Note: Descriptions are shown in the official language in which they were submitted.
1 158856
Background
This invention relates to a method and apparatus for drying fabrics
such as textilesO
Large commercial dryers are used for drying fabrics in a variety of
applications. For example, such dr~ers are used by commercial laundries, towel
ser~ices, diaper ser~ices, and text~le manufacturers and processors.
Much attention has been dlrected to improving the performance of
such dryers. For example, United States Ratent Numbers 1,56~,566; 3,157,391;
3,861,865; and 3,882,613 are all directed tQ improvements in dryers. Also, I
lQ have received United States Patent Numbers 3,419,969; 3,601,903; 3,815,257;
3,831,294; 3,921,308; 3,995,988; and 4,01~,55Q, all of which relate to drying
of textilesO
Commercially available dryers are able to quickly dry large quanti-
ties of fabrics. However, they tend to be inefficient, requiring excessively
la~ge quantities of energy for e~aporating water from fabrics Such ineffi-
cienc~ is particularly troublesome for "pass through" systems, where hot gas
used for drying the fabrics is discharged to the atmosphere, and not recycled
for further dryingO
In addition to inefficiency, another problem noted with commercial
2Q dryers is uneven drying in the drying chamberO This can result in the bulk of
the fabrics in the cha~ber being dry, with a small portion of the fabrics re-
maining wet. The drying cycle needs to be lengthened to dry the wet fabrics,
and this wastes energy and results in inefficient usage of the drying equipment.
It is believed that this problem of uneven drying results from "dead spots~ in
the drying chamber where introduced drying gas is unable to penetrate and cir-
culate.
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Thus, there is a need for an improved drying process and
an improved drying apparatus which are more energy efficient than
commercially available dryers and which provide more even drying
within a drying chamber.
Summary
The present invention provides apparatus. for drying fahrics
comprising: a drying chamber containing wet fabrics and moisture-
laden gas, said moisture-laden gas containing contaminants; means
for withdrawing moisture-laden gas from the drying chamher, heating
a first portion of the withdrawn gas, and recirculating said first
portion of the withdrawn gas back into the drying chamber; a dis-
charge conduit for discharging a second portion of the withdrawn
moisture-laden gas to the atmosphere; and a filter screen located
for filtering said first portion of the withdrawn gas before it is
recirculated into the drying chamber, the screen being located
adjacent to the discharge conduit, wherein the filter screen is
capable of removing and collecting contaminants from said first
portion of the withdrawn gas recirculated back into the dryi.ng
chamber and is capable of releasing contaminants to sai.d second
portion of the withdrawn gas discharged to the atmosphere via the
: discharge conduit.
Preferably, the discharge conduit and filter screen are
located remotely from the means for heating the withdrawn gas and
from the drying chamber.
In a preferred emhodiment, the filter screen has first
and second faces, and in th.e first position recirculated gas passes
through th.e screen from th.e first face to the second face, and in
the second position, discharged gas passes along the first face to
scrub contaminants therefrom.
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Preferably, the discharge conduit is narrower at its
entrance than adjacent to the filter screen.
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~ Drawings
- These and other features, aspects, and advantages of the present
invention will become better understood with reference to the following
description, appended claims, and accompanying drawings where:
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Figure 1 diagrammatically shows a direct fired dryer, partially
cut away, embodying features of'the present invention;
Figures 2 and 3 diagrammatically show the dryer of Figure 1 in a
drying mode;
~igure 4 is a view of the dryer of Figure 1 similar to that of Figure
3 where the dryer is in a cooling mode or ~'open loop" drying mode; ~ '
Figure S is a view similar to that o$ Figure 2 of an indirect fired
dryer embodying features o the present invention;
Figure 6 is a psychrometric chart showing the properties of gas
withdrawn from the drying chamber of the dryer of Figure 1 during the drying of
laundry; and
` Figures 7A, 8A, and 9A show various embodiments of lint filters for
!, use in the dryer of Figure 1 in position for removing contaminants such as lint
from recirculating air, and Figures 7B, 8B, and 9B show the same filters, res-
pectively, in`position for releasing collected contaminants to the atmosphere.
Pescription
The present invention is directed to methods and apparatus for drying
fabrics. By the term "fabrics" there is meant flexible materials which can re-
tain moisture, including, but not limited to synthetic and natural textiles,
2~ fibres, filaments, yarns, and the like. There is also included relatively im-
pervious material such as leather, and cellulosic structures like paper and wood.
Fabrics are dried by introducing a hot drying gas into a drying zone
or chamber containing wet fabrics and mositure-laden gas. In the drying chamber
moisture is evaporated from the fabricsO The pressure of the moisture-laden gas
in the drying chamber is greater than atmospheric pressure so that a portion of
the mo~sture-laden gas can be discharged from the drying chamber directly ~o the
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atmosphere. The nondischarged portion of the gas in the drying chamber is
withdrawn, and at least a portion of it is recirculated for introduction into
the drring chamberO Before it is reintroduced into the drying chamber, the
pressure of the withdrawn gas is increased, the gas is heated, and it is com-
bined with a dilution gas in an a~ount at leasit sufficient to about equal the
a~ount of gas discharged from the dr~ing ~one to reduce the absolute humidity
of the withdrawn gas and to make up what is discharged to the atmosphere.
~ ith reerence to Figures 1 and 2, there is shown a commercial
dryer 1~ embodying ~eatures oP the present inventionO The dryer includes a
1~ ~otatable, perforated drum 12 and ~iltable main housing 13 such as the main
housing shown in United States Patent Number 3,601,9030 The interior of the
drum is referred to as a drying chamber 14 hereinO An exhaust duct 16 connects
the bottom of the main housing 13 with the intake of a main circulating fan or
blower 18. The exhaust duct 16 can be provided with a damper l9o The outlet
o the fan 18 discharges a gas into a discharge duct 20 which leads into a gas
discharge passage 22 contained in a gas flow housing 240 The gas flow housing
24, which is shown in phantom in Figure 1, is rectangular, and is attached to
the top of the discharge duct 200 The housing 24 contains not only the gas
discharge passage 22 but also an air make-up passage 26. The two passages 22
2~ and 26 are partially separated by a vertical wall 27 and are interconnected by
an opening which is covered with an air filter 28, such as a fine mesh screen
o~ 2~ mesh. The gas discharge passage 22 and the make-up air passage 26 are
each provided with a valve-like da~per 30 and 31, respectivel~, each damper be-
ing operated by an air cylinder 32 and 33, respectively. The gas discharge
passage damper 31 is pivotally mounted so as to be able to close off the passage
between the gas discharge passage 22 and the air make-up passage 26.
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115~856
The gas discharge pas~age 3Q is pivotall~ mounted so as to be able to substan-
tially close off the gas discharge pass.age 220 The make-up air passage 26 is
attached to a chamber 34 ~hich can be either ahead of or surrounding a main
burner 36 that supplies the bulk of the energy for drying. The gas discharge
passage 22 and the air make-up passage 26 can be connected to external ducts
38 and 3~, respecti~elyO
~ he air ~ilter 28, ~hich ISA us;ed fox remo~ing lint from circulating
gas, can advantageously be used with a lint disposal apparatus such as the
apparatus described in United States~ Pate~t Number 3,966,4410 In such an
arxangement, the air filter screen 2~ is cylindrically shaped within the opening
between the gas discharge passage 22 and make-up air passage 26, revolves, and
i5 fitted with a small ribbon-type lint burning burner. The lint burner can
provide a portion of the heat required for heating the gas recirculated to the
drying zoneO
The main burner 36 preferably is the burner described in United
States Patent Number 4,128,388, incorporated herein by this reerence. Such a
bu~ner is able to operate both on liquid uels such as fuel oil and gaseous
fuels such as natural gas.
A combustion air fan 40 provides air through a outlet duct 42 to
the burner 360 Fuel introduced to the burner which is not burned immediately
at the burner is consumed in a secondary combustion zone 44. A dryer intake
duct 46 b~ings gases from the chamber 34 surrounding the burner into the hous-
ing 13 and then into the drum 120
The dryer is provided with the housing 13, a safety explosion hatch
50, an access door 52 to the dru~ 12, and a control panel 540 The housing 13
is provided ~ith at leas;t two ~ents 56 to the atmosphere and can be provided
with li~e steam injection bars or ports 580 The vents 56 can be no more than
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random leakage clearances, i.eO "construction clearances" which can result in
fabricating the dryer 1~ without requiring close tolerances. Thus, specially
constructed vents 56 are not required, but instead, random leakage can be
~elied uponO
There are two basic modes in which the dryer 10 can be operated, a
closed loop mode and an open loop modeO The closed loop mode is used for dry-
ing. The open loop mode is used primarily for cooling, but can also be used
for drying. The configuration of the gas flows in the drying mode are shown in
Figures 1-3 and the configuration of gas-flo~s in the open loop drying or cool-
ing mode is shown in Figure 40 During start-up, the dryer is operated in the
open loop mode to avoid the possibility of an explosive concentration of gas
developing in the dryer if the burner fails to igniteO
In the drying mode, moisture is evaporated from wet fabrics 62 in the
drying chamber 140 A portion of the moisture-laden gas in the drying chamber
is vented directly to the atmosphere via the moisture vents 56 in the main
housing 13. As is more fully described below, such venting directly to the
atmosphere does not require suction fans or the like because the drying chamber
is operated under positive pressure. By the term "directly to the atmosphere",
there is meant that discharge of gas to the atmosphere occurs ~ithout passage,
2~ through ducts, suction fans, and the like, but occurs through portions of the
~ain housing proximate to the drying chamberO
The remainder of the moisture-laden gas in the drying chamber 14 are
wi~hdrawn from the chamber 14 by the main circulating fan 18 via the exhaust
duct 16. The damper 19 in duct 16 is in the position sho~n by the solid lines
in Figure 2~
The withdrawn gas is blown by the main circulating fan 18 through
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1 15~856
the discharge duct 20 into the gas discharge passage 220 The gas discharge
damper 30 is maintained in a closed position so that substantially all of the
gas discharged by the fan 18 is blown through the filter 28 to remove lint and
other contaminants The damper 31 for the make-up passage 26 can be closed or
a small gap such as a 3/8 inch gap can be left between the damper 31 and the wall
of the make-up passage for educ*ion of air to be combined with the gas recir-
culated into the drying chamber 140 Hot gaseous combustion products produced
b~ burning of fuel in the burner 36 and the clean gas in the make-up passage 26
are combined in a chamber 34 surrounding the burner 30, and then the combined
gas is introduced into the drying chamber 140 The combustion products have a
xelative humidity that is lower than the relative humidity of the withdrawn gas~Any fine lint and other combustibles which pass through the filter 28
are consumed by the open flame in the burner 360 This reduces the amount of
lint which is recirculated and hence reduces the amount collected on the filter
28 and the amount discharged to ~he atmosphereO
The term "drying gas" as used herein refers to the hot gas introduced
into the drying chamberO As shown in Figure 1, the drying gas can be a combina-
tion of gas withdrawn from the drying chamber, gaseous combustion products of
fuel, and air educted through the make-up passage 260
2Q In the drying mode, a small amount of withdrawn gas can be discharged
to the atmosphere via the gas discharge passage 22 by opening the damper 30
very slightly, in the order of about 318 inch to 1/2 inchO This is done to
maintain the relative humidit~ of the drying gas introduced to the drying chamber
at less than about 10~
The circulating fan 18 increases the pressure of gas withdrawn from
the drying chamber 14 an amount sufficient that ~1) the drying gas is at a pres-
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1 158856
sure greater than atmospheric pressure and ~2) the pressure of the gas in the
drying chamber is maintained greater than atmospheric pressure, and generally
at a pressure of up to about 1 to 2 inches of ~ater.
In the open loop mode, as shown in Figure 4, both the gas discharge
passage damper 30 and the air make-up passage 31 damper are open. This permits
hot gas withdrawn from the drying chamber to be exhausted to the atmosphere
and cool gas to be sucked into the drying chamber via the make-up passage 26
by the circulating fanO The passage of hot gas across the face of the filter
28 creates a low pressure area over the face of the filter which scavanges the
lint and other contaminants from the filterO The contaminants are entrained in
the discharged gas and passed through the discharge duct 38 to atmosphere or a
remote lint collector. This feature of the filter screen is described below in
more detail.
After completion of cooling of the abrics 62 in the drying chamber
14, the gas discharge passage damper 30 and the air make-up passage damper 31
can be closed and the damper 19 in the fan intake duct 16 can be moved to a
closed position as shown by dashed line 64 in Figure 2. The door to the drying
chamber is. then opened, and the air blown by the fan 18 can blow dried fabric
out the doorO
The live steam injection bars 58 fitted near the bottom of the rota-
ting drum 12 can be used for localized contact heating of textiles to speed up
the heating of the fabric to the moisture evaporation point. Preferably, super-
heated steam is used. After cooling of the steam from heat transfer with the
textiles, the steam is simply entrained into the circulating gases in the system.
Hig~ pre~sure steam from the injection bars 58 can provide an ~air-seal~' between
the housing 13 and the rotating drum 12 to prevent by-pass of circulating drying
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1158856
gas around the drumO
The gas discharge damper 30 and the make-up air damper 31 can be
electrically interlocked to flame sensing equipment and combustion controls to
insure that the closed loop mode is operational only after and so long as com-
plete combustion is establishedO Preferably the air filter 28 is provided with
pressure sensing equipment so that if the lint screen is plugged, an alarm goes
off.
Figure 5 diagrammatically shows an indirect heated dryer 66 accord-
ing to the present invention in a closed loop drying mode. Elements in Figure
5 which are the same as elements in Figures 1-4 bear the same reference numer-
alsO The indirect fired dryer 66 differs from the direct fired dryer 10 prin-
cipally in that the burner 36 is replaced with an indirect heating unit 67.
The indirect heating unit 67 can be no more than a plurality of steam or thermal
fluid containing tubes, or electric heaters, or the likeO Because the burner
36 is not required, the indirect fired dryer 66 does not have a combustion air
fan 40. The dryer 66 includes a main housing 80 provided with a cold air door
820
As shown in Figure 5, the gas discharge passage 22 and the air make-
up passage 26 are completely separated by the wall 27. Each passage 22 and 26
is provided with a damper 68 and 69, respectively, across its base portionO The
air make-up passage also has a filter 70 across its base portion and a door 71,
which when closed, separates the make-up passage 26 from the atmosphere. The
door 71 can serve as an explosion hatch. The dryer 66 is sho~n in a closed-
loop drying mode in Figure 50 In this mode the damper 68 and cold air door 82
are substantially closed, the filter 70 is across the opening of the air make-
up passage, and the door 71 is left slightly open. Thus, gas blown by the fan
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1 ~58856
18 is cleaned by the filter 70 and educts air past the door 71 into the heating
unit 67~ Rather than relying on eduction of air into the heating unit 67~ a
make-up air fan 84 can be used to blow air past the door 71 into the heating
unitO
In the cooling mode, the air make-up passage damper 69 is closed
and the gas discharge passage damper 68 is opened to pass hot exhaust gases to
the atmosphereO The filter 70 can be pi~oted to a position across the base por-
tion of the gas air discharge passage 22 for cleaningO The cold air door 82 is
opened wide to the position shown by dashed line 83 in Figure 5. This blocks
the discharge from the heater 67 and permits atmospheric air to be sucked by
the fans 18 into the drying chamber 14 for cooling of the textiles therein.
Although Figures 1-5 only show batch drying, iOeO, the drying of a
batch of fabrics, the recirculating air system, air filter, and positive pres-
sure operation features of the present invention can all be used with continuous
systems such as described in United States Patent Numbers 3~815~287 and 4~010~550.
The psychrometric properties of the gas in the drying chamber are
important to the satisfactory operation of the dryers 10 and 66, particularly
with regard to efficient usage of fuelO It is important according to the pres-
ent invention that the dryers be operated at a high level of fuel efficiency,
i.e., minimization of the number of BTU's required per pound of water evaporated.
It has been determined that if either too little or too much water is evapor-
ated per cubic foot of drying air introduced into the drying chamber, the fuel
utilization of the dryer is unsatisfactoryO Thus, gas withdrawn from the drying
chamber has a relative humidity of at least about 15% and a wet bulb temperature
of at least about 140Fo This corresponds to an absolute humidit~ of about 0.13
pound of water per pound dry airO Also, the relative humidity of the withdrawn
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gas is no more than about 65% and the wet bulb temperature of the withdrawn gas
is no more than about 185Fo These values correspond to an absolute humidity
of about 0 8 pound water per pound of dry air. Within these ranges, fuel
efficiency is generally satisfactory.
When a temperature is presented herein, there is meant the dry bulb
temperature unless indicated otherwiseO A1SQ~ the term "relative humidity" is
defined as the ratio of the amount of water vapor actually present in a gas to
the greatest amount possible at the same temperatureO The term "absolute humid-
ity" refers to the actual amount of water ~apor present in the gas.
Differences have been noted between indirect fired drying and direct
fired dryingO With indirect fired drying, the temperature of the gas in the
drying chamber generally is lower than the temperature of the gas in the drying
chamber with direct fired drying. Thus, with indirect fired drying, as compared
to direct fired drying, there is more tendency for water vapor in gas withdrawn
from the dr~ing chamber to condense on internal, relatively cool surfaces of
the dryer. Such condensation has resulted in slippage of the apparatus used
for rotating the drum. Also, the lower temperatures tend to cause a~lower dry-
ing rate. To avoid these problems, in indirect fired drying, preferably the
withdrawn gas is maintained at a relative humidity of less than about 65~ and a
2~ wet bulb temperature of less than about 185~P, corresponding to an absolute hum-
idity of about 008 pound water per pound of dry air. On the other hand, with
direct fired drying, where these problems do not exist, preferably the withdrawn
gas is maintained at a relative humidity of less than about 55~ and a wet bulb
temperature of less than about 165P, corresponding to an absolute humidity of
about 0.35 pound water per pound dry airO
Differences have also been noted between direct fired drying with oil
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as the fuel versus direct fired dr~ing with gas as the fuel. When drying with
gas, fuel efficiency becomes unsatisfactor~ when the withdrawn gas has a rela-
tive humidity of less than about 35% and a wet bulb temperature of less than
about 15SF. Thereore, when drying with gas, preferably the withdrawn gas
has a relative humidity of at leasit about 35% and a wet bulb temperature of at
least about 155FD This corresponds to an absolute humidity of about 0.23
pounds of water per pound of dry airO
When drying with oil, if the withdrawn gas has too high a water
content, all of the oil is not consumed in the secondary combustion ~one, and
a portion of it can condense on the fabrics in the drying chamber. This can
result ln soiled and smelly fabricsO To avoid this problem, when operating a
direct fired dryer using oil, preferably the gas withdrawn rom the drying
chamber is maintained at a relati~e-humidity of less than about 32% and a wet
bulb temperature of less than about 160F. These values correspond to an absol-
ute humidity of about 0028 pounds of water per pound of dry airO
Figure 6 shows the psychrometric properties of withdrawn gas during
a co~plete cycle of direct fired drying using natural gas as the fuel in the
dryer of Figure lo A test was conducted with about 400 pounds dry weight of
laundry having a water retention of about 65%, i.eO, the laundry when loaded in
the drying chamber contained ~400 pounds~ x ~65%) = 260 pounds of water. The
laundry was dried in about 13 minutesO The curve in Figure 6 shows the psychro-
metric properties of various samples of withdrawn gas during the drying cycle
The samples taken include gas s~mples at the start, when the firing rate was
reduced, when the burner was shut off, and the end of cooling the laundry.
These samples are represented by points 73, 74, 75, and 76, respectively on the
cur~reO
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1 158856
As shown by the curve, during the initial portion of the drying
cycle, the temperature o the withdrawn gas and the moisture content of the
withdrawn gas increased until reaching a maximumO At this maximum, the laundry
had given up the bulk of its moisture. Thereafter, the moisture content of the
withdrawn gas decreased. As the firing rate Nas decreased the dry bulb tempera-
ture of the withdrawn gas also decreasedO Initially, the relative humidity of
the withdrawn gas was lOQ%, but it quickly dropped to about 38~ and then during
the portion of the drying cycle when the burner was operated at full capacity,
it was relatively constant in the ra.~ge of about 33 to 48~o
The curve in Figure 6 shows that both absolute humidity and the dry
bulb temperature of the withdrawn gas changed during the drying cycle with the
relative humidity being maintained relatively co.nstant at a selected range once
it stabilized after the initial start-upO
As noted above, the withdrawn gas i5 subjected to three process steps
before it is reintroduced as drying gas into the drying chamberO First, the
pressure is increased by the an 18 to compensate for pressure drops in the sys-
tem and to maintain the pressure in the drying chamber greater than atmospheric.
As the second and third steps, the withdrawn gas is heated and combined with a
dilution gas.O It is heated in a sufficient amount so that the drying gas has a
2a temperature of at least about 300Fo The higher the temperature of the drying
gas., the better for rapid dryingO Thus, preferably the drying gas has a temper-
ature o$ at least about 450Fo However, at temperatures higher than about 600F~
damage to fabrics, and in particular, damage to synthetic fabrics, can occurO
Therefore, preferably the drying gas is maintained at a temperature of less than
about 600F~
The drying gas is combined with a dilution gas to reduce its absolute
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1 158856
humidity. The dilution gas replenishes the gas discharged directly from the
drying chamber to the atmosphere, and that gas, if any, discharged via the air
discharge passage 22. In the indirect drying process, as shown in Figure 5, all
o the dilution gas is make-up air educted through the make-up air passage 25.
If necessary, external assistance means such as the small fan 8~ can be used for
~roviding the make-up air.
!~ rn a direct fired drying process, preferably the bulk, and more pre-
~erably, all of the dilution gas is provided by the combustion products of the
fuel with air. A small amount of make-up air can be educted as dilution gas
through the make-up air passage 26 by leaving a small gap between the make-up
air da~per 31 and the walls o the make-up air passage 26. A gap in the order
oP about 3/8 inch vas found to be satisfactoryO In such an operation, the dilu-
tion gas includes both the combustion products and educted air~
PrePerably, the dilution gas comprise at least about 5~ by valume of
the drying gas introduced into the drying chamberO If less than about 5% dilu-
tion gas is used, then the gas in the drying chamber has such a high moisture
content that the drying rate becomes unsatisfactorily low and the fuel usage
unsatisPactorily higho Furthermore, when using direct fired drying with oil as
the fuel, if the dilution gas constitutes less than about 5~ of the drying gas,
2a then oil condensation resulting in soiled and foul smelling fabrics in the dry-
ing chamber can result. It is necessary to dilute a sufficient amount when burn-
ing oil to avoid oil condensation on the fabrics in the drying chamber. On the
~ther hand, prePerably the dilution gas comprises at most about 20%, and more
prePerably at most about 10% oP the drying gas introduced into the drying chamb-
er. At dilution levels oP more than 1~%, and particularly at more than about
2Q%, excessive amounts of energy are required for bringing the drying gas up to
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sufficiently high temperatures o at least about 300F for introduction into
the drying chamber. Furthermore, if the dilution gas comprises more than about
10% of the gas introduced into the drying chamber, it is difficult to maintain
~ositive pressure in the drying chamber without using a supplementary fan for
blowing in make-up air. Therefore, the dilution gas comprises from about 5 to
about 20%, and more preferably from about 5 to about 10% by volume of the dry-
ing gas.
The relative humidity of the drying gas is low for rapid drying of
the fabrics in the drying chamber. Preferably the relative humidity of the
dr~ing gas is less than about 10%, and generally in the range of from about 0015
to about 10%o It is undesirable to have the relative humidity of the drying
gas be less than about 0.15% because to achieve this low value, so much dilu-
tion gas is required, excessive amountsof fuel are required for heating the
dilution gas.
The preferred method for controlling the operation of the driers 10
and 66 is to monitor the temperature of the gas withdrawn from the drying cham-
ber. I the temperature of the withdrawn gas is higher than desired, the rate
at which fuel is burned is decreased. If the temperature is lower than desired,
the rate at which fuel is burned is increa~edO
As shown in Figure 4, in the open loop mode, lint is blown from the
lint screen~ As is more clearly shown in Figures 3 and 4, preferably the gas
air discharge passage 22 is narrower across its base or throat 80 than it is
in the ~icinity 82 o the ilter 28 and the filter is recessed relative to the
entrance. This results in the gas discharged via the gas discharge passage
creating a vacuum across the face of the filter. This vacuum assists in scrub-
bing contaminants from the filter for discharge to the atmosphere or collection.
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Three other embodiments of filters according to the present inven-
tion are shown in Figures 7-9, ~ith Figures 7A, 8A, and 9A showing the three
embodiments in a lint collecting mode with the dryer operated in a closed loop
mode, and Figures 7B, 8B, and 9B showing the respective filters in a lint release
mode when the dryer is operating in an open loop mode.
The filter 83 shown in Figure 7A is a rotating cylindrical drum
filter built into the wall 27 separating the gas discharge passage 22 from the
make-up air passage 260 A damper 84 for the make-up air pas,sage is curved so
as to conform to the outer wall of the filter 83 so that passage of gas between
the discharge and make-up passages can be preventedO
The filter 86 shown in Figure 8 is a slidable filter that fits across
either the air discharge passage 22 or the make-up air passage 26. The position
of the filter is controlled by an air or hydraulic fluid mechanism 860
The filter 88 shown in Figure 9 is substantially the same as the
one shown in Figures 3 and 4, except that it is pivotally mounted on the separa-
ting wall 27 so that it can be pivoted into position across the air discharge
passage 22 ~Figure ~B~ so that all gas, discharged through this passage can
sweep contaminants from the filter.
The process and apparatus of the present invention have many advan-
2~ tages compared to prior art processes and apparatusesO For example, excellentfuel utilization is achievedO Operation of a dryer according to the present
in~ention in the closed loop mode with steam coil heat required only about 2,000
B~Us to e~aporate a pound of water, compared to 4,500 BTUs per pound of water
for a conventional open loop sys,temO This amounts to a 55% reduction in fuel
requirements,
~hen operating the direct fired dryer of Figure 1, it has been deter-
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1 158856
mined that as iittle as 1,65Q BTUs are required per pound of water evaporated.
Since the minimum practical heat required to evaporate water in a dryer is
about 1,500 BTUs per poundJ the dryer of the present invention can achieve the
startling high efficiency of about 9~%. For direct fired dryers, improvements
of 25~ are easily obtainableO For a 4QO pound load having a 65% water reten-
tion, the energy savings can amount to 138,000 BTUsO
In addition to fuel sayings, other advantages of the apparatus and
method of the present invention have been noticed. For example, because of
the moisture content of the drying gas, there is a reduced tendency to scorch
the surfaces of textiles in the drying chamberO Fur*hermore, it has been no-
ticed that the abrics in the drying chamber have a "softer touch" due to the
presence of moisture in the drying gasO
Other important advantages result from the use of positive pressure
in the drying chamber. Because of this pressure, more uniform drying occurs,
with all surface areas of the fabrics being available for dryingO Because of
the positive pressure in the dryer, surface evaporation is improved due to the
~omni-directional" gas leakage rom the drying chamber which carrieS off mois-
ture in all directions, whereas negative pressure systems tend to release sur-
~ace moisture only in the direction of circulating air flow. By the term "omni-
directional", there is meant that gas is discharged from the drying zone in aplurality of directions~ This is particularly important in drying impervious
~aterials such as hides, skins, synthetics, and the like~ In addition, uni-
form drying is obtained due to the positive pressure in the drying chamber be-
cause inward leakage of air is prevented, and thus cold air stratification in
the drying chamber is avoided~
Compared to conventional open loop drying systems, the quantity of
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make-up air required is reduced substantiallyO This reduces building heating
and ventilation requirements~ Also, air circulation rate through the fabrics
being dried can be improved. In ~ome open loop operations, large quantities
of make-up air often are not available and the dryer is literally starved for
make-up air.
Another advantage of the closed loop system is that the gas vented
from the drying chamber generally has an absolute humidity greater than about
0.15 pounds of water per pound of dry air. This is sufficientlr high that the
psychrometric properties of the gas withdrawn from the drying chamber and/or the
lQ gas discharged from the drying chamber can be monitored as an indication of the
progress of the drying process~ The highly saturated condition of the small
amount of vented air obtained with the process of the present invention is much
more indicative of the moisture content of the fabrics within the drying chamber
than is the large volume of relatively dry discharge air obtained in convention-
al Qpen loop systems. Thus, the moisture content of gas discharged and/or with-
drawn from the drying chamber can be determined during the drying process, and
the heating of withdrawn gas can be substantially automatically terminated by
appropriate control apparatus when the moisture content of the gas reaches a
preselected value.
Another advantage of the positive pressure system is that convention-
al mechanical wipers or baffles normall~ used in the rotating drying chamber to
prevent by-pass of circulating drying air around the drying chamber are not re-
quiredO
~ hese and other advantages of the present in~ention will become bet-
ter understood from the following examplesD
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Example 1
Four hundred pounds of laundr~ ha~ing a water retention of 65% were
dried in the direct fired dryer of Figure 1 using about 4~ S~FM maximum rate of
natural gas and 5~0 SCFM of airO Drying gas was introduced to the drying cham-
ber at a rate of about 7,000 SCFMo Thus, the dilution gas amounted to about
7.8% ~544/7000 x 100) of the drying gasO The total energy requirements were
about 408 SCFM of natural gas.
Example 2
The test of Example 1 was duplicated except that the natural gas was
replaced with Number 1 fuel oil having an energy content of 137,000 BTUs per
gallon. Fuel oil was burned at a maximum rate of 20 gallons per hour with 550
SCFM of airO The laundry took about 13 minutes to dry and required a total of
3.13 gallons of fuel oil.
Although the present invention has been described in considerable
detail with reference to certain versions thereof, other versions are possible.
For example, all the dryers shown in figures use drying gas entering the top of
the drying chamber. However, the present invention is useful with a bottom
entry "up blast" drying gas dryers and other configurations, including "omni-
directional" air flow.
24 In addition, the gas flow housing 24, which contains the gas dis-
charge passage 22, air make-up passage 26, air filter 28, and valve-like dampers,
can be located remotely from the dryer 10 or 66 by suitable interconnecting duct
work. ~xemplary of this concept is a roof mounted gas flow housing 24.
Furthermore, the method for evaporation of the moisture described
~erein can be enhanced by rapid intermittent iull exchange of circulating gas
to the atmosphere in lieu of or in combination with the previously described
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1 1~8856
constant bleed methodO During these quick intermittent exchanges, which last
from only about 5 to about 20 seconds, the closed loop apparatus dampers can
be switched so as to create a vacuum effect to improve ~he operation. On direct
fired units, the burner can be shut off if a vacuum purge system is used. Dur-
~ng these quick intermittent exchanges, the psychrometric properties and temp-
erature of the drying gas and the gas in the drying zone can, for short periods,
be outside the ranges specifîed aboveO Thus, it should be realized that the
psychrometric properties and temperatures presented herein are time averaged
values.
l~ In addition to using the apparatus and method of the present in-
vention for drying of fabrics, they can also find application in bulking, dye
setting, heatsetting, relaxing, shrinking, and the likeO
In view of these modifications, the spirit and scope of the present
invention should not be limited to the description of the preferred versions
described herein.
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