Box fans as we already know, the required minimum for the ventilation of a current CPU box are two 120 120 mm fans: one of them, located on the front of the box and oriented so that it impulse outside inwards from the box. The air that goes through this fan will form a current which, after renewed from the inside of the box and cool the internal components of the computer, exit through the back fan and power supply. Another fan, placed at the rear, will draw air from inside the box that has been heated in contact with heat sinks and other components that reach measurable temperatures. Currently, some boxes have another fan on the left side, zone where there is enough space to accommodate fans even more than 120 120 mm size minimum recommended, that will produce a large volume of air with a low noise level. Obviously, this solution will be of little use in computers whose box is located in accommodation which obstruct the air intake through the side. Both fans should produce a sufficient air flow to keep within safe limits the temperature of the components that support one greater thermal stress, taking into account that a greater flow, they will produce higher noise level. The direction of rotation of the rotor and the flow of air produced by a fan can be found observing the arrows that have recorded almost all fans on one of the sides. When it comes to replacing a faulty fan that the air flow that produced is unknown, should take into account that this flow is very roughly proportional to the nominal current of the fan intensity, so you will have to install another fan of the same size and voltage rating in place and whose nominal intensity is equal to or greater than the the fan replaced.