Hi all,
I have a couple of questions...
First regarding PACKs. Looking at the schematics of the aircon system in a 767, I still can't work out the job of the ACM. After the bleed air has gone through the ozone removing catalyst, then the primary heat exchanger, it is passed over the compressor of the ACM increasing the air pressure. Why increase the air pressure, surely it is already fairly high as it is bleed air??? Then the air passes over a secondary heat exchanger, and next over a water extractor, then the air reaches the reheater...some of this air passes into the muffler via a condesor to the mix manifold. Some air is passed to the turbine of the ACM then into the muffler. Also, in the schematic it looks like the ACM shaft is connected to the fan for the ram air system for the heat exchanger in the PACK. Can someone please explain exactly the point of the ACM (I assume in cooling air to below ambient on the ground in summer) and in what conditions the flow of air to the turbine of the ACM is increased/decreased. Again I could be completely misunderstanding this. Why can't you just pass the high pressure air through the heat exchangers, allow it to expand and cool (drain any condensate over the heat exchangers), and then bring it back up to a higher temp if required with trim air?
Second question: I read that some jet engines have an active blade clearance control system. I understand this involves controlled passing of cooler air over the area around the outside of the material surrounding the blades, thus controlling its size via usual thermal expansion/ shrinking, and relative proximity to the tips of the blades is therefore controlled. Is this an active control system with sensors, or does it work on a system where a computer calculates that at a set ambient temp, and set engine RPM, it requires x amount of blade tip clearance...
Many thanks for any answers, feel free to pick holes in anything I have said.
tom.
