Page 219 - Chemical process engineering design and economics

202 Chapter 5

1 r (25.56) 2 kg-m 2 0.0893 bar 1x10 5 N 1 m 3 !
= —— | ———————— + —————— ——————— ————— =
2
0.65 L 2 kg-s 2 1 1 m -bar 1.179kg J

4
4
= 1.216xl0 N-m/kg orl.216x!0 J/kg (5.23 Btu/Ib)
The shaft power,
1670 m 3 1 h 1.179 kg 1.216xl0 4 J 1 kW-s
— m W — ____ ____ _______
Pp — HI VV f — —————————— ———————— ———————————— ——————————————— — —————————————
1 h 3600 s 1 m 3 1 kg 1000 J
= 6.651kW

P F = 6.651 kW / 0.7457 kW/hp = 8.919 hp
The electric motor efficiency is 0.95. The motor horsepower,

P E = 8.919/0.95 = 9.388 hp

Therefore, select a standard 10 hp (7.46 kW) motor, which gives a safety
factor of 6.52%.

COMPRESSORS
Figure 5.7 shows that positive-displacement compressors, like vacuum pumps,
are divided into two main classes: reciprocating and rotary. Table 5.4 lists char-
acteristics of these compressors. Ludwig [14] discusses compression equipment
and calculation methods in detail.

Positive-Displacement Compressors
Reciprocating compressors consist of direct-acting and diaphragm types. The
direct-acting compressor consists of one or more cylinders, each with a piston
or plunger that moves back and forth. A gas enters or leaves a cylinder
through valves that are activated by the difference in pressure in the cylinder
and intake or discharge. When the pressure in the cylinder drops below the
inlet pressure, a valve opens allowing gas to flow into the cylinder. After
compressing the gas to a pressure above the discharge pressure, the discharge
valve opens allowing gas to flow out. This is illustrated in Figure 5.8 for a
double-acting reciprocating compressor, i.e., the gas is compressed during
both the forward and backward stroke of the piston. The valves in Figure 5.8
are not shown in any detail. If the piston is just a straight rod, called
a plunger, the compressor cannot be double acting. An advantage of a

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