Setting of the compressor return line
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In these devices, some compressors are equipped with an outlet return line, while others do not. I would like to know what factors are taken into consideration when deciding whether to include such a line or not
There are also economic and safety considerations regarding the compression medium
For centrifugal compressors, the return line is provided mainly to facilitate zero-load startup during startup, as well as to prevent surge during operation； For reciprocating compressors, the return line is provided mainly to facilitate a 0-load start during the startup process ； For centrifugal fans, such as those used in sulfur recovery systems that compress air, the pressure level is around 90 kPa; in such cases, it is sufficient to install a vent line at the outlet. I’ve only seen these types of compressors, and my understanding might not be correct; please add comments and corrections below.
How does the return line of a reciprocating compressor enable the compressor to start without load? Although no fluid enters the system when the return line is fully open, after all, this fluid passes through the compressor, right? Moreover, aren’t we all using unloaders or stepless control these days?
The provision of a return line (return inlet at the outlet) in reciprocating compressors is generally done for safety reasons, mainly to prevent pressure buildup. I’ve also seen ones without a return line, but during the startup process it’s connected to the flare first, and only after that is it switched back to the main line
For reciprocating compressors, there are two considerations regarding the return line: one is 100% backflow for no-load startup (no compression, with all compressors at zero load), and the other is using a control valve to facilitate return flow and thus regulate the inlet pressure of the compressors, ensuring stable inlet pressure (the return valve opens wider when the pressure is low, allowing high-pressure gas to flow back to the inlet). If the inlet pressure is very stable, and the compressor is equipped with a built-in unloading device that allows operation and shutdown at zero load, it may not be necessary to install one. . . But basically, everyone encounters them.
Light-load startup can be set such that the inlet valve closes at start-up, while the vent valve (unloading valve) opens~~
From the perspective of medium properties, compressors used for gases that should not or cannot be released into the atmosphere generally have return lines installed~~For gases that can be released harmlessly into the atmosphere, either a vent system or a return line can be used~~In terms of compressor types, centrifugal compressors are equipped with return lines to prevent surge and to regulate load (flow rate), while positive displacement compressors are primarily used for load regulation~~