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The most common brown stock washing system uses a series of vacuum drum washers. The primary element of a vacuum drum washer is a large compartmentalized cylinder or drum, which has its outer surface covered with a fine mesh. This drum is partially submerged in a vat filled with diluted pulp. As the drum slowly turns through the pulp suspension in the vat, a partial vacuum within the drum causes a mat of fiber to form on the surface as liquor is pulled into the drum. Once out of the vat, liquor continues to be pulled from the mat.