Parallel Tube Passes
Use a single pass whenever possible. In heaters with more than one tube pass, there is the possibility of unequal flows in the passes. It also requires additional and constant operator attention to maintain the balance. Unbalanced flow can lead to overheating of tubes, coking, and “pass stalling” (a sudden decrease or loss of flow in one pass).
To prevent unbalanced flow in multipass systems with a liquid process stream, each pass should have at least a flowmeter, a manual control valve, an outlet temperature measurement, and a low flow alarm. A flow controller is preferred.
For a multipass gas stream, we rely on symmetry of passes to balance flow. It follows that the number of passes will usually be a power of 2 (2, 4, 8, etc.)
Do not split a two-phase process stream into multiple passes because you can’t ensure equal vapor/liquid ratios in the various passes. In some cases, the two-phase stream is the result of combining a liquid and a gas stream. You can split the singlephase streams into multiple passes, as described above, and then combine individual liquid and vapor streams. In other cases, a stream may partially vaporize as it passes through parallel flow control valves; this is all right.