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Bleed air is utilized in a wide variety of ways. Some of the current applications of bleed air are: 

1. Cabin pressurization, heating, and cooling; 

2. Deicing and anti-icing equipment; 

3. Pneumatic starting of engines; and 

4. Auxiliary drive units (ADU). 

## Compressor Types 

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. 

## Centrifugal-Flow Compressors 

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 

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. 

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