Throttling refers to the process where a fluid flows through a partially open valve, causing a significant pressure drop but constant enthalpy. During throttling, velocity may change slightly due to compressibility but enthalpy remains constant. Throttling is commonly used to control steam turbine speed, determine steam dryness, and in refrigeration plants and gas liquefaction. The Joule-Thomson experiment demonstrated that during throttling, enthalpy remains constant through a porous plug, validating the throttling process model.

1. Throttling
Dr. Rohit Singh Lather

2. Throttling process
• A flow is throttled when, for example, it flows through a partially open valve
• When it does so, we notice that there can be a significant pressure loss from one side of the
partially open valve to the other
So, we can say that such a throttling device is one in which pressure drops and enthalpy remains constant
In throttling devices there may be a change in velocity due to compressibility effects, but it is
observed to be small when the flow velocity is much less than the speed of sound.
We shall assume here the velocity is small relative to the speed of sound so as to recover v1 ∼ v2
Thus, h1 = h2
• A throttling process is modeled as steady device with one entrance and exit, with no control volume work or
heat transfer
• Changes in area as well as potential energy are neglected

3. This shows that enthalpy remains constant during adiabatic throttling process.
The throttling process is commonly used for the following purposes :
1. For determining the condition of steam (dryness fraction)
2. For controlling the speed of the turbine
3. Used in refrigeration plants
4. Liquefaction of gases

4. Throttling & The Joule-Thomson Experiment
• Throttling process involves the passage of a higher pressure fluid through a narrow constriction.
• The effect is the reduction in pressure and increase in volume
- This process is adiabatic as no heat flows from and to the system, but it is not reversible.
- It is not an isentropic process
- The entropy of the fluid actually increases
• Such a process occurs in a flow through a porous plug, a partially closed valve and a very narrow
orifice
Gas In Porous Plug
Control Volume
Thermometers
Insulation
T1
T2
Tube

5. • In this experiment gas is forced through a porous plug and is called a throttling process
• In an actual experiment, there are no pistons and there is a continuous flow of gas
• A pump is used to maintain the pressure difference between the two sides of the porous plug
• In this experiment, as pressures are kept constant work is done
iiff
v
0
f
0
v
i vPvPvdPvdPw
f
i
−=+= ∫∫
𝜹𝒒 = 𝒅𝒖 + 𝜹𝒘
iiifffiiffif vPuvPuor)vPvP()uu(0 +=+−+−=
From the definition of enthalpy if hh =
Hence, in a throttling process, enthalpy is conserved

6. 6
• In the region where the atoms or molecules are very close together, then repulsive forces
dominate and as the volume expands, the energy goes down. Thus, for these conditions, pT is
negative
• In the region where the atoms or molecules are close enough that attractive forces dominate,
then as the volume expands, the energy goes up. Thus, for these conditions, pT is positive
• For most gases at not too large pressures, the molecules don't interact very much and so there is
little dependence of energy on volume so pT is very small. In the extreme of zero
interaction, pT is zero. This is the defining condition for an ideal (perfect) gas.

7. 7
Throttling Process
(Joule-Thomson or Joule-Kelvin expansion widely used in refrigerators)
• The pump maintains the pressures Pi and Pf
• In the experiment Pi, Ti and Pf are set and Tf is measured
• Consider a series of experiments in which Pi and Ti are constant (hi constant) and the pumping
speed is changed to change Pf and hence Tf
• Since the final enthalpy does not change, we get points of constant enthalpy

8. 8
Pressuref
Temperaturef
- A smooth curve is placed through the points yielding an isenthalpic curve
- Note that this is not a graph of the throttling process as it passes through irreversible states
•
•
• • •
•
•
•
Pf , Tf
Pi, , Ti
Isenthalpic Curve
Pump
Porous plug
Pi Ti Pf Tf
The enthalpy is the same on the
two sides of the porous plug i.e.,
hf = hi.
Pi > Pf
We plot Tf as a function of Pf

9. 9
We now change Pi and Ti and obtain another isenthalpic curve
Pressuref
Temperaturef
Maximum Inversion T
Cooling
•
d
•
c
•
b
•
a
Heating
Inversion Curve
Inversion Curve
Ideal Gas
We are interested in the temperature change
due to the pressure change, therefore it is
useful to define the Joule-Thomson coefficient
𝝁
This is the slope of an isenthalpic curve and
hence varies from point to point on the graph
( )hP
T
∂
∂
=µ
• A point at which 𝝁 = 0 is called an inversion point
• Connecting all of these points produces the inversion curve

10. 10
Pressuref
Temperaturef
Maximum Inversion T
Cooling
•d
•c
•
b
•
a
Heating
Inversion Curve
Inversion Curve
Ideal Gas
If point a on the diagram
(𝝁 < 0) is a starting point
and point b is the final
point, then the T of the
gas will rise, i.e. we have
heatingIf we start at point c
(𝝁 > 0) and go to point
d, then the T of the
gas will drop, i.e. we
have cooling These curves are horizontal
lines for an ideal gas
As higher initial starting temperatures are used, the isenthalpic curves become flatter and more
closely horizontal
Maximum inversion T, the value of which depends on the gas.
For cooling to occur, the initial T must be less than the maximum inversion T, for such a T the
optimum initial P is on the inversion curve

11. • This also tells us that we cannot just use any gas at any set of pressures to make a refrigerator,
for example
- At a given pressure, some gases may be cooling (m > 0) but others may be heating (m < 0)
• The proper choice of refrigerant will depend on both the physical properties, esp. the Joule-
Thompson coefficient as well as the mechanical capacity of the equipment being used.
• Thus, we cannot just exchange our ozone-depleting freon in our car's air conditioner with any
other coolant unless the two gases behave similarly in the pressure - temperature ranges of the
mechanical device, i.e., they must have the same sign of m at the pressures the equipment is
capable of producing.
• Generally, to use a more environmentally friendly coolant, we need to replace the old equipment
with new equipment that will operate in the temperature range needed to make m positive
• The sign of the Joule–Thomson coefficient, µ, depends on the conditions
• The temperature corresponding to the boundary at a given pressure is the ‘inversion temperature’
of the gas at that pressure

12. Positive Negative
http://faculty.chem.queensu.ca/people/faculty/mombourquette/Chem221/3_FirstLaw/ChangeFunctions.asp
The maximum inversion temperatures of some gases are given below :
(i) He=24K (ii) H2=195K (iii) Air=603K (iv) N2 =261K (v) A=732K (vi) CO2 =1500K

13. 13
To make the discussion clear, we have exaggerated the slopes in the above T-P diagram. In fact, for
most gases at reasonable T’s and P’s the isenthalpic curves are approximately flat and so 0≈µ
It can be shown that µP
h
P
T
c
P
T
c
P
h
−=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
∂
∂
−=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
∂
∂
)(00 Thhsoand
P
h
then
T
==⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
∂
∂
=µ
We now have )1.5oblem(Pr0
P
h
v
u
TT
=⎟
⎠
⎞
⎜
⎝
⎛
∂
∂
=⎟
⎠
⎞
⎜
⎝
⎛
∂
∂
Constant temp. coefficient and can be
determined by Joules Thompson experiment
for an ideal gas
• For a given pressure, the temperature must be below a certain value if cooling is required
but, if it becomes too low, the boundary is crossed again and heating occurs
• Reduction of pressure under adiabatic conditions moves the system along one of the isenthalps, or
curves of constant enthalpy

14. 14
• We have to bring molecules closer so they are held together by intermolecular attraction, in order
to do it we have to reduce the volume
• If we apply pressure we can reduce the volume of gas. Furthermore, we need to reduce
temperature to slow down the speed of molecules
- That means we need to find that particular temperature, pressure and volume in which we can
liquefy real gas, and here Boyle’s law helps us
• Real gases follow ideal behavior and obey Boyle’s law at higher temperature
- V vs p where a straight line is obtained for ideal gas, which means that its volume cannot be
reduced even on applying high pressure or in other words ideal gas cannot be liquefied
- When we reduce the temperature of real gases, they deviated from the Boyle’s law
• The highest temperature at which a real gas shows deviation from ideal behavior for the first
time is the temperature at which we can liquefy a real gas
- This is known as critical temperature (Tc), and corresponding pressure and volume are known as
critical pressure (pc) and critical volume (Vc)
Liquefaction of Gases

15. Critical temperatures and pressures for
common substances
• The figure depicts qualitatively the state point of oxygen at ambient conditions and shows that it
is a superheated gas at this pressure and temperature, existing at well above the critical
temperature but below the critical pressure
State point of oxygen at
ambient temperature and pressure

16. • If the temperature and pressure of a gas can be brought into the region between the saturated
liquid and saturated vapor lines then the gas will become ‘wet’ and this ‘wetness’ will condense
giving a liquid
• Most gases existing in the atmosphere are extremely superheated, but are at pressures well
below their critical pressures
• If it is desired to liquefy the gas it is necessary to take its state point into the saturated liquid-
saturated vapor region
• Some gases can be liquefied in a simple process
- For example, carbon dioxide can be liquefied at room temperature by a simple isothermal
compression to about 60 bar
• To liquefy nitrogen or air is not so simple
- At room temperature, regardless of any increase in pressure, these gases will not undergo a
phase transformation to the liquid state
• First, experience indicates that ‘heat’ has to be taken out of the gas. This can be done by the
following methods

17. Methods
• Cooling the gas by heat transfer to a cold reservoir, i.e. refrigeration
• Compress the gas at temperatures less than its critical temperature
• Make the gas do work against an external force, causing the gas to lose energy and change to a
liquid state
• Make gas do work against its own internal forces, causing it to lose energy and liquefy
• Cascade process - use one liquefied gas to liquefy another
• Joule-Thomson effect - compress and then rapidly expand the gas

18. Liquefaction Temperatures

19. • This method is satisfactory if the liquefaction process does not require very low temperatures. A
number of common gases can be obtained in liquid form by cooling
• Examples of these are the hydrocarbons butane and propane, which can both exist as liquids at
room temperature if they are contained at elevated pressures. Mixtures of hydrocarbons can also
be obtained as liquids and these include liquefied petroleum gas (LPG) and liquefied natural gas
(LNG)
Liquefaction by Cooling - (Refrigeration)

20. Temperature-entropy diagramSchematic of plant
A Simple Refrigerator
• The simple refrigeration system might be used to liquefy substances which boil at close to the
ambient temperature, but it is more common to have to use refrigeration plants in series,
referred to as a cascade, to achieve reasonable levels of cooling.
• The cascade plant is also more cost-effective because it splits the temperature drop between two
working fluids

21. • The difference between the top and bottom temperatures is related to the difference between
the pressures
• To achieve a low temperature it is necessary to reduce the evaporator pressure, and this will
increase the specific volume of the working fluid and hence the size of the evaporator
• The large temperature difference will also decrease the coefficient of performance of the plant
Cascade Refrigeration Cycle

22. • In the cascade arrangement the cooling process takes place in two stages: it is referred to as a
binary cascade cycle
• This arrangement is the refrigeration equivalent of the combined cycle power station
• The substance to be liquefied follows cycle 1-2-3-4-1, and the liquid is taken out at state 1’
• However, instead of transferring its waste energy, Q2, to the environment it transfers it to
another refrigeration cycle which operates at a higher temperature level
• The working fluids will be different in each cycle, and that in the high temperature cycle, 5-6-7-
8-5, will have a higher boiling point than the substance being liquefied
• The two cycles can also be used to liquefy both of the working fluids

23. Liquid Natural Gas (LNG)
• Cooled until it Liquefies @ -160°C
• Reduces volume 600 times
• Colorless, Odorless and Non-Toxic
• Safe to transport and store
• Shipped and Stored at Atmospheric Pressure
Gas Well
Field
Processing
Transmission
Pipeline
Liquefaction
Shipping
Receiving
Terminal
Market
LNG Chain

24. • The liquefaction of natural gas is achieved by means
of a ternary cascade refrigeration cycle
• In this system a range of hydrocarbons are used to
cool each other
• The longer chain hydrocarbons have higher boiling
points than the shorter ones, and hence the alkanes
can be used as the working fluids in the plant

25. Natural Gas Liquefaction Process
Compression
Refrigerant
Loop
LNG
GASGAS
Storage
Treatment
and
Purification
-161ºC
•Removes condensate, CO2,
Mercury, and H2S
•Causes dehydration

26. • At higher temperature range from 50oC to 31oC when pressure is applied it shows perfectly Ideal
behavior as expected by Boyle’s law
• When we reduce the temperature just a bit more to 30.98oC it shows deviation from Boyle’s law
on applying pressure
- In the graph this deviation is clearly recognized by a sudden change in curve
- At this point we get liquid CO2 for the first time
- This temperature 30.98oC is the critical temperature (Tc) of CO2 gas.
- At this temperature on applying pressure CO2 gas gets compressed and transforms into liquid
CO2
• CO2 gas shows different behavior on applying pressure at temperature below 30.98oC
• On compressing, initially CO2 gas remains gas till point ‘B’
• On applying still more pressure it shows deviation from Boyle's law and a little liquid CO2 appears
Let’s take an example of CO2 gas

28. • On further compression the pressure remains constant for a period (point ‘B’ to ‘C’) and we get a
plateau for this phase.
- In this region we get vapour CO2 that means a state in which liquid and gas coexist
- If we further compress it, a steep rise in pressure is observed (point ‘C’ to ‘D’)
- As the plateau ends we start getting liquid CO2
• All real gases show similar behavior as CO2
• CO2 gas represents all real gases, but every gas has a particular set of critical constants
• At critical temperature (Tc) you can liquefy a gas directly into liquid phase
- It means that you can skip transition phase
- But if you carry out liquification at a temperature lower than critical temperature, you will get
the transition phase region or two phase portion in which gas and liquid coexist

29. 29
• A method for these gases, using the throttling process, was invented in 1895 and is called the The
basis idea is to use the gas cooled in the throttling process to precool the gas going towards the
throttle
until the T is below the maximum inversion T
- Starting from room temperature, this cycle can be used to liquefy all gases except hydrogen and
helium
- To liquefy H by this process, it must first be cooled below 200K and to accomplish this liquid N
at 77K is used
- To liquefy He by this process, it must first be cooled below 43K and to accomplish this liquid H
can be used. (A device called the Collins helium liquefier is used to liquefy He)
Hampson-Linde Process

30. Hampson-Linde Process

31. 31
If a throttling process is used to liquefy a gas, the cooled gas is recycled through a heat exchanger to
precool the gas moving towards the throttle. The gas continues to cool and when a steady state is
reached a certain fraction, y, is liquefied and a fraction (1 - y) is returned by the pump.
Using the notation:
- hi = molar enthalpy of entering gas
- hf = molar enthalpy of emerging gas
- hL = molar enthalpy of emerging liquid
Since the enthalpy is constant we have
hi = y h L + (1-y)hf
Of course, as some of the gas liquefies, additional gas must be added to the system.
It should be mentioned that Joule-Thomson liquefaction of gases has these advantages:
• No moving parts that would be difficult to lubricate at low T.
• The lower the T , the greater the T drop for a given pressure drop.

32. Controlling the speed of the steam turbine by Throttling

33. Controlling the speed of the steam turbine by Throttling