Vapor Compression Refrigeration
Vapor Compression Refrigeration
Vapor Compression Refrigeration
plants are among the many types of industrial plants the coil or tubes. This is where the circulating refrigerant
that often utilize large vapor-compression refrigeration rejects heat from the system and the rejected heat is
systems. carried away by either the water or the air (whichever
Refrigeration may be dened as lowering the tempera- may be the case). isotherms
ture of an enclosed space by removing heat from that
P TC TH
space and transferring it elsewhere. A device that per- Critical point
forms this function may also be called an air conditioner, Saturated vapor : any compression will
start making it a little by little into liquid
refrigerator, air source heat pump, geothermal heat pump
Heat out
or chiller (heat pump).
Condensation
TH
1 Description of the vapour- Throttling Compression
compression refrigeration system Evaporation Work in
TC
Liquid Heat in Vapour
Liquid + Vapour
+vap all turns into vapor after
airi passes through it and
V any push decreasing pressure w
of it (saturated liquid)
on is an endothermic
so it steals the energy away
m air A ctitious pressure-volume diagram for a typical refrigeration
cycle so the pressure will
decrease and it will
become partially vapor a
partially liq. but it's all
The condensed liquid refrigerant, in the thermodynamic adiabatic . WHY DOES
decrease? because upo
state known as a saturated liquid, is next routed through an expansion valve the
pressure will decrease a
expansion valve where it undergoes an abrupt reduction in the liquid will evaporate
into a mix of vapour liqu
pressure. That pressure reduction results in the adiabatic mix
1
2 2 THERMODYNAMIC ANALYSIS OF THE SYSTEM
1.1 Refrigerants
"Freon" is a trade name for a family of haloalkane
refrigerants manufactured by DuPont and other compa-
nies. These refrigerants were commonly used due to their
superior stability and safety properties: they were not
ammable at room temperature and atmospheric pres- Figure 2: TemperatureEntropy diagram
sure, nor obviously toxic as were the uids they replaced,
such as sulfur dioxide. Haloalkanes are also an order(s)
of magnitude more expensive than petroleum derived
ammable alkanes of similar or better cooling perfor-
mance.
Unfortunately, chlorine- and uorine-bearing refrigerants
reach the upper atmosphere when they escape. In the culating refrigerant enters the compressor as a saturated
stratosphere, CFCs break up due to UV radiation, releas- vapor. From point 1 to point 2, the vapor is isentropically
ing their chlorine free radicals. These chlorine free rad- compressed (i.e., compressed at constant entropy) and
icals act as catalysts in the breakdown of ozone through exits the compressor as a superheated vapor.
chain reactions. One CFC molecule can cause thousands press is the same
of ozone molecules to break down. This causes severe From point 2 to point 3, the vapor travels through part volume is decreased
temp is decreased
damage to the ozone layer that shields the Earths surface of the condenser which removes the superheat by cooling Q given out
from the Suns strong UV radiation, and has been shown the vapor. Between point 3 and point 4, the vapor travels
to lead to increased rates of skin cancer. The chlorine through the remainder of the condenser and is condensed
will remain active as a catalyst until and unless it binds into a saturated liquid. The condensation process occurs
with another particle, forming a stable molecule. CFC at essentially constant pressure.
refrigerants in common but receding usage include R-11 Between points 4 and 5, the saturated liquid refriger-
and R-12. ant passes through the expansion valve and undergoes
Newer refrigerants with reduced ozone depletion eect an abrupt decrease of pressure. That process results in
such as HCFCs (R-22, used in most homes today) and the adiabatic ash evaporation and auto-refrigeration of a Vaporizing a liquid in such
HFCs (R-134a, used in most cars) have replaced most portion of the liquid (typically, less than half of the liq- a way absorbs heat and
can cause the surrounding
CFC use. HCFCs in turn are being phased out under the uid ashes). The adiabatic ash evaporation process is area to cool drastically
Montreal Protocol and replaced by hydrouorocarbons isenthalpic (i.e., occurs at constant enthalpy).
so no Q out
(HFCs), such as R-410A, which lack chlorine. However, Between points 5 and 1, the cold and partially vaporized
CFCs, HCFCs, and HFCs all have large global warming refrigerant travels through the coil or tubes in the evapo-
potential. rator where it is totally vaporized by the warm air (from
More benign refrigerants are currently the subject of re- the space being refrigerated) that a fan circulates across
search, such as supercritical carbon dioxide, known as R- the coil or tubes in the evaporator. The evaporator oper-
744.[3] These have similar eciencies compared to ex- ates at essentially constant pressure and boils o all avail-
isting CFC and HFC based compounds, and have many able liquid there after adding 4-8 deg kelvin of superheat
orders of magnitude lower global warming potential. to the refrigerant as a safeguard for the compressor as it
cannot pump liquid. The resulting refrigerant vapor re-
turns to the compressor inlet at point 1 to complete the
thermodynamic cycle.
2 Thermodynamic analysis of the
It should be noted that the above discussion is based
system on the ideal vapor-compression refrigeration cycle which
does not take into account real world items like frictional
The thermodynamics of the vapor compression cycle can pressure drop in the system, slight internal irreversibility
be analyzed on a temperature versus entropy diagram as during the compression of the refrigerant vapor, or non-
depicted in Figure 2. At point 1 in the diagram, the cir- ideal gas behavior (if any).
3.2 Rotary screw compressors 3
Hermetic motor, hermetic compressor Reciprocating compressors are piston-style, positive dis-
placement compressors.
Hermetic motor, semi-hermetic compressor
Open motor (belt driven or close coupled), hermetic 3.2 Rotary screw compressors
compressor
5 Control
In simple commercial refrigeration systems the compres-
Figure 4: Operating principle of a Scroll Compressor sor is normally controlled by a simple pressure switch,
with the expansion performed by a capillary tube or sim-
Main article: Scroll compressor ple thermostatic expansion valve. In more complex sys-
tems, including multiple compressor installations, the use
Scroll compressors are also positive displacement com- of electronic controls is typical, with adjustable set points
pressors. The refrigerant is compressed when one spiral to control the pressure at which compressors cut in and
orbits around a second stationary spiral, creating smaller cut out, and temperature control by the use of electronic
and smaller pockets and higher pressures. By the time the expansion valves.
refrigerant is discharged, it is fully pressurized. In addition to the operational controls, separate high
pressure and low pressure switches are normally utilised
3.5 Others to provide secondary protection to the compressors and
other components of the system from operating outside
Main articles: Diaphragm compressor, Axial-ow of safe parameters.
compressor, Diagonal or mixed-ow compressor, Liquid In more advanced electronic control systems the use of
ring compressor and Roots blower oating head pressure, and proactive suction pressure,
control routines allow the compressor operation to be
adjusted to accurately meet diering cooling demands
Diaphragm pump whilst reducing energy consumption.
Axial-ow compressor
mixed-ow compressor 6 Other features and facts of inter-
Liquid ring est
Roots blower
The schematic diagram of a single-stage refrigeration sys-
tem shown in Figure 1 does not include other equipment
items that would be provided in a large commercial or in-
4 Compressor lubrication dustrial vapor compression refrigeration system, such as:
In order to lubricate the moving parts of the compressor, A horizontal or vertical pressure vessel, equipped
oil is added to the refrigerant during installation or com- internally with a demister, between the evaporator
8.2 Disadvantages 5
12 Further reading
Yunus A. Cengel and Michael A. Boles (2008).
Thermodynamics: An Engineering Approach (6th
ed.). McGraw-Hill. ISBN 0-07-352921-4.
13 External links
The ideal vapor compression refrigeration cycle,
University of Nevada (US)
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