Fan unit with improved surge characteristics

Abstract

A fan unit that forms part of a gases supply unit used as part of a breathing assistance system for providing heated gases to a user. The fan has an impeller surrounded by an upwardly sloped surface to facilitate improved airflow performance under surge conditions.

1 . (canceled) 
     
     
         2 . A fan unit comprising:
 a casing comprising a diffuser, a volute, an inlet, and an outlet;   an impeller comprising an inducer and an exducer;   a recirculation passage providing a gases flow path, wherein the recirculation passage is in fluid communication with both the inducer and the exducer; and   wherein a portion of gases exiting the impeller recirculates in the recirculation passage from the exducer to the inducer; and   wherein the portion of gases that recirculates through the recirculation passage is configured to increase when a pressure at the outlet is increased.   
     
     
         3 . The fan unit of  claim 2 , wherein the fan unit is in fluid communication with a breathing assistance apparatus for providing gases to a user, wherein the pressure at the outlet increases when the user exhales. 
     
     
         4 . The fan unit of  claim 2 , wherein a gas flow through the outlet decreases when the pressure at the outlet is increased. 
     
     
         5 . The fan unit of  claim 2 , wherein a gas flow through the impeller is maintained when the pressure at the outlet is increased. 
     
     
         6 . The fan unit of  claim 2 , wherein the inducer is configured to receive a flow of gases from the inlet, and the exducer is configured to expel a flow of gases to the diffuser and the volute. 
     
     
         7 . A fan unit comprising:
 a casing comprising a diffuser, a volute, an inlet, and an outlet;   an impeller comprising an inducer and an exducer;   a recirculation passage providing a gases flow path, wherein the recirculation passage is in fluid communication with both the inducer and the exducer; and   wherein a flow rate of gases flowing through the recirculation passage is configured to increase when a pressure at the outlet is increased.   
     
     
         8 . The fan unit of  claim 7 , wherein the fan unit is in fluid communication with a breathing assistance apparatus for providing gases to a user, wherein the pressure at the outlet increases when the user exhales. 
     
     
         9 . The fan unit of  claim 7 , wherein a gas flow through the outlet decreases when the pressure at the outlet is increased. 
     
     
         10 . The fan unit of  claim 7 , wherein a gas flow through the impeller is maintained when the pressure at the outlet is increased. 
     
     
         11 . The fan unit of  claim 7 , wherein the inducer is configured to receive a flow of gases from the inlet, and the exducer is configured to expel a flow of gases to the diffuser and the volute. 
     
     
         12 . A fan unit comprising:
 a casing comprising a diffuser, a volute, an inlet, and an outlet;   an impeller comprising an inducer and an exducer, wherein the impeller comprises a curved flow directing surface;   a recirculation passage providing a gases flow path between an area proximate the exducer and an area proximate the inducer; and   a wedge member configured to partially encircle at least a portion of the exducer, wherein the wedge member is configured to continue a curvature of the curved flow directing surface.   
     
     
         13 . The fan unit of  claim 12 , wherein the curved flow directing surface and/or wedge member is configured to direct an airflow exiting the exducer in a direction that is acute relative to an axis of rotation of a rotating impeller unit and toward an entry of the recirculation passage. 
     
     
         14 . The fan unit of  claim 12 , wherein the curved flow directing surface and/or wedge member is configured to slope upward at an angle of up to 6 degrees from a plane perpendicular to an axis of rotation of the impeller. 
     
     
         15 . The fan unit of  claim 12 , wherein the curved flow directing surface and/or wedge member is configured to direct a flow of gases exiting the impeller at least partially toward a direction of airflow re-entering the diffuser from the volute. 
     
     
         16 . The fan unit of  claim 12 , wherein the wedge member forms a portion of the diffuser. 
     
     
         17 . The fan unit of  claim 12 , wherein the wedge member is positioned adjacent to an exit from the impeller. 
     
     
         18 . The fan unit of  claim 12 , wherein the impeller is located within the casing and configured to be connected to a motor. 
     
     
         19 . The fan unit of  claim 12 , wherein the recirculation passage comprises a plurality of passages. 
     
     
         20 . The fan unit of  claim 12 , wherein a guide member encircles the inducer and is positioned between the impeller and the casing. 
     
     
         21 . The fan unit of  claim 12 , wherein the wedge member is a separable member. 
     
     
         22 . The fan unit of  claim 12 , wherein the volute comprises a cross-section that increases as it progresses toward the outlet of the casing. 
     
     
         23 . The fan unit of  claim 12 , wherein the volute has a curved inner wall positioned between an outer circumferential wall and a top wall of the casing.