TITLE.PM5

FIRST LAW OF THERMODYNAMICS 161

dharm

/M-therm/Th4-4.pm5

∆PE = 0, ∆KE = 0
W = 0 [Q No work is absorbed or supplied]
Applying the energy equation to the system
h 1 + Q = h 2 ...(4.61)
Q is taken as + ve because heat flows from the surroundings to the system as the tempera-
ture in the system is lower than the surroundings.

4.12.9. Steam nozzle

In case of a nozzle as the enthalpy of the fluid decreases and pressure drops simultaneously
the flow of fluid is accelerated. This is generally used to convert the part of the energy of steam into
kinetic energy of steam supplied to the turbine.

Steam in Steam out

Nozzle

Convergent Divergent part

part

Fig. 4.40. Steam nozzle.

Fig. 4.40 shows a commonly used convergent-divergent nozzle.

For this system,

∆ PE = 0

W = 0

Q = 0

Applying the energy equation to the system,

h 1 +

C 12

2 = h^2 +

C 22

2

or CC^2

2

1

2

22 −

= h 1 – h 2 or C 22 – C 12 = 2(h 1 – h 2 )

or C 22 = C 12 + 2(h 1 – h 2 )

∴ C 2 = Chh 12 +− 2 () 12 ...(4.62)

where velocity C is in m/s and enthalpy h in joules.
If C 1 << C 2 , then
C 2 = 2 ()hh 12 − ...[4.63 (a)]
∴ C 2 = 2 ∆h. ...[4.63 (b)]