Bird Technologies 8640B Series - Manual (Page 14)

3
Chapter 2 Theory of Operation
Resistor
Bird 8640 and 8640B Series Moduloads consist of a thin-film-on-
ceramic resistor immersed in coolant. The coolant flows directly over
the resistor instead of using an intermediate heat transfer system,
reducing the load size to a minimum. After passing over the entire
length of the resistor, the coolant is cooled in a forced air heat
exchanger.
Coolant
The load’s electrical and thermal performance is reduced by
impurities or chemical additives in the coolant, especially ones which
are deposited as scale on the resistor. This increase in the load’s
thermal resistance may cause the load to overheat and fail. Therefore,
the coolant should be distilled water or a mixture of distilled water
and ethylene glycol.
When the coolant is heated, thermal expansion causes an increase in
the internal pressure. The vent plugs relieves this pressure while
protecting the opening from dirt or other contaminants.
Flow Interlock
Dissipation of the heat generated by RF power is critically dependent
on a minimum coolant flow of six gallons per minute at all times,
regardless of coolant temperature. When coolant flow drops below six
gallons per minute, the low flow switch opens causing immediate
transmitter shutdown. The flow switch is a “normally open” type, and
is closed during normal operation.
After flow is restored, a time delay switch keeps the interlock open for
an additional 2 seconds. This ensures proper operation of the cooling
system before RF power is applied to the load, preventing resistor
damage or burnout.
Thermal Interlock
The Moduload is equipped with a normally closed thermoswitch
which opens when the coolant temperature exceeds 79 °C (174 °F),
opening the interlock.