Evan Leafy
Have you ever wondered “how does a refrigerator work?” While rocket science and quantum physics are terribly difficult and complex to comprehend, understanding how a refrigerator works is relatively simple and straightforward. You can understand how a refrigerator works by learning the ideal gas law, phase transitions (liquid to gas and vice versa), the four-step refrigeration cycle, and the main components of a fridge. Understanding the basic mechanics of an important piece of equipment can help you diagnose problems early and save costs on repairs and maintenance.
Understanding the Ideal Gas Law
PV=nRT. Does that ring any bells from high school science classes? This is the Ideal Gas Law. P represents pressure, V is volume, and T is temperature. N is the amount of gas and R is a constant. We don’t need to worry about n and R to understand the refrigeration cycle.
The most important thing to note is that as P increases on the left side of the equation, T increases on the right side of the equation. If you increase pressure, you increase temperature. If you decrease pressure, you decrease temperature. In refrigeration, the pressure is increased and decreased by changing the volume. As the volume decreases, the pressure and temperature increase. As volume increases, the pressure and temperature decrease.
Example
Think about pumping up a bike tire. As you push down on the pump, the volume of the air decreases, and the pressure of that air increases. This increased pressure forces the air into the bike tire tube. If you pump many times, the base of the pump will get warm. Consequently, the pump becomes warm because the pressure of the air inside the pump is increasing. This increased pressure leads to increased temperature based on the ideal gas law.
A diagram of how refrigeration works
Phase Transitions
At the core of every refrigeration system is some basic chemistry: phase transitions. In chemistry, a “phase” is also known as a state of matter. While there are four states of matter, we only need to understand three of them to understand refrigeration: solid, liquid, and gas. Before diving into each individual step, we’ll focus on a high-level overview. Because this process is a cycle, we could theoretically start with any step. You may need to read this a few times!
- Refrigerant, in the liquid phase, enters the fridge and freezer.
- As the refrigerant moves throughout the fridge and freezer, it absorbs heat energy from the space, cooling it.
- Eventually, the liquid absorbs enough heat to undergo a phase transition from a liquid to a gas (this is a bit of a simplification, but this will help you understand).
- The unit’s compressor raises the gas refrigerant’s pressure.
- The hot gas flows through the condenser coils on the outside of the fridge. As the hot gas makes contact with the cooler air of the exterior space, it phase changes back into a liquid.
How Refrigeration Cycle Works
Refrigerators run on a closed-loop system. Running throughout this entire system is the refrigerant, which is often an HFC blend or isobutane these days. This refrigerant is the key to removing heat out of the cooler and rejecting it into the air outside the fridge. To begin with the cycle, we will start at the main power-drawing element of a fridge, which is the compressor.
A small compressor below a household refrigerator. Photo by Kristoferb via Wikimedia.
Compressor – The Pressure Increaser
The compressor is the heart of your fridge and the part that draws the most power. Think of the compressor as a big vapor pump. Just as your heart pumps blood to your arms and feet, the compressor pumps refrigerant throughout the system. If the compressor doesn’t work, the refrigerator doesn’t work either.
The compressor increases the pressure of the refrigerant. This increased pressure also increases the temperature of the refrigerant via the ideal gas law. Cool vapor comes in via the suction line and hot vapor comes out via the condensing line.
A condenser coil on the back of a residential refrigerator. Photo by Juan de Vojnikov via Wikimedia
Condenser – The Heat Ejector
The condenser is the next step in the refrigeration cycle. This component is always outside of the fridge. The condenser usually looks like some sort of radiator with tubes that go back and forth. These tubes are hollow and carry the refrigerant. If you laid out the condenser in one straight line, it would be quite long.
The purpose of the condenser is to passively cool off the refrigerant. The high surface area of the tubes in the condenser gives the system a lot of opportunities to exchange heat with the surrounding ambient air temperature. If you feel the lines of a condenser while the fridge is running, they will feel warm.
As the refrigerant moves from the compressor towards the throttling device (more on that next) it becomes increasingly cooler. In fact, at some point in the condenser, most refrigerants change phase from a gas, which has higher energy, to a liquid, which has lower energy. This phase change is where the condenser gets its name. Condensation is the term for a gas turning into a liquid.
At the end of the condenser, the refrigerant is at moderate warm temperature and very high pressure.
In a condenser, fans must move air through the coils to reject heat from the coils to the outside environment. Photo by Endora 6398 via Wikimedia.
Liquid line
The liquid line is a line that carries the liquid refrigerant from the condenser to the throttling device. The liquid line also ensures that all the refrigerant is in its liquid phase. Sometimes, some of the refrigerants can make it to the end of the condenser without changing from a gas into a liquid. In order for the metering device to work, the refrigerant needs to be liquid.
Metering/Throttling Device – The Pressure Dropper
The metering device, or throttling device, is where the magic of refrigeration happens. The metering device restricts the flow of the refrigerant. The most common types are capillary tubes and expansion valves (TXV). In capillary tubes, which are a common type of metering device, this restricted flow is accomplished by forcing the refrigerant through a long tiny tube. Once the refrigerant makes it to the other side of the throttling device, it encounters a much larger space. Therefore, the volume of the refrigerant increases substantially to fill up this larger space.
Going back to PV=nRT, we can see that if volume increases rapidly and substantially, pressure and temperature will decrease substantially. This is the most difficult theoretical aspect of the refrigeration cycle to understand, so here’s an imperfect analogy.
The Dance Club Analogy
Imagine you are at a crowded club where everyone is dancing. The room gets pretty hot, right? Let’s say you try to leave, but there are a whole bunch of people also trying to leave through the only door. After waiting, it’s finally your turn to exit the club and go through the door. As you emerge onto the other side, there is a big room with few people and lots of space. This room is much cooler, since there is less body heat, and you have plenty of space to move around and dance your heart out.
In this analogy, the door is equivalent to the throttling device. The hot room is the condenser, and the cool room is the evaporator (below). It is crowded and hot on one side of the door, which represents the condenser side of the metering device. This condenser side has high pressure (crowds) and temperature among the dancers. In addition, each dancer is only able to occupy a small volume.
On the other side of the door, which represents the evaporator side of the metering device, there are far fewer people. These fewer people correspond to the lower pressure and lower room temperature on the other side of the door. Each dancer is also able to occupy a much larger volume because there are fewer people to run into!
Granted, this analogy isn’t perfect. Hopefully, it helps you envision how a throttling device works in refrigeration.
Flashing
The concept of flashing is simple but important. It is a common name to describe the change of the state of the refrigerant after it passes the throttling device and makes its way into the evaporator. Refrigerants have the amazing property to flash (boil) at very low temperatures when exposed to low pressures. It is this characteristic that allows the coil to cool down (get frosty) and therefore exchange heat with the air inside the room.
Evaporator – The Heat Absorber
After the metering device, the refrigerant enters the inside of the fridge via the evaporator. The evaporator is similar to the condenser, except it works the other way around. As with the condenser, the evaporator is one long, coiled, hollow tube that loops back on itself. Unlike the condenser, the evaporator makes its journey through the inside of the refrigerator and freezer.
Thanks to the throttling device, the refrigerant is super cold at the beginning of the evaporator. As we push warm air from the room through the coil using the fans of the evaporator, the refrigerant gains heat and turns into gas again as it makes its way out the evaporator coil, effectively cooling the air inside the fridge.The evaporator removes heat from the air inside the fridge.This is why a fridge (or any cooling room) has to be sealed, so that the inside air can be recycled and the evaporator can properly and efficiently remove all the heat contained in the space.
Instead of running from hot to cooler, as the condenser does, the evaporator changes from cool to warm. In the evaporator, the refrigerant changes phases from a liquid back to a gas. This phase change is called evaporation, which gives the evaporator its name.
The evaporator is a common part of the system that causes trouble with coolers. If your refrigerator or freezer doesn’t work, check out the evaporator to see if it is freezing up.
The Suction Line
The suction line connects the evaporator to the compressor. The suction line runs from inside the fridge to outside the fridge. It is called the suction line because the compressor is sucking the refrigerant from the evaporator and pushing it into the condenser.
Other Components That Make A Refrigerator Work
While the refrigeration cycle is critical, the other components that make a refrigerator or freezer unit are also important to understand (thus allowing the refrigerator to work). At the most basic level, the thermostat is the main control on a refrigerator or freezer, however, depending on the system or the application, there can be many other components like: pressure controls, fan delays, heaters, defrost timers, solenoid valves, etc.
The Thermostat
The thermostat inside a fridge or freezer is the main control device and the most common to all of us. It is simply an automatic on/off switch that allows the refrigerator to work. Just like the thermostat of your A/C at home, when the temperature inside the fridge is warmer than the temperature the thermostat is set to, the thermostat kicks the compressor on to start the cooling cycle. The compressor will run, until the thermostat registers the fridge temperature has reached the set point.
When you set the thermostat lower, the compressor will work more frequently because the system needs to remove more heat from inside the fridge. Conversely, if you set the thermostat warmer, the compressor will work less often.
Finding the correct temperature to keep your cooler or freezer will save on energy bills and increase the longevity of your unit. A typical commercial walk-in cooler is designed to work at 35°F and a freezer at -10°F.
Insulation and Structure – Essential to a Working Refrigerator
The structure that contains a fridge or a freezer is incredibly important, and it helps the refrigerator work! Without proper insulation, a fridge or a freezer wouldn’t work efficiently. The layers of insulation that separate the cold environment inside from the ambient air outside keep the space isolated. More effective insulation results in less heat transferred; this, in turn, results in the unit using less energy.
Of course, there are many other aspects of the cooler or freezer structure to consider when you are looking to buy a unit. Some of these include the size, the door type, interior/ exterior finish, and where the condensing unit is situated.
Final Thoughts of How Refrigeration Works
Hopefully, you can answer the question that compelled you to read this article; “How does a refrigerator work?” Now that you understand the basics of the refrigeration cycle and the components of a refrigerator, you should familiarize yourself with common maintenance issues with coolers and freezers. Knowing a few simple cues to look out for could save you money, time, and product!