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Innovative Material (and Application Method) for a Hydrophobic/Oleophobic Coating to an Aluminum-Bodied Heat Exchanger

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: Develop a material (and application method) for a hydrophobic/oleophobic coating to an aluminum-bodied, air-cooled, fluid-managing heat exchanger, with the subject heat exchanger of the tube-and-fin configuration. 

DESCRIPTION: The naval aviation community, as owner and operator of aerospace systems, continuously seeks improvement in the manufacturing arena. As such, the Navy occasionally faces issues with heat exchanger performance in mechanical systems due to the accumulation of dirt and debris on the thermal transfer surfaces. Developing a cost-effective, innovative technology for a coating material and application method, designed to reduce the build-up of organic material on the thermal transfer surfaces of the heat exchanger, would increase the available usage time of a mechanical system. This would result in a decrease in cost to the Government by removing the need to clean or remove components that have diminished heat-rejection capability. An accompanying application method of the coating material must provide even distribution coverage of the coating to the external surfaces that provide the thermal transfer capability. These thermal transfer surfaces could include use in either wet or dry environments. In dry environments, the particulate build-up would be more easily cleaned from the surface when using a water rinse. In wet environments, the particulate matter would have a reduced tendency to adhere to the surface and be primarily carried away in solution with the water. The intent is to provide longer duration of use. The expectation is of longer periods of trouble-free use, which could provide more remote usage of a device. Heat exchanger size could be reduced to account for higher resulting efficiency. The heat exchanger of interest has separate circuits. Specifications are as follows: Heat Exchanger Requirements with Hydrophobic/Oleophobic Coating Design temperatures. The heat exchanger should be capable of functioning in ambient air temperatures between -65ºF and +160ºF, corresponding compartment temperatures between -65ºF and +210ºF, and fluid temperatures between -65ºF and 275ºF. In case of fan failure, the heat exchanger should function properly after prolonged exposure (24 hours) of the fluid inlet temperatures as high as 420ºF at maximum operating pressures of 230psig. Internal Lubricant. The hot side lubricant fluids for the heat exchanger should be any oil conforming to MIL-L-7808, MIL-L-23699 or DOD-L-85734. Internal hydraulic fluid. The hot side fluid for the heat exchanger should conform to MIL-H-5606 or MIL-H-83282. Rated air flow and pressure drop. The heat exchanger is designed for a rated air flow of 7951 cubic feet per minute (CFM) with an inlet temperature of 130ºF and an inlet air pressure of 14.6psia. The corresponding air side total pressure drop should not exceed 8.9 inches of water. Oil heat rejection. Each oil cooler assembly should provide the minimum heat rejection performance specified at the rated oil side and air side flow conditions. Two circuits with the widest ranges are listed: Minimum Heat rejection (BTU/M) 7245 / 490 Rated oil flow (GPM) 36.4 / 4.5 Maximum Oil Out Temp (°F) 230 / 204 Air fins. The minimum opening between fins is 0.032 inches. Length of fins are up to 5 inches. Operating Temperature. Specified performance should be maintained following operation in ambient temperatures between -65ºF (-54ºC) and +160ºF (+71ºC). Non-operating Temperature. Specified performance should be maintained following long periods of exposure to extremes of -85ºF (-65ºC) to +190ºF (+88ºC). Humidity. Specified performance should be maintained during and following exposure to the following relative humidities: Temperature (ºF) Temperature (ºC) Relative Humidity (%) 70 21 45 100 38 95 126 52 80 160 71 20 Salt spray. Specified performance should be maintained during and following exposure to sea salt fallout of 200 parts per billion (PPB). Sand and dust ingestion. The heat exchanger should not leak throughout its operating range at ground environmental conditions with air containing sand and dust in concentrations up to 1.32 x 10-4 pounds of sand and dust per cubic foot. The heat exchanger should be operated for 10 hours in accordance with the endurance test schedule while ingesting the specified concentration of sand and dust. During this 10-hour test, heat transfer performance should be measured at 3.3 hours, 6.7 hours, and at test completion. In addition, heat transfer performance should be measured after cleaning the test unit. This data will be used to determine cleaning intervals. The specified sand and dust contaminant should consist of crushed quartz with the total particle size distribution as follows: Quantity, percent by weight Particle Size, microns finer than size indicated 1,000 ..................................................................... 100 900 ..................................................................... 98-99 600 ..................................................................... 93-97 400 ..................................................................... 82-86 200 ..................................................................... 46-50 125 ..................................................................... 18-22 75 ..................................................................... 3-7 Surface finish The surface roughness of forgings, castings, and machined surfaces cannot be in excess of 250 micro inches. Coating should be erosion resistant and durable for (840 hours). 

PHASE I: Demonstrate the concept and breadboard of the material and application method that allows a determination of distribution and coating quality on a representative surface. Demonstrate and compare the heat-transfer capability to an untreated sample. Demonstrate the hydrophobic/oleophobic-coating performance on a sample to ensure properties are maintained with a developed material and application method. 

PHASE II: Demonstrate the material and application method on representative samples. Demonstrate and compare the heat-transfer capability to an untreated, production-representative sample. Demonstrate the hydrophobic/oleophobic-coating performance on a production representative sample to ensure properties are maintained with a developed material and application method. 

PHASE III: Conduct final testing that includes adhesion and coating consistency evaluation. Transition would be a commercial offering of a coated product either on an individual application basis or as a complete pretreated heat exchanger assembly. Parties interested in licensing this product would include Off-Highway vehicles, mining equipment, and automotive applications intended for off-road use. Devices that use heat exchangers in austere and also wet or day environments would benefit. 

REFERENCES: 

1: Cadogan, D. and Ferl, J. "Dust Mitigation Solutions for Lunar and Mars Surface Systems." SAE Technical Paper 2007-01-3213, 2007. http://papers.sae.org/2007-01-3213/

2:  American Institute of Aeronautics and Astronautics report: "Desert Research and Technology Studies – Exposure of Lotus Coated Electrodynamic Shield Samples", NASA. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110005671.pdf

3:  MIL-H-5606, Hydraulic Fluid, Petroleum Base. http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-H/MIL-H-5606G_5998/

4:  MIL-PRF-83282, Hydraulic Fluid, Fire Resistant, Synthetic Hydrocarbon Base. http://everyspec.com/MIL-PRF/MIL-PRF-080000-99999/MIL-PRF-83282D_7238/

5:  MIL-PRF-7808, Lubricating Oil, Aircraft Turbine Engine, Synthetic Base. http://everyspec.com/MIL-PRF/MIL-PRF-000100-09999/MIL-PRF-7808L_5699/

6:  MIL-L-23699, Lubricating Oil, Aircraft Turbine Engine, Synthetic Base. http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-L/MIL-L-23699E_25009/

7:  DOD-L-85734, Military Specification: Lubricating Oil, Helicopter Transmission System, Synthetic Base. http://everyspec.com/DoD/DoD-SPECS/DOD-L-85734_AMENDMENT-3_24867/

KEYWORDS: Heat Exchangers; Hydrophobic; Oleophobic; Off-Highway; Particulate Accumulation; Coating 

CONTACT(S): 

Kurt Prescher 

(301) 342-0865 

kurt.prescher@navy.mil 

Suresh Verma 

(301) 342-8100 

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