THROTTLING AND FREE EXPANSION PROCESSES CLASS NOTES

 

THROTTLING AND FREE

EXPANSION PROCESSES

CLASS NOTES

 Expansion process at constant enthalpy

is throttling. It is irreversible. No work on

decrease of pressure. This is a necessity in

liquefaction and vapor refrigeration. Free

expansion is at constant internal energy.

This is irreversible non flow process. No

work available.

 

Throttling Process

This is an expansion process at constant enthalpy.

That it exists in refrigeration and air-conditioning. The LIQUID throttles in the expansion valve.

 Liquefaction of gases uses this process.

Throttling is an irreversible process.

No work is obtainable with decrease of pressure. Thus, it is an useless process. It is a compulsion in refrigeration and in the liquefaction of gases. A turbine is not usable for this expansion because of phase change in throttling.  The amount of work obtained will also be extremely small.

Mathematically

dq=0

dw=0

dh=0

h=constant

T changes
u= Internal energy changes.

Joule –Thomson Coefficient ‘μ’ involves throttling.

μ =(бT/бP) h = C

That μ can be positive (+).

Thus  μ can have zero value.

Also μ can be negative (-).

In case of a gas, μ links to the maximum inversion temperature of a gas.

Table below contains the values of maximum inversion temperatures for few gases.

TABLEIMPORTANT PROPERTIES OF GASES

 

N.B.P. C

Freezing Point C

Critical Temperature C

Maximum inversion Temp. C

Air

-191

-212.3

-140.2

330

2

-183

-218.8

-118.8

620

2

-196

-210

-147.0

347.8

2

-252.8

-259.2

-239.9

-77.8

He

-268.9

-269.7

-267.9

-250.0

CO 2

-78.3

——

31.1

1230

Throttling has three possibilities.

Case 1 

 Temperature of gas is lower than the maximum inversion temperature

On expansion (throttling), temperature of the gas will decrease.  Cooling occurs on expansion.

μ is positive.

In an expansion, there is decrease of pressure. Here it causes  decrease in temperature making μ as positive.

It exists in refrigeration and air conditioning.

Case 2 

 Temperature of gas equals maximum inversion temperature before expansion

On expansion (throttling),  no change of temperature will occur. No heating and no cooling.

μ =0

No such case exists in actual practice.

Case 3 

Temperature of gas above the maximum inversion temperature before expansion

On expansion (throttling), temperature will rise. Heating on expansion

μ= — (negative)

It exists in hydrogen or helium liquefaction.

Since the maximum inversion temperature of hydrogen is –77.8C.

Whereas maximum inversion temperatures of Helium is –250C.

FREE EXPANSION PROCESS

It is process at constant internal energy.

Free expansion process is unrestricted process.

When it happens in a thermally insulated tank, it becomes an adiabatic process.

Thus it is an irreversible adiabatic process, non-flow process, and a sudden process.

The  process is useless as no work on expansion. There is no restriction or resistance to free expansion.

dq=0

dw=0

du=0

u= Constant, thus internal energy remains constant. But internal energy is a function of temperature, therefore temperature remains constant during free expansion.

Thus T1 = T2

 It is an isothermal process with zero work.

Examples are

(i)                  Leakage from a system into the atmosphere

(ii)                Leakage into a system working under vacuum conditions.

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