Aerodynamics and Efficiencies of Fan Wheels [Tech Tip]

In our last Tech Tip we looked at the efficiency of different sized fans of the same type. Generally speaking, for the same service, a smaller fan running faster will have a lower first cost than a larger fan running slower. However it will likely consume more power than the larger fan.

Continuing in the efficiency theme we will look at the efficiency of different fan wheel types.

Aerodynamics of a fan has a huge impact on fan efficiency. A radial bladed wheel with an inlet scroll that improves inlet conditions, has a higher efficiency than an open paddle wheel design. A backward inclined fan wheel, also straight bladed, is better yet because of its aerodynamics. The best aerodynamic design – backward curved and air-foil blades – yield the best efficiencies.

While fan efficiency is an important parameter – since it has a direct effect on on-going energy costs – it is still only one of a number of parameters. In fact, there is no one fan for all conditions. Otherwise there would only be one type for all applications.

Let’s look at an example to illustrate this. For a dust collector fan, often a designer will insist that a radial bladed wheel is the only fan type to be used and there are some valid reasons. On a backward curved wheel material in the air-stream can build-up on the underside of the fan’s blade. This can produce imbalance that if left to run for any length of time can damage the fan. As radial bladed wheels have no contours to hold material it is considered to have a ‘self-cleaning’ blade and therefore, they are well suited for handling dust-laden air applications.

But for a fan that is situated after a properly working bag house, where the air is clean, using such a wheel will mean missed energy efficiency opportunities. For instance, using the duty from last episode, 25,000 cfm at 14″ static pressure and comparing a radial and backward curved design we would see the following:

Wheel Type / Size

Wheel Dia. (ins.)

Speed

(rpm)

Efficiency

(per cent)

HP

Installed Motor

Radial Blade / Size 331.

58

902

63.40%

86.9

100

Backward Curved / 7302.

36.5

1641

79.60%

69.1

75

1. The fan size number referred to is the inlet diameter.
2. The fan size number referred to is the wheel diameter x 20.

Looking at purchase and operating costs 3. we see:

(3. based on 10 cents/kwhrs – average of peak and demand charges).

Size

Fan

Motor

Purchase Cost

BHP

Energy Cost Per Year

33

$13,000

$4,000

$17,000.00

86.9

$ 60,221.42

730

$8,750

$3,250

$12,000.00

79.6

$ 47,886.08

In this particular case the backward curved fan is less expensive, although this is not always the case. The part to note is the savings in electricity of $12,335 per year. Many people resist employing more efficient wheels as they wish to avoid potential imbalance conditions, even where the risk tends towards the low side. In this example these savings can justify having a standby fan ready to be dropped into place should a problem occur.

Although one must consider all costs and certainly lost production costs can quickly outstrip savings, with Kyoto becoming a reality, it is likely that we will see more attention paid to exploring areas for energy savings.


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For a quote or for expert help regarding to your industrial operations, call toll free at 1-888-317-8959 ext.26 today.

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