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Filter Presses

Filter presses vary somewhat in form but are based upon the principle of forcing oil under pressure through a series of absorbing and filtering materials, such as paper. Filters of this type are capable of removing carbon, free water, sludge, etc., when they are in suspension, but except for certain special arrangements, they cannot remove them effectively when they are dissolved or in colloidal form. These devices (particularly those with centrifuges) will not remove air but, in fact, tend to aerate the oil. Experience has shown that the most efficient temperature at which to filter insulating oil is between 20 and 40C (68 and 100F). Below 20C, the viscosity increases rapidly, while at temperatures above 40C, the moisture is more difficult to separate from the oil.

Filter Press Operation

When the oil is to be purified by the use of a filter press using blotting paper, the paper should be well dried to obtain the most efficient operation; otherwise, the paper may actually add moisture to the oil. An oven should be used for drying the paper, and the sheets should be separated as they are hung on rods in the oven to permit free circulation of air and to ensure the most rapid drying. The filter paper should be dried from 6 to 12 hours at a temperature of 101 to 105C (214 to 220F). After drying, the paper should be taken from the oven directly to the filter, or it may be stored in dry transformer oil for future use. When transferring the paper, care should be taken to handle it as little as possible to avoid the absorption of moisture from the hands and to minimize the time of exposure to the air.

When purifying oil containing materials such as sludge or small carbon particles, considerable back pressure will develop as the filtering progresses because of the materials clogging up the filter paper. When the back pressure reaches about 517 kPa (75 lb/in2), the paper should be replaced.

When cleaning circuit breaker oil containing a large amount of free carbon, the accumulation of sediment on the surface of the paper makes frequent replacement of the paper necessary. In such cases, it is more economical to remove only the sheet of paper nearest the oil inlet frame in each section (the solid matter collects there) and insert the replacement sheet at the discharge end of the stack.

When purifying very wet oil with a filter press, the back pressure will not increase appreciably as the filter paper absorbs moisture. Therefore, the operator should make frequent dielectric tests of the oil discharged from the filter press to determine when the paper should be replaced.

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