How does a refrigerator on a ship work? Well, the main components needed are a compressor, a condenser, an evaporator, an expansion valve, and some refrigerant (a nontoxic fluid). As well as the fridge box, a box with insulation. Don't worry if you don't know what those are yet.

Conceptually, a refrigerator does not "create cold". Instead, it works by moving heat from inside the fridge box to the outside. The way it does this is by manipulating the processes of evaporation and condensation, switching the refrigerant that passes through the system from a gas to a liquid and back again.

In our refrigerator parts list, we stated that we needed a condenser and an evaporator. You might already be connecting the dots here. The condenser will be the pipes where our refrigerant undergoes condensation, the process where a gas turns into a liquid. In our everyday lives, this happens when there are large differences in temperature. Ever have a can or glass of a cold drink "leak", or form droplets on the outside? That's water vapor (water in a gas form) that was floating around in the air hitting the cold surface and having it's heat stolen by the drink, turning into a liquid in the form of small water droplets. The evaporator pipes will house the opposite process, where liquid turns into a gas. It takes heat for this process to work: a pot of water must come to a boil in order to release steam, changing liquid water droplets to water vapor gas. This is what will make the inside of the refrigerator cold- when the refrigerant turns from a liquid to a gas, it will take heat from it's environment- the inside of the cooler box.

In those examples, evaporation and condensation were triggered by changes in temperature. But we want the reverse: we want to force the processes of evaporation and condensation to happen in order to create a change in temperature. This is where the compressor and expansion valve come in to play. By fiddling around with the pressure of the refrigerant, we can control whether the refrigerant is a gas or a liquid.

The compressor is kind of like a vacuum, or a pump. It's main purpose is to move the refrigerant through the pipe system by sucking in refrigerant from one side and pushing it out the other. Another effect of doing this is that on one side it creates a high pressure by stuffing it with as much gas as possible, while creating a low pressure on the other side by sucking out as much gas as possible. The expansion valve helps keep this difference in pressure: in simple terms, it's a wall in the system with a really small hole. This creates a bottleneck in the cycle, like when lanes of traffic merge together, slowing down everybody. This means a one side is stuck with high pressure while the other side has very low pressure.

Now we have all the components: a condenser outside of the fridge box where gas is under a lot of pressure, forcing it to condense into a liquid and release its heat into the environment. This liquid passes through the expansion valve bottleneck and finds itself in the evaporators inside the fridge box, in a low pressure environment that forces it to expand into gas form again. In order for the liquid to evaporate, it sucks heat from inside the fridge box. The compressor carries the warm gas away out of the fridge box and through the system again!

The last pieces to add to our refrigerator are electric controls to adjust the temperature inside the cooler box by controlling when the compressors kick on and off.