Refrigerators

Discussion

introduction

A refrigerator is any kind of enclosure (like a box, cabinet, or room) whose interior temperature is kept substantially lower than the surrounding environment.

The term "refrigerator" was coined by a Maryland engineer, Thomas Moore, in 1800. Moore's device would now be called an "ice box" — a cedar tub, insulated with rabbit fur, filled with ice, surrounding a sheet metal container. Moore designed it as as a means for transporting butter from rural Maryland to Washington, DC. Its operating principle was the latent heat of fusion associated with melting ice.

The term "air conditioning" was coined by Stuart Cramer in 1905 to describe his system for regulating the temperature and humidity inside a textile factory in the South (the humidity regulation was seen as more important than the temperature regulation). Willis Carrier also designed climate control systems for industry.

One of the first uses of air conditioning for personal comfort was in 1902 when the New York Stock Exchange's new building was equipped with a central cooling as well as heating system. Alfred Wolff, an engineer from Hoboken, New Jersey who is considered the forerunner in the quest to cool a working environment, helped design the new system, transferring this budding technology from textile mills to commercial buildings.

In 1906, Stuart Cramer first used the term "air conditioning" as he explored ways to add moisture to the air in his southern textile mill. He combined moisture with ventilation to actually "condition" and change the air in the factories, controlling the humidity so necessary in textile plants.

An early pioneer who did much to promote "controlled air" was Willis Carrier, a mechanical engineer who worked at the Buffalo Forge Company in Buffalo, New York. Subsequent subsidiary companies carrying his name helped conquer the temperature-humidity relationship, marrying theory with practicality. Starting in 1902, he designed a spray-type temperature and humidity controlled system. His induction system for multi-room office buildings, hotels, apartments and hospitals was just another of his air-related inventions. Many industry professionals and historians consider him the "father of air conditioning."

There are several basic refrigeration techniques:

1. ice box (or dry ice box)
2. cold air systems
3. vapor-compression: the current standard method of refrigeration used in home refrigerators, home air conditioners and heat pumps (Kelvin's idea, refrigerate the environment in the winter, store "cold" in the ground for use in the summer)
4. vapor-absorption: the Electrolux refrigerator with no moving parts
5. thermoelectric

cold air refrigeration

Physician Dr. John Gorrie, Apalachicola, Florida, 1849. Rapidly expanding gases are cooled. Intended to cool hospital wards. Hot air was considered "bad", was thought to be the origin of tropical diseases, thus the name "malaria". Died before commercial models could be made. Design improved by William Siemens of Germany. Dr. Gorrie may have also invented the ice cube tray in its current form.

By widening the vessel… from the bottom upward the removal of the block of ice is… rendered more easy….

To further facilitate the removal of the ice from the vessels [they are] made a little smaller at the bottom than at the top….

schematic diagram

indicator diagram

vapor compression refrigeration

In 1834 an American inventor named Jacob Perkins obtained the first patent for a vapor-compression refrigeration system, it used ether in a vapor compression cycle.

• Joule-Thomson (Kelvin) expansion
• Low pressure (1.5 atm) low temperature (-10 to +15 °C) inside
• High pressure (7.5 atm) high temperature (+15 to +40 °C) outside

Follow along with this discussion using vapor-compression.pdf.

Note: liquids are not ideal gases, liquids are nearly incompressible.

1. compressor
   cold vapor from the evaporator is compressed, raising it temperature and boiling point
   adiabatic compression
   T, b.p. ~ P
   work done on the gas
2. condenser
   hot vapor from the compressor condenses outside the cold box, releasing latent heat
   isothermal, isobaric condensation (horizontal line on PV diagram)
   high temperature
   T (hot)
   latent heat of vaporization Q (hot)
3. expansion valve (throttling valve)
   hot liquid from the condenser is depressurized, lowering its temperature and boiling point
   adiabatic, isochoric expansion (vertical line on PV diagram)
   T, b.p. ~ P
   no work done W = 0
4. evaporator
   cold liquid from the expansion valve boils inside the cold box, absorbing latent heat
   isothermal, isobaric boiling (horizontal line on PV diagram)
   low temperature
   T (cold)
   latent heat of vaporization Q (cold)

indicator diagram

vapor absorption refrigeration

Oliver Evans, USA, 1805, proposed but not built, evaporated sulfuric acid absorbed by water.

The first absorption machine was developed by Edmond Carré in 1850, using water and sulphuric acid. His brother, Ferdinand Carré developed the first ammonia/water refrigeration machine in 1859. Ferdinand Carré, France, ammonia absorption refrigerator, 1859. Established commercial success in the Confederate States during the US Civil War, since Union ice was not being transported to the South.

vapor absorption refrigerators can be powered by any heat Source: natural gas, propane, kerosene, butane?

schematic diagram — vapor-absorption-fridge.pdf

1. generator
   ammonia-water solution heated to generate bubbles of ammonia gas
2. separator
   ammonia gas bubbles out of solution
3. condenser
   ammonia gas condenses
4. evaporator
   ammonia liquid evaporates
5. absorber
   ammonia gas absorbed by water

indicator diagram

performance

Carnot diagrams

Refrigerators aren't described by their efficiency, since their purpose isn't to do work. Instead we use what's called the coefficient of performance, which I often just shorten to performance. This has an informal definition similar to efficiency.

The symbol for the coefficient of performance is the uninspiring initialism COP — or C.O.P., whichever you prefer.

In the case of a vapor-compression refrigerator, the "what you paid for" is the net work (W) done on the refrigerant by the compressor (the thing I pay for is the electrical energy used to run the motor) and the "what you get" is the heat removed from the cold box (Q_C) by the evaporator (the thing I want is cold food).

Since energy must be conserved, the refrigerator dumps more heat into the environment (Q_H) than it extracts from the cold box (Q_C) since net work (W) was done on the refrigerant.

Q_H = Q_C + W

Or, if you prefer, the work done is the difference in the two heats.

W = Q_H − Q_C

The coefficient of performance of a real refrigerator can then also be written like this…

Note the addition of the subscript "real". Why did I do that?

Ignoring all its pieces and parts, a refrigerator is a device that makes heat flow from cold to hot — opposite the way it wants to flow. A vapor-compression refrigerator manages this by doing mechanical work on the refrigerant in the compressor. A refrigerator with better performance needs less work to move more heat. The "perfect refrigerator" would do this for "free" — no work required.

Perfect refrigerators do nto exist. This is another statement of what eventually became known as the second law of thermodynamics. Here's how you say this in more formal language.

Es kann nie Wärme aus einem kälteren in einen wärmeren Körper übergehen, wenn nicht gleichzeitig eine andere damit zusammenhängende Aenderung eintritt.

[Heat can never pass from a colder body to a warmer one unless another related change occurs at the same time.]

Rudolf Clausius, 1854

Perfect refrigerators do not exist, they cannot exist, and we can't even get arbitrarily close to one. The best we can do, theoretically, is the ideal refrigerator. One where the heats being transfered from one place to another are proportional to their absolute temperatures.

The coefficient of performance of an ideal refrigerator would then be written like this…

The coefficeint of performance of every real refrigerator is strictly less than its ideal value.

COP_real < COP_ideal

Not only are there no perfect refrigerators, there are no ideal refrigerators either.

refrigerants

These notes are a disaster.

The first true refrigerator (as opposed to an icebox) was built by Jacob Perkins in 1834. It used ether in a vapor compression cycle. The first vapor absorption refrigerator was developed by Edmond Carré in 1850, using water and sulfuric acid. His brother, Ferdinand Carré, demonstrated an ammonia/water refrigeration machine in 1859. Since 1834 more than 50 chemical substances have been used as refrigerants including…

• amines
  • methyl amine
  • ethyl amine
• chlorides
  • ethyl chloride
  • methyl chloride/methylene chloride
• ethers
  • nitrous ether
  • sulfuric ether/sulfuric (ethyl) ether
• halocarbons
  The current standard refrigerants since the 1940s. See comments below.
  • chlorofluorocarbons (CFCs)
  • hydrochlorofluorocarbons (HCFCs)
• hydrocarbons
  In Europe, and particularly in Germany, simple hydrocarbon compounds are used in small quantities for domestic refrigerators. Due to their flammability and explosive potential, they are not suitable for applications requiring larger cooling capacities.
  • propane
  • butane/isobutane
• sulfur compounds
  • sulfur dioxide
    Sulfur dioxide is a heavy, colorless, poisonous gas with a pungent, irritating odor familiar as the smell of a just-struck match.
  • sulfuric acid
• Miscellaneous
  • ammonia
    Prior to the 1930s and 1940s, ammonia was the primary working fluid for vapor compression refrigeration. Largely abandoned for home use due to its toxicity, but still in widespread use in industrial applications. Also used in vapor-absorption refrigerators.
  • carbon dioxide
    Used under higher pressure than the other fluids.

The first mechanical refrigerators had to be connected to the sewer system to dispose of the refrigerant on a regular basis. In the 1930s and 1940s the halocarbon refrigerants (commonly known by such trade names as "Freon," "Genetron," "Isotron," etc.) were developed, giving the industry a strong push into the household market because of their suitability for use with small horsepower motors.

The most important members of the group have been

• trichloromonofluoromethane (R-11)
• dichlorodifluoromethane (R-12)
• chlorodifluoromethane (R-22)
• dichlorotetrafluoroethane (R-114)
• trichlorotrifluoroethane (R-113)

pause

• appropriately volatile
• low boiling points
• low surface tension
• low viscosity
• non reactive (stable)
• non toxic (vapor may be irritating, however)
• non corrosive
• non carcinogenic
• non flammable

Stable? Yes. Too stable! Stays around and accumulates in the atmosphere. Shifts the equilibrium between O₂ and O₃ in the stratosphere. global warming. Production of chlorofluorocarbons (CFCs) ended in 1995 in developed countries.

Production of R-12 was halted by the Clean Air Act on January 1, 1996. Today the remaining supplies are product which has been recovered and reclaimed back to a Chemically Pure State in accordance to ARI - 700 Standard. The ARI Standard is basically a virgin specification. Persons arguing that the supply of virgin product is still available is probably unrealistic, since most of the reserves were depleted in the 1st year. DoD Public Law prohibits the purchase of R-12 except for existing systems, when retrofit has been determined by technical staff to be prohibitive. Senior or Executive approval of this product to be purchased is required.