Gas Turbines - Compressors — Page 237, Lesson 313

I want to walk you through the multi-spool compressor, because this is the design that solved one of the biggest headaches in early jet engines: compressor stall at low speeds. First, let me set up the problem. Early axial flow engines were built by simply stacking more and more compressor stages onto a single shaft to push the compression ratio higher and higher. More stages meant more compression, which was great for performance. But there was a catch. With all those stages on one shaft, the engine lost operational flexibility in terms of engine speed. Here's why. The compressor blade angles are set so the engine performs at its peak right around maximum rpm. At that speed, two things line up perfectly: the axial velocity of the airflow moving down the compressor, and the rotational speed of the blades themselves. Together they produce the optimum angle of attack of the airflow over the blade. That's the sweet spot where the blades bite into the air cleanly. Now, the moment you reduce engine rpm, that perfect symmetry breaks. The vector diagram that relates the blade's rotational speed to the axial velocity changes shape. The angle of attack is no longer at its optimum value. And when the angle of attack goes wrong, the airflow separates off the blades, and you get stall. So stall became an ever-present problem at lower engine speeds on those single-shaft engines. The fix was to split the compressor. Initially it was split into two sections, and later into three. Each section is driven through its own shaft by its own turbine. So you have separate rotating assemblies, each with its own compressor, its own shaft, and its own turbine. And here's the key relationship: the speed of rotation of each successive compressor increases. The HP compressor — the high pressure compressor — rotates faster than the LP, the low pressure compressor. Now, the whole unit — compressor, shaft, and turbine together — is called a spool. So a two-spool engine has a low pressure spool and a high pressure spool. A three-spool engine adds an intermediate spool in between. Here's the clever part. By designing the engine so that, when you close the throttle, the speed of the low pressure spool falls off more rapidly than the high pressure spools, you can maintain the symmetry of that vector diagram — the one relating to angle of attack — over a much greater range of engine speeds. That greatly reduces the chance of compressor stall. So the multi-spool design isn't just about getting more compression. It's about keeping the airflow over the blades at the right angle across a wide speed range, so the engine stays stable when you throttle back. Now, before we move on, I want to connect this to something you saw just before this passage. When the compressor stalls or operates inefficiently, you get a drop in thrust for a given throttle position. That raises the engine's specific fuel consumption — the sfc — and it also raises the EGT, the exhaust gas temperature, because there's a drop in the amount of cooling air available. That's the penalty you pay for poor compressor operation. And there's a figure here, Figure 15.5, that shows the operation of a compressor bleed valve — that's another device used to manage airflow and prevent stall, working alongside the multi-spool design. So to tie it all together: the multi-spool compressor splits the compression into independently rotating spools, each driven by its own turbine, with the HP spool spinning faster than the LP. When you close the throttle, the LP spool slows down faster than the HP spools, which keeps the blade angle of attack near optimum across a wide speed range, and that's what dramatically reduces the chance of compressor stall.