How does steam flow in a condenser?

Steam flows to the tube surface because of the large change in volume when it goes from the vapor phase to the liquid phase. The condenser operates as a constant pressure device because the steam is constantly replaced and has easy access to all of the tube surfaces per design. They actually build "lanes" into the the tube pattern to allow easy page of steam. If you were to restrict the flow of steam, like have in a pipe, then you would see an actually dP between the steam source and the shell of the heat exchanger caused by the collapse in volume.