WHAT IS FAN-RECIRCULATION?

The purpose of recirculation fans is decrease the use of engine bleed air. If you didn't recirculate some of the air would have to use engine bleed air for all of your cabin air needs. That would increase fuel burn which drives up your costs. The recirculated air goes through HEPA filters to remove airborne particulates.

The recirc fans allow for less airflow from the air conditioning packs by recycling air from the cabin and cockpit (some aircraft may vary) and adding it to fresh pack being delivered by the pack. This increases the efficiency of the system by decreasing the demand of bleed air from the engines. Furhtermore, depending on the aircraft

The result is that the air is somewhat "stale" compared to older aircraft that don't utilize recirculated AIR. A recirculation system reduces bleed air requirements and pack loads, by filtering cabin air and re-introducing it to the mix manifold. Approximately 25% of the cabin air is recirculated. The recirculation fan will switch off if either pack is in HIGH flow, causing a net reduction in ventilation rate of about 15%. Total ventilation rate on a 737-800 is approx 1 m3/sec

The white vapor is visible moisture present in the air after it is removed from the cabin and then recirculated back into the cabin after being cooled and (in theory) filtered. Aircraft designs generally cool and then recycle the air internal to a jet airliner because doing this incurs less penalty in drag than if outside air is brought into the cabin for cooling. All those humans inside the cabin are all exhaling a great deal of invisible moisture with every breath into the cabin air.

*Note

The moisture is invisible so long as it remains at cabin temperature which is well above a temperature known as “dew point”. Dew point is simply the temperature that water vapor becomes visible at when the air is cooled. When the air is cooled in the devices that provide air conditioning for the aircraft, the invisible moisture exhaled by passengers and crew gets lowered to the dew point and becomes visible. Air conditioners can be designed to remove much visible vapor but not all of it. As soon as the visible water vaper gets circulated back into the cabin it is heated up past the dew point and then it becomes invisible again. The cycle repeats for the duration of the flight.

Dew point is a way to measure how much moisture is in the air. You often hear it referred to in weather broadcasts as a percentage of humidity which is derived from the difference between the air's temperature and the dew point (or moisture content) of that air.

An important piece of this is that the warmer the air is, the more water it can hold before being 100% saturated.

When you cool air to its dew point, the cooled air is 100% full of moisture and at this temperature the moisture becomes visible since the air is entirely saturated.

The air close to the outlets for the air conditioning is very cold and cools the cabin air it's meeting quickly down to that air's dew point. That cooled air warms back up as it continues into the cabin and this is why you see the vapor disappear: the air has warmed back above the dew point.

This is the same way clouds and fog are made. If you want to make it happen at home, open the door of your freezer when it's a hot and humid day and you'll see the vapor roll at as the escaping cold air quickly cools the outside air to the dew point. When the vapor disappears, it's because the air has warmed back up enough to be above the dew point again.

Airplanes do have recirculation fans to help with even cooling, but all they do is pull air in from the grills at the bottom of the sidewalls, filter it, and mix it with the air conditioning pack air. The pack air is much higher pressure, so the all the recirc fans do is help even out the temperature in the cabin. The airplane (unless on ground air at the gate) is constantly supplied with high pressure conditioned air from the packs.