Immersion cooling: single-phase and two-phase

Immersion cooling submerges servers in a non-conductive (dielectric) liquid. Every component, including processors, memory, power supplies and drives, transfers heat directly into the fluid. Server fans are removed, the room no longer needs to deliver cold air to the IT equipment, and nearly all the heat is captured in liquid.

There are two families. In single-phase immersion, the fluid stays liquid and is pumped through a heat exchanger. In two-phase immersion, the fluid boils at the hot surfaces and condenses on a coil above the bath. Both can handle very high rack-equivalent densities, but they differ greatly in fluid cost, containment, regulatory exposure and operational practice.

Single-phase immersion

Servers are placed vertically in an open-top tank, sometimes called a bath or tub, filled with a hydrocarbon-based dielectric fluid. Common fluid types include synthetic hydrocarbons, polyalphaolefins (PAO), refined mineral oils and some engineered esters.

Fluid is circulated either by pumps through an external heat exchanger or by a pump and heat exchanger integrated in the tank. Heat passes to a facility water loop, which rejects it through dry coolers, cooling towers or chillers.

Characteristics

• Fluid remains liquid, so loss to the atmosphere is low.
• Fluids are typically viscous compared with water, so flow distribution through servers must be designed carefully.
• Fluids are combustible with relatively high flash points; fire code review by the authority having jurisdiction is needed.
• Hardware must be compatible: some cables, labels, thermal pastes and plastics degrade or leach in certain fluids.

Two-phase immersion

Servers sit in a sealed tank of engineered fluid with a low boiling point, often in the range of about 50 to 60 °C (122 to 140 °F). Heat from chips causes the fluid to boil at their surfaces. Vapour rises to a condenser coil near the top of the tank, which is cooled by facility water. Condensed liquid drips back into the bath.

Boiling is a highly effective heat transfer mechanism, so two-phase systems can manage very high component power with little or no pumping of the dielectric fluid itself.

Characteristics

• Very high heat flux capability and nearly uniform component temperatures.
• Tanks must be sealed and managed carefully to limit vapour loss, especially when lids are opened for service.
• Historically, most two-phase fluids have been fluorocarbon chemistries that fall under the broad PFAS (per- and polyfluoroalkyl substances) category.
• Fluid cost per litre is high, so vapour losses carry real operating cost.

PFAS and fluid availability

PFAS have come under increasing regulatory scrutiny in Canada, the United States and Europe. A major manufacturer of fluorinated heat transfer fluids announced it would exit PFAS manufacturing by the end of 2025, and other suppliers have introduced alternative chemistries. Owners considering two-phase immersion should confirm long-term fluid supply, ask for the fluid's global warming potential and environmental profile, and track federal and provincial regulatory developments. This uncertainty is one reason single-phase immersion has seen wider adoption.

Comparison

Facility implications

Floor loading and layout

Immersion tanks are heavy. A filled tank can weigh several tonnes, and the load is concentrated on a small footprint. Structural review is essential, particularly on raised floors and upper floors. Tanks are laid out horizontally rather than as tall racks, so the floor area per kW is different from conventional rooms, though the overall density is often still higher.

Service access needs clearance above tanks to lift servers out, often with a hoist or lift assist, and space for drip trays while hardware drains.

Facility water

Most immersion systems connect to a facility water loop through an integral or external heat exchanger, similar to a coolant distribution unit. Because fluid temperatures leaving the servers can be high, facility water supply temperatures of 30 to 40 °C (86 to 104 °F) are often possible, depending on vendor requirements. In Canadian climates, that allows dry coolers to handle heat rejection without compressors for most or all of the year.

Room environment

With almost all heat captured in liquid, the room HVAC load drops to envelope gains, lighting, people, network equipment that stays air cooled, and some heat loss from tank surfaces. Ventilation is still needed for occupants, and in single-phase rooms, ventilation helps manage any fluid odour. Humidity control still matters for ancillary air-cooled equipment and for operator comfort.

Fire protection and life safety

The authority having jurisdiction will want to know the fluid's flash point, fire point, quantity and containment. Requirements under the National Building Code, the National Fire Code and provincial adoptions vary, and some jurisdictions may treat larger quantities of combustible liquid as a special hazard. Early coordination with the fire protection engineer and the AHJ avoids late design changes. See fire suppression interactions with data center HVAC.

Operational considerations

1. Hardware compatibility: Use servers qualified by the vendor for immersion. Fans must be removed or disabled, and some components replaced.
2. Service procedures: Technicians need training, gloves, drip trays and fluid handling procedures. Servers come out wet.
3. Fluid management: Test fluid periodically for contamination, moisture and degradation, and keep make-up fluid on hand.
4. Spill response: Provide containment and absorbent materials and plan for fluid disposal according to provincial environmental rules.
5. Warranty: Confirm that hardware warranties remain valid when equipment is immersed.

Heat reuse

Because immersion captures nearly all heat in liquid at useful temperatures, it pairs well with heat recovery. Return water can preheat domestic water, feed building heating loops through a heat pump, or supply a district system. See data center heat reuse and district heating.

Frequently asked questions

Does immersion cooling eliminate the need for chillers?

Often it does, at least for the IT load. Because facility water can run warm, dry coolers or cooling towers can usually reject heat without mechanical refrigeration. Some designs keep a small chiller or trim cooling for peak summer conditions or for ancillary spaces.

Is immersion suitable for an existing data center?

It can be, but floor loading, fluid handling, layout and fire code review are bigger hurdles than for cold plates. Immersion is often deployed in new pods, dedicated rooms or modular enclosures.

Key takeaways

• Immersion cooling submerges entire servers in dielectric fluid and captures nearly all their heat.
• Single-phase systems pump liquid through a heat exchanger; two-phase systems boil and condense the fluid.
• Two-phase fluids face PFAS-related regulatory and supply uncertainty; confirm long-term availability.
• Facility impacts include heavy point loads, service clearances, fire code review and warm facility water.
• Warm return temperatures make immersion a strong candidate for compressor-free cooling and heat reuse in Canada.