Modern Engineering Thermodynamics

Problems 443
exhaust gas is an ideal gas and ignore all kinetic and potential
energy effects.
29. An air–water vapor mixture at 14.7 psia, 100.°F, and 40.4%
relative humidity is contained in a 10.0 ft 3 closed tank. The
tank is cooled until the water just begins to condense.
Determine the temperature at which condensation begins.
30. When the dew point of atmospheric air is between 60.0°F and
70.0°F, the weather is said to be humid, and when it is above
70.0°F it is said to be tropical. If the partial pressure of water
vapor in the air is 0.400 psia when the dry bulb temperature is
80.0°F, determine the relative humidity and whether the weather
is humid or tropical.
31. The volume fractions of the gases in the atmosphere of Mars
measured at the surface of the planet, where the total
atmospheric pressure is 0.112 psia, were found by the early
Viking I mission to be 95.0% CO 2 , 2.50% N 2 , 2.00% Ar,
0.400% O 2 , and 0.100% H 2 O. Determine
a. The partial pressure of the water vapor in the Martian
atmosphere.
b. The mass fractions of all the gases in the Martian
atmosphere.
c. The humidity ratio of the Martian atmosphere (consider the
Martian dry “air” to be everything in its atmosphere except
the water vapor).
32. An engineer at a party is handed a cold glass of beverage with
ice in it. The engineer estimates the outside temperature of the
glass to be 35.0°F and the room temperature to be 70.0°F. What
is the relative humidity in the room when moisture just begins
to condense on the outside of the glass?
33.* The list of cities in Table 12.6 includes representative wet and
dry bulb temperatures. Use the psychrometric chart to determine
their corresponding relative humidity, humidity ratio, dew point
temperature, and water vapor partial pressure.
34. People often clean eyeglasses by holding them near their mouths
and exhaling heavily on them. This usually causes the lenses to
“fog”; the moisture is then wiped off, and this process cleans the
lenses. Assuming the air in the lungs has a relative humidity of
75.0% at a dry bulb temperature of 100.°F, determine the
maximum temperature of the glasses that just causes moisture
droplets (i.e., “fog”) to form when cleaned in this manner.
35.* 7000 m 3 /min of air at 28.0°C and 0.101 MPa with a relative
humidity of 60.0% is to be cooled at constant total pressure to
its dew point temperature. Determine the required heat transfer
rate and indicate its direction.
36. 30 ft 3 /min of air with a dry bulb temperature of 90.0°F and a
relative humidity of 80.0% is to be cooled and dehumidified to
a dry bulb temperature of 65.0°F and a relative humidity of
50.0%. Determine (a) the wet bulb temperature of the air before
dehumidification, (b) the dew point temperature of the air after
dehumidification, (c) the amount of moisture removed during
the dehumidification process, and (d) the amount of heat
removed during the cooling part of the dehumidification
process.
37.* A room containing 275 m 3 of air at 1.00 atm pressure is to be
humidified. The initial conditions are T DB = 24.0°C and
ϕ = 20.0%, and the final conditions are T DB = 20.0°C and
ϕ = 60.0%. Determine the mass of water that must be added to
the room air.
38. 1000. ft 3 /h of moist air at atmospheric pressure, 80.0°F and
70.0% relative humidity is to be cooled to 50.0°F at constant
total pressure. Find whether or not this can be done without the
removal of water from the air. If it cannot, determine the
minimum amount of water that must be removed in lbm/h.
39. A classroom contains 6000. ft 3 of air–water vapor mixture at
1.00 atm total pressure. The dry bulb temperature is 70.0°F and
the wet bulb temperature is 65.0°F. Assuming a closed constant
total pressure system, determine the following:
a. The relative humidity.
b. The partial pressure of the water vapor.
c. The dew point.
d. The amount of water that must be added to or removed
from the air in the room to achieve 40.0% relative humidity
at the same dry bulb temperature.
40. 10,000. ft 3 /h of moist air at 14.7 psia and 75.0°F is to be cooled
to 45.0°F at constant total pressure. Find the amount (lbm/h) of
water condensed if the mole fraction of water in the inlet
mixture is 0.0260.
41.* Atmospheric air can be dehumidified by cooling the air at
constant total pressure until the moisture condenses out.
Suppose that air with a humidity ratio of 5.00 × 10 −3 kg water
per kg of dry air must be achieved by cooling incoming
atmospheric air with a dry bulb temperature of 25.0°C and a
wet bulb temperature of 20.0°C.
a. To what temperature must the incoming air–water vapor
mixture be cooled to achieve a humidity ratio of 5.00 ×
10 −3 kg water per kg dry air?
b. How much water must be removed per kg of dry air to
achieve this state?
42. Outside atmospheric air with a dry bulb temperature of 90.0°F
and a wet bulb temperature of 85.0°F is to be passed through
an air conditioning device so that it enters a house at 71.0°F
and 40.0% relative humidity. The process consists of two steps.
Table 12.6 Problem 33
City T WB T DB ϕ ω T DP P w
Berlin 21.0°C 32.0°C
Chicago 75.0°F 97.0°F
Hong Kong 28.0°C 33.0°C
New York City 75.0°F 93.0°F
Paris 20.0°C 31.0°C
Rome 23.0°C 36.0°C
Tokyo 79.0°F 92.0°F