Refrigeration is a process in which work is done to move heat
from one location to another. The work of heat transport is traditionally
driven by mechanical work, but can also be driven by heat, magnetism, electricity, laser, or other means. Refrigeration has many
applications, including, but not
limited to: household refrigerators, industrial freezers, cryogenics, and air conditioning. Heat pumps may use the heat output of the refrigeration
process, and also may be designed to be reversible, but are otherwise similar
to refrigeration units.
Methods of refrigeration can be
classified as :
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 melting point of 0 °C (32 °F) at sea level. To melt, ice must absorb 333.55 kJ/kg (about 144 Btu/lb) of
heat. Foodstuffs maintained near this temperature have an increased storage
life.
Solid carbon dioxide has no liquid phase at normal atmospheric
pressure, and sublimes directly from the solid to vapor phase at a temperature
of -78.5 °C (-109.3 °F), and is effective for maintaining products at low
temperatures during sublimation. Systems such as this where the refrigerant
evaporates and is vented to the atmosphere are known as "total loss
refrigeration"
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, thethermodynamic
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. Insulationis
used to reduce the work and energy needed to achieve and maintain a lower
temperature in the cooled space. The operating principle of the refrigeration
cycle was described mathematically bySadi
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:
1.
Vapor cycle, and
2.
Gas cycle
Vapor cycle refrigeration can
further be classified as:
1.
Vapor-compression
refrigeration
2.Vapor-absorption refrigeration
Vapor-compression cycle
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.
The thermodynamics of the cycle can be analyzed on a diagram 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 gasbehavior (if any).
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 needed 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) with water (absorbent), and
water (refrigerant) with lithium bromide (absorbent).
Gas cycle
When the working
fluid is a gas that is compressed and expanded but doesn't change phase, the
refrigeration cycle is called a gas cycle. Air is most often this working fluid. As there is no
condensation and evaporation intended in a gas cycle, components corresponding
to the condenser and evaporator in a vapor compression cycle are the hot and
cold gas-to-gas heat exchangers in gas cycles.
The gas cycle is
less efficient than the vapor compression cycle because the gas cycle works on
the reverse Brayton cycle instead of the reverse Rankine cycle.
As such the working fluid does not receive and reject heat at constant
temperature. In the gas cycle, the refrigeration effect is equal to the product
of the specific heat of the gas and the rise in temperature of the gas in the
low temperature side. Therefore, for the same cooling load, a gas refrigeration
cycle needs a large mass flow rate and is bulky.
Because of their
lower efficiency and larger bulk, air cycle coolers are not often used nowadays in terrestrial cooling
devices. However, the air cycle machine is very common on gas turbine-powered jet aircraft as cooling and
ventilation units, because compressed air is readily available from the
engines' compressor sections. Such units also serve the purpose of pressurizing
the aircraft.
