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Gas Turbine Systems Theory 53
generator drive, etc.). Compressor performance is generally shown as
pressure ratio plotted against airflow. (Note: it is more accurate to
use Head instead of pressure ratio, as Head takes into account the
compressibility, molecular weight, temperature, and the ratio of specific
heat of air—and corrected airflow—all at constant speed). This
is discussed in more detail later in this chapter.
Two types of compressors are in use today—they are the axial
compressor and the centrifugal compressor. The axial compressor is
used primarily in medium and high horsepower applications, while the
centrifugal compressor is utilized in low horsepower applications.
Both the axial and centrifugal compressor are limited in their
range of operation by what is commonly called stall or surge. This
phenomenon occurs at certain conditions of airflow, pressure ratio,
and speed (rpm), which result in the individual compressor airfoils
going into stall similar to that experienced by an airplane wing at a
high angle of attack. The stall margin is the area between the steady
state operating line and the compressor stall line.
Considering the Axial Compressor
Air flowing over the moving airfoil exerts lift and drag forces
approximately perpendicular and parallel to the surface of the airfoil
(Figure 4-3). The resultant of these forces can be resolved into two
components:
1. the component parallel to the axis of the compressor represents
an equal and opposite rearward force on the air—causing an
increase in pressure;
2. a component in the plane of rotation represents the torque required
to drive the compressor.
From the aerodynamic point of view there are two limiting factors
to the successful operation of the compressor. They are the angle
of attack of the airfoil relative to the approaching air and the speed
of the airfoil relative to the approaching air (Figure 4-4). If the angle
of attack is too steep, the airflow will not follow the concave surface of
the airfoil. This will reduce lift and increase drag. If the angle of attack
is too shallow, the airflow will separate from the concave surface
of the airfoil. This also results in increased drag.