Condensation heat transfer: for identical operating conditions, the average film coefficient on a horizontal condenser tube is approximately __________ that for a vertical condenser.

Introduction / Context:
During filmwise condensation on tubes, condensate drainage behavior influences film thickness and thus the heat-transfer coefficient. Tube orientation (horizontal vs vertical) changes how the condensate film develops and renews.

Given Data / Assumptions:

• Laminar filmwise condensation on clean, smooth tubes.
• Same fluid, temperature difference, and average vapor conditions.
• No significant non-condensable gases.

Concept / Approach:
On a horizontal tube, condensate tends to drain around the circumference, keeping a relatively thinner average film on the upper regions and shedding at the bottom. On a vertical tube, the film thickens along the length as it flows downward, increasing thermal resistance. Consequently, the average heat-transfer coefficient on a horizontal tube is higher, commonly approximated as about twice that on a vertical tube under comparable conditions.

Step-by-Step Solution:
Step 1: Recognize that thinner condensate films yield higher coefficients (h ∝ 1/film thickness).Step 2: Compare drainage paths: circumferential (horizontal) vs axial thickening (vertical).Step 3: Use the heuristic that h_horizontal ≈ 2 * h_vertical.Step 4: Choose “twice.”

Verification / Alternative check:
Classical Nusselt film condensation analysis and design heuristics support the factor-of-two rule for comparable conditions, acknowledging real-world variations due to surface finish and condensate inundation.

Why Other Options Are Wrong:
Half: Opposite of observed trend.Thrice/Five times: Overstates typical orientation effect.The same: Ignores drainage differences.

Common Pitfalls:
For flooded bundles or with strong shear, the simple ratio can deviate; always validate with manufacturer data for critical services.

Final Answer:
twice