Avionics System Test Chiller Case Study
Sub-Ambient Cooling to Simulate In-Flight Conditions
CASE STUDY: A Chiller for Avionics System Testing
A U.S. defense contractor had requirements to test an avionics system for the next generation multi-role F-35 Lightning II, formerly the Joint Strike Fighter. (See Figure 1.) The avionics on the plane were liquid cooled and the contractor’s engineers needed to simulate in-flight conditions. They turned to Aavid, Thermal Division of Boyd Corporation for a compressor-based recirculating chiller, which 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 is often preferred in military liquid cooling applications due to its dielectric properties, wide operating temperature range, low corrosivity, low toxicity, and hydrolytic stability.
The Challenge with PAO
Cooling with PAO has some obvious benefits but it can be more challenging than cooling with water or ethylene glycol and water (EGW). Wetted materials on the pump, fittings, hoses, and oil filter must be compatible with PAO. Due to its chemical properties, PAO is more likely to leak at pump seals, hose connections, and fittings than water or EGW. Also, due to its dielectric properties, PAO can build up excess static charge that can damage sensitive avionic electronics, posing yet another challenge.
To meet the engineers’ requirements, Aavid designed and manufactured a chiller compatible with PAO. The chiller included a ½ HP centrifugal pump, Parker Push-Lok® hoses, brass fittings, a brass flow switch, a 5-micron filter, an anti-siphon system, and an internal insulation package. The mechanical seals used on the ½ HP centrifugal pump and the compression brass fittings used on the Parker Push-Lok® hoses provided the tight seal that’s necessary when working with PAO. The flow switch made of brass ensured the chiller’s full compatibility with PAO. Lastly, to prevent the build up of static, an all-metal, 5-micron filter housing was selected.
Two additional options were chosen for the PAO chiller. An internal insulation package was necessary because the recirculating chiller was being used for testing. The insulation package was selected in order to minimize the loss of thermal energy to the environment. Also, the anti-siphon system was added. It consists of check and solenoid valves installed on the chiller’s supply and return lines, respectively. This allows the chiller to be installed below the equipment being cooled, preventing backflow and subsequent overflowing of the chiller’s vented tank. Without an anti-siphon system, coolant may flow back into the tank and overflow when the chiller is powered down and it is located below the test setup.
In addition to being compatible with PAO, the recirculating chiller met other requirements. The chiller provides excellent temperature stability by controlling the coolant supply temperature to ± 0.1º C under constant heat load and has an RS232 interface port for diagnostics, remote start feature, external flow valve, and external pressure relief valve.
The recirculating chiller’s compatibility with PAO combined with numerous other features made it a viable thermal solution for cooling the military test electronics.