Page 157 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Be st Practice 3.7
As will be discussed in this section, all types of positive dis-
placement compressors present the designer with a challenge.
Define design features and component function
Present selection and application guidelines This challenge is to provide varying flow requirements to the
Inform regarding field condition monitoring requirements process system in a safe and reliable manner. You will discover
that the screw compressor industry has met the challenge in
a safe, reliable and most efficient way.
Fig 3.7.4 Screw compressor chapter objectives

Principles of operation

Figure 3.7.5 shows the basic principles of operation for a twin
screw compressor. Regardless of type, dry or flooded, the
principle is the same.
As the screws separate at the suction end, the volume be-
tween the male (drive) and female (idle) screw is filled with gas
until the outlet screw flute passes out of the suction volute; the
suction volume is a function of the mating screw volume and the
speed of the compressor.
Once the screw flute passes through the section volute, the
compression phase begins and continues until the screw flute
enters the discharge volute. The designer determines the length
of the compression phase by the machine specified re-
quirements. The compressor ratio is a function of the volume
reduction ratio and the gas characteristics (specific heat ratio
K¼cp/cv). Details concerning performance will be discussed
later in this chapter.
Figure 3.7.7 shows the effects of operating any screw com-
pressor on input power at greater and less than the specified
compressor ratio. This situation is always the actual operating
mode in the field. Since the power required is a function of
volumetric efficiency (related to internal leakage through the
screws), the effects of off-design operation (over or under
Fig 3.7.5 Positive displacement and dynamic characteristics pressurizing) are minimal (usually less than 3%).

Fig 3.7.6 Principle of operation

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