Engineers selecting a blower for a particular installation must have detailed knowledge — or at least estimates — of the application’s operating point as well as a plot of the system’s resistance curve; the prospective blower performance in response to speed changes, static and velocity pressure; and required blower life. Consider the first parameter, performance in response to speed changes. A blower-system operating point is defined as the intersection of the blower’s performance curve with the installation resistance curve, both plotted on a pressure-CFM chart. Therefore, the operating point of a blower depends on two factors:
1) The required CFM flow rate
2) A pressure-change value called head loss or backpressure that’s associated with overall system resistance to flow, thanks to system ductwork, filters, and other obstructions that impose resistance
Manufacturers publish blower performance curves that define maximum performance for blower models at full speed and constant input voltage. Such performance curves express the capability of a blower to deliver flow rate against backpressure all the way from open flow (with zero backpressure) to completely blocked flow, situations in which the treated environment is completely sealed.
Consider one special case — how some manufacturers of BLDC blowers sell these only with adjustable speed control. This ensures that any of one of these blowers can deliver output at any operating point beneath its published performance curve. With knowledge of the target operating point, blower selection is then as simple as browsing a catalog to identify blowers with performance curves that exceed the application’s target operating point.
This sounds simple, right? It is, though as mentioned before, when integrating a blower into a system, OEMs must consider four additional parameters besides the operating point.
1. The system resistance curve
The system expresses pressure drop for a given flow rate. Depending on the setup, it has a mostly linear to second-order relationship to flow rate. Keep in mind that the blower delivers a pressure rise that offsets the pressure loss of system resistance. If the target operating point for the blower is known, then the associated system curve is defined by pressure drop P = Q2 * C, where Q is flow rate and C is the system constant. Superimposed on a candidate blower’s performance curve, this indicates the operating point of that blower in that application at the intersection of the two curves. Tip: This serves as a quick check about whether a blower can satisfy an application by superimposing the curves. Typically, an array of blowers could make a design operate, but only a few will be able to do so efficiently.
2. Blower performance versus speed changes
Blowers respond to speed changes in predictable ways. Refer to the 3 Basic Fan Laws related to blower speed for how the two relate. Note that results from fan-law calculations usually do include negligible errors related to how much motor efficiency varies at different operating points.
3. Static and velocity pressure
Fluid energy in the form of static pressure is force per unit area exerted by a fluid, independent of its motion. If monitored, this pressure would be that measured by a pressure sensor ported normal to flow. Dividing this pressure by fluid density yields a pressure-head value — an energy term per unit mass. In contrast, velocity pressure is a fluid’s kinetic energy — the pressure value of what it would take to slow the stream to zero velocity, expressed as P = 0.5 * V2 * ρ. Static plus velocity pressure is total pressure. One caveat related to blower installation though elevation changes contribute to the stream’s energy, they’re usually small enough to ignore, so blower-performance curves are often plotted as static pressure (or vacuum) rise versus flow rate. Another caveat: Velocity pressure isn’t a useful energy for overcoming flow resistance, because usually resistance results in static-pressure rather than velocity pressure loss. In theory, addition of diverging nozzles convert velocity pressure to static pressure as a way to boost blower flow against backpressure, however this isn’t a common practice.
4. Blower life
Blower life depends on a wide array of design factors including operating point, exposure to contamination (especially at the bearing), temperature, exposure to mechanical shock and blower-motor type. For the latter, options abound, and including switched reluctance (SR), brushed ac and dc — and universal and permanent-magnet brushless motors. Because brushless motors are electronically commutated and have no brushes, they usually deliver the longest life for blower applications, and it’s usually the bearings that ultimately limit brushless-blower life.