I think I know why this is. As the fans are able to withstand a higher static pressure at the same speed when in series, that also means they can reach higher speeds with the same power input through each fan (though in essence you're doubling your power usage total). From an electromechanical perspective that's more solid state, look no further than peltier devices. Stack them in series and you can get higher temperature differences across the system as the temperature difference from one point to the next doesn't need to be as wide for the same temperature difference across the system as it would need for fewer peltier devices. In some ways, the fans are pushing/pulling air through each other, making them operate partly as a generator (or in practice, requiring less wattage for the same linear wind speed than if in parallel), just like the peltier devices can be used as generators. Such is the life of a motor. In parallel, you'll move a greater volume of air at the same speed, while in series, you'll move the same volume of air at a greater speed. Kinda like volts and amps. Then the volumetric speed you're measuring would be the overall power, as in electricity would be measured in watts. This is actually what I love about physics. Everything is actually connected.

Thank you for this video. I suspect you would have gotten even more air flow with the series fans if you had used something like electrical tape to seal off the gap between the two fans. I imagine some of the air flow provided by the first fan escaped from the gap but that's just my instincts talking. It would be interesting to see the actual results. I love KZfaq because I was just pondering what two fans in series would do for a project I'm considering and here you are demonstrating it. Thanks again.

Thanks for posting this video. These are interesting topics. I work for a heavy industrial, centrifugal fan manufacturer. Let me preface by saying that we typically deal with ducted fan systems. IN THEORY, two identical fans in series have a characteristic static pressure curve that has twice the static pressure at each increment of flow, than the static pressure curve of only one fan. It's important to know that this only describes the shape of the "2 Fans in Series" fan curve and does not describe the new Point of Operation. The static pressure generated will not be exactly 200% static pressure compared to one fan, because of the shape of the system resistance curve. (Fans will operate at the intersection of the pressure curve and the system curve) Two fans in series will operate at a slightly higher flow rate, but significantly more static pressure than one fan, which is the goal with fans in series. It would be nice if a could post a snapshot of a typical fans in series fan curve plot, but the comments section only allows for text. IN PRACTICE, there are some other factors that come into play... The higher static pressure (generated by Fan#1) before the inlet of Fan# 2 will result in an air density increase between stages. The air that was just moved by Fan# 1 will occupy a smaller space since it has been compressed. This COULD actually result in a slight reduction in volumetric flow rate of two fans vs. only one fan. They key word here is COULD, and it depends on how much pressure is being generated. The lower pressure example in the video is likely why you are seeing a slight increase in flow (since there is much less compression to the fluid). The point is, the goal of putting two fans in series is to achieve significantly higher pressures than one fan of the same size/design can achieve. It's worth mentioning that if we have an application that requires significantly higher flow rates rather than significant static pressure increases, this is when we would look at potentially putting fans in parallel.

2 fans in series produce more airflow in higher resistance situations like this. In free air flow situations, they do little in series and work better in parallel. I have installed bladed box fans into household evaporative cooler shafts to help increase airflow as these are high resistance situations. The bladed fans augment the squirrel cage fans and do a good job at increasing airflow by between 20 to 20 percent. They also allow the cooler to operate at a lower airflow when used with only the bladed fan on and squirrel cage off, in situations where it's not too hot and we only want a little flow of cool clean air from outside.

great video thanks

It would be interesting to see a follow up with the two fans in parallel through the airflow meter. E.g. one fan puts out 102 linear ft, does two fans in parallel provide for example 170 linear ft (compared to the two in series, which gave 139 linear ft) Thanks for the video!