Refrigeration

It's obvious that the breakthroughs in refrigeration technology in the early part of the 20th century are responsible for a good deal of our way of life. Refrigeration guarantees that we can essentially eat any type of food, at any time during the year, anywhere.

Wow.

Now, I consider myself a science kid -- I know why an airplane wing works (good ol' Bernoulli). I know what the Lockheed SR-71 is made out of (Titanium) and why it leaks when it's on the ground (the fuel tanks only fit together and seal properly at high temperatures when the heat has caused the plane's parts to expand and snug up against each other). I've had what I consider to be a bare-bones primary science education, and I only have that because I was a reading kid as well as being a science kid. My knowlege is just barely enough for me to appreciate the marvels I use every day.

But I never really understood how a household refrigerator worked. I knew there was some kind of gas, and I knew it involved a compressor somehow. I knew it wasn't simply a matter of circulating the magical frost goo through the chassis of the good ol' Frigidaire, but I didn't realized exactly how it worked.

But I was playing with an old portable bicycle pump the other day and it came to me. I was holding my thumb over the nozzle of the thing and compressing the air down as hard as I could. I'd feel the end of the cylinder body turn warm very quickly, because the temperature of a gas varies as its pressure -- the aluminum body of the pump allowed me to feel changes in gas temperature very easily.

Then I simply leaned on the pump, keeping that little bit of compressed air in there for as long as I could. After a moment, I removed my thumb. A little puff of air jumped out, and it was cold.

So I repeated the idle fiddling than had suddenly become an experiment, and got the same results.

I realized that because I'd let the column of compressed air cool a bit before releasing it (that is, I let it radiate the heat that now differentiated it from its thermal environment), when I allowed it to decompress, its new temperature at the normal pressure would be lower than its temperature before I compressed it. This is why aerosol sprays are cold. This is why the bottom (or the top, in the case of my sulfer dioxide 1927 General Electric Refrigerator) of your refrigerator is warm while the inside remains nice and chilly.

It's the little epiphanies in life that make it all worthwhile.

So now I understand how a common household refrigerator works. Now, to get really low temperatures (the kind you need to condense helium into a liquid), some kind of electromagnetic cooling process (that I don't even pretend to understand) is used. Obviously, if you freeze the gas you're using as coolant, none of this works anymore.

Theoretically, you could have self-refrigerating soft drinks, if you had very strong cans and could achieve very high pressures. As I sit here waiting form my Dr Pepper to cool down in the freezer, that seems like a very nice idea.

Domestic and industrial refrigeration works on a principle called the vapour compression cycle. The basic refrigeration system consists of a compressor feeding into a condensor. The output from the condensor, called the liquid line, then feeds into an evaporator. The other end of the evaporator, termed the suction line, leeds back into the compressor. The system is charged with a refrigerant gas which boils at a low temperature.

In addition to the components of a refrigeration system two terms have to be understood. These are latent heat and sensible heat. Latent heat is heat energy gained or lost without a change in temperature. Sensible heat is heat energy gained or lost that results in change in temperature which can be sensed. A change in latent heat is much more efficient than a change in sensible heat and this is the principle on which the vapour compression cycle works.

Starting from the compressor the vapour compression cycle flows as follows.

1) The compressor takes low pressure refrigerant gas and compresses it into a high pressure gas. Since pressure is linked to temperature the gas gains sensible heat during compression.

2) The now high pressure gas is fed into a condensor where it is cooled and also changes state into a liquid. A change of state is a latent process and so the refrigerant must lose latent heat to change from gas to liquid.

3) The liquid refrigerant is now piped through the liquid line to a metering device at the start of the evaporator, usually an expansion valve. Since this valve is much smaller in diameter than the pipe the liquid enters it at high pressure but leaves at low pressure.

4) Now at a low pressure the liquid enters the evaporator where it tries to change state to a gas. This is a latent heat process so the refrigerant draws heat from the surrounding area to fuel its change of state.

5) Once it reaches the end of the evaporator the refrigerant is once again a low pressure gas which is drawn into the compressor and the cycle starts again.

In domestic refrigerators and freezers the compressor and condensor are located on top or in the rear of the case whilst the evaporator is placed inside the case.

In industrial refrigeration systems the compressors are usually located in a plant near the shopfloor and the liquid refrigerant is piped out to the cases. The returning hot gas is piped to a condensor, usually on an outside wall or roof, where it is cooled.

One method of cooling that's commonly overlooked (no doubt in part because of how counter-intuitive it seems) is refrigeration with light. Ordinarily we think of light adding to the temperature of whatever absorbs it, like the difference between shade and sunlight on a warm day. Even a perfect mirror would only stay the same temperature; while it wouldn't get any warmer it wouldn't get any colder either. Physically this kind of heating makes sense too -- photons hitting a surface that aren't immediately reflected instead excite electrons in the material, jostling its atoms and making the material hotter.

In fluorescence, photons of a particular wavelength are able to excite the electron to a higher state. After losing some energy in that state, they fall back to the previous state and emit a photon of lower wavelength than was absorbed. That energy loss is why ultra high-frequency black light can make some materials glow in much more visible (i.e., lower frequency) colors. Most materials release very little light, only that of some set of wavelengths, which look like a specific color to us. The rest is converted to thermal energy, heating the material. Even materials that release many photons at a different wavelength, such as the Day-Glo plus black light example above, convert some of the photon energy to heat. As a group, these properties are called Stokes luminescence, and apply to virtually every substance on earth.

Only recently have manufacturing processes become good enough to create a material that displays just the opposite, anti-Stokes luminescence. In this type of luminescence, the photons are absorbed as usual, but the electron is further excited by heat. When the electron eventually jumps back to the un-excited state, a photon is released with greater energy than the one which was absorbed, and heat is lost from the system. Almost magically, the material glows dimmer -- or at least at a shorter wavelength -- than the laser and becomes cooler. This luminescence is very fragile; any contaminants in the material will absorb energy rather than releasing it, counteracting the cooling effect. Sensitivity of these ultra-pure materials to light is also specific, so a laser tuned to precisely the right wavelength must be used.

Notably, optical refrigeration was used in to cool the Bose-Einstein condensates that have been making news since 1995. Hopes are also high for a process that creates pure enough silicon (and a tight enough laser) that this method can be used to extend Moore's Law a few more years. Cooler still (Hah!) is research being done to find a target material cheap enough to manufacture that it could replace the pressure-based systems used in home refrigerators, air conditioners, etc. Besides the near-100% energy efficiency, this would be silent with no moving parts.

Re*frig`er*a"tion (-?"sh?n), n. [Cf. F. r'efrig'eration, L. refrigeratio.]

The act or process of refrigerating or cooling, or the state of being cooled.

 

© Webster 1913.