Aircraft Pneumatic Systems — Page 211, Lesson 271

Right, let's pick this up with the safety devices on the bleed air supply system, because that's where we left the ducting. We've got the duct pressure gauge, the valve position indicators, and the overheat sensors both inside and outside the supply ducts. Now, the system is also fitted with safety devices to prevent damage to that supply ducting from overpressure or overheat. Let's take overpressure first. Overpressure is usually caused by failure of the high pressure shut-off valve — that's the HPSOV. If that valve fails, a pressure relief valve is fitted to the engine bleed air ducting to protect it. Now here's the clever bit: if the overpressure persists, a sensor bleeds off the HPSOV opening pressure and forces the valve to close. So the sensor doesn't just warn you — it actively dumps the pressure holding the HPSOV open, which forces it shut and isolates the source of the overpressure. Now overheat. An electrical temperature switch is fitted downstream of the bleed air control valve. If the temperature of the air reaches a predetermined level, that switch closes the valve. So the overheat protection shuts the bleed air control valve directly. Both overheat and overpressure conditions are indicated to the pilots by warning lights. And here's the recovery procedure: if an overheat occurrence took place, the bleed valve switch would be selected 'OFF' and the isolation valve opened to restore the lost system. So the pilot manually confirms the bleed valve is off and opens the isolation valve to bring the system back. Now, let's move on to the cooling side — Air Cycle Cooling. This is the preferred system for most modern jet transport aircraft. It uses two principles: energy conversion and surface heat exchange. At the heart of the system is the Cold Air Unit, the CAU, of which the turbo-compressor, or bootstrap, is one basic type. The CAU is often referred to as an air cycle machine, the ACM. Let's look at the turbo-compressor, the bootstrap, in detail. This is the most popular air cycle system in current use. It's used where high pressure bleed air is not available, or where its use is undesirable — as in the case of aircraft using high bypass ratio or small turbo-propeller engines. So the low pressure bleed air, or air from a blower, is pre-cooled in the primary heat exchanger. Then its pressure is boosted by the compressor. This pressure boost is done to make the energy conversion process across the turbine more efficient — that's the conversion of heat and pressure into work. Between the compressor and the turbine is the secondary heat exchanger, which removes any excess temperature rise across the compressor. So the sequence is: primary heat exchanger pre-cools, compressor boosts pressure, secondary heat exchanger removes the temperature rise from that compression, then the turbine expands the air to do work. The key point to note is the pressure rise across the compressor. That pressure rise allows the use of much lower initial tapping pressures, while still achieving a sufficiently high pressure drop across the turbine. So you don't need high-pressure bleed air from the engine — the compressor creates the pressure differential you need. Now, to provide sufficient airflow across the cold air unit when the aircraft is on the ground or at low speed in the air, a fan is provided. That fan draws air in through the ram air or ground cooling air ducts. The ram air doors may be opened and closed according to flap position, or modulated automatically by signals from the temperature control system. And this fan may be electrically powered, or it may be a third wheel of the cold air unit — meaning it's driven by the same shaft as the compressor and turbine. So to tie it together: the bootstrap system takes low-pressure bleed air, pre-cools it, compresses it, removes the compression heat, then expands it across the turbine to produce cold air — and the fan keeps airflow moving through it when you're not moving fast enough to ram air through the ducts.