While studying the air-conditioning of the A320 from the FCOM I came across the information that the pack flow valves are "electrically controlled and pneumatically operated". I can not understand what it means.
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11$\begingroup$ Human bodies are "brain controlled... muscle operated". You think of something to do, the brain sends the signal and the muscles respond to accomplish the action. The electricity (solenoid valve) is the brains of the operation while the pneumatic servo air is the muscle to move the valve $\endgroup$– wbeard52Commented May 28, 2020 at 4:32
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2 Answers
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It means that an electrical switching signal is used to drive the pneumatic or air valves in the air conditioning system.
Think about it like this: We want to use our garden hose to push open a cat flap but we want electricity to control the pressure in the hose. We could apply an electrical signal to a motor which opens the tap and lets water come out. This water pressure could then be used to control how wide the cat flap opens. The only difference in your airbus situation is that it isn't water (which would be hydraulics), it's air that comes out of the tap (Pneumatic means air). So this system would be called an electrically controlled, hydraulically operated system. Substitute the water in the hose for compressed air and you have your answer.
The way electrically operated valves work is generally based on the principles of electromagnetic force by utilising some sort of coil. This setup turns a piece of wire into an electromagnet, which is attached to or interacts with the movable part of the valve. This then ports air pressure to one side or the other, to balance open the valve to a set amount in proportion to the original electrical signal.
So we use electricity >>> this changes the balance of air >>> the air opens or closes the valve.
The valve can be fully open, fully closed or anywhere in between.
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12$\begingroup$ Re, "We want to use our garden hose to push open a cat flap..." Strong contender for the most thought provoking analogy of the day. $\endgroup$ Commented May 28, 2020 at 12:58
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$\begingroup$ Well, I would prefer my cat to water the flower beds:) $\endgroup$ Commented May 28, 2020 at 13:55
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3$\begingroup$ @SolomonSlow Glad you liked the cat flap analogy! I teach avionics for a living and I always used to hate the textbook answer. My instructors really sucked when I went through so I try to make classes as enjoyable as possible, even though the material can be dry sometimes $\endgroup$– CraigCommented May 28, 2020 at 22:51
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2$\begingroup$ So garden hoses are your answer to dry material; yup, that checks out. $\endgroup$ Commented May 29, 2020 at 6:21
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$\begingroup$ @PeterCordes it's an analogy that everyone understands so that is what I went with. If you think there are any better ones out there let me know $\endgroup$– CraigCommented Jun 2, 2020 at 11:44
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They're called "magnet valves". They're basically a valve that's operated by a solenoid.
Also seen on locomotive electrical contactors. The solenoid admits air, which moves an air piston which physically actuates the device. Doing that pure electrical would require gigantic solenoids, heavier wiring, heavier generator.
One thing to note is that unless your RAT generates compressed air, you lose your engines, you lose your air, and the electrical signal will no longer actuate it. I suppose on an A/C pack it hardly matters.
answered May 28, 2020 at 15:06
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1$\begingroup$ Railway trains (electric multiple units mainly) used to have 'straight air', or Westinghouse brakes. The brakes were applied by air pressure, using a 'train pipe' charged with pressurised air. If the driver lowered the pressure in the train pipe, a valve on each coach opened and compressed air from reservoir tanks operated pistons to force friction shoes onto the wheel treads. The reduction of train pipe pressure took a certain time to propagate along the train. and electro-pneumatic air brakes were developed, where solenoid valves on each car operated instantly and all at once. $\endgroup$ Commented May 29, 2020 at 10:32
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$\begingroup$ One reason EP brakes were attractive is that they reduced stopping distance, so trains could run harder, and brake later, between stations, and thus more trains per day could be run on existing infrastructure, so more tickets could be sold. $\endgroup$ Commented May 29, 2020 at 22:54