Bleed Air Sources...

Bleed air can be taken from any engine that is operating. On the ground, pressurized air is also available from ground cart connections or the Auxiliary Power Unit. (The APU on the Boeing 727 cannot be operated in flight.)

On engine 1 and 3, bleed air is taken from the 8th and 13th stages of the compressor section. Although 8th stage air is adequate for most operations, 13th stage can be used to provide additional flow at lower poser settings. Only 8th stage air is available from engine 2.

Bleed air coming from engines 1 and 3 go through precoolers located in each engine strut. Fan stage bleed air (1st and 2nd stage) is passed through the precooler to limit the temperature of 8th and 13th stage air. Engine 2 has no precooler for 8th stage bleed air.

Air Conditioning & Pressurization System...

The Boeing 727 has two air conditioning packs. Air flow from any single pack is adequate for pressurization to above 40,000 feet. Both packs, however, are normally used at all times. An "auto pack trip system" automatically closes both pack valves if power is lost from any engine during takeoff. This allows for the recovery of the bleed air that would have been used for pressurization and results in extra thrust with one engine inoperative. This system is automatically deactivated when the flaps are retracted.

The packs regulate cabin temperature through the mixing of hot bleed air with bleed air that has been cooled. The cooling is done by two heat exchangers and a device called an Air Cycle Machine (ACM). Hot and cold bleed air is mixed by a air mix valve. 

Immediately downstream of the turbine wheel in the ACM is a water separator. This removes the condensed moisture that is a by-product of the ACM's cooling of the air.

The cooling capacity of heat exchangers depends on the amount of ambient air flowing through them. This can be controlled through the positioning of cooling doors on the plane's belly that allows ram air to pass through the heat exchangers. At lower speeds, a fan augments the ram air entering through the belly doors.

The right pack supplies air to the passenger cabin, and the left pack goes to both the flight deck and passenger cabin.

Engine bleeds also provide air for pressurization which is maintained by controlling the rate of cabin air outflow through the pressurization  outflow valves. (The inflow of air through the air conditioning packs is relatively constant.)

Air flows overboard through electronic equipment bays for cooling, the cargo heat outflow valve (for cargo bay heating), and galley, lavatory and other vents.

The cargo heat outflow valve allows air to escape from the forward cargo bay area. This draws warmer air from the cabin around the cargo bay. This valve should be closed if one pack fails.

Normal cabin differential pressure (air pressure inside to air pressure outside) will not exceed 8.6 PSID (Pounds Per Square Inch Differential). Pressure relief valves limit the maximum differential pressure to 9.6 PSID.