A “Fan”tastic Post

Are you a fan of fans? Centrifugal (I prefer to call Centripetal but that’s a tale for another day), Axial, Cross flow, whichever you like, fans can prove to be a challenging topic. A recent series in the Ashrae Journal discusses industrial fans, but what got me thinking about the topic of fans was a much smaller situation. PC computer fans.


Your standard 120 mm PC fan is a fairly homogenous product, but there also exist other fans in the same frame size with considerably higher energy use, cfm, and static pressure capability. These fans can be used to cool equipment, like say a bitcoin miner or a 4U server, sometimes coupled with a direct chip to water to air setup (something like this).

So which use of fans is best? 1 really impressive fan (like this) or 2 run of the mill fans stacked or 2 run of the mill fans next to each other (like these). The single fan consumes 30 watts, while the 2 fans consume 4 watts each. The 30 watt fan moves 240 cfm, while the cheapo fan moves 70 cfm. How much air do you need, and how much static pressure do you need? These fans get slapped in an application but they won’t move that same air volume with any resistance. That’s where the fan curves come into play.

Fan Curves

Here, I have shown a normal CPU fan at 1500 rpm, a normal CPU fan at 3000 rpm (for the ones that can do that, some can’t) listed as 200%, 2 fans in series, and 2 fans in parallel. To find the operating point, select the fan / fan combination you are using, and then the system curve you are using. Where the lines intersect, draw a line down to the bottom to find CFM, then draw a line across to find the static pressure.

Fan Curves op points

This is where things can become interesting. In the low resistance system, the order of air volume is 1 fan, 2 fans in series, 2 fans in parallel, and 1 fan at 200%. In the high resistance system, the order is 1 fan, 1 fan at 200%, 2 fans in series, and 1 fan at 200%. Of the reasonable options, we see that depending on the resistance of the system, sometimes parallel fans are best (perhaps placing them in a “W”or “V” to fit more fans in the same footprint), and sometimes series fans are best (although keep in mind the angles which the air comes off the fan blade can sometimes impact how well the second fan works). In the real world, it is probably best to use “series” fans with 1 before and 1 after the resistance. Also, recall that some fan performances are more impacted by resistance before them rather than after them, as in some are better at blowing than sucking.  But those traits usually become more apparent when looking at how well the fans turn down (deliver less than full rated airflow).

So if you still haven’t become a fanatic, let’s talk about some larger fans. Not even larger, just different styles.

First, let’s address one of the more common fans, and one we discussed above: the Axial fan. The axial fan can move a lot of air, but it fails to generate a lot of static pressure. Place the fan in a resistive environment, and the airflow will plummet. Brake horsepower per CFM will decline, and the fan will start operating in a region of the fan curve known as the stall region. Some fan curves will list this as a “do not operate” region.

You have seen axial fans on your desk, in the back of your computer case, in your ceiling fan, cooling your cars radiator, and many other places. They are common, can be very cheap in construction, and are nearly ubiquitous in applications that demand low static pressure.

In the stall region, air starts finding new paths around the fan blades as they rotate. Instead of moving from suction to discharge. Instead, the air finds the path of least resistance and moves around the fan blade back to the fan suction. Much like a chiller surging, pressure differences cause weird things to happen to the flow. The fan can start buffetting as pressure surges and swells and flow goes back and forth from the normal flow path to the surge flow path.

Additionally, if you speed up an axial fan too much, vortices form in the airpath. This creates turbulence (hurting performance), noise (hurting your ears), and wastes energy as mechanical energy is spent overcoming these factors (impacts on efficiency). Sometimes, a fan gets to a speed and you think “Hey, I can speed this fan up and get more airflow” but it fails to deliver those results. Axial fans are prone to this. Neat feature of axial fans: some can work forward and backwards (not true of centrifugal fans). This is very useful in cooling towers in cold weather when a “defrost” is necessary.

Within axial fans, there are a few types. Axial, vane axial, tube axial are the primary types.

Axial is what your used to.

Tube axial has axial fans inside a tube (often more than 1). The tube shape improves the performance of the fan and the aerodynamics of the airpath.

Vane axial is an upgrade of the tube axial that has vanes inside it. The vanes limit the spinning motion of the airpath (which reduces the usable output of the fans) and redirects it toward the discharge.

Multiple stages in axial fans help make up for the low pressure capability of the fan, but the higher pressure applications quickly give way to other fan types.


Centrifugal fans are the next primary fan type. They can generate higher static pressures and will be seen in many applications requiring ductwork. You might have one of these in your furnace or maybe your blow dryer.

Examples include radial, forward curved, and backward inclined.

The radial fan moves air in a path following its radius. I think they are kind of funny looking. It’s almost like a paddle boat for air. I don’t see these in use much, but I suppose if you have a lot of space they can be decent.

Next, forward curved fans can provide high static pressure. They are often called squirrel cage fans, and need the fan casing to work effectively. They have high horsepower requirements, have a considerable stall region, but are also cheap and deliver high static pressure in a compact footprint. They will require high horsepower in low static pressure applications and have poor overloading characteristics.

Last, backward inclined fans have been increasing in popularity. They do not need a casing, they can achieve very high static pressures, can deliver many different application requirements, and have excellent overloading characteristics. Plenum or plug type fans are backward curved fans.

There are many variations within the backward curved family, allowing fan designers to tease out many different desirable performance traits for various applications. They will tend to be the most costly fans, as a great deal of engineering can go into their impeller design and the construction of that impeller can be costly.

Among the fan types, sometimes designers develop hybrids of one fan type and another. Again, these fan types will require additional design and construction cost as their complexity increases. Modern CFD analysis has allowed new fan styles to grow in popularity as they can be tested before they are built.

A last fan type, often overlooked, is the cross flow fan. A good example of a cross flow fan is the rotating tower fans you may have in your home. Tall and narrow, the cross flow fan moves air across the length of its impeller. This is useful when you want to create a “beam” of air.


Wow, anyway, where am I going with all of this? Mostly thinking out loud about fans and applications, and I thought I would just document some aspects of my thoughts (as I think about these factors relating to some real world challenges I am facing today). I will try to come back to this post and update it with some better direction, so for now just look at it as a resource with some neat factoids about fans. It’s been a long while since I published something, so I wanted to make sure everyone knows I haven’t forgotten about it. New home, new job (with a secretive employer so I can’t talk much about my work on my blog), and a few other things have kept me from posting as much as I’d like. Hopefully I can get back into at least monthly posts. Thanks again for following.





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