PROPERTIES OF PURE SUBSTANCES 93Dharm
\M-therm/th3-2.p652(^34)
1
x 1
h
s
Fig. 3.16. Throttling process.
Hence the dryness fraction is determined and state 1 is defined.
Example 3.26. A throttling calorimeter is used to measure the dryness fraction of the
steam in the steam main which has steam flowing at a pressure of 8 bar. The steam after passing
through the calorimeter is at 1 bar pressure and 115°C.
Calculate the dryness fraction of the steam in the main. Take cps = 2.1 kJ/kg K.
Solution. Condition of steam before throttling :
p 1 = 8 bar, x 1 =?
Condition of steam after throttling :
p 2 = 1 bar, tt 2 =sup 2 = 115°C
As throttling is a constant enthalpy process
∴ h 1 = h 2
i.e., hxh hh cT Tf 1122 +=++ 1 gf f fg ps()sup 22 −s [QT
T
sup
s
2
2
=+=
=+=
115 273 388 K
99.6 273 372.6 K (at 1 bar)]
720.9 + x 1 × 2046.5 = 417.5 + 2257.9 + 2.1(388 – 372.6)
720.9 + 2046.5 x 1 = 2707.7
∴ x 1 2707 7 720 9
2046
= ..−
.5
= 0.97
Hence, dryness fraction of steam in the main = 0.97. (Ans.)
3.18.3. Separating and throttling calorimeter
If the steam whose dryness fraction is to be determined is very wet then throttling to
atmospheric pressure may not be sufficient to ensure superheated steam at exit. In this case it is
necessary to dry the steam partially, before throttling. This is done by passing the steam sample
from the main through a separating calorimeter as shown in Fig. 3.17. The steam is made to
change direction suddenly, and the water, being denser than the dry steam is separated out. The
quantity of water which is separated out (mw) is measured at the separator, the steam remaining,