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Heat Transfer - Engineering

Q1:

For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then

A hf = hd

B hf > hd

C hf < hd

D hf could be greater or smaller than hd

ANS:C - hf < hd

For condensation of pure vapors, the heat transfer coefficients in filmwise and drop-wise condensation depend on various factors and can differ. However, in practice: hf < hd Explanation:

  1. Filmwise Condensation: In filmwise condensation, the condensate forms a thin film on the surface, and heat transfer occurs through this film. The heat transfer coefficient (hf) in filmwise condensation is typically lower compared to drop-wise condensation due to the resistance offered by the condensate film.
  2. Drop-wise Condensation: In drop-wise condensation, the condensate forms as discrete droplets on the surface, which then fall off due to gravity. This process allows fresh surface area to be exposed continuously, resulting in higher heat transfer rates. Consequently, the heat transfer coefficient (hd) in drop-wise condensation is generally higher compared to filmwise condensation.
  3. Comparison: Due to the enhanced heat transfer mechanism in drop-wise condensation, hd is typically greater than hf. However, it's important to note that achieving drop-wise condensation in practice may not always be feasible or practical, and filmwise condensation is more common.
  4. Variability: The actual values of hf and hd can vary depending on factors such as the properties of the vapor, surface characteristics, operating conditions, and presence of additives in the vapor. Therefore, while hf is generally less than hd, there may be exceptions based on specific conditions.
In summary, for condensation of pure vapors, the heat transfer coefficient in drop-wise condensation (hd) is typically greater than the heat transfer coefficient in filmwise condensation (hf), leading to the inequality hf < hd.