Gas Turbines - Bleed Air — Page 396, Lesson 493

Let’s start with the big picture. Bleed air is simply air that we tap off from the engine compressor, and we use it for two very different jobs. Internally, it cools and seals the engine itself. Externally, it services a whole list of aircraft user systems. On a modern turbofan, that external list includes air conditioning and pressurization, hydraulic reservoir pressurization, domestic water tank pressurization, thrust reverser actuation, an air turbine motor — the ATM — which drives a hydraulic pump or an electrical generator, engine and airframe anti-icing, and fuel heaters. Now, where do we take this air from? For external use, we typically bleed from two sources. There is a continual low pressure bleed taken from the outlet of the LP compressor — that’s the low pressure compressor. And that is supplemented, when required, by a high pressure bleed taken from the HP compressor, the high pressure compressor. Here’s the key operating logic. During high power operation, the LP bleed is usually at sufficient pressure to maintain the air conditioning and pressurization system on its own. But during low power operation, the LP bleed pressure falls. When that happens, the HP bleed valve opens to ensure we still have adequate pressure and flow. So the system automatically tops up the low pressure supply with the high pressure source when the engine is spooled down. There’s one more scheduling rule for the HP bleed valve. It is invariably scheduled to open when airframe anti-icing is selected. Why? Because now the requirement is for hot air, and the higher the bleed pressure, the higher the temperature of the air. So when you need heat, you go to the high pressure source. Now, control and safety. All of the bleed air can be shut off from the engine if required, by operating an isolation valve from the bleed air control panel on the flight deck. That same valve is also closed to isolate the engine bleed air when the fire handle is operated. So it’s both a normal control and a fire protection measure. Let me also mention the precooler. The diagram shows fan outlet air being used to cool the bleed air in a precooler. Fan air can also be used for the CSDU — that’s the constant speed drive unit — and for the engine oil coolers. Now, the cost of all this. When we bleed air from the compressor, we are reducing the mass flow through the engine. That has a detrimental effect on thrust, and it reduces the cooling effect in the combustion chamber. The consequences show up on the engine instruments: an increase in rpm, an increase in EGT — exhaust gas temperature — an increase in SFC, specific fuel consumption, and a reduction in EPR, engine pressure ratio. So bleed air is never free; it always costs you performance. Finally, control of bleed air into the cabin is via the pack flow control valves. These let the pilot selectively control the air conditioning system and shut off individual packs in the event of a malfunction, particularly one involving smoke in the cabin. And one more operational point: air conditioning bleeds from the main engines or the APU should also be closed during any ground de-icing operation, to prevent toxic fumes entering the cabin. Now let’s turn to internal air. Air bled for internal use is for internal cooling — for instance, combustion chamber cooling or turbine blade cooling — and for sealing of bearing or turbine disc areas. So internally, bleed air both cools the hot sections and seals the rotating assemblies. That figure shows the bleed air system layout. Let me walk you through the whole picture once more, because it all hangs together. We have two compressor bleed sources, LP and HP. The LP bleed is the normal continuous supply; the HP bleed opens when pressure is low or when you need hot air for anti-icing. The air is cooled in a precooler using fan air, then distributed to the user systems. An isolation valve on the flight deck can shut everything off, and it closes automatically on a fire handle. And every bit of air you take out costs you thrust, raises rpm, EGT and SFC, and lowers EPR. That’s the complete bleed air story.