Choosing flow arrangement — A dilute aqueous process stream (10 kg/s) must be heated using available steam condensate at 95°C (also 10 kg/s) in a 1–1 shell-and-tube exchanger. Which arrangement typically maximizes the thermal driving force and controllability?

Introduction:
For sensible heating with limited source temperature (95°C condensate), maximizing the log-mean temperature difference (LMTD) is crucial. Counterflow generally provides a higher LMTD than parallel flow when inlet temperatures are fixed.

Given Data / Assumptions:

• Single-pass (1–1) shell-and-tube exchanger available.
• Process fluid: dilute aqueous stream to be heated.
• Heating medium: steam condensate at about 95°C (nearly isothermal or narrow range).

Concept / Approach:
Counterflow maintains a larger average temperature difference along the length, especially near the cold-end approach, making it more effective than parallel flow. Placing the process stream in the tubes offers practical benefits: higher tube-side velocities for better heat transfer and easier isolation/cleaning for potentially fouling process streams, while the condensate is conveniently routed on the shell side for distribution and drainage.

Step-by-Step Solution:
Choose counterflow to maximize LMTD at fixed inlet temperatures.Assign the process to the tube side to achieve suitable velocity and controllability.Route condensate on the shell side for good coverage and condensate removal.

Verification / Alternative check:
LMTD calculations comparing counterflow vs. parallel flow at the same terminal temperatures consistently show larger ΔT_lm for counterflow, validating option (b).

Why Other Options Are Wrong:

• (a) Counterflow is good, but process on shell side is less typical for dilute streams requiring velocity.
• (c) and (d) Parallel flow reduces ΔT_lm and thermal effectiveness.
• (e) Crossflow is not the 1–1 arrangement described.

Common Pitfalls:
Overlooking pressure constraints or corrosion that can force side assignment changes; neglecting condensate subcooling needs when setting control strategy.

Final Answer:
Counterflow with the process stream on the tube side