The J shell can be constructed in one of two different fashions - 2 inlet nozzles and one exit OR 1 inlet nozzle and 2 exit nozzles. If the inlet flow is divided by using 2 inlet nozzles then the crossflow velocity is 50% of what it would be in an E shell. If you choose to divide the flow and calculate the pressure drop of each half you cannot add them together to get the total pressure drop. The total pressure drop is equal to the pressure drop of each flow path as they both meet at the center of the exchanger and discharge through the common exit nozzle. Similarly the heat transfer coefficient should be identical for each flow path and not additive to get a total. The area, however, is additive if you choose to look at it as two separate shells.
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