Note: Descriptions are shown in the official language in which they were submitted.
CA 02072297 1997-11-10
1
HIGH DENSITY SINGLE PASS HEAT EXCHANGER
FOR DRYING FRAGMENTED MOISTURE-BEARING PRODUCTS
This is a continuation-in-part application of Serial
No. 07/528,812, filed May 25, 1990.
Background of the Invention
1. Field of the Invention
This invention relates to high density, single
pass heat exchangers especially useful for drying frag-
mented moisture-bearing products such as fibrous materials
in the nature of bakery wastes, alfalfa, peat moss and
wood products.
2. Description of the Prior Art
Drying of large volumes of fragmented fibrous
materials has long been carried out in heat exchangers
consisting of one or more elongated, generally horizontal-
ly oriented drums. Hot gases are caused to flow through
each to remove moisture from the material by heat exchange
between the hot gases and the fibrous product. Generally
speaking, a burner is disposed to direct hot products of
combustion directly into the inlet of the drum which also
receives the moisture-bearing material to be dried. After
removal of the requisite amount of moisture from the
material, it is directed into a collector or other receiv
ing means at the outlet of the heat exchange drum. Often
times, a blower or equivalent device is associated with
the drum to increase the rate of flow of hot gases through
the exchanger.
Alfalfa, for example, has long been dried in
horizontally positioned heat exchange drums which typical-
ly have been from about 8 to approximately 12 feet in
diameter and 20 to 40 feet long. As each drum was rotated
about its longitudinal axis, hot burner gases were direct-
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CA 02072297 1997-11-10
2
ed into the inlet and airflow was increased by a blower
connected to the outlet of the drum. Initially, single
pass drums were the dryer of choice principally because of
economic considerations. The manufacturing cost of the
dryer was a more important overall consideration on an
amortized basis than the amount of fuel burned because of
relatively low natural gas prices. For many years, the
alfalfa dehydration industry advertised that dehydrated
alfalfa was a superior product because it was produced
from alfalfa that was supplied to the dehydrated plant
substantially at fresh cut moisture levels with water then
evaporated from the product to its final desired dry level
of no more than about 8 to 10%.
Even during the era of relatively low fuel
prices, single pass evaporators in the alfalfa dehydration
industry were supplanted in many instances by so-called
three-pass dryers which permitted processing of higher
volumes of the ground alfalfa in essentially the same
overall ground space occupied by then existing single pass
heat exchangers. In three-pass dryers, material was
directed along a generally S-shaped path of travel thereby
providing a longer path and shorter residence time in the
dryer by virtue of increased hot gases velocities.
Single pass dryers have previously been employed
having inwardly directed internal flighting in the drum
which caused the material conveyed through the dryer to be
lifted up somewhat and dropped back into the hot gas
stream rather than simply resting at the bottom of the
drum as it was rotated about its axis. An exemplary
design in this respect is disclosed in applicant's U.S.
Patent No. 4,183,208 which incorporated secondary flight-
ing in the central part of the drum for the purpose of
enhancing heat exchange between the hot gases directed
through the drum and the product to be dried.
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In order to prevent hot gases from being blown
directly through the drum from one end to the other,
single pass dryer designers in the past have resorted to
the use of a transverse plate or plates in the drum to
obstruct the flow of hot products of combustion. The net
result of such construction inevitably was to decrease the
capacity of the dryer while at the same time interfering
with uniform temperature control and preventing mainte
nance of constant material flow rates through the heat
exchanger.
Three-pass dryers, on the other hand, were more
expensive than single pass dryers but found favor because
of the decreased product residence time necessary to
assure adequate drying and more stable temperature control
performance.
As fuel prices have risen, dehydration plant
operators for example have become increasingly more
concerned about fuel costs and have significantly retreat-
ed from a position that alfalfa should be introduced into
the dryer at fresh cut moisture levels approaching 85%.
More and more reliance has been placed on sun curing with
the alfalfa introduced into a dryer being no higher than
about 35 to 75% moisture. Significantly higher fuel costs
have resulted in a substantial retrenchment in the alfalfa
dehydration business with less efficient plants being
closed down rather than updated.
As plants have closed, those still in business
have sought ways to rehabilitate their equipment by
increasing efficiency so that the facilities will accommo-
date greater and greater demand in order to permit contin-
uing operation. New plants also require that a highly
efficient process be provided to effectively compete when
the overall costs of building a totally new plant are
factored into the equation.
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The rotary kiln illustrated a described in U.S.
Patent No. 4,753,019 and which was designed to dry calcium
carbonate sludge, would not be useful to remove moisture
from a material such as alfalfa or the like. The ' 819
patent mixing rods within the drum would not provide ade-
quate agitation of the material to be dried, and especial-
ly would not function to lift the material and provide a
shower thereof which falls through the hot drying gases.
The asphalt dryer of U.S. Patent No. 4,338,732
has internal flighting for lifting of the product, but
does not have adequate means to assure turbulent flow of
hot gases through a product such as alfalfa, and there
would be inadequate residence time of the fragmented
material in the drum to reduce the moisture content of
alfalfa or a similar product to a required level.
Summary of the Invention
This invention relates to a'high density, single
pass heat exchanger for drying fragmented, moisture
bearing products which is especially adapted for either
updating existing three-pass and single pass dyers or for
fabrication of highly efficient new plants. This goal is
accomplished by providing a generally horizontal hollow
drum heat exchanger which has a series of novel, circumfe-
rentially spaced, inwardly directed, product conveying,
showering conductive and connective heat transfer flights
extending inwardly toward the center of the drum where the
total surface area of the flights is at least about as
large as the total heat transfer surfaces of the products
to be dried which are flowing through the heat exchanger
at its maximum rated product throughput capacity.
The single pass dryer therefore has flights
which extend inwardly into the interior of the drum to a
greater degree than in past single pass dryers of this
type while still leaving a flight-free central showering
CA 02072297 1997-11-10
zone of a size allowing adequate heat exchange and convey-
ance of material along the length of the dryer at a
predetermined rate. The unexpected ability of the heat
exchanger to efficiently remove moisture from fragmented
5 products such as alfalfa, bakery wastes, peat moss and
wood materials is in part attributable to the unique
relationship of the diameter of the drum to the diameter
of the internal cylindrical flight-free central showering
and product conveyance zone such that the diameter (R) of
the drum divided by the diameter of the zone (r) yields an
aspect ratio (R/r) within the range of about 1.4 to 2.4.
This critical relationship has been found to be important
in assuring maximum heat exchange without deleterious
effects on the product being dried.
A series of transversely oriented, axially
spaced, circular discs or baffles and associated rings are
mounted in the drum in order to cause the material being
dried and the hot products of combustion introduced into
the dryer to follow an essentially serpentine, generally
helical path longitudinally of the drum. These baffles
are of increasing diameter as the outlet end of the drum
is approached. They are of dimensions such that the
peripheral edges thereof do not overlap with the inner
margins of the internal flighting mounted within the drum.
The drum flights are made up of a series of
metal panel or sheet members which extend longitudinally
of the drum and also project into the interior of the
cylinder. The individual flights comprise a series of
members which in sequence around the circumference of the
drum are made up of straight, radially extending members
on each side of a transversely bent or dog-legged sheet
member oriented to enhance transfer of the fragmented
product to the top of the drum for gravitational fall
through the flight-free zone in a showering pattern that
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results in substantially uniform dispersion or distribu-
tion of the product being dried throughout the extent of
the showering and conveyance zone.
Improved results have also been obtained by
dividing the drum flights into a number of different
sections extending longitudinally of the heat exchange
drum with adjacent flight sections in effect being rotated
with respect to one another during fabrication so that the
angular flight members of each section act somewhat
differently on the product being dried as it is conveyed
through respective flight sections. This enhances mois-
ture removal and at the same time assures uniform convey-
ance of the material along the length of the dryer. An
important feature of the present invention is the fact
that the novel flight defining structure hereof may be di
rectly substituted for the interior components of a three
pass dryer without significant alteration of the dryer
drum itself . At the same time, significantly higher prod
uct throughput capacity is realized to meet the same dried
product specifications.
Brief Description of the Drawings
Figure 1 is a side elevational view of a typical
dehydration plant for removing moisture from various types
of fibrous products such as alfalfa, bakery wastes, peat
moss and wood materials, illustrating from left to right,
the burner assembly, the drum heat exchanger of this
invention, a centrifugal discharge and primary fan, the
primary collector, a cooling and conditioning drum, and a
final processor;
Fig. 2 is an enlarged fragmentary, vertical
cross-sectional view at the inlet end of one embodiment of
a drum heat exchanger embodying the present invention;
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Fig. 3 is a vertical cross-sectional view taken
along the line 3-3 of Fig. 2 and looking in the direction
of the arrows;
Fig. 4 is an enlarged fragmentary, vertical
cross-sectional view at the outlet end of the embodiment
of the drum heat exchanger shown in Figs. 2 and 3;
Fig. 5 is a vertical cross-sectional view taken
along the line 5-5 of Fig. 4;
Fig. 6 is an enlarged, essentially schematic
vertical cross-sectional view similar to Figs. 3 and 5,
illustrating the relationship of the diameter of the drum
to the diameter of the flight-free zone in the interior
thereof, showing the product showering and conveying
region defined by such zone, and also depicting the three
different types of dog-leg shaped sheet members making up
the flighting;
Fig. 7 is a fragmentary, vertical cross-section-
al view through the drum of Figs. 2-6 and illustrating the
way in which the flight sections installation patterns are
rotated relatively along the length of the drum for more
efficient moisture removal and conveyance of product
through the drum;
Fig. 8 is a fragmentary, vertical cross-section
al view of an alternative embodiment of the drum illus
trated in Figs. 2-7 inclusive wherein the radially orient
ed flat sheet members making up the flighting within the
dryer do not have out-turned innermost edges as is the
case with the embodiment of Figs. 1-7 and four different
types of dog-leg shaped flight sheet members are provided;
Fig. 9 is a schematic, vertical cross-sectional
representation on a reduced scale of the dryer drum of
Figs. 1-7 or 8;
Fig. 10 is a schematic representation of the
heat exchanger of Figs. 1-9 and indicating the degree of
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rotation of the flight sections one with respect to the
other along the length of the heat exchanger;
Fig. 11 is a schematic, vertical cross-sectional
representation on a reduced scale of another embodiment of
the drum heat exchanger;
Fig. 12 is a schematic representation similar to
Fig. 10 but in this case illustrating the degree of
rotation of the flight sections one with respect to the
other of a drum embodying flight section structure as
illustrated in the alternate embodiment as shown in Figs.
8 and 11; and
Fig. 13 is a preferred embodiment of an essen-
tially schematic, enlarged, fragmentary vertical cross-
sectional view of a drum heat exchanger constructed in
accordance with the present invention and incorporating a
series of transversely oriented, axially spaced circular
baffles within the drum and an alfalfa stem dryer section
at the outlet end of the dryer;
Fig. 14 is an enlarged vertical cross-sectional
view through the drum of Fig. 13 and taken substantially
on a transverse line to the left of the f first baffle at
the inlet end of the dryer and looking toward the outlet
end of the drum;
Fig. 15 is an enlarged fragmentary of the outlet
portion of the drum dryer and more specifically illustrat
ed the construction of the stem dryer; and
Figs. 16, 17, 18 and 19 are vertical cross-
sectional views taken along respective lines 16-16, 17-17,
18-18 and 19-19 of Fig. 15.
Detailed Description of a First Embodiment of the Inven-
tion
Fig. 1 is an overall essentially schematic
representation of a single pass dehydration system 20 for
evaporating water from a fragmented moisture-bearing
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product such as bakery wastes, alfalfa, peat moss, wood
materials, or similar products. The system shown is of
the type which is especially useful for dehydration of
chopped alfalfa.
Although system 20 is particularly adapted for
drying of chopped alfalfa or similar agricultural prod-
ucts, it is to be appreciated that such system has a wide
variety of uses in instances where it is desirable to
lower the moisture content of a fragmented product to
about 10%-15% levels. In the case of alfalfa, the fibrous
material is chopped to cause the fibers to be no more than
about 3 inches in length. In other instances, the product
can be more finely divided than is the case with alfalfa
to present a larger surface area for contact between the
hot burner gases and the material undergoing drying in
system 20. Oftentimes, the fragmented, moist material is
in the form of small particles having an effective diame-
ter of no more than about 1/16 inch. Wood materials fall
into this category with sawdust being an exemplary product
which benefits from drying before transportation and use
thereof. Dried sawdust has been used extensively for
energy purposes by burning the sawdust to produce heat,
and then using that heat to generate steam for power
generation and similar applications. Bakery wastes, on
the other hand, are dried in order to permit their subse-
quent use as animal feeds and the like. Peat moss is
frequently dried prior to shipment to points of use in
order to reduce transportation costs calculated on a
weight per mile basis. Other moisture-bearing fibrous
products in fragmented form are candidates for drying
using the improved heat exchanger of this invention.
System 20 typically includes a burner assembly
22 adapted to burn natural gas or a similar feed stock to
produce hot products of combustion which are directed into
the inlet end 24 of an elongated, generally horizontal,
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hollow drum heat exchanger broadly designated 26 which is
rotatable about its longitudinal axis. A centrifugal dis-
charge and fan unit 28 is connected to the outlet end 30
of drum 26. Conduit 32 connects the blower unit 28 to the
5 upper end of a conventional primary cyclone collector 34.
The lower extremity of the collector 34 connects to a
conveyor conduit 36 which leads from the bottom of centri-
fugal discharge and blower unit 28 to the inlet end 38 of
a cooling and conditioning drum 40 which is also rotatable
10 about its axis. Outlet end 42 of drum 40 is joined to a
final processor 44 where cubing, packaging or storage of
the dried product may be carried out.
As is best depicted in Figs. 2-5 inclusive, heat
exchange drum 26 has a cylindrical shell 46 which typical
ly is from 8 to 14 feet in diameter, and some 24 to 140
feet long. A series of circumferentially spaced, inwardly
directed, product-conveying, showering and both connective
and conductive heat transfer flights generally designated
48 are mounted within shell 26 for agitating the frag-
mented product to be dried, lifting the product to the top
of the drum for free-fall in the interior space thereof,
and to disperse the product over the central showering
zone for most effective heat transfer and pneumatic selec-
tive conveying between the hot gases from the burner 22
and the moisture-laden, fragmented product to be dried.
As perhaps best shown in the schematic represen-
tation of Fig. 6, the flights 48 are made up of a number
of sheet metal members which project into the interior of
drum 26 from the interior surface of shell 46, and also
extend longitudinally of drum 26. In the preferred
structure of the first embodiment of the invention shown
in Figs. 1-7 inclusive and Figs. 9 and 10, flights 48 have
a series of transversely and longitudinally straight,
relatively narrow width metal sheet members or panels 50
connected to the inner surface of shell 46 and located in
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radial disposition with respect to the axis of shell 46.
Triangular metal support gussets 52 are also welded to the
inner surface of shell 46 and to the trailing surface of
each of the metal sheet members 50, viewing Figs. 3, 5 and
6, stabilize each of the members 50. In an exemplary in-
stallation, three of the gussets are provided for each 48
inch long sheet member 50. The gussets 52 thereby support
a respective sheet metal member 50 and prevent dislod-
gement of such members 50 from their desired position and
prevent significant adverse warpage from occurring by
virtue of contact with the hot gases emitted by burner
assembly 22.
A plurality of radially oriented sheet metal
members 54 are provided to the right of each narrow width
metal sheet member 50 in a counterclockwise direction
viewing Fig. 6. The members 54 project into the interior
shell 46 to a substar~i~lly greater degree than an adja-
cent member 50. MembErs 54 also extend longitudinally of
the drum 46 in generally parallel relationship to respec-
tive shorter members 50. An out-turned leg segment 56
integral with the innermost extremities of each sheet
member 54 and extending the full length thereof, is at an
angle of about 90° with respect to the main body of a
respective sheet metal member 54. As is apparent from
Figs. 3, 5 and 6, the out-turned leg segments 56 are
comparatively short in a direction perpendicular to the
main body of a respective sheet metal member 54. For
example, where the radial width of the member 54 is about
24 inches, the return 56 would typically be about 1 to 1
1/2 inches.
To the right of each member 54 in a counter-
clockwise direction is another relatively short flight
member 50 supported by a series of gussets 52.
The next sheet flight member of maximum radial
width to the right of each straight sheet metal member 54
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is a dog-leg sheet member 58 which has a relatively long
inwardly directed leg segment 58a which is connected di-
rectly to the innermost surface of the shell 46, and an
inner, shorter leg segment 58b. A return leg segment 60
provided at the innermost end of each dog-leg member 58,
projects the same direction as the segment 56 on an
adjacent member 54. The leg segment 58b of each dog-leg
member 58 is bent in the same direction as the return leg
segment 60 to present an obtuse angle between leg segments
58a and 58b. It is also to be observed from Fig. 6 that
the leg segment 58a of each dog-leg member 58 is at an
angle with respect to a radial line between the center of
shell 46 and the point of attachment of a respective leg
segment 58a with shell 46. In the case of a 10 foot
diameter drum 26, it is preferred that each of the seg-
ments 58a be at an angle of about 7° with respect to a
radial line through such flight member and that the obtuse
angle between segments 58a and 58b be approximately 143°.
In that manner, the leg segment 58b is at an angle of +25°
with respect to the radial line. (The +° designation in
this respect is intended to mean that the leg segment 58b
is bent to the right viewing Fig. 6.)
To the right circumferentially of each dog-leg
member 58 as shown in Fig. 6, is another relatively narrow
radially extending sheet member 50 supported by corre
sponding gussets 52.
As if further apparent from Fig. 6, to the right
in a counterclockwise direction from each of the dog-leg
members 58 is a dog-leg flight member 62. The larger leg
segment 62a of each dog-leg member 62 is joined to the
inner surface of shell 46 while the innermost leg segments
62b thereof are bent in the opposite direction from leg
segments 58b of dog-leg members 58 to present what may be
considered a minus angle with respect to a radial line
between the center of the shell 46 and the point of
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connection of each dog-leg member 62 to shell 46. In a
preferred embodiment, the angle of each segment 62a with
respect to corresponding radial lines is about 4°, the
obtuse angle between leg segments 62 and 62b is about 161°
and the angle of segment 62b relative to a respective
radial line is about -15°. The dog-leg member 62 in a
preferred embodiment as illustrated do not have innermost
leg segment returns such as segment 60 of member 58 and
segment 56 of member 54.
Another relatively narrow planar sheet member 50
is provided to the right of each dog-leg member 62 viewed
in a counterclockwise direction which in turn are support-
ed by corresponding triangular gussets 52.
The next flight member to the right of each dog
leg member 62 beyond a respective member 50 therebetween
as shown in Fig. 6 is a dog-leg member designated by the
numeral 64. Each dog-leg member 64 has a large outer leg
segment 64a and a much smaller innermost bent leg segment
64b. A return segment 66 is provided on the outermost
extremity of each leg segment 64b at 90° with respect
thereto and of the same dimensions as segments 56 and 60.
The leg segments 64a of dog-leg members 64, joined to the
inner surface of shell 46, are preferably at an angle of
about 3.5° with respect to a radial line between the
center of shell 46 and the point of connection of a
corresponding leg segment 64a to shell 46. The obtuse
angle between segment 64a and segment 64b of each flight
member 64 is preferably about 131.5° whereby the leg
segments 64b are each at an angle of about 45° relative to
respective radial lines.
Another relatively narrow, gusset-supported
sheet metal member 50 secured to the innermost surface of
shell 46 is provided between each adjacent pair of flight
members 64 and 54.
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14
Viewing Figs. 2 and 4, it can be seen that a
series of ring elements 68 arranged longitudinally of drum
26 are joined to the outer extremities of flight members
54, 58, 62 and 64 for maintaining the circumferential
spacing between the inner ends of such members. It is to
be preferred that one or more support ring element be
provided for each individual sheet member to which they
are joined, along the length of the shell 46. Normally,
the ring elements 68 are located intermediate the ends of
each discrete flight member. All of the sheet metal
members are connected to the inner surface of shell 46 in
equidistantly spaced relationship. In like manner, the
areas of connection where extended sheet metal members 54,
58, 62 and 64 are joined to respective ring elements 68
are in disposition such that the points of securement of
the sheet metal members to the ring elements are equidis-
tantly space.
In view of the fact that the heat exchanger drum
26 is normally of significantly greater length and diame
ter, e.g., 24 to 140 feet in length as compared with 8 to
14 feet in diameter, for fabrication purposes and because
of the tendency of such sheet members to buckle at the
temperatures present inside of the dryer drum during
normal operation, if the members are excessively long, it
is to be preferred that each sheet member be made up of a
number of sections along the length of the shell 46. In
typical installations, the length of each individual sheet
member longitudinally of the drum 26 may be anywhere from
about 30 inches to about 48 inches.
Another advantage of breaking each sheet member
into a number of sections along the length of the drum is
the fact that it is possible to provide a dryer wherein at
least certain of the sheet metal members are of different
transverse angularities and shapes along the length of the
drum, although still longitudinally aligned. This can be
CA 02072297 1997-11-10
accomplished either during fabrication of a new heat
exchanger drum 26, or at the time of rehabilitation of an
existing three-pass dryer to convert it into a single pass
drum. All of the flight members of a particular section
5 may in effect be rotated a certain number of degrees rela-
tive to an adjacent section of flight members, or only
those wider flight members that project inwardly suffi-
ciently that they are supported at their inner ends by
ring elements 68 may be shifted circumferentially of the
10 drum to enhance the drying efficiency of the process,
alter the flow pattern of moisture-bearing products along
the length of the dryer, vary residence time, or provide
compensation for the type of material to be dried. The
narrower sheet members 50 which have gusset supports 52
15 need not be of the same longitudinal length as adjacent
wider sheet metal flight members, even though the wider
members are not of the same angularity or shape throughout
the longitudinal length of a drum installation, because
the narrower flight members 50 have essentially the same
effect on the material throughout the drying process and
there is no need to rotate the positions thereof circumfe-
rentially of the drum along the length of the shell 46.
For this reason, the narrower sheet metal members 50 are
generally fabricated to be longer than adjacent sheet
metal flight members 54, 58, 62 and 64 for efficiency of
installation and minimization of parts. It is to be
understood though that if desired, the longitudinal length
of all of the flight members of discrete circumferentially
extending, longitudinally aligned, end-to-end sections of
the drum heat exchanger may be of the same length.
Because of the fact that the flight members are all spaced
such that the points of attachment thereof to shell 46 are
equidistant from one another, while the paints of attach-
ment of members 54, 58, 62 and 64 to respective ring
elements 68 are also equidistant, it is to be appreciated
CA 02072297 1997-11-10
16
that upon rotation of the wider flight members circumfere-
ntially of the drum from section to section, the degree of
shifting should be equal to the spacing between adjacent
flight members so that longitudinal alignment is main-
s tained throughout the length of the heat exchanger shell
46.
Thus, as is apparent from the schematic repre-
sentation of Fig. 10 which illustrates schematically a
dryer drum 26 as shown in Figs. 1-7 and 9 respectively,
having an internal diameter of about 10 feet with a shell
thickness of about 1/4 inch, and a length of approximately
30 feet, the flights are desirably divided up into eleven
separate circumferentially extending rows in end-to-end
relationship within the drum. In row 1, the radial
flighting pattern may be as schematically depicted in Fig.
9. Under these circumstances, and assuming that the
radius (R) of the drum 26 divided by the radius (r) of the
elements 68, the aspect ratio (R/r) of the dryer is about
1.63.
With that aspect ratio, row 2 of the flight
members as shown in Fig. 9 if the wider flight members
attached at their inner ends to support rings 68 are
rotated counterclockwise at the time of installation so as
to stay in alignment with the f first row of wider f light
elements, then the second row of wider members will have
been rotated or shifted 11.25° counterclockwise. Similar
ly, the flight sections along the length of the dryer drum
26 from the inlet end 24 to the outlet end 30 thereof will
have been during installation to the extent set out in
Fig. 10.
Thus, in order to maintain the wider flight
members in longitudinal alignment throughout the length of
the shell 46 as is best shown by Fig. 7, circumferential
row 1 of flight members if in the pattern as shown in Fig.
9 would result in the wider members of row 2 being rotated
CA 02072297 1997-11-10
17
11.25° counterclockwise, the wider members of row 3
rotated 22.5° counterclockwise as compared with row 1, the
wider members of row 4 rotated 33.75° counterclockwise
with respect to row 1, row 5 having the same flighting
pattern as that of row 1, the wider members of row 6 again
rotated 11.25° counterclockwise from the pattern of row 1,
the wider members of row 7 rotated 22.5° counterclockwise
as compared with row 1, the wider members of row 8 rotated
33.75° counterclockwise from the row 1 pattern, row 9
disposed in the same radial pattern as row 1, the wider
members of row 10 rotated 11.25° counterclockwise from row
1, and the wider members of row 11 rotated 22.5° coun-
terclockwise as compared with row 1.
In the embodiment represented by Fig. 10,
wherein the drum has a cross-sectional diameter of about
10 feet, 4 inches and a length of about 30 feet, a pre-
ferred construction has wider flight members 54, 58, 62
and 64, 32 inches in length and about 24 inches in width
measured from the inner surface of shell 46 to the inter-
nal diameter of ring elements 68. The narrow flight
members 50 on the other hand are each typically 48 inches
long and 12 inches in width, with the members at the ends
of the dryer being of less length if necessary depending
upon the overall length of the drum.
In like manner, the heat exchanger drum 126 of
the alternative embodiment shown in Figs. 8, il and 12,
having a diameter of about 8 feet and a length of approxi-
mately 24 feet, may for example have ring elements 168
which are about 54 inches in diameter. Under these
circumstances, the wider members 154, 162, 164, 176, 178
and 180 may be about 21 inches in width while narrow
members 150 can in the example give be about 12 inches in
width. In this type of installation, the wide flight
members 154, 162, 164, 176, 178 and 180 are all 31 inches
in length while the narrow members 150 are typically 48
CA 02072297 1997-11-10
18
inches in length except for end members which are of
necessary length, as for example, about 36 inches, again
depending on the length of the overall dryer.
As is apparent from the schematic representation
of Fig 10., a rehabbed three-pass dryer which is 8 feet in
diameter and from 24 to 25 feet long would normally be
divided up into nine rows or sections of flights with the
flight pattern of the first row being as shown in Fig. 11.
The wide flight members 154, 158, 162 and 164 in row 2 are
thereby rotated 15° counterclockwise with respect to the
same wider flight members of row 1. The wider members of
row 3 are rotated counterclockwise 45° with respect to the
pattern of row 1, the wider members of row 4 rotated 75°
counterclockwise relative to row 1, the radial flighting
pattern of row 5 are the same as row 1, the wider members
of row 6 are rotated 15° counterclockwise with respect to
rows 1 and 5, the wider flight members of row 7 are
rotated 45° counterclockwise with respect to rows 1 and 5,
the wider flight members of row 8 are rotated 75° counter-
clockwise relative to rows 1 and 5, and the wider flight
members of row 9 are again the same pattern as rows 1 and
5.
In the eight foot diameter embodiment of Figs.
8, 11 and 12, where the drum shell 146 radius (R) is about
4 feet and the radius (r) of the ring elements 68 is about
27 inches, the aspect ratio (R/r) is approximately 1.77.
The dog-leg shaped flight members 176 are constructed such
that the wider legs 176a are at an angle of about 6.25°
transversely of the shell 146 with respect to radial lines
extending from the center of the drum to the point of
attachment of respective legs 176a to the inner surface of
shell 146. The innermost narrow leg segments 176b are in
positions presenting an obtuse angle between corresponding
leg segments 176a and 176b of about 143.75°. This causes
the innermost narrow leg segments 176b to be at an angle
CA 02072297 1997-11-10
19
of about +30° with respect to the associated radial lines.
Dog-leg shaped flight members 162 of each row or
section of flights are positioned such that the outer
wider leg segments 162a are at an angle of about 5° with
respect to corresponding radial lines through the center
of the drum and the point of attachment of respective leg
segments 162a to the shell 146. The obtuse angle between
the outer wide leg segments 162a of members 162 and the
inner leg segments 162b thereof is about 160° with the leg
segments 162b being oriented in directions such that the
leg segments are at an angle of about -15° with respect to
corresponding radial lines.
The flight members 164 have outer wide leg
segments 164a positioned such that they are at an angle of
~5 about 7° with respect to the corresponding radial lines
extending between the center of drum shell 146 and points
of attachment of leg segments 164a to shell 146. An
obtuse angle of about 128° is presented between the
innermost leg segments 164b of dog-leg shaped members 164
and the outer wider leg segments 164a. In this manner,
the inner leg segments 164 are at an angle of about +45°
with respect to corresponding radial lines.
The dog-leg shaped flight members 178 are
disposed such that the outer leg segments 178a thereof are
at an angle of about 5° with respect to a corresponding
radial line, while the outer leg segments 178b of each
member 178 present an obtuse angle relative to a respec-
tive leg segment 178a of about 160°. Thus, the leg
segments 178b are each at an angle of about +15° with
respect to the associated radial lines.
It is to be observed from Fig. 8 that the wide
straight members 154 have short innermost 90° returns 182
thereon, but that wide straight members 180 do not have
such returns. This is in contrast with the embodiment of
CA 02072297 1997-11-10
Figs. 1-7 and .9-10 wherein all of the wide straight
members 54 are provided with returns 56.
The innermost leg segments 164b of wide members
164, short leg segments 176b of wide members 176 and short
5 leg segments 178b of wide members 178 also have narrow 90°
returns 184, 186 and 188 respectively thereon turned to
the right as shown in Fig. 8, as are the returns 182 on
members 154. The return leg segments 182, 184, 186 and
188 are also about 1 to 1-1/2 inches in width.
10 As is most evident from Figs. 2 and 3, material
to be dried is introduced into the drum 26 via inlet
opening 124 which communicates with an inlet conduit 70.
Similarly, as depicted in Figs. 4 and 5, material exits
drum 26 via outlet extension 72 connected to the centrifu
15 gal discharge and blower
assembly 28.
During rotation of the drum 26 as shown in Fig.
6, at a speed of from about 4 to about 10 r.p.m., material
to be dried is elevated by the various flight members
20 until such material may fall by gravity toward the inner
part of the individual flight members. In the case of the
narrower members 150, moisture-bearing fragmented product
lifted by the narrow members 150 eventually slides off
onto the immediately underlying wider flight member 54,
58, 62 or 64. The angularity, and width of the inner leg
segments of respective members 58b, 62b and 64b, as well
as the leg return segments 56 of straight members 54,
return segments 60 of dog-leg members 58, and return
segments 66 of dog-leg members 64 retard release of the
fragmented material to be dried into the product showering
region or zone 74 in a manner such that in conjunction
with the combination of momentum and centrifugal forces
present, there is relatively uniform dispersion of the
material throughout the full circular area of the zone 74.
CA 02072297 1997-11-10
21
As depicted in Fig. 6, assuming that the depic-
tion represents the drum rotating in a counterclockwise
direction but frozen in time for purposes of illustration,
it can be seen that the straight member 54' located at a
3:00 position is in disposition such that there is little
tendency for material supported by the flight member 54'
at that location to allow material to gravitate into the
interior showering zone 74. However, the narrow flight
member 50' immediately thereabove is slightly inclined so
that there will be some flow of product off the end of the
flight member down onto the underlying straight flight
member 54'.
Continuing with the frozen in time sequence, the
dog-leg flight member 58' immediately above narrow flight
member 50' is receiving material from the flight member
50' directly thereabove but the upwardly bent leg segment
58b' of such flight member, and the return segment 60'
thereon prevents significant delivery of fragmented
product into the showering zone unless there is sufficient
material to overflow the flight member 58' which defines
somewhat of a channel shape when disposed in that position
during its rotation.
The flight member 62' directly above flight
member 58' is located in a position of about 2:30 whereby
its outer leg segment 62b' is turned downwardly so that
there is a greater tendency for material supported thereon
to flow off of the upper surface of the flight member and
be released into the showering zone 74. This means that
product to be dried is delivered into the rightmost corner
of the showering zone for heat transfer and pneumatic
separation between the hot products of combustion directed
through the showering zone and the fragmented product.
Material will have also fallen onto the top of dog-leg
member 62 from the overlying narrow flight member 50.
CA 02072297 1997-11-10
22
The dog-leg member 64' at a 2:00 position in
Fig. 6 having a leg segment 64b' is at a greater angle
than the leg segment 58b of the underlying flight member
58', but by virtue of the relatively narrow width of leg
segment 64b' of the flight member 64' when in that dispo-
sition, will somewhat retard release of material but still
allow some product to fall into the showering region 74
immediately to the left of the material released from
underlying flight member 62'.
The overlying straight wide flight member 54 "
at a position of about 1:30 releases material into the
showering zone to the left of that delivered from dog-leg
shaped flight member 54' but the upturned leg segment 56"
again serves to somewhat impede release of material.
The somewhat longer inner leg segment 58b " of
dog-leg member 58 " above straight flight member 54 "
stores material while still allowing some product to flow
into the showering zone to the left of material gravitat-
ing from the outermost end of straight member 54 " . The
greater length of leg segment 58b " assures that material
is collected in the channel-shaped area thereof but still
allow some of the material to be delivered from the
innermost end of the flight member. Return segment 60 "
on the end of dog-leg member 58 " contributes to regu-
lation of flow from the member 58 " since it is to be
noted that at this position of such member, the outermost
major leg segment 58a " is in a relatively upright posi-
tion where material may readily fall therefrom.
The dog-leg shaped flight member 62 " ' in Fig.
6 at a 1:00 position is moving toward vertical disposition
and now will start to deliver virtually all of the materi
al initially contained thereon into the zone 74. Flight
member 64 " ' to the left of member 62"' in the 12:30
position, while delivering material into the showering
zone 74 causes some of such material to be retained by
CA 02072297 1997-11-10
23
virtue of the leg segment 64b"' on the innermost extremity
thereof. Even the straight member 56"" at the 12:00
position will retain some material on the inner extremity
thereof by virtue of the return segment 56"".
The hook-shaped pattern of the dog-leg shaped
members 54, 58, and 64 assist in carrying material beyond
the upright center plane of the showering region 74 so
that there is material delivered into such zone to the
left of such upright center plane as also depicted in Fig.
6. It is to be recognized in this respect that the
depiction of material on the flight members as shown
schematically in Fig. 6 is for generally illustrative pur-
poses only and that by virtue of rotation of the drum 26
in a counterclockwise direction viewing Fig. 6, material
will to a certain extent be thrown toward the left side of
the showering zone 74 by a combination of momentum and
centrifugal force. The vertical dotted lines 74a in Fig.
6 are intended to schematically represent the flight of
fall patterns of moisture-sparing fragmented product which
is delivered from the flir members during rotation of
the drum 26 substantial? e~oughout the horizontal extent
of showering region or gone "4.
Viewing Figs. 6 and 7, it can be seen that in a
preferred arrangement of the flight patterns of rows 1-11
(Fig. 10), the non-linear dispositions of the outer leg
segments of the dog-leg shaped flight members 58, 62 and
64 contribute to prevention of blockage of material
between the flight members and assures that the openings
thereof remain free at all times. For example, the leg
segments 58a are directed away from leg segments 62a to
the right thereof, which are on the opposite side of
respective radial lines to compensate for the fact that
leg segments 58b and 62b of adjacent flight members
project toward one another. On the other hand, leg
segments 62a and 64a of adjacent flight members are non-
CA 02072297 1997-11-10
24
radial toward one another but the opening presented by
their inner leg segments 62b and 64b diverge so that there
is a tendency of the material to flow outwardly therefrom
even though respective adjacent outer leg segments 62a and
64a converge to a limited extent. The intermediate flight
members 50 are no factor insofar as a tendency to bridge
is concerned because of the relatively narrow extent of
such members radially of the drum 26.
In typical three-pass dryers heretofore avail
able, the temperature of the hot products of combustion
directed into the inlet of such dryers sometimes ran as
high as 2,000 to 2,200°F. For fuel efficiency and conser
vation purposes, that temperature is now limited to about
1,200°F. The moisture content of the material to be dried
frequently ran as high as about 85%, but now rarely ex-
ceeds 70% and usually is in the range of 35-60%. Assuming
an output moisture content not exceeding about 10%-15%
which is a frequently encountered product specification
requirement, the three-pass dryers in operation usually
have a hot gas temperature output of about 210°F. Airflow
rates were in the order of 11, 000 cubic feet per minute
for a 8'x24' three pass dryer.
It is desirable to obtain require drying of the
products at the lowest feasible temperature in order to
reduce stress on the material, prevent burning and re
strict the amount of fuel consumed in the process. All of
these parameters must be met at a capital expenditure cost
for equipment which is reasonable.
These objectives are met with the present
invention by providing a maximum area of heat transfer
surface or either metal or product with few if any voids
of holes in the showering zone 74 which would allow hot
gases to pass through without heat exchange with the moist
product. The large area of metal flight surfaces serves
CA 02072297 1997-11-10
to create high connective and conductive heat exchange
areas.
The present invention provides unexpected
benefits in maximizing heat transfer surface area not only
5 of the metal flights but also of the fragmented material
to be dried, but also by doing so in a manner that negates
the necessity of utilizing a three-pass, tortuous path
heat exchanger with its attendant heat stress, inefficien-
cies and relatively costly construction. This objective
10 is met in the present invention by virtue of the fact that
high capacity with minimum heat stress is obtained because
a greater number of pounds of hot gas is forced through
the heat exchanger while exchanging most of the available
heat, and discharging the product at the proper moisture
15 content. For example, in an 8 x 24 three-pass dryer,
which has a 16M btu per hour heat source, and an 11, 000
cfm airflow, the same components will allow an airflow of
15,700 cfm when modified to incorporate the components of
the present invention. Thus, the dryer capacity is
20 proportionately increased with a lower discharge tempera-
ture, less heat stress and more uniform control.
The efficiency of the present single pass dryer
as compared with conventional three-pass dryers has been
demonstrated by conversion of 8' x 24' dryers, as well as
10.4' x 30' three-pass dryers to single pass dryers
incorporating the features and components of this inven-
tion.
CA 02072297 1997-11-10
26
This is illustrated by the
following exemplary
tables of optimum achievable
results:
8'x2 4' 3-Pass
Dryer Conversion
Before After
Airflow at discharge 11,000 acfm 15, 700 acfm
Discharge temperature 160F. 140F.
Drum (heat exchanger)
size 8' dia. same
x 24' long
Primary fan HP require-
ment 50 HP 70 HP
Drum speed 10 rpm same
Heat source maximum
capacity 16 MMBTU/hr. same
Metal heat transfer
surface 3084 ft.2 3900 ft.2
Fragmented product tota'_
heat transfer surface 735 pd.x4 = 1764 pd.x4
=
4 ft.2/pd. 2940 ft.2 7056 ft.2
Retention time 4 min. 6 min.
Total square feet of
heat transfer surface 6024 ft.2 10956 ft.z
25% Moisture
Suncure product input 5 tonnes/hr. approx.8
12% moisture tonnes/hr.
finished prod. 12% moisture
finished prod
CA 02072297 1997-11-10
27
10.5'x 30 3-Pass Dryer Conversion
Before After
Airflow at discharge 28,000 acfm 40,000 acfm
Discharge temperature 210F. 180F.
Drum (heat exchanger) 10'4" dia. same
size x 30' long
Primary fan HP require-
ment 70 HP 125 HP
Drum speed 8 rpm same
Heat source maximum
capacity 28 MMBTU/hr. same
Metal heat transfer
surface 5342 ft.2 7335 ft.2
Fragmented product total
heat transfer surface 760 pd.x4 = 1600 pd. x
4=
@ 4 ft.z/pd. 3040 ft.z 6500 ft.2
Retention time 4 min. 6 min.
Total square feet of
heat transfer surface 8383 ft.2 13735 ft.z
60% Moisture
Wilted dehy product 5.7 ton/hr. 8 ton/hr.
input 12% moisture 12% moisture
Finished prod Finished prod
CA 02072297 1997-11-10
28
Detailed Description of the Preferred Embodiment of the
Invention
a. Drum Dryer
The preferred embodiment of an elongated,
generally horizontal, hollow drum heat exchanger for use
in a single pass dehydration system 20 as depicted in Fig.
1, is illustrated in Figs. 13-19 of the drawings and
designated generally by the numeral 226. It is to be
noted in this respect that drum heat exchanger 226 may
have flighting 248 of the type illustrated in connection
with the embodiment shown in Figs. 2-7 of the drawings,
the embodiment of Figs. 8-10, or in the alternative as
shown in Figs. il and 12.
The drum dryer 226 has a cylindrical shell 246
which is typically of essentially the same dimensions as
those specified above for cylindrical shell 46. Thus, the
shell 246 may have flights 248 of configurations and
dimensions the same as or similar to those previously
described and specifically depicted in Figs. 2-12
inclusive.
The principle difference between drum dryer 226
and the previously described embodiments thereof is the
provision of a series of transversely oriented, axially
spaced, circular discs or baffles 282, 284, 286 and 288
respectively as thus shown in Figs. 13 and 14. It is to
be seen from Fig. 13 that the peripheral edges of each of
the discs 282-288 respectively are spaced inwardly from
the innermost circumferentially extending margin 290 of
the flighting 248 projecting inwardly from the interface
of cylinder 246.
The baffles 282-288 are of successively greater
diameter in a direction extending from the inlet in 238 of
drum 226 toward the outlet end 230 thereof. Viewing Fig.
14 it is to be noted that four struts 292 carry each of
the baffles 282-288 in proper transverse disposition with
the outer end of each of the struts being suitably welded
CA 02072297 1997-11-10
29
or otherwise affixed to the inner surface of cylindrical
drum 246 while the inner end of each strut is welded to
one face of a corresponding disc 282-288. The struts 292
are preferably oriented so that they are essentially
tangential to the margins of respective baffles 282-288 in
order that the discs supported thereby may rotate as they
expand to prevent a fracture of the well joints during
operation of the dryer.
A series of annular deflectors 294, 296 and 298
respectively are mounted within dryer cylinder 246 between
adjacent baffle discs 282-288 inclusive as depicted in
Fig. 13. Each of the deflectors 294-298 is imperforate
except for the central opening defined thereby. Means
such as welding is employed to secure the deflectors 294
298 to the inner surface of cylinder 246 in locations such
that each is mid-way longitudinally of drum 226 between
adjacent discs 282-288.
It is apparent from the schematic depiction of
Fig. 13 that the baffle discs 282-288 and associated
deflectors 294-298 cooperate to cause the material being
dried and the hot products of combustion directed into
drum 246 through inlet 238 to follow an essentially
serpentine path 300 along the length of the dryer 226. It
is to be appreciated in this respect that the serpentine
path 300 of the particles being dried and the hot products
of combustion is also somewhat helical in nature in that
the products, and to a somewhat lesser extent the heated
gases, are caused to rotate to a certain extent within the
interior of the drum 226 as the drum is rotated about its
longitudinal axis.
In a preferred drum dryer which is approximately
10.5 feet in diameter, disc baffle 282 should be about 36
inches in diameter, disc 284 about 42 inches in diameter,
disc 286 about 48 inches in diameter and disc 288 about 54
inches in diameter. Disc 282 is preferably spaced about
CA 02072297 1997-11-10
6 feet from the inlet 238 of drum 226 while the spacing
between adjacent disc baffles is also about 6 feet. Thus,
the annular deflectors 294-298 are also spaced about 6
feet one from another. In the exemplary 10.5 foot
5 diameter dryer, each of the annular deflectors 294-298
extends inwardly of the cylinder 246 to the same extent as
flights 248. Thus, the peripheral margins of discs 282-
288 are spaced from an imaginary cylinder extending
through the inner margins of each of the deflectors 294-
10 298. The diameter of each opening defined by a respective
deflector 294-298 therefore should be about 6 feet 4
inches.
b. Stem Dryer
Stem dryer section 400 located adjacent the
15 outlet end dryer drum 226 is made up of tubular structure
defined by the cylindrical end section 402 of shell 246,
along with annular components making up a series of
annular drying compartments extending circumferentially of
the drum.
20 The innermost annular retainer ring 404 of dryer
section 400 defines the inlet thereof and is positioned
adjacent the dryer outlet end of flighting 248. Retainer
ring 404 is imperforate except for the central opening
therein. Ring 404 preferably should have a height of
25 about 12 inches in the case of a 10.5 foot diameter dryer
drum and therefore defines a cylindrical opening of about
8 feet 2 inches.
The outermost, annular discharge ring 406 of
stem dryer section 400 having an effective height of about
30 25 inches is secured to the inner surface of cylindrical
section 402 and defines a generally circular opening of
having a diameter of about 6 feet 2 inches in the case of
the representative 10.5 foot diameter drum. The discharge
ring 406 has four equally spaced, V-shaped, outwardly
extending, 60° notches 408 in the inner margin thereof.
CA 02072297 1997-11-10
31
The depth of each of the notches 408 is approximately one-
half the height of the ring 406 as is apparent from Fig.
18. Eight elongated, transversely L-shaped scoops 410 of
conventional construction are secured to the outer face of
discharge ring 406 in radial relationship thereto and
located such that they are not in alignment with any of
the notches 408 from Figs. 13 and 15, it can be seen that
discharge ring 406 is located inwardly of the discharge
outlet 230 a sufficient distance to accommodate scoops 410
therebetween.
As can be best observed from schematic sectional
view Fig. 13, it can be observed from that figure that an
intermediate annular ring 412 having an effective height
of about 15 inches is secured to the inner surface of
cylindrical section 246 one-half of the way between rings
404 and 406. In the instance of a 10.5 foot diameter
drum, the opening defined by the inner margin of ring 412
is therefore about 8 feet. Ring 412 is provided with
eight equally spaced, outwardly extending, 30° V-shaped
notches 414 in the inner periphery thereof. As can be
seen from Fig. 17, notches 414 are out of alignment with
the notches 408 in discharge ring 406. Retainer ring 404
and intermediate ring 412 cooperate to define a first,
annular inner stem drying chamber 416, while intermediate
ring 412 cooperates with discharge ring 406 to define an
outer annular stem drying chamber 418.
Annular chamber 416 is provided with
transversely oriented, inwardly directed, axially spaced
heat transfer fin members 420 and 422, while chamber 418
has similar fin members 424 and 426 along with a fin
member 428. Each of the fin members 420-426 has an
effective height of about 12 inches while fin member 428
is about 14 inches high, when fabricated to be
incorporated in a 10.5 foot diameter drum.
CA 02072297 1997-11-10
32
It is also apparent from Figs. 13 and 15 that
fin members 420-428 which are welded to the inner surface
of cylindrical section 402 are equidistantly spaced from
one another and from an adjacent retainer ring,
intermediate ring or discharge ring, longitudinally of the
dryer drum. Viewing Figs. 17 and 18, it can be seen that
each of the compartments 416 and 418 has seven inwardly
directed mixing flight members 430 aligned with seven of
the notches 414, and one dog-leg shaped mixing flight 432
aligned with the other notch 414. The dog-leg flight 432
is of greater height than flights 430. In the exemplary
10.5 foot dryer drum, flights 430 are preferably 2 inches
high, while the flight 432 has a major leg 4 inches high,
with the upper angle segment thereof being at a 45° angle
with respect to the main leg and of an effective height of
about 1 inch.
Operation of the Preferred Embodiment
When drum dryer 226 is used to remove moisture
from alfalfa, the crop to be dried generally has a
moisture content in the range of 35%-80%. That moisture
level should be reduced in the dryer to about 12%. In
order to accomplish this, and in the representative case
where the drum is about 10.5 feet in diameter and 40 feet
long, is rotated at about 8 rpm's, and hot gases at a
temperature in the range of about 700° to about 1800°F at
the inlet of the dryer, and introduced at an airflow
volume rate of about 30,000 cfm, is adequate in most
instances. Many windrowed alfalfa inputs to dehy dryers
today have a moisture content as low as about 35% moisture
to minimize the energy requirements for water removal. In
the case of 35% moisture alfalfa input to the dryer, the
hot gas temperature at the dryer inlet can be maintained
as low as about 700°F. However, it may be necessary to
use an inlet temperature as high as 2000°F to dry very wet
CA 02072297 1997-11-10
33
alfalfa. The temperature of the gases exiting the dryer
usually will be between 200°-250°F and the product at a
temperature of about 110°-130°F. Generally, the residence
time of the product in a dryer of the dimensions indicated
is about 5 minutes. A temperature controller is employed
at the exit of the dryer which regulates the temperature
of the heated gases introduced into the inlet of the dryer
by turning the burner up or down as necessary to assure
proper drying of the alfalfa product.
The provision of baffle discs 282-288 and
associated deflectors 294-298 increase the air path
through drum to an extent which is about 50% greater than
is the case without such baf f les and def lectors . Thus ,
the residence time and material is about a medium value
between that of a conventional single pass dryer and a
three pass dryer. However, there is only about a 30%
decrease in air volume. For example, if the air volume in
is approximately 50,000 cfm it has been found that the air
volume out will be about 30,000 cfm. The baffle discs
282-288 increase in diameter as the discharge end of the
dryer is approached in order to gradually increase to
maintain the heated gas velocity notwithstanding the fact
that the flow rate of the material being dried remains
essentially uniform, noting in this respect that the
weight of the product being dried decreases as the product
traverses the length of the dryer. Thus, the heated air
velocity will increase in a typical system from about 380
ft/m adjacent disc 282 to about 460 ft/m at the disc 288.
The longer serpentine and helical path that the material
is required to follow through dryer 226 causes the product
being dried to be exposed to the hot gases for a longer
time period, and at the same time allows the metal to
absorb heat from the gas for a greater period thus
increasing the heat transfer efficiency from the metal to
the product.
;i~~ ed W~ ?a:~a ~!c_~.;ty. ET RL. 8i6 4r~4 '305r
CA 02072297 1997-11-10
34
The Iea~,res of the alfalfa dry mare readily than
thQ stoma. When the inlet gas temperature is raised to a
level to effectively dry the stems to the desired 1~%
moisture contsnt, the leaves may be subjeLted to excessive
tetaperature for a tire such that burning of the leaves can
occur, or the full nutriea~t value of the leaves degraded
or partially or completely destroyed.
The provision of stem dryer section 400 all;~ws
the leaves to be discharged from the dryer more quickly
1~ than the stems so that the alfalfa stems may be s~:bjected
to the reaGad gas for a ls~nger interval of time than the
leaves. Ta thifi end, leaves Which have reahed the
desired moisture level, tend to directly exit from the
dryer w~:en the leaves reach the stem dryer section.
i~ However, those westerns which are heavier than the dry
leaves at t:~aat point in the dryer gravitate toward the
bottom of the campartmEnts 416 and 418 and remain in such
r.:ampartmsnts until the required 12% moisture level is
at~ained.
20 When the stems accumulate in chambers 416 and
4I8, rotation of the drum in a direction indicated by the
arrow in Fig. 19, causes the steno to tend to collect and
move upwardly in the directicn of rotation reflected by
the accumulated body of material 43ti in F;g. 19. The
25 purpose cf f.in mema~ers 42~-.~28 is to transfer thermal
energy fron respective fin meabers to the alfalfa stems.
Rs the drum rotates, the fin members are subjP~~ted to the
hot products of cembustion flowing through the ste~x dryer
s~actia~ 400 during the arcuate path of the fin manubers
30 reflected by the numeral 436 of Fig. 19, thus causing the
fin members to be raised to a temperature approaching that
of the hvt gases. During the arGUate rotational path o°
th; drum represented by the numeral 438 in Fig. 19, the
heat stored in the fin members is transferred by
TJL 0b '9c 1'r7:34 H.~'~~~, ~T ~L. 8':.6 ::''4 9~~T P.
CA 02072297 1997-11-10
3S
conduction to the a~.falfa stems thus proz~ating enhanced
drying of the products.
Flights 430 and 432 ir. chambers 416 and 418 not
only agitate the stems for most efficient transfer of heat
to the stems from t;le fin members, but also enhance
migration of the stems during dryiric~ thereof from chamber
416 to chamber 418 via notches 414 when the latter axe
positioned at the bottom of their paths of travel and
during the g~eriasi such notches are in the area represented
by the accua~ulatvd body of material 434. in like manner,
notches 408 in discharge ring 406 allow migrat~vn df
material from chamber 418 into the discharge in 234 of the
dryer 226. Scoops 410 assure that prbduct will empty from
the dryer and also kick out any rocks that might tend to
accumulate in stem dryer 400.
It has been determined that there is a gov3
correlation between the temperature of the heat transfer
surfaces of the fin rnombars 420-428 and the rings 404, 406
and 412, the temperature of the product to be dried, the
product volume, the residence time of the material in the
chambers 416 and 418, and the amount of moistsre which is
removed per unit of time. Thus, if more water must be
removed for d particular type of product, this can simply
be accomplisred by adding mare fin members in compartments
2~ 416 and 418, or to provide additional compartments with a
requisite number of fin members as determined by
4alculation of the paran~,etars as set forth above.
The spacing of fin members 420-428 as depicted
in the drawings is what has been found most .effective for
rQmoving moisture from alfalfa. However, if the dryer 226
is to be employed for removing moisture from fin~aly
camminuted ma~~erials such as bakery wastBS, wand products
or the like, again more effective results oan be Obtained
by adding more closely spaced fin members to vhe chambers
416 and 4~.8.
