Photo credit: Pixabay
Mother Nature, in her wisdom, has created laws that help define the world around us. Gravity acts downwards, opposites attract, heat flows from hot to cold. For the most part we accept and abide by these laws, and even take advantage of them to ease our day to day lives. Yet sometimes the rules get broken, Nature is flipped on her head and the Miracle Box is created. The power to direct heat contrary to the dictation of scientific law is now widely available to the majority of mankind: in fact, we use it to make sure the milk doesn't go bad.
OK, the above is hyperbole in the extreme and technically no scientific laws are broken, but it is true that engineers have created some very intelligent and impressive workarounds to the problems we face when we try to go against the grain. The refrigerator, or Miracle Box for the dramatists amongst us, is one such creation. If you put a cold drink in a warm room and leave it a while, it too will be warm when you return to it. Heat will not spontaneously flow from a cold object to a hot one; it takes energy to go against the natural flow. This is where the refrigeration unit comes in.
The job of a refrigerator is to take heat from somewhere cool and expel it somewhere comparatively warm. To do this, fridges use a middleman, a refrigerant, to transfer the heat from the cool environment to the hot one. There are many different methods of refrigeration that fall under four different categories: non-cyclic, cyclic, thermoelectric and magnetic. The method you'll most likely find in your kitchen is vapour-compression refrigeration, which falls under the cyclic category.
The refrigertion cycle. Credit: Steven M Gordon (Wikimedia Commons).
The diagram above helps explain how the vapour-compression cycle got its name; the refrigerant used goes through both the gaseous and liquid phase, and compression occurs. The refrigerator unit consists of four main parts: the compressor, condenser, throttle (or expansion valve) and evaporator. The refrigerant cycles through these parts and goes through a process that allows a cool temperature to be maintained. The interesting part of any trick is, however, how it's done. Yet, before the cycle and the processes involved can be explained, it is important to note that temperature and pressure are both determining factors in defining the boiling point of a substance. For example, water boils at 100°C, but that is only at atmospheric pressure. It will boil ten degrees lower, 90°C at around 0.69atm, the pressure halfway up Mount Everest. At a lower pressure, less energy is required to cause boiling. Of course, the inverse is also true, and the vapour-compression cycle uses this secret complete the trick of taking heat from cool place and dumped in a warm place.
Point 1 - 2 Starting at Point 1, the refrigerant is a vapour at a low pressure and a low temperature, and thus has low internal energy. The vapour passes through the compressor which does work on it by compressing it. This increases the pressure and the temperature, and thus the internal energy, of the vapour, but it remains a vapour.
Point 2 - 3 The hot vapour continues on through to the condenser, where it is condensed into a hot, high pressure liquid. This is an example of the effect pressure has on boiling point. Because the vapour was at a high pressure, it has a high boiling point. This means it can be condensed, thus losing internal energy, and still be relatively hot. The condenser is the part of the unit exposed to the hot environment, ie, the back of the fridge that is exposed to the air. At this point the refrigerant is hotter than its surroundings so can lose energy to them.
Point 3 - 4 After the condenser the refrigerant passes though the throttle valve, or expansion valve. Here the pressure is rapidly dropped, which in turn rapidly decreases the boiling point of the refrigerant. This causes flash evaporation, and part of the refrigerant turns into vapour but most of it remains a liquid. The refrigerant is now a cool liquid at a low pressure, and thus a low internal energy.
Point 4 - 1 It continues on to the evaporator where the liquid is completely vaporised by taking in energy from its surroundings. As the pressure of the refrigerant is very low, its boiling point is low as well, allowing it to lose energy to the cool environment. The evaporator is the part of the unit exposed to the cool environment, ie, the inside of the fridge. After exiting the evaporator, the refrigerant is a cool vapour at a low pressure and has returned to the start of the cycle.
By repeating this process over and over, the unit can keep the fridge cool. So yes, it is possible to transfer heat from a cold space to a hot place on a macroscopic scale but when you get into the nitty-gritty, heat still flows from hot to cold. Energy has to be put into the system, in this case the energy put into the system by the compressor upon compression. So no miracles occur and the laws of thermodynamics remain as undeniable as ever; yet I believe that the fridge is an excellent example of how any problem can be solved and any hurdle can be circumnavigated, and of what can be achieved through engineering. So next time you’re in the hot sun enjoying your cool drink, take time to ponder the next ‘miracle’ in the works.