Air initially at 101. 3 kPa and 40°C and with a relative humidity of 50%, is cooled at constant pressure to 30°C. The cooled air has a

ANS:C - higher relative humidity

To determine the changes in the air properties when cooled at constant pressure, we can use psychrometric principles. Given:

• Initial conditions: 𝑃1=101.3 kPaP1​=101.3kPa, 𝑇1=40°𝐶T1​=40°C, 𝑅𝐻1=50RH1​=50
• Final conditions: 𝑇2=30°𝐶T2​=30°C

1. Dew Point:
   • Dew point is the temperature at which air becomes saturated with moisture and starts to condense. It depends only on the moisture content of the air.
   • If the air is cooled at constant pressure, its dew point will decrease because cooling reduces the air's ability to hold moisture. Therefore, the cooled air will have a lower dew point.
2. Absolute Humidity:
   • Absolute humidity is the mass of water vapor present in a unit volume of air.
   • At constant pressure, the absolute humidity remains unchanged during cooling. Therefore, the cooled air will have the same absolute humidity as the initial air.
3. Relative Humidity:
   • Relative humidity is the ratio of the partial pressure of water vapor to the vapor pressure of water at the same temperature, expressed as a percentage.
   • If the actual water vapor content remains the same and the air is cooled, its relative humidity will increase because cooler air has a lower capacity to hold moisture. Therefore, the cooled air will have a higher relative humidity.
4. Wet Bulb Temperature:
   • Wet bulb temperature is the temperature measured by a thermometer that has its bulb covered with a wet wick. It is an indication of the evaporative cooling effect in the air.
   • If the air is cooled at constant pressure, its wet bulb temperature will decrease because cooling reduces the air's capacity to hold moisture. Therefore, the cooled air will have a lower wet bulb temperature.

• The cooled air will have a lower dew point, unchanged absolute humidity, higher relative humidity, and lower wet bulb temperature.a