Exercise 17.6 Air at 20°C and 60% relative humidity is heated to 90°C and then enters a continuous dryer in an adiabatic operation. The air exits at 40 °C. Calculate: a) the inlet volumetric flow rate of air required to remove 20 kg of water/h from the product and b) the required energy to heat the air.

Elements Of Electromagnetics
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Exercise 17.6
Air at 20°C and 60% relative humidity is heated to 90°C and then enters a
continuous dryer in an adiabatic operation. The air exits at 40 °C. Calculate: a)
the inlet volumetric flow rate of air required to remove 20 kg of water/h from
the product and b) the required energy to heat the air.
Transcribed Image Text:Exercise 17.6 Air at 20°C and 60% relative humidity is heated to 90°C and then enters a continuous dryer in an adiabatic operation. The air exits at 40 °C. Calculate: a) the inlet volumetric flow rate of air required to remove 20 kg of water/h from the product and b) the required energy to heat the air.
Solution
Step 1
Draw the process diagram:
m
Heater
Dryer
H2
H1
Wi
VH1
H2
Step 2
Find the properties of the air at the inlet using a psychrometric chart or the
spreadsheet program Moist air properties.xls.
Air at 20 °C dry bulb temperature and 60% relative humidity:
Humidity W
Enthalpy H1
Humid volume VHI =
%3D
Air at 90°C dry bulb temperature and humidity W1:
Enthalpy H2 =
Air at 40 °C dry bulb temperature and enthalpy H2:
Humidity W3
Stęp 3
Write a mass balance on the moisture between points 2 and 3:
m. (W, - W)
Substitute values and calculate ma:
ma =
Use the humid volume to calculate the volumetric flow rate:
Step 4
Write an enthalpy balance on the air between points 1 and 2 and calculate the
required energy:
Transcribed Image Text:Solution Step 1 Draw the process diagram: m Heater Dryer H2 H1 Wi VH1 H2 Step 2 Find the properties of the air at the inlet using a psychrometric chart or the spreadsheet program Moist air properties.xls. Air at 20 °C dry bulb temperature and 60% relative humidity: Humidity W Enthalpy H1 Humid volume VHI = %3D Air at 90°C dry bulb temperature and humidity W1: Enthalpy H2 = Air at 40 °C dry bulb temperature and enthalpy H2: Humidity W3 Stęp 3 Write a mass balance on the moisture between points 2 and 3: m. (W, - W) Substitute values and calculate ma: ma = Use the humid volume to calculate the volumetric flow rate: Step 4 Write an enthalpy balance on the air between points 1 and 2 and calculate the required energy:
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