Page 181 - Fluid mechanics, heat transfer, and mass transfer

FANS, BLOWERS, AND COMPRESSORS 159

& To save on equipment costs, it is desirable to use as & Surge happens at low flow rates, often when the
few stages of compression as possible. downstream demand decreases.
& As a rule, compression ratio is limited by a practical & >When the flow decreases below a certain minimum
desirability to keep outlet temperatures below 150 C point, flow patterns within the compressor become

or so to minimize the possibility of ignition of unstable and fluid can move back through the com-
lubricants, as well as the effect that power require- pressor from the high-pressure side to the low-
ments go up as the outlet temperatures go up. pressure side.
& For minimum equipment cost, the work requirement & Figure 5.38 illustrates surge point on the centrifugal
should be the same for each stage. pump operating curve.
& For ideal gases with no friction losses between & In a centrifugal compressor, surge is usually initiated
stages, this implies equal compression ratios. at the exit of the impeller or at the diffuser entrance
for impellers producing a pressure ratio of less than
. Does compression ratio differ from stage to stage for a
3:1. For higher pressure ratios, the initiation of surge
multistage compressor? Give the equation.
can occur in the inducer.
& Normally compression ratio should be about the
& Flow reversals are temporary and forward flow is
same in each stage of a multistage compressor.
quickly reestablished once the compressor discharge
& Compression ratio can be represented as
pressure drops.
& The pattern starts again, setting up a large-scale flow
1=n
Ratio ¼½P n =P 1 Š ; with n stages: ð5:33Þ
oscillation in the system.
. How are efficiencies related to compression ratios for & Thus, surge process is cyclic in nature and if allowed
reciprocating compressors? to cycle for some time, irreparable damage can occur
& Efficiencies increase with increase in compression to the compressor.
ratios as illustrated in Table 5.11. & The set of operating conditions under which surge
. What is meant by surge in a compressor? Characterize begins, at a givenrotational speed, is referred to as the
surge. surge point. Surge normally occurs at about 50% of
the design inlet capacity at design speed.
& Surging is defined as a self-oscillation of the dis-
& Because surge is fast, high-energy phenomenon, it
charge pressure and flow rate, including a flow
can introduce excessive dynamic loads on internal
reversal. Every centrifugal or axial compressor has
components, such as thrust bearings, seals and
a characteristic combination of maximum head and
blades, and introduce pipe vibrations.
minimum flow. Beyond this point, surging will occur.
During surging, a fast flow reversal through a com- & Over a period of time, surge can lead to fatigue
pressor is often accompanied by a pressure drop. This failures that can damage the compressor.
flow reversal is accompanied with a very violent . What is surge limit line (SLL)? Illustrate the difference
change in energy. between surge limit line and surge control line (SCL).
& If the demand for gas is reduced, the operating point & Surgelimit line is the locus of surge points at different
will move toward the surge point. If the load is values of rpm.
sufficiently reduced, the compressor loses the ability
to increase the discharge pressure and flow reversal
occurs.
& The compressor surgelimit is a complex functionthat
is dependent on the gas composition, rpm, suction
temperature, and pressure.
& Symptoms of surge are excessive vibration and a
large audible sound.

TABLE 5.11 Compression Ratio Versus Efficiency
Compression Ratio Efficiency (%)
1.5 65
2.0 75
3–6 80–85
FIGURE 5.38 Centrifugal compressor operating curve.