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Hank: It’s summer in the northern hemisphere, which means picnics, sunscreen, and lots of time outdoors. At least... until it gets too hot and everyone wants to go back inside. Depending on where you live, heat and humidity can become really unbearable – even dangerous. And things like electric fans, or a trip to the pool, can help you keep cool, but in many places, you can just duck into an air-conditioned building.

We take air conditioners for granted, but they’re one of the most influential inventions of the 20th century. They’ve changed how we work, where we spend our free time, and even where people can live in the first place. As far back as the 19th century, inventors were experimenting with fans and ice to create some sort of machine that cooled air. Because, everyone wants to be comfy. But the reason for inventing air conditioning was, ultimately, an economic one.

In 1902, a publishing company in New York was having problems with the hot, humid summer air wrinkling its magazine pages, blurring its ink and messing up its printing machinery. So it asked young engineer Willis Carrier to figure out a solution. Carrier came up with an “Apparatus for Treating Air,” which was designed to purify the air inside a building and adjust the amount of water vapor in it – the humidity. His apparatus worked by using a fan to push air through multiple chambers. In one chamber, air could be sprayed with water, before moving through plates that acted as filters to take out dust. Then, the air could blow over a set of coiled pipes with a chemical inside that could either heat or cool the air, to alter its humidity. If the chemical was a coolant, it made the air colder, and caused some of the water vapor in the air to condense into a liquid – lowering the air’s humidity.

Over the decades, lots of engineers refined this technology, and eventually the first compact, modern air conditioner was made. Today, the air conditioner you might have in your house or office works basically like Carrier’s. And it’s all based on the thermodynamic fact that heat energy flows from hot areas to cold areas. The a/c you have sticking in your window right now basically uses a fluid – which can be either a liquid or a gas – to move the heat that’s inside your room and take it outside. The fluid is the key. It can be one of any number of chemicals that converts easily between a gas and a liquid, and the rest of your a/c unit just moves that chemical around, making it either expand or compress, at just the right times.

The system starts with a fan that blows the warm air in your room over a coil that’s filled with the liquid. The liquid is cooler than the ambient air, so the heat from the warm air transfers into the liquid, and then a fan blows the cooled air back into your room. At the same time, warm water vapor also condenses on these cool coils and collects in a drain pan, which reduces the humidity, and causes the dripping you often see from some a/c units.

But all the heat that’s been absorbed has to go somewhere. And when the liquid has received enough heat energy, it evaporates – turns into a gas. That gas in the coils is then funneled into another section of the air conditioner, where it’s compressed back into a liquid. And as the chemical condenses, it releases heat energy – that heat is then transferred to the outside air that’s blowing over the coils. Then, the coolant is ready to cycle back through and remove more heat.

Initially, air conditioners like Carrier’s used chemicals like ammonia or propane to transfer heat, but these were really dangerous if they leaked. By the mid-1930s, most refrigerators and air conditioners used chlorofluorocarbons – sometimes known under the trademark Freon – which were non-flammable, and safer to use. But scientists later realized that these compounds can release chlorine that reacts with ozone molecules in the atmosphere, depleting the Earth’s ozone layer, which protects us from UV radiation from the sun. So, in the 1980s, manufacturers gradually stopped using CFCs and replaced them with hydroflurocarbons or HFCs, which are similar to CFCs, but don’t contain chlorine.

So that’s how a relatively simple idea invented for a New York print shop transformed the world – from California to the Arabian Peninsula. Of course, today’s air conditioning units aren’t without their drawbacks – many refrigerants, if they leak into the atmosphere, act as potent greenhouse gases, and the amount of energy that air conditioners use is a real challenge for many communities. That’s why new standards are evolving, and more energy efficient technologies are being developed. These include air conditioners that can use heat pumps and water, instead of chemicals like HFCs as a refrigerant. There are even designs in the works that take out the fluid-filled coils altogether, and use metal rods that are heated and cooled using magnetic fields. So, the future is still looking pretty cool. It just might be cool in different and better ways.

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