look at this doc it may be helpful for your topic . Good luck 

The fluid flow rate is linked to the pressure difference and the fluid pressure is linked to the static pressure in the tank

pressure = density * acceleration due to gravity * height from the top of tank

In general, the flow direction is always from high to low pressure side. But, If there is a device (pump or blower  that make a pressure difference that is required to overcome the pressure losses in the direction of flow), then the flow direction will be from low to high pressure side.

For vertical air receiver there is slight pressure different between top and bottom. Another factor is that water will condensed to thebottom of the vessel and the water will be removed as air is drawn out..that is why there is a drier at the outlet.