Forced-circulation long-tube vertical (LTV) evaporators vs. natural circulation: which statements are correct?

Introduction / Context:
Evaporators remove solvent (often water) by boiling. Two common types are natural-circulation long-tube vertical (LTV) and forced-circulation LTV. The choice impacts heat-transfer coefficients, fouling behavior, boiling control, and installed/operating costs.

Given Data / Assumptions:

• Comparison focuses on hydraulic/thermal features, not specific duty economics.
• Forced-circulation units have an external pump loop.
• Vapor disengagement occurs in a vapor drum or separator.

Concept / Approach:
Forced-circulation evaporators drive liquid through tubes at high velocity using a circulation pump. This suppresses nucleate boiling inside the tubes (often boiling is allowed mainly in the separator), increases heat-transfer coefficients, and reduces fouling. As a result, for the same duty, less heating surface may be required than in natural circulation. The hydraulic loop is typically arranged with a pump drawing from the vapor drum down-take and discharging to the tube-bundle inlet.

Step-by-Step Solution:
Evaluate (b): High velocity → higher h and often less area → correct.Evaluate (c): Loop arrangement with centrifugal pump between the down-take and bundle inlet → correct.Evaluate (a): “More economical in operation” is not universally true; pumps add power and maintenance, though benefits may offset in fouling/viscous services. Hence (a) is not a blanket statement.Therefore, select “Both (b) & (c).”

Verification / Alternative check:
Design texts and vendor datasheets consistently depict the external recirculation pump loop and report higher film coefficients with forced circulation.

Why Other Options Are Wrong:
(a) Over-generalizes economics; depends on service.Single selections (b) or (c) omit the other true statement; (d) captures both correct points.

Common Pitfalls:
Allowing boiling inside tubes in forced-circulation units with fouling fluids; undersizing the pump causing insufficient velocity and poor performance.

Final Answer:
Both (b) & (c)