To test an avionics system for the next generation multi-role F-35 Lightning II U.S. aircraft, a government-contracted company turned to Lytron Inc., Woburn, Mass., for the right equipment.

The plane's avionics were liquid cooled, and the contractor's engineers needed to simulate in-flight conditions. Lytron supplied a compressor-based recirculating chiller that provides precise temperature control and cooling below ambient temperature. One of the critical requirements was that the thermal solution be compatible with polyalphaolefin (PAO), a synthetic hydrocarbon heat transfer fluid. PAO often is preferred in military liquid cooling applications due to its dielectric properties, wide operating temperature range, low corrosivity, low toxicity and hydrolytic stability.

Cooling with PAO can be more challenging than cooling with water or ethylene glycol and water. Wetted materials on the pump, fittings, hoses and oil filter must be compatible with the fluid. Due to its chemical properties, PAO is more likely to leak at pump seals, hose connections and fittings than is water or ethylene glycol and water. Also, because of its dielectric properties, PAO can build up excess static that can damage sensitive avionic electronics.

To meet the engineers' requirements, Lytron designed and manufactured a chiller compatible with PAO. The unit includes a 0.5 hp centrifugal pump, hoses, brass fittings, brass flow switch, 5 μm filter, anti-siphon system, and internal insulation package. The mechanical seals on the pump and the compression brass fittings on the hoses provide the tight seal that is necessary when working with PAO. The brass flow switch ensures the chiller's full compatibility with PAO. Lastly, to prevent the static buildup, an all-metal filter housing is used.

The internal insulation package was necessary because the recirculating chiller was being used for testing and it would minimize the loss of thermal energy to the environment. The anti-siphon system consists of check and solenoid valves installed on the chiller's supply and return lines, respectively. This allows the unit to be installed below the equipment being cooled, preventing backflow and subsequent overflowing of the vented tank. Without an anti-siphon system, coolant could flow back into the tank and overflow when the chiller is powered down.

In addition to being compatible with PAO, the recirculating chiller meets other requirements. It provides excellent temperature stability by controlling the coolant supply temperature to ± 0.1°C (under constant heat load) and has an RS232 interface port for diagnostics, remote start feature, external flow valve, and external pressure-relief valve.