FIELD OF THE INVENTION:
The present invention relates to a method of, and an apparatus for drying
ceramic bodies such as station post solid core porcelain insulators having a
complex shaped uneven surfaces and large section thicknesses.
BACKGROUND OF THE INVENTION:
In order to dry the complex shaped ceramic bodies such as solid core having a
complex surface with significant unevenness traditionally a combination of
natural drying and a hot air circulation or humidity is adapted. Most important in
the prior art process is the correct control of temperature and humidity inside the
drawing chamber during a critical phase of shrinkage development. Any
exhibition of cracks during this stage indicates an uneven drying throughout the
body. This can be avoided either by increasing the relative humidity or reducing
the temperature in the dryer or both. The prior art processes are difficult to
control and many times the yields are very poor, and influenced by the
environment such as humidity. This is because; the surface topography of the
complex shaped body has a large surface area per unit volume. The temperature
distribution in the shaped ceramic body as a whole becomes non-uniform, and
the convex regions in the complex surface are heated to a higher temperature
than the concave regions so that "drying cracks" tend to be formed in the convex
regions.
The conventional drying process (Ref.Tl) requires a substantial drying time,
ranging several hundred hours, besides that the drying state becomes unstable
due to environmental influences and is not easy to control. This results in a poor
yield and many times the drying cracks and differential moisture deep inside the
core results in development of firing defects which further results in wasted
energy.
Conventionally, humidity drying (Ref: Tl) is the most common process adapted
in porcelain insulator manufacturing of station post and hollow insulators. While
microwave drying has been adapted for green ceramic articles like honeycombs,
there is no prior art available which adapts microwave drying for thick section
porcelain insulators being difficult to dry.
The applicant of the instant patent application for the first time had earlier
reported the laboratory level developmental activity on the microwave drying
technology in both continuous mode and in pulsed mode as can be evidenced in
the two patent applications filed earlier (Ref. PI and P2). These methods
highlight the potential of microwave drying technology for high voltage porcelain
insulators. In said two cases, the applications were limited to small disc
insulators, bus bar insulators and small segment of solid core insulators. The
actual solid core station post insulators with differential profile was not covered
in these patent applications due to unavailability of an appropriate apparatus for
the same. Thus, the utilization of pure microwave energy for homogeneous
drying, a large ceramic body has not been attempted for the large cross section
and large L/D ratio porcelain insulators. Thus, the present disclosure emphasizes
adaptation of microwave drying technology, and design and configuration of an
apparatus for implementation.
OBJECT OF THE INVENTION:
It is therefore an object of the present invention to propose a method of drying
ceramic bodies such as station post solid core porcelain insulators having a
complex shaped uneven surfaces and large section thicknesses, which eliminates
the drawback of the prior art.
Another object of the invention is to propose a method of drying ceramic bodies
such as station post solid core porcelain insulators having a complex shaped
uneven surfaces and large section thicknesses, which eliminates development of
any drying crack within the complex-shaped ceramic bodies.
A still another object of the invention is to propose a method of drying ceramic
bodies such as station post solid core porcelain insulators having a complex
shaped uneven surfaces and large section thicknesses, which enables a
relatively quick drying of the ceramic bodies.
A further object of the invention is to propose an apparatus for drying ceramic
bodies such as station post solid core porcelain insulators having a complex
shaped uneven surfaces and large section thicknesses.
SUMMARY OF THE INVENTION:
To this end, according to one aspect of the present invention, there is provided a
method of drying a complex shaped ceramic body by removing moisture at a
controlled rate volumetrically using a pure microwave energy.
According to another aspect of the present invention, there is provided an
apparatus for drying a complex shaped porcelain/ceramic bodies having uneven
surfaces; the apparatus comprising a multiple source of microwaves which is
radiated inside by a plurality of ports arranged on either side of the walls in a
particular configuration.
Therefore, according to the present invention, the drying process can be
performed without allowing the development of drying cracks and within a
shortened time, and with uniform drying of the core adapting a pure microwave
energy in a closed chamber.
Advantageously, the temperature distribution of the shaped ceramic body as a
whole is made uniform, and it becomes possible to more positively prevent
formation of drying cracks, particularly at the convex regions in the second
surface of the body.
Advantageously, the shaped ceramic body is preferably dried only by pure
microwaves and does not require any auxiliary source such as hot air supply.
Advantageously, the shaped ceramic body is dried by volumetrically heating the
body at a temperature up to a peak temperature range of 100 -120 deg C
and, except for the object to be dried, no other element in the chamber is
susceptible to heating, thereby resulting in a full energy utilization.
The shaped ceramic body is dried within a closed chamber.
According to the invention, the station post solid core insulators are typically of
dimension which can be 200 cm long or more and core diameter ranging from
100-200 cm based on the HV ratings.
The inventive method does not require hot air or auxiliary heating or steam as
required in conventional drying process.
The present invention achieves a reduction in drying time in the range of 55-
60% in comparison to conventional humidity drying process and even for
complex shaped porcelain insulators of large size and section thicknesses.
The invention described herein is not limited to porcelain insulators alone, but
also to ceramic green bodies of any shape and size and with large section
thickness that makes it difficult to dry by conventional drying process.
The velocity of the exhaust fans is limited to remove the excess moisture from
condensing within the walls of the chamber and does not affect the equilibrium
moisture of the core and surface and thus avoiding shed falling.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS :
The present invention is explained in further detail below, with reference to the
preferred embodiments illustrated in the accompanying drawings, in which:
Figure 1 - Shows a typical complex shaped porcelain insulators arranged in a
loading trolley; the insulators being held in a solid wooded trolley using C-clamps
made of wood.
Figure 1(a) - Shows a cross sectional view of the complex shaped porcelain
insulator with a core-shed structure.
Figure 2 is a schematic frontal view of an applicator showing a sine wave
applicator, a mode stirrer, a plurality of magnetron positions and a loading
trolley, according to the invention.
Figure 3 - A schematic illustration of a Microwave Drying system exhibiting a
control system, the applicator and other accessories like chilling unit including
the trolley containing green insulators, according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
OF THE INVENTION:
There is shown, in figure 1, a microwave compatible trolley (1) containing non
metallic wheels and wooden frame (2). A plurality of insulators (3) are placed on
the trolley (1) and tied to the frame of the trolley for restricting the movement of
the insulators (3) during transfer. The complicated profile of the insulators (3) is
shown in figure 1 (a) comprising a core (4a) and shed structure (4b) in a cross-
sectional view. In figure 2, there is shown an embodiment of a microwave drying
applicator (5) for carrying out the present invention. The applicator (5) is formed
of stainless steel material for drying the insulators (3) tied-up in the loading
trolley (1). The applicator (5) exhibits the positions of one each 3 kW microwave
source (6) disposed on either side of at least one each cavity walls of a chamber
(2). Two hinged doors (7) both on front and rear side of the box fitted with limit
switches including microwave compatible seals (8) are used to close the cavity
and avoid microwave leakage during operation. Further, a large mode stirrer (9)
is used on the top which maintains uniformity of microwave zone inside the
cavity. Two ports (10) for temperature sensors are located at six different points
in the cavity and the temperature sensors are located in these ports for
temperature measurement of different samples in different segments. There are
two exhaust ports (11) in the cavity on the top, which allow the moisture
removal during operation. The most important mechanism for maintaining the
microwave homogeneity inside the cavity is to position a sign wave reflector
means (12) on both the interior sides of the cavity. A microwave drying with the
applicator system can be viewed in figured 3. The multiple microwave source
constitutes a microwave power pack of 10 x 3 kW i.e. 30 kW source. The flexible
microwave drying system has at least ten microwave launchers directly mounted
to a plurality of ports provided on the wall of the chamber and connected to a
PLC based power pack through HV cables. A control cubical houses the PID
controller for programming the heating cycle, and the PID output drives to
control the microwave power to the applicator(5). Furthermore, the control
cubicle houses all interlocks such as doors, chilled water supply and microwave
leakage sensor and control. The system comprises a PLC panel assembly
containing a PLC panel (13), and a Touch screen panel (13a), a control panel for
the system (14), and a chilling system (15) for cooling the magnetrons (6)
during operation. The main panel system (14) comprises a PID controller (14a),
a controller for Microwave leak detector (14b), and a paperless recorder (14c)
which monitors the uniformity in temperature in different zones of the cavity
during operation.
The multiple microwave sources (6) are arranged such that the drying objects
receive microwave radiation uniformly on the entire volume. The shaped ceramic
body in this embodiment is a semi-finished product for manufacturing a
suspension insulator having a shed portion. In the drying chamber 2, the
multiple microwave feed ports are arranged to create a uniform zone in which
the objects to be dried are positioned.
According to the present invention, since the shaped ceramic body (3) is dried
mainly from the multiple microwaves so distributed from side surfaces, the
shaped reflected walls (12) ensure uniform field avoiding locally overheated
regions. It is thus possible effectively to prevent formation of drying cracks.
Moreover, the present invention makes it possible to complete the drying within
a markedly shortened time of approximately 140-150 hours, in contrast to
drying according to the conventional humidity drying process and natural drying
process which require approximately 300-350 hours.
The drying method is controlled based on the temperature of the body (3),
determined depending upon parameters of the shaped ceramic body, such as
shape, moisture content and the like. For example, when drying is to be
performed with respect to a shaped ceramic body for a suspension-type
insulator, having moisture content of approximately 20%, the microwave power
is controlled by the time temperature relation which dictates the control release
of moisture from the green body to avoid rupture/drying cracks. In the first
region, up to 50 deg C, the microwave heats the body as well as moisture inside
the body, however avoiding excessive evaporation. In the region at 50 deg C the
temperature is maintained and heated from 50 to 60 degrees at a rate of 0.1
deg/hr for a period wherein the moisture content is reduced by around 14%.
Subsequently the body can be heated (which is the resultant of the steam
temperature of the evaporating of moisture as well) to a temperature range
of 100-120 deg C at a faster rate of 3 deg/hr and soaked at the peak
temperature for removing the moisture and/or reducing the moisture to less
than 1% throughout the entire length of ~ 2m of all the
insulators . The volumetric and controlled drying limits uneven temperature of
the objects and does not require high temperatures as in the conventional
process. This is because the conventional process requires external thermal
energy to evaporate and drive moisture from the core. The temperature profile is
different in case of microwave drying wherein the core temperature is slightly
higher due to volumetric heating and ensuring a continuous evaporation of the
moisture from deep inside and providing a differential pressure for the vapor to
come to the surface resulting more effective and uniform drying.
In order to verify the advantageous effects of the present invention, drying
experiments were performed with respect to shaped ceramic bodies, by changing
the heating profile as well as the operating conditions. In this instance, the
shaped ceramic bodies were for silica-alumina-clay-based suspension-type
insulators with moisture content of approximately 18%. The system can
accommodate a load of 1 MT consisting of individual insulators up to 2.3 m in
height and in a base of 1.1 m x 1.1 m. The Trolley (1) containing these insulators
(3) are kept inside the cavity as described earlier. A multi segment programmed
heating and cooling cycle is fed to the PID controller which is linked to a PLC and
controls the drying rate by providing appropriate power level to the magnetrons
(6). Typically, the drying cycle consists of a slow heating intermittent region
followed by soaking at peak temperature in the region of 100-120 deg C for
long hours. A typical heating cycle using the present method is in the range of
140-150 hrs in contrast to the conventional drying cycle in the range of 300-
350 hrs. The typical moisture content throughout the insulators in this new
method can be maintained in the range of 0.5 - 1%. The other most important
factor is the yield. In the conventional drying process for long hours, the yield is
very low and typically lies in a wide range of 20-50%. However, using the
present method, the yield can be in the range of 80-100%. Further, the new
method as described in the present embodiment, does not require separate
facility for humidity during drying as practiced in the conventional process and as
outlined in the published literature. In the new method, the heating cycle, the
moisture removal rate at a particular temperature range and the exhaust rate are
so well balanced that no separate humidity control is required for drying these
insulators.
According to the present invention, wherein the heating cycle is appropriately
determined, it is possible to complete drying of the shaped ceramic bodies within
a relatively short time and with a relatively high yield. During the period in which
shaped ceramic body (3) is dried in accordance with the present invention, it is
advantageous to continuously suction by fans so that the vapor generated from
the shaped ceramic body (3) is discharged out of the drying chamber to prevent
condensation on the walls of the apparatus. When the shaped ceramic body to
be dried is for a suspension-type insulator, it is difficult to effectively discharge
the vapour out of the drying chamber (2) under the speed of hot air which is
below 0.1 m/s, while cracks tend to be formed due to the wind under the speed
which exceeds 0.2 m/s. Therefore, it is preferred that the suction is relatively
very low and a diffused air flow is maintained.
As explained above in detail, the present invention makes it possible to quickly
dry the shaped ceramic bodies (3) having a complex surface (4b) with enriched
unevenness, without developing drying cracks and within a shortened time. The
present invention is applicable not only to shaped ceramic bodies for suspension-
type insulators, but also generally to other shaped ceramic bodies.
While the present invention has been explained above with reference to certain
preferred embodiments, the same system and technology can be utilized for
other similar materials and components by making minor modifications and/or
alterations, which shall be construed as falling under the scope and protection of
the invention.
REFERENCE PATENTS
PI A novel environment-friendly microwave assisted method for fast drying of
porcelain insulators Indian patent field 784/Kol/2007(BHEL)
P2 System and method for generating pulsed microwave source for drying
large section thickness ceramic bodies Indian patent field
1356/ Kol /2008 (BHEL)
OTHER REFERENCES :
Tl. Technical information on drying from M/s. NOVOKERAM Max Wagner
GmbH, Germany.
WE CLAIM :
1. A method of drying complex shaped porcelain insulators, having an
uneven surface with sheds formed to maintain a creepage distance for
high voltage (HV) application requirement, wherein said insulators are
dried by coupling microwave radiation source to a green body of the
insulator with moisture;
allowing a volumetric heating by the microwaves causing a
simultaneous generation of steam by evaporating the moisture deep
inside the core of the body to set up a flow path;
characterized in that said shaped ceramic body is dried by controlling
the microwave power and the time temperature profile of the insulator
limited to a peak temperature range of 100-120C, and in that said
shaped ceramic body is dried by maintaining a maximum temperature
difference between the group of insulators from bottom to top to less
than 5 degree C.
2. The drying method as claimed in claim 1, wherein said complex
porcelain insulators are dried using a pure microwave radiation and
wherein the time temperature profile is controlled for removal of
moisture from deep within the green body (3) which avoids rupture
due to excessive pressure built up.
3. The drying method as claimed in claim 2, where said pure microwave
radiation is controlled based on a feedback temperature data of the
core of the insulator.
4. The drying method as claimed in claim 1, wherein a control output
from a PID based on a difference between the actual temperature and
a programmed temperature generates a signal to simultaneously
control the group of microwave generators.
5. The drying method as claimed in claim 1, wherein the measurement of
temperature inside the core of the insulator at different locations of
the cavity is obtained by a plurality of mineral-filled metallic
thermocouples.
6. The drying method as claimed in any of the preceding claims, wherein
the surface temperature and temperature uniformity within the
chamber is monitored and controlled respectively by a plurality of infra
red pyrometers, and paperless recorder disposed at several locations.
7. The drying method as claimed in any of the preceding claims, wherein
the excess moisture is removed at a controlled speed of less than
O.lm/sec to avoid condensation inside the chamber.
8. An apparatus for drying porcelain insulators comprising an applicator
made of stainless steel and accommodating a trolley having a plurality
of green body of porcelain insulators; a multiple magnetron sources for
radiating microwave inside the applicator through several ports which
are arranged on either side of the walls for drying; one each sign wave
reflector means disposed on side walls of the applicator cavity to
maintain microwave homogeneity; and a PID controller to monitor
and control the heating cycle.
9. The drying apparatus as claimed in claim 8, comprising a plurality of
exhaust fans.
10. The apparatus as claimed in claim 1, comprising a single large mode
stirrer to further homogenize the field to obtain uniform drying.

The invention relates to a method of drying complex shaped porcelain insulators,
having an uneven surface with sheds formed to maintain a creepage distance for
high voltage (HV) application requirement, wherein said insulators are dried by
coupling microwave radiation source to a green body of the insulator with 18%
moisture; allowing a volumetric heating by the microwaves causing a
simultaneous generation of steam by evaporating the moisture deep inside the
core of the body to set up a flow path; The shaped ceramic body is dried by
controlling the microwave power and the time temperature profile of the
insulator limited to a peak temperature range of 100-120C. The shaped ceramic
body is dried by maintaining a maximum temperature difference between the
group of insulators from bottom to top to less than 5 degree C. The invention
further discloses a drying apparatus to implement the inventive method.