How to Measure A Fan for Retrofit or Replacement

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Replicating fans or retrofitting fan parts and accessories doesn’t have to be difficult. Senior Application Engineer Chet White explains seven key areas of measurement for accurate replication, retrofitting, or replacement.

Seven key sets of measurements are required to retrofit or replace an existing fan:
1. Inlet flange dimensions
2. Outlet flange dimensions
3. Centerline height
4. Inlet flange face to the centerline of the housing
5. Outlet flange face to the centerline of the housing
6. Centerline of the inlet to the centerline of the outlet
7. The fan footprint


Full Transcript:

If you're trying to measure the size of your fan, you're likely doing it for one of two reasons. Either one, you want to retrofit a product that you have in the field that you may not have a drawing for, maybe really old and it needs to be replaced, or two, you're just trying to find the size so you can buy another one or buy a replacement for that one and go back to the manufacturer with it. So, let's look at what you need to do in measuring to determine the size of your fan first in a retrofit. So, if you're trying to retrofit and match a fan into an existing location, you need to make sure to get seven key measurements. So, the first one, if you can isolate your fan from the system like this one is, that'll make it a lot easier. Otherwise, you might have duct work on the inlet, duct work on the outlet, will make it a little bit more challenging to get some of these dimensions.

So, the first one we want is the inlet flange dimension. So, you want to get an OD, an ID, and then a bolt circle and a number of bolts. And you also want to determine if your bolts are on the center line or if they're straddling the center line like this one. Then you want to get your outlet flange dimension that's over here. You want to get, typically on a fan, you're going to have a rectangular outlet, a round inlet on a blower. You might have a round inlet and a round outlet that often match. But on this one we have a rectangular outlet. So, you want to get a width, a height, you want to get the OD and the ID on those. And then you also want to get your bolt dimensions, so your bolt circles, and that defines your outlet flange dimension.

Okay, the next thing you want to get is a center line height. This is critical. So, from the floor to the center line, what is the height of your fan? The next dimension you want to get is the inlet flange face to the center line of the housing. So again, we're looking at center of the housing to the inlet flange face. This will allow your fan to actually meet up with the inlet duct work. The next dimension you want to get is the outlet flange face to the center line of the housing, so outlet, flange to the vertical center line of the housing. The last dimension on the airstream that we want to get is the center line of the inlet to the center line of the outlet. In this case, that dimension is approximately two here. If you have a straight edge, it makes it a lot easier in the field.

Your seventh and final dimension that you want to get for a retrofit is the footprint. You want this thing to sit on the same bolts potentially that you have in your current foundation. So go around measuring your bolt hole diameters, your width plus your length. This way, if I show it on this side, looking at this way, you want to get your number of foundation holes, the diameter, and then your fan can drop in place on the current foundation and connect to the current connection points. Now, if you're just trying to find the size of the fan so that you can report back to the manufacturer, "I need another of this size." Your manufacturer likely sizes your fan in one of two ways. They either size by the inlet diameter or they size by the wheel diameter like Air Pro typically does. So, the inlet diameter you just get by measuring the ID of the inlet, and that often is the fan size, especially for general industrial fans.

If getting the wheel diameter, you have to access the wheel. Now, if you have a fan like this where you've got an inlet panel like this one where, call it an inlet cover plate, or you have an access door like this one, you can get at the diameter of the wheel by pulling the access door, maybe finding the circumference, using a flexible tape or some type of string, and then measuring the circumference dividing by pie. That's a fun way to do it. Or you can pull off your inlet panel like I'm going to do on this one. Now we have access to the wheel and we want to measure the diameter. So, all we're going to do is measure from that side to that side and get the diameter of the wheel. If we get the diameter of the wheel, you can typically report that back to the manufacturer and they know the size of the fan. So that's how you measure a fan.