| 1. Define the coolant and temperature range first | Coolant chemistry, operating temperature, freeze protection and peak temperature | Common coolant: water-glycol mixture, often approximately 30–50% glycol by volume.
Typical liquid-cooling operating range: about −40°C to 65°C, with short-duration peaks that may be higher depending on the system. | PA12, PPA, PPS, PVDF, PTFE or 316L stainless steel | Material temperature limits must exceed the maximum coolant and surrounding-part temperature. A 50% glycol mixture generally has higher viscosity and lower heat capacity than water alone. | Confirm compatibility with glycol, corrosion inhibitors, biocides and additives. Compatibility can change with concentration, temperature and exposure time. | Perform immersion testing using the actual coolant at the maximum expected temperature and duration. Check mass change, swelling, tensile strength and leakage. |
| 2. Match thermal conductivity to the heat-transfer objective | Heat transfer through the pipe wall versus electrical isolation | Approximate thermal conductivity at room temperature:
316L stainless steel: 14–16 W/m·K
Aluminum alloys: approximately 150–220 W/m·K
Engineering polymers: approximately 0.15–0.40 W/m·K | Metal for high wall heat transfer; PA12, PPA, PPS or PVDF when electrical isolation and low weight are priorities. | Higher conductivity does not automatically produce better cooling; wall thickness, coolant velocity, contact area and fin or cold-plate design are also decisive. | Polymer pipes reduce electrical-conduction paths and galvanic-coupling risk. Metal pipes require attention to electrical isolation and galvanic corrosion when joined to dissimilar metals. | Compare complete thermal resistance, including pipe wall, contact interfaces and coolant-side convection, rather than selecting by conductivity alone. |
| 3. Select a material with sufficient continuous-temperature capability | Continuous service temperature, short-term peak temperature and aging margin | Indicative continuous-use ranges vary by grade and design:
PA12: commonly about 80–100°C
PPA: commonly about 120–150°C
PPS: commonly about 150–200°C
PTFE: commonly about 200–260°C | PA12 for moderate temperatures; PPA or PPS for higher-temperature polymer applications; 316L or PTFE for specialized high-temperature requirements. | Temperature ratings depend on pressure, stress, reinforcement, fluid exposure and required service life. Do not use a material at its published maximum without a design margin. | Long-term heat exposure can reduce polymer strength, increase permeation and accelerate additive extraction or oxidation. | Use pressure-temperature derating curves from the selected grade and validate after thermal cycling between the lowest and highest design temperatures. |
| 4. Check chemical resistance and permeation | Coolant compatibility, water absorption, swelling, extractables and gas or moisture permeation | PA12 generally offers good chemical resistance but can absorb moisture.
PPS and PVDF provide strong resistance to many automotive fluids.
PTFE has very broad chemical resistance but may have higher deformation under load.
316L requires protection from chloride-rich conditions. | PPS, PVDF or PTFE for demanding chemical environments; PA12 or PPA for many glycol-water systems after validation; 316L where metal compatibility is confirmed. | Swelling or softening can reduce wall strength and alter dimensions, especially at elevated temperature. | Chlorides can promote localized corrosion in stainless steel. Additives, cleaning agents and assembly lubricants can also affect polymer seals and liners. | Test the complete pipe, liner and seal assembly in the actual coolant formulation. Measure dimensional change, permeation, burst pressure and post-aging appearance. |
| 5. Size the pipe for flow, pressure and pressure drop | Inside diameter, wall thickness, flow rate, pressure rating and bend radius | Coolant flow is commonly selected from the thermal load and allowable temperature rise:
Q = ṁ × Cp × ΔT
Pressure loss increases as inside diameter decreases and generally rises sharply with flow velocity. | Reinforced PA12, PPA, PPS or PTFE for flexible or electrically isolated routing; 316L for high-pressure, low-permeation metal routing. | A smaller pipe may reduce packaging space but increase pumping power and heat generation. Excessive velocity can increase erosion, noise and vibration. | Design against maximum pump pressure, water-hammer events, vibration and temperature-related softening. Maintain adequate wall thickness after bending and forming. | Calculate straight-pipe and fitting losses, then conduct hydrostatic, burst, pulsation and vibration testing at the maximum operating temperature. |
| 6. Consider electrical isolation and galvanic corrosion | Dielectric strength, isolation resistance, metal pairing and grounding strategy | Nonconductive polymer pipes can help isolate coolant from high-voltage components. Metallic pipes conduct electricity and may require controlled bonding or insulated joints. | PA12, PPA, PPS, PVDF or PTFE for electrical isolation; 316L when conductivity, grounding and corrosion control are deliberately designed. | Thermal cycling can damage insulating layers, seals or overmolds if the coefficients of thermal expansion differ significantly. | Prevent coolant leakage near energized components. Dissimilar-metal joints should be evaluated for galvanic corrosion, especially in conductive aqueous coolants. | Measure isolation resistance after thermal cycling, vibration, coolant aging and pressure testing. Inspect joints for corrosion and insulation breakdown. |
| 7. Validate joining, sealing and lifetime reliability | Connector compatibility, seal material, assembly method, fatigue life and serviceability | Common joining methods include compression fittings, barbed connections with clamps, crimped fittings, welded polymer joints and formed metal connections. | PA12, PPA and PPS can support molded or welded components depending on grade; PTFE often requires mechanical retention; 316L supports brazed, welded or mechanically sealed designs. | Repeated heating and cooling can cause differential expansion between the pipe, fitting and seal. Design the joint to avoid excessive strain and permanent compression loss. | Seal selection must account for coolant chemistry, temperature, compression set, pressure pulses and vibration. Avoid relying on friction alone for safety-critical retention. | Conduct leak, pull-off, torsion, vibration, thermal-cycle, pressure-pulse and burst tests on production-intent assemblies. Confirm compliance with the applicable vehicle and battery safety requirements. |
