Chapter04 Compression Systems and Compressors

7/29/2019 Chapter04 Compression Systems and Compressors

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Althouse Turnquist Bracciano

PowerPoint Presentation by:Associated Technical Authors

Publisher

The Goodheart-Willcox Company, Inc.

Tinley Park, Illinois


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Chapter 4


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Compression Systems

Compressors


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Study the five thermal laws relating torefrigeration.

Explain the compression cycle for a

domestic refrigerator.List the components of a refrigeration

compression system.

Explain the operation of each component of

a compression system.

Trace the flow of refrigerant through acomplete refrigeration system.


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Name the two types of motor controls anddiscuss their operation and purpose.

Describe the five principal types of

refrigerant controls and their operation.Name four different types of compressors.

Explain how compressors operate.

Identify the internal parts of a compressor.Follow approved safety procedures.


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Chapter 4


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4.1

Fluids absorb heat while changing from a liquid to avapor state.

Fluids give up heat in changing from a vapor to a

liquid. The temperature at which a change of state occurs is

constant during the change, providing pressureremains constant.

Heat flows only from a body that is at higher

temperature to a body that is at a lower temperature(hot to cold).

Laws of Refrigeration


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4.1

Metallic parts of the evaporating and condensingunits use metals that have high heat conductivity(copper, brass, aluminum).

Heat energy and other forms of energy (electrical,mechanical) are interchangeable.

Laws of Refrigeration


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Compression Cycle

The compressor changes the refrigerant vapor fromlow pressure to high pressure during the

compression cycle. This transfers heat from the inside of the cabinet to

the outside.

Refrigerating system consists of a high-pressure sideand a low-pressure side.

4.2


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Compression Cycle4.2


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4.2.1

Operation ofCompression Cycle

Note high side and low side.

The evaporator is on the lowside.

Heat is absorbed in the lowside.

The condenser is in the high

side.

This is where the heat isreleased from the refrigerant.


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4.2.2

Temperature and Pressure Conditionsin the Compression Cycle

Upon starting, compressor moves refrigerantmolecules from the low-pressure side to the high-pressure side.

It is necessary to speed up the molecules byincreasing their temperature.

With each stroke of the compressor, more vapormolecules squeeze into the condenser, increasingpressure and temperature.

During compression, pressure increases.

At the same time, temperature increases.


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4.2.2



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4.2.2

Temperature and Pressure Conditions

in the Compression Cyclecontinued

The higher temperature causes a flow of heat to thesurrounding metal and air.

Heat moves from the vaporized refrigerant to thecooling medium.


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4.2.2



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4.2.2


Cooling continues until heat loss causes some vapormolecules to become liquid molecules.

The liquid molecules collect and flow into the liquidreceiver.

Temperature and pressure increase until a balance isreached.


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4.2.2



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When does a fluid give up latent heat?

Questions

When it changes from a gas to a liquid.

Heat always flows from a __________ substance to a______________ substance.

warmer

What is the purpose of a compressor?

It changes a low-pressure, low-temperature gas toa high-pressure, high-temperature gas.

Into which part of the system is the heat absorbed: thehigh side or the low side? The low side.

cooler


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What main component absorbs heat?

Questionscontinued

The evaporator.

During the compression cycle, as the refrigerantpressure increases, its ____________ also increases.temperature

When heat transfers from the high-pressure high-temperature refrigerant gas as it flows through thecondenser, what happens to the refrigerant?

It changes to a liquid.


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4.3

Evaporator

Two main types of evaporators:

Dry System Evaporators are fed refrigerant as

quickly as needed to maintain desired temperature.

This system usually has a superheated gasleaving the evaporator.

Flooded System Evaporators are always filled withliquid refrigerant.

The type of refrigerant control used determines thetype of evaporator used.


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4.3Evaporator

continued

Four styles of evaporators for residentialrefrigerators/freezers:

Shell-type.

Shelf-type.

Wall-type (used with chest freezers).

Fin tube-type with forced circulation (used withfrost-free construction).


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4.3

Evaporator


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4.3

Evaporator


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4.3

Evaporator


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4.3

Evaporator


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4.4Accumulator

A safety device.

Prevents liquid refrigerant from flowing back into thecompressor.

Liquid refrigerant that flows into the accumulator willbe evaporated.

This vapor will flow into the suction line.


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4.4Accumulator


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4.5

Suction Line

Carries the refrigerant vapor from the evaporator tothe compressor.

Must be large enough to avoid resistance of

refrigerant 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 to

reduce flash gas in evaporator.


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4.5.1Low-Side Filter-Drier

Included at the compressor end of the suction line onsome systems.

May be placed in the system for a short period to clean

the refrigerant within the system.

Should offer little resistance to vaporized refrigerantflow.


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4.5.1Low-Side Filter-Drier


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4.5.2

Compressor Low-Side orSuction Service Valve

Allows the technician to connect gauges to thesystem.

Allows for checking pressures and adding or removing

refrigerant or oil.

Sealing caps protect the opening when valve is not inuse.

Most new domestic models do not have service valves.

Saddle valves are used instead.


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4.5.2

Compressor Low-Side or SuctionService Valve


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What are two main types of evaporators?

Questions

A dry system evaporator and flooded system evaporator.

Which component determines whether an evaporator isconsidered dry or flooded?

The metering device.

Name four styles of evaporators for residentialrefrigerator/freezers.

Shell-type, shelf-type, wall-type, and fin tube-type.


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What is the purpose of an accumulator?

Questionscontinued

It prevents liquid refrigerant from flowing back into the

compressor. Where is the suction line located on a refrigeration system?

Between the evaporator and the compressor.

Where is the low-side filter-drier placed in a system?

In the suction line.


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Questionscontinued

What is the purpose of the suction service valve?

To allow a technician to access the system for

charging, removal of refrigerant, evacuation, andadding refrigerant oil.


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4.6

Compressor

A motor-driven device.

Removes heat-laden vapor refrigerant from the

evaporator.

Compresses the vapor into a small volume at a hightemperature.


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4.6.1

Compressor High-SideService Valve

Provides a shutoff between the compressor and thecondenser.

Provides an opening for a high-pressure gauge or agauge manifold.


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4.6.1

Compressor High-SideService Valve


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4.7

Oil Separator

Separates the oil from the hot, compressed vapor.

Placed between the compressor exhaust and thecondenser.

Contains a series of baffles or screens that collect theoil.

Oil is returned to the compressor crankcase by the useof a float-type valve.

Commonly used in large commercial installations.


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4.7Oil Separator


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4.8

Condenser

Removes the condensation heat from the refrigerantvapor.

This heat is picked up in the evaporator and the

compressor.


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4.8Condenser

Condensers commonly used in domestic refrigeration:

Finned-static (natural convection).

Finned-forced convection. Wire-static.

Plate-static.


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4.8Condenser


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4.8Condenser


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4.8Condenser

Condensers commonly used in commercial systems:

Finned-static, air-cooled.

Finned-forced convection, air-cooled. Water-cooled, tube-in-a-tube, shell and coil, shell

and tube, and evaporative type.

Plate-static.


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What is the purpose of the compressors high-side servicevalve?

Questions

It provides shutoff between the compressor and thecondenser and provides access to the high side for aservice technician.

What is the purpose of an oil separator?

It separates the refrigerant oil from the high-pressuregas leaving the compressor.


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Where is an oil separator located?

Questionscontinued

In the discharge line between the compressor and the

condenser. What type of valve is used in an oil separator?

A high-side float.

What is the purpose of the condenser?

It removes the heat that is picked up in the evaporatorand the compressor.


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Name two condensers commonly used in domesticrefrigeration.

Questionscontinued

Wire-static and finned-force convection. Name two types of condensers used in a commercial system.

Finned-force convection and water-cooled.


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4.9

Liquid Receiver

A storage tank for liquid refrigerant.

Most have service valves.

Often found in systems using a low-side float orexpansion valve-type refrigerant control.

Not used in capillary-tube systems.

Seldom used in domestic systems or small commercialunits.


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4.9

Liquid Receiver


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4.10

Liquid Line

Usually made of copper tubing.

Domestic units use steel.

Used to carry liquid refrigerant from the condenser tothe evaporator.

Avoid pinching or buckling these lines.


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4.10.1

Liquid Line Filter-Drier

Often installed in liquid line.

Keeps moisture, dirt, and metal from enteringrefrigerant flow control.

Drying element in filter removes moisture.

Some equipped with sight glass to indicate refrigerantlevel.

May contain chemical that changes color to indicate

moisture in system.


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4.10.1

Liquid Line Filter-Drier


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What is the purpose of a liquid receiver?

Questions

It is a storage tank for refrigerant within the system.

Does a capillary tube system require a liquid receiver? No.

Does a TXV system require a receiver? Yes.

Where is the liquid line located on a refrigeration system?

Between the condenser (or receiver) and themetering device.


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What is the purpose of a liquid line filter drier?

Questionscontinued

To clean the system of moisture and small particles.

What is commonly used with a filter drier that determinesrefrigerant charge in a system? Sight glass.


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4.11

Refrigerant Flow Control

Allows liquid refrigerant to enter the evaporator.

Maintains the required evaporating pressure in theevaporator.


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4.11Refrigerant Flow Control

continued

There are five types of refrigerant flow controls:

Capillary (CAP) Tube.

Automatic Expansion Valve (AEV).

Thermostatic Expansion Valve (TEV).

Low-Side Float (LSF).

High-Side Float (HSF).


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4.11Refrigerant Flow Control

continuedCapillary (CAP) Tube

Long length of small diameter tubing.

Reduces pressure by reducing the flow of

refrigerant through its length. Does not use a check valve or a direction control

valve.

High and low pressures equalize during the offpart of the cycle.


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4.11.1

Capillary (CAP) Tube


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4.11.2Refrigerant Flow Control

continuedAutomatic Expansion Valve (AEV)

Used only with the temperature-operated motorcontrol.

Maintains constant pressure in the evaporatorwhen the system is running.

Operates independently of the amount ofrefrigerant in the system.

Division point between high side and low side.

Adjustable to the correct evaporator pressure.

Refrigerant flows only when the compressoris running.


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4.11.2

Automatic Expansion Valve(AEV)


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4.11.3

Refrigerant Flow Controlcontinued

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.

Th i i V l


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4.11.3

Thermostatic Expansion Valve(TEV)


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continuedLow-Side Float (LSF)

Used on a flooded system.

May use either a temperature- or pressure-operated motor control.

Usually has a large liquid receiver.

Can be used in multiple evaporator systems.


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4.11.4Low-Side Float (LSF)


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continuedHigh-Side Float (HSF)

Float is located in the liquid receiver tank or in achamber in the high-pressure side.

Float controls level of liquid refrigerant on the high-pressure side.

Amount of refrigerant in system must be carefullymeasured.

Extra refrigerant will overcharge the evaporator

and cause frosting of the suction line. Can be used with a pressure- or temperature-

operated motor control.


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4.11.5High-Side Float (HSF)


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What is the purpose of a refrigerant flow control?

Questions

It separates the high side from the low side and changes ahigh-pressure liquid to a low-pressure liquid.

On a capillary tube system, what happens to the high- andlow-side pressures during the off cycle? They equalize.

What is the purpose of an AEV (automatic expansionvalve)? It maintains a constant pressure in the evaporator.

Which type of compressor motor control is used with anAEV? A temperature-operated motor control.


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What are three operating pressures of a TEV (thermostaticexpansion valve)?

Questionscontinued

Sensing bulb (opening force), the spring, and evaporatorpressure (closing force).

Which type of motor control is used with a TEV?

A pressure- or temperature-operated motor control.


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Which type of system uses a low-side float?

Questionscontinued

A flooded evaporator system.

Name two metering devices that are used on criticallycharged refrigeration systems.

The high-side float and the capillary tube system.


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4.12Motor Control

Two types:

Temperature-operated motor control(thermostatic).

Sensing bulb connected to a toggle or snap-action switch.

Adjustment permits differences in operatingtemperatures.

Pressure-operated motor control (low-sidepressure).


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4.12Motor Control


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Chapter 4


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4.13

External-Drive Compressors

Bolted together.

Crankshaft extends through the crankcase and isdriven by a flywheel (pulley) and belt.

May be driven directly by an electric motor or gasengine.

Requires a crankshaft seal.

Seldom used today.


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4.13

External-Drive Compressors


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4.14

Hermetic Compressors

Motor is sealed inside a dome or housing.

Motor is directly connected to the compressor nocrankshaft seal needed.

Usually is spring-mounted inside the hermetic dome,reducing vibration.


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4.14

Hermetic Compressors


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4.15

Types of Compressors

Reciprocating.

Rotary.

Scroll.

Screw.

Centrifugal.


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4.15.1

Reciprocating Compressor

Used in majority of domestic, commercial, andindustrial HVAC systems.

Basic components include a cylinder and a rod.

Can be classified by cylinder arrangement, number ofcylinders, type of crankshaft, or construction.

May be multicylinder, thereby increasing pumpingcapacity.

Cylinders are usually made of cast iron. Usually, crankcase is part of same casting as the

cylinder.


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4.15.1



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4.15.1



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4.15.1

Reciprocating Compressorcontinued

Piston is designed to come as close as possible to thecylinder head without touching it. This forces as muchof the vapor into the high-pressure side as possible.

A valve plate under the cylinder head contains boththe intake and exhaust valves.

A connecting rod attaches the piston to thecrankshaft.


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4.15.1



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4.15.1



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4.15.1Reciprocating Compressor

continued

Several compressor valve designs are used.

The intake valve operates at a relatively cooltemperature and is constantly lubricated by oil

circulating with the refrigerant vapors. The exhaust valves operate at high temperatures and

must be leakproof.

Valves open about .010" (.254mm). If movement isgreater, valve noise develops.


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4.15.1



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continued

If a system uses an external motor to drive thecompressor, a crankshaft seal is necessary. All sealsuse two rubbing surfaces that must be lubricated toprevent wear and leakage.

External drive compressors are driven by a V-belt.These are usually driven at less than the motor speed.Therefore, the motor belt pulley will be smaller than

the compressor pulley. More than one belt may be used in large-capacity

installations.


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4.15.1



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continued

Some compressors use an eccentric-type crankshaftmechanism. This construction reduces vibration andthe need for connecting rod caps and bolts.

A scotch yoke mechanism may be used to connect thepiston to the crankshaft. No connecting rod is used.The cylinder and piston are quite long.

The scotch yoke is popular in small high-speedcompressors.


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4.15.1



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4.15.1



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continued

The reciprocating compressor used on automobile airconditioning systems is known as a swash plate (orwobble plate) compressor. No connecting rod isused.

The swash plate compressor usually has three or morecylinders arranged in a circle around the drive shaft.

The swash plate compressor is double-acting athree-cylinder compressor gives a pumping action like

a six-cylinder conventional compressor.


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4.15.1



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continued

Compressor housings support the cylinders,crankshaft, valves, oil pump, lubrication lines, and therefrigerant inlet and exhaust openings.

In hermetic compressors, the housing also supportsand aligns the driving motor.


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4.15.2

Rotary Compressor

Two basic types:

Stationary blade.

Rotating blade.


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4.15.2

Rotating Blade

Blades rotate with the shaft.

Operation:

Low-pressure vapor from the suction line is drawn

into the opening.

The vapor fills the space behind the blade as itrevolves.

Trapped vapor in the area ahead of the blade is

compressed until it can be pushed into the exhaustline to the condenser.


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4.15.2

Rotating Blade


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4.15.2Rotating Blade

continued

Frequently used as the booster compressor incascade systems.

Advantages:

Provides a large-size opening into the suction line.

Provides large inlet port openings.

Has a very small clearance volume.


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4.15.2

Stationary Blade

Blade remains stationary and is part of the housingassembly.

Operation:

An eccentric shaft rotates an impeller in a cylinder.The impeller constantly rubs against the outer wallof the cylinder.

The blade traps vapor as the impeller revolves. Thevapor is compressed into a smaller space. The

pressure and temperature increase.

The vapor is forced through the exhaust port,entering the condenser.


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4.15.2

Stationary Blade


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4.15.2

Stationary Blade


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Rotary Compressor


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4.15.2

y pConstruction

Rotating blade compressor rotor is a fixed part ofshaft.

Stationary blade compressor rotor accurately fits

the eccentric. The eccentric is a fixed part of the shaft.


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4.15.3

Scroll Compressor Operation

Generates a series of crescent-shaped gas pocketsbetween two scrolls.

One scroll remains stationary.

The other scroll the orbiting scroll rotatesthrough the use of the swing link.

Pockets of gas are slowly pushed to the center,reducing their volume.

When the pocket reaches the center of the scroll, thegas is at high pressure.

Discharge is through the center port.


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4.15.3Scroll Compressor Operation


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4.15.4

Screw Compressor

Two basic types:

Open, externally driven compressor, used mostoften with ammonia systems.

Hermetic, internally driven compressor, usinghalocarbon refrigerants.


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4.15.4

Screw Compressor Operation

AMale rotor. BFemale rotor


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4.15.4Screw Compressor Operation

continued

The rotors provide a continuous pumping action,trapping and compressing the refrigerant.

A capacity control device provides for variableunloading and accurate temperature control of aconditioned space.

C if l C


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4.15.5

Centrifugal Compressor

Used in large-capacity systems from 50 to 5,000 tons.

Simple design no valves, pistons, or cylinders.

Pumping efficiency increases with speed.

Centrifugal CompressorO i


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4.15.5

Operation

Vapor is fed into housing near the center of thecompressor.

Rapidly rotating impellers move the vapor outward,forcing it against the outer diameter and increasing itspressure.

Several impellers are put in series to multiply thepressure increase and further compress the vapor.

Centrifugal CompressorO i


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4.15.5

Operation

C M t


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4.16

CompressorMotors

External drive compressor.

Motor connected to compressor by one or moreV-belts or by direct drive.

Hermetic compressor.

Motor is mounted under the same dome ascompressor.


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4.18Mufflers

Used on most hermetic units and many external-drivesystems to reduce noise.

May be located at both the intake and the exhaustopenings of the compressor.

Constructed of brazed cylinders with baffle platesmounted inside.


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4.18Mufflers

Note discharge andsuction (intake)

mufflers.


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Compressor VolumetricEffi i


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4.21

Efficiency

For efficient operation, the volumetric efficiency mustbe as high as possible.

Items such as head pressure, clearance space, andsize and condition of the valve openings impactvolumetric efficiency.

Formula:

Volumetric efficiency =__Actual volume__Calculated volume( ) X 100

C i R ti


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4.22

Compression Ratio

The relationship of the high-side absolute pressure tothe low-side absolute pressure.

Ratios vary up to 10:1.

Check Val es


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4.23

Check Valves

Prevent refrigerant vapor, oil, or liquid refrigerantfrom backing up into the evaporator or other devices.

Check Valves


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4.23

Check Valves

Unloader


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4.24

Unloader

Makes it easier to start the compressor by temporarilyreducing high-side pressure at the cylinder head.

May be operated mechanically, electrically,hydraulically, or by a solenoid valve.

Unloader


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4.24

Unloader

Gaskets


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4.25

Gaskets

May be made of special paper, synthetic material,plastic, or lead.

Used between bolted parts such as cylinder heads,valve plates, and crankcase openings.

O ring


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4.26

O-ring

Sealing device.

Material used for O-ring depends on temperature,pressure, fluids to be controlled, and useful liferequired.

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O ring


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4.26

O-ring

Crankcase Heater


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4.27Crankcase Heater

Evaporates liquid refrigerant trapped in the oil.

Required on remote installations where ambienttemperature is lower than evaporator temperature.

Reduces risk of oil slugging.

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Crankcase Heater


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4.27

Crankcase Heater

Questions


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Name two types of compressor motor controls.

Questions

Temperature-operated and pressure-operated.

Name two types of compressor drives.Direct drive and belt drive.

Name five types of compressors.Reciprocating, rotary, scroll, screw, and centrifugal.

On a reciprocating compressor, where are the discharge

and suction valves located?In a valve plate under the compressor head.

Safety


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Always be alert to hazardous situations or proceduresand equipment that may be dangerous.

Always follow recommended procedures.

Always wear goggles when working on a refrigeratingsystem.

Handle all parts with care. Clean and dry each part thatis dismantled.

Do not allow moisture to enter the refrigerating

mechanism.

Safety4.28

Safety


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Make sure settings and adjustments are done correctly.Improper temperatures cause food to spoil.

Take care when tightening tube connections, installinggaskets, replacing electrical terminals, and solderingfittings.

Safetycontinued

4.28

Glossary


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Glossary

accumulatorStorage tank that receives liquid refrigerant from the evaporator andprevents it from flowing into the compressor before vaporizing.

centrifugal compressorPump that compresses gaseous refrigerants by centrifugal force.

check valvesAllow fluid to flow in only one direction in a system.

clearance space

The space left in the cylinder when the roller or piston ends itscompression stroke.

Glossary


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Glossary

compression ratioRatio of the total volume of the cylinder to the clearance space.

compressor

Pump of a refrigerating mechanism that draws a low pressure on thecooling side of the refrigerant cycle and squeezes or compresses thegas into the high-pressure or condensing side of the cycle.

compressor volumetric efficiencyThe relationship between the actual performance of a compressor andtheoretical performance of the pump based on its displacement.

Glossary


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Glossary

condenserThe part of the refrigeration mechanism which receives hot, high-pressure refrigerant gas from compressor and cools gaseousrefrigerant until it returns to its liquid state.

evaporatorPart of a refrigerating mechanism in which the refrigerant vaporizesand absorbs heat.

Glossary


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Glossary

head pressureThe pressure the compressor must pump against.

liquid receiver

Cylinder (container) connected to condenser outlet for storage ofliquid refrigerant in a system.

oil separatorDevice used to remove oil from gaseous refrigerant.

rotary compressor

Mechanism that pumps fluid by using rotating motion.

Glossary


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Glossary

screw compressorCompressor constructed of two mated revolving screws.

scroll compressor

A compressor that uses the interaction of two spiral coils (scrolls) tocompress a vapor.

suction lineTube or pipe used to carry refrigerant gas from evaporator tocompressor.

unloaderTemporarily reduces the high-side pressure at the cylinder head duringcompressor startup.

Chapter04 Compression Systems and Compressors

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Transcript of Chapter04 Compression Systems and Compressors