Current applications of refrigeration
Probably the most widely used current applications of refrigeration are for the air-conditioning of private homes and public buildings, and the refrigeration of foodstuffs in homes, restaurants and large storage warehouses. The use of refrigerators in kitchens for the storage of fruits and vegetables has permitted the addition of fresh salads to the modern diet year round, and to store fish and meats safely for long periods.
In commerce and manufacturing, there are many uses for refrigeration. Refrigeration is used to liquify gases like
oxygen,
nitrogen,
propane and
methane for example. In compressed air purification, it is used to
condense water vapor from compressed air to reduce its moisture content. In
oil refineries,
chemical plants, and
petrochemical plants, refrigeration is used to maintain certain processes at their required low temperatures (for example, in the
alkylation of
butenes and
butane to produce a high
octane gasoline component). Metal workers use refrigeration to temper steel and cutlery. In transporting temperature-sensitive foodstuffs and other materials by trucks, trains, airplanes and sea-going vessels, refrigeration is a necessity.
Dairy products are constantly in need of refrigeration, and it was only discovered in the past few decades that eggs needed to be refrigerated during shipment rather than waiting to be refrigerated after arrival at the grocery store. Meats, poultry and fish all must be kept in climate-controlled environments before being sold. Refrigeration also helps keep fruits and vegetables edible longer.
One of the most influential uses of refrigeration was in the development of the sushi/sashimi industry in Japan. Prior to the discovery of refrigeration, many sushi connoisseurs suffered great morbidity and mortality from diseases such as
hepatitis A[citation needed]. However the dangers of unrefrigerated sashimi was not brought to light for decades due to the lack of research and healthcare distribution across rural Japan. Around mid-century, the
Zojirushi corporation based in Kyoto made breakthroughs in refrigerator designs making refrigerators cheaper and more accessible for restaurant proprietors and the general public.
[edit] Methods of refrigeration
Methods of refrigeration can be classified as
non-cyclic,
cyclic and
thermoelectric.
[edit] 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".
[edit] Cyclic refrigeration
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
[edit] Vapor-compression cycle
-
- (See Heat pump and refrigeration cycle and Vapor-compression refrigeration for more details)
The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. Figure 1 provides a schematic diagram of the components of a typical vapor-compression refrigeration system.
Figure 1: Vapor compression refrigeration
The
thermodynamics of the cycle can be analyzed on a diagram
[11][12] as shown in Figure 2. 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.
Figure 2: Temperature–Entropy diagram
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).
More information about the design and performance of vapor-compression refrigeration systems is available in the classic
Perry's Chemical Engineers' Handbook.
[13]
[edit] Vapor absorption cycle
In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used. After the development of the vapor compression cycle, the vapor absorption cycle lost much of its importance because of its low
coefficient of performance (about one fifth of that of the vapor compression cycle). Today, the vapor absorption cycle is used mainly where fuel for heating is available but electricity is not, such as in
recreational vehicles that carry
LP gas. It is also used in industrial environments where plentiful waste heat overcomes its inefficiency.
The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapor from the high-pressure liquid. Some work is required by the liquid pump but, for a given quantity of refrigerant, it is much smaller than needed by the compressor in the vapor compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent), and water (refrigerant) and lithium bromide[absorbent]
