COURSE: THERMODYNAMICS
COURSE CODE: FD-201
CLASS: SE (FOOD ENGINNERING)
SEMESTER: FALL 2020
TEACHER: SALMAN ALI KHAN
1
One form of mechanical work frequently
encountered in practice is associated with the
expansion or compression of a gas in a
piston–cylinder device.
During this process, part of the boundary
(the inner face of the piston) moves back and
forth.
Therefore, the expansion and compression
work is often called moving boundary
work, or simply boundary work (Fig. 4–1).
Some call it the PdV work for reasons
explained later.
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The moving boundary work associated with real engines or
compressors cannot be determined exactly from a
thermodynamic analysis alone because the piston usually
moves at very high speeds, making it difficult for the gas
inside to maintain equilibrium.
Then the states through which the system passes during the
process cannot be specified, and no process path can be
drawn.
Work, being a path function, cannot be determined
analytically without a knowledge of the path.
Therefore, the boundary work in real engines or
compressors is determined by direct measurements.
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We analyze the moving boundary work
for a quasi equilibrium process, a
process during which the system remains
nearly in equilibrium at all times.
Consider the gas enclosed in the piston–
cylinder device shown in Fig. 4–2.
The initial pressure of the gas is P, the
total volume is V, and the cross sectional
area of the piston is A.
If the piston is allowed to move a
distance ds in a quasi-equilibrium
manner, the differential work done
during this process is
4
That is, the boundary work in the differential form is equal
to the product of the absolute pressure P and the differential
change in the volume dV of the system.
This expression also explains why the moving boundary
work is sometimes called the P dV work.
Note in Eq. 4–1 that P is the absolute pressure, which is
always positive.
However, the volume change dV is positive during an
expansion process (volume increasing) and negative during
a compression process (volume decreasing).
Thus, the boundary work is positive during an expansion
process and negative during a compression process.
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Therefore, Eq. 4–1 can be viewed as an expression for
boundary work output, Wb,out. A negative result
indicates boundary work input (compression).
The total boundary work done during the entire process
as the piston moves is obtained by adding all the
differential works from the initial state to the final
state:
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However, when performing a general analytical study or
solving a problem that involves an unknown heat or work
interaction, we need to assume a direction for the heat or work
interactions.
In such cases, it is common practice to use the classical
thermodynamics sign convention and to assume heat to be
transferred into the system (heat input) in the amount of Q and
work to be done by the system (work output) in the amount of
W, and then to solve the problem. The energy balance relation
in that case for a closed system becomes
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13
We know from experience that it takes different amounts of energy
to raise the temperature of identical masses of different substances
by one degree.
For example, we need about 4.5 kJ of energy to raise the
temperature of 1 kg of iron from 20 to 30 C, whereas it takes about
9 times this energy (41.8 kJ to be exact) to raise the temperature of 1
kg of liquid water by the same amount (Fig. 4–17).
Therefore, it is desirable to have a property that will enable us to
compare the energy storage capabilities of various substances. This
property is the specific heat.
14
The specific heat is defined as the
energy required to raise the
temperature of a unit mass of a
substance by one degree (Fig. 4–18).
In thermodynamics, we are interested
in two kinds of specific heats: specific
heat at constant volume cv and
specific heat at constant pressure cp.
15
Physically, the specific heat at constant
volume cv can be viewed as the energy
required to raise the temperature of the unit
mass of a substance by one degree as the
volume is maintained constant.
The energy required to do the same as the
pressure is maintained constant is the
specific heat at constant pressure cp. This is
illustrated in Fig. 4–19.
The specific heat at constant pressure cp is
always greater than cv because at constant
pressure the system is allowed to expand
and the energy for this expansion work
must also be supplied to the system.
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Now we attempt to express the specific heats
in terms of other thermodynamic properties.
First, consider a fixed mass in a stationary
closed system undergoing a constant-volume
process (and thus no expansion or
compression work is involved).
The conservation of energy principle ein-eout
= Δesystem for this process can be expressed in
the differential form as
17
Observation that can be made from Eqs. 4–19 and 4–20 is that cv
is related to the changes in internal energy and cp to the changes
in enthalpy. In fact, it would be more proper to define cv as the
change in the internal energy of a substance per unit change in
temperature at constant volume.
18
Both the internal energy and enthalpy of a substance
can be changed by the transfer of energy in any form,
with heat being only one of them. Therefore, the term
specific energy is probably more appropriate than the
term specific heat, which implies that energy is
transferred (and stored) in the form of heat.
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20
A 0.5-m3 rigid tank contains refrigerant-134a initially at 160 kPa and 40
percent quality. Heat is now transferred to the refrigerant until the pressure
reaches 700 kPa. Determine(a) the mass of the refrigerant in the tank and (b)
the amount of heat transferred. Also, show the process on a P-v diagram with
respect to saturation lines.
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22
A piston–cylinder device contains steam initially at 1 MPa, 450 C, and 2.5
m3. Steam is allowed to cool at constant pressure until it first starts
condensing. Show the process on a T-v diagram with respect to saturation
lines and determine (a) the mass of the steam, (b) the final temperature, and
(c) the amount of heat transfer.
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