Solucionario Himmemblau Chapter 5

111 CHAPTER 5
Problem 5.1A
A 100-hp engine is used to pump ground water into an irrigation channel. Calculate the
rate at which the pump is doing work in
(a) Btu/hr (b) J/s (c) kW.
Solution Basis: 100 hp
a. 100 hp 0.7068 Btu/s
1 hp
3600 s
1 hr = 2.544 ´ 105 Btu/hr
b. 100 hp 745.7 J/s
1 hp = 7.457 ´ 104 J/s
c. 100 hp 0.7457 kW
1 hp = 745.7 kW
Problem 5.1B
Classify the followingvariables as intense or extensive.
a) temperature d) specific heat capacity
b) density e) refractive index
c) mass
Solution
a. Temperature: intensive property
b. Density: intensive property
c. Mass: extensive property
d. Specific heat capacity: intensive property
e. Refractive index: intensive property
Sec. 5.1 Energy Balances Concepts and Units 112
Fig 19a Cutaway view of two-vaneimpeller with open eye (courtesy
of Nagle Pumps)
Fig. 19d Gear pump (courtesy of the Hydraulic Institute)
Fig 19b Semi-open impeller for handling
fluids containing pulpy solids
(courtesy of Nagle Pumps)
Fig. 19e Flexible impeller pump (courtesy of the Hydraulic
Institute)
Fig 19c Volute pump
PUMPS
Numerous designs of pumps exist among which a few are illustrated in the figures above
forpumping liquids. Centrifugal pumps have fixed impellers with fairly close clearance between
the impeller and the casing. Flexible impeller pumps have rubber blades that are squeezed
between the shell and the impeller hub. Gear pumps represent a class of positive displacement
pumps whose teeth mesh and force liquid into the outlet. Reciprocating piston pumps operate
much like the piston in anautomobile except the latter pressurizes gases.
PUMPS AND BLOWERS
113 Sec. 5.1 Energy Balances Concepts and Units
Problem 5.1C
Make a rough sketch of each of the following processes. Show the system boundary, and
classify the system as open or closed.
a) pump b) storage tank c) windmill d) internal combustion engine
Solution
The dashed lines represent the respective system boundaries.
a.pump
Liquid in
Liquid out
Electric motor
open system: mass is transferred across the system boundary
b. Storage tank
Storage
Tank
Closed system: the tank does not exchange mass or energy with
surroundings (except when it is filled or emptied)
Sec. 5.1 Energy Balances Concepts and Units 114
c. Windmill
Open System: wind moves in and out of the system
d.
Fuel IC Engine
air
exhaustOpen System: fuel and air enter the engine, and exhaust gases leave the
engine.
Problem 5.1D
Calculate the potential energy of 1 kg of water located at the surface of a lake that is 100
meters above a water turbine which is 200 meters away and used to generate electricity.
Solution
PE = mgh =
1 kg 9.80 m
s2
100 m 1 J
1 (kg)(m2)
s2
= 980J
115 Sec. 5.1 Energy Balances Concepts and UnitsProblem 5.1E
An ideal gas in a tank at 500°C and 100 kPa is compressed isothermally to 1000 kPa.
What was the work (in J/g mol) of compression?
Solution
We have to assume that the work done on the gas represents energy transferred solely to
the gas, and no work is done on the equipment containing the gas during the compression.
Basis: 1g mol gas
Isothermal compression
W = pdV
V1
V2
=V1
V2
nRT
V
dV = nRT ln
V2
ò ò V1
p2V2
p1V1
= n1RT1
n2RT2
so that
ˆV
2
ˆV
1
= p1
p2
= 100
1000
= 0.10
W =
1 g mol 8.314J
1 (g mol)(K)
(500 + 273.15) K
ln (0.10) = 14,800J
(the positive sign designates work done on the system)
Problem 5.1F
A wall of an annealing oven is made of 10 inch thick forebrick. If the interior of the wall
is at 2550°F and the exterior of thewall is at 392°F, what is the heat loss per hour? The relation
for heat transfer is Q = kA (T2 – T1)/Dx where k is the thermal conductivity, T is the temperature
in °F, and Dx is the wall thickness.
Solution
Basis: 1 hour
You have to look up the thermal conductivity in a handbook. It varies with temperature
but assume an average value of 0.81 Btu/(hr)(ft2)(°F)/ft.
Thus, Q =
0.81(Btu)(ft)...