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driven p ower pumps for building normal pressure. But similarities do exist in some components. 

Air Compressors On some aircraft, permanently installed air compressors have been added to recharge air bottles whenever pressure is used f or operating a unit. Several types of compressors are used for this purpose. Some have two stages of compression, while others have three, depending on the maximum desired operating pressure. 

## Relief Valves 

Relief valves are used in pneumatic systems to prevent damage. They act as pressure limiting units and prevent excessive pressures from bursting lines and blowing out seals. 

## Control Valves 

Control valves are also a necessary part of a typical pneumatic system. Figure 12-72  illustrates how a valve is used to control emergency air brakes. The control valve consists of three-port housing, two poppet valves, and a control lever with two lobes. 

In Figure 12-72A,  the control valve is shown in the off position. A spring holds the left poppet closed so that 

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compressed air entering the pressure port cannot flow to the brakes. In Figure 12-72B,  the control valve has been placed in the on position. One lobe of the lever holds the left poppet 

open, and a spring closes the right poppet. Compressed air now flows around the opened left poppet, through a drilled passage, and into a chamber below the right poppet. Since the right poppet is closed, the high-pressure air flows out of the brake port and into the brake line to apply the brakes. 

To release the brakes, the control valve is returned to the off position. [Figure 12-72A] The left poppet now closes, stopping the flow of high-pressure air to the brakes. At the same time, the right poppet is opened, allowing compressed air in the brake line to exhaust through the vent port and into the atmosphere. 

