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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to glass coated, separable impellers for use
in closed, glass lined vessels to mix and agitate corrosive, adhesive,
abrasive or easily contaminated ingredients such as acids, polymers,
pharmaceuticals, dyes, food stuffs and the like.
Closed, glass lined vessels as used by the chemical, pharmaceutical or food
industry may range in size to several thousand gallons and in the case of
reactors, may be able to withstand relatively high internal pressures. In
such vessels, it is desirable to have as small an access opening as
possible to increase the integrity of the vessel and reduce the overall
cost of vessel construction.
Since impellers used in these vessels may have an effective diameter or
span equal to 40% to 60% of the vessel diameter the problem is how to
provide such a vessel with an impeller of this size while limiting access
openings to the size of manholes.
In the prior art this is accomplished by one of several means. For example,
the impeller could have a split hub wherein the separate hub pieces, each
with an impeller blade or two, could pass easily through a manhole size
opening. The split hub would then be assembled to the end of the agitator
drive shaft within the vessel by bolting or clamping the split hub pieces
to the shaft. This is not entirely satisfactory because the fasteners used
may loosen and those made of reactive metals to resist corrosion are
frequently weak and always expensive. Also, protruding bolts or clamps
provide places where product can accumulate as when mixing polymers.
Another way is to provide several hubs each with one or two impeller
blades. These pass easily through a manhole opening and are stacked on the
impeller drive shaft to provide a balanced three or four bladed impeller.
This latter method is generally described in U.S. Pat. Nos. 2,811,339 and
3,494,708. While such construction has the advantage of the exposed
surfaces being glass coated, the surface finishes and mechanical means
needed to provide a gasketless, leak-proof connection are difficult to
manufacture economically.
The present invention avoids the problems of split hubs or of the need to
stack hubs by providing an impeller of unitary construction having four
blades extending from a single hub in a unique, balanced arrangement which
permits the impeller to pass through a manhole size opening even though
the span of the unitary impeller is substantially greater than any manhole
diameter.
SUMMARY OF THE INVENTION
The present invention may be characterized in one aspect thereof by the
provision of a glass coated impeller for a separable blade agitator, the
impeller being of one piece construction including a hub and four impeller
blades extending from the hub, the effective diameter of the impeller
being substantially larger than a manhole size opening in the vessel. The
four impeller blades are grouped in two pairs fixed to and extending from
generally opposite sides of the hub to provide a balanced structure,
wherein the included angle between the impeller blades of a pair is about
60.degree. and the included angle between adjacent impeller blades of each
pair is about 120.degree..
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view of a closed pressure vessel partly broken away and in
section showing an agitator having the four-bladed, one piece impeller of
the present invention,
FIG. 2 is a plan view of the impeller of the present invention;
FIG. 3 is a elevation view of the impeller; and
FIG. 4 shows how the impeller being passed through a manhole size opening.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, FIG. 1 shows the separable, one piece
four-bladed impeller generally indicated at 10 attached to the end of a
drive shaft 18 within a closed pressure vessel 12. The vessel itself may
be briefly described as a standard reactor used in the chemical industry
for carrying out various reactions involving corrosive, adhesive,
pharmaceutical or abrasive material. Such a vessel may also be used to
process various food stuffs. These vessels ordinarily would be able to
withstand relatively high internal pressures, have a capacity of several
thousand gallons and would have their internal surfaces provided with an
easily cleaned corrosion and abrasion resistant coating such as glass or
the like. To simplify the description, the coating will be described
herein as being "glass" but it should be understood that this term is to
include any of the vitreous, partly devitrified ceramic or enamel coatings
well known in the art as being applied to vessels, agitators and baffles
of the type described herein.
These vessels would have one or more top located openings including an
opening 14 for a baffle or temperature probe, an opening 16 to accommodate
the passage of the agitator drive shaft 18 and a manhole opening 20.
Typically the manhole opening may range in size from a 12.times.14 inch
elipse to a 24 inch round opening. A drain or other clean out opening 22
could be provided adjacent at the bottom of the vessel. It should be
appreciated that all of these openings can be sealed to retain the
internal pressure, the seal between opening 16 and shaft 18 being a rotary
seal (not shown) to permit some drive means outside of the vessel to
rotate agitator shaft 18 and impeller 10 for purposes of mixing the
contents of the vessel.
As the effective diameter of an impeller 10 is preferably 40 to 60% of the
vessel diameter, the problem is how to carry an impeller which may have a
span of 30 to 50 inches into the vessel through manhole opening 20 which
may be no larger than 24 inches in diameter. As set out above, this is
accomplished in the prior art by having one or two blades on each part of
a split hub or on separate hubs which are then stacked one above the other
on the shaft.
As shown in FIGS. 1 and 2, however, impeller 10 of the present invention
has a single hub 24. Joined to this hub so as to provide a unitary
construction, are four impeller blades 26, 28, 30 and 32. Preferably, each
blade is a retreat curve. In addition the blades are grouped in two pairs
each extending generally from opposite sides of the hub. Thus, when
considering the direction of rotation as being counterclockwise as viewed
in FIG. 2, the leading and trailing blades 26, 28 respectively of one pair
are on one side of the hub while the leading and trailing blades 30, 32
respectively of the other pair are on the opposite side of the hub.
The blades 26, 28, 30, 32 are arranged about the hub to provide a balanced
structure. In this respect, the included angle B between the leading blade
(26 or 30) of one pair and the trailing blade (28 or 32) of the other pair
is between 105.degree. and 130.degree. and preferably about 120.degree..
Concurrently, the included angle A between the leading and trailing blades
of each pair is between 50.degree. and 75.degree. and preferably, about
60.degree..
With impeller blades 26, 28, 30 and 32 disposed about hub 24 in this
fashion, the impeller can be said to be generally rectangular in plan
view. The outward ends 36, 38, 40 and 42 of each of the impeller blades
26, 28, 30 and 32 respectively lie at the corners of the rectangle and the
effective diameter or span of the impeller is the length of the diagonal
of the rectangle. This effective diameter may be much larger than the
largest manhole opening and still the impeller may be carried into the
vessel in one piece through such an opening and attached to drive shaft
18.
The preferred embodiment is to have the impeller blades arranged such that
the included angle between the impeller blades of a pair (angle A) is
about 60.degree. and the included angle between adjacent blades of each
pair (angle B) is approximately 120.degree..
Also, it is preferred that each impeller blade follow a retreat curve which
is a circular arc section as shown in FIG. 2. With this arrangement the
largest vessel manhole opening needed to accommodate passage of the
impeller is about equal to the distance indicated at 44 which represents
the length of the perpendicular line extending from the end of the leading
blade of either pair to the trailing blade.
As a practical matter where the blades are curved, the actual diameter of
the manhole opening must be slightly larger than theoretical in order to
allow for the vessel wall thickness and the depth of any collar about
manhole opening 20. This is because the curved impeller is rotated as it
is fed through the vessel opening as set out below.
To pass the impeller having the preferred curved blades through manhole 20,
the impeller is oriented as shown in FIG. 4 and snaked through the
opening. First, the end 38 of trailing blade 28 is inserted into the
manhole at one side of the opening and the impeller tipped clockwise as
end 38 and a portion of trailing blade 28 passes through the opening. End
36 of the leading blade 26 then passes through at the opposite side of the
manhole. The impeller continues to tip clockwise until hub 24 is passed.
Thereafter, it is tipped counterclockwise to pass first the leading blade
30 and then the trailing blade 32 through the opening.
In the preferred embodiment, each blade as shown in FIG. 2 has a constant
radius of curvature from its point of attachment to the hub to its outer
end. Tests have shown that an impeller of this design wherein angles A and
B are 60.degree. and 120.degree. respectively consumes more power and
therefore is generally more effective than a typical three bladed impeller
of the same span and retreat curve design wherein the three blades are
disposed at 120.degree. intervals. This preferred four bladed design,
however, does not consume as much power as a more conventional four bladed
impeller of the same effective diameter and with the blades set at
90.degree.. However, and more importantly, if each of the conventional
three or four bladed impellers described above is made of one piece
construction, it could not be passed into a closed vessel through a
manhole size opening substantially smaller than the effective diameter of
the impeller. The impeller of the present invention not only has the
advantage of being made in a one piece construction, it also is able to
fit through a manhole opening which is considerably smaller than the
effective diameter of the impeller as shown in Table I below.
TABLE I
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Vessel Impeller
Diameter
Capacity Manhole Size Needed
Diameter
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48" 500 Gals. 12" .times. 16" oval
32"
60" 1000 " 14" .times. 18" oval
33"
78" 2000 " 14" .times. 18" oval
34"
96" 4000 " 18" round 42"
132" 10,000 " 24" round 56"
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In contrast Table II below shows the relatively larger opening sizes need
to accommodate a unitary three bladed impeller as conventionally used.
TABLE II
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Vessel Impeller
Diameter Diameter Opening
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48" 36" dia. 24" round
60" 44" 30" round
78" 44" 30" round
96" 54" 36" round
132" 72" 54" round
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Comparing Tables I and II it is seen that the four bladed impeller of the
present invention is able to fit through considerably smaller diameter
openings or ovals than a standard three bladed impeller of like diameter
or span. For example, a four bladed impeller of the present invention
having a 56" span will fit through the same 24" size opening as a
conventional three bladed impeller having only a 36" span.
It is preferred that angle A range from 50.degree. to 75.degree. and that
angle B range from 105.degree. to 130.degree.. Increasing angle A (or
decreasing angle B) to about 90.degree. greatly increases the diameter of
the vessel opening needed to pass the impeller. Conversely, decreasing
angle A (or increasing angle B) brings the blades of each pair closer
together so that the impeller approaches the shape of a simple paddle
having two blades disposed 180.degree. apart. Such a paddle is
significantly less efficient and dynamically less stable than either the
three or four bladed design.
In order to mount impeller 10 to the end of drive shaft 18 within the
closed vessel, hub 24 is provided with an axial bore 46 adapted to receive
the lower end of the shaft 18 (FIGS. 1 and 3). While any suitable means
well known in the art may be used to affix impeller 10 to the lower end of
the drive shaft 18, it is preferred that the connection be made by an
interference fit. Such an interference fit may be effected by cooling the
lower end of shaft 18 to shrink its diameter by an amount sufficient to
permit insertion of the shaft through bore 46. Thereafter, when the shaft
warms to ambient temperature and expands, it will provide an interference
fit between the shaft and the impeller hub. In order to accomplish this
interference fit it should be appreciated that the outside diameter of at
least the lower portion of shaft 18 and the inside diameter of bore 46
should be machined to close tolerances. It has been found that an
interference fit of approximately 0.00025 to 0.00075 inches per inch
diameter is sufficient to provide a strong interference fitted joint.
Another advantage of the one piece construction is that the entire outer
surface of the impeller including the hub, the impeller blades and the
internal surface of bore 46 can be provided with a continuous unbroken
glass coating which adds to the integrity of the one piece construction of
the four bladed impeller of the present invention.
If the bottom of vessel 12 is dished, FIG. 3, shows that the blades may be
tilted upwardly from hub 24 so that the full length of the impeller can
lie close to the vessel bottom. The preferred angle of this tilt is about
15.degree.. This allows agitation and mixing of material which lies on or
near the bottom of the vessel.
While the preferred design is shown having each of the four blades curved
in a continuous circular arc section, it should be appreciated that other
curvatures could be used. Also, blades 26, 28, 30 and 32 could be
straight. However, straight blades increase the size of the manhole
opening needed over the same diameter impeller having curved blades
because in the straight blade design, distance 44 is greater.
Thus, it should be appreciated that the present invention provides a
unique, four balded, glass coated impeller of one piece construction which
is able to fit through a manhole vessel having a substantially smaller
diameter than the span of the impeller.
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