IMPROVED DRYING METHOD
Field of the Invention
[0001] The present invention relates t o the drying of textile fibres and fabrics i n a
tumble dryer using a system which utilises only limited quantities of energy, and which
reduces drying-related creasing and associated textile fabric damage. Specifically, the
invention provides a method adapted for use i n this context.
Background to the Invention
[0002] Tumble drying processes are a mainstay of both domestic and industrial textile
fabric cleaning procedures and typically involve placing the textiles i n a container such
as a perforated cylindrical drum which i s rotated i n alternating clockwise and anti¬
clockwise cycles whilst hot air is introduced into the drum through the perforations. A
combination of the hot air treatment and the mechanical action of the tumbling process
causes water t o be expelled from the textile materials i n order that drying i s achieved.
[0003] However, such processes, though generally very effective, are usually
characterised by high levels of energy consumption, both i n terms of effecting rotation
of the container and, most particularly, i n generating heated air. Typically, prior art
processes may involve prolonged treatments at high temperatures i n order t o effect the
required degree of drying. Clearly, however, the lower are the energy requirements of a
system, the more efficient is the system and its associated drying process.
Consequently, there i s a desire t o reduce both the time of such drying treatments and
the temperature at which they are carried out i n order t o provide more efficient
processes, whilst maintaining equivalent drying performance.
[0004] Current efficient domestic tumble dryers are graded i n terms of energy
consumption according t o E U Directive 92/75/EEC and, more specifically, Directive
95/13/EEC, with category 'A' dryers being the most efficient, and category 'G' the least
efficient. Hereinafter, energy consumptions are quoted for the cotton drying cycle for
each machine type, i n kWh/kg of drying load. Thus, for vented tumble dryers, 'A' class
consumption i s <0.51 kWh/kg, 'C class (most common) i s between 0.59 and 0.67
kWh/kg, whilst 'G' class i s >0.91 kWh/kg. These values differ slightly for condenser
tumble dryers, with 'A' class at <0.55 kWh/kg, ' C class (most common) at between 0.64
and 0.73 kWh/kg, and 'G' class at >1.00 kWh/kg. With average domestic dryer
capacities now at around 8.0 kg, this equates to a typical consumption for a ' C class
vented tumble dryer of 4.7-5.4 kWh/cycle; a n 'A' class equivalent machine would run at
<4.1 kWh/cycle. Some vented domestic dryers are now capable of performing beyond
this lower limit and, at the time of writing, the energy labelling system i n the European
Union i s being adjusted i n line with this, such that tumble dryers will soon move to A +
and A++ labels. Performance levels i n the domestic sector generally set the highest
standard for a n efficient fabric drying process. Energy consumption i n industrial tumble
drying i s usually higher, due to the need for faster cycle times. I t i s also noteworthy
that, overall, tumble drying i s significantly less efficient than washing as a component
part of the laundry process i n either sector.
[0005] Heating of the circulating air i s the principal use of energy i n such tumble
dryers and the present inventors have therefore sought to effect improvements i n the
prior art processes by reducing the temperature levels required i n such processes. This
has been possible by means of changes made to the mechanical action of the process
o n the fabric i n the drying load. Mechanical action i n a conventional, horizontal axis
tumble dryer i s generated by the forces acting o n the fabric through falling and hitting
either other fabric or the dryer inner drum surface, whilst the fabric is interacting with the
forced hot air flow. This results i n release and evaporation of water from within the
fabric, and hence drying. In the method herein provided, alteration of the mechanical
action of the process i n order to promote more localised release and evaporation of
water at the fabric surface has resulted i n lower drying temperatures. As a further
potential benefit, i t has been found that the changes made can also reduce the degree
of fabric folding, and hence the level of creasing associated with tumble drying.
Creasing, which concentrates stresses during this drying process, i s a major source of
localised fabric damage. Ironing at high temperatures i s then the conventional means
used to remove such creasing and this, too, brings a fabric damage penalty.
Prevention of fabric damage (i.e. fabric care) i s of primary concern to the domestic
consumer and the industrial user. Furthermore, i f creasing i s reduced, there is also the
secondary benefit to the user of convenience resulting from less ironing.
[0006] Hence, the present inventors have sought to devise a new approach to the
drying problem, which allows the above deficiencies associated with the methods of the
prior art to be overcome. The method which is provided eliminates the requirement for
the use of high drying temperatures for extended periods of time, but is still capable of
providing a n efficient means of water removal, so yielding economic and environmental
benefits. The method which is provided also promotes fabric care through reduced
creasing and fewer requirements for subsequent ironing.
[0007] In WO-A-2007/1 28962 there i s disclosed a method and formulation for
cleaning a soiled substrate, the method comprising the treatment of the moistened
substrate with a formulation comprising a multiplicity of polymeric particles, wherein the
formulation i s free of organic solvents. In preferred embodiments, the substrate
comprises a textile fibre and the polymeric particles may, for example, comprise
particles of polyamides, polyesters, polyalkenes, polyurethanes or their copolymers, but
are most preferably i n the form of nylon particles.
[0008] The method disclosed i n this prior art document has been highly successful i n
providing a n efficient means of cleaning and stain removal which also yields significant
economic and environmental benefits due to its use of a cleaning formulation which
requires the use of only limited amounts of water. The present inventors have now
sought to provide a drying process which adopts a similar approach to that disclosed i n
WO-A-2007/1 28962, and which offers benefits i n terms of reduced energy
requirements, whilst still providing a n acceptable level of performance, and have
succeeded i n achieving at least equivalent drying performance whilst employing
significantly reduced process temperatures. Thus, a process i s provided wherein the
drying effect achieved as a consequence of mechanical interaction of a wet substrate
with physical media i s optimised, such that excellent drying performance may be
achieved at much lower temperatures (i.e. low energy) without extending drying times.
Additional benefits have also been observed i n terms of the reduction of fabric creasing
and associated fabric damage.
Summary of the Invention
[0009] The present invention derives from a n appreciation o n the part of the inventors
that optimum drying performance can be achieved as a result of improved mechanical
interaction between substrate and physical media. This can be effected by the use of
solid particles i n the drying process and i s a function of the number, size and mass of
the particles and the free volume within the vessel i n which the drying operation takes
place, i n addition to the G force dictated by its speed of rotation. Free volume i n this
context refers to the space inside the vessel which remains unoccupied by wet
substrate or particulate media, and G force i s defined o n the basis of the centripetal
forces which are acting.
[0010] Thus, according t o a first aspect of the present invention, there i s provided a
method for the drying of a wet substrate, said method comprising treating the substrate
with a solid particulate material at ambient or elevated temperature, said treatment
being carried out i n a n apparatus comprising a drum comprising perforated side walls,
wherein said drum comprising perforated side walls i s rotated so as t o facilitate
increased mechanical action between said substrate and said particulate material.
[0011] In a n embodiment of the invention, said drum comprising perforated side walls
has a capacity of between 5 and 50 litres for each kg of substrate. Typically, said drum
i s rotated at a speed which generates G forces i n the range of from 0.05 t o 0.99 G .
[0012] In certain embodiments of the invention, the drum comprising perforated side
walls comprises a rotatably mounted cylindrical cage.
[0013] Typically, said solid particulate material comprises a multiplicity of particles
which may be polymeric, non-polymeric or mixtures thereof, and which may be added at
a particle t o fabric addition level of 0.1 : 1 -10:1 by mass.
[0014] The size of said particles, i n combination with their material density and the
total particle t o fabric addition level, determines the number of particles which are
present i n a process according t o the invention. Each particle may have a smooth or
irregular surface structure, can be of solid or hollow construction, and i s of such a
shape and size t o allow for good flowability and intimate contact with the soiled
substrate, which typically comprises a textile fabric. A variety of shapes of particles can
be used, such as cylindrical, spherical or cuboid; appropriate cross-sectional shapes
can be employed including, for example, annular ring, dog-bone and circular. Most
preferably, however, said particles comprise cylindrical or spherical particles.
[0015] Polymeric particles typically have a n average density i n the range of 0.5-2.5
g/cm
3
, more typically from 0.55-2.0 g/cm
3
, more typically from 0.6-1 .9 g/cm
3
. Nonpolymeric particles generally have a n average density i n the range of from 3.5-12.0
g/cm
3
, more typically from 5.0-10.0 g/cm
3
, most typically from 6.0-9.0 g/cm
3
. The
average volume of both the non-polymeric and polymeric particles i s typically i n the
range of 5-275 mm
3
, more typically from 8-140 mm
3
, most typically from 10-120 mm
3
.
[0016] In the case of cylindrical particles - both polymeric and non-polymeric - of oval
cross section, the major cross section axis length, a , i s typically i n the range of from 2.0-6.0 mm, more typically from 2.2-5.0 mm, most typically from 2.4-4.5 mm, and the minor
cross section axis length, b , i s typically i n the range of from 1.3-5.0 mm, more typically
from 1.5-4.0 mm, and most typically from 1.7-3.5 mm (a > b). The length of such
particles, h , i s typically from 1.5-6.0 mm, more typically from 1.7-5.0 mm, and most
typically from 2.0-4.5 mm (h/b i s typically i n the range of from 0.5-10).
[0017] For cylindrical particles - both polymeric and non-polymeric - of circular cross
section, the typical cross section diameter, d
c
, is i n the range of from 1.3-6.0 mm, more
typically from 1.5-5.0 mm, and most typically from 1.7-4.5 mm. The typical length, h
c
, of
such particles i s again from 1.5-6.0 mm, more typically from 1.7-5.0 mm, and most
typically from 2.0-4.5 mm (hc/d
c
is typically i n the range of from 0.5-10).
[0018] In the case of both polymeric and non-polymeric spherical particles (not perfect
spheres) the diameter, d
s
, is typically i n the range of from 2.0-8.0 mm, more typically i n
the range of from 2.2-5.5 mm, and most typically from 2.4-5.0 mm.
[0019] In embodiments where the particles, whether polymeric or non-polymeric, are
perfect spheres, the diameter, d
ps
, is typically i n the range of from 2.0-8.0 mm, more
typically from 3.0-7.0 mm, and most typically from 4.0-6.5 mm.
[0020] Polymeric particles may comprise either foamed or unfoamed polymeric
materials. Furthermore, the polymeric particles may comprise polymers which are either
linear or crosslinked.
[0021] Preferred polymeric particles comprise polyalkenes such as polyethylene and
polypropylene, polyamides, polyesters or polyurethanes. Preferably, however, said
polymeric particles comprise polyamide or polyester particles, most particularly particles
of nylon, polyethylene terephthalate or polybutylene terephthalate.
[0022] Optionally, copolymers of the above polymeric materials may be employed for
the purposes of the invention. Specifically, the properties of the polymeric materials
may be tailored to individual requirements by the inclusion of monomeric units which
confer particular properties o n the copolymer. Thus, the copolymers may be adapted to
attract moisture by comprising monomers which, inter alia, are hydrophilic through being
ionically charged or including polar moieties or unsaturated organic groups.
[0023] Non-polymeric particles may comprise particles of glass, silica, stone, wood, or
any of a variety of metals or ceramic materials. Suitable metals include, but are not
limited to, zinc, titanium, chromium, manganese, iron, cobalt, nickel, copper, tungsten,
aluminium, tin and lead, and alloys thereof. Suitable ceramics include, but are not
limited to, alumina, zirconia, tungsten carbide, silicon carbide and silicon nitride. I t i s
seen that non-polymeric particles made from naturally occurring materials (e.g. stone)
can have various shapes, depending o n their propensity t o cleave i n different ways
during manufacture.
[0024] In further embodiments of the invention, said non-polymeric particles may
comprise coated non-polymeric particles. Most particularly, said non-polymeric particles
may comprise a non-polymeric core material and a shell comprising a coating of a
polymeric material. In a particular embodiment, said core may comprise a metal core,
typically a steel core, and said shell may comprise a polyamide coating, for example a
coating of nylon.
[0025] In accordance with the present invention, the selection of specific particle type
(polymeric and non-polymeric) for a given drying operation i s particularly important i n
optimising fabric care. Thus, particle size, shape, mass and material must all be
considered carefully i n respect of the particular substrate which i s t o be dried, so that
particle selection i s dependent o n the nature of the garments t o be dried, i.e. whether
they comprise cotton, polyester, polyamide, silk, wool, or any of the other common
textile fibres or blends which are commonly i n use.
[0026] The generation of suitable G forces, i n combination with the action of the solid
particulate material, i s a key factor i n achieving a n appropriate level of mechanical
action o n the wet substrate. G is a function of the drum size and the speed of rotation
of the drum and, specifically, is the ratio of the centripetal force generated at the inner
surface of the cage t o the static weight of the wet substrate. Thus, for a cage of inner
radius r (m), rotating at R (rpm), with a load of mass M (kg), and a n instantaneous
tangential velocity of the cage v (m/s), and taking g as the acceleration due t o gravity at
9.81 m/s
2
:
Centripetal force = Mv / r
Load static weight = g
v = 2TTrR 60
Hence, G = 4
2
r R /3600rg = 4TT rR /3600g = 1.18 x 10 rR
2
When, as i s usually the case, r is expressed i n centimetres, rather than metres, then:
G = 1. 1 18 x 10
5
rR
2
Hence, i n a preferred embodiment of the invention, for a drum of radius 37 c m
(diameter 74 cm) rotating at 48 rpm, G = 0.95. Typically, for such a drum, optimum
speeds of rotation are i n the range of from 10 t o 49 rpm.
[0027] In preferred embodiments of the invention, the claimed method additionally
provides, o n completion of the drying process, for separation and recovery of the
particles comprised i n the solid particulate material, which are then re-used i n
subsequent drying procedures.
[0028] Said rotatably mounted cylindrical cage i s comprised i n any suitable tumble
drying apparatus comprising a housing and access means, allowing access to the
interior of said cylindrical cage. In a preferred embodiment, said apparatus may
comprise:
(a) housing means, having:
(i) a first upper chamber having mounted therein said rotatably
mounted cylindrical cage, and
(ii) a second lower chamber located beneath said cylindrical cage;
(b) recirculation means;
(c) access means;
(d) pumping means; and
(e) delivery means,
wherein said rotatably mounted cylindrical cage comprises a drum comprising
perforated side walls, wherein up to 60% of the surface area of said side walls
comprises perforations, and said perforations comprise holes having a diameter of no
greater than 25.0 mm.
[0029] Said drying process also comprises the introduction of either ambient or heated
air into said drum comprising perforated side walls. I f said air i s heated, this i s achieved
by means of any commercially available air heater and circulated using a fan so as to
achieve a temperature of between 5 ° and 120°C, preferably between 10° and 90°C,
most preferably between 20° and 80°C i n the apparatus. The temperature of ambient
air i s dependent o n the surroundings i n which the drying process i s running, but this can
typically vary from 5-20°C.
[0030] I t should be particularly noted that heating the air naturally results i n heating of
the particulate media i n the drying process. This heat then i s retained by the particles
o n completion of a drying cycle and, hence, i f the next drying cycle occurs within the
time taken for the particles to cool down, there will be a transfer of this retained heat t o
that subsequent drying process. There is, therefore, a n even greater level of drying
efficiency achievable i n the event that multiple drying cycles are run consecutively. This
is, of course, applicable to both the domestic and industrial laundry sectors - but, most
particularly, to the latter. Rapid turnaround of drying cycles and high load throughput
are both key factors i n this kind of drying operation i n a n industrial scenario.
[0031] As a consequence of employing the method of the present invention, excellent
drying performance may be achieved whilst using reduced temperatures (i.e. lower
energy consumption), without increasing drying times. Thus, drying operations
according to the invention are typically carried out at temperatures which are 20°C lower
than with prior art processes, whilst achieving equivalent drying performance for the
same time of treatment.
Brief Description of the Drawings
[0032] Embodiments of the invention are further described hereinafter with reference
to the accompanying drawings, i n which:
Figure 1 i s a diagrammatic representation of particles which are employed i n
the method of the invention;
Figure 2 i s a graphical representation of the efficiency of the drying process
according to a n embodiment of the invention; and
Figure 3 a graphical representation of the efficiency of the drying process
according to a further embodiment of the invention.
Detailed Description of the Invention
[0033] In apparatus employed i n the method of the invention, said access means
typically comprises a hinged door mounted i n the casing, which may be opened to allow
access to the inside of the cylindrical cage, and which may be closed i n order to provide
a substantially sealed system. Preferably, the door includes a window.
[0034] Said rotatably mounted cylindrical cage i s mounted horizontally within said
housing means. Consequently, i n preferred embodiments of the invention, said access
means i s located i n the front of the apparatus, providing a front-loading facility.
[0035] Rotation of said rotatably mounted cylindrical cage i s effected by use of drive
means, which typically comprises electrical drive means, i n the form of a n electric
motor. Operation of said drive means i s effected by control means which may be
programmed by a n operative.
[0036] Said rotatably mounted cylindrical cage i s of the size which i s to be found i n
most domestic or industrial tumble driers, and may have a capacity i n the region of 50
t o 7000 litres. A typical capacity for a domestic machine would be i n the region of 80 to
140 litres and, for a n industrial machine, this range would typically be from 170 to 2000
litres.
[0037] Said rotatably mounted cylindrical cage i s located within a first upper chamber
of said housing means and beneath said first upper chamber i s located a second lower
chamber which functions as a collection chamber for said solid particulate material.
[0038] Said housing means is connected to standard plumbing features, thereby
providing recirculation means, for returning said solid particulate material from said
lower chamber, and delivery means, by virtue of which said solid particulate material
may be returned to said cylindrical cage.
[0039] In operation according to the method of the invention, agitation i s provided by
rotation of said rotatably mounted cylindrical cage and by the introduction of heated air.
Thus, said apparatus additionally comprises means for circulating air within said
housing means, and for adjusting the temperature therein. Said means may typically
include, for example, a recirculating fan and a n air heater. Additionally, sensing means
may also be provided for determining the temperature and humidity levels within the
apparatus, and for communicating this information to the control means.
[0040] Said apparatus comprises recirculation means, thereby facilitating recirculation
of said solid particulate material from said lower chamber to said rotatably mounted
cylindrical cage, for re-use i n drying operations. Preferably, said recirculation means
comprises ducting connecting said second chamber and said rotatably mounted
cylindrical cage. More preferably, said ducting comprises control means, adapted to
control entry of said solid particulate material into said cylindrical cage. Typically, said
control means comprises a valve located i n feeder means, preferably i n the form of a
feed tube attached to the apex of a receptor vessel located above, and connected to
the interior of, said cylindrical cage.
[0041] Recirculation of solid particulate matter from said lower chamber to said
rotatably mounted cylindrical cage is achieved by the use of pumping means comprised
i n said recirculation means, wherein said pumping means are adapted to deliver said
solid particulate matter to said control means, adapted to control the re-entry of said
solid particulate matter into said rotatably mounted cylindrical cage. Preferably, said
recirculation means comprises a vacuum pumping system.
[0042] In operation according t o the method of the invention, during a typical cycle,
cleaned garments containing residual moisture are first placed into said rotatably
mounted cylindrical cage. The cylindrical cage i s caused t o rotate and ambient or
heated air i s introduced via the perforations i n the cage before the solid particulate
material i s added. During the course of agitation by rotation of the cage, water i s
caused t o be removed from the garments by evaporation and a quantity of the solid
particulate material falls through the perforations i n the cage and into the second
chamber of the apparatus. Thereafter, the solid particulate material i s re-circulated via
the recirculation means such that i t i s returned, i n a manner controlled by said control
means, t o the cylindrical cage for continuation of the drying operation. This process of
continuous circulation of the solid particulate material occurs throughout the drying
operation until drying i s completed.
[0043] Thus, the solid particulate material which exits through the perforations i n the
walls of said rotatably mounted cylindrical cage and into said second chamber i s carried
t o the top side of said rotatably mounted cylindrical cage, wherein i t i s caused, by
means of gravity and operation of the control means, t o fall back into said cage, thereby
t o continue the drying operation.
[0044] Preferably, pumping of fresh and recycled solid particulate material proceeds at
a rate sufficient t o maintain approximately the same level of material i n said rotatably
mounted cylindrical cage throughout the drying operation, and t o ensure that the ratio of
particulate material t o substrate stays substantially constant until the cycle has been
completed.
[0045] On completion of the cycle, feeding of solid particulate material into the
rotatably mounted cylindrical cage ceases but rotation of the cage continues so as t o
allow for removal of the solid particulate material. Air heating and re-circulation may
also be stopped at this point. After separation, the solid particulate material i s
preferably recovered i n order t o allow for re-use i n subsequent drying operations. Said
separation of particulate material removes >99% of these particles, and typically
removal rates approach, or actually reach, 100%.
[0046] Generally, any remaining solid particulate material o n said at least one
substrate may be easily removed by shaking the at least one substrate. I f necessary,
however, further remaining solid particulate material may be removed by suction
means, preferably comprising a vacuum wand.
[0047] Said rotatably mounted cylindrical cage more preferably has a volume of
between 5 and 50 litres for each kg of fabric i n the load. Preferred rates of rotation of
said rotatably mounted cylindrical cage are sufficient to give G forces of between 0.05
and 0.99 G . Typically the drying process and the subsequent separation of the
particles from the fabric are both carried out within this G range. After separation, the
particles are recovered for use i n subsequent drying procedures.
[0048] According to the method of the invention, said apparatus operates i n
conjunction with wet substrates and drying media comprising a solid particulate
material, which i s most preferably i n the form of a multiplicity of particles which may be
polymeric, non-polymeric, or mixtures of both polymeric and non-polymeric particles. All
particles may be solid or hollow i n their structure and the polymeric particles may be
foamed or unfoamed and linear or crosslinked. These particles are required to be
efficiently circulated to promote optimum performance and the apparatus, therefore,
preferably includes circulation means. Thus, the inner surface of the cylindrical side
walls of said rotatably mounted cylindrical cage preferably comprises a multiplicity of
spaced apart elongated protrusions affixed essentially perpendicularly to said inner
surface. Preferably, said protrusions additionally comprise air amplifiers which are
typically driven pneumatically and are adapted so as to promote circulation of a current
of heated air within said cage. Typically said apparatus comprises from 3 to 10, most
preferably 4 , of said protrusions, which are commonly referred to as lifters.
[0049] The method of the invention may be applied to the drying of any of a wide
range of substrates including, for example, plastics materials, leather, metal or wood. In
practice, however, said method is principally applied to the drying of wet substrates
comprising textile fibres and fabrics, and has been shown to be particularly successful
i n achieving efficient drying of textile fabrics which may, for example, comprise either
natural fibres, such as cotton, or man-made and synthetic textile fibres, for example
nylon 6,6, polyester, cellulose acetate, or fibre blends thereof.
[0050] Most preferably, the solid particulate material comprises a multiplicity of
particles which may be polymeric, non-polymeric, or mixtures thereof. Typical polymeric
particles may comprise polyamide or polyester particles, most particularly particles of
nylon, polyethylene terephthalate or polybutylene terephthalate, or copolymers thereof,
most preferably i n the form of beads, which may be solid or hollow i n their structure.
The polymers may be foamed o r unfoamed, and may be linear o r crosslinked. Various
nylon o r polyester homo- o r co-polymers may be used including, but not limited to,
Nylon 6 , Nylon 6,6, polyethylene terephthalate and polybutylene terephthalate.
Preferably, the nylon comprises Nylon 6,6 homopolymer having a molecular weight i n
the region of from 5000 t o 30000 Daltons, preferably from 10000 t o 20000 Daltons,
most preferably from 15000 t o 16000 Daltons. The polyester will typically have a
molecular weight corresponding t o a n intrinsic viscosity measurement i n the range of
from 0.3-1 .5 dl/g as measured by a solution technique such as ASTM D-4603.
[0051] Suitable non-polymeric particles may comprise particles of glass, silica, stone,
wood, o r any of a variety of metals o r ceramic materials. Suitable metals include, but
are not limited to, zinc, titanium, chromium, manganese, iron, cobalt, nickel, copper,
tungsten, aluminium, tin and lead, and alloys thereof. Suitable ceramics include, but
are not limited to, alumina, zirconia, tungsten carbide, silicon carbide and silicon nitride.
I t i s seen that non-polymeric particles made from naturally occurring materials (e.g.
stone) can have various shapes, depending o n their propensity t o cleave i n different
ways during manufacture.
[0052] Said solid particulate cleaning material may be comprised entirely of polymeric
particles o r entirely of non-polymeric particles, or may comprise mixtures of both types
of particles. In embodiments of the invention wherein said solid particulate cleaning
material comprises both polymeric particles and non-polymeric particles, the ratio of
polymeric particles t o non-polymeric particles may be anywhere from 99.9%:0.1% t o
0.1%:99.9% w/w. Certain embodiments envisage ratios of from 95.0%:5.0% t o
5.0%:95.0% w/w, o r from 80.0%:20.0% t o 20.0%:80.0% w/w, of polymeric particles t o
non-polymeric particles.
[0053] The ratio of solid particulate material t o substrate i s generally i n the range of
from 0.1 : 1 t o 10:1 w/w, preferably i n the region of from 1.0:1 t o 7:1 w/w, with particularly
favourable results being achieved using polymeric particles at a ratio of between 3:1
and 5:1 w/w, and especially at around 4:1 w/w. Thus, for example, for the drying of 5 g
of fabric, 20 g of polymeric particles would be employed i n one embodiment of the
invention. The ratio of solid particulate material t o substrate i s maintained at a
substantially constant level throughout the drying cycle.
[0054] The method of the present invention may be used for either small o r large
scale batchwise processes and finds application i n both domestic and industrial drying
processes.
[0055] As previously noted, the method of the invention finds particular application i n
the drying of textile fabrics. The conditions employed i n such a system do, however,
allow the use of significantly reduced temperatures from those which typically apply to
the conventional tumble drying of textile fabrics and, as a consequence, offer significant
environmental and economic benefits. Thus, typical procedures and conditions for the
drying cycle require that fabrics are generally treated according t o the method of the
invention at, for example, temperatures of between 20 and 80°C for a duration of
between 5 and 55 minutes. Thereafter, additional time i s required for the completion of
the particle separation stage of the overall process, so that the total duration of the
entire cycle is typically i n the region of 1 hour.
[0056] The results obtained are very much i n line with those observed when carrying
out conventional tumble drying procedures with textile fabrics. The extent of water
removal achieved with fabrics treated by the method of the invention is seen to be very
good. The temperature requirement i s significantly lower than the levels associated
with the use of conventional tumble drying procedures, again offering significant
advantages i n terms of cost and environmental benefits.
[0057] The method of the invention also shows benefits i n terms of reducing dryingrelated fabric damage. As previously observed, fabric creasing readily occurs i n
conventional tumble drying, and this acts to concentrate the stresses from the
mechanical action of the drying process at each crease, resulting i n localised fabric
damage. Prevention of such fabric damage (or fabric care) is of primary concern to the
domestic consumer and industrial user. The addition of particles according to the
method of the invention effectively reduces creasing i n the process by acting as a
pinning layer o n the fabric surface i n order to help prevent the folding action. The
particles also inhibit interaction between separate pieces of fabric i n the drying process
by acting as a separation or spacing layer, thereby reducing entanglement which i s
another major cause of localised fabric damage. In the presently disclosed method,
mechanical action i s still present but, critically, this i s much more uniformly distributed
as a result of the action of the particles. I t i s the localised aspect of the damage that
determines the lifetime of a garment under multiple drying processes.
[0058] Thus, the method of the present invention provides for enhanced performance
i n comparison with the methods of the prior art under equivalent energy conditions;
alternatively, equivalent drying performance may be achieved at lower levels of energy,
together with reduced fabric damage.
[0059] During the drying cycle, the solid particulate material is continually falling out of
the rotatably mounted cylindrical cage through its perforations, and i s being recycled
and added, together with fresh material, via the control means. This process may either
be controlled manually, or operated automatically. The rate of exit of the solid
particulate material from the rotatably mounted cylindrical cage i s essentially controlled
by means of its specific design. The key parameters i n this regard include the size of
the perforations, the number of perforations, the arrangement of the perforations within
the cage and the G force (or rotational speed) which is employed.
[0060] Clearly, i t i s required that the perforations should be sized so as t o be at least
the size of the largest dimension of the particles comprised i n the solid particulate
material, i n order that these particles are able to exit from the cage. For the preferred
particle size range, however, optimum separation of particles from fabric i s achieved
when the perforations are sized at around 1-3 times the largest particle dimension
which, typically, results i n perforations having a diameter of between 2.0 and 25.0 mm.
In one embodiment of the invention, a rotatably mounted cylindrical cage would be
drilled so that only around 34% of the surface area of the cylindrical walls of the cage
comprises perforations. Whilst restricting air flow, this allows for greater retention of
solid particulate material i n the drying load. The perforations may be banded i n stripes
or distributed evenly over the cylindrical walls of the rotatably mounted cylindrical cage,
or could even be exclusively located, for example, i n one half of the cage.
[0061] Conventional commercial vented tumble dryers (e.g. Danube™ - Model
Number TD2005/10E), typically have perforations of 6.5 mm diameter, and these are
drilled at maximum areal density, such that they are distributed closely packed ( 1 mm
apart) over the cylindrical cage wall. This equates to some 56% of the surface area of
the cylindrical walls of the cage comprising perforations which ensures good air flow
through the drying load, and this cage geometry i s also found t o be suitable for the
successful performance of the method of the present invention.
[0062] The rate of exit of the solid particulate material from the rotatably mounted
cylindrical cage i s also affected by the speed of rotation of said cage, with higher
rotation speeds increasing the G force, although at G > 1 the fabric adheres to the
sides of the cage and prevents exit of the particulate material. Hence, slower rotational
speeds have been found to provide optimum results i n this regard, as they allow the
particles to fall from the fabric and through the perforations as the fabric opens out
more during tumbling. Rotational speeds resulting i n a G force of < 1 are therefore
required (< 42 rpm i n a 98 cm diameter cage, for example). The G force (or rotational
speed) i s also controlled so as t o maximise the beneficial effect of the mechanical
action of the particulate material o n the substrate, and the most suitable G is generally
found t o be i n the region of 0.9 G (e.g. 40 rpm i n a 98 c m diameter cage).
[0063] On completion of the drying cycle, addition of solid particulate material t o the
rotatably mounted cylindrical cage is ceased, but the rotation G and rotational speed
are maintained at the same values of < 1 and low (40) rpm as i n the drying cycle i n
order t o effect the removal of particulate material; this removal of particles generally
takes around 5-20 minutes, with the drying cycle i n a typical operation typically taking
40-55 minutes, giving a total overall cycle time i n the region of 1 hour.
[0064] The method of the invention has been shown t o be successful i n the removal
of particulate material from the dried substrate after processing and tests with cylindrical
polyester particles, and nylon particles comprising either Nylon 6 or Nylon 6,6 polymer,
have indicated particle removal efficacy such that o n average < 5 particles per garment
remain i n the load at the end of the particle separation cycle. Generally, this can be
further reduced t o a n average of < 2 particles per garment and, i n optimised cases
wherein a 20 minute separation cycle i s employed, complete removal of particles i s
typically achieved.
[0065] Additionally, i t has been demonstrated that re-utilisation of the particles i n the
manner described operates well, so that particles can be satisfactorily re-used i n
subsequent drying procedures. Indeed such re-utilisation offers further advantages i n
terms of energy efficiency, as heating the air naturally results i n heating of the
particulate media i n the drying process. This heat then i s retained by the particles o n
completion of a drying cycle and, hence, i f the next drying cycle occurs within the time
taken for the particles t o cool down, there will be a transfer of this retained heat t o that
subsequent drying process. There is, therefore, a n even greater level of drying
efficiency achievable i n the event that multiple drying cycles are run consecutively. This
is, of course, applicable t o both the domestic and industrial laundry sectors - but, most
particularly, t o the latter. Rapid turnaround of drying cycles and high load throughput
are both key factors i n this kind of drying operation i n a n industrial scenario.
[0066] The method of the invention i s believed t o comprise the mechanical action of
the particles against a cloth so as t o liberate the moisture trapped between fibres, and
the pick up of this moisture o n the particle surface, wherein rapid evaporation occurs of
the thin film of water which i s formed. Certain polymeric particles also have the ability
t o absorb moisture t o a larger extent (Nylon 6 and Nylon 6,6 being examples). I t may
be the case, therefore, that some such absorption i s also contributing to the drying
mechanism.
[0067] The invention will now be further illustrated, though without i n any way limiting
the scope thereof, by reference to the following examples and associated illustrations.
Examples
Example 1
[0068] A drying procedure was carried out by adding a solid particulate material
comprising 4 kg of Nylon 6,6 particles (DuPont Zytel
®
101 NC010) to a mesh bag with 1
kg (dry mass) of a cloth substrate, which had been wetted with 10°C water. Details of
the particles are set out i n Table 1 and a n illustration of these cylindrical particles i s
provided i n Figure 1.
TABLE 1 PARTICULATE MATERIAL
The substrate was made up of the same type of article i n each case (cotton
pillowcases). This bag was then loaded into a conventional commercial vented tumble
dryer (Danube™ - Model Number T D 2005/1 0E). The dryer was set to rotate at 48 rpm
which, with a drum diameter of 74 cm, resulted i n a centripetal force o n the bag and its
contents of 0.95 G . The dryer operating temperature was set to 20°, 30°, 40°, or 60°C
for individual separate drying tests, and repeat experiments were performed without
particles present (i.e. fabric only) to act as controls. The heat up rate programmed into
the dryer was 2.0°C/min. and experiments were run for various times up to 3 hours, i n
order to be able to extrapolate accurately the overall drying efficiency, which i s
expressed as % water removed/minute of drying time. The substrate was uniformly
wetted out t o -60% w/w moisture content at the start of each test (measured
individually). The results are set out i n Table 2 and are illustrated i n Figure 2 .
TABLE 2 DRYING TEST RESULTS
[0069] Here i t can be seen that i n all cases the addition of particles has reduced the
drying time at the same drying temperature. Even at 20°C (effectively ambient
temperature with the heaters i n the dryer switched off), there is a significant reduction i n
drying time (defined as the time t o reach 5% moisture retention - touch dry). In terms
of drying efficiency (% water removed/minute of drying time) at 20°C this has increased
with particles from 0.19 t o 0.28% water/min (+47%); at 30°C the increase i s from 0.59 t o
0.71% water/min (+20%), whilst at 40°C the increase i s from 0.91 t o 1.05% water/min
(+15%), and at 60°C the increase is from 1.10 t o 1.28% water/min (+16%). The most
interesting comparison, however, is that the test denoted 'Particles/40°C' has the same
drying time as the test denoted 'Particles/60°C' - or, put another way, the same drying
time (-52 mins) i s achieved with the use of particles, but at a 20°C lower drying
temperature. This i s extremely beneficial considering the energy consumptions of such
machines as previously described - even when the most efficient domestic models are
considered. I t appears therefore, that the extra thermal mass of the polymer particles
(i.e. their mass x specific heat capacity) i s not hindering improved drying performance,
although this clearly becomes more of a consideration as the drying temperature
increases, as can be seen the relative % improvements i n drying efficiency which are
shown.
Example 2
[0070] Table 3 and Figure 3 provide a comparative illustration of the drying efficacy
which i s achieved when heated particles are employed. These data effectively provide
a n illustration of the benefits associated with heat retention i n the particles for a
subsequent drying process. Here, however, the particles were pre-heated i n a separate
tumble dryer to 60°C (measured by a n in-situ remote temperature recorder) i n order to
simulate heated particles from a previous cycle. These hot particles were then quickly
added to the mesh bag with wet cloth as before, and tumbled i n the Danube™ dryer at
20°C (the test denoted 'Particles 60°C/Dryer 20°C). As previously therefore, this was
effectively ambient temperature with the heaters i n the dryer switched off. With heated
particles, the drying efficiency increased to 0.48% water removed/minute, vs. the test
from Example 1 with the particles at 20°C, which gave only 0.28% water/min.
TABLE 3 DRYING TEST RESULTS
Hence, the heated particles clearly improve the drying efficiency as might be
anticipated; perhaps less expected, however, i s the extent of the improvement - some
71%. Clearly therefore, this i s a n alternative drying approach which also has merit, but
the key here will be the energy consumed i n heating the particles vs. the same energy
used to heat the air i n the dryer. The low specific heat capacity of the polymeric
particles i n particular should, however, prove advantageous i n this regard. The obvious
advantage of such particle drying is the ability to transfer heat between drying cycles something which is inherently lost with air heating.
[0071] Throughout the description and claims of this specification, the words
"comprise" and "contain" and variations of them mean "including but not limited to", and
they are not intended to (and do not) exclude other moieties, additives, components,
integers or steps. Throughout the description and claims of this specification, the
singular encompasses the plural unless the context otherwise requires. In particular,
where the indefinite article is used, the specification i s t o be understood as
contemplating plurality as well as singularity, unless the context requires otherwise.
[0072] Features, integers, characteristics, compounds, chemical moieties or groups
described i n conjunction with a particular aspect, embodiment or example of the
invention are to be understood to be applicable to any other aspect, embodiment or
example described herein unless incompatible therewith. All of the features disclosed
i n this specification (including any accompanying claims, abstract and drawings), and/or
all of the steps of any method or process so disclosed, may be combined i n any
combination, except combinations where at least some of such features and/or steps
are mutually exclusive. The invention i s not restricted to the details of any foregoing
embodiments. The invention extends to any novel one, or any novel combination, of
the features disclosed i n this specification (including any accompanying claims, abstract
and drawings), or to any novel one, or any novel combination, of the steps of any
method or process so disclosed.
[0073] The reader's attention is directed to all papers and documents which are filed
concurrently with or previous to this specification i n connection with this application and
which are open to public inspection with this specification, and the contents of all such
papers and documents are incorporated herein by reference.
CLAIMS
1. A method for the drying of a wet substrate, said method comprising treating the
substrate with a solid particulate material at ambient or elevated temperature, said
treatment being carried out i n a n apparatus comprising a drum comprising perforated
side walls, wherein said drum comprising perforated side walls i s rotated so as to
facilitate increased mechanical action between said substrate and said particulate
material.
2 . A method as claimed i n claim 1 wherein said solid particulate material
comprises a multiplicity of particles.
3 . A method as claimed i n claim 2 wherein said particles comprise polymeric
particles, non-polymeric particles, or mixtures of polymeric and non-polymeric particles.
4 . A method as claimed i n claim 1, 2 or 3 wherein said substrate is a textile fabric.
5 . A method as claimed i n any one of claims 1 to 4 wherein said drum comprising
perforated side walls comprises a rotatably mounted cylindrical cage.
6 . A method as claimed i n any preceding claim wherein said drum comprising
perforated side walls has a capacity of between 5 and 50 litres for each kg of substrate.
7 . A method as claimed i n any preceding claim which additionally comprises
separation of the solid particulate material from the dried substrate o n completion of the
drying process and recovery of said solid particulate material for re-use i n subsequent
drying procedures.
8 . A method as claimed i n claim 7 wherein said drying process and said
separation of the solid particulate material from the dried substrate are carried out by
rotation of said drum comprising perforated side walls at a speed which generates G
forces i n the range of from 0.05 t o 0.99 G .
9 . A method as claimed i n any preceding claim wherein said solid particulate
material comprises a multiplicity of particles which are added at a particle t o fabric
addition level of 0.1 :1-10:1 by mass.
10. A method as claimed i n any one of claims 2 t o 9 wherein said particles are
elliptical, cylindrical, spherical or cuboid i n shape.
11. A method as claimed i n any one of claims 2 t o 10 wherein said particles
comprise solid particles.
12. A method as claimed i n any one of claims 2 t o 11 wherein said particles
comprise hollow particles.
13. A method as claimed i n any one of claims 3 t o 12 wherein said particles
comprise mixtures of polymeric and non-polymeric particles said the ratio of said
polymeric particles t o said non-polymeric particles i s from 99.9%:0.1% t o 0.1%:99.9%
w/w.
14. A method as claimed i n claim 13 wherein said ratio i s from 95.0%:5.0% t o
5.0%:95.0% w/w of polymeric particles t o non-polymeric particles.
15. A method as claimed i n claim 13 or 14 wherein said ratio is from 80.0%:20.0%
t o 20.0%:80.0% w/w of polymeric particles t o non-polymeric particles.
16. A method as claimed i n any one of claims 3 t o 15 wherein said polymeric
particles have a n average density i n the range of 0.5-2.5 g/cm
3
.
17. A method as claimed i n any one of claims 3 to 16 wherein said non-polymeric
particles have a n average density i n the range of from 3.5-12.0 g/cm
3
.
18. A method as claimed i n any one of claims 2 to 17 wherein the average volume
of said particles is i n the range of 5-275 mm
3
.
19. A method as claimed i n any one of claims 2 to 18 wherein said particles are
cylindrical particles of oval cross section and have a major cross section axis length i n
the range of from 2.0-6.0 mm, a minor cross section axis length i n the range of from
1.3-5.0 mm and a length of from 1.5-6.0 mm.
20. A method as claimed i n any one of claims 2 to 18 wherein said particles are
cylindrical particles of circular cross section and have a cross section diameter i n the
range of from 1.3-6.0 mm and a length of from 1.5-6.0 mm.
2 1 . A method as claimed i n any one of claims 2 to 18 wherein said particles are
non-perfect spherical particles and have a diameter i n the range of from 2.0-8.0 mm.
22. A method as claimed i n any one of claims 2 to 18 wherein said particles are
perfect spheres and have a diameter i n the range of from 2.0-8.0 mm.
23. A method as claimed i n any one of claims 3 to 22 wherein said polymeric
particles comprise foamed polymeric materials.
24. A method as claimed i n any one of claims 3 to 22 wherein said polymeric
particles comprise unfoamed polymeric materials.
25. A method as claimed i n any one of claims 3 to 24 wherein said polymeric
particles comprise linear polymers.
26. A method as claimed i n any one of claims 3 t o 24 wherein said polymeric
particles comprise crosslinked polymers.
27. A method as claimed i n any one of claims 3 t o 26 wherein said polymeric
particles comprise beads of polyalkenes, polyamides, polyesters or polyurethanes.
28. A method as claimed i n claim 27 wherein said polyamide comprises Nylon 6 or
Nylon 6,6.
29. A method as claimed i n claim 28 wherein said polyamide comprises Nylon 6,6
homopolymer having a molecular weight i n the region of from 5000 t o 30000 Daltons.
30. A method as claimed i n claim 27 wherein said polyester comprises
polyethylene terephthalate or polybutylene terephthalate.
3 1 . A method as claimed i n any one of claims 3 t o 30 wherein said non-polymeric
particles comprise particles of glass, silica, stone, wood, metal or ceramic material.
32. A method as claimed i n claim 3 1 wherein said metal i s selected from zinc,
titanium, chromium, manganese, iron, cobalt, nickel, copper, tungsten, aluminium, tin
and lead, and alloys thereof.
33. A method as claimed i n claim 3 1 wherein said ceramic material i s selected
from alumina, zirconia, tungsten carbide, silicon carbide and silicon nitride.
34. A method as claimed i n one of claims 3 t o 11 or 13 t o 33 wherein said nonpolymeric particles comprise coated non-polymeric particles.
35. A method as claimed i n claim 34 wherein said non-polymeric particles
comprise a non-polymeric core material and a shell comprising a coating of a polymeric
material.
36. A method as claimed i n claim 35 wherein said core comprises a steel core and
said shell comprises a coating of nylon.
37. A method as claimed i n any preceding claim which i s carried out at a
temperature of between 5 ° and 120°C.
38. A method as claimed i n claim 37 which attains said temperature by the
provision of a n air heater and a recirculating fan i n said apparatus.
39. A method as claimed i n claim 37 which attains said temperature by the
provision of solid particulate material retaining heat from a previous drying cycle.
40. A method as claimed i n any one of claims 5 to 39 wherein said rotatably
mounted cylindrical cage i s comprised i n a n apparatus comprising a housing and
access means, allowing access to the interior of said cylindrical cage.
4 1. A method as claimed i n any one of claims 5 to 40 wherein said rotatably
mounted cylindrical cage i s mounted i n a first chamber within said housing means,
which also comprises a second chamber located adjacent said cylindrical cage.
42. A method as claimed i n claim 40 or 4 1 wherein said apparatus additionally
comprises recirculation means and delivery means.
43. A method as claimed i n claim 40, 4 1 or 42 wherein said apparatus additionally
comprises pumping means, and wherein said rotatably mounted cylindrical cage
comprises a drum comprising perforated side walls, wherein up t o 60% of the surface
area of said side walls comprises perforations, and said perforations comprise holes
having a diameter of no greater than 25.0 mm.
44. A method as claimed i n any one of claims 40 t o 43 wherein said access means
comprises a hinged door mounted i n the housing which may be opened t o allow access
t o the inside of the cylindrical cage.
45. A method as claimed i n any preceding claim wherein said apparatus comprises
circulation means, adapted t o promote circulation of said solid particulate material.
46. A method as claimed i n claim 45 wherein said circulation means comprises a
multiplicity of spaced apart elongated protrusions affixed essentially perpendicularly t o
the inner surface of the cylindrical side walls of a rotatably mounted cylindrical cage.
47. A method as claimed i n any one of claims 5 t o 46 wherein said rotatably
mounted cylindrical cage comprises a 74 cm diameter cage and the speeds of rotation
are i n the range of 10-49 rpm.
48. A method as claimed i n any preceding claim wherein said apparatus
comprises:
(a) housing means, having:
(i) a first upper chamber having mounted therein said rotatably
mounted cylindrical cage, and
(ii) a second lower chamber located beneath said cylindrical cage;
(b) recirculation means;
(c) access means;
(d) pumping means; and
(e) delivery means,
wherein said rotatably mounted cylindrical cage comprises a drum comprising
perforated side walls, wherein up t o 60% of the surface area of said side walls
comprises perforations, and said perforations comprise holes having a diameter of no
greater than 25.0 mm.
49. A method as claimed i n any preceding claim for use i n small or large scale