There is often quite a bit of confusion regarding the proper identification of blower wheels used in our industry. Most of what is sold are standard, squirrel cage type centrifugal wheels. There are several other types: forward, and backward inclined wheels, radial wheels (such as our S58-462 used on Broan [model 663, 688] bathroom fans), but here we're only going to discuss centrifugal wheels, such as those found in most residential and light commercial furnaces, air handlers and fan coil units.

HOW DO THESE WHEELS MOVE AIR?

Centrifugal blower wheels move air by increasing the pressure on the concave surface of each blade as the wheel rotates. This increase in pressure on the concave side creates low pressure on the back (convex) side of each of these blades. This low pressure causes air in the center of the wheel to rush towards the edge of the wheel. The low pressure condition created by a blade on the right side of a clockwise turning wheel will immediately be pressurized by the blade to the immediate left as the wheel continues to turn. Low pressure will then be created on the left (convex) side of this blade, and so on... The centrifugal affects on the moving air as it travels from the center inlet(s) of the wheel cause it to be thrown out from the edge of the wheel in to the blower housing. This air remains pressurized (by the constant reintroduction of more air) as it makes its way out of the discharge portion of the snail-shell shaped housing. Since fan blades (propellers) cannot develop and maintain the necessary static pressure to move equivalent amounts of air through ducts, filters and registers, centrifugal blowers are used for these applications.

The KEY identification steps to proper blower wheel identification are as follows:

1] Single or Double Inlet Wheel? If it is a double inlet wheel, there will be a centered (not always exactly in the center) plate containing the hub which mounts to the shaft, and the air enters from both ends of the wheel. If it is a single inlet wheel, the plate will be at one edge of the wheel opening, and the air can only enter from the edge opposite the hub plate.

2] Diameter and Width: Always refer to the size of a wheel by diameter first, and then width. Diameter First, Width Last. This is easy to remember because the D & F both come before W & L in the alphabet. By always doing this, we will eliminate the possibility of our vendors sending us an incorrect wheel.

3] Determine Rotation CW vs. CCW: This is often an area of mass confusion, but if you already read about how these wheels work, the following should be easy to understand: However, just because you do, doesn't mean it will be easy to explain it to your customers. The best way to get this information is to ask them to draw a circle within a circle. Say a 2" circle around a centered 1" circle, and the 1" circle represents the hub. The 2" circle represents the outside edge of the wheel. Then have the customer draw a letter "C" at 6 o'clock and another "C" at 12 o'clock along the edge of the large circle. Now, ask the customer which "C" represents the customer's wheel. If it's the 12 o'clock "C", his wheel is CW (clockwise). If it is the 6 o'clock "C", then his wheel is CCW (counter clockwise). One last but VERY IMPORTANT word of CAUTION: The industry standard for determining the proper rotation of a single-inlet blower wheels is while viewing the back plate, or opposite the air inlet end. You need to remember this, because your customers probably won't. A double-inlet wheel's direction of rotation is viewed from the hub end (that with the set-screw lock. Although all double-inlet wheels can be reversed, it may be quite difficult to access the set-screw to tighten the wheel onto the blower or motor shaft.

4] Shaft Bore: Obviously, this must match that of the motor. Remember: if you're also selling a new motor, you don't have to stay with the old bore size. As long as the bore matches the size of your new motor you're OK.

5] Material: Most wheels are either of galvanized steel, painted steel, plastic or aluminum. There are very few reasons why somebody with a plastic or metal wheel should not be able to accept a substitute material. Only severe environmental conditions or possible smoke/fire concerns could dictate one over the other. If anybody specifies a "spark proof" application, you must avoid steel wheels. Always check with your customer.

6] On some small blowers and residential furnaces, the hub plate may be either concave or convex, relative to the inlet of the wheel containing the hub and locking (set) screw. If you can physically fit a concave wheel in place where there was a convex wheel (and visa versa), you'll probably be OK. But you should always verify these features of your replacement wheel whenever you notice a hub plate that is not straight (perpendicular to the blades).

David Whitnack

Johnstone-Philadelphia