Liquid Cooling: What are the options?

As processing-intensive workloads continue to rise, data center operators are increasingly turning to liquid cooling technologies to boost energy efficiency. The prevalence of liquid cooling is now on the up, driven largely by the need for scalable and reliable cooling methods in high performance computing.

Traditional air cooling systems simply can’t keep up with the heat generated by high-density racks. The new generation of central processing units (CPUs) and graphics processing units (GPUs) generate far more heat than their predecessors, and because of the demand being placed on such equipment, manufacturers are increasingly having to pack more CPUs and GPUs into each rack. That’s why liquid cooling is becoming ever more important.

Liquid cooling provides options for data centers looking for a way to keep up with the amounts of heat being generated by equipment, and it does so in a way that’s both cost-effective and sustainable.

However, there are choices to be made when it comes to the use of liquid cooling, so it’s vital that businesses have a good understanding of what it does and how it works before any decisions are made.

In this article, we’ll talk you through some of the key options you have when deciding on whether or not to use liquid cooling, from the challenges such cooling systems present to the opportunities they provide.

The three types of liquid cooling

Liquid cooling is now being used to protect rack-mounted servers in data centers all over the world. And it’s inevitably used in one of three ways. Below, we’ll talk through the three key types of liquid cooling and explain how they work.

Rear-door heat exchangers:

Passive or active liquid heat exchangers are used in place of the rack’s rear door in this form of liquid cooling. Liquid is then used to absorb heat from air as it’s taken out of the equipment.

Systems that use rear-door heat exchangers can effectively supplement traditional air cooling methods, and are becoming increasingly important in facilities where air cooling is no longer sufficient to manage the heat being generated as demand increases.

Rear-door heat exchangers are particularly well suited to environments with mixed rack densities. For more information, take a look at our complete guide to rear door heat exchangers.

Direct-to-chip liquid cooling:

In direct-to-chip liquid cooling, systems use cold plates which are mounted directly on heat-producing components.

Cold plates transfer heat using liquid, in single-phase cooling, or can use evaporation in a two-phase system. In doing so, they can efficiently remove between 70 and 75% of heat generated by equipment. When combined with air cooling systems to manage the other 25%, direct-to-chip liquid cooling is brilliantly effective.

Immersion cooling:

Immersion cooling comes in two key forms: single-phase and two-phase. This option sees equipment, servers and other components submerged in a thermally conductive dielectric fluid, which effectively manages all required cooling.

Liquid used in immersion cooling systems has incredible heat-transfer capabilities, and it’s ideal for businesses looking for optimal energy efficiency. With immersion cooling, there’s no need for additional air cooling systems, however the majority of data centers will have such systems in place to manage airflow throughout the facility.

What to consider before introducing liquid cooling in data centers

Liquid cooling comes with a whole host of incredible benefits, but introducing it in an existing data center is far from a simple task.

There’s plenty to think about before liquid cooling can be set up in an existing facility, from piping and plumbing considerations to planning for the level of demand that will be placed on new cooling systems.

If you’re thinking about making the switch to liquid cooling, here are a few key things to consider before you do so.

Heat load-to-liquid ratio

Think about your heat load-to-liquid ratio, as this plays a key role in determining how a system should be laid out and set up.

In some cases, liquid cooling can dramatically reduce the need for other air cooling systems, giving data centers the option to reallocate space that would’ve been taken up by such systems. But other forms of liquid cooling are best used in partnership with air cooling systems for optimal results.

The type of liquid cooling you choose therefore plays a huge role. Weigh up your options with regards to heat load, liquid flow rates and pressure before you make any decisions.

Piping and plumbing

New facilities are being designed with liquid cooling in mind, which means plumbing is prioritised from the very start. But for existing facilities, this can be more of a challenge.

Liquid cooling of course means plenty of piping, which must be arranged in such a way that cooling fluid can move to the rack and back again seamlessly. If plumbing isn’t managed well, it can cause obstructions to airflow. For best results, we recommend trying out some simulations before you begin any plumbing work to check that it won’t have a negative impact.

Existing data centers usually opt for a phased approach when setting up the plumbing infrastructure needed to accommodate liquid cooling systems. This helps to minimise disruption and gives the data center a chance to upscale its liquid cooling offering in line with rising customer demand.

Don’t forget to plan for potential leaks when introducing liquid cooling systems, either. Drip pans should be installed under all fittings to catch leaks before they cause any damage. It’s also recommended that such fittings are selected with their planned use in mind; choose fittings designed for such an environment to avoid future problems.

Secondary cooling loops

Secondary cooling loops are used to control the distribution of fluid in liquid cooling. At the heart of these loops is the CDU, which controls flow rate and temperature, while also filtering the liquid to keep the entire system in good health for as long as possible.

CDUs use a liquid-to-liquid heat exchanger to capture heat generated by equipment and move it through a chilled water system, thus absorbing the heat and reducing the temperature of the fluid. Typically, CDUs are positioned near the racks they’ve been set up to cool, but if this isn’t an option they can be situated further away.

Calculating demand

Liquid cooling can handle a lot of heat, but like everything it does have its limits. So, begin by determining exactly how much heat each cooling system will need to cope with and what the capabilities of your chosen liquid cooling option are.

You’ll need to be aware of the level of demand that will be placed on existing air cooling systems once your liquid cooling is up and running. If you’re opting for liquid cooling in the form of rear-door heat exchangers, for example, you’ll still need air cooling systems to manage the heat generated by racks whenever those doors are open. Indoor chillers are usually your best option here.

Leak detection and risk management

Liquid cooling isn’t without its risks, and this too is something to be aware of from the very beginning.

There have always been concerns over what might happen if something goes wrong while liquid is making its journey to the rack, and efforts have been made over the years to reduce that risk. But leaks can still happen, so data center staff need to know how to detect and fix them at a moment’s notice.

Leak detection technology is vital for the protection of all components. The use of dielectric fluids also mitigates such a risk, as these fluids won’t cause damage to equipment. However, they’re expensive, so leak detection remains a priority for systems using these fluids.

Choosing heat-rejection systems

There are many things to consider when it comes to choosing your heat-rejection system. For best results, these systems need to align with the demands and scale of deployment and its location. The type of liquid being used in your cooling system also influences which heat-rejection solution will be most effective.

In existing data centers, cooling towers or dry coolers can be used to facilitate heat rejection, but data centers often need to make changes to these to get the best results from any new liquid cooling systems being set up.

 
 
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 If you’d like to know more about liquid cooling and how it’s used here at TRG, get in touch with our team. We can talk you through everything you need to know about liquid cooling, from how it works to how to get the best results from it.
 
Our team is here to help with any questions you might have, contact us to find out more.