# Direct-to-Chip vs Immersion vs Rear-Door Cooling Compared

Compare the three main liquid cooling approaches for AI and high-density data centers: direct-to-chip cold plates, single-phase and two-phase immersion, and rear-door heat exchangers.

Updated 2026-11-01. Also on the [web page](https://liquidcooling.fyi/guides/direct-to-chip-vs-immersion-vs-rear-door).

Three liquid cooling approaches dominate AI and high-density data centers: **direct-to-chip cold plates**, **immersion cooling** (single-phase and two-phase), and **rear-door heat exchangers**. Each has distinct density limits, retrofit characteristics, and operational requirements.

## Quick comparison

| Aspect | Direct-to-chip cold plates | Single-phase immersion | Two-phase immersion | Rear-door heat exchangers (RDHx) |
|--------|---------------------------|------------------------|---------------------|----------------------------------|
| Typical rack density | 30-150+ kW | 50-150+ kW | 100-200+ kW | 30-75 kW |
| Coolant | Water / glycol (facility loop) | Dielectric hydrocarbon fluid | Dielectric fluorocarbon fluid | Water / glycol (facility loop) |
| Server modification | Cold plates replace air heatsinks | Remove fans, immerse fully | Remove fans, immerse fully | No server modification |
| Facility loop | CDU separates IT and facility loops | CDU or tank-integrated HX | CDU or tank-integrated HX | Connects to facility water |
| Retrofit difficulty | Moderate (server downtime) | High (full immersion) | High (full immersion) | Low (no server changes) |
| PUE potential | 1.05-1.15 | 1.02-1.08 | 1.01-1.05 | 1.15-1.30 |
| OEM warranty | Increasingly supported | Limited, case-by-case | Limited, case-by-case | No impact |

Sources: NVIDIA GB200 NVL72 specs (NVIDIA, 2026); Mordor Intelligence Data Center Liquid Cooling Market Report (2026); CoolIT Systems CDU Guide (2025); Allied Control PUE 1.02 claim (Wikipedia PUE, 2026).

## Direct-to-chip cold plates

Cold plates mount directly on CPUs, GPUs and other high-heat components. A coolant distribution unit (CDU) circulates water or water-glycol through the plates, transferring heat to a facility water loop via a heat exchanger. The IT loop and facility loop remain separate.

- **Density**: Supports the highest rack densities. NVIDIA GB200 NVL72 racks reach ~140 kW using direct-to-chip cooling (TrendForce, 2024; NVIDIA, 2026).
- **Retrofit**: Requires server downtime to replace heatsinks. Cold-plate loops and manifolds must be installed in each rack.
- **Warranty**: Growing OEM support. Accelsius offers server-level warranties for two-phase direct-to-chip systems (Mordor Intelligence, 2026).
- **Fluid**: Typically water-glycol on the facility side; some vendors use dielectric fluid in the IT loop.

## Immersion cooling

Servers are fully submerged in dielectric fluid. Heat transfers directly from components to fluid.

### Single-phase immersion
Fluid stays liquid. Pumped through a heat exchanger (CDU or tank-integrated) to reject heat to the facility loop.

- **Density**: 50-150+ kW per rack/tank.
- **Fluid**: Synthetic hydrocarbons (PAO), esters. Shell S3 X uses GTL-based hydrocarbon (Mordor Intelligence, 2026).
- **Operating temperature**: 15-65°C typical (Wikipedia Immersion Cooling, 2026).
- **Advantage**: No cold-plate machining; entire server cooled uniformly.

### Two-phase immersion
Fluid boils at low temperature (typically 30-50°C). Vapor rises to a condenser, returns as liquid.

- **Density**: 100-200+ kW per tank.
- **Fluid**: Fluorocarbons (for example Chemours Opteon). Chemours and Navin Fluorine agreed to manufacture Opteon two-phase fluid, with capacity from 2026 (Chemours, 2025). Confirm current supply with the fluid maker — 3M exited fluorochemical production, so Novec fluids are being phased out.
- **Advantage**: Highest heat flux; passive phase change drives circulation.
- **Challenge**: Fluid loss, sealed tanks required, higher fluid cost.

## Rear-door heat exchangers (RDHx)

A heat exchanger replaces the rear door of an air-cooled rack. Facility water circulates through the door; rack fans push air through the coil. Heat transfers from air to water without changing the servers.

- **Density**: 30-75 kW per rack. Limited by air-side heat transfer.
- **Retrofit**: Easiest. No server modification. NorthC installed Legrand USystems ColdLogik RDHx at Münchenstein while the site stayed live (Legrand/NorthC, 2025).
- **Use case**: Brownfield sites, incremental density increases, mixed air/liquid rows.
- **PUE**: Higher than direct-to-chip or immersion because rack fans remain.

## Choosing an approach

- **New-build AI cluster, 100+ kW/rack**: Direct-to-chip or two-phase immersion. NVIDIA GB200 NVL72 uses direct-to-chip (NVIDIA, 2026).
- **Retrofit, limited downtime, up to 75 kW/rack**: Rear-door heat exchangers.
- **Maximum density, PUE priority, willing to qualify fluids**: Two-phase immersion.
- **Operational simplicity, hydrocarbon fluid preferred**: Single-phase immersion or direct-to-chip.

## Sources

- NVIDIA. "NVIDIA GB200 NVL72 Specifications." 2026. https://www.nvidia.com/en-us/data-center/gb200-nvl72/
- TrendForce. "NVIDIA Blackwell's High Power Consumption Drives Cooling Demands." 30 July 2024. https://www.trendforce.com/presscenter/news/20240730-12232.html
- Wikipedia. "Immersion cooling." Last edited 30 October 2026. https://en.wikipedia.org/wiki/Immersion_cooling
- Chemours. "Chemours and Navin Fluorine Announce Agreement to Manufacture New Liquid Cooling Product." 6 May 2025. https://www.chemours.com/en/news-media-center/all-news/press-releases/2025/chemours-and-navin-fluorine-announce-agreement-to-manufacture-new-liquid-cooling-product
- Legrand / NorthC. "Powering AI with High-Density Data Centre Infrastructure — NorthC Münchenstein." November 2025. https://www.legrand.com/datacenter/int-en/reference-projects/northc-legrand-the-partnership-that-makes-ai-possible
- Wikipedia. "Power usage effectiveness." Last edited 17 October 2026. https://en.wikipedia.org/wiki/Power_usage_effectiveness
- CoolIT Systems. "Coolant Distribution Units Explained for Liquid Cooling" 3 July 2025. https://www.coolitsystems.com/resources/news/coolant-distribution-units-liquid-cooling/
- Mordor Intelligence. "Data Center Liquid Cooling Market Report." 2026. https://www.mordorintelligence.com/industry-reports/data-center-liquid-cooling-market

## Questions

### Which cooling method is best for a 100 kW rack?

Direct-to-chip and immersion cooling both support 100 kW+ racks. Rear-door heat exchangers typically handle up to 50-75 kW per rack. The choice depends on retrofit constraints, fluid compatibility and OEM warranty requirements.

### Does immersion cooling void server warranties?

It can. Some OEMs now offer warranties for validated immersion configurations. Accelsius reported offering server-level warranties for two-phase direct-to-chip cooling in March 2025 (Mordor Intelligence, 2026). Always confirm with the OEM before immersing servers.

### Can rear-door heat exchangers be added without downtime?

Yes. Rear-door heat exchangers are often used as a retrofit because they can be installed on existing rack rows without interrupting operations. NorthC deployed Legrand USystems rear-door units at Münchenstein without interruption in November 2025 (Mordor Intelligence, 2026).