Equipment manufacturers have modified the CPI configuration for horizontal water flow perpendicular to the axis of the corrugations in the plates as shown in Figure 7-8. This allows the plates to be put on a steeper angle to facilitate sediment removal, and to enable the plate pack to be more conveniently packaged in a pressure vessel. The latter benefit may be required if gas blowby through an upstream dump valve could cause relief problems with an atmospheric tank.
Cross-flow devices can be constructed in either horizontal or vertical pressure vessels. The horizontal vessels require less internal baffling as the ends of nearly each plate conduct the oil directly to the oil-water interface and the sediments to the sediment area below the water flow area. However, the pack is long and narrow and, therefore, it requires an elaborate spreader and collection device to force the water to travel across the plate pack in plug flow. There is a possibility of shearing the inlet oil droplets in the spreader, which would make separation more difficult.
Vertical units, although requiring collection channels on one end to enable the oil to rise to the oil-water interface and on the other end to allow the sand to settle to the bottom, can be designed for more efficient sand removal. CPI separators are generally cheaper and more efficient at oil removal than cross-flow separators. However, cross-flow should be considered where the use of a pressure vessel is preferred and sediment laden water is expected.