Liquid Cooling_
Rear-Door Heat Exchanger (RDHx) Install & Facility Water Tie-In
A step-by-step field guide for installing rear-door heat exchangers on GPU racks and tying them into facility water loops, covering mechanical mounting, water connections, condensation prevention, and commissioning tests.
Key facts
- RDHx units typically reject the majority of the rack heat load (often 60–80% depending on design), reducing demand on facility cooling.
- Facility water supply temperature must be above the rack dew point to prevent condensation; typical setpoints range from 18–22°C depending on ambient humidity.
- Water flow rate per RDHx is determined by the heat load and desired ΔT per ASHRAE guidelines; for example, a 40 kW rack might require 8–12 GPM at a 10°C ΔT.
- Condensate drain lines must be sloped at least 1/4 inch per foot (per standard plumbing codes) and routed to a floor drain or dedicated condensate pump.
- RDHx units are mounted using the rack’s threaded mounting rails; torque for M6 screws is typically 5–7 Nm per OEM spec to avoid stripped threads.
- Pressure testing of the water loop is performed at 1.5× the design pressure for 30 minutes with no measurable drop, per ASHRAE guidelines.
- Dielectric unions are required at the rack-to-facility water interface to prevent galvanic corrosion between copper piping and stainless steel RDHx connections.
Mechanical Mounting and Rack Preparation
Before any water line is touched, the rack must be leveled and anchored. Use a precision level on the rack frame; shim the base if needed. The RDHx unit is heavy—typically 80–120 kg—so a lift cart or overhead hoist is mandatory. Remove the rear door of the rack (if present) and attach the RDHx mounting brackets to the rack’s vertical rails using M6 screws. Torque to the OEM spec (typically 5–7 Nm) using a calibrated torque wrench. Overtightening strips threads in aluminum rails, a common field failure.
Once the brackets are secure, lift the RDHx onto the brackets and slide it into the rack’s rear opening. The unit must sit flush against the rack frame. Secure it with the provided locking pins or screws. Verify that the RDHx does not obstruct any rear cable management or power distribution. If the rack has a rear door that must close over the RDHx, check clearance—some units add 150–200 mm to rack depth. Always confirm with the rack OEM that the combined depth fits within the data center aisle constraints.
Facility Water Tie-In and Piping
The facility water supply and return lines must be terminated at the rack location with isolation ball valves and a drain port. Use dielectric unions at the connection point to prevent galvanic corrosion between copper facility piping and stainless steel RDHx connections. Connect flexible braided hoses from the RDHx to the facility lines; these hoses absorb vibration and allow for minor misalignment. Flow direction is critical—supply goes to the bottom port, return from the top port, to ensure full coil coverage and prevent air pockets.
After all connections are made, open the supply valve slowly to fill the RDHx coil. Purge air from the system using the manual vent valve at the top of the RDHx. Once water flows steadily from the vent, close it. Check for leaks at every joint with a dry paper towel. Then pressurize the loop to 1.5× the design pressure (per ASHRAE guidelines) using a hand pump and hold for 30 minutes. A pressure drop greater than 5% indicates a leak that must be found and repaired before proceeding.
Condensation Management and Dew Point Control
Condensation is the single biggest operational risk with RDHx. The facility water supply temperature must always remain above the rack’s ambient dew point. Measure dew point at the rack inlet using a handheld psychrometer or the facility BMS. A typical safe margin is 2–3°C above dew point. If the supply water is too cold, condensation will form on the RDHx coils and drip onto servers. Install a condensate drip tray under the RDHx with a drain line sloped at least 1/4 inch per foot to a floor drain or condensate pump. Test the drain by pouring water into the tray and verifying it flows freely.
For high-humidity environments (dew point > 20°C), consider adding a condensate pump with a high-level alarm. Some RDHx units have built-in humidity sensors that can trigger an alarm or shut off the water valve if condensation risk is detected. Commission the condensation detection system by simulating a high-humidity condition (e.g., using a wet cloth near the sensor) and verifying the alarm triggers in the BMS.
Flow Balancing and Pressure Testing
Each RDHx in a row must be flow-balanced to ensure even cooling. Use a calibrated flow meter or the RDHx’s built-in flow indicator. Adjust the balancing valve on the return line until the flow matches the design value (e.g., 10 GPM for a 40 kW rack). If multiple RDHx units are on the same branch, balance them one at a time, starting with the unit farthest from the supply. Record the final flow and pressure drop for each unit in the commissioning report.
After balancing, perform a final pressure test at the system level. Close the isolation valves at each rack and pressurize the facility loop to the design pressure. Check for leaks at all joints, especially the dielectric unions and hose connections. A common failure is a loose compression fitting on the braided hose—tighten to the OEM torque spec, not by feel. Document all test results per ASHRAE Standard 90.4 or the facility’s commissioning plan.
Common Failure Modes and How to Catch Them
The most frequent field failure is condensation due to supply water temperature set too low, often because the chiller temperature is set without checking the actual dew point at the rack. Always measure dew point at the rack inlet before commissioning. Second: air pockets in the RDHx coil reduce heat transfer and cause flow alarms. Purge air thoroughly during fill and listen for gurgling sounds. Third: overtightened fittings on dielectric unions can crack the plastic insulator, leading to slow leaks. Use a torque wrench set to the OEM spec (typically 20–30 Nm for 1-inch unions).
Another common issue is condensate drain blockage from debris or improper slope. Test the drain by pouring water into the tray during commissioning. If the drain line is too long or has too many bends, install a condensate pump. Finally, galvanic corrosion at the dielectric union can occur if the union is installed backward or the gasket is missing. Inspect union orientation before pressurizing. Catching these issues during commissioning saves hours of troubleshooting later.
Commissioning Checklist and Documentation
A thorough commissioning checklist ensures nothing is missed. Verify: rack level and anchored, RDHx mounted with correct torque, water connections with dielectric unions, supply/return flow direction, air purged, pressure test passed, flow balanced to design value, condensate drain tested, dew point measured and supply temperature set above it, and all alarms functional. Document each step with photos of the connections and the flow meter reading. Leviathan Systems uses a digital commissioning app that timestamps each check and stores it in the project record.
After commissioning, label each RDHx with the rack ID, flow rate, and supply/return temperature setpoints. Provide the facility team with a summary of the dew point margin and any alarms configured. This documentation is critical for ongoing operations and troubleshooting. A well-documented install reduces mean time to repair by giving the facility team clear baselines.
Standards referenced: ASHRAE Standard 90.4 (Energy Standard for Data Centers) · ASHRAE TC 9.9 (Liquid Cooling Guidelines) · ISO 14644-1 (Cleanroom standards for particulate control, applicable to water quality) · NFPA 75 (Standard for the Fire Protection of Information Technology Equipment)
Frequently asked_
What is the minimum water temperature I can safely use with an RDHx?
The minimum water temperature is determined by the dew point at the rack inlet. You must maintain a margin of at least 2–3°C above the dew point to prevent condensation. For example, if the dew point is 18°C, set the supply water to 20–21°C. In practice, most facilities run supply water in the 18–22°C range. Always measure dew point on-site with a psychrometer, not from a remote sensor.
Can I install an RDHx on a rack that already has servers installed?
Yes, but you must power down the servers first to avoid thermal shock and to safely access the rear of the rack. The RDHx adds significant weight and depth, so verify the rack’s load capacity and floor loading. Also, ensure there is enough clearance for the RDHx to open or swing out for service. Leviathan Systems typically installs RDHx units on empty racks before server deployment to avoid downtime.
How do I test for leaks in the water loop without damaging the servers?
Perform a dry pressure test before any servers are installed. Pressurize the loop to 1.5× the design pressure with compressed air or nitrogen and hold for 30 minutes. Use a soap solution on all joints to check for bubbles. If the pressure holds, then fill with water and repeat the test. Only install servers after the water loop passes both tests. Never pressure test with water above the rack if servers are present.
What should I do if I see condensation on the RDHx during operation?
Immediately raise the supply water temperature by 2–3°C and check the dew point at the rack inlet. If condensation persists, inspect the condensate drain for blockage and ensure the drip tray is properly sloped. In extreme cases, you may need to install a condensate pump or add a humidity sensor that can shut off the water valve. Document the incident and adjust the facility water setpoint permanently.
How often should the RDHx coils be cleaned?
Cleaning frequency depends on the data center’s air quality. In a typical environment with MERV-13 filters, clean the coils every 12–18 months. Use a soft brush or low-pressure compressed air to remove dust from the fins. Avoid water or chemical cleaners unless the OEM approves them, as residue can corrode the aluminum fins. Leviathan Systems recommends scheduling coil cleaning during server maintenance windows.