Page 128 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 3 .1 Compressor Best Practices

Fig 3.1.10 Single stage high speed compressor (Courtesy of
Sundstrand Corp.)
Fig 3.1.11 Centrifugal multi-stage horizontal split (Courtesy of
Mannesmann Demag)

Compressor characteristics
per minute, and let us assume that the temperature of com-
In this section we will discuss the two principle compressor pression remains constant. If the compressor in question has
characteristics: positive displacement and dynamic compres- a compression ratio of two, one actual cubic foot per minute
sion. In addition, the concepts of volume flow, mass flow and entering the compressor will be compressed to a discharge
standard flow will be covered. Although this chapter covers volume of exactly one half of a cubic foot per minute assuming
compressors, the characteristics of positive displacement and that the gas is dry.
dynamic are equally applicable to pumps. Standard volume is defined as one volume, always referenced
Positive displacement compression is defined as the increase to the same pressure and temperature conditions. In customary
in pressure of a gas caused by operating on a fixed volume in units, standard pressure is defined as 14.7 pounds per square
a confined space. Types include reciprocating, rotary liquid inch and standard temperature is defined as 60 F. A measure-

piston, rotary lobe and screw compressors. This concept can ment in standard cubic feet then is the ratio of the actual
best be envisioned by using a simple syringe. As one moves the pressure to the referenced standard pressure and the referenced
plunger into the syringe, the volume inside changes. It will be standard temperature to the actual temperature multiplied by
displaced regardless of the resistance under which the com- the actual volume. Referring back to the previous example of
pressor operates, provided sufficient power is available and the a compressor with a compression ratio of two and no com-
design of the compressor can meet the pressure requirements. pression temperature increase, one can see that the standard
Looking at a schematic of a positive displacement reciprocating cubic feet per minute in this compressor would remain the
compressor shows that gas will not enter the cylinder until the same, assuming a dry gas. This is because even though the actual
pressure inside it is lower than the suction pressure. Conversely, volume of the gas does decrease by one half, the discharge
gas will not exit the cylinder until the pressure inside the standard volume is the ratio of the discharge pressure to the
cylinder is greater than the discharge pressure. The valves shown standard atmospheric pressure multiplied by the discharge
in this figure are merely check valves. The suction valves act as volume. This will result in the same exit standard volume as the
check valves, preventing the compressed gas from escaping back inlet.
into the suction line. The characteristics, then, of a positive Mass flow is the product of the actual volume flow and the
displacement compressor are fixed volume, variable pressure density of the specific gas. The concept of mass flow and stan-
capability (energy or head) and not self-limiting. By this, we dard volume flow are the same. That is, the mass flow into the
mean the compressor will stall or damage itself unless a pressure compressor example cited above will be exactly equal to the exit
or power limiting device is included in its design. This is usually mass flow provided the gas is dry.
achieved by using a relief valve. Both standard volume flow and mass flow are used to
Before proceeding to the concept of dynamic compression, describe process capacities and are used in power calculations.
actual flow, mass flow and standard flow will be discussed. In After all, it would be very difficult to charge customers for
the design of any compressor, actual volume flow must be used. produced products (gas) unless either a standard volume
This is necessary since the design is based on an optimal gas measure or weight measure were used.
velocity. Gas velocity is the result of a given volume flow acting We now consider dynamic compressors. These increase the
in a specific area. Think of any compressor, dynamic or positive pressure of a gas by using rotating blades to increase its velocity.
displacement, compressing a volume of one actual cubic foot They can be either axial or centrifugal compressors.

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