In general, the pneumatics system provides compressed air at a constant flow rate to each of the air conditioning units, where its temperature is controlled and it is ducted into the cabin.  Boeing refers to the air conditioning units as packs.  The pressurization control system restricts the escape of this air from the cabin to maintain proper pressurization in the cabin.  The pneumatic system also provides compressed air for engine starting.
The engine compressors. auxiliary power unit, or a ground unit can be used to supply the pneumatic system.  The pneumatic manifold is normally supplied from engines one and three, with backup from engine two.  Engines 1 and 3 are usually referred to as the "pod" engines.  The APU or an external air source may also be used.  AC powered valves in the pneumatic manifold control the delivery of engine bleed air into the manifold.  The engine 2 bleed valves, when closed, isolate the two sides of the pneumatic manifold so that each air conditioning pack is supplied by a separate air source.
The bleed air switches flight engineers panel normally control the engine bleed valves.  The engine 2 bleed switches also control the APU's bleed air valve.  The fire switches will close the bleed valves when those switches are pulled.  The number 2 fire switch closes both engine 2 bleed valves.  In order for the engine bleed valves to respond to the positions of the bleed switches and fire switches, the AC buses must be powered.
There are two bleed valves an each pod engine.  The 8th stage bleed normally provides most of the air to the manifold except at low engine power, at which time the 13th stage valve opens automatically to augment the flow.  In normal operation, pneumatic flow is arranged so that engine one provides air for the left air conditioning pack, and engine three provides the right pack.  As the compressed air passes from the pneumatic manifold to an air conditioning pack, it is fed through a flow sensing venturi.  If the flow rate is sensed to be too low, the venturi signals the 13th stage bleed valve to open and increase flow.

At high engine powers there is more than sufficient air-flow from an 8th stage bleed for the associated air conditioning pack.  As the compressed air leaves the pneumatic manifold it passes through a modulating and shut off valve on its way to the flow sensing venturi.  The modulating and shut off valve is signaled from the flow sensing venturi to control the flow at high power settings.
As the 13th stage valve on a pod engine opens or closes, the temperature of the air from that engine varies.  The temperature of this bleed air must be controlled for air conditioning pack operation.  This is accomplished automatically on engines one and three by a pre-cooler on the bleed lines from each engine.  The pre-cooler uses fan stage air to cool the bleed air.  A temperature sensitive valve controls the rate of flow of fan stage air through the cooler.  There is no flight deck control for the pre-cooler.
To protect against excessively high temperature in the pneumatic duct from a pod engine, an automatic trip off feature is installed.  When the temperature of the bleed air is too high, the bleed air valve closes and a trip off light next to the affected bleed switch illuminates.
After a trip has occurred and the temperature of the bleed air has dropped sufficiently, pressing the reset button on the air conditioning panel will return the bleed valve to normal operation.  If the condition that caused the trip to occur still exists, however, the bleed valve trip will reoccur.
Engine No, 2 supplies air only from the 8th stage.  Since supplemental air is not supplied from the 13th stage on this engine, a pre-cooler is not fitted.  To warn of excessively high temperature in the engine No 2 bleed system, a high temperature light is provided.  No automatic trip off is associated with illumination of this light.
Opening the engine No. 2 left bleed switch will open the left engine 2 bleed valve to supply air to the left air conditioning pack.  The engine 1 bleed switch should be closed in this case so that only one engine is supplying bleed air to that pack.
Pressure in the bleed air distribution system can be read on the duct pressure gauge at the flight engineers panel.  There are duct pressure transmitters installed on both sides of the ducting.  If both engine 2/APU bleed switches were open, the left and right pressures would be equal indicating common pressure.
There is provision for using an external air cart for pneumatic supply.  The external air cart is connected to the bleed air distribution system between the right No. 2 and No. 3 bleed valves.  On the exterior of the airplane this connection is on the aft right side of the fuselage.
The APU can be used on the ground to deliver compressed air to the air conditioning packs or the pneumatic manifold.  The APU bleed air valve, which bleeds compressed air from the APU will open when either (or both) engine 2/APU bleed switch is in the open position.
On the 200 series aircraft there is a flow multiplier.  The purpose of the flow multiplier is to augment the bleed air output of the APU so that there will be sufficient quantity of compressed air to operate both air conditioning packs from the APU for ground operation.
With both air conditioning packs operating the augmented APU bleed air is ducted directly to both packs.  If the air conditioning packs are not operating, APU bleed air travels back through the ducting, which contains the flow sensing venturi and modulating and shutoff valves to the pneumatic manifold.  This allows the APU bleed air to be used for engine starting.
With the APU operating, when one air conditioning pack is turned on, both modulating and shutoff valves close to isolate the air conditioning packs from the pneumatic manifold.  Therefore, with at least one engine 2/APU bleed switch open and one air conditioning pack on, there is no flow of air through either modulating and shutoff valve.  The duct pressure gauges are installed in the pneumatic manifold. If the APU is the only source of bleed air to the pneumatic system, and at least one air conditioning pack is turned on, no compressed air will reach the gauges, and they will read zero pressure.
If the air conditioning packs are not operating, the APU provides compressed air to the pneumatic duct through the modulating and shutoff valves.  This is the normal configuration for engine starting.
Heat from a broken pneumatic or anti-ice bleed air duct could cause damage to the airplane structure.  Three detection systems are installed in the areas of these ducts to give warning of duct failures.
A detection system is installed in each pod engine strut area inboard of its engine firewall.  A third system combines several sensors to detect overheat in what is referred to as lower aft body.  These lower aft body sensors are located on either side of the aft airstairs, above the ceiling of the aft cargo compartment, and in the fuselage keel beam.  An overheat sensed by any of the three detection systems is reflected in the flight deck by illumination of the appropriate amber warning light on the flight engineers panel.  The adjacent test button is used to test simultaneously the light bulbs and the continuity of the overheat detection sensors for the struts and lower aft body.