How to find dryness fraction of steam using throttling calorimeter.

Aim: To measure the dryness fraction of steam using throttling calorimeter.

Theory:
A throttling calorimeter used to determine the dryness fraction of steam is shown in Fig.
It consists of a separator A into which steam is admitted through a control valve from the
steam main. The pressure and temperature are measured by the pressure gauge and the
thermometer T1 provided in this section. It may be noted that temperature recorded by T1
is same as the saturation temperature corresponding to the pressure of steam in
calorimeter B.
Fig

This steam is then throttled through a narrow aperture of restricted valve openings, its

total heat remaining constant. The steam is in the superheated state after throttling at a

lower pressure than previous. The temperature and pressure of steam leaving the

calorimeter B is noted by the thermometer T2 and manometer respectively.

Let P1 = Pressure of steam before throttling, i.e. pressure of steam main,

P2 = Pressure of steam after throttling,

hf1 = Sensible heat of water at pressure P1, … (From steam tables)

hfg1 = Latent heat of steam at pressure P1, … (From steam tables)

hg2 = Total heat of dry steam at pressure P2, … (From steam tables)

tsup = Temperature of superheated steam after throttling,

t2 = Saturation temperature at pressure P2, … (From steam tables)

cp = Specific heat of superheated steam, and

x = Dryness fraction of steam before throttling.

Since the steam has undergone a throttling process, therefore

Total heat before throttling = Total heat after throttling

hf1+ x hfg1 = hg2 + cp (tsup – t2)

The condition for the successful operation of this calorimeter is that the stam must be

superheated after throttling. This condition requires a high dryness fraction of steam

before throttling. This calorimeter can not be used if the dryness fraction of the steam s

above 0.95. The minimum dryness fraction of the steam can be measured by throttling

calorimeter depends upon the initial pressure of the steam as the pressure after throttling

remains near atmospheric.

Precautions:

1. The whole apparatus must be perfectly dry as the presence of moisture shows

differ results.

2. The connection should be leak proof.

3. The sampling pipe fitting with steam main should be proper.

4. The thermometer should be placed properly.

5. The reading of water thermometer should be read accurately.