Question 1

A room is filled with saturated moist air at 20°C and a total pressure of 90 kPa. If the mass of dry air in the room is 100 kg, the mass of water vapor is

Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
T1=20 "C"
P=90 "kPa"
m_air=100 "kg"
RH=1
P_g=PRESSURE(Steam_IAPWS,T=T1,x=0)
RH=P_v/P_g
P_air=P-P_v
w=0.622*P_v/(P-P_v)
w=m_v/m_air
 
"Some Wrong Solutions with Common Mistakes:"
W1_vmass=m_air*w1; w1=0.622*P_v/P "Using P instead of P-Pv in w relation"
W2_vmass=m_air "Taking m_vapor = m_air"
W3_vmass=P_v/P*m_air "Using wrong relation"

Question 2

A room contains 40 kg of dry air and 0.5 kg of water vapor at 20°C and 95 kPa total pressure. The relative humidity of air in the room is

Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
T1=20 "C"
P=95 "kPa"
m_air=40 "kg"
m_v=0.5 "kg"
w=0.622*P_v/(P-P_v)
w=m_v/m_air
P_g=PRESSURE(Steam_IAPWS,T=T1,x=0)
RH=P_v/P_g
"Some Wrong Solutions with Common Mistakes:"
W1_RH=m_v/(m_air+m_v) "Using wrong relation"
W2_RH=P_g/P "Using wrong relation"

Question 3

A 35-m³ room contains air at 25°C and a total pressure of 90 kPa with a relative humidity of 85%. The mass of dry air in the room is

Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
V=35 "m^3"
T1=25 "C"
P=90 "kPa"
RH=0.85
P_g=PRESSURE(Steam_IAPWS,T=T1,x=0)
RH=P_v/P_g
P_air=P-P_v
R_air=0.287 "kJ/kg.K"
m_air=P_air*V/(R_air*(T1+273))
 
"Some Wrong Solutions with Common Mistakes:"
W1_mass=P_air*V/(R_air*T1) "Using C instead of K"
W2_mass=P*V/(R_air*(T1+273)) "Using P instead of P_air"
W3_mass=m_air*RH "Using wrong relation"

Question 4

A room contains air at 25°C and a total pressure of 87.0 kPa with a relative humidity of 85%. The partial pressure of dry air is

Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
T1=25 "C"
P=87 "kPa"
RH=0.85
P_g=PRESSURE(Steam_IAPWS,T=T1,x=0)
RH=P_v/P_g
P_air=P-P_v
 
"Some Wrong Solutions with Common Mistakes:"
W1_Pair=P_v "Using Pv as P_air"
W2_Pair=P-P_g "Using wrong relation"
W3_Pair=RH*P "Using wrong relation"

Question 5

The air in a house is at 25°C and 72 percent relative humidity. Now the air is cooled at constant pressure. The temperature at which the moisture in the air will start condensing is

Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
T1=25 "C"
RH1=0.72
P_g=PRESSURE(Steam_IAPWS,T=T1,x=0)
RH1=P_v/P_g 
T_dp=TEMPERATURE(Steam_IAPWS,x=0,P=P_v)
"Some Wrong Solutions with Common Mistakes:"
W1_Tdp=T1*RH1 "Using wrong relation"
W2_Tdp=(T1+273)*RH1-273 "Using wrong relation"
W3_Tdp=WETBULB(AirH2O,T=T1,P=P1,R=RH1); P1=100 "Using wet-bulb temperature"

Question 6

On the psychrometric chart, a cooling and dehumidification process appears as a line that is

Accepted Answer

A cooling and dehumidification process on a psychrometric chart appears as a line moving diagonally downwards to the left (SW direction), indicating a decrease in both temperature and humidity.

Question 7

On the psychrometric chart, a simple heating process (no humidification or dehumidification) appears as a line that is

Accepted Answer

In a simple heating process, the dry bulb temperature increases while the specific humidity remains constant, which is represented by a horizontal line to the right on the psychrometric chart.

Question 8

Air in a space heating system is heated while being humidified by steam injection. On the psychrometric chart, this process will appear as a line that is

Accepted Answer

When air is heated and humidified by steam injection, both the temperature and humidity increase, which is represented by a diagonal line moving upwards to the right (NE direction) on a psychrometric chart.

Question 9

Air is cooled and dehumidified as it flows over the coils of a refrigeration system at 90 kPa from 25°C and a humidity ratio of 0.021 kg/kg dry air to 15°C and a humidity ratio of 0.015 kg/kg dry air. If the mass flow rate of dry air is 0.7 kg/s, the rate of heat removal from the air is

Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
P=90 "kPa"
T1=25 "C"
w1=0.021 
T2=15 "C"
w2=0.015
m_air=0.7 "kg/s"
m_water=m_air*(w1-w2)
h1=ENTHALPY(AirH2O,T=T1,P=P,w=w1)
h2=ENTHALPY(AirH2O,T=T2,P=P,w=w2)
h_w=ENTHALPY(Steam_IAPWS,T=T2,x=0)
Q=m_air*(h1-h2)-m_water*h_w
 
"Some Wrong Solutions with Common Mistakes:"
W1_Q=m_air*(h1-h2) "Ignoring condensed water"
W2_Q=m_air*Cp_air*(T1-T2)-m_water*h_w; Cp_air = 1.005 "Using dry air enthalpies"
W3_Q=m_air*(h1-h2)+m_water*h_w "Using wrong sign"

Question 10

Air at a total pressure of 85 kPa, 15°C, and 70 percent relative humidity is heated and humidified to 25°C and 60 percent relative humidity by introducing water vapor. If the mass flow rate of dry air is 4.6 kg/s, the rate at which steam is added to the air is

Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
P=85 "kPa"
T1=15 "C"
RH1=0.70 
T2=25 "C"
RH2=0.60
m_air=4.6 "kg/s"
w1=HUMRAT(AirH2O,T=T1,P=P,R=RH1)
w2=HUMRAT(AirH2O,T=T2,P=P,R=RH2)
m_water=m_air*(w2-w1)
 
"Some Wrong Solutions with Common Mistakes:"
W1_mv=0 "sine RH = constant"
W2_mv=w2-w1 "Ignoring mass flow rate of air"
W3_mv=RH1*m_air "Using wrong relation"

Quiz 14: Gas-Vapor Mixtures and Air-Conditioning