What is the Liebert XD?
Liebert XD is a high heat density cooling solution that offers solutions utilizing Open or Closed Architecture, Water-based or Pumped Refrigerant-based technology, and a wide range of equipment configurations.
- Open architecture systems utilize cooling coils near the heat load, either inside or outside the open server rack, and utilize the room air volume as a thermal storage to ride through short power outages.
- Closed architecture systems fully enclose the rack with the cooling coils inside. Other provisions are required for power-loss ride-through.
- Pumped Refrigerant technology eliminates the presence of water in a data center. It operates at low pressure in the piping system and becomes a gas at room temperatures, if a leak would occur. It utilizes micro-channel coil efficiency and low pressure drop.
- Water-based systems offer a cooling alternative where electrical hazards are minimal or the use of water is not a concern. They are also designed to work in any size space from a small computer room to a large data center.
The Liebert XD is further described in the Liebert XD Brochure (SL-11265).
Which is the most energy efficient solution within the Liebert XD product family?
This depends a lot on site specific conditions. In general, cooling solutions that take the cooling fluid closer to the heat load are more efficient. Therefore, a Closed architecture solution is typically more energy efficient than an open architecture solution.
However, a solution using the Pumped Refrigerant technology is more energy efficient than a solution using water-based technology. The reason for this is that the phase changing of the fluid that takes place with the pumped refrigerant allows the fans in the evaporator modules and the circulating pumps to be smaller.
Which Liebert XD cooling module should I use for a Data Center?
This depends on site specific conditions. Liebert XDO, XDV and XDH all use the Pumped Refrigerant technology and they are designed for the hot aisle/cold aisle principle.
Liebert XDO is mounted above the cold aisle, on rods supported in the ceiling. It has a nominal capacity of 20kW (at 55ºF entering fluid temperature, EFT, and 92ºF entering air temperature, EAT). If XDOs are combined with XDVs, they can cool more than 20kW per rack. Since the XDO module is located further away from the racks with heat load than the XDV and XDH, it is rated at 92ºF entering air temperature.
The XDV is mounted on top of the rack or suspended on rods in the ceiling. The XDV has a nominal capacity of 10kW (at 55ºF EFT and 98ºF EAT) and can cool racks with more than 20kW heat load if the XDVs are double stacked.
The XDH is mounted on the floor, between the racks, and the high capacity version has a nominal capacity of 30kW (at 55ºF EFT and 98ºF EAT). Therefore, it can cool racks with more than 30kW heat load if each XDH has only one rack to cool.
In addition to the XDO, XDV and XDH modules, the Liebert XD CoolFrame module is available. It also uses the Pumped Refrigerant technology and is specially designed to attach directly to the rear of the Egenera BladeFrame EX system. It allows these racks to be placed in a data center environment around other heat-generating equipment without adding to the heat load in the room.
For Water-Based systems, the Liebert XDR-W (Rack Cooler) is available. It has a nominal capacity of 8kW (at 55ºF EFT and 100ºF EAT) and is attached to the back door of the enclosure. Fans in the module move air from the equipment in the rack, through a cooling coil and expel it from the back of the unit, chilled to the point where the impact on the room is close to neutral.
Can the cooling modules in Liebert XD be combined in an installation?
Yes, they can be combined so they are connected to the same piping circuit supported by an XDP/XDC. Combining XDO with XDV is recommended.
Since the XDH modules have a horizontal air flow pattern, and the XDV and the XDO have a vertical air flow pattern, supplying cold air down into the cold aisle, it is typically not recommended to combine XDH with XDV or XDO.
When do I need an XDC or an XDP?
If a building chilled water system is available, the Liebert XDP could be used. It serves as an isolating interface between the building chilled water system and the pumped refrigerant circuit.
If a building chilled water system is not available, a chiller, Liebert XDC, is required. It connects directly to the Liebert XD Cooling Modules and provides chilled pumped refrigerant circulation and control.
Both the XDP and the XDC ensure that the refrigerant supplied to the modules is constantly above the actual dew point in the room, eliminating concern about condensation.
Why is Liebert XD called Supplemental High Heat Density Cooling?
The Liebert XD system is doing sensible cooling (the temperature of the cooling fluid supplied to the cooling modules is always above the actual dew point in the room so no condensation can occur on the coils in the modules or on the piping) to match the high heat density load. It supplements the precision cooling equipment which typically still is required for humidity control and air filtration.
When do I need to use a high heat density cooling solution?
A high heat density cooling solution is deployed to extend the cooling beyond the limits of traditional solutions.
The limit for traditional solutions capability is typically depending on the air distribution to the racks and there are many factors to consider; raised floor/plenum/duct height, how air tight the raised floor/plenum/ducts are, obstructions in the raised floor/plenum/ducts, layout of the perforated tiles/vents, location of the CRAC units, hot-cold aisle layout, etc. In general, it is recommended that a high heat density cooling solution is considered when the heat load per rack is more than 5kW; the user is currently experiencing spot or zone heat problems; using or planning to use blade servers or other high density servers; or is losing data center floor space by spreading out the heat load.
What are the alternatives to installing a dedicated high heat density cooling solution?
If the heat loads in the racks are higher than current cooling system can handle, there are a few basic options; spread out the load; add traditional cooling capacity; and/or optimize current infrastructure (see separate question below).
- Spreading out the heat load assumes that floor space is available and cabling and other related issues can be handled.
- Adding traditional cooling capacity also assumes that floor space is available and that distribution of the additional cold air is possible in an efficient way. Therefore, adding cooling capacity often also requires additional actions (see separate question “What can I do to accommodate high heat…”).
What can I do to accommodate high heat density loads in my data center?
If the cooling capacity is enough for the load in the room, the air distribution could possibly be improved as follows. Make sure that:
- the raised floor plenum is sealed at cable penetrations and along the walls
- the room is sealed (doors, windows) and has a good vapor barrier
- hot aisle/cold aisle layout is used for the racks
- blanking panels are being used in the racks (if they are not filled completely) and between the racks (if there are empty spots in the row) so no bypass of hot and cold air inside the rack, or between the racks, is possible
- the raised floor (or the plenum/ducts) is as clean as possible and no cables or piping are obstructing the air flow
- the Computer Room Air Conditioning (CRAC) units are installed at the end of the hot aisle so the possibility of mixing the hot air and the cold air is minimized
- the perforated tiles are located where needed. The best location of the perforated tiles depends on many site specific factors. CFD (Computational Fluid Dynamics) can be used to optimize the perforated tile locations
- the possibility to utilize the overhead plenum for hot air return is reviewed. Since this will give a higher return air temperature to the CRAC unit, it is important to verify that the CRAC unit is sized for this air temperature.
A professional Site Assessment can be a good way to identify, and document, opportunities for improvements.
Should I use a water based technology or a pumped refrigerant technology in my high heat density cooling system?
Each system has its advantages. In general, the pumped refrigerant allows the modules to be mounted over head, the piping to be routed above the racks without drip pans, does not interfere with the electronic equipment in case of a leak, is more energy efficient and also more space efficient than water. However, water costs less than refrigerant, is space and energy efficient and the water based technology does not have any limitations to room size.
What is the difference between Open and Closed architecture and which architecture is recommended?
Each architecture has its advantages.
In general, an Open architecture allows the room to be a heat sink (for ride through during failures or power loss), does not limit the rack selection, has a less complex emergency mode and allows the racks to “borrow” cold air from each other.
A Closed architecture solution has low audible noise, is deployable as a single rack, minimizes the air circulation in the room and does not require hot/cold aisle arrangement.
How can I design a Liebert XD high heat density cooling solution with built in redundancy?
Redundancy can be designed into a solution utilizing the open architecture version of Liebert XD by combining CRAC units and interlaced pipe connection between the cooling modules and the supporting XDP/XDC. The interlaced connection means that cooling modules next to each other are connected to different XDP/XDC units. For the XDH, which has dual piping circuits, this interlacing can even be done within each module.
What is the TCO for Liebert XD?
The TCO for a cooling solution depends a lot on site specific factors and has to be calculated in each specific case.
A Liebert XD solution typically has a little higher first cost (equipment and installation) than traditional solutions. However, continuing costs (energy and maintenance) are typically lower so the calculated pay back period is short. If the floor space savings (opportunity costs) for Liebert XD is considered in the calculation, the pay back period is even shorter and in some cases immediate.
It is assumed that the salvage value is the same for both alternatives above.
How much time do I have until the servers start to shut down in case of a cooling system failure?
For a closed architecture cooling solution, how fast this situation might occur depends in general on the load in the rack, the thermal mass in the rack and the heat exchange with the surroundings (leaks). In extreme situations, an over temperature situation can occur within seconds.
For an open architecture solution, similar factors have an impact on how fast this situation would occur. However, since the room works as a heat sink, it will typically take minutes until an over temperature is reached. Full scale tests that Liebert have done together with Hewlett Packard showed that 104ºF entering air temperature to the server (which is the temperature when most servers start to shut down) was reached after 23 minutes at 3.5kW per rack heat load in the room. At 7kW per rack heat load, it was reached after 7 minutes; and at 11kW per rack, 104ºF was reached after 2 minutes.
What are the restrictions regarding the pumped refrigerant piping in Liebert XD?
The pumped refrigerant piping should be done with refrigerant quality brazing techniques, the piping should run overhead to allow a small piping slope from the modules back to the pumping unit (XDP) or the chiller (XDC). In general, the max distance between the XDP/XDC and the furthest located modules is 175 feet.
Pipe sizing and other system design guidelines are described in the Liebert XD System Design Manual (SL-16655).
Which refrigerant is used in the Liebert XD pumped refrigerant circuit and what are the pressure and temperature in the circuit?
Liebert uses R134a in the pumped refrigerant circuit. It is an environmentally friendly refrigerant with no ozone depletion potential and very modest global warming potential. This is an off-the-shelf refrigerant that in the Liebert XD system operates at low pressure and becomes a gas at room temperatures should a leak occur. At typical operating conditions, the temperature in the circuit is 55-60F and the pressure is 60-80 psig. Due to the phase changing of the refrigerant in the circuit, the pressure and the temperature does not change during the refrigeration cycle.
Which are the components in the Liebert XD pumped refrigerant circuit?
The Liebert XD system is very simple, and includes cooling modules and pumping (XDP) or chiller (XDC) units connected with piping.
The components in the XDP/XDC are; Heat exchanger (R134a to Chilled water in the XDP and R134a to R407C in the XDC), Receiver tank and Pumps. The XDC includes a complete DX chiller with dual R407C circuits that may be air, water, or glycol cooled.
The components in the cooling modules are; Filter drier, Flow regulator and Evaporator. Since no compressor is needed in the circuit, oil and oil traps are not required in the Liebert XD pumped refrigerant circuit.
How is the temperature/capacity controlled in the pumped refrigerant version of the Liebert XD system?
Since the Liebert XDH (and XDO and XDV) are using the pumped refrigerant technology, they have built-in automatic capacity/temperature control.
The self-regulating capacity function is achieved through the phase-changing of the fluid as it passes through the cooling module. If there is a low heat load and low entering air temperature to the cooling module, the cooling capacity of the module goes down. At high heat load and higher entering air temperature to the cooling module, the capacity goes up. In addition, the controls in the Pumping Unit, Liebert XDP (and the refrigerant chiller, Liebert XDC), that supplies pumped refrigerant to the modules can be set so the fluid temperature to the modules is never below a value decided by the user.
Which of the Liebert XD cooling modules is most energy efficient?
The XDO is the most energy efficient module. At the nominal capacity, the efficiency ratios for XDO and XDV are 17.5W and 19W input power per kW cooling, respectively. The efficiency ratios for XDH20 and XDH32 are 27W and 40W input power per kW cooling, respectively.
The XD CoolFrame module does not include a fan.
Why do the modules in Liebert XD not have any air filters?
Due to the dew point control of the fluid temperature in the Liebert XD system, the coils are always kept dry so dust does not clog the coils. Therefore, no filters are needed in the XD cooling modules - XDH, XDV and XDO.