The film co-efficient between condensing vapour and metal wall increases with

The correct answer is:

✅ D. Increasing temperature drop
🔍 Explanation:
In filmwise condensation, the heat transfer rate is governed by the temperature difference between the condensing vapor and the metal wall. The film heat transfer coefficient increases as this temperature drop increases, since:

𝑞
=
ℎ
⋅
𝐴
⋅
Δ
𝑇
q=h⋅A⋅ΔT
Where:

𝑞
q = heat flux

ℎ
h = film heat transfer coefficient

Δ
𝑇
ΔT = temperature difference between vapor and wall

When the temperature drop increases, more heat is transferred per unit area, implying a higher effective film coefficient.

🔻 Why the others are incorrect:
A. Increasing temperature of the vapour: Not necessarily true unless wall temperature is fixed; effectiveness depends on temperature difference, not absolute value.

B. Decreasing temperature of the vapour: This would reduce the temperature difference and hence reduce the film coefficient.

C. Increasing viscosity of the film: Higher viscosity reduces film coefficient due to increased resistance to flow in the condensate film.

✅ Final Answer: D. Increasing temperature drop

option B is correct because h is increases with decreases in Temp. drop ( Tv - Tw )^(1/4) .
Since, Tv vapor Temp. must be decreases for increase in h .