Throttling
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THROTTLING. Throttling is a type of expansion where steam passes through a narrow passage and expands with a fall of pressure without doing an external work. In this case, there is no interchange of heat similar to an adiabatic process. The enthalpy remains constant during this operation. So

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THROTTLING

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Throttling

THROTTLING

Throttling is a type of expansion where steam passes through a narrow passage and expands with a fall of pressure without doing an external work.

In this case, there is no interchange of heat similar to an adiabatic process.

The enthalpy remains constant during this operation.

So

Enthalpy before throttling =Enthalpy after throttling

In this process, steam becomes drier and nearly saturated steam becomes, superheated

Dept. of Mech & Mfg. Engg.


Measurement of dryness fraction

Measurement of Dryness Fraction

Dryness fraction of a sample of steam may be determined by means of steam calorimeters.

Types of calorimeters:

  • Tank calorimeter.

  • Throttling calorimeter.

  • Separating calorimeter.

  • Combined calorimeter.

Dept. of Mech & Mfg. Engg.


Throttling

Limitations of Throttling calorimeters:

  • This type of calorimeter fails when the steam is not superheated after expansion.

    Limitations of Separating calorimeters:

  • Only an approximate dryness fraction is measured, because the steam discharged from the calorimeter is not absolutely dry.

Dept. of Mech & Mfg. Engg.


Combined calorimeter

COMBINED CALORIMETER

Dept. of Mech & Mfg. Engg.


Combined calorimeter1

COMBINED CALORIMETER

Separating calorimeter does not give an accurate result and the throttling

calorimeter fails if the steam is not superheated after throttling.

A combination of separating and throttling calorimeter is therefore found

most suitable for accurate measurement of dryness of steam.

Dept. of Mech & Mfg. Engg.


Combined calorimeter2

COMBINED CALORIMETER

The steam first passes through a separating calorimeter and then

through a throttling calorimeter.

Let mf = mass of water collected in the separating calorimeter

Let mg = mass of steam passed from the separating calorimeter into the throttling calorimeter.

It is then finally condensed in a condenser.

Dept. of Mech & Mfg. Engg.


Combined calorimeter3

COMBINED CALORIMETER

Therefore

mf + mg = total mass of steam entered into the separating

calorimeter,

So, x2 = mg /(mf +mg )

Dept. of Mech & Mfg. Engg.


Throttling

Let be the dryness fraction of wet steam which enters into the throttling calorimeter. This dryness fraction can be determined with the help of equation:

Dept. of Mech & Mfg. Engg.


Throttling

This dryness fraction can be determined with the help of equation:

Dept. of Mech & Mfg. Engg.


Throttling

Now if x be the actual dryness fraction at the steam main, then

Then x = x1 * x2

Dept. of Mech & Mfg. Engg.


Throttling

Example 4.9 : A combined calorimeter is used to determine the dryness' fraction of steam at a pressure of 10 kgf/cm2 abs. (9.8 bar abs.). The steam is discharged from the calorimeter at atmospheric pressure and at a temperature of 105°C. During the test 0.2 kg of water is collected into the separating calorimeter and 2 kg of steam is discharged from the throttling calorimeter. Find the dryness fraction of steam. Assume cp=0.48 kcal/kgK (2.01 kJ/kgK)

Dept. of Mech & Mfg. Engg.


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