Chapter 14 | 74714–113 A wet cooling tower is to cool 25 kg/s of cooling
water from 40 to 30°C at a location where the atmospheric
pressure is 96 kPa. Atmospheric air enters the tower at 20°C
and 70 percent relative humidity and leaves saturated at
35°C. Neglecting the power input to the fan, determine
(a) the volume flow rate of air into the cooling tower and
(b) the mass flow rate of the required makeup water.
Answers:(a) 11.2 m^3 /s, (b) 0.35 kg/s
14–114 A natural-draft cooling tower is to remove waste heat
from the cooling water flowing through the condenser of a
steam power plant. The turbine in the steam power plant
receives 42 kg/s of steam from the steam generator. Eighteen
percent of the steam entering the turbine is extracted for vari-
ous feedwater heaters. The condensate of the higher pressure
feedwater heaters is trapped to the next lowest pressure feed-
water heater. The last feedwater heater operates at 0.2 MPa and
all of the steam extracted for the feedwater heaters is throttled
from the last feedwater heater exit to the condenser operating
at a pressure of 10 kPa. The remainder of the steam produces
work in the turbine and leaves the lowest pressure stage of the
turbine at 10 kPa with an entropy of 7.962 kJ/kgK. The cool-
ing tower supplies the cooling water at 26°C to the condenser,
and cooling water returns from the condenser to the cooling
tower at 40°C. Atmospheric air enters the tower at 1 atm with
dry- and wet-bulb temperatures of 23 and 18°C, respectively,
and leaves saturated at 37°C. Determine (a) the mass flow rate
of the cooling water, (b) the volume flow rate of air into the
cooling tower, and (c) the mass flow rate of the required
makeup water.
Review Problems
14–115 The condensation of the water vapor in compressed-
air lines is a major concern in industrial facilities, and the
compressed air is often dehumidified to avoid the problems
associated with condensation. Consider a compressor that
compresses ambient air from the local atmospheric pressure
of 92 kPa to a pressure of 800 kPa (absolute). The com-
pressed air is then cooled to the ambient temperature as it
flows through the compressed-air lines. Disregarding any
pressure losses, determine if there will be any condensation
in the compressed-air lines on a day when the ambient air is
at 20°C and 50 percent relative humidity.
14–116E The relative humidity of air at 80°F and 14.7 psia
is increased from 25 to 75 percent during a humidification
process at constant temperature and pressure. Determine the
percent error involved in assuming the density of air to have
remained constant.
14–117 Dry air whose molar analysis is 78.1 percent N 2 ,
20.9 percent O 2 , and 1 percent Ar flows over a water body
until it is saturated. If the pressure and temperature of air
remain constant at 1 atm and 25°C during the process, deter-
mine (a) the molar analysis of the saturated air and (b) the
density of air before and after the process. What do you con-
clude from your results?
14–118E Determine the mole fraction of the water vapor at
the surface of a lake whose surface temperature is 60°F, and
compare it to the mole fraction of water in the lake, which is
very nearly 1.0. The air at the lake surface is saturated, and the
atmospheric pressure at lake level can be taken to be 13.8 psia.
14–119 Determine the mole fraction of dry air at the sur-
face of a lake whose temperature is 18°C. The air at the lake
surface is saturated, and the atmospheric pressure at lake
level can be taken to be 100 kPa.
14–120E Consider a room that is cooled adequately by an air
conditioner whose cooling capacity is 7500 Btu/h. If the room
is to be cooled by an evaporative cooler that removes heat at the
same rate by evaporation, determine how much water needs to
be supplied to the cooler per hour at design conditions.
14–121E The capacity of evaporative coolers is usually
expressed in terms of the flow rate of air in ft^3 /min (or cfm),
and a practical way of determining the required size of an
evaporative cooler for an 8-ft-high house is to multiply the
floor area of the house by 4 (by 3 in dry climates and by 5 in
humid climates). For example, the capacity of an evaporative
cooler for a 30-ft-long, 40-ft-wide house is 1200 4 4800
cfm. Develop an equivalent rule of thumb for the selection of
an evaporative cooler in SI units for 2.4-m-high houses
whose floor areas are given in m^2.
14–122 A cooling tower with a cooling capacity of 100 tons
(440 kW) is claimed to evaporate 15,800 kg of water per day.
Is this a reasonable claim?COOL
WATER34 °C
90%WARM
WATERAIR
INLET60 kg/s
40 °C1 atm
Tdb = 22°C
Twb = 16°CMakeup
water26 °CAIR
EXITFIGURE P14–112