Chapter 2 Theory Of Operation; Resistor; Coolant; Flow Interlock - BIRD Moduload 8640 Series Operation Manual

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 or pressure switch opens causing immediate transmitter shutdown. The flow or pressure switch or
pressure switch is a "normally open" type, and is closed during normal operation.
After flow is restored, a time delay relay 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.
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