Bird Technologies 8730 Series - Manual (Page 15)

3
Chapter 2 Theory Of Operation
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 "Coolant" on page 5 and to "Specifications" on page 20.
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 thermo-
switch. Normally closed, it opens at a preset temperature, turning off trans-
mitter 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 trans-
mitter 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 ambi-
ent 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
out
= Water temperature at the output of the load
T
in
= Water temperature at the input to the load.
F = Water flow rate in gallons per minute
P k T
out
T
in
– ( ) F Ч Ч =