Gas Turbines - Compressors — Page 232, Lesson 302

I want to walk you through the compressor section of the gas turbine engine. This is where we start the whole powerplant story, because before we can burn fuel and expand gas through the turbines, we have to squeeze the air first. That's the fundamental job: the air must be compressed before fuel is added in the combustion chambers, and before that hot gas expands through the turbines. No compression, no thrust. Now, there are basically two types of compressor in use in engines today. One allows axial airflow through the engine — that's the axial flow compressor. The other creates centrifugal flow — that's the centrifugal compressor. In both cases, the compressor is driven by a turbine, and the turbine is coupled to it by a shaft. So the energy to spin the compressor comes from the turbine, through that shaft. Let's start with the centrifugal compressor, because it came first historically. Its big advantage is that it's much more robust than the axial flow compressor. It's also the easiest and cheapest of the two types to manufacture. That's why it was so popular in early gas turbine engines. But it has disadvantages that pushed it to second place in large modern engines. Here's the key comparison: if we take two compressors with the same frontal area — one centrifugal, one axial — we find two things. First, the axial flow compressor can consume far more air than the centrifugal. Second, much higher compression ratios can be attained in the axial flow compressor. And since the thrust an engine generates depends partly on the mass of air flowing through it, the centrifugal compressor engine will produce less thrust than an axial flow compressor with the same frontal area. That's the fundamental reason the axial type dominates big engines today. Now let's get into how the centrifugal compressor actually works. The action of the turbine rotates the impeller of the compressor at high speed. The impeller is the rotating part. Air is introduced continuously into the eye of the impeller — that's the centre of the impeller — by rotating guide vanes. Then centrifugal force causes the air to flow outwards towards the tip. Here's where the pressure rise begins. Because of the divergent shape of the vanes — divergent meaning the passages widen as you go outwards — the pressure of the air increases as it flows outwards. And because we're adding energy into the equation, the air's velocity also increases. So both pressure and velocity are going up as the air moves from the eye to the tip. The air then leaves the tip of the impeller and passes into the diffuser section. The diffuser is a system of stationary divergent ducts. Its job is to convert kinetic energy — that's the velocity — into potential energy, which is pressure. So the diffuser slows the air down and turns that speed into pressure. Here's a useful practical figure: approximately 50% of the pressure rise across the compressor occurs in the impeller, and the other 50% occurs in the diffuser section. So half the compression happens in the rotating part, half in the stationary part. Now, what's the compression ratio of a single stage centrifugal compressor? It's in the region of 4:1. That means the outlet pressure of the compressor stage is approximately four times greater than the inlet pressure. So if air comes in at one atmosphere, it leaves at about four atmospheres. If we want greater engine compression ratios using centrifugal compressors, we'd have to use two of them in series with each other — one feeding into the next. But here's the limitation: in practice, it has not been found feasible to use more than two centrifugal compressor stages together. Excessive impeller tip speeds and extreme centrifugal loading prohibit efficient operation of a third stage. So you can't just keep stacking them — the mechanical stresses and the tip speeds become prohibitive beyond two stages. That's the centrifugal compressor in full. Next we'll look at the axial flow compressor and how it compares.