Direct-to-Chip Liquid Cooling and CDUs
Direct-to-chip cooling carries heat from cold plates to a coolant distribution unit. Flow, temperature, pressure and coolant quality all matter to the design.
TL;DR: A cold plate transfers heat from a chip into a liquid loop. A CDU can separate that controlled loop from facility water. For water carrying 120 kW with a 10 K temperature rise, the heat balance gives about 172 liters per minute. The installed system must also meet the vendor’s pressure, temperature, coolant and redundancy requirements.
A CDU (coolant distribution unit) pumps the rack's coolant loop and passes its heat to the building's water through a heat exchanger, so the two liquids never mix.
Estimate flow from the heat balance
heat carried by a liquid loop
Q = m_dot x c_p x delta_T
Q heat removed, in watts
m_dot mass flow rate, kg/s
c_p specific heat of water, about 4,180 J per kg per K
delta_T temperature rise from supply to return, in K
solve for flow at a 120 kW rack and a 10 K rise
m_dot = 120,000 / (4,180 x 10) = 2.87 kg/s
water is about 1 kg per litre, so 2.87 L/s = 172 L/min
the same rack at other temperature rises
delta_T = 5 K: 344 L/min more pumping power, gentler on components
delta_T = 10 K: 172 L/min the common design point
delta_T = 20 K: 86 L/min less flow, hotter return, tighter margins at the chip
sanity: halving the flow doubles the temperature rise for the same heat, so flow and delta_T
trade directly, and the chip's maximum inlet temperature is what bounds the choice
cross-check against the published figure
120 kW x 3,412 BTU/hr per kW = 409,440 BTU/hr
this is a unit conversion, not an independent measurement of cooling performance
The two loops, and why they are separate
Cold plates collect heat on the rack side. The CDU passes that heat to the facility side for rejection outdoors.
The paired lines represent each loop’s supply and return. The divider inside the CDU marks the separation between fluids.
Rack loop
- Controlled coolant through cold plates and the rack manifold.
- Check flow, temperatures, pressure and leak signals.
Facility loop
- Building water carries the transferred heat to the plant.
- Supply conditions must stay within the CDU and system requirements.
The separation exists so that the fluid touching a cold plate is one the operator controls. Facility water carries whatever the building's loop carries: particulates, dissolved minerals, and biology. A cold plate has channels measured in fractions of a millimetre and fouls quickly. The CDU's plate heat exchanger moves the heat across without mixing the two, and everything on the rack side is treated, filtered and monitored.
What the requirements list actually contains
| Requirement | Typical figure | Why it exists |
|---|---|---|
| CDU capacity | vendor-rated load with required margin and redundancy | Margin for peak draw and for fouling as the loop ages |
| Facility supply temperature | within the equipment’s supported range | Warm water can often be made with dry coolers, avoiding chiller power |
| Filtration | vendor-specified rating | Use the cold-plate manufacturer’s filtration requirement |
| Conductivity monitoring | continuous | A change can indicate contamination; interpret it against the coolant specification |
| Corrosion inhibitor | per the coolant specification | Mixed metals in the loop corrode without it |
| Leak detection | rack and row level | The failure that takes out hardware rather than just performance |
| Quick disconnects | dripless, rated for the cycle count | Every node service event opens and closes them |
The warm-water point is the one that surprises people coming from air-cooled halls. Whether supply water needs refrigeration depends on outdoor conditions and the required approach temperature. A 35-degree supply can sometimes use dry coolers, which is why liquid cooling can lower total facility power even though it adds pumps.
The first minute of a cooling failure
Loss of flow or a leak can require a rapid automatic response. The available reaction time depends on heat load, coolant volume, thermal mass and the equipment’s protection behavior. Do not assume there is enough time to save a checkpoint before shutdown.
Monitor flow, pressure, temperatures and leaks at the relevant loop and device boundaries. Follow the supported control sequence for power reduction, isolation and shutdown, then test that sequence under an approved commissioning procedure. Thermal, Power and Cooling Events covers how to connect those signals to affected workloads.
What interviewers are listening for
Derive the flow estimate from heat load, specific heat and temperature rise, then state its limits. The calculation does not size pumps or cold plates by itself. Explain how inlet temperature, pressure drop, coolant compatibility and failure response constrain the final design.
Key takeaways
- The heat balance is Q = mass flow x specific heat x temperature rise.
- For water at a 120 kW heat load and a 10 K rise, the estimate is 2.87 kg/s, or about 172 liters per minute.
- A liquid-to-liquid CDU transfers heat while keeping the facility and rack fluids separate.
- Warmer supply water can reduce refrigeration needs when equipment limits and outdoor conditions allow it.
- Use the equipment vendor’s coolant, filtration and monitoring requirements; there is no single specification for every loop.