SNS3Guppy I don't know are just won't understand me or am I writing some none understandable language!
Three basic things that environment control system have to provide in the cabin:
1. safe pressure 750-1000mbar
2. safe temperature 18-26°C
3. safe CO2 or O2 level (minimum O2 level that human can still breathe is 17,5%)
My point was and I can back it up with some basic data, that fulfiling all three basic task of ECS could be less energy greedy if the close loop heating system will be used and open loop system for fresh air delivery.
Meaning that rough bleed air (hot and pressurized), will heat the cabin via air to air heat exchangers layed down the cabin flor, then still pressurised will return to the engine and small amount of fresh air will be deliverd from ACM via ceiling vents.
Cooling will aslo be done throug ACM, (not really efficient way, because energy is wasted) but because of simlicity and fact that plane doesn't operate much on the ground.
Same is applied in the BUS or a train where hot engine water is heating the inside of a bus, fresh air is deliverd separatly via ceiling vents, cooling on the other hand is one more separate subsystem - buses usualy have evaporator with fan on the back of the roof and airstream is also layed to the ceilling vents.
Data from this table shows how much air is needed to maintain safe O2 level in the cabin and how much more is needed to warm the cabin durring flight 36.000ft above.
If you cool the bleed air prior it is introduced to the cabin energy is wasted and if you heat the big volume of fuselage with mixed air from ACM and engine that you will directly introduced to the cabin (can't be very hot) you need much higher mass of air because heat was wasted in heat exchangers.
wiki link to ECS (usful data)
Environmental Control System - Wikipedia, the free encyclopedia