Question

Moist air at 30°C, 1 atm, and 42% relative humidity enters an evaporative cooling unit operating at steady state consisting of a heating section followed by a soaked pad evaporative cooler operating adiabatically. The air passing through the heating section is heated to 45°C. Next, the air passes through a soaked pad exiting with 50% relative humidity. A. Determine the specific humidity of the entering moist air (to the heating section). (5 of 45 points. Please input your final answer in the box below) B. Determine the rate of heat transfer to the moist air passing through the heating section, in kJ per kg of the entering mixture. (20 of 45 points. Please input your final answer in the box below)

Answer 1

Given data 1

db1 30 °C

01 0.42

db2 45 °C

03 = 0.50

$\textbf{Psychometric chart}\rightarrow$

-10 0 10 20 30 40 50 60 115 120 725 130 135 140 110 35 °C 105 N 100 ora 0.03 yol 140 es 0.03 90 Hulu 735 ha=h3 85 30 °C 80 130 75 125 hi Special Enthalpy (kJ/kg) 65 70 120 60 25 °C 115 0.02 55 We 0.02 Humidity Ratio (kg of moisture per kg of dry air) -20% 110 $ ulus 105 3 40 Wet-Bulb Temperature (°C) 7 100 W3 SS 15 "C 1 80 0.01 W1=Wz 0.01 VAN 10% as 10°C 80 . 75 0°C Constant Refative Humidity -5 °C 70 -10 -10C 65 0.00 0.00 60 10 $ S 10 25 30 55 US 90 ss So 75 20 Specific Enthalpy (kJ/kg + H 40 -10 0 10 Tobz 50 60 20 30 Tdbi Dry Bulb Temperature °C VDAS" Produced by TecQuipment's VDAS system TECQUIPMENT.COM

$\textbf{Solution}\rightarrow$

$\textup{Specific enthalpy and specific humidiity at point 1 corresponding to 30 }^{o}\textup{C and }\phi _{1}=0.42$$h_{1}=59\textup{ kJ/kg}$

$\omega _{1}=0.017\textup{ kg/kg of dry air}$------------------------------ Ans A

$\textup{from psychometric chart, }\omega _{1}=\omega _{2}=0.017\textup{ kg/kg of dry air}$

$\textup{Specific enthalpy at point 2 corresponding to 45 }^{o}\textup{C and }\omega _{2}=0.017\textup{ kg/kg of dry air}$

$h_{2}=74\textup{ kJ/kg}$

$\textbf{Rate of heat transfer through heating section}\left ( q \right )\rightarrow$

$q=\left ( h_{2}-h_{1} \right )$

$q=74-59$

$q=15\textup{ kJ/kg}$--------------------------------------- Ans B

$\textup{specific humidity at point 3 correspondings to }\phi _{3}=0.5\textup{ and }h_{3}=74\textup{ kJ/kg}$

$\omega _{3}=0.016\textup{ kg/kg of dry air}$---------------------------- Ans C

$\textup{dew point temperature at point 3 correspondings to }\phi _{3}=0.5\textup{ and }h_{3}=74\textup{ kJ/kg}$

$T_{db3}=32\: \: ^{o}\textup{C}$------------------------------- Ans D