Basic Mechanical Engineering (3110006)

BE | Semester-1   Summer-2020 | 11-06-2020

Q5) (a)

What is throttling calorimeter? Explain its limitation.

Construction

  • Figure shows throttling calorimeter which essentially consists of throttle valve, pressure gauge, thermometer and manometer.
  • Through sampling tube steam is taken to throttle valve where steam is throttled from higher pressure to lower pressure.
  • Pressure gauge is used to measure pressure before throttling and manometer is used to measure pressure after throttling. Thermometer is used to measure temperature after throttling.

Construction

  • With full open steam stop valve, steam is allowed to throttle until steady pressure and temperature is reached. At steady state condition pressure before throttling (p1) and temperature after throttling is to be measured.

Calculation

  • As we know that during throttling process enthalpy remains constant. This fact is used to measure dryness fraction of wet steam.
Enthalpy before throttling = Enthalpy after throttling
hf1 + x2 hfg1= hg2 + CpsTsup - Tsat
 
x2 =  hg2 + CpsTsup - Tsat - hf1 hfg1
 
Where, x2 unknown dryness fraction
Where, hf1 enthalpy of saturated liquid at p1, kJkg
Where, hfg1 latent heat of steam at p1, kJkg
Where, hf2 sensible heat of water at p2, kJkg
Where, hfg2 latent heat of steam at p2, kJkg
Where, Cps specific heat of superheated steam assumed 2.1 kJ/kg k
Where, Tsup temperature of steam measured by thermometer after throttling
Where, Ts2 saturation temperature of steam at p2

Limitations

  • This calorimeter is used when the dryness fraction is greater than 0.95.
  • To use this calorimeter condition of steam after throttling must be superheated.