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TM 10-6640-233-10 e.    Purge   System.     The   purge   system   is   used   to   exhaust   potentially   dangerous   contaminated   air   inside   the laboratory to the outside.  The system consists of the following components: (1)   Power control relay located in panelboard assembly number 1. (2)   Time delay relay located in explosion proof distribution box A3. (3)   Blower control relays located in explosion proof distribution box. (4)   Two purge intake doors and two purge exhaust doors located on the outside wall of the laboratory. (5)   Four purge door limit switches. (6)   Two intake dampers and two exhaust dampers located behind the external purge doors. (7)   Three ventilation dampers mounted in the ECU plenum located in the mechanical room. (8)   Seven purge damper motors. (9)   One purge alarm buzzer located at the power entrance panel. A  purge  cycle  can  be  initiated  in  two  ways,  one  of  which  is  by  the  application  of  external  power  after  a  power shutdown,  the  other  is  by  an  alarm  generated  by  the  gas  detector  system.    When  external  power  is  first  applied,  the power required for a purge cycle is picked off ahead of the main circuit breaker.  Main circuit breaker is held open by power control relay.  This prevents power from being applied to any equipment or lighting in the laboratory until the purge cycle is completed.  Relay routes the power that was picked off ahead of the main circuit breaker to start the 5-minute time delay relay and to energize the four blower control relays which provide power to the ECU blowers.  Power is also routed  to  the  purge  door  damper  motors  and  to  the  ventilation  damper  motors  so  that  the  purge  door  dampers  are opened  and  the  ventilation  dampers  are  closed.    The  ECU  blowers  suck  outside  air  into  the  ECU  ducting  where  it  is dumped into the laboratory creating a pressure which forces any gases or contaminated air to the outside via the purge exhaust doors.  After 5 minutes have elapsed, time delay relay de-energizes.  This causes power control relay to change state  which  de-energizes  relays.    Power  is  removed  from  the  ECU  blowers,  the  dampers  are  driven  to  their  normal positions and the main circuit breaker may now be closed to supply normal power to the laboratory.  If at the end of the purge cycle the gas detector alarm system is in an alarm condition, the time delay relay is activated causing the cycle to be repeated.  This will continue until the alarm clears.  When external power is applied and a purge cycle initiated, one of the external purge doors is closed, the purge alarm buzzer will be activated by the purge door limit switch and will remain activated until the door is opened. f. Air System (Figure 1-7).  The air system consists of the air compressor, air tank, pressure switch, moisture trap, and associated piping and valves.  The air compressor is located in the mechanical room and operates on 208V, 60Hz, 3-phase  power.    It  can  supply  air  at  120  psi  to  the  system.    In  automatic  operation,  the  compressor  is  controlled  by  a pressure switch which shuts it off when the pressure has reached 120 psi and turns it back on when the pressure in the system drops to 60 psi.  The air tank relief valve is set to operate at 150 psi.  There are air service valves located within the main laboratory work area and are color coded orange. 1-18


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