Safety in the use of RF heaters and sealers
The lossy material is often placed in the upper portion of the vestibule; the higher the percentage of this material that occupies the entire height, the greater the attenuation. Water, or special tiles, is often used. The resistance of the material must be sufficient to provide the required attenuation. Caution should be exercised to avoid the possibility of fires when materials such as carbon-loaded foam or other flammable absorbing materials are used.
The amount of attenuation needed from vestibules is also affected by the amount of stray radiation emitted by other parts of the equipment. Doors or windows in the shield walls may allow radiation to escape, adding to the total amount emitted by the dielectric heater. Vestibule design should provide enough attenuation that the escape of RF energy from other parts of the dielectric heater does not result in exposing the workers to RF levels exceeding recommended levels.
Air ducts can act as good vestibules provided they are properly connected to the shielding on the main heater chassis. The length of the air duct should be equal to twice its diameter or greatest cross-sectional dimension. The duct should be made of a material with good electrical conductivity and connected to the shield with a minimum of slots between fasteners. To ensure high conductivity and to avoid corrosion, at least the first length of the duct closest to the dielectric heater should be made of aluminium. However, some air ducts have become coated with a flammable material that can catch fire and burn the aluminium ducts. To prevent fire damage in these cases, the first length of the duct closest to the dielectric heater should be made of stainless steel and cleaned frequently. No paint should be used because it will impede current conduction and it is flammable. Stainless steel has a sufficiently high melting point that it will not be destroyed by a fire. If the air duct is used, little need exists for a screen over the opening in the shield where the duct is connected. In very wide ducts, connecting straps across (perpendicular to) the narrow dimension of the shield opening is sometimes desirable to shorten long current paths. This will decrease leakage and other undesirable effects such as a voltage variation in an electrode system.
Although vestibules made of a very lossy material would seem to reduce the need for further attenuation, this was not supported by experimental results; perhaps the wall currents were not sufficient for this to occur. Also, the product being heated will cause some attenuation when it extends into the vestibule. However, if the material's resistance is sufficient for it to be heated with an RF field, its resistance will be too high for it to act as an effective attenuator for vestibules.
High frequency power connections
Metallic conductors run between the generator and the applicator of a dielectric heater and carry high currents in most cases. These ungrounded conductors usually have a high voltage as well as a high current. They can also be sources of intense RF leakage unless shielded which involves enclosing them in a duct (see figure A-8). The enclosing duct must have high conductivity contacts with both the generator cabinet shield and the applicator housing. Fasteners used to connect the duct to the generator cabinet and the applicator housing must be closely spaced because any slot between them is perpendicular to the normal current path and will emit radiation. On high-powered heating equipment, a staggered, double row of bolts is sometimes necessary to provide a higher conductive connection and reduce RF leakage to an acceptable level.
The power output conductors (cables, straps, wires, etc.) often have bends to reach between the desired positions of the generator and the applicator. Such bends may create gaps in the conductors that are across the current path and are parallel to the inner conductor of the connecting system. Therefore, connections to bends such as elbows must also have high
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