How do fridges work and staying cool in summer
With summer on the doorstep and this Wednesday hitting 30° C you might consider using the fridge as the last resort like this guy:
Although it seems very tempting, unfortunately the coolness would not last long as your average fridge can’t cool that well.
But how does a fridge even work, how can one cool things down. The temperature of an object is defined by how much the molecules/atoms inside the object are moving around (or jiggling in case of a solid). It is easy to imagine how to heat something up, putting it over a fire for example. But when you want to cool something you must extract energy and make the molecules in it move slower. You need something cooler than the object to cool it down. Well, but how do you cool that something down in the first place.
Through the magic of physics.
Evaporation and Condensation
We humans are a great example; we can cool down by sweating. The excess heat of our body gets transferred to the water in the sweat which then evaporates (evaporation just means that the molecules go from a denser state (liquid) to a less denser state (gas)). The body lost its heat to the water on its skin, which “used” the heat to evaporate. This process is called evaporation.
The opposite is called condensation. A substance going from the gaseous state into the liquid state while losing heat. This is what happens when you take a hot shower. When the hot water vapour from your shower hits the cold mirror the vapour loses its heat to the mirror. This cooling leads the water vapour to turn into liquid water, which you then see as droplets against the mirror.
These to processes are crucial to understand how a fridge works. The only way to cool down an object is to transfer its heat to another object/substance.
Changes in pressure also lead to changes in temperature. This is why, a bicycle pump gets hot when you compress the air and a spray deodorant gets cold when the compressed liquid expands into gas by opening the nozzle. Fridges exploit these principles to cool your food.
Basic principle of a fridge
The fridge has a system of tubes containing a liquid, called refrigerant (or coolant) running continously in a closed loop from the inside to outside and back inside.
- The liquid refrigerant is released via an expansion nozzle (1) into the evaporator tube.
- There is low pressure in the evaporator tube (2) causing some of the liquid refrigerant to evaporate immediately and cool down. The rest of the refrigerant gradually evaporates by taking heat from the inside of the fridge (just like we cool down with sweat).
- All of the refrigerant reaching the compressor (3) is now gaseous. As the name suggests, the compressor compresses the gaseous refrigerant heating it up in the process.
- The hot compresses gaseous refrigerant now travels through the condenser (4). The surrounding kitchen air is much colder than the refrigerant and cools down the walls of the condenser tube. On the inside of that tube the hot gaseous refrigerant loses its heat to the tube and condensates on the tube (just like the water vapour on your bathroom mirror). This tube is often connected to grill rack like structures to increase heat transfer to the ambient air.
- The liquid refrigerant travels through the insulation of the fridge and reaches the expansion valve and the cycle starts again.
Now that you know how a fridge works you should have realized that leaving a fridge open is not going to help. The fridge cools by transfering heat from the inside to the outside ambient air using a refrigerant as “transport vehicle” for heat. This is why you can get a portable heater for the winter but not a portable air conditioning system in the summer. Air conditioning systems need a connection to the outside environment to lose their heat to.
But, Alex! What if I don’t have built in air conditioning? How am I going to survive the summer, if I can’t even sit in the fridge??
The best thing you can do to stay cool in the summer is get yourself a fan. It will increase the heat transfer from your body to the surrounding air by providing fresh air that you haven’t heated up yet. And if it’s over 37°C, so hot that there is no considerable heat transfer it will at least increase the evaporation of the sweat on your skin.