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Liquid and Air Systems
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TM-5-4210-205-12 Truck Fire Fighting; Powered Pumper; Foam and Water 500 G.M.P.; Centrifugal Pump Power Take Off Driven; 400 Gal. Water Tank 40 Gal. Manual
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Figure 71.8. Battery compartment shut-of valve.
f. Non-Fire Relay. The non-fire relay is a thermal-
to either ON position. In the ON-LO position
the control switch closes a circuit to the fuel
type, time-delay relay which is a safety device
regulator to reduce fuel flow.
incorporated into the heater electrical system to stop
(4) The indicator lamp glows after the heater
fuel flow if ignition fails to occur two minutes after the
reaches
operating
temperature
and
heater is turned on.  The relay heater element is
continues  to  glow  until  the  heater
energized by a circuit through the power relay and the
extinguishes and the combustion chamber
flame switch in cold position. If the flame switch does
cools. The indicator lamp also provides a
not trip to hot position, the relay will break the circuit to
check of the power supply to the heater. A
the fuel regulator valve.
spring-loaded terminal on the lamp socket
g. Ignition System. The ignition system consists of
is connected to the circuit breaker, so the
a spark plug, ignition coil, condenser, and contact set
lamp glows when pressed in if the circuit
similar to automotive systems.  The contact set is a
breaker button is in and power is available.
mechanical interrupter, operated by a cam collar on the
b.  Flame Switch Assembly.  The flame switch
combustion air motor shaft, which causes the ignition
assembly is a mechanically operated device which
coil to energize the spark plug to ignite the fuel vapor.
controls the electrical circuits during heater operation. It
The spark plug, located in the burner, creates a spark
consists of a frame supporting a microswitch and a heat-
between its electrode and the ground electrode in the
resisting tube encasing a quartz rod. The tube extends
burner base, causing fuel ignition. The ignition system
into the combustion chamber and is subjected to the
is  thoroughly  shielded  for  suppression  of  radio
heat resulting from combustion.  The difference in
frequencies.
expansion of the metal tube and the quartz rod when
h. Limit Switch. The overheat limit switch shuts the
heated trips the microswitch. The switch automatically
heater off by breaking the primary power circuit if the
restores itself to the normal position when it cools.
temperature of the liquid rises beyond a safe limit. The
c. Preheater Assembly. The preheater assembly is
switch incorporates a manual reset button which must
an imbedded coil of resistance wire encased in a metal
be pressed to close the circuit again for further heater
sheath and mounted within the burner. It is energized
operation.
only when the heater is initially started. The preheater
i. Thermostat. The thermostat is an adjustable heat
heats the burner in preparation for the incoming fuel for
sensor which is electrically connected to the control box.
combustion. This vaporizes the fuel so that it can easily
When the liquid has attained the preset temperature, the
be ignited.
electric circuit is opened, stopping the heater operation.
d. Preheat Relay. The preheat relay is a thermal-
The circuit is closed when the liquid temperature is
type, time-delay relay which delays energizing the power
below the thermostat adjustment allowing the heating
relay that controls the ignition system, for approximately
cycle to be completed.
one minute until the preheater sufficiently heats the
j.  Motor Switch.  A two position switch permits
burner to allow fuel vapor to ignite.
operation of the liquid pump without the heating cycle.
e. Power Relay. The power relay is a coil operated
When the switch is in the automatic position, the
relay that controls the circuits to the combustion air
electrical circuit is connected directly to the liquid heater
blower, ignition system, and the fuel regulator valve
system.  With the switch in the manual position, the
during normal heater operation.  The flame switch
electrical circuit is disconnected from the liquid heater
supplies energy to the power relay coil through the
system and a circuit is completed to an electrical
preheat relay.
source, allowing the pump to circulate fluid.
AGO 5667A
9

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