Immersion Cooling Tank

Overview

Immersion cooling submerges entire servers in a non-conductive, thermally conductive dielectric fluid, enabling heat dissipation directly from CPUs, GPUs, and memory to a heat exchanger without air gaps or fins. A single immersion tank removes 50–200 kW from 1–4 multi-socket nodes, delivering 4–6× higher cooling density than air-cooled racks and reducing data center footprint by 30–50%.

Servers are mounted on sliding rails inside a sealed stainless-steel vessel filled with fluid (typically synthetic ISO VG 32 dielectric oil or ester). A centrifugal pump recirculates fluid through a coolant distribution unit (CDU) containing a brazed plate heat exchanger that rejects waste heat to facility chilled water or air. A bypass filtration loop continuously removes particles and moisture, maintaining dielectric strength >30 kV.

The approach eliminates CPU coolers, RAM heatsinks, and most case fans, leaving only power supplies and peripheral cards exposed. Reliability improves because fluid carries heat away from hotspots faster than still air, reducing thermal transients and extending component life by 5–10 years in production fleets.

How it works

Fluid circulation: The pump draws 25–30 °C fluid from the tank sump and forces it at 1–3 bar through the distribution manifold. Nozzles spray fluid over server components, CPU sockets, DIMMs, and backplanes, at 10–30 m³/h. Heat diffuses through the fluid via conduction and natural convection. Warm fluid (35–40 °C) pools in the sump and enters the CDU's heat exchanger inlet.

Heat rejection: Inside the CDU, fluid passes through a brazed aluminum or copper plate HX in counter-flow against facility chilled water (12–18 °C inlet). Thermal delta is 15–20 °C across the plates. Return chilled water (15–22 °C) and lukewarm dielectric fluid (20–25 °C exiting the HX) are recirculated. A turbine flow meter monitors circulation; if flow drops below 8 m³/h, a low-flow alarm triggers.

Fluid maintenance: A bypass filtration cart (3 µm pleated cartridge + molecular sieve dehydrator) runs offline 2–4 hours per month. As fluid passes through, particles and dissolved water (hygroscopic absorption from air during refills) are removed. A dielectric breakdown voltage probe auto-tests fluid at 1 kV steps every shift; if BDV drops below 25 kV, the fluid has absorbed water and requires immediate dehydration or replacement.

Tank sealing & access: The bolted lid with silicone O-ring maintains a sealed environment, preventing moisture ingress and fluid evaporation. A ball valve with cam-lock coupler allows safe fluid withdrawal for filtration or replacement. Server modules slide in and out via quick-disconnects (power) and flexible hoses.

Electrical: A 60 A main disconnect and 3-phase contactor protect the pump motor. The monitoring PLC displays fluid temperature, level, and BDV in real time via 7" touchscreen. Modbus TCP reporting to the data center BMS enables automated shutdown if BDV falls below critical or if pump pressure exceeds 3.5 bar (cavitation warning).

Integration Scenarios

Dedicated cooling loop: A facility chiller supplies 12 °C fluid to the CDU inlet; multiple tanks in parallel draw from the same loop. The BMS staggers pump start/stop to balance facility load.

Air-side economizer hybrid: During cold months, outside air fans cool a plate HX upstream of the immersion tank CDU, displacing chiller load. Tanks reduce demand by 30–50% vs. air-cooled racks.

Containerized deployment: Pre-filled, pre-filtered tanks ship ready to install; only facility chilled water and power connections are required. First-fill usually takes 6–8 weeks to condition fluid (water content drops from 100 ppm to <50 ppm).

Thermal Performance

• Heat transfer coefficient (fluid → CPU): 5–15 kW/m² °C (vs. 50–100 W/m² °C for air).
• Thermal resistance (CPU to sump): <0.002 K/W per watt (extreme efficiency).
• Supply fluid temp to CPU: 25–30 °C; no thermal cycling of silicon.
• Reliability gain: Component life extends 5–10 years due to stable thermal environment and elimination of air-side corrosion (salt, dust, thermal cycling).

Fluid Chemistry & Safety

Accepted fluids:

• Synthetic PAO or ester dielectric: ISO VG 32, >30 kV fresh, non-flammable (ISO 6743-4 Type HEK).
• Specific heat: 1.8–2.0 kJ/kg·K.
• Thermal conductivity: 0.12–0.14 W/m·K (vs. 0.026 W/m·K for air).
• Viscosity @ 40 °C: 28–36 cSt (pumpable at 3 bar; avoids bearing wear).

Moisture limits:

• New fluid: <100 ppm water (verified before tank fill).
• Operational: <300 ppm (BDV >25 kV acceptable).
• Rejectable: >500 ppm water (BDV <20 kV; replace fluid).

Flame point / flash point: >300 °C (safe for data centers; no increased fire risk vs. air-cooled).

Maintenance & Consumables

Monthly: Filter and dehydrate offline; check BDV. Quarterly: Inspect gasket for leaks; verify pump pressure and flow. Annual: Replace filter cartridge; clean distribution manifold nozzles if buildup detected. Every 3–5 years: Full fluid replacement (degradation, additive depletion).

Replacement HX modules and pump assemblies are plug-and-play; typical mean-time-to-repair (MTTR) is <2 hours with on-site spares.

Standards & Certification

• ASTM D2619: Thermal stability of dielectric fluids.
• IEC 60156: Dielectric breakdown voltage testing.
• IEEE 1415: Guide for maintenance of immersion cooling equipment.
• Data center compatibility: Most facilities allow immersion tanks as long as fluid and spill containment meet local environmental regulations (EPA, state waste oil codes).