Tank-Mounted and Immersion U-Tube Bundles

Short bundles hang from the mounting flange alone; longer ones get a support leg or bracket at the bend end that rests on the tank floor or a clip on the tank wall. The unsupported length is limited by deflection of the bundle under its own weight plus the product that fills it, and by the vibration that steam flashing in the tubes can set up. A sagging bundle loads the tube joints at the tubesheet. Tell us the tank depth and whether an internal support is acceptable, since some asphalt and tallow tanks cannot take welded attachments.

Yes, because the tube wall runs close to the steam saturation temperture, and a product that chars or degrades at the wall will coat the tubes. Heavy fuel oil, asphalt and tallow each have a film temperature above which they lay down a hard carbon layer. Lower steam pressure keeps the wall temperature down and the bundle gets more surface to compensate. Hot water or thermal oil is used where even low-pressure steam runs too hot. The mistake is choosing the plant's main steam pressure for convenience and finding the bundle coked after one season.

The coldest product temperature the bundle has to work against, which is usually the tank after a winter weekend with no heat, not the normal operating temperature. Natural convection on the tank side is weakest when the product is cold and most viscous, so that condition sets the surface. Sizing for the running temperature gives a bundle that keeps a warm tank warm but cannot recover a cold one. Give the lowest expected ambient, the tank insulation, the volume and the time allowed for heat-up, and the bundle will be sized for recovery rather than just holding.