AN OSCILLATOR CIRCUIT FOR A VARIABLE LOAD RADIO FREQUENCY (RF) DRYER

FIELD OF THE INVENTION

The present invention relates to an oscillator circuit for a variable load radio frequency (RF) dryer suitable for drying textile materials, food materials, ceramic materials and other materials in a fast and economic manner,

BACKGROUND OFTHE INVENTION

Drying of materials is one of the activities in many manufacturing processes. Textile manufacturing is no exception. Efficient drying under room temperature enhances the fabric appearance, reduces chances of yellowing and avoids the decomposition of dyes. Regular steam drying or blowing of hot air as practiced in stentor in the conventional method involves elevated temperature with the dominant drying effect at the surface of the product and deeper drying taking place due to the capillary effect whereby the internal moisture migrates form the center to the surface of the product. Drying dyed yarns in package form requires high volume airflow in order to penetrate the surface of the package. The process is slow and requires significant energy input. This drying method requires the packages to be stored within the stentor for a considerable period of time. It is therefore self-evident that there is need for a fast and controllable energy efficient method to remove moisture at low to moderate temperature. Furthermore it is of great benefit to be able to control the amount of moisture retained in the yarn or fabric, in gray or dyed form. In an attempt to achieve this one could think of using a centrifuge or other mechanical means and a radio frequency drying in sequence or in combination. So that bulk moisture is removed in centrifuge, or by other mechanical means, and the retained small fraction of moisture can be removed by the application of radio frequency energy. A further advantage of radio frequency is that the retained moisture level inside the textile material or package can be precisely controlled.

The above arrangements as applied to yarn and fabric holds good for drying food stuffs, ceramics and others where precise moisture control is needed.

Radio frequency drying uses the dielectric heating technique. Water has a high dielectric constant of approximately 80 and will heat when subjected to a sufficiently strong high frequency electric field. These systems are typically arranged to be centered on 27.12MHz in accordance with the internationally agreed I. S. M. (Industrial Scientific and Medical) allocation of 27.12MHz +/-0.6%.

Under the high frequency field the dipolar water molecules will swing to and fro in attempt to align themselves with the alternating field polarity. This action gives rise to friction that in turn causes heating. There is an additional heating mechanism in the form of resistive heating, in this mechanism the 'heat' is generated by the reactive losses created by the flow of an electric current through the dielectric formed by the dryer electrodes and the product (and the water content thereof). Since radio frequency drying is a direct form of heating that does not rely upon thermal conduction the moisture removal takes place at a lower temperature, typically less than 60 deg C; a significant point when the degradation of the product to be dried is to be considered. Fiber damage due to thermal stress is greatly reduced at lower temperature and is virtually eliminated where the maximum temperature is limited to 6OC.

The process of radio frequency drying depends upon agitation of the constituent dipolar molecules of the water present and in that process, the attached as well as bonded, water molecules escape the material to be dried. If a small draft is maintained with the help of a fan (or fan's) the free water molecules that have escaped from the body of the package, fabric or the other materials to be dried, are completely removed. If the material is first centrifuged (or subject to some other mechanical extraction method) the bulk of the moisture is removed by the brute physical force it then forms the ideal feed stock for radio frequency drying.

The same principal applies to drying (or moisture reduction or moisture leveling) of foodstuff, ceramics, wood or any item to be dried or the moisture level to be maintained at a precise desired level. The heating method employed here is a direct heating method and therefore the efficiency of the transfer of energy is high. Being a direct heating effect it may be accurately controlled by controlling the energy by the careful monitoring and control of both the RF power and drying time.

The oscillating circuit of the conventional radio frequency dryer shown in figure 1 employs predominantly capacitive coupling (Fig.7) which leads to improper coupling, and increased likelihood of sparking and energy loss. Being a C class high frequency electronic oscillator circuitry (Fig 1), the stability of the oscillation is a very important parameter. The construction of the oscillator design circuit of the conventional RF dryer shown in Fig.7 will change the frequency as the oscillator is loaded, create instability and unnecessarily leads to sparking condition. Once sparks appear inside the equipment it has the potential to become a fire hazard, it terminates the manufacturing routine and leads to loss of the value added manufactured material. Also sparks can lead to the creation of a plasma or corona discharge inside the enclosed area and therefore the oscillator element of the valve is subject to abnormal load conditions and in extreme situations can cause premature failure of the valve. The High-energy oscillator is the single most expensive component of this drying gadget and breakdown time increases the cost of drying.

Further, in the conventional RF dryer the oscillating tank circuit is made up of many elements in the form of plates Fig.7, with many fastening elements in the form of screws and fasteners. This construction arrangement under high frequency conditions can create local resonant circuits each of which may oscillate at that particular frequency, this can lead to a condition known as - parasitic oscillation - and thus cause a range of effects such as reduced efficiency, instability, arcing and many other problems. The presence of parasitic oscillations thus created by the current state of art leads to unstable oscillator conditions and it absorbs part of the power supplied to the tank. The result is that the dryer fails to develop its expected

rated power. In other words the parasitic oscillations does not help towards drying but drain part of the supplied energy. Parasitic oscillation may develop a climate conducive to the production of sparks and thus help towards the breakdown of the gadget and the drying process.

Another disadvantage of the oscillating circuit Fig.7 of the conventional RF dryer is that its load capacity cannot be varied. In the conventional design the tank circuit architecture is in such a way that one has to have different number of plates for each load and the load coupling arrangement changes depending upon the rating of the dryer namely 10 KW, 20 KW and 40 KW etc. Therefore, a separate RF circuit needs to be designed for every new load.

SUMMARY OF THE INVENTION

The object of the present invention is to design the geometry of the tank circuit in such a way that the arcing is virtually eliminated, the efficiency of converting electrical energy into radio frequency energy is increased so that the cost of drying is reduced.

Another object of the present invention is to provide an oscillator circuit in which the coupling device Ll, Fig.l can be moved in and out of the tank inductive element L2, Fig 1 in such a way that its position decides the coupling factor of the dryer. Thus a RF dryer with the radio frequency oscillator of the present invention can be considered to be transformer coupled to the load and be adjusted to effect the drying process on different loads such as 1OkW, 2OkW, 4OkW etc.

Thus another object of the present invention is to provide a variable load radio frequency dryer incorporating the new oscillator coupling circuit of the present invention.

Accordingly, the present invention provides an oscillator circuit for a variable load radio frequency dryer, the said oscillator comprising an inductor (L2) in the form of a bent plate with plurality of slots therein; capacitor (C3) in the form of one or more parallel plates; and load coupler (Ll) in the form of one or more loops passing through the slots of the inductor bent plate (L2); wherein each of the loop of the load coupler (Ll) being arranged to advance or retract from the bent plate (L2) for varying the load of the dryer. The coupling is now predominantly inductive rather than predominantly capacitive as in the conventional design.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated below with reference to an exemplary embodiment shown in the accompanying drawings.

Fig.1 shows a general radio frequency oscillator circuit.

Fig.2 shows a radio frequency dryer with the oscillating circuit of the present invention.

Fig.3 shows a side view of the oscillator circuit of the present invention.

Fig.4 shows a front view of the oscillator circuit of the present invention.

Fig.5 shows a perspective view of the oscillator circuit of the present invention.

Fig.6 shows the different views of the oscillator circuit of the present invention. Fig.7 shows the oscillator circuit of the conventional dryer.

DETAILED DESCRIPTON OF THE DRAWINGS

Fig 1 represents an oscillator circuit, which adequately describes the principle of operation of the radio frequent dryer. C3 and L2 form the tank circuit and it forms the magnetron elements for radio frequency oscillation. The geometry and architecture of this tank circuit is critical and it decides the frequency of the oscillation. The arrangement of the plates, the shape and load coupling of the inductor decides the fidelity and efficiency of the frequency of the operation. The

effectiveness of the coupling is decided by the disposition of the plates and the energy loss due to heating is decided by the parasitic oscillation. Parasitic oscillation occurs due to ineffective coupling of the load circuit and due to the presence of too many fastening elements. Thus the cardinal element of the new gadget is the design and architecture of the tank circuit comprising of elements C3, Ll and L2 shown in Fig 1. When parasitic oscillations exist the load gets less amount of its share of energy and to achieve the load level one has to increase the voltage which when reaches the insulation breaking stage sparks occur. Sparks sometimes short the oscillator valve and the valve fails.

The conventional oscillator tank circuit shown in fig.7 utilizes capacitance coupling. The conventional oscillator design consists of several plates with number of screws and fasteners as the capacitance element C3, Fig.7. The inductive element is in the form of semi circular bent plate L, Fig.7. The coupling is done by an outside bend plate Ll, Fig.7 and the coupling is capacitative. The disadvantage of this arrangement is that the coupling level cannot be varied and for one set of arrangement the coupling is constant and the associated load is fixed. Thus the circuit design is not flexible.

The oscillator circuit of the present invention designed to avoid the above- mentioned drawbacks is shown in Fig 2 in which C3 represents the capacitance. This capacitance can be created with different types of plate arrangement. For effective and efficient operation the capacitance and inductor plate can be assembled to achieve optimal performance. The number of plates and their dimensions are critical and these plates together form the capacitor element C3 and the bend plate L2 forms the inductor element. These together form the capacitor elements C3 and L2 of the tank circuit for oscillation in the desired radio frequency range and in the present case it is 27.12 megacycles. L2 is made of metal plate, preferably an aluminum plate, of critical dimensions and C3 is preferably comprised of eight parallel plates. For different frequency of oscillations the shape and number of capacity plates and the inductor plate has to be changed. The efficiency of the tank circuit depends upon the

arrangement, the separation and the area of the plates as well as the geometry of the element L2. The efficiency of the power transfer depends upon the coupling between Ll and L2. This could be capacitative and or inductive.

The highlight of the oscillating circuit of the present invention is that Ll to L2 coupling is predominantly inductive and partially capacitative. When two inductive elements are juxtaposed the partial capacitative effect cannot be avoided at higher frequencies of operation. Thus this is a hybrid kind of set up. Ll is composed of three loops made of metal and it penetrates into L2 through' three slots of L ₅ Fig.5 cut in the plate L2 (Figs.3 and 5). The Ll loops are mounted on a pedestal and they can be moved up and down (H) in side the L2 through these slots Fig.4. In the present case the movement (H) is restricted to 10 mm to 100 mm though higher movement can be incorporated for larger capacity dryers with adequate changes in the tank circuit on the above lines.

The advantage of this arrangement is that by advancing Ll in side L2 and keeping its movement (H) restricted the effectiveness of the coupling can be varied or in other words the load can be varied. Depending upon the requirement, Ll can have any number of bent metal plates moving up and down inside L2.

Thus the present geometry and architecture of the tank circuit help to change the load on the RF dryer. By advancing Ll into L2 the load can be increased and by withdrawing Ll from L2 the load can be decreased. This movement (H) changes the coupling and thus effectively changes the load. Thus with the same electronic set up and circuitry, machines with different drying capacities can be designed. In the present case the load variation can be from 1OkW to 10OkW. By changing H the user can increase or decrease the drying capacity depending upon his requirement. Extending the same logic higher load dryers can be designed.

Further attention has been paid to the operational efficiency of the system by the careful design of the oscillator and ancillary components, all major components are designed to be water-cooled and the water-cooling system has been designed to maintain the water temperature to enable effective cooling and allow recovery of thermal energy via a water to air heat exchanger the warm exhaust of which is vented into the dryer oven (1 in Figs 3, 4 and 6) and is then drawn over and through the product to be dried. The warm airflow is to recover energy that is used to assist the RF drying effect. The cooling system provides a means to optimize recovery of thermal energy normally last in cooling the water.

The present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope of the present invention.