Gusto niyo ng TITILLATING? haha. ITO! (Patulong naman! Sige na! Wala pa akong tulog!)

Solve the following problems. Write your solutions at the back of this page. (20 points each)

1. A humidifier is conditioning air to 120 °F dry-bulb and 90 °F wet-bulb by heating outside air and then passing it through a spray chamber (i.e., adiabatic saturation) in which it reaches 90 % RH, and then reheating it to the desired temperature. The outside air is at 60 °F, carrying as liquid 0.0004 lb water / ft³ dry air. What temperature (of air) must be reached in each heating operation, and how many Btu (of heat) is required in each heating stage per 100 lb of dry air entering from the outside?

2. A porous solid (initially weighing 400 kg), spread evenly in the dryer with a uniform thickness of 5 cm, is dried under constant drying conditions, with only the top surface exposed to the air flowing parallel to it. It takes 9 h to remove 75% of its total moisture content, starting with a wet solid with 30% moisture. The density of the wet solid is 2000 kg/m³. From separate drying experiments using the same material under same constant drying conditions, the following data were reported: equilibrium moisture content – 5%, critical moisture content – 15%.

• Estimate R_C.

• Same material is dried under constant drying conditions, but starting with only 200 kg of the material. Initial moisture content is same (i.e., 30%) and the thickness of the solid as spread in the dryer is still 5 cm. The total drying time is 7.2 h. What is the final moisture content, in %?

3. A counterflow induced-draft cooling tower operates with inlet and exit temperatures of 45 °C and 35 °C when the air has a dry-bulb and wet-bulb temperatures, respectively, of 35 °C and 30 °C. The tower has 1.8 m of stacked plastic fill and the flow rates are G_x = 85 kg/h×m² and G_y = 95 kg/h×m²_.

• Determine the number of transfer units, the height of a transfer unit based on the overall gas-phase driving force, and the temperature approach.

• Determine (minimum) flow rate of air needed for the given G_x so that the tower height requirement is large.

Note: The plastic stack height is the height of the tower.