At equal mass velocities, how do typical convective film heat-transfer coefficients for gases compare with those for liquids?

Introduction / Context:Convective film coefficients depend on fluid properties and flow regime. Understanding the relative magnitude for gases versus liquids guides exchanger area estimates and selection of compact geometries when dealing with gas streams.

Given Data / Assumptions:

• Comparison at the same mass velocity (kg/m^2·s) and similar hydraulic diameter.
• Fully developed turbulent region assumed for both.
• Typical ambient-pressure gases and water-like liquids.

Concept / Approach:Dimensionless correlations (e.g., Dittus–Boelter type) show h ∝ k * Nu / L, with Nu depending on Re and Pr. Gases generally have much lower thermal conductivity k and higher kinematic viscosity, with Prandtl numbers near 0.7, while liquids like water have higher k and Pr of order 2–10. As a result, for similar Re at the same mass velocity, liquid-side coefficients are typically an order of magnitude larger than gas-side coefficients.

Step-by-Step Solution:Compare properties: k_liquid ≫ k_gas; Pr_liquid > Pr_gas.Use Nu correlations to infer h_liquid ≫ h_gas for equal mass flux.Conclude gas-side h is lower than liquid-side under comparable conditions.

Verification / Alternative check:Typical exchanger design tables list gas film coefficients in the tens–hundreds W/m^2·K, versus liquids in the hundreds–thousands W/m^2·K for similar geometry.

Why Other Options Are Wrong:Higher or same: contradicts property-based scaling and common data.Unpredictable: while exact values require calculation, the trend is well established.

Common Pitfalls:Comparing at equal volumetric flow instead of mass velocity; overlooking large property changes at high pressure (supercritical gases may deviate).

Final Answer:Lower