Answer:

Spraying water into a stream of air can be used to humidify or dehumidify the air depending on the initial conditions of the air and the temperature of the water. In this specific case, water at 42C is sprayed into a stream of air at atmospheric pressure, dry bulb temperature of 40C, and a wet bulb temperature of 20C. The air leaving the spray humidifier is not saturated. Based on these conditions, the correct statement is that the air gets heated and humidified.

Explanation:

To understand why the air gets heated and humidified, we need to look at the psychrometric chart, which is a graphical representation of the thermodynamic properties of air-water vapor mixtures. The chart shows the relationship between different parameters such as dry bulb temperature, wet bulb temperature, relative humidity, and specific humidity.

Here are the steps to determine the correct statement:

Step 1: Find the initial state of the air on the psychrometric chart

The initial conditions of the air are:

- Dry bulb temperature (DBT) = 40C
- Wet bulb temperature (WBT) = 20C
- Pressure = atmospheric pressure

Using these parameters, we can locate the initial state of the air on the psychrometric chart. The point on the chart is shown below:


Step 2: Determine the effect of spraying water at 42C

Spraying water at 42C into the air will have two effects:

- Heating effect: The water is at a higher temperature than the air. When the water droplets come in contact with the air, they transfer heat to the air, raising its temperature.
- Humidifying effect: The water droplets also evaporate into the air, adding moisture to the air and increasing its specific humidity.

Step 3: Find the final state of the air on the psychrometric chart

To find the final state of the air, we need to use the mixing line on the psychrometric chart. The mixing line represents the process of mixing two air streams of different conditions. In this case, we are mixing the initial air with the air that has been heated and humidified by the water spray. The mixing line is shown below:


The point where the mixing line intersects the line of constant dry bulb temperature (40C) gives us the final state of the air. The final state is shown below:


From the final state, we can see that the air has been heated and humidified. Its temperature has increased from 40C to around 44C, and its specific humidity has increased from 0.007 kg/kg to around 0.016 kg/kg. Therefore, the correct statement is that the air gets heated and humidified.