A wide variety of equipment and systems, such as portable and desktop computers, mainframe computers, communication infrastructure frames, automotive equipment, etc., include heat-generating components in their casings. One method used to remove heat from such equipment is to have a tube-axial fan draw air from exterior of the housing to blow cooling air over the heat-generating components. It has also been the trend to make electronic equipment more compact with the casings or housings therefor becoming smaller for consumer use and transport with highly complex circuits and miniaturized integrated electronic components used therewith, creating high-density electronic systems in the small casings. Thus, the heat generated in the casing can become relatively intense and requires high performance from the fan to meet the additional cooling demand.
A known problem that axial fans face relates to the amount of airflow that can be generated by the fan before the system resistance creates a static pressure drop or stall effect, especially in high-density applications. This typically is shown in a static pressure (P) vs. quantity of air flow (Q) graph plotting fan performance and system impedance pressure curves thereon. Greater heat load systems, such as the aforedescribed high density electronic systems, require a larger amount of airflow against high pressure drop to provide the desired cooling for the system. In such applications, this increased cooling demand is typically not available in an efficient manner from a conventional fan because of the relatively early stalling effect of the fan when it cannot overcome the pressure increasing in the system as it is being cooled.
More specifically, it is the section of the fan performance curve between the point where the static pressure is zero, and the point where the static pressure is at its peak value and before system pressure begins to reduce the static pressure from its maximum, which is the efficient or preferred operating area of the fan and which is desired to be maximized in terms of fan performance. At the high pressure end of the fan preferred operating area, the fan operation is in a "centrex region" which is an area proximate and at the intersection of both the fan performance and the system pressure curves. In the centrex region, as the output pressure from the fan drops with decreasing flow rate, the fan begins to show stall characteristics. Fan operation in the centrex region is in a highly unstable and inefficient manner. As flow reduces and output pressure increases after the centrex region, the fan operates in a highly turbulent region and mixed mode of operation which can increase vibrations and power requirements for the fan.
A known method to meet higher heat loads is to employ multiple fans aligned in series in a push/pull configuration. The series multiple-fan system has the ability to raise the maximum pressure before stall. The shortcomings with series multiple fans include the increased space necessary for incorporating multiple fans into the smaller casings of high density electronic system and the additional power requirements. Space requirements in electronic devices such as microsite communication infrastructure systems are of paramount importance, and accordingly space allocation for an additional fan may not be feasible. Multiple fans also produce an increased level of noise which is undesirable, particularly where the fans are incorporated into consumer devices. At the bottom line, the cost associated with multiple fans is also increased.
U.S. Pat. No. 5,707,205 discloses a fan with a special construction including annular plates disposed about the fan blades and spaced axially by an optimum amount to improve flow. The fan of the '205 patent with the annular plate construction is disclosed as improving the so-called P-Q characteristic of the fan but has been found to lack the desired airflow in the preferred operating area of the fan (i.e., from zero static pressure to the maximum pressure before stall) and specifically at lower pressures thereof because of the oversized nature of the circumferentially extending spaces between adjacent plates.
Thus, there is a need for an improved air moving device, such as a fan used to cool high-density electronic systems, that has an increased maximum pressure before the stall effect. Further, a fan would be desirable that does not reduce airflow in the preferred operating area. Also, it is desired that the fan be able to be incorporated into the smaller housing including high density electronic systems therein in a cost-effective manner and without creating excessive noise.