Refrigeration is the process of removing heat from an enclosed space, or from a substance, and moving it to a place where it is unobjectionable. The primary purpose of refrigeration is lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. The term cooling refers generally to any natural or artificial process by which heat is dissipated. The process of artificially producing extreme cold temperatures is referred to as cryogenics.
The Refrigeration Cycle
The refrigerator in your kitchen uses a cycle that is similar to the one described in the previous section. But in your refrigerator, the cycle is continuous. In the following example, we will assume that the refrigerant being used is pure ammonia, which boils at -27 degrees F. This is what happens to keep the refrigerator cool:
1. The compressor compresses the ammonia gas. The compressed gas heats up as it is pressurized (orange).
2. The coils on the back of the refrigerator let the hot ammonia gas dissipate its heat. The ammonia gas condenses into ammonia liquid (dark blue) at high pressure.
3. The high-pressure ammonia liquid flows through the expansion valve.
You can think of the expansion valve as a small hole. On one side of the hole is high-pressure ammonia liquid. On the other side of the hole is a low-pressure area (because the compressor is sucking gas out of that side).
4. The liquid ammonia immediately boils and vaporizes (light blue), its temperature dropping to -27 F. This makes the inside of the refrigerator cold.
5. The cold ammonia gas is sucked up by the compressor, and the cycle repeats.
By the way, if you have ever turned your car off on a hot summer day when you have had the air conditioner running, you may have heard a hissing noise under the hood. That noise is the sound of high-pressure liquid refrigerant flowing through the expansion valve.
Pure ammonia gas is highly toxic to people and would pose a threat if the refrigerator were to leak, so all home refrigerators don't use pure ammonia. You may have heard of refrigerants know as CFCs (chlorofluorocarbons), originally developed by Du Pont in the 1930s as a non-toxic replacement for ammonia. CFC-12 (dichlorodifluoromethane) has about the same boiling point as ammonia. However, CFC-12 is not toxic to humans, so it is safe to use in your kitchen. Many large industrial refrigerators still use ammonia.
In the 1970s, it was discovered that the CFCs then in use are harmful to the ozone layer, so as of the 1990s, all new refrigerators and air conditioners use refrigerants that are less harmful to the ozone layer.
To understand what is happening inside your refrigerator, it is helpful to understand refrigerants a little better. Here are two experiments that help you see what is happening.
(This is exactly what is happening when we humans deal with liquid nitrogen. We feel comfortable at 70 F. Liquid nitrogen boils at -320 F. So if you had a pot of liquid nitrogen sitting on the kitchen table, its temperature would be -320 F, and it would be boiling away -- to you, of course, it would feel incredibly cold.)
Modern refrigerators use a regenerating cycle to reuse the same refrigerant over and over again. You can get an idea of how this works by again imagining our oven creature and his cup of water. He could create a regenerating cycle by taking the following four steps:
1. The air temperature in the oven is 400 degrees F. The water in the cup boils away, remaining at 212 F but producing a lot of 400 F steam. Let's say the creature collects this steam in a big bag.
2. Once all the water boils away, he pressurizes the steam into a steel container. In the process of pressurizing it, its temperature rises to 800 F and it remains steam. So now the steel container is "hot" to the creature because it contains 800 F steam.
3. The steel container dissipates its excess heat to the air in the oven, and it eventually falls back to 400 F. In the process, the high-pressure steam in the container condenses into pressurized water (just like the butane in a lighter -- see sidebar).
4. At this point, the creature releases the water from the steel pressurized container into a pot, and it immediately begins to boil, its temperature dropping to 212 F.
By repeating these four steps, the creature now has a way of reusing the same water over and over again to provide refrigeration.
Now let's take a look at how these four steps apply to your refrigerator.
There are any number of ways to configure a large, free-standing icemaker -- all you need is a refrigeration system, a water supply and some way of collecting the ice that forms.
One of the simplest professional systems uses a large metal ice-cube tray, positioned vertically. You can see how this system works in the diagram below http://home.howstuffworks.com/icemaker2.htm
In this system, the metal ice tray is connected to a set of coiled heat-exchanging pipes like the ones on the back of your refrigerator. If you've read How Refrigerators Work, then you know how these pipes work. A compressor drives a stream of refrigerant fluid in a continuous cycle of condensation and expansion. Basically, the compressor forces refrigerant through a narrow tube (called the condenser) to condense it, and then releases it into a wider tube (called the evaporator), where it can expand.
Compressing the refrigerant raises its pressure, which increases its temperature. As the refrigerant passes through the narrow condenser coils, it loses heat to the cooler air outside, and it condenses into a liquid. When the compressed fluid passes through the expansion valve, it evaporates -- it expands to become a gas. This evaporation process draws in heat energy from the metal pipes and the air around the refrigerant. This cools the pipes and the attached metal ice tray.
The icemaker has a water pump, which draws water from a collection sump and pours it over the chilled ice tray. As the water flows over the tray, it gradually freezes, building up ice cubes in the well of the tray. When you freeze water layer by layer this way, it forms clear ice. When you freeze it all at once, as in the home icemaker, you get cloudy ice (see How do you make clear ice cubes? for more information).
After a set amount of time, the icemaker triggers a solenoid valve connected to the heat-exchanging coils. Switching this valve changes the path of the refrigerant. The compressor stops forcing the heated gas from the compressor into the narrow condenser; instead, it forces the gas into a wide bypass tube. The hot gas is cycled back to the evaporator without condensing. When you force this hot gas through the evaporator pipes, the pipes and the ice tray heat up rapidly, which loosens the ice cubes.
Typically, the individual cube cavities are slanted so the loosened ice will slide out on their own, into a collection bin below. Some systems have a cylinder piston that gives the tray a little shove, knocking the cubes loose.
This sort of system is popular in restaurants and hotels because it makes ice cubes with a standard shape and size. Other businesses, such as grocery stores and scientific research firms, need smaller ice flakes for packing perishable items. We'll look at flake icemakers next.