TITLE.PM5

206 ENGINEERING THERMODYNAMICS

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(^11)
2
2
p 1
p 2
T
s s
h
2 (wet)
2 (superheated)
p 2
p 1
1
1
Fig. 4.73. T-s diagram. Fig. 4.74. h-s diagram.
Now since Q = 0, and W = 0, then
h 1 + C^1
2
2
= h 2 + C^2
2
2
When the velocities C 1 and C 2 are small, or when C 1 is very nearly equal to C 2 , then the
K.E. terms may be neglected.
Then h 1 = h 2 ...(4.81)
i.e., For a throttling process :
Initial enthalpy = Final enthalpy.
The process is adiabatic but highly irreversible because of the eddying of the fluid around
the orifice at X. Between sections 1 and X the enthalpy drops and K.E. increases as the fluid
accelerates through the orifice. Between sections X and 2 the enthalpy increases as K.E. is destroyed
by fluid eddies.
During throttling pressure always falls.
The throttling process is used for the following purposes :

1. To determine the dryness fraction of steam.


2. To control the speed of the engine and turbine.


3. To reduce the pressure and temperature of the liquid refrigerant from the condenser
   condition to evaporator condition in a refrigeration system.

+Example 4.61. Steam at 18 bar is throttled to 1 bar and the temperature after throt-
tling is found to be 150°C. Calculate the initial dryness fraction of the steam.
Solution. Pressure of steam before throttling, p 1 = 18 bar
Pressure of steam after throttling = 1 bar
Temperature after throttling = 150°C
Initial dryness fraction, x 1 :
From superheat tables at 1 bar and 150°C, we have
h 2 = 2776.4 kJ/kg
Then for throttling, h 1 = h 2 = 2776.4
But h 1 = hxhf 11 + 1 fg