Check Valves Restrictors Variable Restrictor Filters Desiccant/Moisture Separator 12-50 Figure 12-74.  Pneumatic orifice valve. Figure 12-73.  Flap-type pneumatic check valve. Figure 12-75.  Variable pneumatic restrictor. Check valves are used in both hydraulic and pneumatic  systems.  Figure 12-73  illustrates a flap-type pneumatic check  valve. Air enters the left port of the check valve, compresses  a light spring, forcing the check valve open and allowing air to flow out the right port. But if air enters from the right, air  pressure closes the valve, preventing a flow of air out the  left port. Thus, a pneumatic check valve is a one-direction  flow control valve. Restrictors are a type of control valve used in pneumatic  systems.  Figure 12-74  illustrates an orifice-type restrictor  with a large inlet port and a small outlet port. The small outlet  port reduces the rate of airflow and the speed of operation  of an actuating unit. Another type of speed-regulating unit is the variable  restrictor.  [Figure 12-75]  It contains an adjustable needle  valve, which has threads around the top and a point on the  lower end. Depending on the direction turned, the needle  valve moves the sharp point either into or out of a small  opening to decrease or increase the size of the opening.  Since air entering the inlet port must pass through this  opening before reaching the outlet port, this adjustment also  determines the rate of airflow through the restrictor. Pneumatic systems are protected against dirt by means of  various types of filters. A micronic filter consists of a housing  with two ports, a replaceable cartridge, and a relief valve.  Normally, air enters the inlet, circulates around the cellulose  cartridge, and flows to the center of the cartridge and out  the outlet port. If the cartridge becomes clogged with dirt,  pressure forces the relief valve open and allows unfiltered  air to flow out the outlet port. A screen-type filter is similar to the micron filter but contains  a permanent wire screen instead of a replaceable cartridge.  In the screen filter, a handle extends through the top of the  housing and can be used to clean the screen by rotating it  against metal scrapers. The moisture separator in a pneumatic system is always  located downstream of the compressor. Its purpose is to  remove any moisture caused by the compressor. A complete  moisture separator consists of a reservoir, a pressure switch,  a dump valve, and a check valve. It may also include a  regulator and a relief valve. The dump valve is energized  and deenergized by the pressure switch. When deenergized,  it completely purges the separator reservoir and lines up to  the compressor. The check valve protects the system against  pressure loss during the dumping cycle and prevents reverse  flow through the separator. Outlet port Inlet port Flap Spring Housing Pressure port Inlet port Adjustable needle valve