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Mass Transfer - Engineering

Q1:

Unsaturated air (with dry bulb temperature and dew point being 35°C and 18°C respectively) is passed through a water spray chamber maintained at 15°C. The air will be cooled

A and humidified.

B and dehumidified with increase in wet bulb temperature.

C at the same relative humidity.

D and dehumidified with decrease in wet bulb temperature.

ANS:D - and dehumidified with decrease in wet bulb temperature.

When unsaturated air is passed through a water spray chamber maintained at a temperature below its dew point, it will undergo cooling and humidification. Here's why:

  1. Cooling: The air temperature will decrease towards the temperature of the water spray, which is 15°C. This happens because heat is transferred from the air to the water as it comes into contact with the cooler water droplets.
  2. Humidification: As the air temperature decreases towards the dew point (18°C in this case), the air becomes increasingly closer to saturation. Since the air was initially unsaturated, it can absorb moisture from the water spray. This leads to an increase in the absolute humidity of the air.
So, the correct statement is: "The air will be cooled and humidified." Regarding the other options:
  • The air will not be dehumidified because it will gain moisture from the water spray.
  • The wet bulb temperature will decrease due to the cooling effect, not increase.
  • The relative humidity may change depending on the final conditions, but it will likely increase due to the addition of moisture to the air.