When estimating the overall design heat-transfer coefficient, in which situation must a fouling factor definitely be included?

Introduction / Context:
Fouling is the unwanted accumulation of deposits (scale, biofilm, particulates) on heat-transfer surfaces. It adds thermal resistance and increases pressure drop. A realistic fouling allowance is built into the overall design heat-transfer coefficient U to ensure adequate thermal performance over time and reasonable cleaning intervals.

Given Data / Assumptions:

• Conventional shell-and-tube or plate heat exchanger design.
• Service may contain suspended solids.
• Design objective is reliable long-term heat duty with manageable maintenance frequency.

Concept / Approach:
Suspended solids at low velocity tend to settle or adhere more readily, promoting deposit growth. Conversely, high velocities provide a scouring action that mitigates fouling formation (within erosion limits). While viscosity and specific gravity affect film coefficients and hydraulics, they are not primary determinants for invoking a fouling factor; the risk is fundamentally tied to deposit propensity and local shear.

Step-by-Step Solution:

Verification / Alternative check:
Operating data routinely shows higher U degradation in low-velocity solids service; design handbooks assign larger fouling resistances for dirty, slow-moving streams.

Why Other Options Are Wrong:

• High velocity reduces fouling via scouring.
• High viscosity or high specific gravity affect heat transfer coefficients but do not, by themselves, mandate a fouling allowance without fouling tendency.

Common Pitfalls:
Overlooking asymmetry between shell and tube side fouling; ignoring seasonality or biogrowth; setting velocities too low to save pump power at the expense of severe fouling.

Final Answer:
Liquid containing suspended solids flows at low velocity.