Discuss various prometric processes that can be achieved by using an air washer.
1 Answer

Air Washer:

Air washer as shown in figure involves flow of air through a spray of water. During the flow, air may be cooled or heated, humidified or dehumidified or simply adiabatically saturated depending on the water temperature.

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Water is accordingly cooled or heated externally, or simply recirculated by a pump. Make-up water is added in case of humidification of air.

Figure shows the thermodynamic changes of state of air along paths 1-2 in an air washer. The actual temperature of water is $t_s$ and of air is $t_1$. The WBT of air is $t_1^{'}$ and DPT is denoted by $t_d$. The following psychrometric processes are possible:

Process 1-2A: Heating and humidification $(t_s \gt t_1)$ The temperature of water is greater than the DBT of air. Water is externally heated.

Process 1-2B: Humidification $t_s=t_1$

The water temperature is equal to DBT of air. The enthalpy of air increases and hence water is required to be externally heated.

Process 1-2C: Cooling and humidification $(t_1^{'}\lt t_s \lt t_1)$ The water temperature is less than DBT of air but greater than the WBT of air. Though the air is cooled, its enthalpy increases due to humidification. Hence water is required to be heated externally.

Process 1-2D: Adiabatic saturation $(t_1^{'}=t_s)$ In this case water is recirculated by pump without any external heating or cooling. The recirculated water reaches the equilibrium temperatures which is equal to WBT of air.

Process 1-2E: Cooling and humidification $(t_d \lt t_s \lt t_1^{'})$ Process is similar to 1-2C except that the enthalpy of air decreases. Hence water is required to be externally cooled.

Process 1-2F: Cooling $(t_s=t_d)$ Temperature of water is equal to the DPT of air. Water is required to be cooled.

Process 1-2G: Cooling and dehumidification $(t_s \lt t_d)$ The water temperature is lower than DPT of air and so air is simultaneously cooled and dehumidified.

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