Bird Technologies 8930 Series - Manual (Page 13)

3
Chapter 2 Theory of Operation
Load Resistor
Bird 8930 Loads consist of a thin-film-on-ceramic resistor immersed
in a dielectric coolant. The resistor, individually selected for its
accuracy, is enclosed in a special housing. When surrounded by the
coolant, this produces a uniform, practically reflectionless line
termination over the specified frequencies.
Coolant
The load is cooled by forced air and natural fluid convection currents.
The coolant, chosen for its desirable dielectric and thermal
characteristics, carries heat from the resistor to the walls of the
cooling tank, where radiator fins surrounding the tank transfer the
heat to the forced air flow.
When the coolant is heated, thermal expansion causes an increase in
the internal pressure. A vent plug relieves this pressure while
protecting the opening from dirt or other contaminants.
Fans
Forced airflow is provided by three axial fans beneath the heat
exchanger. Baffles direct the air over the radiator fins for more
efficient cooling. A passive, normally open control thermoswitch
closes when the coolant reaches 60 °C (140 °F), turning the fans on.
Power Rating
Reduction
The baffles interfere with the free flow of normal air currents, causing
a 75% reduction in heat transfer efficiency if the forced airflow is
stopped. The maximum power dissipation when the fans are not
functioning is 2.5 kW.
Thermal Interlock
When installed, a passive, normally closed overtemperature thermo-
switch opens above the maximum safe load temperature of 226 °C
(439 °F), turning off transmitter power. The interlock will not permit
use of the transmitter until the load has reached a safe temperature.