Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 Control of Drying Operation
The invention relates to the control of drying or other
moisture control operations by the circulation of air or
other suitable gas over or through the material the
moisture in or on which is to be controlled.
The ability of relatively dry air to extract water from
surfaces with which it makes contact is employed in a wide
range of industrial and other drying operations. The
drying capacity or drying potential of air being determined
solely by its temperature and existing moisture content,
drying operations can be and often are precisely thermally
controlled in an attempt to optimise efficiency.
However important the thermal efficiency may be, the prime
consideration must always be product quality, and the
present invention arose from a consideration of, but is not
necessarily confined to, circumstances in which control
of drying potential of the drying air is of supreme
importance to the quality of the product.
According to one aspect of the invention there i5
provided a method of controlling moisture in or on
materials when passing gas over or through the materials
wherein the specific enthalpy of the gas is controlled
independently of the drying poten~ial thereof. The value
of specific enthalpy relative to drying capacity may be
controlled by monitoring the temperature and moisture
content adding heat and moisture as required.
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1 According to another aspect of the invention there is
provided a method of drying a material by passing a gas over
or through the material wherein the specific enthalpy of the
gas is controlled independently of the drying potential
thereof, comprising the successive steps of:
(a) warming the material without substantial
removal of water therefrom by the use of
gas at a first enthalpy level and a first
drying potential and
(b) removing a first quantity of water from
the material by increasing the drying
potential of the gas relative to that
utilised in step (a) and by increasing
the enthalpy level above said first
enthalpy level.
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1 The method may be applied to the drying of material or,
alternatively, to the maintenance of a controlled property
of moisture on or in materials.
According to a further aspect of the invention there is
provided a method of controlling the moisture on or in
materials when passing gas over or through the materials
comprising means for controlling the specific enthalpy of
the gas independently of the drying potential thereof.
Preferably the addition of heat and moisture is controlled
automatically by signals representing temperature and
moisture content.
Before discussing a specific embodiment of the present
invention, a brie review of some basic concepts will
follow.
Specific enthalpy H is the quantity of recoverable energy
available in a certain mass. Basically, specific enthalpy H
is defined as follows:
H = U + PV (1)
where U is the internal energy, P is pressure and V is
volume. ~ore particularly, specific enthalpy H can broken
down into different components as follows:
a g g (2)
where H is the specific enthalpy of moist air per kilogram
of dry air, Ha is the specific enthalpy of dry air, Hg is
the specific enthalpy of water vapor, all measured in kJ/kg,
and g is the moisture content per kilogram of dry air,
measured in kg/kg.
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l In the present invention, the method of drying a material is
used with a fixed pressure and volume. Since the internal
energy U is a function of pressure and temperature, that is,
U = f(P,T), from equations (1) and (2), the following are
derived:
Ha=f(U,P,V)=f(P,V,T) (3)
Hg=f(U,P,V)=f(P,V,T) (4)
Combining equations (2), (3) and (4), the specific enthalpy
of the system is derived as follows:
H=f'(Ha Hg g)=f'(P,V,T,g) (5)
Since the present invention utilizes a constant pressure and
volume, equation (5) is reduced as follows:
H=constant x f"(g,T)(6) (6)
From equation (6), it is readily seen that specific enthalpy
H is a function of moisture content g and temperature T. It
will be remembered that drying potential of air is also
dependent on the moisture content and temperature of the
air. However, the enthalpy and drying potential thereof
vary differently, although depending on the same quantities,
that i9, temperature T and moisture content g. A simple
example readily illustrates this concept. For example,
saturated air at 10C. has approximately the same enthalpy H
as air at 20C. and 25~ humidity, since the air in both
situations has the same heat content and the same potential
for recoverable heat. However, the 10C. air has no drying
potential, since it is saturated, while the 25C. air does
have a drying potential. The values of enthalpy H as a
function of temperature T and moisture content g are readily
obtainable from prepared tables, such as those published in
the "IHVE Guide, Cl & 2, Properties of Humid Air, Water and
Steam", 1975, by The Institution of Heating and Ventilating
Engineers, ~9 Cadogan Square, London SWlXOJB, and printed by
the Curwen Press Ltd., London E13.
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- 2b -
1 The invention will now be further described by reference to
a number of non-limiting examples and the accompanying
drawings of which the single figure shows schematically an
installation for the drying of material by the circulation
of air therethrough.
In the non-limitlng example described, the material to be
dried consists of green malt, which is a substance which is
susceptible to damage if subjected to incremental changes in
enthalpy and drying rate greater than predetermined
values. It will be assumed that the initial moisture
content of the malt is 45% and that it is required to reduce
this to something like 4~.
For the sake of the example, it will also be assumed that
the ambient air has a temperature of 15C and a dew point of
10C, though if the drying cycle continues into the night,
the moisture content may drop to a dew point of 0C and the
temperature, having also dropped to around 0C may,
typically, recover to some 3C by morning.
In the present example, the method of drying will be
described, for the sake of simplicity, as comprising three
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distinct phases, thougll it should be understood that in
pract:ice the phases may overlap at. least. to some exten~.
As shown in ~he figure the installation for drying
05 comprises a chamber 2 for containing a quanti-ty of material
4 to be driecl, in this example green malt. Air i5 passe~1
int.o the chamber 2 through an inle-t 6 by means of a fan ~,
and having passed over or t~rough -the material 4 the air is
exhausted through outlet 10 and either voided to atmosphere
~0 at opening 12 or ret.urned to the fan 8 via branch ~ipe 14
for recirculation. A valve 16 controls the ~roportion of
air recirculated with respect. to the proportion voided ~o
atmosphere.
lS In its passage from the fan 8 to the inlet 6 the air is
capable of being heated by means of a heater 18. A mixing
unit 20 is also provided in the path between the fan 8 and
the inlet 6 providing facility for the introduction of
moisture, for example in the form of steam or liquid water
through moisture inlet 22 controlled by valve 24.
The rate oE air flow produced by the Ean 8 is controlled by
means of a con-troller 26, and to the extent that the
exhaust side of the fan is not satisfied with recirculated
air from branch pipe 14, air is introduced through inlet 28
via valve 30~ The air in~roduced through inlet 28 will
normally be moist ambient air hut in certain circumstances
may be rela-~ively or absolutely dry air.
The temperatures t. of the air circula~ing in the apparatus
is determined at various posit.ions by means of sensors Tl,
T2, T3 and T4. The moisture conten-t m of the circulating
air is similarly determined at corresponding locations by
moisture meters Ml, M2, M3 and M4.
In t~he first, initia~ion, phase of drying, ambient air is
drawn from inlet 28 through valve 30 ~y fan 8 and, heated
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1 in heating unit 18 and moistened as required in the mixing
unit 20 so as to produce the desired enthalpy and drying
potential, is introduced into the material 4 in chamber 2
via inlet 6.
As the initiation phase continues, the enthalpy of the input
air is increased gradually whilst controlling the drying
potential so as not to damage the material 4.
The warming of the material 4 increases the transpiration of
moisture within the material but, although the specific
enthalpy of the air is increased, the drying capacity of the
air is not allowed to increase in this example. The process
is infinitely variable and in other examples the drying
capacity may be allowed to increase but not in
correspondence with increasing specific enthalpy.
This can be contrasted with normal drying where, for
example, ambient air with a temperature of 15C and a dew
point of 10C is merely warmed prior to passing through the
malt. During the night the dew point may be reduced to 0C,
and under these conditions the control of input temperature
alone fails to control the drying process adequately and
damage to the product can occur.
The control of drying capacity independently of specific
enthalpy is effected by introduction into the air entering
the chamber of quantities of moisture controlled by signal
ml provided by sensor Ml. The moisture introduced may
originate from the atmosphere, any recirculated exhaust air
or, predominantly at this phase, by injection into the
mixing unit 20, and the input air moisture signal is
therefore used to control the valves 16, 24, and 30.
The initiation phase therefore proceeds with gradual
3S warming of the material without drying. The exhaust air
leaving the chamber 2 via outlet 10 may be recirculated or
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voided to atmosphere. The process is continued un~..il the
conditions recorded by sensors M3 and T3 h~ve reached
predetemlined values or until the condi-tions at a
predetermined zone within the material 4, as determinecl by
05 sensors M2 and T2, have reached prede-termined values. In
practice, the temperature and the moisture content of the
air within the material 4 may be determined by a series of
probes at dif~erent levels, and the sensors T2 and M2 shown
in the figure are to be taken as representative of such
probe~.
The second phase of the process is one of free drying. The
specific entllalpy is furtl~er increased, but now the drying
capacity of the air is also allowed to increase according
]5 to the requirements of the process and the material, for
example as to whether the main requirement is for thermal
efficiency or the safety of the product. It is to be
understood that the requirements may vary throughout the
process.
During this phase, the increase of drying capacity is still
controlled independently of the increase of specific
enthalpy. At least some of the exhaust air is
conveniently recirculated during this process, and the
requirement for the injection of moisture through valve 24
is accordingly adjusted. In excep-tional circumstances dry
air will be introduced through inlet 28. Data used in the
control of ~dle valves accordingly may include signals
indicative of the moisture content for the time being of
bo~h illpU~: and recirculated exhaust air.
During the phase of free drying, evaporative cooling
generally means that the t-emperat-ure of ~he product does
not rise t.o t.he value of t.he input air. Conversely,
when t.he t:emperature of the produc~: as measured by probe
T2 begins to increase without any correspondin~ increase in
t~le moist.ure con~ent. of -~.he air in the material as
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de~ermined by M2, the end of free drying is indicat.ed and
t.he inset of the t.hird phase - restric~ed drying - in which
water removal cannot continue at the former rate. At this
st.age, ~he moisture content of the material may be,
05 typically, 7~, and in order to reduce this to the 4~
required, it is convenien-t to increase the drying potential
of the air so as to shorten the process. ~is may be
effected by:-
I0 (a) increasing the t.emperature of t.he circulating airfurther by operat.ion oE the heater )8 whils-t maintaining
its moisture cont.ent;
(b) main~aining the temperat.ure and, for example by
int.roducing dry air through inlet 28, reducing the
moisture con~ent; or
(c) varying the temperature, the moisture content, and
the flow rate o the circulat.ing air by means oE controller
26.
Whilst in the above example the ~hree phases of the
operation have been described as being quite distinct from
one anot.her, in practice the free drying phase may be
integra~ed with the initiation phase lnasmuch as some
drying could take place from the outset. Nevertheless,
the drying capacity o~ the air will be controlled together
with, but independently of, specific enthalpy.
If the process of free drying cont.inues to a point at which
the mois~ure content of the material is reduced to the
required Level, the third phase will of course be dispensed
with.
The example described above is one of batch drying wherein
t~le three drying phases - distinct or overlapping - take
place in the sarne spatial zone but at. different (distinct
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or overlapping) periods of time. However, -the inven-tion is
equally applicable to con-tinuous processing wherein a
por~.ion of a mat.erial is made subject to the conditions of
the first. phase in a first zone and is then moved
05 successively to second and third zones maintained at the
conditions respectively-of the second and third phases.
Successive portions of the material follow the first
mentioned portion t.hrough the three zones. In the
continuous process the e~uivalent of the overlapping of the
)0 phases in the batch process can be afforded by providing
zones int.ermediate the above-mentioned ~ones and providing
in the intermediat.e zones conditions int.ermediate those of
t.he adjacent zones.
The invention has been described primarily in relat.ion to
drying and this is likely to be it.s ma~or application.
However, in the storage of cert.ain products 5UC}l as fruit
or vegetables, it may be required to maintain or even
increase the moisture content of the product whilst
circulating air or other gas over or through it, and the
invention is to be underst.ood as extending to such
application.
Again, ~he above descriptions have related primarily to
cont.rol of moisture within materials, but it i9 to be
understood that the invention is equally applicable to the
control of moisture on the sur~ace of materials~
