Corona Treating Basics
Components in a corona treating system
Each corona treating system consists of:
- Power Supply
- Treater Station
- Exhaust System
- The Power Supply
The purpose of the power supply is to supply a sufficient amount of power to the treater station to generate a corona. Low frequency power, commonly supplied by the local utility, is converted to high frequency power ranging between 3-340 kilohertz (kHz) depending on the generator. Solid state generators, which are the most common, range between 3-40 kHz. The power supply can also be divided into three basic components; the input transformer, the high voltage transformer, and the control box. First, the primary function of the input transformer is to raise or lower the input voltage to the power supply. A secondary function of the input transformer is to isolate the power supply from the incoming power line so that it can be utilized as a filtering device. It also smooths out any power surges that may be present. Input transformers may be incorporated into the power supply cabinet, depending on the size. Larger sized input transformers are usually in separate housing. Occasionally, the input voltage is the same as the required voltage, so that input transformers are not used. Other power supply designs incorporate a chopper circuit which is used in place of the input transformer, but is frequently unreliable. Next, the high voltage transformer takes high current, low voltage power and converts to low current, high voltage power. Once converted the voltages can range from 10,000 to 30,000V. Last, the control box takes incoming power (in watts) and converts it to direct current. This is a conversion from low frequency, high current, low voltage; to direct current; and finally is chopped into very high frequency ranging from 3kHz to 50kHz. Essentially, the control box converts the incoming power from low frequency to high frequency. There are monitoring functions of operation; such as, power adjustment knobs, on/off buttons, loss of treatment knobs, etc.
The Treater Station
There are many different configurations of treater stations, but only two basic types. These treater stations are designed to treat conductive materials (bare roll), and those designed to treat nonconductive materials (covered roll). The primary different between the two systems is the location of the dielectric material, and the method used to cool the dielectric. All treater stations have a high voltage electrode, a ground electrode, and a solid dielectric material to cover one of the two electrodes in order to generate a corona atmosphere. To sum, the treater station is composed of a high voltage electrode, a ground path, dielectric, air gap, and housing. The sole function of the high voltage electrode is to present the high voltage electrical field across the air gap and the substrate. Although there are many types, the basic electrodes are normally metal for covered roll systems and ceramic tubes filled with conductive particles for bareroll systems. The metal electrodes are usually fixed bars, shoes, segments or fins. The ground path or ground electrode is typically a roller that can be metal or dielectrically covered depending on the specific system. It is used to provide an electrical path to the ground. Covered roll applications would usually use an aluminum or steel roll, depending on the dielectric coating used and the roll length. Bareroll systems usually employ bare aluminum or stainless steel rolls. The aluminum rolls are occasionally coated with either chrome or an electrically conductive ceramic in order to reduce wear and dirt build up. The dielectric materials’ sole function is to evenly distribute the HV field across the substrate. A dielectric material is any material which is nonconductive. This material is needed to cover one of the two electrodes in order to generate a corona atmosphere. The dielectric is used to buffer the voltage field to create a capacitor. The air gap is one of the most critical components of the treater station, since it is the air gap where the ionization (corona) occurs. Corona will occur anywhere air is present between the two electrodes. The distribution of power across the gap will only be even if the distances between the two electrodes is even. This is the area where corona is generated. The housing to the station serves two purposes. The first is to capture and contain any of the ozone or gas by products that are produced. The second function is for safety so that operators do not have access to high voltage/currents. The style and shape of the housing is dependent on the type of treater, the web path and the application. The open style station is the more popular one for flexo applications due to the limited space available on the press.
The exhaust system
The exhaust system is composed of piping and usually an exhaust blower. There are several by products that are produced during the process of corona such as undesirable gases. The most notable gas produced is ozone. It is for these reasons, that an exhaust system is required. The most common exhaust system incorporates piping and some exhaust ducting which leads out to the roof. In some instances, the by products are released into the air. A more sophisticated exhaust system is an ozone destruct unit. This unit will exhaust by products through a filtration process. Since these devices are intended to breakdown the ozone only, it is not recommended to return the exhaust air back into the plant.
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