Figure 4-30.A recycling solenoid valve.
Figure 4-31.A diaphragm gas valve.
gas supply is allowed to be applied to the top of the
diaphragm, forcing it down and sealing the main valve.
PILOT LIGHT.The gas pilot light in a
gas-heating unit is a small flame that burns
continuously and lights the main burner during normal
operation of the heating unit. It is located near the main
burner, as shown in figure 4-27.
The gas flow to the pilot light is, in some cases,
supplied by a small, manually operated gas shutoff
valve on the main gas line above the main gas valve. In
other cases, the gas can be supplied from the pilot
tapping on a solenoid gas valve, as shown in figure
4-29. In more expensive heating units, the gas for the
pilot light is often supplied by a thermocouple-
probably the simplest unit in the electrical field that is
used to produce an electric current by means of heat. It
is constructed of two U-shaped conductors of unlike
metals in the form of a circuit, as shown in figure 4-32.
If these conductors were composed of copper and
nickel, respectively, and are joined as shown in the
figure, two junctions between the metals exist. If a
flame heated one of these junctions, a weak electric
current would be produced in the circuit of these
conductors. A series of junctions can be arranged to
form a thermopile to increase the amount of current
produced, as shown in figure 4-33.
In the heating field, thermocouples and
thermopiles are used to produce the electrical current
used to operate such units as gas valves, relays, and
other safety devices.
The thermocouple is located next to the pilot light
of the main gas burner, as shown in figure 4-27. It
generates the electric current (usually 50,000
microvolts) which holds open a main gas valve, a relay,
or any other safety devices, permitting gas to flow to
the main burner. Soon after the pilot light is
extinguished, current ceases to flow to these safety
devices, thus causing them to shut off the gas to the
heating unit. These safety devices will not operate
again until the pilot light is lighted and current is again
generated by the thermocouple.
THERMOCOUPLE CONTROL RELAY.
The thermocouple-operated relay shown in figure 4-34
is a safety device used on gas-fired heating equipment.
The thermocouple, when placed in the gas pilot flame,
generates electricity. The electric current energizes an
electromagnet that holds a switch or valve in the OPEN
position as long as the pilot flame is burning. When the
pilot flame goes out because of high drafts or fuel
failure, the electromagnet is de-energized, thus closing
and preventing the opening of the switch or valve. The
Figure 4-32.The principle of a thermocouple.