Page 248 - Chemical process engineering design and economics

Compressors, Pumps, and Turbines 229

Drivers can be grouped according to the type of energy supplied - electrical,
expansion of a high pressure gas, and expansion of a high pressure liquid. An
important consideration in the selection of a driver is to match the speed of the
driver with the speed of the machine. If it is necessary to run both units at differ-
ent speeds for technical or economic reasons, then gears will be needed to increase
or decrease the speed of the driver. Fans for many applications are V-belt driven.

Electric Motors

Most chemical-plant-size compressors are electrically driven [43]. Moore [25]
discusses the characteristics of squirrel-cage induction and synchronous electri-
cal motors. Wound rotor induction motors have not been used for compressor
drives. For 370 to 4500 kW (500 to 6,000 hp), the induction motors are the first
choice. The squirrel-cage induction motor is the most commonly used driver in
the process industries from 1/8 to 1,5000 hp (0.0932 to 1,120 kW [25]. From
15,000 hp (149 to 11,200 kw) the synchronous motor could be used [25]. If the
compressor is operated at 7,500, 11,000, and 23,000 rad/s (1,200, 1,800 and
3,600 rpm), no step-up gears are required. The least costly speed for an induc-
tion motor is 1,000 rad/s (1800 rpm) so that this speed is usually selected. Step-
up gears are used to obtain higher speeds.
To calculate the size of an electric motor, divide the compressor shaft power
by an electric-motor efficiency. Efficiencies for electric motors are given in Table
5.9. The size of electric motors are standardized according to horsepower, as
shown in Table 5.10. If less than the standard horsepower is calculated, then the
next standard horsepower is selected.

Table 5.10 Standard Electric-Motor Sizes

Horsepower 3

1/20,1/12, 1/8, 1/6, 1/4, 1/3, 1/2, 3/4, 1/2, 1

1-1/2, 2, 3, 5, 7-1/2, 10,15,20, 25, 30,40, 50, 60, 75, 100

125, 150,200,250, 300, 350,400, 450, 500, 600, 700, 800, 900,1000

1250, 1500, 1750, 2000, 2250, 2500, 3000, 3500, 4000, 4500, 5000

and up to 30,000

a)To convert to kW multiply by 0.7457.

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