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Feature

How to analyze bid differences in blower package horsepower requirements
Brad Schultz, Smoot Co., div. Magnum Systems

photo When you solicit bids from various equipment suppliers for your pneumatic conveying system, there may be some obvious differences—like conveying line diameters and construction materials—among the proposals you receive. But aside from these, the one difference that's likely to jump out at you is the variation in the motor horsepower requirements for the blower packages. (Besides the rotary-lobe positive-displacement blower and motor, a typical standard blower package includes a drive assembly, inlet and outlet silencers, switches, and gauges, all mounted on a base.)

So how can the blower package horsepower requirement vary widely among the bids for the same conveying system? Equipment suppliers have different approaches to sizing the blower motor based on what safety factor (that is, extra capability over the minimum required) they elect to build into the blower package and its components. Some suppliers are more liberal with this safety factor, and some are more conservative. Examining how blower horsepower requirements for a pneumatic conveying system are calculated will help explain why.

Some calculations
Consider this example: We want the blower in our proposed conveying system to produce a 400-cfm air volume at 8-psig pressure. We can use this air volume and pressure information with a blower horsepower chart or software program to calculate how much horsepower (here called brake horsepower 1) the blower will require at its shaft for this air volume and pressure combination. (Blower horsepower charts and software programs are available on many blower supplier websites.) In this case, the blower requires 18 brake horsepower. Because most blowers are belt-driven, we also need to figure in the blower's belt and sheave horsepower losses. These losses run at about 10 percent (in this case 2 horsepower), so now we're looking at a 20-horsepower requirement.

Sounds simple, doesn't it? But a supplier never sizes a blower package based on the exact air volume and pressure your conveying system is expected to run at. The risk is that if your conveying conditions change or the conveying system is altered at or after installation, such as with additional bends or line sections, the blower won't have enough power to run the system. That's why the blower package and its components need to have some built-in safety factor—and how a supplier applies this safety factor is where the huge differences in motor horsepower requirements can come in during the bidding process.

How safety factors affect blower motor size
For instance, some suppliers would add a safety factor from 2 psig to as much as 4 psig to your conveying system's expected 8-psig pressure before sizing the blower motor. Other suppliers may maximize the motor size to achieve the maximum pressure level the conveying system can handle.

With factoring in belt and sheave losses of 10 percent, this means the supplier's safety factor for sizing the blower motor can run from 12 psig to 15 psig.

So here's the motor size we started at, without any safety factor:

• 8 psig = 18 brake horsepower + 10 percent = 20 horsepower, so we'd use a 20-horsepower motor.

And here's how adding different safety factors increases the motor size:

• 8 psig+4 psig safety factor=12 psig = 26 brake horsepower + 10 percent = 29 horsepower, requiring a 30-horsepower motor.

• 8 psig+7 psig safety factor=15 psig = 37 brake horsepower + 10 percent = 41 horsepower, requiring a 40- or 50-horsepower motor.

Moving from the 30-horsepower motor to the 50-horsepower unit involves not only a huge difference in the motor's initial cost, but also in the size of the blower package's electrical components and the package's potential electrical power use. Of course, you want to ensure that the blower motor supplies enough power to successfully run your conveying system. So which motor is right for this system?

It all depends on your confidence in the supplier's motor sizing accuracy for your system's pressure requirement. To be comfortable with the blower motor size in a bid, ask how the supplier arrived at this size. What safety factor did the supplier use? Why? What experience does the supplier have with systems like yours that makes this safety factor reasonable?

You can get further assurance that the motor size in the bid is right for your conveying system by observing pilot-plant tests of the proposed system in the supplier's test lab.

Reference
1. Brake horsepower is the actual power required at the blower shaft, including all the loads imposed on the blower by the downstream process as well as by the blower itself. This brake horsepower is then corrected for the motor's efficiency and the type of drive arrangement to come up with the necessary motor horsepower. (Click here to return to the text.)

For further reading
To learn more about pilot-plant tests and making your blower package decision, you can read the complete article, "How to analyze bid differences in blower package horsepower requirements" by Brad Schultz, Smoot Co. You can also find articles listed under "Pneumatic conveying" in PBE's Article Index.

Brad Schultz, product manager of pneumatic conveying systems at the time of this article's first publication, is currently sales manager at Smoot Co., div. Magnum Systems.

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