Thermal 07

Unit No:01
Thermodynamics
Prof. Yash B. Parikh
M.Tech (Computer Integrated Manufacturing)
B.E.(Mechanical Engineering)
Assistant Professor
Department of Mechanical Engineering

Throttling Process
2
 Whenever a substance expands from a region of
relatively high pressure across a restriction or small
opening under a steady flow without heat transfer.
e.g. the puncture of a tyre is a throttling process.
 The pressure drop takes place so rapidly that there is
neither sufficient time nor surface area for heat
transfer to take place.
 Throttling process is a.k.a. isenthalpic process.
 Expansion of a gas through a large opening is called
a free expansion.
e.g. bursting of tyre is the case of free expansion
process.

Throttling Process
3
 Only difference between the two is in free expansion
gas leaves with larger velocity whereas in throttling
change in velocity is negligible.

Limitations of First Law of Thermodynamics
4
 Heat is transferred from hot body to cooler
surroundings. However reverse of this is not possible.
 In Joules experiment, reverse process is not possible.
i.e. raising the weight by transferring heat from fluid
to the paddle wheel is not possible.
1. The processes can proceed in a particular direction
only.
2. All processes involving conversion of heat into work
& vice-versa are not equivalent.

Second Law of Thermodynamics
5
 According to First law of thermodynamics heat and
work are mutually interchangeable.
 This law does not mention about the direction flow of
heat.
 Second law of thermodynamics gives the direction of
flow of heat energy.
 It is important to understand how heat energy is
converted into mechanical work.

Heat Engine
6
 Heat engine is a device used for converting heat
energy constantly into mechanical work.
 Three essential parts of a heat engine are :
Source (Hot Body) , Sink (Cold Body) & Working
Substance.

Heat Engine
7
 Heat Source : A heat reservoir which supplies heat to
a system.
 Heat Sink: A heat reservoir which absorbs heat from
a system.
 Heat Reservoir : The source of infinite heat energy.
Finite amount of heat absorbed or rejected will not
affect its temperature
e.g. Ocean, Lakes, Atmosphere.

Thermal Efficiency of a Heat Engine
8
 Heat energy is never converted completely into
mechanical energy.
i.e. efficiency of heat engine is never equals to 100%.
It’s given by,
Efficiency = External W.D. / Heat absorbed from Source

Statements of Second Law
9
1. Clausius Statement :
It is impossible to construct a device operating on a
cycle whose sole effect is the transfer of heat from a
low temperature heat reservoir to a higher
temperature heat reservoir.
2. Kelvin-Planck Statement :
It is impossible to construct a heat engine operating
on a cycle whose sole effect is the transfer of heat
from a single heat reservoir & its conversion into
equal amount of work.

Refrigerator & Heat Pump
10
Refrigerator :
It’s a device operating on a cycle which removes heat
from a low temperature body and rejects it to a body
at high temperature on the expense of external work.
Heat Pump :
If the system delivers heat energy at higher
temperature than that of ambient temperature.
e.g. heater

Coefficient of Performance (C.O.P)
11
 The efficiency of refrigerator & heat pump is
expressed in terms of C.O.P.
(C.O.P)refrigerator = Desired Effect / Energy input
= Q2 / W
(C.O.P)heat pump = Desired Effect / Energy input
= Q1 / W
(C.O.P)heat pump - (C.O.P)refrigerator = 1

Example
12
 Heat pump is used to maintain house at 23 degree C.
The house is losing heat to outside air through walls
at 60000 kJ/hr. While energy generated in house by
various appliances is 4000 kJ/hr. For a C.O.P of 1.5,
find required power input in kW, supplied to the heat
pump. 6 Marks

Thermal 07

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Thermal 07