In a motor driven fan of typical construction having a motor operatively connected to drive an impeller within a housing which defines a passage for directing a flow of air generated by the impeller, it is common practice to mount the motor within the passage so that the motor may be cooled by the flow of air through the passage. While this approach is simple to incorporate and has proven to be quite acceptable for providing motor cooling in a majority of applications, it has proven inadequate and may result in motor damage due to overheating in those applications where the fan must be operated continuously for extended periods with the flow of air severely restricted or entirely blocked.
One well known example of such an application is a gasper vent system which has been utilized for many years in aircraft to provide a source of ventilation for passengers in the form of an air jet issuing from small individually adjustable air nozzles, commonly known as "gasper vents", positioned above each passenger seat, which may be adjusted by each passenger according to his or her personal whim to restrict or to entirely block the air stream. Such a system typically includes a fan, commonly called a gasper boost fan, positioned in a flowpath defined by a duct configured to supply air to a large number of such gasper vents.
Since the amount of air required may vary considerably due to restriction and blockage of the individual air jets supplied by the duct, and since it is desirable to avoid annoyance to the passengers caused by either a lack of sufficient airflow or the need for continually readjusting his or her individual gasper vent, gasper boost fans must typically supply airflow at a relatively constant pressure over a wide range of flow rates. In addition, since it is desirable that the system supply air essentially instantaneously when the passenger opens a vent, it is generally a requirement of such systems that the gasper boost fan run continuously, even when all of the gasper vents are fully closed.
One approach utilized in prior gasper vent systems to ensure a flow of air for motor cooling during extended periods of operation with all gasper vents blocked has been the inclusion of an air recirculation duct either integral with or external to the gasper boost fan to allow means for air to recirculate from the outlet to the inlet of the fan due to the pressure rise across the impeller. While this approach has been utilized for reasonably extended periods of operation with all gasper vents blocked, the length of time is limited by the fact that, once the system is blocked, no new air enters the system, and eventually the temperature of the air being recirculated will be raised to the point that motor overheating will occur.
In another approach utilized in prior gasper vent systems, a pressure responsive relief valve is included in the duct to provide an alternate outlet for air flowing in the duct which opens when a predetermined pressure is reached inside the duct. With this approach, even though all of the gasper vents are blocked, a flow of air through the fan is maintained to provide motor cooling. While this approach has been utilized successfully, the inclusion of the valve adds cost and weight and reduces reliability of the gasper vent system.
The instant invention is directed to overcoming one or more of the aforementioned problems and disadvantages.