fixed orifice (B) in the valve body for this purpose. It is expanded to atmospheric pressure through the fixed orifice, which controls the purge airflow. The purge air back  flows  to  atmosphere  through  each  desiccant chamber during half of each cycle. The dry purge air enters the top of the chamber. The oil and water that are removed from the chamber are discharged from the bottom  of  the  chamber  through  the  purge  muffler. The key to the simplicity and unusual efficiency of the dehydrator is its unique valving system. A particular point to notice is that the solenoid control valves handle only the purged air. (The main airflow through the unit does not pass through them as in older units employing three-way  valves.)  This  means  that  simple, direct-acting, large-orifice, two-way valves can be used to  eliminate  the  sources  of  high-pressure  drops.  In addition, this type of valve is more dependable. Let us take a detailed look at the operation of a dehydrator.   We’ll   consider   a   complete   cycle   of operation. Refer to figure 2-6, phases I, 11, and III for the  following  descriptions. PHASE I—DRY/PURGE.— The solenoid valve (F)  is  closed.  Solenoid  valve  (D)  is  open.  Incoming moist air, under pressure, flows through chamber (A), where it is dried. Most of this dry air is passed through the outlet conduit to be put to use. A small quantity, however,  bypasses  the  closed  ball-check  valve  (E) through a small orifice (B) into chamber (C). Here it picks up moisture from the partially saturated desiccant and passes out of the unit through the open valve (D). Since  the  chamber  being  purged  is  at  near-atmospheric pressure, the ball-check valves (E) and (G) are held in place by the pressure of the main airflow. PHASE  II—DWELL.—  The  timer  closes  both solenoid valves (D) and (F). This allows the pressure in the regenerated chamber (C) to build up through the orifice to approximately that of chamber A. PHASE   III—DRY/PURGE.—   In  figure  2-6 (phase III), the solenoid valve (F) is opened by the timer; solenoid  valve  (D)  remains  closed.  The  pressure  in chamber (A) drops immediately, causing the ball-check valves (E) and (G) to move rapidly to the right, sealing off chamber (A) and directing the main airflow through chamber (C) where it is dried. A portion of this dry air, leaking  through  orifice  (H),  passes  through  chamber(A) where  it  picks  up  moisture  and  is  emitted  to  the atmosphere   through   valve   (F).   After   completing another  dwell  period,  the  cycle  repeats. The most important single aspect of maintaining the dehydrator is to assure proper operation of the automatic dump traps on the fluid separator and the telltale oil filter units. If oil, water, and sludge are not expelled the accumulated liquids may fill the trap, back up into the air lines, and pass into the dehydrator. Complete failure of the desiccant material of the dehydrator to dry and purify quickly follows. If this happens, you will have to completely disassemble and clean the fluid separator, telltale  oil  filter,  both  dump  traps,  the  pressure  regulator, the dehydrator, and all the interconnecting piping. Also, the desiccant in the dehydrator chambers will have to be replaced. Dust Filter The dust filter (fig. 2-1) consists of a separable housing, which contains a replaceable falter cartridge, constructed of pleated paper (5 microns). The design is such that air flows from the outer (housing) side inward to a hollow center that is connected to the outlet port. Dust is then collected on the outer surface of the filter cartridge. Since the collected dust is dry, any substantial accumulation falls to the bottom of the housing and lessens the possibility of clogging the filter. Flow Limiter A flow limiter (fig. 2-1) is installed between the dust filter and output of the dryer. It is designed to limit the output  of  the  dryer,  should  the  output  of  air  increase beyond the limits of the dryer capability. The flow limiter is a spring-restrained poppet valve with  an  orifice  in  the  poppet  valve.  The  poppet  valve offers no appreciable resistance to the flow of air during normal operation. When the flow exceeds the design of the dryer, the poppet valve closes, and the flow is then limited by the orifice in the poppet valve. MOISTURE  MONITOR A moisture monitor probe is installed downstream from the dryer to monitor the dew point of the dry air. The monitor gives you a visual reading in ppm (parts per million) or in dew point (for example, 40°F). Most monitors have a built-in alarm system. It provides both audible and visual alarms when the air quality is less than the value that it is set to. When the monitor unit is designed to read the moisture content of the air in dew point, a conversion chart is included with the unit. ‘This chart converts the individual reading to a standard dew-point reading, which is at atmospheric pressure; for example, a dryer feeding three waveguide systems at 10, 20, and 30 psig. 2-8