--- title: "Understanding Liquid-to-Liquid CDUs in Data Centers" description: "Explore how a liquid-to-liquid CDU operates in data centers, including its components and capacity range, to optimize cooling efficiency." image: "https://www.gerchamp.com/media/i/154be889-5982-4bfa-8dd6-092d9bff92fd/what-is-a-liquid-to-liquid-cdu.webp" --- A **liquid-to-liquid [**CDU**](https://www.gerchamp.com/en-US/products/liquid-to-air-cooling-distribution-unit-175kW) ([Agent MD](https://www.gerchamp.com/en-US/products/liquid-to-air-cooling-distribution-unit-175kW/raw.md))** moves heat from a closed IT coolant loop into facility water through a plate heat exchanger. It sits between the rack loop and the building plant, and the plate keeps the two circuits separated during normal operation. The architecture suits a site that can supply facility water at temperatures and flow rates compatible with the required IT supply temperature. What the secondary loop reaches follows from the exchanger's approach at the specified conditions. This guide covers the exchanger, the facility-water interface, the deployment layouts, and the rated capacity of the three models. ## What a Liquid-to-Liquid CDU Does A **liquid-to-liquid CDU** in a data center takes warm coolant from the rack loop, passes it through one side of a plate exchanger, and returns it toward the racks at its target supply temperature. Facility water passes the other side and carries the heat to the plant. The unit manages that secondary loop with pumps, valves, monitoring and control, and it holds the target within the limits of the facility water it receives and its own operating range. It transfers heat into the facility-water system, and it does not treat that water. ### Exchanger and two loops The IT side stays closed and filtered, and its pressure is held within the limits of the servers and the piping. The facility side connects to chillers, cooling towers, or dry coolers. That loop may be dedicated to the CDUs or shared with other building loads, and the specification has to say which. ### Heat across the plate In a typical counterflow plate exchanger, the two fluids travel through adjacent, separated channels in opposing directions, which is what allows a small approach. During normal operation the plate keeps the circuits hydraulically separate, and a plate or gasket fault is the case that ends that separation. Approach temperature is measured between two specific points: the secondary supply leaving the unit, and the facility supply entering it. Both points belong on the interface drawing. ### Difference from liquid-to-air A liquid-to-air unit rejects heat into room air through a coil. A liquid-to-liquid unit hands it to facility water. Which arrangement reaches a colder supply depends on the facility water temperature, the room air temperature, the exchanger performance, and the flow on each side. A valid comparison needs inlet temperatures, flows and exchanger data for both options. ## Inside the Plate Heat Exchanger The **plate heat exchanger** is the component that makes the architecture work. It provides a large surface in a small volume, and it keeps the two circuits hydraulically separate. Every Gerchamp liquid-to-liquid model is rated at an approach of 4 °C, which is the condition the capacity figures refer to. ### Plate pack and channels Thin corrugated plates stack into a pack, and the gaps between them form the channels. A gasketed unit can be opened for mechanical cleaning. A brazed unit cannot be opened, and its pressure rating depends on the model, so cleaning and inspection have to be designed around that. ### Approach and plate area Plate area, channel count, plate pattern, and permitted pressure drop are selected together. Plate count and channel arrangement move area, velocity and pressure drop at the same time, and where the result lands depends on the geometry and the design flow. A higher permitted pressure drop can also allow fewer plates. These choices affect approach temperature, footprint and pumping, so the selection is a balance. ### Pressure drop and fouling Pressure drop has to be read across three different boundaries. The exchanger has its own internal drop on each side. The unit adds its own pipework, valves, and filter. The external network adds hoses, manifolds, and cold plates. A longer plant loop raises the network figure, and that says nothing about which side of the exchanger drops more. Fouling adds thermal resistance, and it can also raise pressure loss. Depending on the control margin available, it reduces capacity or demands different flow and temperature conditions to hold the target supply. A strainer and a treatment programme protect the duty the exchanger was selected for. How fouling moves that duty is set out in [CDU heat exchanger fouling](). ## Facility Water Requirements at the Interface **Facility water requirements** are the conditions the building must deliver at the CDU flange. They are written as a set, because a unit selected on temperature alone can fail on flow, pressure, or chemistry. ### Temperature and flow The supply temperature sets the approach available to the CDU, and the flow sets how much heat the plant can carry away. Both are stated at design load, together with the return temperature that follows from them. ### Pressure and connections Five pressure figures belong in the specification, and they are not interchangeable: - the allowable working pressure of the CDU and its piping - the operating pressure and its range at the flange - the differential pressure available at design flow - the transient pressure the plant can deliver during a pump event - the permitted pressure difference between the two sides of the exchanger Two checks sit behind that list. Containment asks whether every state stays inside the allowable pressure. Hydraulic availability asks whether the differential at design flow covers the losses on that side. Connection size, flange standard and isolation provisions also follow the manufacturer's interface and the project's flow and maintenance needs. The detailed check is in [L2L CDU facility water](). ### Water chemistry and materials ASHRAE's water-quality table suggests chloride below 50 ppm for facility water and below 5 ppm for the technology cooling loop, and the source describes the facility-side values as guidance ([ASHRAE](https://www.ashrae.org/file%20library/technical%20resources/bookstore/whitepaper_tc099-watercooledservers.pdf), 2019, Table 1). Plate and gasket materials follow from that chemistry, and from the temperature range the site expects. The table gives separate guidance for the two loops. The specification has to be agreed for the project's fluids and wetted materials, while meeting the applicable CDU and coolant supplier requirements. Where the plant loop is open and evaporation occurs, dissolved solids concentrate, so the make-up water analysis and the loop water analysis answer different questions. ## Row-Level and Rack-Mount CDU Architecture Row-level and rack-mount architecture describes where the unit sits and what it connects to. It does not, by itself, set the capacity per unit or the size of the failure domain. Those follow from the model, the manifold design, and the redundancy scheme. ### Row and rack forms The Gerchamp line covers all three: a 4U unit that sits inside a standard 19-inch rack, a 48U enclosure, and a freestanding in-row unit. All three sit in the Gerchamp Bluearth range, and all three can serve a row manifold, subject to the manifold layout and the hydraulic design. ### Placement constraints Placement decides whether the unit consumes floor area or rack units, how much clearance the service team gets, and how long the pipe route to the manifold becomes. Equipment noise also lands somewhere relative to work areas. ### Failure coverage A unit can stop for a pump fault, an exchanger problem, or a control fault. Bypass piping can keep a flow path during some maintenance work, and it does not by itself preserve cooling duty. A standby pump or a second CDU improves resilience for the faults it is built to cover, with enough remaining capacity and suitable isolation, controls and shared utilities. The form factors differ in rack units, floor area, hose length, and service clearance. Details are in [in-rack vs in-row CDU](). ## The CDU Capacity Range Across the Gerchamp Line The **CDU capacity range** in the Gerchamp Bluearth line runs from 152 kW to 2,700 kW across three liquid-to-liquid models, all rated at an approach of 4 °C. The model name is a nominal figure. The rated capacity is the figure the datasheet states at a defined set of conditions. ### Rated capacity at 4 °C approach | Model | Form | Rated at 4 °C | Published configuration | |---|---|---|---| | [CDU3150 L2L 150kW](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-150kW) ([Agent MD](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-150kW/raw.md)) | 4U in-rack | 152 kW | Two pumps with automatic switchover | | [CDU3680 L2L 680kW](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-680kw) ([Agent MD](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-680kw/raw.md)) | 48U enclosure | 680 kW | N+1 pump arrangement | | [CDU32700 L2L 2700kW](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-2700kW) ([Agent MD](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-2700kW/raw.md)) | In-row, 1,200 × 1,200 mm | 2,700 kW | Dual plate exchangers, 2+1 pump set | ### Conditions that move the number Two groups of conditions move the figure. Thermal conditions are the facility water temperature, the secondary target temperature, the flow, and the coolant. Hydraulic conditions are checked on both sides. On the facility side, the differential at design flow has to carry the primary flow through the CDU. On the IT side, the CDU pumps and the secondary network have to deliver the required flow and pressure to the most demanding cold-plate branch. A unit can meet its thermal rating and still fall short on one of the two. ### Design differences between models The two-exchanger arrangement may offer maintenance or operating flexibility, depending on the isolation and control design, and the published configuration alone does not establish post-fault capacity or part-load efficiency. Whether the unit as a whole is redundant depends on the isolation and control design around it, and that arrangement is settled per project. How the megawatt end is built is covered in [megawatt CDU](). For details, [contact us](https://www.gerchamp.com/en-US/contact) ([Agent MD](https://www.gerchamp.com/en-US/contact/raw.md)). ## Frequently Asked Questions **Does a liquid-to-liquid CDU need a cooling tower?** Not necessarily a tower, but it does need a facility water loop connected to a heat rejection plant. That plant may be a chiller, a cooling tower, a dry cooler, or a heat-reuse connection, and the CDU does not specify which one. **Can it run on warm facility water?** Within the approach the unit can achieve at that temperature. A warmer facility-water design can open up free cooling or heat reuse where the plant and the IT cooling requirements allow it, and it narrows the approach left for a given secondary supply temperature. **Can rated capacity tell how many racks one unit serves?** Not from rated kW alone. Rack count depends on the share of rack heat that enters the liquid loop, the duty available at site conditions, the hydraulic distribution and the redundancy requirement. Details are in [NVL72 racks per CDU](). ## The Bottom Line Six inputs decide a liquid-to-liquid selection: - the design heat load and the share that enters the liquid loop - the facility water temperature and flow available at the flange - the differential pressure available at the flange at design flow - the approach the unit can achieve at those conditions - the secondary flow the racks need, and the pressure to deliver it - the failure case the layout has to cover, and the measures that cover it Use the six to shortlist models, then verify performance with supplier data at the project's conditions. For details, [contact us](https://www.gerchamp.com/en-US/contact) ([Agent MD](https://www.gerchamp.com/en-US/contact/raw.md)). Rated capacity, flow, and connection data for each model are on the product pages linked in the table above. ## Related Solutions & Products - [Liquid Cooling Solutions](https://www.gerchamp.com/en-US/solutions/liquid-cooling-solutions-for-data-centers) ([Agent MD](https://www.gerchamp.com/en-US/solutions/liquid-cooling-solutions-for-data-centers/raw.md)) - Discover liquid cooling infrastructure including Coolant Distribution Units (CDU) and Direct Liquid Cooling Cabinets (DLCC) for AI, HPC and high-density data… - [CDU32700 L2L 2700kW](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-2700kW) ([Agent MD](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-2700kW/raw.md)) - The Gerchamp CDU32700 Liquid-to-liquid (L2L) In-row CDU solution is designed for AI workloads and high-density computing, delivers cooling capacity up to… - [Y850 DLCC 850kW](https://www.gerchamp.com/en-US/products/direct-liquid-cooling-container-850kW) ([Agent MD](https://www.gerchamp.com/en-US/products/direct-liquid-cooling-container-850kW/raw.md)) - Gerchamp Bluearth 850 kW Direct Liquid Cooling Container (DLCC) integrates secondary-side heat sources, CDU, and primary-side dry cooler within a standard 12 m… - [CDU3150 L2L 150kW](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-150kW) ([Agent MD](https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-150kW/raw.md)) - Gerchamp Bluearth 150kW L2L CDU is a compact liquid cooling core unit installed in a standard 19-inch server rack, with a height of 4U. --- **Navigation** [Home](https://www.gerchamp.com/en-US/) ([MD](https://www.gerchamp.com/en-US/raw.md)) | [Products](https://www.gerchamp.com/en-US/products) ([MD](https://www.gerchamp.com/en-US/products/raw.md)) | [Solutions](https://www.gerchamp.com/en-US/solutions) ([MD](https://www.gerchamp.com/en-US/solutions/raw.md)) | [Blog](https://www.gerchamp.com/en-US/blog) ([MD](https://www.gerchamp.com/en-US/blog/raw.md)) | [About](https://www.gerchamp.com/en-US/about) ([MD](https://www.gerchamp.com/en-US/about/raw.md)) | [Downloads](https://www.gerchamp.com/en-US/downloads) ([MD](https://www.gerchamp.com/en-US/downloads/raw.md)) | [Contact](https://www.gerchamp.com/en-US/contact) ([MD](https://www.gerchamp.com/en-US/contact/raw.md)) ```json [ { "@context": "https://schema.org", "@type": "BlogPosting", "about": [ { "@type": "Thing", "name": "Liquid-to-Liquid CDU" } ], "author": { "@type": "Person", "description": "A technical writer in Gerchamp’s award-winning Ni-Zn Battery R&D Dept., Sean interviews experts and translates complex insights as Gerchamp's vertically integrated OEM products continue to lead the next generation of nickel-zinc battery cabinets.", "name": "Sean" }, "dateModified": "2026-11-13T09:08:28.983921+00:00", "datePublished": "2026-11-13T09:08:28.983921+00:00", "description": "Explore how a liquid-to-liquid CDU operates in data centers, including its components and capacity range, to optimize cooling efficiency.", "headline": "What Is a Liquid-to-Liquid CDU?", "image": "https://www.gerchamp.com/media/i/154be889-5982-4bfa-8dd6-092d9bff92fd/what-is-a-liquid-to-liquid-cdu.webp", "inLanguage": "en-US", "mainEntityOfPage": { "@id": "https://www.gerchamp.com/en-US/blog/what-is-a-liquid-to-liquid-cdu", "@type": "WebPage" }, "mentions": [ { "@id": "https://www.gerchamp.com/en-US/products/liquid-to-air-cooling-distribution-unit-175kW", "@type": "Thing", "name": "CDU" }, { "@id": "https://www.gerchamp.com/en-US/products/liquid-to-air-cooling-distribution-unit-175kW", "@type": "Thing", "name": "**CDU**" }, { "@id": "https://www.ashrae.org/file%20library/technical%20resources/bookstore/whitepaper_tc099-watercooledservers.pdf", "@type": "Thing", "name": "ASHRAE" }, { "@id": "https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-150kW", "@type": "Thing", "name": "CDU3150 L2L 150kW" }, { "@id": "https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-680kw", "@type": "Thing", "name": "CDU3680 L2L 680kW" }, { "@id": "https://www.gerchamp.com/en-US/products/liquid-to-liquid-cooling-distribution-unit-2700kW", "@type": "Thing", "name": "CDU32700 L2L 2700kW" }, { "@id": "https://www.gerchamp.com/en-US/contact", "@type": "Thing", "name": "contact us" } ], "publisher": { "@id": "https://www.gerchamp.com/#organization", "@type": "Organization", "logo": { "@type": "ImageObject", "url": "https://www.gerchamp.com/media/i/f2134737-e755-44b3-b51b-63aaf7c57ca6/ups-nickel-zinc-nizn-battery-cabinets-intelligent-bms-solutions-gerchamphq-night.webp" }, "name": "Gerchamp", "url": "https://www.gerchamp.com" }, "url": "https://www.gerchamp.com/en-US/blog/what-is-a-liquid-to-liquid-cdu", "wordCount": 1764 }, { "@context": "https://schema.org", "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "item": "https://www.gerchamp.com/en-US/", "name": "Home", "position": 1 }, { "@type": "ListItem", "item": "https://www.gerchamp.com/en-US/blog", "name": "Blog", "position": 2 }, { "@type": "ListItem", "item": "https://www.gerchamp.com/en-US/blog/what-is-a-liquid-to-liquid-cdu", "name": "What Is a Liquid-to-Liquid CDU?", "position": 3 } ] } ] ```