Heat-transfer enhancement in stirred tanks: which change typically increases heat-transfer performance at the vessel wall or in coils?

Introduction / Context:Overall heat-transfer coefficient U in stirred vessels depends on film coefficients on both the process side and the jacket/coil side as well as wall resistance. Agitation affects the process-side film coefficient by changing turbulence near the wall and around internal coils.

Given Data / Assumptions:

• Standard baffled, mechanically agitated tank.
• Cooling medium flow is adequate.
• Process fluid is not excessively viscous.

Concept / Approach:Increasing stirrer speed intensifies turbulence and wall shear, thinning the thermal boundary layer on the process side and elevating the process-side heat-transfer coefficient h. In contrast, removing baffles increases swirling and can lower effective turbulence near surfaces. A higher coolant inlet temperature reduces the driving temperature difference, decreasing heat flux. Slower agitation reduces h and mixing uniformity.

Step-by-Step Solution:1) Identify U drivers: U depends on h_process, h_jacket, and conduction through the wall.2) Higher rpm → higher h_process via stronger turbulence.3) Therefore, at fixed coolant conditions, increasing rpm increases heat-transfer rate.

Verification / Alternative check:Empirical correlations for h around coils and walls scale positively with Reynolds number based on impeller speed and diameter.

Why Other Options Are Wrong:Option A reduces driving force; option C harms mixing pattern; option D lowers h; option E risks poor jacket-side performance.

Common Pitfalls:Ignoring jacket-side limits; if jacket flow is insufficient, gains from higher rpm may be muted.

Final Answer:Increasing the stirrer speed