Quote from vp1:
rog747 asked a question regarding air quality... ...Unlike modern aircraft,the B707 (and DC8) did not use engine bleed air to ventilate the cabin. Engine bleed air from three engines was used to drive turbocompressors. The turbocompressors then provided air to pressurise the cabin.
[Unquote]
vp1 is spot-on re the B707. The TCs (turbo-compressors) were on engines 2, 3, & 4. But on the later models (don't know about the early ones), with the JT3D turbofan engines, you had the option of shutting down a TC in the cruise, and replacing it with engine bleed-air from the same engine. This saved a bit of fuel.
The Conway-powered VC10 used dedicated compressors, taking in fresh air, each driven mechanically by the associated engine's accessory gearbox. This was partly because, on those 1950s and early 1960s engines, the low-pressure compressor stages did not process a big enough mass of air to run the air-conditioning/pressurisation of the cabin.
It might appear that the old-fashioned systems, as above, would produce cleaner air than present-day bleeds from engine comperessors, but this does not necessarily follow. Just as the bearings of engine shafts have to be lubricated, so did the shaft on a dedicated compressor. There is nothing uniquely dirty about the air in the compressor stages of a jet engine.
Have you checked the oil-seals on the bearings of your office air-conditioning system recently?
[clevlandHD, you are wrong about "older aircraft" flying lower. On the 707, we regularly reached F/L 410 (41,000 ft), and F/L 430 on the VC10. The early Comets liked to fly even higher, their turbojet engines being very thirsty lower down. The evolution of massive fans and by-pass ratios led us, if anything, to fly lower. The Olympus engine on Concorde is not a turbofan.]