Regulating Dynamic Compressors
In a lot of cases, applications require constant pressure in the compressed air system. This, in turn, requires that the compressed air flow from the compressor center is regulated. In this article we will talk about flow regulation in dynamic compressors.
How are dynamic compressors regulated?
Inlet regulation
Throttling the inlet:
The inlet can be throttled on a dynamic compressor to continuously reduce the capacity of the compressor. The minimum flow is determined when the pressure ratio reaches the pump limit and the machine becomes unstable (surge). The regulation range is determined by the design of the machine (e.g. the number of stages and the impeller design) but also to a large degree by external factors such as counter pressure, suction temperature, and coolant temperature. The minimum flow often varies between 60% and 85% of the maximum flow.
Inlet guide vanes:
Vanes arranged as radial blades in the intake cause the drawn-in gas to rotate while the flow is throttled. The method has the same impact as throttling, but offers a greater regulation range and features improved energy utilization. Regulation down to 50–70% of the design flow is a typical value. There is also the possibility of increasing slightly the capacity and pressure of the compressor to a certain degree, by turning the vanes in the opposite direction. This, however, may impair performance to a certain degree.
Outlet regulation
Variable outlet guide vanes (diffuser):
To further improve the regulation range, the flow in the compressor stage's diffuser may also be controlled. Regulation down to 30% with maintained pressure is common. Usage is limited to single-stage compressors, due to the complexity and increased costs.
Pressure relief:
The original method for regulating dynamic compressors was to use a pressure relief valve or blow off valve to release excess compressed air into the atmosphere. In principle, this method works identically as with pressure relief on a displacement compressor.
Load-unload-stop
Previously mentioned regulation methods can be combined to control the compressor unit. Two modes are commonly used:
Modulating: The excess flow is released into the atmosphere (or the inlet), but energy consumption is unchanged.
Auto dual: The flow turndown of the unit will be limited to the turndown of the intake valve and/or the outlet guide vanes for flows below the turndown limit. The regulation system fully closes the inlet valve at the same time as the compressor's outlet is opened to the atmosphere (compare with the displacement compressor).
The off-loading power is still relatively high, representing 20% of the full load power, depending on the design of the impeller etc.
Speed regulation
Speed regulation has a similar effect as the use of inlet guide vanes. Flow can be varied with constant pressure within the compressor turndown range. At higher powers, speed variation is less advantageous, due to the high cost of the required drive installation.
