Hypothetically speaking, would you rather risk a fire in your data center or accidentally release gases into the environment that would contribute to global warming?
This might sound like a trick question, but they’re actually considerations that data center managers need to make when weighing the relative trade-offs associated with different approaches to deploying immersive liquid cooling to their data center.
Immersive liquid cooling is a technique in which the server is literally submerged in a dielectric fluid as a way of dissipating heat, and the practice is emerging as a long-term replacement for traditional air cooling. (Dielectric fluids are non-conductive so they can safely come into contact with electronic systems).
Download White Paper 291: Comparison of Dielectric Fluids for Immersive Liquid Cooling of IT Equipment
Liquid cooling has traditionally been used in high-performance compute scenarios, but it is now moving toward mainstream application. Intel’s newest chips, slated to start shipping next year, require liquid cooling. And as data centers shift toward higher densities and hotter types of chip configurations, such as GPUs, liquid cooling will become more of a necessity. Ultimately, self-contained, liquid-cooled servers will allow data center operators to phase out the cumbersome, expensive and less efficient air-cooling infrastructure of chillers and fans.
Oil vs. Fluorocarbon for Data Center Cooling
It’s important to understand the pros and cons of the two main categories of dielectric fluids used for liquid cooling: oils and fluorocarbons. When evaluating the differences, organizations should consider four attributes: safety and environmental impact, total cost of ownership (TCO), material compatibility, and IT maintenance and handling.
1. Safety and Environmental Impact
As we noted, oil-based systems can catch on fire, and fluorocarbons, if they accidentally escape into the air, negatively affect the environment because they have a global warming potential (GWP). But those are worst-case scenarios. Great strides have been made in fluorocarbon GWP over the past 20 years with new formulations being developed that are < 1.
Both oils and fluorocarbons pose little health risks to humans during normal operations when they are handled and disposed of properly. The types of synthetic oils used for immersive cooling are technically combustible, but only at temperatures near or above the 200°F/93°C range, which is clearly far higher than what is likely to be encountered in a data center.
2. Total Cost of Ownership
When it comes to initial cost, synthetic oils can be 5-to-10 times less expensive than fluorocarbons. Different cooling technologies require different amounts of fluid, so be sure to consider that in your evaluation. Also, if you decide to go with two-phase cooling then you must use fluorocarbons.
In terms of operating costs over time, oils have a very low rate of evaporation, while fluorocarbons will evaporate quickly when exposed to the atmosphere. Fluorocarbon systems are typically sealed to prevent evaporation, but you can still expect that a well-sealed, two-phase immersive liquid cooling system will lose 1-2% of its liquid per year.
Both oils and fluorocarbons have a long-life span: synthetic oils generally last for 10-20 years while fluorocarbons can last more than 30 years. In both cases, the dielectric fluid will generally outlast the piece of IT hardware that it is protecting.
3. Materials Compatibility
This is an area where oils have historically received a bad rap for being, well, oily. There have been issues with oils being incompatible with plastic cabling, certain thermal greases and label adhesives. However, modern synthetic oils have improved material performance over older mineral and white oil solutions that were used in the past, and because of this, most compatibility issues have been greatly reduced.
Fluorocarbons, some of which have been used in the manufacturing of IT equipment for 50 years, have superior materials compatibility.
On the technology side, most liquid cooled servers on the market today have been converted from air cooled models, so some minor issues need to be addressed. Fans need to be removed or disabled, SSD or sealed hard drives installed and optical networking connections need to be tightly sealed.
4. IT Maintenance and Handling
Immersive liquid cooling technologies render the servers warm or even hot to the touch, so IT staffers might need to let equipment cool for a minute or two once removed from the system. Wearing nitrile gloves to handle the equipment is recommended for any fluid. Oil-based systems seem messy and will take some getting used to. IT equipment removed from a fluorocarbon system dries quickly, which makes servicing easier than with oil-based systems.
So, what’s the best dielectric fluid for immersive cooling? Of course, “it depends.” If I were put on the spot to choose one, today I would lead with a synthetic oil. It has the shortcomings of being messy to work with, but it’s cost and excellent environmental profile are attractive. But progress does not stop and I expect innovative fluid providers are not far from a molecule that provides great characteristics across the board.
Do your research. Learn more about dielectric choices with this white paper.
Still not sure about the right approach for your data center? For a more in-depth analysis aimed at helping you choose the right liquid cooling technology for your applications, read White Paper 291: Comparison of Dielectric Fluids for Immersive Liquid Cooling of IT Equipment.
 Before a chemist points out my error, these two categories are used to simplify the conversation. There are more classes of dielectric fluids (synthetic ester is a good example), which we touch on in the white paper.