Why do the APUs operation create smoke puff?
The vapor clouds are coming from the Water Spray Boilers (WSB) that are used to cool the lube oil of the APU turbines by spraying a small bit of water on the heat exchanger lines. The water vaporizes and the overpressure gets vented to the outside. The turbine exhaust of each APU on the other hand is invisible. The lube oil of each APU is circulated through a heat exchanger in a corresponding water spray boiler. Three water spray boilers (WSBs), one for each APU, cool the lube oil systems. The hydraulic fluid of each hydraulic pump driven by an APU is also circulated through a hydraulic heat exchanger in the corresponding water spray boiler to cool hydraulic fluid during hydraulic system operation. The three WSBs are also located in the aft fuselage of the orbiter.
