the vapor compression 1

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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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



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-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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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



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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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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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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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the vapor compression 1

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