There is no better example to start a topic about cooling liquids than aircrafts. We all have today the option to travel by plane anytime and anywhere in the world. When you do that, of course maybe the only inconvenient you might have is that you must sit and keep the seatbelt tighten during the flight and probably you get bored when the flight is on long distance and it takes couple of hours. Sometimes you get up from your seat because you need to go to the toilet, but in rest you don’t have other discomfort being in the cabin. You can still enjoy the voyage. You can enjoy listening to music, reading something or even watching a movie provided by the aircraft screen placed on the backside of the seat in front of you. But for this things to occur very normal and pleasant, there is a lot of reasearch and development behind all that and obviously a lot of material science.
The first thing to observe while you are inside the cabin is how the exit doors of the aircraft are made. They all have a porthole with a large red handle on it. This is one of the main element responsible for your comfort during the flight, becasue it completelly seals the inside of aircraft from the outdoor environment. This red handle must stay locked for the entire journey. If you are stupid enough to pull this handle open, then what will happen next is that the air inside the cabin would be sucked out, along with you and anyone else not wearing a seatbelt. Everyone who is strapped in would stay put, but the air temperature on the plane would drop to approximately -50°C, and the air pressure would also drop, making it very diflicult to breathe. At this point, as we know from the pre-flight safety briefing, the oxygen masks would fall from their overhead lockers.
The low air pressure at altitude is, of course, the very reason why we fly so high; the lower density of the air provides less resistance to our passage, making the aircraft more fuel-efficient and allowing it to fly further. Nevertheless it presents a dual problem for aircraft engineers: they have to find ways of keeping their passengers from asphyxiating and developing hypothermia. They’ve achieved this through air conditioning, the history of which involves some of the most dangerous liquids on the planet.
The turbulence at high altitudes is caused by changes in the density of the air we are flying through; because of weather patterns below, we are passing through a mixture of low- and high-density air. As the plane hit pockets of high-density air, it slows, because of the increased drag on the aeroplane. Then when it comes upon the low-density pockets, it will drop ‘suddenly, as lower-density air provides less lift to the wings.
But despite the rapid changes in air pressure outside, our breathing is fairly normal; the cabin pressure, though lower than we are used to, is not fluctuating. This is thanks to the air conditioning, a field of engineering so specialized that even Einstein became interested in it, in his day, and was awarded several patents for his innovations, although at the time he was more interested in saving lives on the ground, rather than allowing people to breathe during long-haul flights. The problem Einstein was trying to solve was this:
In the 1920s, refrigerators, newly invented, were gaining popularity, and ice boxes, which had been the way to keep things cool for hundreds of years, were being phased out of homes. But these early fridges were not very safe. Einstein had been shocked to read in the newspaper that a family living in Berlin, with several children, had been poisoned because the pump in their fridge leaked. At the time, refrigerators used one of three types of liquid coolant: methyl chloride, sulphur oxide or ammonia – all are toxic. They’d been chosen for manufacture, though, because of their low boiling points.
Refrigerators work by pumping liquids through a series of pipes contained within them. If the temperature in them is warmer than the boiling point of the liquids, they boil. Boiling requires input of energy to break the bonds between the molecules in the liquid (called latent heat) and this heat is taken from the air inside the fridge, cooling it down. Thus the need for low-boiling-point liquids: they need to boil at temperarures inside a fridge, around 5°C. But for a fluid really to be useful in a fridge you need to be able to turn it back into a liquid again by compressing it using a pump.
In order to compress a gas into a liquid, you have to remove of the latent heat from it – essentially, the heat is squeezed out of the gas. This happens at the back of a fridge – when the compressor is running you can hear it; it is that hum that your fridge intermittently emits. It’ s why the back of your fridge is hot, and also why leaving your refrigerator open won’t cool your home; whatever cooling is caused by the door being open is more than compensated for by the heat produced at the back by the pump, a manifestation of the first law of thermodynamics, which states that “if we make something cool by taking energy’ out of it, then that energy has to go somewhere – it can’t just disappear”. So, in this case, the energy comes out of the back of the fridge.
It may sound easy to put a pump on to a set of tubes containing a liquid and then add a valve to allow that liquid to turn into a gas, but it presents a considerable engineering challenge. The gas is under pressure, so the molecules are in constant motion, colliding against the inside of the tubes. Wherever the tubes connect with the pump there are weak points, places where, without the right materials, the constantly clamouring molecules, expanding and escaping, give way to material failure. Which is exactly what happened with early fridge designs. In the middle of the night, the ammonia leaked out and killed whole families in their beds. Einstein wanted to do something about this, and having been a patent lawyer, he understood the technical intricacies of mechanical and electric machines. He began working with a physicist named Leo Szilard and they set about trying to invent a new type of fridge, one that would be safer for families to use in their hornes. They wanted to get rid of external pumps altogether, along with all the connectors that came with them, and instead make a system with no moving parts, which would hence be much less likely to fail.
From 1926 to 1933 Szilard and Einstein worked together to develop different ways of manipulating liquids into gases, and then back again, to create a working fridge. Of course, as we just discovered later, a liquid evaporating into a gas cools its surroundings. But going the other way, reclaiming the liquid, had always been done with a pump that forced gas molecules back into close proximity with another, compressing them into a liquid again. There had to be a different way. Szilard and Einstein had many ideas. They built working prototypes and filed for several patents.
One design used heat to drive liquid butane around a series of tubes, where it combined with ammonia to become a gas, creating a cooling effect; the gas was then mixed with water, which absorbed the ammonia and allowed the butane to be recirculated through the pipes, continuing the refrigeration process.
The second had liquid metal, initially mercury, running through a series of tubes, which they vibrated using electromagnetic forces; the oscillacions of the vibrating liquid acted as a piston to compress the refrigerant from a gas into a liquid – essentially creating refrigeration by having one liquid act on another liquid, without any moving solid parts. As with their other designs, the working fluids were hermetically sealed in tubes, and so, supposedly, safer than the models in use at the time.
While there was commercial interest in their prototypes – a Swedish company, Electrolux, bought up one patent and a german company, Citogel, developed another – time was running out for the Szilard-Einstein partnership. By then the Nazi party was gaining popularity in Germany and it was becoming more and more difficult for Jewish people like Szilard and Einstein to live and work within the country. Szilard moved to Britain, where he came up with an invention that would change the course of history – not by cooling things down, but by heating them up. It was the principle behind the atomic bomb: the nuclear chain reaction.
Meanwhile, Einstein toured Europe while an increasingly hostile Nazi party grew in power. Both Einstein and Szilard eventually ended up in America, where they were able to continue their collaboration, but by then it was too late. Scientists in America had also been working to make refrigerators safer, but they’d approached the problem the other way round – making the working fluids safer, rather than eliminating pumps.
In 1930, the chemist Thomas Midgley invented FREON liquid; it was hailed as safe and cheap and put Einstein and Szilard out of the refrigeration business. Unfortunately, it turned out freon wasn’t safe at all, but it was fifty years before that came to light, even though Thomas Midgley was known for creating dangerous liquids.
In the 1920S, while working at General Motors, Thomas Midgley discovered a liquid called TETRAETHYLLEAD, which when added to petrol made it burn more completely, thus increasing the performance of petrol engines. Tetraethyllead worked well, but it contained lead, which is highly toxic. Midgley poisoned himself while working with it. “After about a year’s work in organic lead,” he wrote in January 1923:
“I find that my lungs have been affected and that it is necessary to drop all work and get a large supply of fresh air”.
Despite the clear dangers he pressed on. It took many years, during which some of the production workers suffered from lead poisoning, hallucinations and death, but eventually, in 1924, Midgley held a press conference, demonstrating the safety of tetraethyllead. He poured the liquid over his hands and inhaled the vapour. Once again, he suffered from lead poisoning, but it didn’t stop him from putting tetraethyllead into commercial production.
Tetraethyllead was subsequently used as an additive to petrol around the world, but from the 1970s it started to be phased out, due to cumulative evidence of its toxicity (it was only completely banned in the UK on 1 January 2000). As a result, there was a dramatic drop in the rates of lead concentration in the blood of children, for example, and the social, effects were widespread. A statistically significant correlation was found between the usage rate of leaded fuel and violent crime, for instance. Such is the potency of lead as a neuro-degenerative substance; scientists have even speculated that banning leaded gasoline brought about a significant increase in the IQ level of people living in urban areas.
But that was all after Midgley began working on the problem of safe refrigeration. By the late 1920s, he’ d found a solution. His team focused on small hydrocarbons like butane that had low boiling points. The downside of these substances was that they were all highly flammable and potentially explosive, which is why they’re used as fuels in cigarette lighters and camping stoves. They replaced the hydrogen atoms on the hydrocarbon molecules with fluorine and chlorine, thus creating a new family of molecules called CHLOROFLUOROCARBONS (CFCs). In doing this they were potentially making something even more dangerous than the small hydrocarbons they’d started with; if these new molecules were to decompose, they would form hydrogen fluoride, an extremely corrosive and toxic substance. But Midgley’s team thought that kind of decomposition was highly unlikely because the fluorine-carbon bond was so strong, the liquid would be inert. And so it proved: chlorofluorocarbons are indeed chemically inert.
They seemed to be the perfect chemical solution to the problem of refrigeration because if they leaked out of the back, they wouldn’t kill anyone. Midgley was right about this, but he was otherwise wrong about the safety of CFCs. Ever since their introduction, CFCs had been leaking from the back of fridges, but it seemed the main effect of this was just that the refrigerators would malfunction – they didn’t kill anyone. And because they were so cheap to produce, they brought about a huge surge in the popularity of refrigerators. In 1948, just 2% of the UK owned a fridge; by the 1970s, pretty much everyone did. It was a miracle really. Britains went from a nation that ran on larders and cool boxes to a place where everyone had the means to cool and store their food and drinks. It made fresh food distribution radically more eflicient, cutting food waste in fish, dairy, meat and vegetables, and so making food cheaper. It was no less than a refrigeration revolution, all thanks to the seemingly innocuous CFCs.