I am trying to find out what happens to the air flow within a pipe if a fan cannot generate the required pressure to overcome the friction within the pipe.
After much research I am aware that air moves from a point of high pressure to a point of low pressure. Also as the length of pipe increases, the pressure drop due to the friction of the pipe on the air also increases.
So as a working example, assume that I have a fan that has an airflow rating of 1 m3/s and generates a static pressure of 50 Pa. The fan and pipe have a diameter of 30cm and finally the pipe has a pressure drop of 7 Pa/m.
If the pipe has a length of 1 meter, then the air pressure at the end of the pipe would be:
$$50\, Pa - ( 7\, Pa\cdot 1 ) = 43\, Pa.$$
So the air flow would continue past the end of pipe into the surrounding environment.
If the pipe had a length of say 10 meters, then this image gives a clearer idea of what I'd imagine happening with the air pressure:
As each meter of pipe drops the air pressure by 7 Pa, then just after 7 meters of pipe the air pressure would be 0 - essentially neutral to the surrounding air.
And because air flow moves from a point of high pressure to a point of low pressure, then there would be no reason for air to flow within the pipe after 7 meters. Since the fan will still be generating 50 Pa, then air will still flow through the fan into the end of the pipe connected directly to the fan.
So, what happens then to the air that is already within the pipe but has no pressure difference to move and what happens to the air that continues being pulled into the pipe?
Clearly I'm not much up on fluid dynamics and physics, so would appreciate a simplistic answer to the question. What am I missing here?