Question

Air (at atmospheric pressure) at a dry bulb temperature of ${40}^{\circ }C$ and wet bulb temperature of ${20}^{\circ }C$ is humidified in an air washer operating with continuous water recirculation. The wet bulb depression (i.e., the difference between the dry and wet bulb temperatures) at the exit is 25% of that at the inlet. The dry bulb temperature at the exit of the air washer is closest to

• A. ${10}^{\circ }C$
• B. ${20}^{\circ }C$
• C. ${25}^{\circ }C$
• D. ${30}^{\circ }C$

Answer 1

The correct option is C ${25}^{\circ }C$

Given data:

$(DBT{)}_i={40}^{\circ }C$

$(WBT{)}_i={20}^{\circ }C$

Web bulb depression,

$(WBD{)}_i=(DBT{)}_i-(WBT{)}_i$

$=40-20={20}^{\circ }C$

Wet bulb depression at outlet,

$(WBD{)}_0=(DBT{)}_0-(WBT{)}_0$

By given condition,

$(WBD{)}_0=25\%(WBD{)}_i$

$\therefore (DBT{)}_0-(WBT{)}_0=0.25\times 20$

$(DBT{)}_0=0.25\times 20+(WBT{)}_0=5+(WBT{)}_i$

Where $(WBT{)}_0=(WBT{)}_i$ for humidified in an air washer

$\therefore (DBT{)}_0=5+20={25}^{\circ }C$



POINTS TO REMEMBER

Air Washer: An air washer is a device for conditioning air. In an air washer, air comes in direct contact with a spray of water and there will be an exchange of heat and mass (water vapour) between air and water. The outlet condition of air depends upon the temperature of water sprayed in the air washer. hence, by controlling the water temperature externally, it is possible to control the outlet conditions of air, which then can be used for air-conditioning purposes.