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8/2/2019 The Vapor Compression 1
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REFRIGERATION SYSTEM
AND COMPONENTS
CYRIL G. FABREA, MSME, PME
AssociateProfessor
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Vapor Compression Cycle
Compressor
ThrottlingDevice
Evaporator
Condenser
Liquid Side
Gas Side
Low Pressure Side
High Pressure Side
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Pressure
(P)
Pressure
Enthalpy Diagram
Enthalpy (H)
Liquid-VaporMixture Region
LiquidRegion
Vapor
Region
SaturatedLiquid
SaturatedVapor
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Pres
sure
(P)
Enthalpy (H)
A
D
C
B
A
BC
D
Discharge / Condensing Pressure
Suction / Evaporating Pressure
Compressor Work
W
Condenser Heat RejectionQ
Refrigeration LoadQ
Vapor Compression Cycle
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Major Components in Vapor Compression Cycle
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Refrigeration Cycle: PH diagramEvaporator
A diagram of a typical vapor-compression refrigeration cycle issuperimposed on a pressure-enthalpy (P-h) chart to demonstratethe function of each component in the system.
The pressure-enthalpy chart plots the properties of a refrigerant
pressure (vertical axis) versus enthalpy (horizontal axis).
The cycle starts with a cool, low-pressure mixture of liquid andvapor refrigerant entering the evaporator where it absorbs heatfrom the relatively warm air, water, or other fluid that is beingcooled.
This transfer of heat boils the liquid refrigerant in the evaporator,and this superheated refrigerant vapor is drawn to the compressor.
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Refrigeration Cycle: PH diagramCompressor
The compressor draws in the
superheated refrigerant vapor andcompresses it to a pressure andtemperature high enough that it canreject heat to another fluid.
This hot, high-pressure refrigerantvapor then travels to the condenser.
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Refrigeration Cycle: PH diagramCondenser
Within the condenser, heat is transferredfrom the hot refrigerant vapor to relatively
cool ambient air or cooling water. This reduction in the heat content of the
refrigerant vapor causes it to desuperheat,condense into liquid, and further subcoolbefore leaving the condenser for theexpansion device.
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Refrigeration Cycle: PH diagramExpansion Device
The high-pressure liquid refrigerant flows throughthe expansion device, causing a large pressure
drop that reduces the pressure of the refrigerant tothat of the evaporator.
This pressure reduction causes a small portion ofthe liquid to boil off, or flash, cooling the
remaining refrigerant to the desired evaporatortemperature.
The cooled mixture of liquid and vapor refrigerantthen enters the evaporator to repeat the cycle.
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Major Components in Vapor Compression Cycle
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CompressorsKompressorerVerdichter
- Classification- Types- Application
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Compressors Basics
Task:The compressor has the task to compress a certainamount of the refrigerant (or gas) from a lower to ahigher pressure.
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Compressors Classification
Accessibilityhermetic
semi-hermetic
open
Type of compressionstatic (displace) => positive displacement compressors
reducing the volume of the compression chamber
dynamic => centrifugal (turbo) compressors
continuous transfer of angular momentum from the rotatingparts to the gas followed by conversion of momentuminto a pressure rise
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Positive Displacement Compressors:
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Reciprocating Compressors:
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Reciprocating Compressor
The refrigerant vapor is compressed by a piston that is located
inside a cylinder.
The piston is connected to the crankshaft by a rod.
As the crankshaft rotates, it causes the piston to travel back
and forth inside the cylinder.
Suction valve and the discharge valve, are used to trap therefrigerant vapor within the cylinder during this process.
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Reciprocating Compressor
Intake stroke
The piston travels away from the discharge valveand creates a vacuum effect
Reduction in the pressure within the cylinder tobelow suction pressure forces the suction valve toopen and the refrigerant vapor is drawn into thecylinder.
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Reciprocating Compressor Compression stroke & discharge
The piston reverses its direction and travels toward the discharge
valve, compressing the refrigerant vapor
The suction valve is then closed, trapping the refrigerant vaporinside the cylinder.
As the piston continues to travel toward the discharge valve, therefrigerant vapor is compressed.
The discharge valve is forced open and the compressedrefrigerant vapor leaves the cylinder.
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Compressors Types
Reciprocating Trunk Piston Hermetic
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Compressors Types
Reciprocating Trunk Piston Open
http://www.daikin.be/internet/denv/content.nsf/557d7fb9835ac0d1c12569df003b23b2/58b4fda1c4454926c1256ab500313822?OpenDocument
no built-in
pressure ratio starting torque
reduction capacity control
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Compressors Types
Reciprocating Axial Piston
small size many pistons => low vibrations and pulsations
mobile AC
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Compressors Classification
Rotating Compressors:
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Trochoidal:
low vibrations
no suction valve
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Scroll compressor (contd)
The refrigerant vapor enters through the outer edge ofthe scroll assembly and discharges through the port at
the center of the stationary scroll. The orbiting motion causes the relative movement
between the orbiting scroll and the stationary scroll sothat the pockets of refrigerant moving towards the
discharge port at the center of the assembly, Hence, there is a gradually decreasing in refrigerant
volume and increasing in pressure.
Scroll compressors are widely used in heat pumps,rooftop units, split systems, etc.
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Screw Compressor
Screw compressortraps the refrigerant vapor andcompresses it by gradually shrinking the volume of the
refrigerant.
This particular screw compressor design uses twomating screw-like rotors (male and female rotors) toperform the compression process.
Only the male rotor is driven by the compressor motor.The lobes of the male rotor engage and drive the femalerotor, so that the two parts counter-rotate.
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Screw Compressor (Contd)
Refrigerant vapor enters the compressorhousing through the intake portand fills thepockets formed by the lobes of the rotors.
As the rotors turn, they push these pockets ofrefrigerant toward the discharge end of thecompressor.
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Continued rotation of the rotor lobes drives thetrapped refrigerant vapor toward the discharge
end of the compressor. (i /e., compressing therefrigerant).
When the pockets of refrigerant reach the
discharge port, the compressed vapor is This action progressively reduces the volume of
the pockets (released and the rotors force theremaining refrigerant from the pockets.
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Centrifugal Compressor The centrifugal compressor adopts the principle of dynamic
compression by converting kinetic energy to static energy to
increase the pressure and temperature of the refrigerant.
A centrifugal compressor comprises rotating impeller thecentre of which is fitted with blades that draw refrigerant vaporinto radial passages that are internal to the impeller body.
The rotation of the impeller causes the refrigerant vapor toaccelerate within these passages that leaves the impeller andenters the passages. These passages start out small andbecome larger as the refrigerant travels through them. As thesize of the passage increases, the kinetic energy of therefrigerant decreases.
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Open, hermetic, and semi hermetic.A reciprocating compressor
Open compressor
An open compressoris driven by an external powersource, such as an electric motor or an engine.
The motor is coupled to the compressor crankshaft by a
flexible coupling. The coupling needs precise alignment.
The shaft protrudes through the compressor housing andhence a seal is used to prevent refrigerant from leaking
out of the compressor housing , and these seals are aprime source of oil and refrigerant leaks.
This motor is cooled by air that is drawn in from thesurrounding space.
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H i
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Hermetic compressor
A hermetic compressor seals the motor within thecompressor housing.
This motor is cooled by the refrigerant, either byrefrigerant vapor that is being drawn into the compressorfrom the suction line or by liquid refrigerant that is being
drawn from the liquid line. The heat from the motor is then rejected by the
condenser. Hermetic compressors eliminate the need for the shaft
couplings. However, if the motor burns out, a system with a
hermetic compressor will require thorough cleaning thatis not needed for an open compressor.
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Semi hermetic compressor
The motor for a semi hermetic compressor isalso contained within the compressorhousing and is cooled by the refrigerant.
The term semi hermetic means that thesealed housing is designed to be opened torepair or checking the compressor or motor.
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Compressors Application
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Compressors Application
0,1
1
10
100
1000
10000
Recipr
oc.
Rotar
yva
ne
Stat.van
e
Trocho
idal
Scroll
Screw
Centri
fugal
Compressor Type
Shaft
Power/kW
Compressors Application
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Compressors Application
-150 -100 -50 0 50 100
Reciproc.
Rotary vane
Stat. vane
Trochoidal
Scroll
Screw
Centrifugal
Evaporating Temperature / C
Chemestry AirConProcess Cool.
Freezing AirConNR
AirCon
AirConHH
Mob. AC
?
Whole Refrigeration and AirCon
NR
NR = Normal Refrigeration
HH = Household Application
Tunnel
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Condenser
Removes the condensation heat from the refrigerantvapor.
This heat is picked up in the evaporator and thecompressor.
Condensers commonly used in domestic refrigeration:
Finned-forced convection
Wire-static
Finned-static (natural convection)
Plate-static
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Finned-forced convectionFinned-static (natural convection)
Wire-static Plate-static
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Condensers commonly used incommercial systems:
Finned-static air-cooled
Finned-forced convection, air-cooled
Water-cooled, tube-in-a-tube, and shell andcoil, shell and tube and evaporative type.
Plate-static
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Condensers commonly used in commercialsystems:
* Finned-forced convection, air-cooled
* Finned-static air-cooled
* Water-cooled, tube-in-a-tube, and shell and
coil, shell and tube and evaporative type.* Plate-static
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Air-Cooled Condensers
A typical air-cooled condenseruses propeller-type fans to
draw outdoor air over a finned-tube heat transfersurface.
The resulting reduction in the heat content of therefrigerant vapor causes it to condense into liquid.
Within the final few lengths of condenser tubing (subcooler), the liquid refrigerant is further cooled below thetemperature at which it was condensed.
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Water-Cooled Condensers
The shell-and-tube is the most common type. Water is pumped through the tubes while the refrigerant
vapor fills the shell space surrounding the tubes.
As heat is transferred from the refrigerant to the water,
the refrigerant vapor condenses on the tube surfaces. Hot refrigerant vapor enters the water cooled condenser
at the top
The condensed liquid refrigerant then falls to the bottomof the shell at which is sub cooled by the sub cooler
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Evaporative Condensers
Within evaporative condenser, the refrigerant flowsthrough tubes and air is drawn or blown over the tubesby a fan.
Water is sprayed on the tube surfaces.
As the air passes over the coil, a small portion of thewater evaporates.
Evaporation of water absorbs heat from the coil thatcauses the refrigerant vapor within the tubes tocondense.
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Evaporator
Two main types of evaporators:Dry System Evaporators are fed refrigerant
as quickly as needed to maintain desiredtemperature.
This system usually has a superheated gasleaving the evaporator.
Flooded System Evaporators are alwaysfilled with liquid refrigerant.
The type of refrigerant control useddetermines the type of evaporator used.
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Four styles of evaporators forresidential refrigerator/freezersShell type
Shelf-type
Wall-type (used with chest freezers)
Fin tube-type with forced circulation (usedwith frost-free construction).
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Shell type
Shelf-type
Wall-type (used with chest freezers)
Fin tube-type with forced circulation
Evaporator
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Evaporator.
The evaporator is a heat exchanger thattransfers heat from air, water, or some otherfluid to the cool liquid refrigerant.
Two common types of evaporators arefinned-tube and shell-and-tube.
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Fi d T b E
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Finned-Tube Evaporators
A finned-tube evaporator includes rows of tubes passingthrough sheets of formed fins.
Liquid refrigerant flows through the tubes, cools the tube andfin surfaces.
When air passes through the coil and comes into contact withthe cold fin surfaces, heat is transferred from the air to therefrigerant.
The refrigerant to boil and leave the evaporator as vapor asheat is transferred.
The fins of the coil are formed to produce turbulence as the air
passes through them. This turbulence enhances heat transfer,preventing stratification within the coil-leaving air stream.
Producing cooled air comparing with shell-and-tube type whichis for chilled water.
Shell-and-Tube
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Shell-and-TubeEvaporators
A shell-and-tube evaporator is used to produce chilled water.
The cool liquid refrigerant flows through the tubes and waterfills the shell space surrounding the tubes.
As heat is transferred from the water to the refrigerant, therefrigerant boils inside the tubes. Water enters the shell at one end and leaves at the opposite
end.
This chilled water is pumped to one or more heat exchangers tohandle the system cooling load.
Baffles within the shell direct the water in a rising and fallingflow path over the tubes that carry the refrigerant. This createsturbulence and results in improved heat transfer.
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Flow Refrigerant Control
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Flow Refrigerant Control
Allows liquid refrigerant to enter the evaporator. Maintains the required evaporating pressure in the
evaporator.
There are five types of refrigerant flow controls:
1. Capillary (CAP) Tube
2. Automatic Expansion Valve (AEV)
3. Thermostatic Expansion Valve (TEV)
4. Low-Side Float (LSF)
5. High-Side Float (HSF)
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Refrigerant Flow Control
1. Capillary (CAP) Tube
Long length of small diameter tubing.
Reduces pressure by reducing the flow ofrefrigerant through its length.
Does not use a check valve or a direction controlvalve.
High and low pressures equalize during the off
part of the cycle.
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Capillary (CAP) Tube
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Refrigerant Flow Control
2. Automatic Expansion Valve (AEV) Used only with the temperature-operated motor
control.
Maintains constant pressure in the evaporator
when the system is running. Operates independently of the amount of
refrigerant in the system.
Division point between high side and low side.
Adjustable to the correct evaporator pressure. Refrigerant flows only when the compressor
is running.
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Automatic Expansion Valve (AEV
Refrigerant Flow Control
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g
3. Thermostatic Expansion Valve (TEV) Sensing bulb mounted at the evaporator outlet.
Bulb temperature controls the operating of thethermostat valve needle.
Sensing bulb is the opening force; spring andevaporator pressure are the closing forces.
Evaporator fills more quickly and permits moreefficient cooling.
Used with pressure- or temperature-operatedmotor control.
Can be used with a multiple evaporator system.
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Thermostatic Expansion Valve (TEV)
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R f i t Fl C t l
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Refrigerant Flow Control
4. Low-Side Float (LSF) Used on a flooded system.
May use either a temperature- or pressure-operatedmotor control.
Usually has a large liquid receiver. Can be used in multiple evaporator systems.
L Sid Fl (LSF)
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Low-Side Float (LSF)
R f i Fl C l
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Refrigerant Flow Control
5. High-Side Float (HSF) Float is located in the liquid receiver tank or in a
chamber in the high-pressure side.
Float controls level of liquid refrigerant on the
high-pressure side. Amount of refrigerant in system must be carefully
measured.
Extra refrigerant will overcharge the evaporatorand cause frosting of the suction line.
Can be used with a pressure- or temperature-operated motor control.
Hi h Sid Fl t (HSF)
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High-Side Float (HSF)
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A l t
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Accumulator
S ti Li
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Suction Line
Carries the refrigerant vapor from the evaporator tothe compressor.
Must be large enough to avoid resistance ofrefrigerant flow.
Should slope from the evaporator or accumulatordown to the compressor to avoid oil pockets.
May be in contact with all or part of liquid line toreduce flash gas in evaporator.
L Sid Filt D i
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Low-Side Filter-Drier
Included at the compressor end of the suction lineon some systems.
May be placed in the system for a short period toclean the refrigerant within the system.
Should offer little resistance to vaporized refrigerant
flow.
Compressor Low-Side or
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Suction Service Valve
Allows the technician to connectgauges to the system.
Allows for checking pressuresand adding or removingrefrigerant or oil.
Sealing caps protect the openingwhen valve is not in use.
Most new domestic models donot have service valves. Saddle
valves are used instead.
Compressor High-Side
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p g
Service Valve
Provides a shutoff between the compressor and thecondenser.Provides an opening for a high-pressure gauge or agauge manifold.
Oil S t
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Oil Separator Separates the oil from the hot, compressed vapor.
Is placed between the compressor exhaust and thecondenser. Contains a series of baffles or screens which
collect the oil. Oil is returned to the compressor crankcase by the
use of a float type valve. Commonly used in large commercial installations.
Liq id Recei er
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Liquid Receiver
A storage tank for liquid refrigerant Most have service valves. Often found in systems using a low-side float or
expansion valve-type-refrigerant control. Not used in capillary-tube systems. Seldom used in domestic systems or small
commercial units.
Liquid Line
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Liquid Line
Usually made of copper tubing. Domestic units use steel.
Used to carry liquid refrigerant from the condenserto the evaporator.
Avoid pinching or buckling these lines.
Liquid Line Filter Drier
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Liquid Line Filter-Drier
Often installed in liquid line. Keeps moisture, dirt, and
metal from enteringrefrigerant flow control.
Drying element in filterremoves moisture.
Some equipped with sightglass to indicate refrigerantlevel.
May contain chemical thatchanges color to indicatemoisture in system.
Moisture-Indicating Sight Glass
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A moisture-indicating sight glass is
installed in the liquid line, upstream ofthe expansion valve,
It enables the operator to observe thecondition of the Refrigerant: Indicate its moisture &
Detect the presence of bubbles in theliquid line
prior to entering the expansion valve.
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