Many centrifugal compressors run in applications such as gas gathering, gas storage, or pipeline compression where a wide operating range is required. However, when specifying the desired flow range of a compressor, operators sometimes confuse the function of a compressor with that of a blower.
There are good reasons to occasionally operate a compressor at high flow and low head. But a compressor, by definition, is designed to compress gas by an elevated pressure ratio. A blower (or fan), on the other hand, essentially produces volume flow over a minimal pressure differential or head.
One of the first questions is: Why does it matter? When the compressor runs at high flow and low head, it will have a low efficiency. It is usually not economical to operate there. However, during start-up operations, such as when a pipeline or storage facility is to be filled, the machine will run at a very low pressure ratio. The capability to maximize flow is advantageous.
Overall centrifugal compressor performance is mechanically limited by its maximum allowable speed, the torque its shaft can handle, the power provided by its driver, and several other factors. But beyond that, the aerodynamic performance is defined by the head versus flow characteristic map where, at a given speed, maximum flow is limited. This limit is often referred to as choke, stonewall, or overload limit.
Many manufacturers define a distinct overload limit, and others provide performance maps where the speed lines end somewhere in mid-space. In tests and actual operation, we find compressors where the head at constant speed drops vertically at high flows, while other compressors show a more gradual reduction. The rapid vertical drop, usually called stonewall, is due to the flow choking when it reaches sonic speed.
The purist’s definition of maximum flow is the flow at no pressure rise over the compressor. That’s not practically feasible since there are always pressure losses that increase with flow. The only exception may occur when running a compressor, against atmosphere, with both suction and discharge nozzle open. However, that’s not a common application. So generally, maximum flow is a somewhat arbitrary definition at best.
Understanding the physics of the range limit of a centrifugal compressor requires understanding its behavior as it enters the high-flow region. As the compressor moves away from the design point toward higher flows, the flow into the compressor components like the impeller or diffusor, becomes aerodynamically less favorable with high incidence angles, separation, blockage, and increased viscous losses.
“It doesn’t matter whether the aerodynamic maximum flow is limited by choke or an increasingly inefficient impeller.”
Classical choke means that a shock forms across the flow channel when the flow reaches the speed of sound inside a restricted passage. This defines the maximum actual volume flow through that flow channel, and in particular, this volumetric flow will not increase even if the downstream pressure is reduced.
This stonewall limit on the head-flow characteristic map shows as a rapid drop of the head as the compressor transitions into the high flow areas. However, this type of choke does not occur within many compressors, especially when lighter gases are compressed, or the pressure ratios are low. Rather than going rapidly vertical, the head gently slopes downward as flow is increased. In these cases, the flow never chokes; instead increased aerodynamic losses are responsible for the gradual head reduction.
In many cases the maximum flow is limited by other considerations, such as thrust load or the excitation of aero components, including impeller blade flutter. The axial thrust in a compressor is created by the pressure difference between the front and the backside of each impeller, which changes with the operating point of the individual impeller. Most of this thrust is balanced by a balance piston (in the case of a back-to-back machine, by other impellers). The residual thrust acts against the thrust bearing and, depending on thrust bearing capacity can overload the bearing when the compressor operates near choke.
Similarly, high-flow incidence angles on the impeller blade leading edges can generate unsteady aerodynamic forces that can excite the natural frequencies of the blades and cause structural damage at high flows. Most manufacturers define a conservative maximum flow limit that takes all these limitations into account.
Where does that leave us? It doesn’t matter whether the aerodynamic maximum flow is limited by choke or an increasingly inefficient impeller. More serious concerns are mechanical in nature, such as thrust load or aerodynamic excitation, because they may require additional flow restricting valves in the system, especially during startup and shutdown operations.
But one should always remember: A compressor is not a blower. ■