Chapter Two – Humidification , Cooling Tower ………… 1 ………… 2.1- INTRODUCTION Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 2 ………… 2.2- HUMIDIFICATION OPERATION Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 3 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 4 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 5 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 6 ………… 2.3- MIXING OF HUMID STREAM Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 7 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 8 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 9 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 10 ………… Unit Operations 2.4- HUMIDIFICATION PROCESSES THEORY Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Chapter Two – Humidification , Cooling Tower ………… 11 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 12 ………… Unit Operations 2.5- COOLING TOWER PRINCIPLES AND OPERATION Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Chapter Two – Humidification , Cooling Tower ………… 13 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 14 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 15 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 16 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 17 ………… 2.6- DESIGN COOLING TOWER Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 18 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 19 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 20 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 21 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 22 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 23 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 24 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 25 ………… 2.7- MINIMUM GAS (AIR) FLOW 2.8- COOLIN RANGE AND APPROACH Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 26 ………… Unit Operations 2.9- ADIABATIC HUMIDIFICATION - COOLING Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Chapter Two – Humidification , Cooling Tower ………… 27 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 28 ………… Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 29 ………… 2.10- DEHUMIDIFICATION TOWER Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Unit Operations Chapter Two – Humidification , Cooling Tower ………… 30 ………… Unit Operations Examples Chapter Two:-Humidification dehumidification and cooling tower Example(1): Section (1) Air containing 0.005 kg water vapour per kg of dry air is heated to 325 K in a dryer and passed to the lower shelves. It leaves these shelves at 60 per cent humidity and is reheated to 325 K and passed over another set of shelves, again leaving at 60 per cent humidity. This is again repeated for the third and fourth sets of shelves, after which the air leaves the dryer. On the assumption that the material on each shelf has reached the wet-bulb temperature and that heat losses from the dryer may be neglected, determine: (a) the temperature of the material on each tray; (b) the amount of water removed in kg/s, if 5 m /s moist air leaves the dryer; (c) the temperature to which the inlet air would have to be raised to carry out the drying in a single stage. Example(2): In an air-conditioning system, 1 kg/s air at 350 K and 10 per cent humidity is mixed with 5 kg/s air at 300 K and 30 per cent humidity. What is the enthalpy, humidity, and temperature of the resultant stream? Example(3): 0.15 kg/s steam at atmospheric pressure and superheated to 400 K is bled into an air stream at 320 K and 20 per cent relative humidity. What is the temperature, enthalpy, and relative humidity of the mixed stream if the air is flowing at 5 kg/s? How much steam would be required to provide an exit temperature of 330 K and what would be the humidity of this mixture? Example(4): Fresh air at 21.1 C in which partial pressure of water vapor is 0.018 atmosphere is blown at the rate of 214 m3/h first through a preheater and then adiabatically saturated in spray chambers to 100% saturation and again reheated . this reheated air has humidity of 0.024 kg water vapor per kg dry air. It is assumed that the fresh air and the air leaving the re-heater have the same percentage humidity. Determine:a- The temperature of preheater, spray chamber and re-heater b- Heat requirement for preheating and re-heating Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar Chapter Two – Humidification , Cooling Tower ………… 31 ………… Unit Operations Example(5): Humidifier is conditioning 15000 kg moist air per hr at 49 C dry bulb and 32C wet bulb temperature by heating outside air , passing it through an adiabatic spray chamber in which it reaches 90% Relative humidity and then reheating to the desired temperature . The outside air is at 4.5 C and is fogy carrying 0.006kg of liquid water /m3. To what temperature must the air be heated in the first heating operation ? what is the temperature of air as it emerges from the spray chamber before the heating operation? Calculate the heat suppled in the first and final heating?. Examples Chapter Two:-Humidification dehumidification and cooling tower Example(1): Section (3) (Adiabatic humidification and cooling of air): Air has to be humidified and cooled adiabatically in a horizontal spray chamber with recirculated water. The active part of the chamber is 1 m # 2 m # 1.5 m long. Under the operating conditions, the coefficient of heat transfer is expected to be 1300 kcal/(hr)(m2)(°C). 200 m3/min of air at 60 °C and 1 atm pressure with a humidity of 0.018 kg water/kg dry air is to be blown through the spray chamber. Calculate the following: (a) the temperature and humidity of the exit air (b) make-up water to be supplied, windage and blow down are neglected (c) the expected gas-phase mass transfer coefficient, kya (d) the temperature and humidity of the exit air if an identical spray chamber is added in series with the existing one. Example(2): A horizontal spray chamber with recirculation water is used for adiabatic humidification and cooling of air. The chamber has a cross section of 2 m2 with air rate of 3.5m3/sec at dry bulb temperature of 65C and absolute humidity of 0.017 kg water/kg dry air, the air is cooled and humidified to dry bulb temperature of 34C and leaves at 90% saturation. For the system the volumetric mass transfer coefficient may be taken as 1.12 kg/m3 sec (mole fraction) . The density of the air is 1.113kg/m3. Determine the length of the chamber for the requirements. Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar
