What happens when two or more fans are in a row?

I’m sure I’m using some incorrect terminology here, so please forgive any egregious errors.

Say I have a wall unit blowing cold air. I place a portable fan several feet in front of the unit so that the cold air blows in the direction of the fan.

The unit is pushing air with some amount of force. The fan is pushing air with some other amount of force. Maybe more, maybe less. The fan is smaller than the unit.

What happens when the moving air hits the fan?

Does the fan add its power to the moving air, increasing the acceleration regardless of whether its total thrust is greater or less than the unit?

Is the force of the moving air hijacked as it’s caught by the fan’s input, then outputting at whatever the fan normally pushes air at?

Is there some kind of inefficiency caused by the interaction of the the fan and the unit, actively decreasing the flow of air?

What about distance? Does the fan increase the distance that the unit is able to move the cold air? Or does the fan help to guide the cold air in a particular direction without appreciably increasing how far the cold air moves? Or do the interactions of two different outputs just cause some kind of clusterfuck of alternate air patterns? Since the fan is smaller than the unit, cold air is certainly flowing around it as well as through it.

This post brought to you by a working wall unit in one room taking over the duties of a broken wall unit around a corner until I get around to calling an electrician.

What happens when two or more fans are in a row?

They argue about whether the USS Enterprise could beat a Star Destroyer or not?
:rocket: :vulcan_salute:

Seriously, I suspect that a lot of energy & thrust is wasted sideways, and your multiple fan situation would come to nothing.

As the jet of air gets further and further from the wall unit, it slows down and broadens more and more due to mixing with the quiescent ambient air. Your second fan will grab some of this and kick it a little further across the room, but it’ll be a warmer blend because of the aforementioned mixing.

You aren’t likely to decrease the total flow of air with your two-fan system (compared to what the wall unit can put out by itself), but you aren’t going to particularly increase it either. To do that, you’d need to put your second fan directly downstream of the wall unit, close enough so that it can actually lower the downstream pressure felt by the wall unit’s blower. Even then, the effect would be very limited, unless you enclosed both units in a duct. This is a common way to get more flow in long HVAC ducts, and in fact you can buy these things to help your home’s HVAC system get more hot/cold air to the farthest rooms in your house:

In industry, multi-stage axial compressors and turbines are commonplace, whether for stationary power generation (e.g. steam turbines in power plants) or for propulsion (e.g. jet aircraft). These things are really just what you’re talking about, a bunch of fans placed in series (inside a snugly fitting duct) so that each fan can add its power to the moving fluid - although to be fair, those fans are much more refined/optimized and designed to work in conjunction with each other, and they include curved stator vane arrays between each fan stage to straighten out the rotation caused by the previous fan.

One thing that’s for sure: if the motor of the downstream fan is in the path of the airflow of the wall unit, the air passing the downstream fan is going to get warmed up. It may get a velocity change, but it will be warmer.

I was gonna say: They do the wave.

When I saw the topic name, I thought about the promo for those Dyson hoop fans. They had a circle of them in a big room, and released a balloon behind one of them. The balloon got passed from one fan to the next, even up and down stairs.

In some countries, a spectacular punch-up.


I know that in PCs sometimes two fans are used to move air through the same spot (usually a heat exchanger…read heat sink). I have heard that, ideally, the fans are contra-rotating (opposite directions). This increases the static pressure across the heat sink and improves efficiency. The alternative is high air flow. I confess I am not clear why you want one or the other but there are reasons to favor one or the other depending on what is trying to be achieved.

There are some examples of planes with two sets of propellers one in front of the other such as the Soviet TU-95 bomber. Those are basically big fans. Also supposedly an ungodly loud plane and the crew suffered for it.

Hope to not make this a hijack - seems to me that every fan will contribute some heat, and that may be offset by improved airflow and circulation.

I have one large AC unit cooling my downstairs, and I use five small fans on low speed to drive the conditioned air from my LR, through the DR, to the kitchen, then down the hallway and back into the LR.

My downstairs is always comfy, largely due to the air movement; the AC cycles on and off, staying ahead in my 100-year-iold minimally-insulated house, here in the sweaty old mid-Atlantic.

My Kill-A-Watt tells me that each fan consumes 40W. Is there a a formula to describe how much of that is waste heat?


Ideal power consumption = pressure increase times airflow. Extra power will be lost as heat…

I DID ask for this…

How about a simple seat-of-pants estimate - little 10 or 12 inch table fan, capacitor start, worst likely type of motor, fan housing barely gets warm…

Out of 40w, perhaps 5w, maybe 10w of waste?


Reminds me of the coaxial counter rotating rotors in some helicopters:

They need not be noisier than the usual design and you can get rid of the tail rotor.

My wild guess is that it would not be nearly that efficient. Maybe only 15% efficient? Most of it waste, in other words. I’m no HVAC engineer though

High static pressure is needed to keep up flow if there’s a lot of resistance to air movement from obstacles, like a very dense heat sink, intake vents, filters, bends/curves, etc. Otherwise you get blowback around the vanes of the fan and little actual air movement despite a lot of spinny spin. This is a big issue in servers which are crammed full of components in a very short (1U rack-mount especially) chassis where you can only fit tiny diameter fans. That’s why they sound like a vacuum cleaner. Low-resistance high-flow fans are preferable because they’re quieter and also use less power. Think Mac Pro or Noctua tower coolers, where large diameter fans spin relatively slow and blow straight through fairly shallow radiators.

Distance depends a lot on what kind of fan you have. Box fans churn air, but mostly around the fan itself. They are good for mixing the air up in the room. You won’t feel the force very far from the fan. Other fans (like Vornado fans), create a strong column of air that can shoot far ahead. These Vornado-type fans would be better at throwing the cold air from one room into another. But both types of fans will help get the cold air from the AC farther into the rest of the house. It’s like putting a drop of dye into some water. If you just let it sit, it will take a while for the dye to completely mix. But if you stir the water, the dye will quickly mix.

Ideally, you would want to to create a circulation pattern with an inflow and outflow so that the air is like on a conveyor belt that goes through your whole house. Have a fan pushing cold air out of the room with the working AC, but also have a fan pushing warm air from the house into the room with the AC. Your house would need to have a loop-like design for this to really work. Like if there are two doorways, have a fan pushing air out of one doorway and another fan pushing air into the other doorway. You can do this with a single doorway, but you’d want to do something like have a fan on the ground pushing cold air out and a fan up high pushing warm air into the room. Since you are trying to cool other parts of the house, it’s more important to circulate air in and out of that room rather than trying to get the air from the AC to be thrown as far as possible from the unit.

I did something like this in my first apartment where the A/C was in the living room but the bedroom had nothing and got hit by the afternoon/evening sun and would be noticeably warmer. I just had a small vornado-type fan on the floor in the living room (conveniently under a table I never sat at) blowing into the bedroom through the door. That was adequate enough because it would also push the hotter air back out the top of the same door (the air going in has to be balanced by the air going out). Yes if you had a fan blowing hot air out of the bedroom through the top of the door that would reduce the resistance the fan at the floor was having to push against, but I didn’t find it necessary. One stronger directional fan is better than two weaker ones.

What happens when two or more fans are in a row?

I imagine, in most cases, a punch-up. But it may depend on the sport.

It would all depend. If the velocity of air on the inlet side of the 2nd side of the fan is moving slower than the design air velocity of the 2nd fan, then the 2n fan will increase the air velocity. If the velocity is highter than the fan design the fan becomes a resisstace to movement.

The example of a Axial compressor or a turbine The fan blades get bigger moving through the blades. The large baldes have a higher tip velocity so they can increase the velocity or pressure of the air moving from tiop to tip.