A liquid-to-air CDU moves heat from a closed IT coolant loop into room air through an integral coil and fan set. It sits beside or inside the row, and it takes no facility-water branch.
The architecture is common in retrofits, in halls that were never designed for liquid cooling, and in sites where plant water is unavailable or expensive to bring in.
This guide covers the heat path, the airside limits, the retrofit fit, and the site checks that come before delivery.
What a Liquid-to-Air CDU Does
A liquid-to-air CDU in a data center takes warm coolant from the rack loop, passes it through a coil, and returns it at setpoint temperature. Fans move room air across that coil, and the room’s air handling carries the heat away.
The unit holds one closed liquid circuit that reaches the cold plates. Its second medium is room air.
Coil and fan set
The coil provides the surface, and the fan set provides the airflow across it. Coil face area, fin spacing, and fan curve together set how much heat leaves at a given air temperature and fan speed. Fin spacing trades surface area against cleanability, and that choice shows up later in maintenance intervals.
The closed liquid circuit
On the liquid side the unit works like any other CDU. A pump set circulates coolant through a filter, the coil, and the external loop that reaches the cold plates. The loop stays closed and pressurised. Gerchamp’s CDU4175 L2A 175kW is specified for a secondary side below 2 bar. That field sets the containment limit. The control setpoint is a separate setting. How the two loops divide the work is set out in L2A CDU architecture: two loops, two media.
Differences from liquid-to-liquid
A liquid-to-liquid unit hands heat to facility water through a plate exchanger. A liquid-to-air unit rejects it to the room. That single change removes the facility-water branch to the CDU and its treatment programme, and it adds an airside load to the hall. The IT loop still needs coolant management, hose and coupling connections, fill and venting.
The two paths differ in the water connection, the airside load, and the retrofit work.
Room Air Heat Rejection Through the Coil
Room air heat rejection is the step where the coolant gives up its heat to the air that passes the coil. The air then carries that heat to the return path and out of the hall.
The unit moves heat into the room. It does not remove it from the building.
Heat path through the coil
Warm coolant enters the coil header and distributes across the tubes. Air passes the fins, picks up heat, and leaves warmer. Coolant leaves the coil at the supply temperature the controller targets. Normal operation holds the coolant supply above the room dew point, so the coil stays dry. Condensation becomes a risk during a cold start or an abnormal low-temperature event, and a drain with a dew-point alarm covers that case.
Airflow and coil surface
Capacity responds to airflow and coil surface, and the response is not free. More airflow raises fan power and noise. A denser coil transfers more heat per unit of face area and adds air pressure drop, so the fan has to work harder. How much airflow to use and how dense to build the coil is a selection decision, and the airflow figure itself is worked through in L2A CDU airflow.
Removal by the hall system
The room’s cooling system takes the heat from the air. That system may be a CRAC unit, a CRAH loop, or a larger air handling arrangement. Its spare capacity and its return path decide whether the added load stays inside the design envelope.
Airside Capacity Limits in a Real Hall
The airside capacity of a liquid-to-air CDU is the heat it can move at a stated room temperature, airflow, and coolant condition. Catalogue numbers describe a point inside that envelope.
A rating taken at 35 °C room air holds at that temperature. In a cooler hall the gap between coolant and air widens, so the same unit carries a little more, and the figure for that hall is calculated at its air state.
Rating conditions
Liquid-to-air ratings use the room air temperature as the reference for approach temperature. The published table for Gerchamp’s CDU4175 L2A 175kW lists 175 kW at an approach of 10 °C and 125 kW at 5 °C, with a 35 °C room condition. The full test conditions behind each figure, covering coolant, flow and fan state, follow the project. The conditions depend on the project. For details, contact us.
Hall airflow and return path
The coil can only reject what the room air can absorb. The risk is exhaust air returning to the coil inlet, or a blocked path around the unit. Either one raises the local inlet temperature, and capacity falls. A hall average hides the effect because it is local, so airflow and temperature are measured at the unit inlet.
Stratification and dead zones
Warm air collects at high points and behind obstructions. A unit placed in a dead zone sees warmer inlet air than the hall average, which reduces its effective capacity. Placement is a capacity decision as much as a layout decision.
Where L2A Fits in a Retrofit
Where L2A fits depends on the hall and on the airside load it can still take. A site with airside headroom can use the architecture even where facility water exists, because the tie-in window, the distance to the plant, or a pilot phase can rule out a water connection.
A site without headroom cannot use it, whatever its water situation.
Retrofits without plant water
A site with no facility-water loop avoids new pipework, water treatment, and a plant shutdown. The CDU is positioned, connected to power and controls, and commissioned against the room’s existing air system. Where that route pays off is covered in sidecar CDU advantages.
Mixed density and heat load
Liquid-cooled racks and air-cooled racks can share a room. The liquid-cooled racks take the dense load, and the air side continues to serve standard equipment. This pattern is common in the first phase of a retrofit. Start with the racks that cannot be air-cooled, then add liquid capacity as the mix changes.
The airside load is not only the new racks. Five items belong in that calculation:
- heat captured by the liquid loop, which the CDU returns to the room
- IT heat the liquid loop does not capture, which stays in the room
- heat from the CDU’s own power draw, inside the same boundary
- the net change between added racks and any racks removed
- the hall margin at design ambient and at the required redundancy state
Limits of the fit
Two different constraints get confused here. An airside shortfall is a cooling problem, and it calls for added air handling capacity or a different architecture. A floor-loading shortfall is a structural problem, and it calls for a different position, additional support, or a structural review. Neither one fixes the other.
L2A Installation Requirements on Site
L2A installation requirements are mostly physical checks. The unit needs air, power, controls, clearance, and a floor that can carry it.
These checks belong in the site survey, before the delivery date is fixed.
Airside checks
Confirm the airflow available at the intended position, the return path, and the local ambient at full load. Check that the coil inlet is not in a recirculation zone. Where the hall uses containment, confirm the unit sits inside the intended air path. The full checklist is in L2A CDU installation requirements.
Power and controls
Confirm the supply capacity for pumps and fans, the earthing arrangement, and the control interface. The unit runs locally without a BMS link. Remote monitoring is a project requirement, and where the site wants it the controller connects over Modbus, SNMP, or BACnet. Protocol and point list belong in the specification, and a DCIM platform needs its tags agreed before delivery.
Placement and floor loading
Check the footprint, the service clearance, and the route into the hall. Three figures answer different questions. Gerchamp’s CDU4175 L2A 175kW occupies 900 × 1300 × 2300 mm and stands 52U high. Its assembly weight is 485/501 kg. Its published wet floor load is 625 kg/m². The structural check uses the floor load and the point load at each foot or castor, and neither follows from the assembly weight. Noise also belongs in the placement check, since fan speed at full load sets the sound level near work areas.
| Check | What to confirm |
|---|---|
| Airside | Available airflow, return path, local ambient at full load |
| Power | Supply capacity, protection, earthing |
| Controls | BMS protocol, point list, alarm mapping |
| Placement | Footprint, service clearance, rigging route |
| Floor | Wet floor load, point load, structural allowance |
| IT liquid circuit | Coolant and material compatibility, hose and coupling range, fill and vent |
| Liquid duty | Design flow and available external pressure at the racks |
| Pressure | Design pressure on the secondary side, test requirement |
| Water | No facility-water branch for the CDU itself |
Frequently Asked Questions
Does a liquid-to-air CDU need outdoor equipment? The CDU itself needs no outdoor unit and no facility-water connection. The room cooling system that absorbs its heat still rejects that heat from the building, through chillers, dry coolers, or an air handling plant.
Can it run in a warm hall? Yes, within the rating envelope. Capacity falls as room air temperature rises, because the approach between coolant and air narrows. Check the capacity at the site’s design ambient, and confirm the coolant supply temperature stays above the room dew point. The conditions depend on the project. For details, contact us.
How much floor space does one take? A 175 kW class sidecar unit occupies roughly 1.2 m² of footprint at 900 × 1300 mm, before service clearance. Clearance for doors, filters, and coil access adds to that figure. How a sidecar of this class is deployed in an AI hall is covered in a 175 kW sidecar CDU for AI halls.
The Bottom Line
Five checks decide a liquid-to-air installation:
- the airside headroom in the hall at design ambient
- the return path and the local airflow at the unit position
- the power and control interfaces
- the floor loading and the rigging route, with the wet floor load separate from the assembly weight
- the liquid-side connection, design flow, and pressure at the racks
With those five answered, the capacity figure applies to the design, once the liquid side matches the same flow and temperature basis.
For the full rating table at both approach temperatures, see Gerchamp’s CDU4175 L2A 175kW product page.