1. First and Second Law of Thermodynamics
Lecture 3
First and Second Law of Thermodynamics

2. First Law of Thermodynamics FOR OPEN SYSTEMS
Inserting expression for flow work
and regrouping terms
Recall enthalpy defn.:
h=u+pv
For rate processes dividing both sides by t and letting t0
For large processes provided all inlet/outlet conditions are steady (not changing with time) integrate both sides

3. Conservation of mass and energy for a steady flow process
Conservation of energy

4. Applications Nozzles and diffusers (e.g. jet propulsion)
Turbines (e.g. power plant, turbofan/turbojet aircraft engine), compressors and pumps (power plant)
Heat exchangers (e.g. boilers and condensers in power plants, evaporator and condenser in refrigeration, food and chemical processing)
Mixing chambers (power plants)
Throttling devices (e.g. refrigeration, steam quality measurement in power plants)

5. Applications in pictures
Heat exchangers
Throttling devices
Source: internet

6. nozzles/diffusers
Single stream

7. turbines
Usually

8. compressors
Usually

9. heat exchangers hot (h) cold (c)
Take CV enclosing the stream that is hot at inlet
Take CV enclosing the stream that is cold at inlet

10. Mixing chambers or “direct contact heat exchangers”3 2 1
Conservation of mass
Conservation of energy

11. Principles of Thermodynamics

12. Thermal efficiency, where,
W = Net work transfer from the engine, and Q1 = Heat transfer to engine.
Q2 = Heat transfer from cold reservoir,

13. Clausius Statement
“It is impossible for a self acting machine working in a cyclic process unaided by any external agency, to convey heat from a body at a lower temperature to a body at a higher temperature”.
In other words, heat of, itself, cannot flow from a colder to a hotter body

14. Kelvin-Planck Statement
“It is impossible to construct an engine, which while operating in a cycle produces no other effect except to extract heat from a single reservoir and do equivalent amount of work”.

16. Why does Q flow from hot to cold?
Consider two systems, one with TA and one with TB
Allow Q > 0 to flow from TA to TB
Entropy changed by: DS = Q/TB - Q/TA
If TA > TB, then DS > 0
System will achieve more randomness by exchanging heat until TB = TA

17. Efficiencies of Engines
Consider a cycle described by: W, work done by engine
Qhot, heat that flows into engine from source at Thot
Qcold, heat exhausted from engine at lower temperature, Tcold
Efficiency is defined:
Qhot
engine
Qcold W
Since ,

18. Carnot Engines
Idealized engine
Most efficient possible

19. Application of 2nd law Carnot Engine isothermal compression adiabatic
expansion TA TB
1-2
2-3
3-4
4-1
Q12
Q34
W12
W23
W34
W41
Carnot
Engine
2T engine

20. Carnot Cycle

21. Efficiency of a Carnot engine
apply 1st law for this cycle:
then energy conversion efficiency is:
for a reversible process:

22. Refrigerators Given: Refrigerated region is at Tcold
Heat exhausted to region with Thot
Find: Efficiency
Qhot
engine
Qcold W
Since ,
Note: Highest efficiency for small T differences

23. Heat Pumps Given: Inside is at Thot Outside is at Tcold
Find: Efficiency
Qhot
engine
Qcold W
Since ,
Like Refrigerator: Highest efficiency for small DT

24. Entropy Total Entropy always rises! (2nd Law of Thermodynamics)
Adding heat raises entropy
Defines temperature in Kelvin!