Beverage Break

So much for those PowerBars. Solids only account for half the magnets’ diet. With all that exertion, they need to keep up their fluid intake, too. And that’s where the magnet equivalent of Gatorade comes in.

We’ve learned how massive amounts of energy, in the form of electricity, are pumped into the magnets. Now we will learn how all that energy – in the form of heat, or thermal energy – makes its way back out.

Three separate plumbing loops, totaling some 762 meters (2,500 feet) of pipe, tag team this task, helped by more than 35 pumps along the way. We’ll explain each loop in turn, starting with the hottest: The water coming right out of the magnet, the magnet-cooling system.

Any of our resistive magnets, on the receiving end of multiple megawatts, would quickly heat up to more than 1,000 degrees Celsius (almost 2,000 degrees Fahrenheit), melting into a copper puddle. That’s what would happen, anyway, if they weren’t cooled. So cold water is pumped right through the metal coils of the magnet, cooling it off as it heats up. To carry off the intense heat, the water rushes through the coils with three times the force of a fire hose: about 7,500 to 15,000 liters (2,000 to 4,000 gallons) of water per minute at pressures of 17 to 31 bar (250 to 450 pounds per square inch).

Sound like a recipe for electrocution? It might be, if the water came from the faucet. But this is deionized water: all the extra ions have been removed. Regular tap water conducts electricity – and would interfere with a magnetic field – because of those ions. Pure H₂0, however, does not. (Sadly, this fact messes with our Gatorade analogy, because that sports beverage is pumped full of electrolytes, thirst-quenching ions such as sodium and chloride). A water-treatment apparatus in this magnet-cooling system keeps the water deionized by passing it over a 60-cubic-foot bed of mixed resin, which draws off impurities.

After exiting the magnet, the water flows to one of two heat exchangers that, like the radiator in your car, transfer heat from one thing to another. In this case, the heat is transferred from the magnet cooling loop to the second water loop in this series – the chilled-water system. So magnet cooling water entering the heat exchanger at about 49 degrees Celsius (120 degrees Fahrenheit) exits at a much chillier 6 degrees Celsius (42 degrees Fahrenheit), then heads back to the magnet for its next cooling shift. Together, the heat exchangers can remove 56 MW of heat – the thermal equivalent of all the electricity coming into the DC Field Facility.

Two features work in parallel in the chilled water loop to keep the water cold: water storage tanks and chillers. Let’s talk about the tanks first.

Two, four-story water tanks behind the lab hold between them 16.3 million liters (4.3 million gallons) of water – the equivalent of about six and a half Olympic-size swimming pools. But you might not want to swim in them – the water is kept at 4.4 degrees Celsius (40 degrees Fahrenheit).

The volume of water in the tanks never changes; its purpose isn’t to water gardens or fill bathtubs, but to store thermal energy and make cold water available at a moment’s notice. Warm water enters at the top of the tank; cold is dispensed from the bottom. (Because this water does not come into contact with the magnets, it’s regular ground water – ions and all.) If only one smaller magnet is in use, these tanks are sufficient to keep the system cool. But with multiple or bigger magnets, the chillers kick in.

These are no household appliances; with a rating of 2,000 tons (a reference to chilling capacity, not weight), each of our four chillers is 400 times more powerful than a residential A/C system. If the lab was in the ice business, we could crank out 2 billion cubes a day. In fact, in addition to cooling magnet water, this system air-conditions the entire MagLab – no small feat during sultry Tallahassee summers.

Each 16.5-ton (as in weight) chiller puts about 3,625 kg (8,000 pounds) of refrigerant (R-22) through the same heat transfer cycle repeated (on a much smaller scale) inside your fridge dozens of times a day: it’s compressed, condensed, expanded and evaporated, drawing heat away from the warm water.

Thus the heat from the chilled-water system is transferred to the condenser-water system on the other side of the chillers. This water, at some 30 degrees Celsius (85 degrees Fahrenheit) now comfortably swimmable, is flushed into four cooling towers. Nine meters (30 feet) tall and stretching 44 meters (144 feet) along one side of the MagLab, these towers collectively can hold some 750,000 liters (200,000 gallons) of water. From here whatever energy is left in the water has nowhere to go but up … Up, up and away, coaxed by fans bigger than helicopter blades, into the atmosphere.

This is the end of our chapter on that 56 MW of energy that began at Tallahassee substation #31. But it’s not the end of the story. After all: Energy can be changed from one form to another, but it cannot be created or destroyed. The heat rejected into the atmosphere above the Magnet Lab will continue its adventures, onward and upward. Onward and upward, too, will go our knowledge of physics, biology, chemistry, materials science and engineering – thanks to experiments conducted in our powerful magnets.

Go, Team Tesla!

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Thanks to the Magnet Academy’s scientific adviser on this article, Bryon Dalton, head of Magnet Operations at the Magnet Lab.