Application Digest

Providing cooling in tight

Arthur Ward, Applications Engineer, NMB

Many times, an engineer completes an electronics enclosure design only to
find it needs additional cooling. If a low or medium speed fan was initially
selected, then the simple solution is to substitute a high-speed fan with the
same dimensions. However, if no higher-airflow fan is available, a simple fix
can become a major redesign.

Rather than redesign the enclosure to increase airflow or accommodate a larger fan, multiple fans can often be used with minimal changes. Adding a second identical fan results in only a 3-dBA increase in noise. But engineers must choose between operating the fans in parallel or series.

In a low-impedance system, two fans can work in parallel to increase airflow. This approach doubles the effective airflow rate in free air. As system impedance increases, added airflow from the second fan decreases. Care must be taken in mounting two fans side-by-side. The mounting must be secure to eliminate the possibility of vibrational resonance between the fans.

In systems with high impedance, two fans applied in series are most effective. Theoretically, the system back pressure can be doubled, resulting in increased airflow.

The best method is to mount one fan at the intake and one at the exhaust. Mounting the fans back-to-back is less effective because the angular airflow component from the first fan reduces the second's efficiency.

To speak with an NMB Technologies' application engineer, call: (818) 341-3355.

Don't Underestimate Screw Pumps

Luca Cozzi, Chief Engineer, SEIM srl

Though positive-displacement, self-priming screw pumps have been available for many years, ongoing R&D has led to remarkable performance improvements. So much so that, nowadays, screw pumps are used in applications ranging from lubrication schemes to actuating systems and oil-hydraulic test benches.

The most important features of screw pumps-nearly constant flowrate, silent operation, and high mechanical efficiency-also make them excellent contributors to off-line filtration systems. Consider: The tooth-profile characteristics of gear pumps and the interaction of the two gears can lead to temporary rupture of the lubricating fluid film. Likewise the pressure in the sealing area of vane pumps and the reciprocating motion of the vane into the rotor can rupture the lubricating film. In both cases, this rupture can cause wear and release contaminating particles into the fluid current.

With screw pumps, no contaminant particles are released because:

The rolling diameters are equal, hence the relative speed between driving screw and idler screw is nil.

  • The epicycloidal screw profile engages the corresponding profile of the idler screw via a rolling contact.

  • There are no trapping areas for contaminant particles-the sort typically found in other types of pumps-crossing the screw pump.

  • There is no contact between the driving screw and idler screws, or the three screws and the pump casing.

In short, screw pumps-thanks to their low sensitivity to polluting particles and simple construction-offer long service life for maximum filter performance.

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