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Steam also has mass-related requirements; there must be a space for gas-liquid separation at the top to prevent steam from containing water.

One type is the vertical thermosyphon type, which relies on density differences and often uses a reboiler at the bottom of the tower; the other type is the kettle type. Two specific descriptions: Pot still reboiler: A pot still reboiler has an enlarged shell, within which the vapor-liquid separation process takes place. The liquid level is maintained by a vertical baffle to ensure that the tube bundle is completely submerged in the liquid. The tube bundle usually takes the form of a U-shaped tube with two tube passes, or it can also be a floating-head structure with multiple tube passes. Hydrodynamic effects have little impact on kettle reboilers, so their performance is relatively reliable; in particular, their performance is even better under high vacuum conditions. By increasing the spacing between tubes, a high heat flux density can be achieved, allowing for good operating conditions under small temperature differences. The disadvantage of kettle-type reboilers is that they tend to scale easily; they are generally the most prone to scaling among all types of reboilers. In addition, their shells are large, resulting in higher costs. The optimal application of kettle reboilers is for clean fluids under low pressure, with a narrow boiling point range, and in conditions of either small or large temperature differences. For near-critical pressure conditions, although the shell is larger and the cost is higher, the performance remains reliable. Thermal siphon reboiler: The feed is introduced into the reboiler through the downcomer at the bottom of the tower; the liquid boils in the shell side and vaporizes, forming a vapor-liquid mixture with lower density. Due to the difference in density between the liquid in the feed pipe and that in the discharge pipe, a static pressure difference is created, which serves as the driving force for the natural circulation of the fluid. The heating medium flows inside the tubes, and the tube side can be of single-pass or multi-pass type. Its advantage is a high cycle rate, which results in a high flow velocity and a lower outlet temperature; this prevents the accumulation of high-boiling-point components and reduces the rate of scaling. Since the tube bundle is arranged horizontally and the flow area is easy to control, a lower static head is required. Its disadvantage is that it is difficult to clean once scaling occurs in the shell side. Due to the effect of baffle plates and support plates, local drying out can occur under high heat flux conditions. For large thermal siphon reboilers, multiple nozzles and connecting fittings are required to ensure a uniform flow distribution, which inevitably increases the cost of the reboiler. For fluids with a wide boiling range, horizontal baffles should be installed to prevent the light components from flashing at the inlet and the heavy components from concentrating at the outlet. To prevent flow blockage and flow instability, the maximum heat flux density should be limited. This reboiler is suitable for applications with medium pressure, medium temperature difference, and low static head.

Mainly, steam requires a gas phase space to prevent it from carrying water