can be of any suitable
material. These tubes 62a, 62b allow fluid communication
between the vacuum pressure chamber 60 and the inlet 16.
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Though Fig. 1 shows two tubes, the number of such tubes
could be one or more without departing from the scope of
the present invention.
Fig. 3 shows the vacuum pressure seal 50
enlarged and before it is mounted to the casing 12,
including a rectangular front plate 54 and a circular
rear_ plate 56. The rectangular plate 54 includes holes
68 for passage of bolts 52 for mounting the vacuum
pressure seal 50 on a centrifugal gas exhaust fan. Both
the rectangular plate 54 and the circular plate 56 also
include aligned openings 70 for passage of the shaft 26.
Though the openings 70 are shown as being the same
diameter, they could be a different size in the case
where the impeller hub 24 extends past the rectangular
plate 54 when it is mounted as shown in Fig. 2. Also
shown in Fig. 3 are spigots 64a, 64b including ribbed
portions 72 for secure attachment of tubes 62a and 62b
respectively.
Referring now to Fig. 4, there is shown the
vacuum pressure seal 50 taken along line 4-4 of Fig. 3
and mounted to a plate 74 which in turn is mounted to the
casing 12. Between the plate 74 and the casing 12, there
is shown the seal 21 and the mounting plate 18. Arrows
indicate the direction of flow of gas leaking from the
rear- of the fan when the vacuum pressure seal 50 is
operational. The arrows also indicate the path of the
gases from the vacuum pressure chamber 60 through spigots
64a and the tubes 62a.
When the centrifugal gas exhaust fan 10 is in
operation, the impeller 22 pumps gases from the inlet 16
and through the outlet 14. Because of the funnel shape
of the inlet 16, the speed of the air passing through the
inlet 16 increases with increasing proximity to the
impeller 22. Another effect of the shape of the inlet 16
is 'that the static pressure of the gas therein at the
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pos_Ltion of the spigot 66 is relatively low as compared
to i:he gas pressure adjacent the impeller 22.
Since the gas which leaks from the rear of the
fan is at approximately the pressure of the gas adj acent
to the impeller 22, the gas pressure in the vacuum
pressure chamber 60 is higher than the gas pressure at
the spigot 66 in the inlet 16 at the position of spigot
66 whenever the fan is in operation. Therefore, when the
tube 62a or 62b is connected to the vacuum pressure
chamber 60 and to the inlet 16, allowing fluid
communication between the two, gas in the vacuum pressure
chamber 60 tends to be drawn through the tube 62 to the
inlet 16.
In order to control the flow of gas through the
tubE~ 62, a conventional regulating valve (not shown) may
be added at some point thereon.
It should be understood that the configuration
of the centrifugal gas exhaust fan 10 and the vacuum
pressure seal 50 could be varied without departing from
the scope of the invention.
For example, the impeller hub 24 attached to
the impeller 22 may extend to the exterior of the casing
12 of the fan 10, as shown in Fig. 2. In such a case,
any conventional seals used to inhibit the path of the
leakage of gas from the interior of the fan would be
attached to the casing 12 and in rotating relation with
the impeller hub 24. However, another arrangement is
shown in Fig. 4, whereby the shaft 26 extends through the
opening in the rear of the fan and the impeller hub 24
does not extend to the exterior of the fan. In this
case, any conventional seals which are similarly attached
to the casing 12 should be in rotating relation with the
shaft 26.
A similar variation is possible in respect of
the vacuum pressure chamber 60, wherein the impeller hub
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24 may be arranged such that it extends to the exterior
of the rear of the fan but does not extend to the
exterior of the vacuum pressure chamber 60, all as shown
in Fig. 2. Another possible arrangement would involve
the impeller hub 24 extending beyond the plate 56 of the
vacuum pressure chamber 60. Such an embodiment could be
employed without departing from the scope of the
invention.
It should also be understood that the
configuration of the inlet 16 need not involve a portion
thereof having a gradually reduced cross-section with
increasing proximity to the impeller 22, as shown in Fig.
2. The gas static pressure in the inlet 16 during
operation of the fan would remain below the gas pressure
adjacent the impeller 22 and therefore in the vacuum
pre:>sure chamber 60 even without this feature, albeit the
pressure difference would be reduced.
In addition, it should be understood that
either the inner seal 30 or the outer seal 61 or both
could be omitted from the fan without departing from the
scope of the present invention. However, in order for
the vacuum pressure seal described herein to operate
efficiently, it is preferable that there be some
conventional seal between the fan casing 12 and either
the shaft 26 or the impeller hub 24, as the case may be,
to reduce the amount of leaked gas with which the vacuum
pre:~sure chamber 60 is required to deal.
It should also be understood that the
front plate 54 of the vacuum pressure seal 50 may have a
shape other than rectangular without departing from the
scope of the invention. Similarly, the rear plate 56 of
the vacuum pressure seal 50 may have a shape other than
circular without departing from the scope of the
invention.
