Single-stage pressure blowers, the most common and least expensive design choices for industrial systems, can offer tremendous operating cost savings in low pressure systems. Companies use very expensive compressed air for various drying, dust blowoff, air flotation, and cleaning operations. These blowers can also be used for vacuum service for parts hold down, pneumatic conveying, and dust collection.
Pressure blowers provide relatively high pressures at low volume when compared to other types of centrifugal fans. For purposes of this discussion, fans with volumes of 10,000 cfm to pressures of 140 in. wg (5.0 psig) are considered pressure blowers. Single-stage pressure blowers can be provided in alloy and temperature ranges that are not available with multiple-stage and regenerative blowers. Single-stage centrifugal blowers can be designed with extra heavy duty casings used in series to boost pressures beyond normal single-stage maximums while maintaining single-stage efficiencies.
Pressure Blower Selection
Selecting pressure blowers or any other type of fan for applications involving relatively high pressure requires some special considerations. Pressure blowers are generally used with the pressure entirely on the inlet or entirely on the outlet. Air is compressed as it passes through the fan, lowering the volume and raising the density. In negative pressure systems air is rarified to become less dense.
The extent to which the effects of compression and rarefaction must be considered depends largely on the degree of accuracy employed in the actual system design and calculation process. During compression there is also a temperature rise associated with the energy expended to overcome the system resistance and blower inefficiency to determine the proper air volume for selection purposes. The effect on density of both compression and temperature must be considered.
Typical applications require constant pressure throughout the system’s operating range. The simplest pressure blowers employ straight radial wheel design. These radial blade wheels are inherently stable and perform with moderate efficiency. Lower in specific RPM, these designs produce higher pressure and noise levels than other designs.
Alternative Designs
Fan manufacturers have developed alternative designs with blades that are curved slightly, either backward or forward. Volume and pressure characteristics remain close to the same as radial blade wheels without sacrificing stable performance conditions. These fan designs are generally direct coupled to a motor through shaft and bearings or directly mounted on the motor shaft itself, resulting in greater reliability for continuously operating systems.
Standard single-stage backward curve blade centrifugal fans can be supplied with efficiencies up to 80%, and provide oil-free, stable (no surge) airflow with a non-overloading horsepower characteristic. Fans of this size are found in printing dryer and metal finishing systems with wide continuous conveyors. They can also be used for vacuum hold down and air flotation chores in these same systems.
Heating and cooling of the drying air can be accomplished for additional cost. High efficiency filtration and silencing accessories are available. Direct drive industrial fans and blowers that use standard frame motors are readily maintained and provide dependable service in 24/7 operating conditions.
A primary concern in many of today’s industrial applications is the noise level criteria. To meet these requirements many pressure blowers require sound attenuation. The backward curved wheel design can result in an 8–10 dB noise reduction over traditional straight blade designs. In some cases this may make the difference between a suitable or unsuitable installation.
Another source of concern is the sound level of the electric motors driving the blowers. In some cases traditional absorptive silencers are not sufficient and complete noise enclosures are required. These noises enclosures in turn can require ventilation air in order to cool the machinery itself. A blower/silencer package of this type is best designed at the time of equipment purchase in order to gain minimum footprint and maximum noise level reduction.
