Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

236 | Thermodynamics

Dividing by the mass flow rate m.and substituting, h 2 is determined to be

Then,

Discussion Note that the temperature of steam drops by 38.0°F as it flows

through the nozzle. This drop in temperature is mainly due to the conversion

of internal energy to kinetic energy. (The heat loss is too small to cause any

significant effect in this case.)

2 Turbines and Compressors

In steam, gas, or hydroelectric power plants, the device that drives the elec-

tric generator is the turbine. As the fluid passes through the turbine, work is

done against the blades, which are attached to the shaft. As a result, the

shaft rotates, and the turbine produces work.

Compressors, as well as pumps and fans, are devices used to increase the

pressure of a fluid. Work is supplied to these devices from an external

source through a rotating shaft. Therefore, compressors involve work inputs.

Even though these three devices function similarly, they do differ in the

tasks they perform. A fanincreases the pressure of a gas slightly and is

mainly used to mobilize a gas. A compressoris capable of compressing the

gas to very high pressures. Pumpswork very much like compressors except

that they handle liquids instead of gases.

Note that turbines produce power output whereas compressors, pumps,

and fans require power input. Heat transfer from turbines is usually negligi-

ble (Q

.

0) since they are typically well insulated. Heat transfer is also neg-

ligible for compressors unless there is intentional cooling. Potential energy

changes are negligible for all of these devices (pe 0). The velocities

involved in these devices, with the exception of turbines and fans, are usu-

ally too low to cause any significant change in the kinetic energy (ke 0).

The fluid velocities encountered in most turbines are very high, and the

fluid experiences a significant change in its kinetic energy. However, this

change is usually very small relative to the change in enthalpy, and thus it is

often disregarded.

EXAMPLE 5–6 Compressing Air by a Compressor

Air at 100 kPa and 280 K is compressed steadily to 600 kPa and 400 K.

The mass flow rate of the air is 0.02 kg/s, and a heat loss of 16 kJ/kg occurs

during the process. Assuming the changes in kinetic and potential energies

are negligible, determine the necessary power input to the compressor.

P 2 200 psia

h 2 1354.4 Btu/lbm

f¬T 2 662.0°F¬¬ 1 Table A–6E 2

1354.4 Btu/lbm

 1 1371.4
1.2 2 Btu/lbm

1 900 ft/s 22 
 1 134.4 ft/s 22

2

a

1 Btu/lbm

25,037 ft^2 /s^2

b

h 2 h 1 
qout

V 22 
V 12

2