The First Law of Thermodynamics

1
The First Law of Thermodynamics

• Open System (Control Volume)

2
First low of thermodynamics for open Systems

• Reminder of an open System.
• Open system Control volume
• It is a properly selected region in space.
• Mass and energy can cross its boundary.

3
Control volume involves two main processes

• Steady flow processes.
• Fluid flows through the control volume steadily.
• Its properties are experiencing no change with
  time at a fixed position.
• Unsteady flow processes.
• Fluid properties are changing with time.

4
Mass balance for steady flow processes

• We already showed that for steady flow
• Many engineering devices involve a single stream
  (one inlet and one exit only).

5
Energy Balance for Steady-Flow Systems
0
6
Let us look at some common steady flow devices
More than one inlet and exit
Only one in and one out
7
For single stream steady flow devices, the 1st
low becomes
Often the change in kinetic energy and potential
energy is small.
Per unit mass
8
Nozzles
A1
A2
A nozzle is a device that increases the velocity
of a fluid at the expense of pressure
9
Diffusers
A diffuser is a device that slows down the
velocity of a fluid causing an increase in its
pressure
10
Diffusers
11
Nozzles and Diffusers (1st low analysis)
NO
Is there work in this system?
let us say NO
Is there heat transfer?
In fact, it depends on the problem!
Does the fluid change elevation?
NO
12
In a nozzle, enthalpy is converted into kinetic
energy
How can you find the mass flow rate in a nozzle?
13
Example (4-9) Deceleration of Air in a Diffuser
Air at 10oC and 80 kpa enters the diffuser of a
jet engine steadily with a velocity of 200 m/s.
The inlet area of the diffuser is 0.4 m2. The air
leaves the diffuser with a velocity that is very
small compare to the inlet velocity. Determine (1
) The mass flow rate of the air and (2) The
temperature of the air leaving the diffuser.
14
Example (4-10) Acceleration of Steam in a Nozzle
Steam at 1.72 Mpa (250 psia) and 371?C (700 ?F)
steadily enters a nozzle whose inlet area is
0.019 m2 (0.2 ft2.) The mass flow rate of the
steam through the nozzle is 4.54 kg/s (10 lbm/s)
. Steam leaves the nozzle at 1.38 Mpa (200 psia)
with a velocity of 274.3 m/s (900 ft/s). The
heat losses from the nozzle per unit mass of the
steam are estimated to be 2.8 KJ/kg (1.2
Btu/lbm). Determine (a) the inlet velocity and
(b) the exit temperature of the
steam. Answers a) 41.0 m/s or 134.4
ft/s, b) 350 ?C or 661.9 ?F
15
Solution of Example Acceleration of Steam in a
Nozzle
Note that there is heat transfer (Q).So, you
have to go back to the general form of the 1st
low for single stream devices and get the
following
16
Turbines
A turbine is a device that produces work at the
expense of temperature and pressure.
As the fluid passes through the turbine, work is
done against the blades, which are attached to a
shaft. As a result, the shaft rotates, and the
turbine produces work.
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
Compressors
A compressor is a device that increases the
pressure of a fluid by adding work to the
system. Work is supplied from an external source
through a rotating shaft.
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
Turbines and Compressors
Yes!
Is there work in this system?
Is there heat transfer?
Negligible because of insulation. Exception
Internal cooling in some compressors.
Does the fluid change elevation?
NO
Usually it can be ignored
Does the kinetic energy change?
25
Example (4-12) Power Generation by a Steam
Turbine
The power output of an adiabatic steam turbine is
5 MW, and the inlet and the exit conditions of
the steam are as indicated in the figure on the
right. a) Compare the magnitude of ?h, ?ke, and
?pe. b) Determine the work done per unit mass of
the steam flowing through the turbine. c)
Calculate the mass flow rate of the steam.
Answers a) ?h -885.9 kJ/kg, ?ke 14.95
kJ/kg, ?pe -0.04 kJ/kg, b) 871.0 kJ/kg, and c)
5.74 kg/s
26
Throttling Valve
A throttling valve reduces the fluid pressure.
P1gtP2
It is small device and thus the flow through it
may be assumed adiabatic (q0) since there is
neither sufficient time nor large enough area for
any effective heat transfer to occur.
27
Throttling Valve
NO
Is there work in this system?
Is there heat transfer?
Usually it can be ignored
NO
Does the fluid change elevation?
Usually it can be ignored
Does the fluid change velocity?
28
What happens to the fluid temperature a cross
throttling Valves ?
29
Throttling Valves (incompressible substance )
For incompressible substance (like water), ? is
constant
and
For incompressible substance only!
30
Throttling Valves (compressible substance Vapor)
Example (4-13) Expansion of Refrigerant-134a in
a Refrigerator
Refrigerant-134a enters the capillary tube of a
refrigerator as saturated liquid at 0.8 MPa and
is throttled to a pressure of 0.12 MPa.
Determine the quality of the refrigerant at the
final state and the temperature drop during this
process. ltAnswers 0.339, -53.69oCgt
31
Solution of Example Expansion of
Refrigerant-134a in a Refrigerator
Notice that T2 ltT1
32
Throttling Valves (Compressible substance an
Ideal Gas)

• What happens if the gas is ideal?
• For ideal gases
• ?h Cp ? T But ? h 0
• So ? T 0
• The inlet and outlet temperatures are the same!!!

33
Mixing Chamber
Mixing two or more fluids is a common engineering
process
The mixing chamber does not have to be a distinct
chamber. An ordinary T-elbow, or a Y-elbow in a
shower, for example, serves as the mixing chamber
for the cold- and hot-water streams as shown in
the figure (Left).
34
Mixing Chamber
We no longer have only one inlet and one exit
stream
Is there any work done?
No
Is there any heat transferred?
No
Is there a velocity change?
No
No
Is there an elevation change?
35
Mixing Chamber

• Material Balance
• Energy balance

36
Example(4-14) Mixing of Hot and Cold Waters in a
Shower
Consider an ordinary shower where hot water at
140oF is mixed with cold water at 50oF. If it is
desired that a steady stream of warm water at
110oF be supplied, determine the ratio of the
mass flow rates of the hot to cold water. Assume
the heat losses from the mixing chamber to be
negligible and the mixing to take place at a
pressure of 20 psia. ltAnswer 2.0gt
37
Heat Exchanger
A heat exchanger is a device where two moving
fluids exchange heat without mixing.
38
Heat Exchangers
Your analysis approach will depend on how you
define your system
39
Heat Exchangers System (a) entire HX

• Mass Balance
• Divide into two separate streams with equal inlet
  and outlet flow rates
• Energy balance
• Two inlets
• Two outlets

40
Heat Exchangers System (b) Single stream

• Mass Balance
• Considering one single stream with one inlet and
  one outlet flow rates
• Energy balance
• One inlet
• One outlet
• Plus heat transfer

41
Example Cooling of Refrigerant-134a by Water
Refrigerant-134a is to be cooled by water in a
condenser. The refrigerant enters the condenser
with a mass flow rate of 6 kg/min at 1 MPa and
70oC and leaves at 35oC. The cooling water enters
at 300 kPa and 15oC and leaves at 25oC.
Neglecting any pressure drop, determine (a) the
mass flow rate of the cooling water required and
(b) the heat transfer rate from the refrigerant
to water. ltAnswers a) 0.486 kg/s, b) 20.35
kJ/sgt