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Chapter 2
Resistor
Bird 8730 Series Econoloads consist of a thin-film-on-ceramic resistor immersed in externally-supplied 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 leaves the load and can either be
discarded or cooled in a heat exchanger and reused.
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 may cause the load to overheat and fail. Salt water
will have a similar effect and should not be used. For recommended coolant, refer to
"Specifications" on page
Optional Flow Interlock
A minimum coolant flow of four gallons per minute is required at all times to dissipate the heat from RF power.
When the flow rate drops below this point, the 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 12 seconds. This ensures
proper operation of the cooling system before applying RF power, preventing resistor burnout.
Optional Thermal Interlock (8732A Only)
The 8732A can be supplied with an optional passive overtemperature thermoswitch. Normally closed, it opens at a
preset temperature, turning off transmitter power. The temperature setting is preset at the factory for either 72 or
79 °C (162 or 175 °F). The interlock system will not permit use of the transmitter until the load has reached a safe
temperature.
Calorimetry
Almost all the RF power in the load is transferred to the coolant as heat. There is no heat transfer to the outer
housing of the load, leaving it at ambient temperature even at full power. The flow rate, and the difference
between the input and output coolant temperatures, can be used to calculate the power dissipated in the load with
the following formula:
where
P = Power in kilowatts
k = 0.263 for temperature in °C, 0.146 for temp. in °F
T
= Water temperature at the output of the load
out
T
= Water temperature at the input to the load.
in
F = Water flow rate in gallons per minute
16.
P
=
k
Theory Of Operation
T
–
T
F
out
in
3
"Coolant" on page 4
and to
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