CoolSim models the air side of a liquid-cooled room. For each rack you tell it how much of the heat goes into liquid, and it keeps only the rest in the room air. It does not simulate the coolant loop: coolant temperatures and flow rates, CDU capacity, pipework pressure drop, facility water temperature, or what happens when a pump fails. Size those with your equipment vendor’s tools.
Getting the air side right depends on where the liquid picks up its heat. This page covers the common cases.
The Three Rack Inputs
Open the rack’s editor and go to the Specifications tab:
- Heat Load Per Rack: the full IT load of the rack, liquid and air together.
- % Heat rejected by liquid: the share of that load that leaves in liquid.
- Expected Temperature Rise: how much the air warms up as it passes through the rack.
CoolSim works out the rack’s airflow from these: the heat left in the air, divided by the temperature rise. Flow Rate Per Rack shows the result; you cannot type into it. With Individual Rack Specification checked, the same three inputs are set rack by rack.
Direct-to-Chip Cold Plates
Cold plates carry most of the heat away in coolant, and the server fans still move air across everything else.
- % Heat rejected by liquid: the liquid share from the server vendor’s data, typically 85–90% for current AI racks.
- Expected Temperature Rise: the air temperature rise across the servers in their liquid-cooled configuration, from the same data. Don’t reduce it to account for the liquid; the percentage already does that.
Rear-Door Heat Exchangers
A rear door is a water coil on the back of the rack. The servers heat the air as usual and move their full airflow; the door then takes a share of the heat out of the exhaust before it re-enters the room.
Entering the door’s share and the servers’ temperature rise gets this wrong: CoolSim would size the airflow for the heat left after the door, as if the servers had slowed their fans. Enter it this way instead:
- % Heat rejected by liquid: the door’s share of the rack’s heat, at your water temperature and server airflow.
- Expected Temperature Rise: the servers’ temperature rise × (1 − the door’s share).
For example, a 30 kW rack whose servers warm the air by 20°F (11°C), with a door that removes 80%:
| Entered | Rack airflow | Exhaust above inlet |
|---|---|---|
| 80% and 4°F (2.2°C) | about 4,700 CFM | 4°F |
| 80% and 20°F | about 950 CFM | 20°F |
The first row is what the rack does. The second models one fifth of the air the rack pulls from the cold aisle, and that hides the air starvation dense rows are prone to.
CoolSim needs some heat in the air to size the airflow. If the door is rated to take all of it, enter 95%: the 5% left in the room errs on the warm side.
Cold plates and a rear door on the same rack: enter the cold-plate share plus the door’s share of the remainder (85% on cold plates and an 80% door: 85 + 0.8 × 15 = 97%), and the servers’ temperature rise × (1 − the door’s share).
Liquid-to-Air CDUs
A liquid-to-air CDU (or a sidecar heat exchanger) cools the rack’s coolant with room air, so the liquid’s heat ends up back in the room. The rack inputs above take that heat out of the room, so add it back:
- Enter the racks as above, with their liquid share.
- Place an HDU where the CDU stands. The HDU is the object CoolSim uses for PDUs, UPSs and other equipment that gives off heat.
- Set its Heat Load to the liquid heat of every rack it serves, and its DeltaT and flow direction to match the unit’s datasheet.
Leaving the HDU out removes most of the room’s heat. Eight 100 kW racks at 80% liquid put 640 kW back into the room through their CDUs; without the HDUs, the model carries 160 kW.
A liquid-to-liquid CDU sends the heat to facility water. Only its pump and casing losses reach the room. If the datasheet gives them, add them as an HDU in the same way.
Immersion Tanks
CoolSim does not model the tank or the coolant. What the room sees is the heat the tank loses to the air. Place an HDU the size of the tank, set to Up Flow, with Heat Load equal to the tank’s loss to the room from the vendor’s data, not the IT load.
Don’t enter an immersion tank as a rack at 100% liquid. With no heat in the air, CoolSim has nothing to size the rack’s airflow from.
Checking the Results
The report shows the heat taken by liquid for each rack and in total (Total Heat Rejected by Liquid Cooling). The rack heat load totals include the liquid share.
The heat removed by the cooling units should closely match the heat left in the air: the rack heat load, minus the heat rejected by liquid, plus any HDUs. If the two are far apart, don’t base a decision on the run; ask support to look at it.
Summary
| Equipment | % Heat rejected by liquid | Expected Temperature Rise | Also add |
|---|---|---|---|
| Direct-to-chip cold plates | Vendor’s liquid share | Servers’ air rise | |
| Rear-door heat exchanger | Door’s share (at most 95%) | Servers’ rise × (1 − door’s share) | |
| Liquid-to-air CDU | Rack’s liquid share | Servers’ air rise | HDU at the liquid kW it serves |
| Liquid-to-liquid CDU | Rack’s liquid share | Servers’ air rise | HDU at its losses, if known |
| Immersion tank | Not a rack | HDU at the tank’s loss to the room |
For questions about a specific configuration, contact our engineering support team.