Refrigeration

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Refrigeration

Refrigeration is a process in which work is done to remove heat from one location to another.

Refrigeration has many applications including but not limited to; household refrigerators, industrial freezers,

cryogenics, air conditioning, and heat pumps.

In order to satisfy the Second Law of Thermodynamics , some form of work must be performed to accomplish this.

The work is traditionally done by mechanical work but can also be done by magnetism, laser or other means.

History

The first known method of artificial refrigeration was demonstrated by William Cullen at the University of Glasgow in Scotland in 1756. Cullen used a pump to create a partial vacuum over a container of diethyl ether, which then boiled, absorbing heat from the surrounding air. The experiment even created a small amount of ice, but had no practical application at that time.

In 1758, Benjamin Franklin and John Hadley, professor of chemistry at Cambridge University, conducted an experiment to explore the principle of evaporation as a means to rapidly cool an object. Franklin and Hadley confirmed that evaporation of highly volatile liquids such as alcohol and ether, could be used to drive down the temperature of an object past the freezing point of water. They conducted their experiment with the bulb of a mercury thermometer as their object and with a bellows used to "quicken" the evaporation; they lowered the temperature of the thermometer bulb down to 7 °F (−14 °C) while the ambient temperature was 65 °F (18 °C). Franklin noted that soon after they passed the freezing point of water (32 °F) a thin film of ice formed on the surface of the thermometer's bulb and that the ice mass was about a quarter inch thick when they stopped the experiment upon reaching 7 °F (−14 °C). Franklin concluded, "From this experiment, one may see the possibility of freezing a man to death on a warm summer's day".

In 1805, American inventor Oliver Evans designed but never built a refrigeration system based on the vapor-compression refrigeration cycle rather than chemical solutions or volatile liquids such as ethyl ether.

In 1820, the British scientist Michael Faraday liquefied ammonia and other gases by using high pressures and low temperatures.

An American living in Great Britain, Jacob Perkins, obtained the first patent for a vapor-compression refrigeration system in 1834. Perkins built a prototype system and it actually worked, although it did not succeed commercially.

In 1842, an American physician, John Gorrie, designed the first system for refrigerating water to produce ice. He also conceived the idea of using his refrigeration system to cool the air for comfort in homes and hospitals (i.e., air-conditioning). His system compressed air, then partially cooled the hot compressed air with water before allowing it to expand while doing part of the work required to drive the air compressor. That isentropic expansion cooled the air to a temperature low enough to freeze water and produce ice, or to flow "through a pipe for effecting refrigeration otherwise" as stated in his patent granted by the U.S. Patent Office in 1851. Gorrie built a working prototype, but his system was a commercial failure.

The first gas absorption refrigeration system using gaseous ammonia dissolved in water (referred to as "aqua ammonia") was developed by Ferdinand Carré of France in 1859 and patented in 1860. Due to the toxicity of ammonia, such systems were not developed for use in homes, but were used to manufacture ice for sale. In the United States, the consumer public at that time still used the ice box with ice brought in from commercial suppliers, many of whom were still harvesting ice and storing it in an icehouse.

Thaddeus Lowe, an American balloonist from the Civil War, had experimented over the years with the properties of gases. One of his mainstay enterprises was the high-volume production of hydrogen gas. He also held several patents on ice making machines. His "Compression Ice Machine" would revolutionize the cold storage industry. In 1869 he and other investors purchased an old steamship onto which they loaded one of Lowe’s refrigeration units and began shipping fresh fruit from New York to the Gulf Coast area, and fresh meat from Galveston, Texas back to New York. Because of Lowe’s lack of knowledge about shipping, the business was a costly failure, and it was difficult for the public to get used to the idea of being able to consume meat that had been so long out of the packing house.

Domestic mechanical refrigerators became available in the United States around 1911.

Commercial utilization

With the invention of synthetic refrigerants based mostly on a chlorofluorocarbon (CFC) chemical, safer refrigerators were possible for home and consumer use. Freon is a trademark of the Dupont Corporation and refers to these CFC, and later hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC), refrigerants developed in the late 1920s. These refrigerants were considered at the time to be less harmful than the commonly used refrigerants of the time, including methyl formate, ammonia, methyl chloride, and sulfur dioxide. The intent was to provide refrigeration equipment for home use without danger: these CFC refrigerants answered that need. However, in the 1970s the compounds were found to be reacting with atmospheric ozone, an important protection against solar ultraviolet radiation, and their use as a refrigerant worldwide was curtailed in the Montreal Protocol of 1987

Method of Refrigeration

Non-cyclic refrigeration

In non-cyclic refrigeration, cooling is accomplished by melting ice or by subliming dry ice (frozen carbon dioxide). These methods are used for small-scale refrigeration such as in laboratories and workshops, or in portable coolers.

Ice owes its effectiveness as a cooling agent to its constant melting point of 0 °C (32 °F). In order to melt, ice must absorb 333.55 kJ/kg (approx. 144 Btu/lb) of heat. Foodstuffs maintained at this temperature or slightly above have an increased storage life.

Solid carbon dioxide has no liquid phase at normal atmospheric pressure, so sublimes directly from the solid to vapor phase at a temperature of -78.5 °C (-109.3 °F), and is therefore effective for maintaining products at low temperatures during the period of sublimation. Systems such as this where the refrigerant evaporates and is vented into the atmosphere are known as "total loss refrigeration".

Cyclic Refrigeration.

Main article: Heat pump and refrigeration cycle

This consists of a refrigeration cycle, where heat is removed from a low-temperature space or source and rejected to a high-temperature sink with the help of external work, and its inverse, the thermodynamic power cycle. In the power cycle, heat is supplied from a high-temperature source to the engine, part of the heat being used to produce work and the rest being rejected to a low-temperature sink. This satisfies the second law of thermodynamics.

A refrigeration cycle describes the changes that take place in the refrigerant as it alternately absorbs and rejects heat as it circulates through a refrigerator. It is also applied to HVACR work, when describing the "process" of refrigerant flow through an HVACR unit, whether it is a packaged or split system.

Heat naturally flows from hot to cold. Work is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. Insulation is used to reduce the work and energy required to achieve and maintain a lower temperature in the cooled space. The operating principle of the refrigeration cycle was described mathematically by Sadi Carnot in 1824 as a heat engine.

The most common types of refrigeration systems use the reverse-Rankine vapor-compression refrigeration cycle although absorption heat pumps are used in a minority of applications.

Cyclic refrigeration can be classified as:

Vapor cycle, and
Gas cycle

Vapor cycle refrigeration can further be classified as:

Vapor-compression refrigeration
Vapor-absorption refrigeration

Vapor compression Refrigeration Cycle

The vapor-compression uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 1 depicts a typical, single-stage vapor-compression system. All such systems have four components: a compressor, a condenser, a Thermal expansion valve (also called a throttle valve), and an evaporator.

Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with typically available cooling water or cooling air. That hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case).

The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated.

The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.

To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor.

The thermodynamics of the cycle can be analyzed on the diagram as shown .

In this cycle, a circulating refrigerant such as Freon enters the compressor as a vapor.

From point 1 to point 2, the vapor is compressed at constant entropy and exits the compressor as a vapor at a higher temperature, but still below the vapor pressure at that temperature.

From point 2 to point 3 and on to point 4, the vapor travels through the condenser which cools the vapor until it starts condensing, and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature.

Between points 4 and 5, the liquid refrigerant goes through the expansion valve (also called a throttle valve) where its pressure abruptly decreases, causing flash evaporation and auto-refrigeration of, typically, less than half of the liquid.

That results in a mixture of liquid and vapor at a lower temperature and pressure as shown at point 5.

The cold liquid-vapor mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes.

The resulting refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

The above discussion is based on the ideal vapor-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

Refrigeration Ton

The British Thermal Unit ( BTU ) is defined as the amount of heat necessary to raise the temperature of one pound of water one degree Fahrenheit from 58.5 to 59.5 degrees Fahrenheit.

Most consumers were used to iceboxes and cold storage ice warehouse facilities were common. So, in a stroke of marketing genius, the recognizable measurement for "coldness" was defined as a relationship to melting ice!

Melting Ice at 32 degrees Fahrenheit requires 144 BTU's per pound to become liquid at 32 degrees Fahrenheit.

The Refrigeration Ton was defined as the heat ABSORBED by one ton of ice (2000 pounds) causing it to melt completely by the end of one day (24 hours).

Therefore:

1 Refrigeration Ton = 2000 Pounds x 144 BTU per Pound / 24 hours

The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems.

Above Figure provides a schematic diagram of the components of a typical vapor-compression refrigeration system.

Unit of Refrigeration

The units of refrigeration are always a unit of power. Domestic and commercial refrigerators may be rated in kJ/s, or Btu/h of cooling.

For commercial and industrial refrigeration systems most of the world uses the kilowatt (kW) as the basic unit refrigeration.

Typically, commercial and industrial refrigeration systems North America are rated in Tons of Refrigeration (TR). Historically, one Ton of Refrigeration was defined as the energy removal rate that will freeze one short ton of water at 0 °C (32 °F) in one day.

This was very important because many early refrigeration systems were in ice houses. The simple unit allowed owners of these refrigeration systems measure a days output of ice against energy consumption and compare their plant to one down the street. While ice houses make up a much smaller part of the refrigeration industry than they once did the unit of Tons of Refrigeration has remained in North America.

The unit's value as historically defined is approximately 11,958 BTU/hr (3.505 kW) has been redefined to be exactly 12,000 BTU/hr (3.516 kW)

A much less common definition is: 1 tonne of refrigeration is the rate of heat removal required to freeze a metric ton (i.e., 1000 kg) of water at 0 °C in 24 hours.

Based on the heat of fusion being 334.9 kJ/kg, 1 tonne of refrigeration = 13,954 kJ/h = 3.876 KW.

As can be seen, 1 tonne of refrigeration is 10 percent larger than 1 ton of refrigeration.

= 12000 BTU's per Hour

= 3516 Watts (3.516 KW)

Note that the Refrigeration Ton is a defined as a BTU's per hour which is work units divided by time units. Work done in a time frame is defined as power.

Therefore, the Refrigeration Ton can be related directly to other definitions of power such as horesepower or watts.

1 Refrigeration Ton = 4.72 Horsepower (HP)

1 Refrigeration Tonne (metric)= 5.27 HP (metric)

= 3.876 KW

Rules of thumb (Assuming Usual Ceiling Heights):

1 ton of air conditioning for every 400 square feet of floor space.

Each ton of air conditioning requires 400 CFM airflow.

Walk in coolers: one ton to 175 square feet of floor space.

Walk in freezers: one ton to 85 square feet of floor space.

Refrigerants and Safety

"Freon" is a trade name for a family of haloalkane refrigerants .

These refrigerants were commonly used due to their superior stability and safety properties:

They were not flammable nor obviously toxic as were the fluids they replaced, such as sulfur dioxide.

Unfortunately, these chlorine-bearing refrigerants reach the upper atmosphere when they escape. In the stratosphere, CFCs break up due to UV-radiation, releasing their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, thus causing severe damage to the ozone layer that shields the Earth's surface from the Sun's strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule.

CFC refrigerants in common but receding usage include R-11 and R-12. Newer refrigerants that have reduced ozone depletion effect include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use.

HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine. However, CFCs, HCFCs, and HFCs all have large global warming potential.

Potential Health Effects ( R - 22 )

Potential Health Effects ( R - 134a )

Inhalation of high concentrations of vapor is harmful and may cause heart irregularities, unconsciousness or death.

Intentional misuse or deliberate inhalation may cause death without warning.

Vapor reduces oxygen available for breathing and is heavier than air.

Liquid contact can cause frostbite.

HUMAN HEALTH EFFECTS:

Inhalation

Gross overexposure may cause: Central nervous system depressionwith dizziness, confusion, incoordination, drowsiness orunconsciousness.

Irregular heart beat with a strange sensation in the chest, "heart thumping", apprehension, lightheadedness, feeling of fainting, dizziness, weakness, sometimes progressing to

loss of consciousness and death.

Suffocation, if air is displaced by vapors.

Skin contact

Immediate effects of overexposure may include: Frostbite, if

liquid or escaping vapor contacts the skin.

Eye contact

"Frostbite-like" effects may occur if the liquid or escaping

vapors contact the eyes.

Increased susceptibility to the effects of this material may be observed in persons with pre-existing disease of the: central nervous system, cardiovascular system.

HUMAN HEALTH EFFECTS:

Skin contact with the liquid may include frostbite.

Prolonged overexposure may cause defatting or dryness of the skin. Eye contact with liquid may include eye irritation with discomfort, tearing, or blurring of vision.

Inhalation may include temporary nervous system depression with anesthetic effects such as dizziness, headache,confusion, incoordination, and loss of consciousness.

Higher exposures may lead to temporary alteration of the heart’s electrical activity with irregular pulse,palpitations, or inadequate circulation.

Fatality may occur from gross overexposure.

Individuals with preexisting diseases of the central nervous or cardiovascular system may have increased susceptibility to the toxicity of excessive exposures.

First Aid

INHALATION

If inhaled, immediately remove to fresh air. Keep person calm. If not breathing, give artificial respiration.

If breathing is difficult, give oxygen. Call a physician.

SKIN CONTACT

In case of contact, flush area with lukewarm water.

Do not use hot water. If frostbite has occurred, call a physician.

EYE CONTACT

In case of contact, immediately flush eyes with plenty of water for at least 15 minutes. Call a physician.

INHALATION

If high concentrations are inhaled, immediately remove to fresh air. Keep person calm.

If not breathing, give artificial respiration. If breathing is difficult, give oxygen. Call a physician.

SKIN CONTACT

In case of contact, immediately flush skin with plenty of water for at least 15 minutes, while removing contaminated clothing and shoes. Call a physician.

Wash contaminated clothing before reuse. Treat for frostbite if necessary by gently warming affected area.

EYE CONTACT

In case of contact, immediately flush eyes with plenty of water for at least 15 minutes. Call a physician.

Fire and Explosion Hazards:

Cylinders may rupture under fire conditions.

Decomposition may occur.

Contact of welding or soldering torch flame with high

concentrations of refrigerant can result in visible changes

in the size and color of torch flames. This flame effect

will only occur in concentrations of product well above the

recommended exposure limit, therefore stop all work and

ventilate to disperse refrigerant vapors from the work area

before using any open flames.

HFC-134a is not flammable in air at temperatures up to

100 deg. C (212 deg. F) at atmospheric pressure.

However, mixtures of HFC-134a with high concentrations of air at elevated pressure and/or temperature can become combustible in the presence of an ignition source.

HFC-134a can also become combustible in an oxygen enriched environment (oxygen concentrations greater than that in air).

Whether a mixture containing HFC-134a and air, or HFC-134a in an oxygen enriched atmosphere become combustible depends on the inter-relationship of

1) the temperature

2) the pressure

3) the proportion of oxygen in the mixture.

In general, HFC-134a should not be allowed to exist with

air above atmospheric pressure or at high temperatures; or

in an oxygen enriched environment.

For example HFC-134a should NOT be mixed with air under pressure for leak testing or other purposes.

Other burning materials may cause "FREON" 22 to burn weakly.

Chlorodifluoromethane is not flammable at ambient temperatures and atmospheric pressure. However, chlorodifluoromethane has been shown in tests to be combustible at pressures as low as 60 psig at ambient temperature when mixed with air at concentrations of 65 volume % air.

Experimental data have also been reported which indicate combustibility of "FREON" 22 in the presence of certain concentrations of chlorine.

Cylinders may rupture under fire conditions. Decomposition may occur.

Storage ( R - 134 a )

Storage ( R- 22 )

Store in a clean, dry place. Do not heat above 52 C (125 F).

Valve protection caps and valve cutlet threaded plugs must

remain in place unless container is secured with valve

outlet piped to use point.

Do NOT drag, slide or roll cylinders.

Use a suitable hand truck for cylinder movement.

Never attempt to lift cylinder by its cap.

Use a pressure reducing regulator when connecting cylinder to lower pressure (>3000 psig) piping or systems.

Do NOT heat cylinder. Use a check valve or trap in the discharge line to prevent hazardous back flow into the cylinder.

Cylinders should be stored upright and firmly secured to

prevent falling or being knocked over.

Separate full containers from empty containers.

Storage area temperatures should not exceed 125 deg F (52 deg C) and should be free of combustible materials.

Avoid area where salt or other corrosive materials are present.

Avoid excessive inventory and storage time.

Use a first-in first-out system. Keep accurate inventory records.

Store in a clean, dry place. Do not heat above 52 C (126 F).