PACKS, ACM's and active blade clearance

[tom775257]

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. :)

[gas path]

Tom
In the air conditioning packs the pneumatic bleed air is passed from the pack flow control valve through the first stage of a dual heat exchanger, where it gives up some of it's heat. This charge air is further reduced in temperature by the ACM. The now cooler air is heated again by virtue of passing through the compressor but gives up some of this heat again as it crosses the second part of the heat exchanger, now the greatest reduction in temperature is achieved by the expansion of the air across the turbine which gives up it's (heat)energy in driving the compressor.(bootstrap cycle)
The ACM also drives a fan in the inlet duct, this is there purely for use on the ground to move air across the heat exchangers and is redundant in flight as ram air is the cooling medium for the heat exchangers.
FWIW the packs are very good at reducing the temperature and if uncontrolled will spit ice. Look at the mix manifold temp!
The moisture removed by the water separators is sprayed onto the surface of the heat ex. to futher reduce the temperature by evaporation and the reheater is there to inject a bit of warm air to prevent freezing as mentioned above.
The cabin temperature control system controls the pack outlets to the cabin zone that requires the least amount of heat and trim air is added to the other zones as required.
Active clearance control is there to reduce tip losses and therefore improve the SFC.
Depending on the engine will dictate how sophisticated the system is but basically for a modern powerplant the engine EEC will control the valve(s) based on inlet temp. and n1 and n2 shaft speeds. For example LPT Acc. the EEC will use n1 speed and open the valve fully when t.o. power is set.
HPT Acc. the EEC uses inlet temp. n1 and n2 speed and the valve will open fully (briefly) once t.o. power is set and then modulate. It's all done by calculation!!
Hope that helps I'm :confused: myself now :)

[411A]

ACM's have been used for a very long time, starting in jets with the B707 and are very efficent as they draw no aircraft power (other than bleed air). Boeing tried freon air conditioning on some models of the 707 but the early systems were certainly not very reliable. PanAmerican had some of the later freon systems and they could get the aircraft VERY cold but were a high maintenance item.

[DoctorA300]

Tom,
The primary reason for the pack layout of modern aircraft, with heatexchangers, acm´s and so on, is to remove as much heat AND water as possible.

As you correctly state, it is possible to use heatexchanger cooling only, and if you could keep a sufficient cooling flow over the heatexchangers this would be a good cheap alternative, the problem is the amount of cooling air needed, you would only get this amound with a relative high speed, probably 150 kts or you could inrease the size or amount heatexchangers.
An ACM is used primarily because for a low weight you can remove alot of heat from the air, so much that you need to reheat it to stop ice from forming. To answer you question about wether or not the bleed air pressur is sufficient to drive the cooling turbine, No it is not. For the turbine to become effective you need to have it loaded, hence the connection to the compressor and cooling fan, and it needs an inlet pressure around 200 to 250 psi to give a propper expansion and thereby cooling.

Regarding your question about Tip clearance in jet engines, there are systems to activily measure the tip clearence, the use eddy current. But due to their size they have no aviation application. Most modern jet engines GE90, PW4000, V2500, CFM56-5/7 and RR RB211/Trent have a "Heat expantion" schedule programmed into their Engine controller and the cooling is then based upon this schedule biased by RPM and Altitude. Older engines like PW JT9D-59A/70 or CFM56-3C used a trigger signal (RPM, Slats, Airborne and so on) to start a Timer that would give cooling air for a certain time, it wasn´t as accurate as todays but it was better than nothing.

Hope this wasn´t too long an answer.

Brgds
Doc

[gas path]

Doc
Thanks for that.I forgot the most important thing. It's all about cooling!

[tom775257]

Thanks all for the information!
Most interesting. :cool: