Pushing Out Heat to Keep Refrigerators Cold
Most of us have used a refrigerator before, whether it's grabbing a cold drink after working in the yard or playing sports, putting last night’s dinner or leftovers in there, or getting our favorite frozen treats from the freezer. They’re a part of everyday life for so many of us that we just expect a refrigerator to be in our homes, our offices and work places, restaurants, and medical care centers and hospitals. Many of us don’t think about what could happen if the fridge or freezer stopped working—until it does. However, this was not always the case as there were times when these machines weren’t even an idea.
Ancient Preservation for Survival
According to Brian A. Nummer, Ph.D. from the University of Georgia, there were several methods of preserving food in ancient times that best fit with their environment. This included Middle Eastern and oriental cultures as early as 12,000 B.C. drying foods, Romans drying fruit, and “still houses” in the Middle Ages to dry fruits, vegetables and herbs. Cellars, caves and cool streams were used in cold or freezing environments to keep food cold or frozen to help keep them fresh for longer periods of time.
The continuing ancient practice of drying food
Fermenting was thought to have been discovered around 10,000 B.C. to make beer as a nutritious drink that rewarded the consumer with an elevated spirit thanks to a special ingredient known as alcohol. Romans used a pickle sauce, known as “garum”, to preserve food in it.
There were other methods used as well by our ancient ancestors to preserve foods. These methods demonstrated mankind’s ability to adapt to the environments in which they lived. However, as time moved on and progress was being made, new methods and more pragmatic ways of food preservation were being explored and developed to keep pace with the newly industrialized world.
An Industrial and Cultural Revolution
As time progressed into the late 1700s and as the Industrial Revolution was about to begin, Americans began storing food in pits buried in the ground with huts built above them known as icehouses. These pits contained ice that was gathered from nearby lakes and were able to remain frozen during the hot summer months. Unfortunately, icehouses weren’t accessible to everyone as only those who were in the upper class were able to afford these icy pits.
Thankfully, as time progressed to the late 1800s, people who weren’t so well off were now able to buy a device known as an icebox. As the name implies, an icebox stored ice at the top and kept food chilly in the storage below for a reasonable period of time. The inside lining was made of tin or zinc, which are cheap metals that can conduct, or transfer, the cool temperatures from the ice to the bottom storage compartment to keep perishable food colder for longer periods of time. Straw, sawdust, cork or seaweed was used as insulation to help keep the heat out and have the icebox stay cold on the inside.
As time carried on and advancements were made through the late 1800’s through the early to mid 1900’s, society changed as well as its needs. So, when World War II ended, the middle class had extra money and could afford nicer things, like a refrigerator. These became the hallmark of a home’s kitchen in so many ways. You could now store your vegetables, fruit or some other delicious food in your General Electric or Philco fridge or freezer and rest assured that it would stay fresh for a good while longer.
A typical 1950’s refrigerator storing fruits and vegetables
A More Modern Approach
As with all things, the refrigerator wasn’t even close to being in its final form. The 1980’s saw refrigerators en masse incorporate water dispensers that could give you cold water and ice by just pressing the button. The late 90’s to early 2000’s was a time that stainless-steel refrigerators were becoming popular. Even though the first smart refrigerator was introduced by LG in June of 2000, they didn’t really take off and become popular until the late 2010’s to early 2020’s
The LG Internet Digital DIOS (Courtesy of Wikipedia)
A Reliable Solution for Business and Health
Not only did advancements in refrigeration help with everyday living, but they also helped in other spaces as well: medicine and dining. Medical-grade refrigerators are present in nearly every health clinic and hospital across the world to store patients’ medicines and treatments. Restaurants rely on these devices to keep high-quality food fresh longer and reduce the chance of spoilage.
Fundamentally the Same Throughout Time
Even though the refrigerator has advanced over time with new materials and technology, the fundamentals are still the same. What is a refrigerator anyway? Well, it’s a machine that’s able to transfer warm air or heat from an area of low temperature to an area of higher temperature. Yes, refrigerators now most likely work better than they did back in the day, but the fundamental components and engineering behind them hasn’t changed.
These devices still use the same basic components to keep your food and drinks cold and fresh for longer periods of time. They still use evaporator coils, a compressor, a condenser, and an expansion device of some kind. These devices still use the exact same engineering principles from thermodynamics, heat transfer, fluid dynamics, and materials science to keep all your favorite foods and drinks fresh and cold for longer periods of time. Let’s take a dive into these components and see the engineering principles behind them.
A Smooth, Fluid-Like Operation
Starting With the Basics
Each refrigerator has four basic components to it: the evaporator coils, the compressor, the condenser, and some kind of an expansion device. The expansion device is mainly either a capillary tube or a thermal expansion valve (TXV); usually refrigerators used in restaurants or medical settings have a TXV since they tend to regulate temperature more effectively while a residential refrigerator would have a capillary tube due to manufacturing costs. Each of these components are connected to each other transporting refrigerant throughout the system. Let’s explore that process and see how it works.
So, what do these different parts do? Let’s start with the evaporator coils, which is the component responsible for extracting heat from the refrigerator compartment and replacing it with cold air. The refrigerant then moves on to the compressor, which pumps the refrigerant throughout the system while also increasing the temperature and pressure of the refrigerant. The refrigerant is then carried to the condenser, where the heat being carried by the refrigerant is released into the environment. Then, the refrigerant moves to the expansion device, where the liquid refrigerant experiences a decrease in pressure and becomes a low-quality mixture of liquid and vapor.
Here are articles from KaTom and Whirlpool that help explain these components in more detail.
The refrigerant then fully returns to a vapor in the evaporator coils and the cycle keeps repeating. Now that the cycle has been outlined, how does this work, exactly? How is it calibrated so well yet so simple? Why don’t we take a look at the thermodynamics —yes, we are going through the thermodynamics of it, but it won’t be too bad, I promise!—regulating these systems and incorporate a little bit of fluid dynamics in it as well.
The Power From Within
The compressor doesn’t just pump the refrigerant through the system on its own and have the refrigerant work special magic through the system, unfortunately. It has to depend on thermodynamic principles to make this work. There are two main cycles that are used to describe refrigeration—The Reverse Carnot Refrigeration Cycle and the Ideal Vapor-Compression Refrigeration Cycle.
Thankfully, there are phase diagrams that help explain these cycles so we can better understand how they operate. If you’re not familiar with the term phase diagram, it’s a diagram that shows the different stages of the refrigerant in each cycle with respect to the evaporator coils, the compressor, the condenser, and the expansion device. Now the moment we’ve all been waiting for: a deep dive into the thermodynamics of both the Reversed Carnot Refrigeration Cycle and the Ideal Vapor-Compression Refrigeration Cycle with clear phase diagrams to help visually explain each cycle.
The first cycle we’ll discuss is the Reversed Carnot Refrigeration Cycle. It begins at the evaporator coils where the heat is absorbed from the cold area and ends in the cycle’s expansion device—a turbine. Yes, a turbine is what’s used as the expansion device in this system.
Let’s break down the diagram. The refrigerant is represented by the arrowed lines within the temperature (T) vs. entropy (s) curve. The numbers at the four corners of the square lines represent the different phases of the refrigerant. In between each phase is the component of the Reversed Carnot Refrigeration Cycle: the evaporator (1-2), the compressor (2-3), the condenser (3-4), and the turbine (4-1). The arrows on the lines represent the direction of the refrigerant’s flow. There are also the two symbols shown in the diagram: QL and QH. QL represents the heat being absorbed by the evaporator coils and QH represents the heat being dissipated into the environment from the condenser.
Great, now we know what each phase is, but what is entropy (s), exactly? Well, in plain terms, it’s a measure of disorder or chaos in a system. What does that mean? It means the molecules in the system are moving randomly and at high speeds. To help me explain entropy a bit further and to help you understand entropy more, I’m about to show you an equation and break it down in a way you can understand:
s = kb ln W
This, ladies and gentlemen, is what’s known as The Boltzmann Equation. Discovered by the brilliant Austrian mathematician and physicist Ludwig Boltzmann, this equation explains how entropy (s) works in a system. In order to find entropy (s), you will need to know the Boltzmann Constant (kb) and also the number of microstates within a macrostate (W). To explain more clearly what W means, it’s simply stating how many smaller groups of molecules exhibit different patterns of randomness and chaos within the group of total molecules in the whole system.
Thankfully, in the context of refrigerators, the Boltzmann Constant (kb) is determined to be 1.38 x 10^-23 J/K. This value was approved in November 2018 by the Système International d’unités (SI), which is also known as the International System of Units. Notice how the units are J/K (Joules—energy—per Kelvin—temperature) and how it fits with the phase diagram’s use of temperature.
The Reversed Carnot Refrigeration System is the most efficient refrigeration system because it is known as a “reversible” cycle, meaning that it can complete all four phases without any losses in an ideal setting. However, nothing is ideal in the real world. Let’s revisit the phase diagram for the Reversed Carnot Refrigeration Cycle and explain why this is the case.
When we look at the refrigerant moving through the compressor (2-3), the refrigerant will be a liquid-vapor mix instead of a vapor. Liquid in a device such as a compressor can potentially cause damage and affect reliability of the system. They’re not designed to have substantial amounts of both liquid and fluid move through them like the amazing valve from Alfa Laval is.
Having the liquid refrigerant move through a turbine (4-1) is also not practical. While there are examples of this that take place in the real world, having a turbine as an expansion device is just not very cost effective, especially compared to a TXV or a capillary tube. It’s a neat idea, but it’s also overkill.
Nonetheless, the Reversed Carnot Cycle is still used, but only for theoretical purposes. This cycle helps determine how efficient a current refrigeration system is and whether or not an engineering team should reassess its design. Hopefully that section wasn’t too bad, because we will now move to a more realistic refrigeration cycle: The Ideal Vapor-Compression Refrigeration Cycle.
The Ideal Vapor-Compression Refrigeration Cycle is really the cycle that much more accurately demonstrates the phases of the refrigerant in a refrigerator. Unlike the Reversed Carnot Refrigeration Cycle, the Ideal Vapor-Compression Refrigeration Cycle begins at the compressor and ends at the evaporator coils. Since the refrigeration cycle uses pressure and temperature, not only does it have a T-s phase diagram, but it also has a P (Pressure) vs h (enthalpy) phase diagram used by AC technicians and engineers alike. Let’s break down both diagrams and see how they compare.
Notice how the location of the numbered phases has changed to reflect the change in where the cycle starts and stops in both phase diagrams. In the Ideal Vapor-Compression Refrigeration Cycle, the refrigerant moves through the compressor (1-2), then the condenser (2-3), then the expansion device (3-4) and finally the evaporator coils (4-1). QL and QH are the same in the Ideal Vapor-Compression Refrigeration Cycle as they are in the Reversed Carnot Refrigeration Cycle.
So, we understand that the first phase diagram is a comparison of Temperature vs. entropy in the system, but what about the second phase diagram? Why is it pressure (P) vs. enthalpy (h)? What is it supposed to tell us about the Ideal Vapor-Compression Refrigeration Cycle?
This phase diagram uses enthalpy (h) and pressure (P) as another method to determine the phase changes in the system. It is a method that engineers and technicians use in the real world. This is because the enthalpy (h) and pressure (P) phase diagram shows how pressure and energy changes affect the refrigerator’s performance. To understand this better, let’s break down what enthalpy is and how it is calculated.
What is enthalpy, you ask? Well, enthalpy (h) is the sum of the total internal energy (U) and the product of the pressure and volume of the system (PV). When writing that statement as an equation, here is what it looks like:
h = U + PV
In this case, the value for the internal energy (U) is predetermined based upon the vapor used in the system and the thermodynamic state the vapor is in. This formula is always true whether or not the internal energy (U), pressure (P) or volume (V) vary in a system. So what if there is a change in either of those variables? How could we calculate the change in enthalpy (h)?
Δh = ΔU + ΔPV
That’s the equation for a change in entropy, where Δh is the change in enthalpy, ΔU is the change in internal energy and ΔPV is the change in pressure and/or volume; if pressure, volume, or both pressure and volume change, it’s still reflected in the equation as ΔPV. Wait, if the internal energy is predetermined by the type of refrigerant used, how can we calculate what the change in internal energy will be? Feast your eyes on this equation and fear no more:
ΔU = Q + W
Now we can solve for the change in internal energy, ΔU. We need to know how much heat the system is receiving from its surroundings (Q) and how much work is being done on the system by its surroundings (W). This equation is from the First Law of Thermodynamics.
So now we’ve got the thermodynamics behind refrigerators down, right? Well, almost, but that’s the Ideal Vapor-Compression Refrigeration Cycle, not the Actual Vapor-Compression Refrigeration Cycle. Don’t worry, I won’t draw more phase diagrams, because both are governed by the same school of thought and both start and end at the same location.
What is the difference between the two? The technical reason is because of irreversibilities, or inefficiencies in the actual cycle itself. The two main ones are heat transfer and friction. If we take a quick trip down memory lane and visit an article I wrote where I discuss heat transfer in homes, we can see why this would be an irreversibility or inefficiency in the process.
With heat transfer, there are three components: conduction, convection, and radiation. Conduction is when energy is transferred along either a solid such as a metal or stationary liquid like water. Convection is when heated particles in a fluid such as air or water move through the liquid and transfer heat. Radiation is when an object emits heat from its surface. Since these are present in actual refrigerators, it is easy to see why these are irreversibilities.
Well, how does friction play into all of this? This is where fluid dynamics comes into play. From fluid dynamics, it is known that as a fluid moves through an enclosed system, the pressure changes from one end to the other:
Δp = p1 - p2
Why is that? That is because in another application of Δp, there is also a component known as shearing stress at the wall of the pipe. A shearing stress is a stress that is applied parallel to the surface of a fluid. Because of that shearing stress, there is friction—the second main irreversibility in the system.
There, you made it! Congratulations on getting through some pretty deep thermodynamic theory! I told you it wasn’t that bad…right? Now that we’ve got the thermodynamic theories down, let’s take a moment to look at the different types of refrigerants that are currently being used.
When it comes to refrigerant, the ones that comprise 90% of the total market are R-11, R-12, R22, R-134a, and R-502. R-11 refrigerant is used in large-capacity water chillers that are used in AC building systems. R-134a refrigerant is used in applications like domestic refrigerators, freezers, automotive A/C systems, and as a replacement for the toxic R-12 refrigerant.
R-22 refrigerant is used in things like window air conditioners, AC systems for commercial buildings, large refrigeration systems, and as an alternative to ammonia-based refrigerants. R-502 refrigerant, which is a mix of R-115 and R22 refrigerants, is used in refrigeration systems of large supermarkets such as Walmart, H-E-B, Publix, etc. There, now we’re completely done with thermodynamics and we’ll continue by discussing the materials that are used to make refrigerators carry out all the thermodynamics they need to work properly.
Made From the Good Stuff
In order to have the refrigerator operate with the least amount of irreversibilities in the system, the refrigerator needs to be made from high-quality materials that minimize irreversibilities. Even though it won’t be perfect, it is essential that the refrigerator is made well to maximize its efficiency and effectiveness without making it too expensive to make or too cheap to be taken seriously. Let’s take a deep look into the materials that help make the refrigerator into a modern-day necessity.
Built to Industrial Grade
Industrial refrigerators used in restaurants need to be made with high-quality materials in order to help their users be as productive as possible. The exterior is usually made from stainless steel in order to make it easier to clean. The insulation usually consists of a foam like polyurethane since it’s very good at minimizing heat transfer.
When dealing with tubing, that’s usually made from copper since the metal is an excellent conductor and bodes well against oxidation. The coils and fins in industrial refrigerators are usually made of aluminum since they’re lightweight and cost-effective. The gaskets that keep the warm air out are made from materials such as PVC or silicone and are good at keeping an air-tight seal.
Industrial grade refrigerator (Courtesy of BuildOps)
Made For Your Home
Similar to industrial refrigerators, refrigerators are made from similar materials. The doors are usually made from aluminum, steel or stainless steel for easy cleaning. The inside of the refrigerator is either made of sheet metal like what’s found on the doors or some kind of plastic, depending on the function of the refrigerator and the budget allotted to make the refrigerator.
Filling the gaps between the refrigerator’s exterior and interior is insulation material like fiberglass or a polyfoam. When it comes to the piping, it's made from copper like in the industrial grade refrigerators. Also like in the industrial grade refrigerator, the fins and coils can be made from aluminum, but can also be made from copper or an appropriate alloy.
An elegant modern-day refrigerator
Another important point to make is that sometimes, spills happen. This is why tempered glass is used in modern-day home refrigerators. Not only does it clean more easily, it's also created to withstand temperature fluctuations, thus increasing the refrigerator’s durability.
Those plastic bins that are usually at the bottom? Yeah, they’re made from plastic for a reason as well. They’re lighter, more durable to impacts like dropping oranges or carrots compared to glass, and are more energy efficient.
The Spirit Behind It All
Speaking of energy, let’s talk about how this is all able to take place. As I mentioned before in How Homes Help Us Beat the Heat, this process is the reverse of what happens in nature and needs energy to make it happen. However, it’s important that this energy is used efficiently as well and we need to look at how this is being measured.
Efficient Performance
I know we got through some deep theory in thermodynamics, but I’m going to revisit it just to bring out a formula that’s known as Coefficient of Performance. Let’s look at it and break it down:
COPR = QL /Wnet, in
In this example, COPR is the Coefficient of Performance for a Refrigerator, QL is the heat extracted from the refrigerated space (take a look at the phase diagrams if you need a refresher), and Wnet, in is the work (energy) needed to power the refrigerator.
The way this works is the higher the COPR, the more energy-efficient the refrigerator is. In case you’re wondering, the average COPR for a refrigerator is around 2.5. You remember the Reversed Carnot cycle? Well, it’s back, but it’s not nearly as in depth as the thermodynamic explanation was, so let’s dig in:
COPR, Carnot = (1)/((Th/Tl)-1)
Let me write it in words to help better explain this: the Coefficient of Performance for the Reversed Carnot refrigerator (COPR, Carnot) is one (1) divided by the difference between the division of the high temperature (Th) by the low temperature (Tl) and the number one (1). This is the highest COPR value that the refrigerator can get, but can never attain because the Carnot cycle is an idealized theoretical model.
Shoot For the Stars
I will say this, though: the refrigerators whose COPR values are closest to their respective COPR, Carnot values have most likely received an Energy Star Certification Mark. Even though they use different criteria, Energy Star certifications are still the gold standard when determining a refrigerator’s energy efficiency. This level of efficiency will definitely help save on energy costs in the future.
The industry’s gold standard in energy efficiency (Courtesy of Energy Star/US EPA)
There’s a Cost to Everything
Welcome Home!
Speaking of costs, a good refrigerator is going to come at a price. An average refrigerator for your home will cost you $2,500.00 with installation, with some going for far more than that. According to the Social Security Administration, the average income in 2023 was $66,621.80. This means that a quality refrigerator is at least 3%-4% of a person’s annual income, representing a significant cost for many.
Home installation crew (Courtesy of Arnold’s Appliance)
What’s also worth mentioning is that average maintenance and repair costs for refrigerators is around $650.00, but your mileage may vary. Depending on the quality of your refrigerator, this figure might be more or less than the average figure posted here.
Mise en Place
Those figures were for residential refrigerators, but what about restaurants, the places we like to go to when we don’t feel like cooking or wanting to catch up with a friend? How much do they have to shell out on refrigerators? Well, the answer may or may not surprise you if you’ve worked in the food industry.
Restaurant-level equipment always comes at a higher cost (Courtesy of Auguste Escoffier School of Culinary Arts)
The average cost to start a restaurant is approximately $375,000. A typical restaurant that can be started with this budget can also serve 75 people on an average day and somewhere between 150-200 on a busy day. In that case, the restaurant might need 3 refrigerators and 2 freezers.
Let’s break down the cost of these refrigerators and freezers, shall we? Well, for a restaurant-grade refrigerator, it’s most likely going to cost somewhere between $5,000-$7,500 per refrigerator. It’s similar to how much a restaurant-grade freezer will cost you, which is somewhere between $4,000-$7,000.
Now that we have the numbers, let’s do some light math. On the “cheap” side:
3 refrigerators x $5,000 per unit + 2 freezers x $4,000 per unit = $23,000
Now let’s focus in on the “expensive” side:
3 refrigerators x $7,500 per unit + 2 freezers x $7,000 per unit = $36,500
If we use these numbers, the cost of just purchasing the necessary amount of refrigerators to sustain business operations will be somewhere between 6% to 8% of the total $375,000 budget. That’s just refrigerators. By the way, that doesn’t include repairs that can range between $100-$1200 per unit. That’s also not accounting for lost revenue due to faulty refrigerators/freezers spoiling the ingredients you might need to cook with.
So, for those of you who want to start a restaurant someday, just make sure to never underestimate the importance of a refrigerator!
A Component of the Backbone
As outrageous as those prices and costs for restaurant-grade refrigeration and freezing equipment are, there is an environment where the costs and the stakes are even higher, which can be found at your local medical clinic or hospital. The labs in these environments are the support system behind all the operations which take place, from running diagnostics to storing patient medicines and treatments. Anything that could go wrong here could not just cause a slight stomach bug, but could seriously harm the health of all the patients medical clinics and hospitals serve.
To make sure these facilities are maintained at the highest standard, an average 5,000 square-foot medical facility will cost somewhere between $1 million to $2 million. Since the stakes are higher, medical-grade equipment is required and will cost you quite a bit more. You can see here that prices for medical-grade refrigerators can only be found by requesting a quote. Like the old saying goes, “If you have to ask, you can’t afford it”.
Figures aren’t given, but it’s not unreasonable to assume that a 5,000 square foot facility would need 2 refrigerators and 1 freezer to safely and adequately store patient products. There aren’t any figures available for refrigerator or freezer repairs, but it’s also not unreasonable to assume that they are most likely just as much as a restaurant-grade refrigerator or maybe even more. Making sure these devices run properly isn’t just good practice in these environments, it also comes with serious responsibility to the health and welfare of the patients.
Compressing It All Together
Hopefully by now you realize all that goes into a refrigerator. They are devices that are the result of centuries of refinement and evolution, from the earliest icehouses to the modern-day smart refrigerator. Their “basic” design is simple to follow, yet the science and engineering behind it is what makes them cost as much as they do.
Nevertheless, these devices are essential for living in modern society. They store our favorite leftovers, the food we plan to cook for our families and help us maintain healthy lifestyles by keeping high-quality food fresh for longer periods of time. They also help maintain human well-being by storing essential medicines and treatments for patients that need it most.
If poorly manufactured, it could lead to detrimental outcomes. Not only will it come at a financial cost, but it can seriously impact the quality of life of the people who depend on these devices. The food and medicines stored in a poorly manufactured fridge could spoil or be compromised, affecting bottom lines and patient health outcomes. Furthermore, poorly maintained equipment could also cause the same detrimental outcomes as those seen in poorly manufactured refrigerators.
Let’s close with these questions: can we make refrigerators more energy efficient? Can we make them as effective and reliable as they are with fewer materials? How can we make them more affordable for everyone? Let us know in the comments!
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