Figure 8-79. Bleed air from turbojet engines can be used to provide pressurized air for medium-pressure systems.
Figure 8-80. Vane-type air pumps are used to provide low pressure air on aircraft fitted with reciprocating engines.
Sliding vanes are rotated by the driveshaft and as the shaft turns, the chambers located at positions A and B become larger, while those at positions C and D decrease in size. Air is pulled into the pump at the position the chambers enlarge, and it is moved out as they decrease
There are two types of air pumps used to provide instrument airflow, and both are vane-type pumps.
One is called a "wet" pump and the other a "dry"
pump, based on their method of lubrication.
"Wet" vacuum pumps use steel vanes moving in a sealed cast-iron housing and are lubricated by engine oil metered into the inlet air port. This oil is discharged with the air and is removed with an oil separator before the air is either used for inflating de-icer boots or is pumped overboard. A schematic drawing of this type of system is shown in figure 8-81 on the following page.
The more modern instrument air systems use "dry"
pumps that have carbon vanes and rotors and require no external lubrication. These pumps may be used to drive the instruments by producing a vac-uum and pulling air through them or by using the output of the pump to force the air through the instruments. [Figure 8-82]
PNEUMATIC SYSTEM COMPONENTS
Pneumatic systems are often compared to hydraulic systems, but such comparisons can only hold true
Figure 8-81. Wet-type vacuum systems utilize an air-oil separator to remove lubricating oil from the system.
Figure 8-82. A dry-type pump can be used in the vacuum mode as seen in (A) or in the pressure mode as seen in (B).
in general terms. Pneumatic systems do not utilize reservoirs, hand pumps, or accumulators.
Similarities, however, do exist in some components.
RELIEF VALVES
Relief valves are used in pneumatic systems to pre-vent damage. They act as pressure-limiting units and prevent excessive pressures from bursting lines and blowing out seals. [Figure 8-83]
Figure 8-83. A pneumatic -system relief valve is utilized to prevent excess pressures from damaging the system.
At normal pressures, a spring holds the valve closed, and air remains in the pressure line. If pres-sure grows too high, the force it creates on the disk overcomes spring tension and opens the relief valve. Then, excess air flows through the valve and is exhausted as surplus air into the atmosphere. The valve remains open until the pressure drops to normal.
CONTROL VALVES
Control valves are also a necessary part of a typical pneumatic system. The control valve consists of a three-port housing, two poppet valves, and a control lever with two lobes. [Figure 8-84]
In view (A), the control valve is shown in the "off"
position. A spring holds the left poppet closed so that the compressed air entering the pressure port cannot flow to the brakes. In view (B), the control valve has been placed in the "on" position. One lobe
Figure 8-84. This flow diagram of a pneumatic control valve shows how a valve is used to control emergency air brakes.
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.
CHECK VALVES
Check valves are used in both hydraulic and pneumatic systems. In a flap-type pneumatic check valve, air enters one port of the check valve and compresses a light spring, forcing the check valve open and allowing air to flow out the other port. However, if air enters from the other direc-tion, air pressure closes the valve, preventing a flow of air out the intake port. Thus, a pneumatic
Figure 8-85. A pneumatic check valve utilizes a weak spring to restrict the direction of air flow.
RESTRICTORS
Restrictors are a type of control valve used in pneu-matic systems. One type of orifice restrictor has 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. [Figure 8-86]
Figure 8-86. A large inlet combined with a small outlet impedes the flow of air through a restrictor.
Another type of speed-regulating unit is the variable restrictor. 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. [Figure 8-87]
FILTERS
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 filter cartridge, and a relief valve. Normally, air enters the inlet, circulates around the cellulose car-tridge, then 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
Figure 8-87. A variable orifice enables the variable pneumatic restrictor to be set for a wide range of airflow, from the max-imum provided by the air source down to no flow at all.
allows unfiltered air to flow out the outlet port.
[Figure 8-88]
A screen-type filter is similar to the micronic 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. [Figure 8-89]
DESICCANT/MOISTURE SEPARATOR Moisture in a compressed air system will condense and freeze when the pressure of the air is dropped for actuation and, for this reason, every bit of water must be removed from the air. A moisture separator collects the water that is in the air on a baffle, and holds it until the system is shut down. When the inlet pressure to the separator drops below a preset value, a drain valve opens and all of the accumu-lated water is blown overboard. An electric heater built into the base of the separator unit prevents the water from freezing.
After the air leaves the moisture separator with about 98% of its water removed, it must pass through a desiccant, or chemical dryer, to remove the last traces of moisture. This unit consists of a tubular housing with inlet and outlet ports and con-tains a desiccant cartridge. These replaceable car-tridges consist of a dehydrating agent (MIL-D-3716) and incorporate a bronze filter at each end. Any moisture not removed by the separator will be absorbed by the dehydrating agent.
check valve is a one-direction flow-control valve.
[Figure 8-851
Figure 8-88. The micronic filter is equipped with a replace-able paper element.
SHUTTLE VALVES
Shuttle valves may be installed to allow a pneu-matic system to operate from a ground source.
When the pressure from the external source is higher than that of the compressor, as it is when the engine is not running, the shuttle slides over and isolates the compressor. The pneumatic systems may then be operated from the ground source.
Shuttle valves may also be used to provide an emer-gency pneumatic backup for hydraulically operated landing gear or brake systems. [Figure 8-90]