Evaporator configurations: in which unit is heating steam inside the tubes while the process liquor flows on the tube exterior (shell side)?

Introduction / Context:
Evaporator selection depends on viscosity, fouling tendency, temperature difference, and required residence time. A key configurational distinction is whether heating steam flows inside tubes or on the shell side. Recognizing where steam is inside the tubes helps in predicting film behavior, heat-transfer coefficients, and maintenance needs.

Given Data / Assumptions:

• Conventional industrial evaporators without exotic internal enhancements.
• Steam heating and sensible/latent boiling of the liquor in standard arrangements.

Concept / Approach:
Horizontal-tube evaporators classically pass heating steam through the horizontal tubes while the liquor wets and boils on the outside (shell side). In contrast, long- and short-tube vertical designs commonly place steam in the shell and liquor inside the tubes, enabling vigorous two-phase upflow and high heat-transfer coefficients in the tubes.

Step-by-Step Solution:

Verification / Alternative check:
Standard schematics show steam channeled through tube sheets into tubes in horizontal units, with liquor boiling as a shell-side pool or film around tubes.

Why Other Options Are Wrong:

• Long-tube vertical and short-tube vertical typically place steam on shell side and liquor inside tubes.
• “None of these” is incorrect because one option matches the configuration.

Common Pitfalls:
Confusing “liquor inside the tube” (vertical units) with “liquor outside” (horizontal units); overlooking the cleaning and fouling implications of external boiling films.

Final Answer:
Horizontal-tube evaporator