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5 things you need to know about connector reliability: quick disconnect coupling and thermal management.

The use of liquid cooling to mitigate heat generated by electronics can be found in a wide variety of applications, including gaming computers, supercomputers, and medical equipment. The widest acceptance of the technology is in today's data center industry, where server manufacturers are turning to liquid cooling for cost-efficient and effective thermal management. Here, in particular, quick disconnect couplings are playing an integral role in cooling system performance and reliability.

Implementing an efficient, effective, and safe cooling system to remove waste heat is an ever-growing concern for data centers. Energy consumption by servers has risen dramatically within the last decade as power densities of server racks have increased. A decade ago, server racks consumed 250 W to 1.5 kW each, current power racks consume around 10 kW. Projections for the next decade put power consumption at 50 kW. Further, it is estimated by 2020 the data center industry will generate as much C[O.sub.2] as the airline industry, according to Data Center Journal.

Traditionally, data centers have relied on air for cooling, but forcing cold air through server racks is an inefficient and costly method to remove heat. Sixty percent of cooled air is lost as a result of bypass and through air mixing, and cooling costs can exceed the costs of running the servers.

As computer servers are pushed to run faster, and data center rack densities increase, it is becoming impractical to expect moving air to provide sufficient cooling.

Water, on the other hand, is 3,500 times more effective than air (on a volume basis) at storing and transferring heat. In fact, 41 percent of respondents in a recent industry-wide survey expect a combination of air and liquid to be the primary method of data center cooling, according to Data Center 2025: Exploring the Possibilities. Liquid cooling represents the best alternative to raising cooling efficiency and reducing air conditioning costs.

Using liquid to cool computers goes back to the mid 1960s when IBM implemented liquid cooling for its 704 mainframe. Nearly 50 years later, IBM recently employed processor-level liquid cooling for a supercomputer located at a Swiss university, where water is circulated to cool the processors and then pumped to the underfloor heating system where the waste heat can be used for building heat. Compared with air-cooled servers, IBM estimates that the liquid-cooled system uses 40 percent less energy, and the additional use of the waste heat lowers the carbon footprint by an estimated 85 percent, according to CNET.

A critical component for liquid cooling reliability

One of the critical components in liquid cooling systems is the quick disconnect coupling. These couplings are used to link together flexible tubes that carry cooling liquid to individual servers and central manifolds on the server rack connected to central heat exchangers. The critical contribution of quick disconnect couplings becomes more evident when considering that these couplings need to stay reliably connected for long durations, but then easily disconnect without valve failure. Furthermore, when servers need to be serviced or changed out, the couplings must easily disconnect without dripping or spilling coolant on sensitive electronics.

The chief concern of server designers and data center operators is system reliability. According to the Ponemon Institute, the average cost for any type of data center outage is more than $690,000. One of the basic methods of achieving high reliability is through redundant components that take over if primary components fail. In liquid cooling systems, redundant pumps are often employed to prevent loss of circulation. However, individual quick disconnect couplings in the cooling system represent a single point of failure. A faulty connector has no backup, so a leak might cause the entire server rack to fail. This puts a critical focus on the design, construction, and quality of the quick disconnect coupling.

When selecting quick disconnect couplings for liquid cooling of servers and other high-end computers, it is important to look for the following five attributes to ensure reliability.

1. Robust design and construction

Liquid cooling system quick disconnect couplings need to be durable and robust. They also need to be designed for low-pressure applications and able to withstand long periods of connection before being disconnected. Couplings should also be designed for low pressure drop in order to minimize flow restriction and the burden on cooling system pumps.

Some liquid cooling systems employ couplings developed for the hydraulics industry because of their perceived higher durability. However, the seals and internal valves of these couplings were not designed for low-pressure applications or where couplings are required to be connected for months or years at a time. When selecting quick disconnect couplings for liquid cooling systems, look for couplings designed specifically for low-pressure applications and feature robust construction.

2. "Dry disconnect" capability

Technicians need to be able to easily disconnect cooling loops from servers without risk of water or other coolant leaking on to sensitive electronics.

Quick disconnect non-spill couplings can feature two styles of internal valves that automatically shut off the flow of liquid when the couplings are disconnected. One style of double-shutoff coupling features poppet valves that, by virtue of its design, trap a small amount of liquid within the coupling body. This liquid can drip out when the coupling is disconnected. While the leakage is not a large amount--likely just a drop or two--it might be sufficient to raise safety or reliability concerns around electronics.

In contrast, true dry-disconnect couplings feature flush-face valves that allow no more than a coating of cooling fluid on the valve surfaces. This virtually eliminates the possibility of dripping onto vital components. Look for couplings that feature precision-made, internal flush-face valves and, ideally, couplings designed specifically for liquid cooling applications.

3. Ease of use

It is important to be able to easily disconnect the cooling loops from servers with no spills. Whether it is a push-to-connect or twist-lock type of coupling, the valves need to reliably close when disconnected, even after a few months, or years. Another aspect of ease of use is the availability of color-coded couplings, which help ensure hot-to-hot and cold-to-cold connections. When a cooling system is equipped with properly designed quick disconnect couplings, it simplifies maintenance and serviceability. Technicians accustomed to connecting and disconnecting Ethernet connections and power cables will have little trouble with intuitively designed quick disconnect couplings.

4. Resilient seals

Couplings used in liquid cooling applications remain in a connected state for extended periods. Upon disconnection and reconnection, they need to function perfectly so there are no drips or leaks of coolant. Seals that are not properly formulated or designed can take a "set" that causes leaks upon disconnection. The seals also have to be compatible with the cooling liquid to prevent seal swell, shrink, brittlement, or other distortions. In some cases, couplings may feature dual seals to enhance sealing capabilities in the quest for higher reliability. However, dual seals can introduce greater friction to the system and might actually reduce reliability if not designed properly.

5. Materials compatibility

Most plastics and metals are compatible with cooling liquids, such as water-glycol mixtures. If your cooling system uses a proprietary cooling liquid, be sure to determine whether the coolant is compatible with coupling housings, seals, valves, and tubing. Use particular caution when selecting metal couplings for the cooling system that might come into contact with a server chassis, as dissimilar metals can create electrolysis and corrosion where they touch. Couplings developed from metal ball-and-sleeve hydraulic couplings are not intended to be used for cooling applications, so materials compatibility should be carefully checked. Compatibility issues can be avoided by using metal couplings designed specifically for liquid cooling.

Liquid cooling of computers and servers is the most efficient and cost-effective method of dissipating heat generated by today's faster and higher-density electronics. By providing cooling at the source of the heat, air conditioning demands can be reduced along with the attendant costs and carbon impact.

With cooling system designs still evolving, it is important to understand the role that quick disconnect couplings play in these systems and how they contribute to system reliability. Evaluating couplings based on their robustness, valves, seals and materials compatibility can help ensure long-term performance.

By David Vranish, Applications Engineer, Colder Product Company (CPC)
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Title Annotation:Equipment Protection
Author:Vranish, David
Publication:ECN-Electronic Component News
Date:Oct 1, 2014
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