coolingzone conducted two one-day courses on liquid cooling in 2005 instructed by dr. cathy biber. during the first session of this course at the coolingzone's 2005 summit, we asked the attendees about the concerns they had when considering the use of liquid cooling.

in this article, we list some of these concerns under various headings as follows:

quick disconnects

in rack systems where many shelves are housed in a large rack or for the purpose of maintenance, the liquid hoses must connect to various components (the cold plate, the radiator, the pump) a safe and reliable quick disconnect may be required. here are the concerns when using quick disconnects:

• higher pressure drop than other fittings
• pressure drop data from manufacturer are required
• "positive locking mechanism, so they can be used with confidence"
• higher cost -- 10-100x barbed fittings
• live on ends of hoses
• adds to total number of joints

concerns about leaks

leakage of the coolant is probably the most sited concern when it comes to liquid cooling. here are some recommendations for dealing with coolant leakage:

• use a drip pan
• usespray shield
• "avoid drips behind fan, which will cause spray"
• use coolant additive to stop leaks as in car radiators?
• form hose and strain relieve to avoid stress concentration at hose clamp
• use barbed fittings to avoid hose clamps
• engineer joints correctly
• add o-ring between hose barbs
• use sleeve or collar over hose in barb area to increase local hoop strength
• use quick disconnects with check valves
• "avoid condensation by keeping surface temperatures above dewpoints (insulate surfaces if needed, keep chiller t higher than dewpoint)"

filtration

if the cold plate you are using is of micro-channel variety, you must filter the fluid ahead of the cold plate.

standards

• picmg atca air cooling standard exists; liquid cooling discussion ongoing
• "ashrae data center cooling subcommittee may get to discussing this in 2006 (allowable water t range, cleanliness, ph, possibly fluid connectors, allowable supply pressures)"
• look to igbt solution set for standards ideas. igbt users have been doing liquid cooling for a while and are several years ahead of the rest of the market.

reliability solutions

• use finger guard near heat exchanger
• "biocides -- dow makes some, search on ""closed loop water treatment""; also see process cooling & heating magazine"

corrosion/erosion concerns

• keep fluid velocity through tubes low enough -- same speed for water in other materials besides copper (1.5 m/s)
• to avoid erosion with sea water use cu-ni tubing
• use anti-corrosion additives
• avoid mixing metals likely to induce galvanic corrosion

implementation concerns

these are the concerns raised by the attendees:

• leaks
• management resistance (we don't want water next to our electronics)
• cost
• field service
• corrosion
• mold growth
• reliability -- many single points of failure
• supply chain -- doesn't have multiple selection of well-tested products
• "lack of standards -- part interchangeability, interfacing and mating parts"
• "thermal interface -- mating parts, cte issues"
• electrical conductivity of system
• serviceability
• filtration
• ruggedness of heat exchanger
• ee resistance (conductive fluid)
• space
• weight
• "portability (weight, size, power consumption)"
• design complexity
• "coolant limitations (few choices, drawbacks)"

most of these concerns are being addressed by researchers and vendors around the world. as the power density of electronic component rises above the capability of conventional solutions,the need to resolve these concerns becomes even greater.

in the future issues of coolingzone's online magazine, we will begin addressing some of these concerns and discuss the available and emerging solutions. meanwhile, if you have thoughts or ideas about any of these items, we would love to hear from you.