THE COOLING LIQUIDS STORY – Part 2 – How cooling liquids that make refrigerators + air-conditioning safe and effective could have the potential to make liquid-breathing possible?

Air-conditioning systems are essentially refrigerators for air. In your car, for instance, the air-conditioning system passes the cabin air over copper tubes containing refrigerant, thus cooling the air. Cool air can’t maintain a high concentration of water, which is why water droplets form on air conditioners (this is also why clouds form as air rises and becomes cooler). Hence a by-product of air conditioning is that it dehumidifies the air.

In hot, humid countries air conditioning is often the only way to make travelling by car, bus or train tolerable. But it also consumes a huge amount of energy. In Singapore, for instance, cooling accounts for about 50% of the energy consumption in homes and offices. In the US, the entire transport sector, including trains, planes, ships, trucks and cars, accounts for 25% of the country’s energy use, while the heating and cooling of buildings through air conditioning accounts for nearly 40%.

And just as the back of your fridge gets hot as a result of cooling the interior, so too does air conditioning a vehicle or building release that heat back into the environment, raising outside air temperatures. The overall effect of this isn’t huge except in dense cities, where the temperature rise due to air conditioning is appreciable. Scientists at Arizona State University have shown that, solely because of air conditioning average night-time temperatures have increased by more than 1°C in urban areas. That doesn’t sound like a lot, I admit, but, remember, even a 2°C increase in global average temperatures is likely to lead to severe climate change. Making air conditioning more energy-efficient is thus a global challenge.

To increase the efliciency of cooing systems, the heat has to conduct quickly through the metal pipes, which is why we use copper for air-conditioning pipes. Copper may be expensive, but it’s a very good conductor of heat. But on a very hot day in a stuffy office with the outside temperatures approaching 40°C, even copper sometimes isn’t enough to keep the room cool, The way the liquid coolant flows through the tubes can tip the balance though.

Uniform flow, like water coming out of a pipe, is predictable, but its speed is inconsistent within the stream. Generally the outer part of the flow, the part nearest the pipe – also called the boundary layer – is slower than the inner part. There isn’t much thermal interaction between these two layers, which decreases the speed at which the heat is conducted away. The cooling system is considerably more efficient if you can achieve what’s known as a turbulent flow. This is a chaotic state of flow, where the liquid tumbles and creates vortexes, mixing everything together quite thoroughly. Increasing the pressure is one way to get turbulence (turning the tap on all the way, so the water comes tumbling out of the pipe chaotically), but that uses up a lot of energy. It’s better if you can disrupt the boundary layer, which we accomplish by making helical grooves inside the copper pipe so that they break up the uniform flow by constantly mixing the liquid. This has become the preferred means of getting a turbulent flow, which allows the cooling liquid to extract heat more efliciently, radically increasing the efficiency of air conditioning without any extra energy expenditure. Genius, eh?

Einstein missed this idea. This system of creating a turbulent flow was invented in the twentieth century, at a time when Einstein was already dead and the state of the air-conditioning sector hadn’t progressed beyond it. Energy efficiency was becoming a more important issue and there as a lot of pressure to lower the costs of making the spiral, helical-grooved, copper tube. Grooved copper tubes are made through a process that’s pretty similar to squeezing toothpaste – just imagine that, instead of toothpaste, there’ s a bullet inside the tube, with a diameter that’s slightly greater than the nozzle so it doesn’t squirt out when you squeeze. Instead, the bullet gets pushed against the nozzle, and the tube flows around it, which stretches out the copper. But because there are helical grooves on the bullet, as you squeeze, the bullet spins and carves it grooves into the inside of the copper tube. Magic! The only problem is that the bullet had to be made by bolting together several components made from a super-hard material called tungsten carbide, and inside the massive copper-squeezing machine the pressure often got so high that the bolts snapped off, the bullet fell apart, and the whole thing ended up in a big mess and cost millions of pounds to sort out.

Miraculously, the liquid that solved the problem was found. It was determined that we could bond the two halves of the tungsten carbide bullet together by turning the inside of the material into liquid, while keeping the rest of the material solid. It’s a kind of very precise welding. And like a lot of discoveries, once you know the trick, it’s easy to do. The engineers who found this solution just had to compress the two parts together and put them into a high-temperature furnace. This caused liquid to form inside the material; it flowed between the two pieces and then joined them together. Once it all cooled down, you were left with single, seamless piece of tungsten carbide. But that didn’t mean the bullets would hold together through practical use. So they did a trial and good news was that it worked perfectly and they filed for a European patent, as “Method of liquid phase bonding”.

Finding ways to cool more efficiently is all weIl and good, there were larger problems looming. So much work had gone into making cooling systems work better, but no-one had thought about what would happen when the fridges and the air conditioners stopped working. They just went to rubbish dump, where the valuable metals were salvaged -the steel from the frame of the fridge, and the copper tubes. No one collected the ChloroFluoroCarbons (CFCs); they evaporated quickly, as soon as the copper pipes were cut, cooling them one last time as the liquid evaporated into thin air. No one was worried about them. CFCs were already being used as propellants in cans of hairspray and other disposable items: they were supposedly inert, so what harm could they do? It was just assumed that once they became a gas, they’d be dispersed by the wind. Which is exactly what happened.

But over the course of decades they found their way into the stratosphere, where they started to get broken down by the ultraviolet light from the sun, into molecules that could do us a lot of harm. The sun emits light we can see and light we can’t see. Ultra-violet light is the latter. It’s the light that gives us a tan, and because it has so much energy, it can and does burn us: prolonged exposure can damage your DNA, and eventually causes cancer. This is why wearing sun cream is essential; the job of this liquid is to absorb ultraviolet light before it hits your skin. But there’s another barrier between you and the ultraviolet light that’s a lot more effective – the ozone layer.

Ozone is like a sun cream for the planet, and like sun cream you can’t really see it once it’s been applied. In fact, when a plane is flying through the ozone layer, but looking out of the window, you’d have no idea. Ozone is related to oxygen. The oxygen we breathe is a molecule made up of 2 oxygen atoms bonded together (O2); Ozone is a molecule made up of 3 oxygen atoms bonded together (O3). It’s not very stable, and, being highly reactive, it doesn’t stick around for long. Ozone also has a smell, which you can sometimes detect during the production of sparks – some of the O2 in the air is transformed into O3 as it encounters the spark’s high energy, and the resulting reaction produces a strange pungent smell. But while there’s not a lot of ozone in the air we breathe down on terra firma, up in the stratosphere there’s enough ozone to form a protective layer that absorbs ultraviolet light from the sun. But when CFC molecules find their way into the ozone layer, they break down after interacting with the high-energy rays of light emitted from the sun. This creates highly reactive molecule called free radicals; these then react with the ozone an decrease its concentration, thus depleting our ozone layer.

By the 1980s, atmospheric scientists had begun to realize that the effect of CFCs on our ozone layer was significant and had huge consequences. In 1985, scientists from the British Antarctic Survey reported that there was a hole in the ozone layer, spanning 20 million square kilometres, above Antarctica and not long afterwards it was determined that, across the globe, the thickness of the ozone layer was degrading. CFCs are, by and large, to blame for this, and so an intenational ban, called the Montreal Protocol, was put in place and took effect in 1989. CFCs in refrigeration were banned, was their use in dry cleaning, where they were used instead of water to clean clothes. But despite the swift response of the global community, there are still CFCs in circulation, and other holes have opened up in the ozone layer. In 2006, a hole of 2.5 million square kilometres big was found over Tibet, and in 2011 there was a record loss of ozone over the Arctic, which suggests we won’t be able to recover from all this damage until the end of the 21st century.

But back in the CFCs’ heyday chemists spent a lot of time exploring the properties of carbon-and-fluorine-based molecules. They discovered an amazing family of molecules called PERFLUOROCARBONS, or PFCs. Unlike CFCs, PFCs don’t contain any chlorine – they’re liquids made entirely of carbon and fluorine atoms. The simplest PFCs resemble hydrocarbons, in which all of the hydrogen atoms have been replaced by fluorine atoms.

Fluorine bonds are extremely strong, so they’re also very stable, making PFCs very inert. You can dunk pretty much anything you like into them with impunity, even your phone, which will continue to operate as if nothing had happened. You could put your laptop in a bucket of PFC – and people do, because the liquid cools them down during operation much more efficiently than their intern al fans, allowing the computers to operate at much higher speeds. But even more miraculous than that is the fact that PFCs are able to absorb a high concentration of oxygen – up to 20% of their volume in fact – which means they can act as artificial blood. Blood substitutes have a long history.

Blood loss is a major cause of death and the only way to get more blood into people is through a transfusion. But for a successful transfusion, you can’t use just any blood. Human blood isn’t all the same type; transferring blood from one person to another is only successful if their blood type matches. A scientist named Karl Landsteiner discovered blood types in the 1900s, and classified them as A, B, 0 and AB. In 1930 he was awarded the Nobel Prize for this insight, and a decade later the enormous casualty count of the Second World War led to the establishment of the world’s first blood banks. But, because of the challenges of matching donated blood with patients, scientists have been on the hunt for a reliable synthetic blood, which would eliminate the need to match blood types, and remove some of the strain on blood banks.

In 1854 some doctors used milk, with a degree of success, but it was never taken up by the medical establishment at large. Some people have also tried to use blood plasma extracted from animals, but that was found to be toxic. In 1883 a substance called Ringer’s solution was developed, a solution of sodium (Na), potassium (K) and calcium ‘Ca) salts that’s still used today, but as a blood-volume expander, rather than a true substitute for blood.

It wasn’t until PFCs came along, though, that people really started to believe a viable artificial blood could be created. In 1966 Leland C. Clark Jr. and Frank Gollan, two medical scientists from the USA, began studying what would happen to rats if they inhaled liquid PFCs. They found that the mice were still able to breathe, even when fully submerged in a bath of liquid PFC, and then were able to breathe air again upon removal – effectively transitioning from a fish-like existence, where they obtained their oxygen from the PFC liquid, back to a mammalian one, where they got their oxygen from air. This so-called ‘liquid-breathing‘ appears to work not just because their lungs could obtain oxygen disoved in the PFC, but also because the liquid can absorb all the carbon dioxide the mice were exhaling.

Further studies have shown that mice can Iiquid-breathe for hours, and research continues, with the ultimate aim of figuring out how humans might be able to liquid-breathe. In the 1990s the human trials were conducted. Patients with lung problems were asked to Iiquid-breathe, using PFCs that were loaded up with medication for their lungs. The therapy seems to work, but, for the moment, not without side-effects. No one is quite sure where this strange technology might lead, but if PFCs do become prevalent in one way or another, we’ll need to work out their potential environmental impact.

The world has managed to avoid catastrophic loss of the ozone layer by banning CFC liquids and replacing them with fluids less damaging to the environment – these days the refrigerant in your fridge is likely to be butane. It’s a highly flammable liquid, and if it leaks from the back of your fridge it could be hazardous, but it’s still safer than the liquids used in Einstein’s day, and it’s a much better bet for the planet. Our protective sunscreen layer of ozone is too precious to destroy with CFCs. But while the risk of using butane may be small enough for refrigerators, it’s still too great for the engineers of aircraft. These days liquid refrigerants aren’t used in aircraft air-conditioning systems. Instead, air is actually sucked in from outside the plane, and through a series of compression and expansion cycles is used to cool the interior – it’s very cold out there, after all.

The downside of this, though, is that when the plane is on the tarmac, the air conditioning doesn’t work very well because the air on the ground is warmer. Which is why, adding to the general pleasures of sitting on a delayed flight, when you’re stuck on a plane on the tarmac, waiting for takeoff, it can be sweltering. A plane’s air-conditioning system does more than just regulate temperature and humidity, though; it’s also set to equilibrate the air pressure inside the cabin. At 12.000 meters altitude, the air outside doesn’t have enough oxygen for people to breathe easily – or at all. So the air pressure inside the cabin has to be a lot higher than the air pressure outside. This puts the skin of the fuselage in essentially the same stress state as a balloon, causing the aircraft to bulge. The bulging can lead to cracks, so to minimize the chances of their formation the air-conditioning system makes a compromise: the pressure is set to be high enough to allow people to breathe normally, but not so high that the aircraft skin is put under undue stress. As the plane descends, the air-conditioning systems pump more air into the cabin to equilibrate to pressure levels on the ground, which is why your ears pop.

Planes don’t carry liquid oxygen on board for emergencies. In case of a loss of cabin pressure, the masks that drop from the overhead locker will supply you with oxygen made by a chemical oxygen generator – they create oxygen gas through a chemical reaction, allowing them to be very compact and lightweight, both essential features for anything carried on board an aircraft. I’ve never been on a flight where the oxygen masks have been deployed, and I’m fascinated by how well those systems are hidden.

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