What Does A Centrifugal Fan Do?

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When you ask the question “What does a centrifugal fan do,” are you asking how it works, or what it accomplishes? Either way, there are seemingly simple answers with a lot of complexity behind them.
In this whiteboard video, AirPro Sales Manager / Senior Application Engineer Chet White provides a hypothetical example to touch on an essential “what” and a bit of the how - just enough to breathe life into a static situation.
Full Transcript:
When I get the question, “What do I do” I like to tell people I work with fans, and of course, that opens their mind up to whatever they think a fan is. Now a lot of us have an idea that a fan is a fan, it’s something that we sit on the ground, we turn it on, we blow air, it cools us down. So some people think that I sell those kinds of fans.
But, when I answer the question “What does a centrifugal fan do?” Simply, it moves gas. A centrifugal fan moves gas. But every fan moves gas. So why a centrifugal fan? Well, talked about it in some other videos, but a centrifugal fan looks like this. It takes air into the inlet and blows air out of the outlet. So why would you need one? I mean, what is the reason for a centrifugal fan? What does it do?
Well, I’m standing in a room right now and I just want you to go through an exercise with me. Take whatever dimensions in the room you’re in, just mentally (stick with me here). Let’s say you’re in a room that is 10 feet wide, and you’re in a room that is 20 feet long, and 10 feet high. So do some quick math on that, we got 10 feet wide by 20 feet long, so our area on our floor is 200 feet squared. Now we take 10 feet high and we multiply that by our 200 feet squared, and we have 2000 cubic feet in the room in which we are standing.
So put yourself through an exercise and say “What if I was in this room and there was no way for the air to ever change?” Alright, something every single person who is watching this video is doing is breathing. I am as well. We don’t think about it too much but we’re all breathing. As we breathe, we’re exhaling, and what we exhale, we don’t really want to breathe again. So if I was in a room that had no way to change the air of 2000 cubic feet, at some point, I would die due to a lack of oxygen. So, at its core, a centrifugal fan can help you not die. That’s really cool.
But why would you need a centrifugal fan? I mean we’re all in rooms all the time, we got doors we can open and stuff like that. But let’s say the room I’m in right now is 100 feet underground. Now I got a different situation. Now you don’t maybe want to put your centrifugal fan 100 feet underground. You want to put it up above ground. So let’s say this guy is above ground, and we’ve got ground right here. And this room that you’re in is down here. And this room has 2000 cubic feet. That’s how much air makes up this little room. Well, we’ve got to find a way to get from this room to the fan that is ventilating that room, and we need air that can get to the room.
But a centrifugal fan is rotating, and it’s moving - maybe a high volume of air (the volume can be variable) but it’s moving a higher static pressure than an axial fan. That’s good when you’ve got a long run of duct or pipe or whatever going from where you need the air to where the fan is. And so in this case, let’s say through all this ductwork, you’ve got 200 feet of resistance or something like that of duct run with all these turns, that’s what we calculate as static pressure. So when you’re looking at a centrifugal fan and you’re saying “I need this much volume and this much static pressure,” this is practically what it means. You’ve got a room down here that needs 2000 cubic feet, you’ve got a run of all this ductwork that requires pressure driving from the fan to get everything through all that, and now you’re going to be able to ventilate this room.
So then your calculation of well, what do I need from my volume from my centrifugal fan? In this case, let’s say you need to change the air rate in this room once every two minutes. That’s 2000 cubic feet, so you would need 1000 cfm, (so in 2 minutes, you’d move 2000 cubic feet) and whatever the calculation of static pressure is of this entire run of duct all the way to the fan, say for the sake of this video it’s 20 inches of water column, you would need 1000 cubic feet per minute at 20 inches of water column in order for this centrifugal fan to let you live way under the surface in this room.