TITLE.PM5

FIRST LAW OF THERMODYNAMICS 157

dharm

/M-therm/Th4-4.pm5

Applying energy equation to the system.

Here, Z 1 = Z 2 (i.e., ∆ Z = 0)

h 1 + C^1

2

2 – Q = h^2 +

C 22
2 + W ...(4.55)
The sign of Q is negative because heat is rejected (or comes out of the boundary).
The sign of W is positive because work is done by the system (or work comes out of the
boundary).

4.12.3. Centrifugal water pump

W

Boundary

Z 2

Z 1

Electric motor

Fig. 4.34. Centrifugal water pump.
A centrifugal water pump draws water from a lower level and pumps to higher level as
shown in Fig. 4.34. Work is required to run the pump and this may be supplied from an external
source such as an electric motor or a diesel engine.
Here Q = 0 and ∆ u = 0 as there is no change in temperature of water ; v 1 = v 2 = v.
Applying the energy equation to the system

or p 1 v 1 + Z 1 g + C^1

2

2 = p^2 v^2 + Z^2 g +

C 22

2 – W ...(4.56)

The sign of W is negative because work is done on the system (or work enters the boundary).

4.12.4. Centrifugal compressor

Refer to Fig. 4.35. A centrifugal compressor compresses air and supplies the same at mod-
erate pressure and in large quantity.