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4. Water at 24C is flowing by gravity through a filter bed of small particles having an equivalent diameter of 60 um. The void fraction of the bed is measured to be 0.42. The packed bed has a depth of 1.5 m. The pressure at the top of the bed is 4.0 kPa gauge and the column empties to the atmosphere. What is the water velocity based on the empty cross section of the bed