Understanding Air Washer Processes

An air washer is a device used in air conditioning systems to clean, humidify, or dehumidify air, and sometimes cool or heat it. The process that occurs within an air washer depends primarily on the temperature of the sprayed water relative to the properties of the incoming air, specifically its dry bulb temperature (DBT) and dew point temperature (DPT).

Analyzing the Given Scenario

We are given the following conditions for the air entering the air washer:

• Dry Bulb Temperature (DBT) = $35\text{^\circ C}$
• Dew Point Temperature (DPT) = $20\text{^\circ C}$

The temperature of the water sprayed in the air washer is given as:

• Water Temperature = $25\text{^\circ C}$

Determining the Air Washer Process

The type of process occurring in an air washer depends on how the water temperature compares to the air's state. Let $T_w$ be the water temperature, $T_{db}$ be the dry bulb temperature, and $T_{dp}$ be the dew point temperature. The general cases are:

• If $T_w > T_{db}$: Heating and humidification.
• If $T_w < T_{dp}$: Cooling and dehumidification.
• If $T_{dp} < T_w < T_{db}$: Cooling and humidification.
• If $T_w$ is approximately equal to the air's wet bulb temperature (WBT): Adiabatic saturation (cooling and humidification towards saturation).

In this specific case, we have:

• $T_{db} = 35\text{^\circ C}$
• $T_{dp} = 20\text{^\circ C}$
• $T_w = 25\text{^\circ C}$

Comparing these values, we see that the water temperature ($25\text{^\circ C}$) is greater than the dew point temperature ($20\text{^\circ C}$) and less than the dry bulb temperature ($35\text{^\circ C}$). This fits the condition $T_{dp} < T_w < T_{db}$.

Under this condition, heat is transferred from the warmer air (initially at $35\text{^\circ C}$) to the cooler water ($25\text{^\circ C}$), causing the air's dry bulb temperature to decrease (cooling). Simultaneously, water evaporates from the spray into the air because the water temperature is higher than the air's dew point temperature, increasing the air's specific humidity and thus its dew point temperature (humidification).

Conclusion

Based on the analysis of the temperatures, the air washer process in this scenario is cooling and humidification.

Let's look at the options:

1. sensible cooling: Incorrect, as humidification is also occurring.
2. cooling and dehumidification: Incorrect, as humidification is occurring (water temperature is above DPT).
3. cooling and humidification: Correct, as the water temperature is between the air's DPT and DBT.
4. cooling and constant dew point temperature: Incorrect, as humidification increases the dew point temperature.

Therefore, the process is cooling and humidification.