Basic Mechanical Engineering (3110006)

Question-5a

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

Q5) (a)

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.

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.

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

h_{f 1} + x_{2} h_{fg 1} = h_{g 2} + C_{ps} (T_{\sup} - T_{sat})

x_{2} = \frac{h_{g 2} + C_{ps} (T_{\sup} - T_{sat}) - h_{f 1}}{h_{fg 1}}

Where,

x_{2}

unknown dryness fraction

Where,

h_{f 1}

enthalpy of saturated liquid at

p_{1}, \frac{kJ}{kg}

Where,

h_{fg 1}

latent heat of steam at

p_{1}, \frac{kJ}{kg}

Where,

h_{f 2}

sensible heat of water at

p_{2}, \frac{kJ}{kg}

Where,

h_{fg 2}

latent heat of steam at

p_{2}, \frac{kJ}{kg}

Where,

C_{ps}

specific heat of superheated steam assumed

2.1 kJ / kg k

Where,

T_{\sup}

temperature of steam measured by thermometer after throttling

Where,

T_{s 2}

saturation temperature of steam at

p_{2}

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.