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If the ram air is used for air conditioning, the airplane doesn't need a heat exchanger, cooling fan and cool the bleed air from compressor. So why do modern planes use bleed air cooled by ram air?

My guess would be mass of the air. The compressed air has more mass in small volume.

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    $\begingroup$ What would you do for A/C when you're sitting on the ground? $\endgroup$ – jamesqf Sep 16 at 17:29
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The problem is pressure. In order to use outside air for the cabin, it needs to be pressurized. Otherwise you could not reach higher pressures inside the cabin than outside.

The advantage of using bleed air is that this air is already pressurized, so you don't need to do any additional work. It is however too hot to be used as cabin air directly, so a PACK (Pneumatic Air Cycle Kit) is used to reach lower temperatures using ram air for cooling.

Ram air cannot be directly used without increasing its pressure. On the Boeing 787, which is equipped with bleed-less engines, the outside air therefore needs to be pressurized with electrically driven compressors:

In the no-bleed architecture, electrically driven compressors provide the cabin pressurization function, with fresh air brought onboard via dedicated cabin air inlets. This approach is significantly more efficient than the traditional bleed system because it avoids excessive energy extraction from engines with the associated energy waste by pre-coolers and modulating valves. There is no need to regulate down the supplied compressed air. Instead, the compressed air is produced by adjustable speed motor compressors at the required pressure without significant energy waste. That results in significant improvements in engine fuel consumption.

(Boeing Aero Quarterly - 787 No-Bleed Systems, emphasis mine)

Note however that this compression also adds heat to the air, so the air still needs to go through a PACK before it can be used as cabin air:

In the 787 electrical architecture, the output of the cabin pressurization compressors flows through low-pressure air-conditioning packs for improved efficiency. The adjustable speed feature of electrical motors will allow further optimization of airplane energy usage by not requiring excessive energy from the supplied compressed air and later regulating it down through modulating valves resulting in energy loss.

Avoiding the energy waste associated with down regulation results in improvements in engine fuel consumption, and the environmental-control system air inflow can be adjusted in accordance with the number of airplane occupants to achieve the lowest energy waste while meeting the air-flow requirements.

(Boeing Aero Quarterly - 787 No-Bleed Systems, emphasis mine)

You can see the inlet for the compressors and the ram air inlet in the following picture:

B787 Air Inlets
(image source)

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  • $\begingroup$ An interesting follow up question: Is the CAC Deflector Door ever opened in flight, or only when it's on the ground getting AC/Heat from ground support? $\endgroup$ – FreeMan Sep 16 at 15:52
  • $\begingroup$ @FreeMan To be honest, I don't know too much about the 787 and I don't have access to an FCOM. This Facebook post (where I got the image from) says: "Ahead of the CAC inlet is its deflector door. It is there to prevent FOD from entering the CAC during landing and ground operations but when the outside temperature is too high (about 95°F ) or there is a fault, the deflector door will close." $\endgroup$ – Bianfable Sep 16 at 17:10
  • $\begingroup$ Well, I wasn't really expecting an answer (I know better than to really ask a new question in the comments :D ), but that is interesting. FOD deflection makes sense since it's a pretty low inlet. Makes you wonder why they didn't use a ram air intake (or NACA duct) like the opening above. (Really, who doesn't love a good NACA duct?) $\endgroup$ – FreeMan Sep 16 at 18:08
  • $\begingroup$ I'm not sure the physics is quite right here. Even for a plane flying at sea level, the air is moving very fast (in the plane's frame of reference). That means that if you want to slow the air down and bring it to rest (relative to the plane), you have to dissipate a lot of energy, which means heat. Yes, compression will also involve heating, but the need for compression is not the only reason for heating. $\endgroup$ – Ben Crowell Sep 16 at 20:49
  • $\begingroup$ Thermal coefficient of air is around 1kJ/(kg)(K). So KE at 400mph is sufficient to change temperature of the same mass by around 16K. $\endgroup$ – BowlOfRed Sep 16 at 21:39

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