Drying and gas or vapor contact with solids – Apparatus – Vacuum
Reexamination Certificate
1997-08-20
2001-01-30
Bennett, Henry (Department: 3744)
Drying and gas or vapor contact with solids
Apparatus
Vacuum
C034S077000, C034S075000
Reexamination Certificate
active
06178659
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a vacuum apparatus for multiple-bed industrial hide driers, and to a drier that includes the apparatus.
2. Description of the Related Art
It is known that the vacuum method of drying hides has, with respect to conventional methods that entail laying the hides on frames placed in ventilated and dehumidified rooms or in heated tunnels, the advantage of a hide that is smoother and flatter and also better adaptable to the subsequent finishing operations. Furthermore it allows shorter drying times, on the order of minutes instead of hours, and lower labor costs. Finally, modern driers entail lower energy consumption and occupy far less space than barometric-pressure systems.
However, vacuum driers have the drawback of higher initial and running costs and of lower final hide quality, especially for thinner hides with finer grain, due to the considerable heat-induced stresses on the hides and to the depletion of fatty materials contained in their fibers. In vacuum driers, the hides are in fact arranged on beds which are heated to approximately 60-80° C., i.e. to temperatures that are considerably higher than the “body” temperature of the animals from which they are obtained, in order to make the residual moisture evaporate as quickly as possible. In order to lower the evaporation temperature, the beds are hermetically enclosed by airtight lids so as to form a series of evaporation chambers that are connected to a vacuum pump by means of a circuit along which one or more condensers and/or condensate separators are arranged as shown in FR-A-2557888. The vapor state diagram clearly shows that the higher the vacuum produced in the sealed chambers, the lower the evaporation temperature at equilibrium.
For this purpose a high vacuum is produced in the vapor circuit by means of vacuum pumps of the liquid-ring type or of the variable-chamber cylinder type, using water or oil as working fluids; these pumps can produce vacuum up to 95% with residual absolute pressures of less than 30 mbar.
In practice, the pressure is gradually reduced down to a vacuum of more than 90%, which corresponds to an absolute pressure of approximately 80 mbar, with a vapor equilibrium temperature of approximately 45° C. Accordingly, the beds are heated to a temperature of at least 60-70° C. to produce a thermal gradient that allows evaporation of the residual moisture of the hides in an acceptable time. In order to reduce the evaporation time it is obviously possible to increase the temperature of the beds to more than 80° C. so as to increase the thermal gradient and thus the transfer of heat toward the hides, but this entails the risk of creating irreparable damage to their fibers.
Furthermore, in these conditions, and by cooling the condensers with water at a temperature of approximately 15° C., it is possible to dry the hides to a residual moisture content of approximately 30%. In order to provide more intense drying it is necessary to extend the time for which the hides remain on the beds, negatively affecting both the productivity of the apparatus and the quality of the dried product.
U.S. Pat. No. 3,027,651 describes an apparatus for removing condensable vapors including a Roots-type compressor arranged upstream of of a two-step condenser and of a gas ballast press. The compressor acts on the vapors as a supercharger to a low-temperature condenser unit operating between −20° C. and −40° C.
BRIEF SUMMARY OF THE INVENTION
The aim of the present invention is to eliminate the drawbacks described above by providing an apparatus with lower evaporation temperatures than in the conventional types, with obvious advantages from the point of view of production.
An object of the invention is to drastically reduce the evaporation temperature and consequently the temperature of the beds, so as to avoid any damage to the hides and give them maximum grain softness, with obvious advantages from the point of view of quality.
The invention achieves this aim and this object by means of a vacuum apparatus for industrial hide driers, including a circuit with an open end and having arranged in series: at least one manifold associated with each bed to collect the vapors released by the hides; a first condenser inserted in each manifold; a first condensate separator at the output of the various condensers; at least one main vacuum pump which is suitable to reduce the absolute pressure in the circuit to a first upper value which corresponds to a given vapor equilibrium temperature; characterized in that it has a secondary aspiration device which is arranged upstream of the main vacuum pump, said device operating in series to the vacuum pump when the first upper value of the absolute pressure is reached, said device being suitable to further reduce the absolute pressure to a second lower value that allows to instantly increase the evaporation and/or reduce the vapor equilibrium temperature to a second value which is lower than the first one.
With a vacuum system of the type specified above it is possible to drastically reduce drying times with respect to those of the past, with relatively low evaporation temperatures.
As an alternative, it is possible to significantly reduce the temperature of the beds, improving the final quality of the hides without negatively affecting the evaporation times.
It may also be possible to combine the above described effects, reducing both the evaporation times and the bed temperatures, with positive results in terms of both productivity and quality of the dried hides.
Preferably, the upper value of the absolute pressure reached by the main vacuum pump in the steady state is 100 to 40 mbar.
The lower value of the absolute pressure reached in the steady state by placing the secondary aspiration device in series to the main pump can be of 15 to 1 mbar.
Conveniently, the secondary aspiration device is of the high-capacity, low-head type, with an average capacity of 300 to 1000 m
3
/h and a pressure differential of 30 to 100 mbar.
The secondary aspiration device can be constituted by a positive-displacement compressor of the Roots or vane type or by a gas or steam ejector.
Advantageously, the intake and delivery ducts of the secondary aspiration device are connected by a bypass circuit with a controlled electric valve interposed.
In operation, the secondary aspiration device runs continuously: the electric valve of the bypass circuit is normally open when the absolute pressure in the circuit is higher than the first upper value and is closed for equal or lower values.
REFERENCES:
patent: 3027651 (1962-04-01), Nerge
patent: 3302303 (1967-02-01), Aupoix
patent: 3987556 (1976-10-01), Tissot et al.
patent: 5033206 (1991-07-01), Corner
patent: 5056239 (1991-10-01), Corner
patent: 5184487 (1993-02-01), Corner
patent: 5288471 (1994-02-01), Corner
patent: 2557888 (1898-07-01), None
Corner Antonia Maria
Corner Antonio
Corner Carlo
Corner Laura
Corner Maria Luisa
Bennett Henry
Coleman Henry D.
Corner Antonia Maria
Corner Carlo
Corner Laura
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