A positive-displacement compressor is characterized by a pressure rise volume curve that is almost vertical. (It is not completely vertical because there is mechanical clearance, and slip and leakage from the discharge to the suction; the slip increases as the compression ratio rises.) The compressor delivers its gas against any pressure head up to the limit of its mechanical strength and drive capacity. Capacity is almost directly proportional to speed.
The characteristics of a centrifugal compressor are appreciably different. Generally, the pressure rise–volume curve is quite flat (Figure 1.4a). (It may be somewhat steeper if a heavier gas is being compressed.) A small change in the compression ratio produces a marked effect on the compressor output. As the discharge pressure increases, the flow decreases, and if the flow decreases too much, the machine will start to surge.
Surging occurs when the velocity of gas leaving an impeller wheel is too low to move the fluid through the machine. With no gas leaving the impeller, the discharge pressure may drop. Should this occur, the machine will again start to compress gas, and the cycle will be repeated. Such intermittent operation may severely damage a compressor. The characteristic curve can be modified by the installation of adjustable inlet guide vanes (Figure 1.4b). These are most effective on machines having a few stages. Adjustable diffuser vanes have been used on some machines.
In some installations, process requirements may dictate that the compressor be run at the far right of the characteristic curve, where it is very steep. Operating in this area requires careful control and is accomplished at some penalty of compressor efficiency.
The volumetric capacity of a centrifugal compressor is almost directly related to its speed; its developed head, to the square of the speed. (The horsepower requirements are thus related to the cube of the speed.) The efficiency of centrifugal compressors is lower than that of reciprocating machines by perhaps 5 to 20 percent.
These characteristics establish the sensitivity of the compressor to variations in flow conditions. For example, a change in the density of the fluid being compressed will have little effect on either the volume of gas pumped or the discharge pressure developed by a reciprocating machine, although one would have to be sure that no component parts of the compressor were being mechanically overstressed. Any variation in the density of a gas being compressed will result in a proportionate change in the weight of gas pumped.
On the other hand, because the head developed by a centrifugal compressor depends only on the velocity developed, a change in gas density will be directly reflected by a proportionate change in the developed discharge pressure. However, at a given density, if the discharge pressure can be permitted to change slightly, one can obtain large variations in volumetric gas flow through the compressor.
The axial compressor has a very steep characteristic curve (Figure 1.4c). The unit’s surge capacity is thus close to its operating capacity. However, by providing a method of adjusting the angle of stator blades and inlet guide vanes, a greater operating range can be obtained (Figure 1.4d).
Generally, the efficiency of an axial compressor exceeds that of a multistage centrifugal machine by perhaps 5 to 10 percent. The axial compressor does not contain diaphragms that expand radially as the compressed gas gets hot. This mechanical factor, combined with higher efficiency, leads to greater freedom from temperature limits and permits a higher compression ratio per case than do centrifugal units.