Question

Atmospheric air with Dry Bulb Temperature (DBT) of 24°C and Relative Humidity of 35%, entering in a circular duct (assume no pressure drop in the duct) is heated by an electrical resistance arrangement inside the duct. The DBT of air measured at the outlet of the duct is equal to 30°C. Considering the flow to be steady, which of the following statement(s) is (are) correct as regards to the outlet air, with respect to the inlet air?

• A. There is no change in the Relative Humidity
• B. There is no change in the Dew Point Temperature
• C. There is no change in the Specific Humidity
• D. There is no change in the Specific Enthalpy

Hint:

Heating air in a duct without adding or removing moisture results in a decrease in Relative Humidity but no change in Specific Humidity or Dew Point Temperature.

Answer 1

The Correct Option is B, D

We are given that air is entering a duct with a Dry Bulb Temperature (DBT) of 24°C and Relative Humidity (RH) of 35%, and is being heated to a DBT of 30°C. The flow is steady, and the pressure drop is assumed to be negligible.

Step 1: Analyze the effect of heating on the air properties.
When air is heated in a duct without any change in pressure, there is a change in DBT, but the relative humidity (RH) will decrease. However, since no water is added or removed from the air, the specific humidity (which is the mass of water vapor per unit mass of dry air) remains constant.

Step 2: Evaluate the given options.
- (A) There is no change in the Relative Humidity: This statement is incorrect. As the air is heated, the capacity to hold moisture increases, causing a decrease in RH.
- (B) There is no change in the Dew Point Temperature: This statement is correct. The Dew Point Temperature (DPT) depends on the specific humidity, which remains constant as no moisture is added or removed. Therefore, the DPT remains unchanged.
- (C) There is no change in the Specific Humidity: This statement is correct. Since no moisture is added or removed from the air, the specific humidity remains the same.
- (D) There is no change in the Specific Enthalpy: This statement is incorrect. The specific enthalpy will change as the temperature of the air increases.

Step 3: Conclusion.
The correct answers are (B) and (C) because the Dew Point Temperature and Specific Humidity remain unchanged as the air is heated.

Final Answer: \text{(B) There is no change in the Dew Point Temperature}
\text{(C) There is no change in the Specific Humidity}