My last post for the basic concepts series involved phases of matter and transformations from one phase to another. This post will look at how a phase change can be put to practical use in a common household appliance — the freezer. My aim here is to give you a good thermodynamic feel for how a freezer works. As a bonus, I’ll explain why leaving the freezer door open is a futile strategy for cooling down a hot kitchen.
If you’re constructing a freezer (or a refrigerator — the same basic set up, but with a different temperature range), the goal is to keep the temperature in your cold compartment low enough so that your ice cream stays frozen. (A temperature of roughly -50C would probably do the job.) That means you need some way to take the heat that gets into the cold compartment (say, when you open the freezer door to find that bag of peas) and get it out of the cold compartment!
Let’s say a few words about heat. The “natural” way for heat to behave is to flow from a hotter part of the system to a colder part of the system. Left to come to equilibrium, you’d expect all the parts of the system to end up at the same temperature. A freezer is a set up where we want to maintain a cold part of the system and keep it cold — which is to say, some cunning is required to perform a task that seems to go against the natural flow of things. A good freezer needs to be one that sets up conditions where any heat in the system that might flow into the cold compartment to melt you Chunky Monkey has something better to do.
So, it’s worth noticing what heat can do. If you put heat into a collection of molecules, it can:
- raise the temperature of those molecules (which amounts to raising their average velocity in the system)
- be a source of energy with which to break up some intermolecular attractions (which could bring about a phase change)
- be a source of energy to drive a chemical reaction
To understand refrigeration, we don’t need to worry about chemical reactions, so set that third option aside. Instead, the system that keeps your ice cream cold depends on some clever arrangements of heat flows and phase changes.
The standard clever arrangement is a cold compartment surrounded by a liquid refrigerant. The refrigerant is a compound whose boiling point is the temperature at which you want to maintain the cold compartment. That way, when the opening the freezer door lets some kitchen-temperature air into the cold compartment, the heat that comes in with that kitchen-temperature air has something to do besides raise the temperature of the cold compartment contents.
Instead, that heat flows from the cold compartment to the liquid refrigerant, where it breaks up intermolecular forces holding the refrigerant molecules together in the liquid phase. (It takes energy to break up attractive forces, and heat is a form of energy.) Once those intermolecular bonds are broken, the refrigerant is in gas phase — but it’s still at the boiling point temperature. (The heat here is used to break intermolecular bonds, not to raise the temperature of the refrigerant molecules.) And, it’s worth noting that transforming the refrigerant from a liquid to a gas is a process that comes with an increase in entropy, because the gas phase is more disordered than the liquid phase. (You probably heard somewhere that the “natural” direction of things is in the direction of more entropy.)
We’ve dealt with the heat threatening our ice cream in a way that keeps the cold compartment at the desired temperature. But you know someone’s going to open that freezer door again.
This means that if we want to be able to move more heat out of the cold compartment, we need to have more liquid refrigerant at the ready. The most practical way to do this is to convert the gaseous refrigerant back into liquid refrigerant by compressing it. Squeezing the molecules closer together lets them form intermolecular attractions to each other again, putting them back into liquid phase so they’re ready to divert extra heat from the cold compartment to undergo vaporization once again.
You’ll notice that my diagram of the system includes an electrical plug. Running the compressor to convert gaseous refrigerant back to liquid refrigerant requires energy.
Let’s say it’s a brutally hot day in your one room apartment. You have no air conditioning. You’ve eaten all your ice cream. As you stare into the empty cold compartment of your freezer, you hatch an idea to cool your living space: you’ll throw open the freezer door and let the freezer be your A/C. But while this may seem like a reasonable plan to your heat-addled brain, it will make your apartment even hotter.
Here’s why: When you throw open the freezer door, the compartment from which the freezer is trying to remove heat is the interior of the cold compartment plus the whole room. That’s a lot of heat to try to move out. Meanwhile, to shift that heat out of the (cold compartment + hot apartment), you need to keep regenerating your liquid refrigerant by running the compressor. An unavoidable by-product of running the mechanical compressor is heat; usually it’s vented out to the room near the bottom of the freezer unit. (Stand near your refrigerator or freezer on a hot day and you’ll notice the hot air coming out of the vent.) So, because the compressor is dissipating some heat back into the hot apartment, you now have even more heat to shift out of the (cold compartment + hot apartment). And the longer you run the freezer this way, the more extra heat you’ll be putting into the room.
Your best option with no A/C and an empty freezer would be to unplug the freezer before opening the freezer door. Then, since heat tends to flow from hotter parts of a system to cooler parts of a system, until the whole system reaches its equilibrium temperature, the still-cold cold compartment will be warmed up by heat from the hot apartment, making things a little bit cooler in that apartment.