Bleed air is utilized in a wide variety of ways. Some of the  current applications of bleed air are: for minimum flow restriction and turbulence. In most types, the impeller is fabricated from a single forging.  This type impeller is shown in  Figure 1-46 . The impeller,  whose function is to pick up and accelerate the air outwardly  to the diffuser, may be either of two types—single entry or  double entry. The principal differences between the two  types of impellers are size and ducting arrangement. The  double-entry type has a smaller diameter but is usually  operated at a higher rotational speed to assure sufficient  airflow. The single-entry impeller, shown in  Figure 1-47 ,  permits convenient ducting directly to the impeller eye  (inducer vanes) as opposed to the more complicated ducting  necessary to reach the rear side of the double-entry type.  Although slightly more efficient in receiving air, the single- entry impeller must be large in diameter to deliver the same  quantity of air as the double-entry type. This, of course,  increases the overall diameter of the engine. The two principal types of compressors currently being  used in gas turbine aircraft engines are centrifugal flow and  axial flow. The centrifugal-flow compressor achieves its  purpose by picking up the entering air and accelerating it  outwardly by centrifugal action. The axial-flow compressor  compresses air while the air continues in its original direction  of flow, thus avoiding the energy loss caused by turns. The  components of each of these two types of compressor have  their individual functions in the compression of air for the  combustion section. A stage in a compressor is considered  to be a rise in pressure. The centrifugal-flow compressor consists of an impeller  (rotor), a diffuser (stator), and a compressor manifold.  [Figure 1-46]  Centrifugal compressors have a high pressure  rise per stage that can be around 8:1. Generally centrifugal  compressors are limited to two stages due to efficiency  concerns. The two main functional elements are the impeller  and the diffuser. Although the diffuser is a separate unit and is  placed inside and bolted to the manifold, the entire assembly  (diffuser and manifold) is often referred to as the diffuser.  For clarification during compressor familiarization, the units  are treated individually. The impeller is usually made from  forged aluminum alloy, heat treated, machined, and smoothed  1-40 Figure 1-47. Single-entry impeller. Centrifugal-Flow Compressors Compressor Types 1.  Cabin pressurization, heating, and cooling; 2.  Deicing and anti-icing equipment; 3.  Pneumatic starting of engines; and 4.  Auxiliary drive units (ADU). Figure 1-46. (A) Components of a centrifugal-flow compressor; (B) Air outlet elbow with turning vanes for reducing air pressure losses. Compressor manifold Impeller Diffuser Air outlet elbow with turning vanes  for reducing air pressure losses A B