Many people know that work on nuclear weapons enabled the development of the first electronic computers. But its no less true that the humble refrigerator, in a roundabout way, enabled the development of the first atom bomb. While reading the newspaper one morning in 1926, Albert Einstein nearly choked on his eggs. An entire family in Berlin, including several children, had suffocated a few nights before when a seal on their refrigerator broke and toxic gas flooded their apartment. Anguished, the forty-seven-year-old physicist called up a young friend of his, the inventor and scientist Leo Szilard. There must be a better way, Einstein pleaded. Szilard, a stocky man of 28, had first impressed Einstein six years earlier by proving him wrong on a certain scientific point. (That didnt happen often.) Szilard also had a knack for turning esoteric ideas into useful gadgets. In later years, he became a sort of Thomas Alva Edison of high-energy physics, sketching out the first electron microscope and particle accelerator; he and Einstein had bonded in part over their love of such mechanical devices. (Although a theorist and somewhat flighty, Einstein came from a family of tinkerershis uncle Jakob and father Hermann had invented new types of arc lamps and electricity metersand hed worked in the Swiss patent office for seven years.) So when Einstein called Szilard that morning, the two men agreed to collaborate and build a better, safer refrigerator. This wasnt as odd as it might sound: in the previous half century, refrigeration had become serious science. The study of thermodynamics and heat had led to the concept of absolute zerothe coldest possible temperatureand several labs around the world were racing to reach the bottom of the thermometer. Some of the best science revolved around attempts to liquefy certain gases: nitrogen, oxygen, hydrogen, methane, carbon monoxide, and nitric oxide. Throughout the 1800s this sextetthe so-called permanent gaseshad resisted all efforts to liquefy them. This stubbornness had led some scientists to declare that these six gases could never be liquefied, that they somehow stood apart from the rest of matter. Other scientists said baloneythat powerful new cooling methods would eventually condense them. In particular, the latter group pinned their hopes on a clever, cyclical cooling process that involved removing heat from substances in several stages. Stage one involved filling a chamber with a gas that was easy to liquefy. Call it A. Scientists first compressed A with a piston, then cooled down the compression chamber with an external jacket of cold water. As soon as A had chilled down, a valve opened. This dropped the pressure on A and allowed it to expand into a larger volume. The key point is that expanding into a larger volume takes energy, takes work. (Its similar to how a litter of puppies, if locked in a broom closet, would suddenly expend a lot more energy if you opened the door and let them run free inside the house.) And in this situation, the only energy A can draw on to expand and spread is its own internal store of heat energy. But depleting its internal store of heat energy inevitably cooled A down even more, and it eventually condensed into a liquid at around 100°F. Now came the clever part. The next stage involved a chamber of gas B, which was tougher to liquefy. Scientists once again compressed B with a piston to start. But for the cooling jacket this time, instead of cold water they ran liquid A through the jacket. This dropped gas Bs temperature to 100°F. Opening a valve then caused B to expand, which forced B to deplete its internal store of heat energy. Its temperature plunged to around 180°F, whereupon it also liquefied.
Liquid B could now be used in another cooling jacket to liquefy a more stubborn gas, C, and so on alphabetically. This bootstrapping process finally reached temperatures so low (circa 420°F) that not even permanent gases could resist, and all six were eventually liquefied. Especially beautiful was liquid oxygen, which glowed faintly blue, like liquid sky. Gas refrigeration remained a mere curiosity, however, until the Guinness Brewing company invested in the technology around 1895. Before this, breweries generally brewed beer only in the winter and stored it. (Lager means storage in German.) Refrigerators let Guinness make beer year-round, thank goodness. As a knock-on technology, the rest of the world got commercial refrigerators, like the one in your home right now. All modern fridges rely on the same general principles of gaseous cooling. If you tore out the inner panels on your fridge, youd see a series of tubes. Inside the tubes youd find a liquid (call it Z) with a low boiling point. As the casseroles and other leftovers inside your fridge emit heat, Z absorbs the heat through the fridge walls and warms to a boil. The resulting gaseous Z then floats away through other tubes, carrying the heat with it. Next, Z enters a compression chamber, which compacts the gas with a piston. (The motor that runs the compressor causes the characteristic hum of refrigerators.) The compressor now pushes warm gas Z through still more tubes behind the fridge, which allows Z to jettison heat to the outside world. At this point the gas has successfully removed heat from inside the unit and dumped it out back. And after Z dumps enough heat, it condenses back into liquid. Now Z passes through an expansion device that lowers its pressure, cools it further, and completes the cycle. Liquid Z re-enters the tubes inside the fridge panels, reboils, and resumes sucking out heat. Now, one detail here might sound suspicious. Youre boiling a liquid (Z), so shouldnt everything heat up? Not quite. The liquid heats up, yes. But in an enclosed unit like a refrigerator, the liquid can warm itself up only by stealing heat from your casserole: warming the one necessarily cools the other. And the boiling is indeed crucial. Remember James Watts old bête noire, latent heat? This principle says that liquids changing into gases absorb ridiculous amounts of energy. In Watts engines this was a bug, but fridges make it a benefit: absorbing heat and whisking it away is exactly what refrigerators aim to do, and nothing does that better than liquids changing into gases. (This same general process explains why liquid sweat, when it evaporates, cools you on a summer day.) By the 1920s gas-compression refrigerators had replaced iceboxes all across Europe and North America. There was only one problem. All three gases commonly used as coolants thenammonia, methyl chloride, and sulfur dioxidewere toxic and occasionally killed whole families. (Methyl chloride sometimes exploded, too, just for fun.) Hence Einsteins vow to find a better way. He knew the weak point in home refrigerators was the compressor, whose seals often cracked under pressure. So he and Szilard designed a fridge without a compressor, a so-called absorption fridge.
In the simplest type of absorption fridge, you start with two liquids mixed together in a chamber, the absorbent and the refrigerant. The key to the design is that, at low temperatures, these substances mix readily. But if you raise the temperatureusually by warming the chamber with a small methane flamethe refrigerant boils out as gas, leaving the absorbent behind.
The refrigerant gas now goes on a long and tortuous journey. It first flows into tubes behind the fridge and dumps the heat it absorbed from the flame; this step simultaneously cools the refrigerant back into liquid. This liquid flows via gravity into the panels inside the fridge, where it sucks the heat out of yet another casserole. Absorbing this heat causes the liquid to reboil, and the resulting gas whisks the latent heat away, removing it from the units interior. (In some designs, the gas then heads to still more tubes behind the fridge, to jettison heat one last time.) Meanwhile, back in the original chamber, the methane flame has switched off, allowing the absorbent there to cool down. A jacket of cold water then cools the absorbent further. The absorbent cools so much, in fact, that when the refrigerant gas finally wends its way back into the chamber, the absorbent condenses it into liquid again and reabsorbs it. You therefore end up back where you started, with a mix of two liquids that you can separate with a flame. Overall, absorption fridges and regular fridges cool things down the same way, by boiling gases. But they use a different process to recycle the refrigerant. Again, though, this probably sounds like cheating: a flame can cool my beer? But thats the magic of gases. Really, the flame here isnt so much adding heat as doing physical workseparating the refrigerant from the absorbent by turning the refrigerant into gas. And once you have a free gas in the system, you have oodles of options. Indeed, the art of refrigeration consists of manipulating gases to absorb heat energy here, carry it there, and dump it somewhere else. Hearkening back to Thomas Savery, you could call the Einstein-Szilard refrigerator an engine for freezing water by fire. The Einstein-Szilard fridge actually used three liquids and gases, not two, making it a tad more complicated than the scheme above. But their design did have several advantages over regular fridges. With no motor, it made no noise and rarely broke down. It also used no electricity (just methane), and it avoided the seals that all too often broke and leaked toxic gas. In looking back on this episode, some historians have assumed that Einstein merely offered advice on the patent applications or used his celebrity to lure investors, leaving the real work to Szilard. In truth Einstein laboured over the project, and the duo ended up receiving dozens of patents in six countries on different fridge components. (An American patent attorney reviewing applications did a double take, as well he might, when he noticed Einsteins signature.) The duo ended up selling several patents and collecting a nice check for $750 (around $10,000 today); they subsequently opened a joint checking account, like a married couple. Szilard collected an additional $3,000 per year in consulting fees. Like any married couple, though, they clashed sometimes. Szilard had an engineers appetite for complexity and kept adding new valves and cooling lines to the fridge. Einstein, meanwhile, longed for simplicity and elegance no less in his home appliances than in his physics. (He would have hated working with James Watt.) The need for simplicity eventually drove Einstein and Szilard to invent two other cooling units, each of which worked on a different physical principle. In one they replaced the piston in a standard fridge with molten sodium, which magnets pumped up and down to compress gases. The other device used water pressure from a kitchen faucet to power a small vacuum pump; the pump then cooled things by evaporating methanol. Einstein called the latter device Der Volks-Kühlschrank, the peoples fridge. In the end, sadly, none of the three Einstein-Szilard fridges ever made it into anyones home. Not surprisingly, the molten-sodium pump proved a wee bit impractical for your average kitchen (though it later found use in nuclear power plants). The faucet cooler failed because German apartment buildings had lousy water pressure, which hindered the vacuum pump. And absorption fridges simply burned too much fuel to compete with compression fridges; the Einstein-Szilard design seemed like a Newcomen engine in comparison.
Even the biggest objection to conventional fridges, the lethal gases, became moot in 1930 with the debut of a new and nontoxic cooling gas, Freon. Within a decade, virtually all home units had switched to this chlorofluorocarbon, and the Einstein-Szilard fridge was rendered a historical relic. Of course, Freon did have one pesky drawback. When old refrigerators went to the junkyard, the Freon leaked out and climbed into the stratosphere. There, ultraviolet light cleaved the chlorine atoms off, creating free radicals that chewed through ozone molecules with sickening efficiency: each chlorine radical can destroy 100,000 O3 molecules over its lifetime. This destruction eventually opened up a hole in the ozone layer that still exists and that wont recover for decades, if ever. Humanity might have saved itself a lot of trouble in the long run by investing in the Einstein-Szilard approach to cooling water with fire. So was the Einstein-Szilard fridge a waste of these mens time and talent? Not entirely. Einstein found the work a refreshing break from his futile search for a Theory of Everything. With two families to support and a crumbling German economy, Einstein also enjoyed the extra cash. Szilard needed the money even more, especially after he fled Nazi Germany for London in 1933. (He was part Jewish.) He spent the next few years living off his fridge proceeds, and he used his sudden freedom to take long walks and ponder what the next big thing in physics might be. The answer came to him one afternoon in September 1933, as he stepped off a curb near the British Museum. Hed been hearing about some experiments involving the release of subatomic particles called neutrons. He started wondering what would happen if, say, a uranium atom split and released multiple neutrons. Other nearby uranium atoms might absorb them, become unstable, and release neutrons themselves when they split. These secondary neutrons would destabilise more atoms, which would release tertiary neutrons, and so on. Each atom that split would alsoaccording to his patent partners famous equation, E = mc2release energy in an ever-growing cascade . . .By the time he crossed the street, Szilard had worked out the principle behind the first nuclear chain reaction. And unlike his clever fridges, this invention became all too pervasive in the turbulent decades to followdecades that would shatter not only the publics belief in benevolent science, but scientists belief in a neat, tidy, predictable universe.
Wired, 23 July 2017 ; http://www.wired.com/news