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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

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THROTTLING

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  1. 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.

  2. 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.

  3. 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.

  4. COMBINED CALORIMETER Dept. of Mech & Mfg. Engg.

  5. 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.

  6. 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.

  7. COMBINED CALORIMETER Therefore mf + mg = total mass of steam entered into the separating calorimeter, So, x2 = mg /(mf +mg ) Dept. of Mech & Mfg. Engg.

  8. 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.

  9. This dryness fraction can be determined with the help of equation: Dept. of Mech & Mfg. Engg.

  10. Now if x be the actual dryness fraction at the steam main, then Then x = x1 * x2 Dept. of Mech & Mfg. Engg.

  11. 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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