The Cold Truth about Refrigeration
The science behind an essential technology
Suppose your air conditioner breaks down in the middle of summer. You are sweaty and miserable, but you can’t find anyone to come fix it the same day. You just want some relief from the heat. Here’s a little puzzle for you. What would happen if you left the door open on your refrigerator? Would the room get cooler?
It might surprise you that the opposite would happen: the room would get even hotter. But why? To understand this, let’s divide a refrigeration system into its parts and see how each one works. With a few scientific principles and a basic understanding of refrigeration machinery, the answer will become clear.
The Key Concept: Phase Changes
Air conditioners, refrigerators, and freezers all work the same way. Their goal is to extract heat from inside a thermally-insulated box (the enclosure) to make its contents cooler. These machines all implement a refrigeration cycle, a four-step process where a substance called a refrigerant is pumped through copper tubing in a loop.
I’ll come back to the refrigeration cycle later. First we need to talk about what happens when a liquid evaporates into a gas, and what happens when that gas condenses back to a liquid. The phase of a substance is what state it is in: solid, liquid, or gas. For example, ice is an example of water in its solid phase. When solid ice melts into liquid water, we say that its phase is changing from solid to liquid.
In general, when a substance changes phase from solid to liquid (melting), or liquid to gas (evaporating or boiling), it absorbs a large amount of heat from the surroundings. The opposite is true too: when a gas condenses into a liquid, or a liquid freezes into a solid, it releases a large amount of heat to the environment around it.
One thing that often confuses people is the difference between heat and temperature. During a phase change, the temperature remains constant, but a lot of heat energy is released or absorbed. For example, if you have ever defrosted some frozen soup on the stove, you know it takes a long time. The stove burner keeps putting heat energy into the soup to make it melt, but the soup stays near 0°C the whole time the melting is in progress. It takes a lot of heat energy to convert 0°C ice to 0°C water, even though the temperature isn’t changing at all. Once the soup completely melts, the temperature starts to increase quickly.
To do their job, refrigerators, freezers, and air conditioners take advantage of phase changes in the refrigerant they use. Let’s talk more about refrigerants.
A refrigerant is a substance that can be compressed from a gas to a liquid, then allowed to expand back to a gas. The refrigerant must do this within a practical range of temperatures and pressures for the intended cooling application. Modern refrigerators and air conditioners use hydrofluorocarbons, or HFCs, as refrigerants.
HFCs are useful as refrigerants because of how they change phases. They boil from liquid to gas and condense from gas to liquid at just the right pressures and temperatures to be useful inside a refrigerator.
The most common HFC used in refrigerators is 1,1,1,2-tetrafluoroethane, commonly known as R-134a. R-134a molecules attract each other strongly enough to form a liquid at high pressures, but weakly enough to become a gas at low pressures, even when very cold.
That’s enough background theory. Now we’re ready to start talking about the machinery.
The Refrigeration Cycle
The refrigeration cycle consists of four major components. The refrigerant moves in a continuous loop through these components in the following order:
- Expansion Valve
As shown in the diagram above, the refrigerant is flowing in an endless counterclockwise loop, so there’s no start or end to the refrigeration cycle. But to explain it, I have to start somewhere. I’ll start with the compressor because it is the driving force behind the entire cycle.
The compressor is an electrically-powered pump that pulls cool, low-pressure refrigerant gas from copper tubing inside the enclosure. The compressor squashes the refrigerant into a hot, high-pressure mixture of liquid and gas and sends it toward the condenser. The compressor also pushes the refrigerant through the whole cycle, just like your heart pumps blood throughout your body.
Any time you compress a gas, its temperature increases. You may have noticed this while pumping air into a bicycle tire: if you touch the needle where compressed air enters the tire, it will feel warm. Temperature is a measure of the average speed of the molecules in the gas. As the gas is forced into a smaller volume, its molecules bounce off each other with more speed. As the refrigerant is compressed partly into a liquid phase, its density increases greatly and so does its temperature.
It takes a lot of energy to power the compressor. In fact, this is by far the largest consumer of electric power in the entire system. Other electrical devices in the refrigerator, like fans and light bulbs, use a tiny fraction of the energy that the compressor does.
The condenser acts like a car’s radiator. Its job is to cool off the hot mixture of liquid and gas coming out of the compressor. The condenser is a long piece of copper tubing outside the enclosure. Usually the condenser follows a serpentine pattern to allow the long tube to fit into a limited space.
Usually, a fan (not shown in the diagram) blows air across the tubing to carry heat away from the condenser. The air gets warmer as the refrigerant inside the copper tube gets cooler. Thus the condenser removes heat from the refrigerant and releases it into the surrounding air.
Copper is an ideal metal for the tubing because it’s an excellent conductor of heat. It allows heat to flow rapidly from the refrigerant inside it to the cooler, surrounding air.
As the hot refrigerant passes through the condenser and loses heat, it changes completely into liquid. The phase change greatly enhances the refrigerant’s ability to release heat.
The expansion valve is a device that allows the refrigerant to expand and decrease in pressure. You can think of the expansion valve as a constriction, like a kink in a hose, that allows only a small amount of refrigerant to squeeze through it at a time. The refrigerant entering the expansion valve from the condenser is a warm liquid, still under high pressure from the compressor.
As the refrigerant squeezes through the expansion valve, it sprays out the other side with a lot less pressure. Under lower pressure, the liquid converts into an extremely cold mixture of liquid and gas. This process is the inverse of what happened in the compressor.
The mixture of cold liquid and gas coming out of the expansion valve enters another long, serpentine copper tube inside the enclosure called the evaporator. The refrigerant inside the evaporator is even colder than the air and food around it. Heat always flows from warmer substances to cooler substances. Therefore heat flows from the air in the enclosure into the refrigerant.
The refrigerant absorbs heat, and the contents of the enclosure get cooler. Heat entering the refrigerant is enough to completely evaporate all the remaining liquid into gas. The phase change greatly enhances the amount of heat the refrigerant inside the evaporator absorbs.
The gas leaving the evaporator is not as intensely cold as the mixture entering it, but it is still fairly cool. This cool gas enters the compressor, and the cycle starts all over. As the refrigeration cycle runs, the refrigerant keeps moving in a loop, continuously absorbing heat from the air and food inside the refrigerator and dumping it into the air outside the refrigerator.
The Net Result
The overall effect of the refrigeration cycle is to transfer heat from inside the enclosure to the surrounding air in the room. Both heat transfer components — evaporator and condenser — rely on the fact that, when two materials at different temperatures come in contact, heat spontaneously flows from the warmer material into the cooler material.
Inside the enclosure, as cold as the air is, it’s still warmer than the extremely cold refrigerant flowing through the evaporator. And as warm as the air is in the room, it’s still cooler than the hot refrigerant being pumped through the condenser. Without both of these temperature differences, the refrigerator simply wouldn’t work.
In addition to the compressor, there are are some other components in a refrigerator that help it work properly. As mentioned above, usually there is a fan that blows air across the condenser tubing to help transfer heat to the air outside the enclosure. Likewise, often there is a fan inside the enclosure that blows on the evaporator tubing to help transfer heat from the enclosure into the refrigerant. (Some refrigerators omit one or both of the fans and rely on passive convection for the heat transfer.)
To keep the refrigerator’s contents within a desired temperature range, a thermostat circuit turns the compressor and fans on when the enclosure’s temperature gets too warm. The thermostat turns the compressor and fans back off once the temperature gets cold enough. The thermostat conserves energy and prevents the refrigerator from getting too cold.
Solving the Puzzle
Now we come back to the puzzle we started with. If you leave the refrigerator door open, why does the room get even hotter? A key observation is that energy is conserved. This means that energy cannot be created or destroyed.
There are two kinds of energy involved in the refrigerator’s operation: electricity and heat. Electrical energy enters the compressor from the electrical outlet. Heat energy from the food and air inside the enclosure enters the evaporator coils. And finally, heat energy leaves the refrigerator from the condenser coils.
Because energy is conserved, the total energy flow must add up like this:
(heat leaving enclosure) + (electricity) = (heat entering room)
Thus the amount of heat entering the room must be greater than the amount of heat extracted from the enclosure. The waste heat that leaves the refrigerator includes all of the heat from inside the enclosure, plus additional heat energy from the electricity we put into the compressor. Here’s another way to look at it: not only does the refrigerator have to cool off the food and air inside its enclosure, it needs to cool off the compressor.
When you leave the refrigerator door open, the air inside and outside the enclosure starts mixing. The refrigerator keeps pumping heat from the mixed air, converting electricity to more heat inside the compressor, and finally dumping the total heat into the room. The hotter air keeps mixing with the air inside the enclosure through the open door. The net result is that you are using the compressor as a heater to make the room warmer. The refrigerator keeps running because its thermostat senses that the interior is too warm. The longer you leave the door open, the hotter the room gets.
This thought experiment illustrates that a refrigerator doesn’t “create” coldness. Instead, a refrigerator is a heat pump. It transports heat from one place to another, but it doesn’t destroy any heat.
There is a natural tendency of materials at different temperatures to come to a common middle temperature. As effective as the refrigerator’s thermal insulation might be, if you unplug the refrigerator and wait long enough, eventually heat from the outside will leak through the insulation and bring the inside of the refrigerator to room temperature. The refrigeration cycle consumes electrical energy to work against that natural tendency. Just like it takes energy to pump water uphill, it takes energy to pump heat from a colder environment to a warmer environment.
So now you know why leaving the refrigerator door open makes the room hotter. It’s also why your air conditioner exhausts heat outdoors. When your air conditioner is working, your home’s interior is like the inside of a big refrigerator. And your little food refrigerator is inside that big refrigerator, contributing part of the heat that the air conditioner pumps outdoors.
In theory, with your broken air conditioner, you could use the refrigerator to cool the house, if you could figure out how to expose the refrigerator’s condenser coils to the outdoors while leaving the opened compartment indoors.
Maybe you can cut a rectangular hole in the wall behind the refrigerator, push the back of the refrigerator into that hole, seal up the boundary with duct tape, and…. Wait, on second thought, that’s probably a bad idea. I recommend you tough it out and wait for the repairman to show up. Be strong. You can do it!