Compressor Design

Compressor Design

Compressor Design

Compressor Design

Refrigeration Cycle

Compressor Design

Mass Flow Rates @ Conditions

Specific Volume

Comparison: Pressure-Sp. Vol.

Specific Volume

Mass Flow Rate

Compressor Capacity

Compressor Displacement

Pressure To Capacity Ratio

Compression Ratio

Comfort Conditions

Compressor Types

Compressor Types

Compressor Design

•Positive Displacement Pump

•Non-Positive displacement Pump

Reciprocating Compressor

Cylinder Volume/Pressure

Re-Expansion

Suction

Compression

Discharge

Actual Compression Cycle

Work

Work

Result of re-expansion

Due to excessive

Discharge pressure

Reciprocating Compressors

Hermetic Compressor

•Hermetic compressors employ a an electric

motor and compressor unit sealed inside a

steel enclosure.

•These compressors can NOT be taken apart

for service, the compressor pump is directly

driven by the electric motor.

Welded Hermetic Design

Semi-Hermetic Type

•Semi-hermetic compressors are bolted

together using gaskets to form a hermetic seal.

•The electric drive motor is located within the

casing of the compressor, along with the

compressor pump.

•These compressors can be taken apart for

service.

Semi-Hermetic Design

Open Drive Compressors

•The open drive type compressor contains

utilizes a separate drive motor.

•The drive shaft of the open type compressor

exits the side of the compressor connecting to

the drive system.

•The open drive uses a bolted together

construction, implementing seals and gaskets

to prevent refrigerant and oil leakage, as well

as contaminant infiltration.

Open Drive

Open Drive/ Direct Coupled

Belt Drive

Positive Displacement

•Reciprocating

•Rotary

•Both of these designs employ a mechanical

method where a volume of refrigerant is

reduced, thereby increasing it’s pressure.

Positive Displacement

•In a reciprocating compressor, a piston

moving in a cylinder reduces the volume

occupied by the refrigerant.

•Rotary compressors use a roller, vane, lobe or

scroll to reduce the volume of space occupied

by the vapor.

Open and Closed Types

•Compressors are further classified by their

construction types.

•Open Drive

•Hermetic

•Semi-Hermetic

Reciprocating Compressors

•The reciprocating compressor is the most

widely used compressor in the refrigeration

industry.

•The recip is used with all types of refrigerants,

and a wide range of applications.

•The reciprocating compressor can be divided

into two categories, the single or double acting

type.

Reciprocating Compressors

•The single acting reciprocating compressor

only compresses vapor on one side of the

piston.

•The double acting type compresses vapor on

both sides of the piston.

•The single acting reciprocating compressor is

the commonly utilized design in the

refrigeration industry today.

Rolling Piston Rotary

Sliding Vane Rotary

Rotary Compressors

•The rotary compressor can be classified into

four categories.

•The rolling piston

•Rotating Vane

•Helical Rotary (Screw)

•Scroll

Rotary Compressors

•The rotary compressor produce a smoother,

nearly constant flow of the discharge and

suction vapors.

•This operating characteristic reduces vapor

pulsations and noise by these compressors.

•Like reciprocating compressors rotary

compressors suffer from efficiency losses, but

they are generally less.

Rolling Piston Rotary

•Also known as the stationary vane rotary

compressor.

•This design employs a roller on an eccentric

shaft that glides the roller against the

stationary vane squeezing the refrigerant.

Rotating Vane Rotary

•This compressor is similar to the Rolling Piston

design.

•Except the roller has several spring loaded

vanes that are rotated on the eccentric shaft.

•As the space between the roller and the vane

is narrowed, this increases the pressure of the

refrigerant.

Helical Rotary (Screw)

•In this design compression is accomplished by

the meshing of two mating helically grooved

steel rotors.

•The male rotor consists of a series of lobes

along it’s length that mesh with the formed

helical flutes on the female rotor.

•The female rotor is driven by the turning

motion of the male rotor.

Screw Compressor

Screw Compressor Design

Screw Compressor Design

Screw Compressor Design

Screw Compressor Design

Helical Rotary (Screw)

•As the rotors turn suction vapor is drawn into

the space between the female flute and the

cylinder wall.

•This vapor becomes sealed between an

interlobe space and the cylinder wall as the

male lobe meshes with the female flute.

•The space occupied by the refrigerant vapor is

continuously reduced as the lobes mesh.

Helical Rotary (Screw)

•Twin screw compressors employ a unique

design of capacity control.

•In this design a sliding valve is utilized to open

a by-pass passage that allows refrigerant to

enter in the compression cycle later.

•The turn-down ratio on a capacity slide valve

is 90%.

Helical Rotary (Screw)

•Rotary compressors utilize very elaborate

lubrication systems.

•The use an oil pump, oil separator, oil receiver,

means of oil cooling, and all associated filters and

safeties.

•Rotary compressors utilize oil injection lubrication,

to lubricate and provide internal cooling, preventing

compressor overheating.

• This allows for compression ratios as high as 25 to

1, this allows for single stage compression.

Screw Capacity Control

Scroll Compressor

•In a scroll compressor the compression unit

consists of two matching scrolls, which when

placed together form a series of vapor pockets.

•The top scroll remains stationary, while the

bottom scroll is driven by the drive motor.

•The bottom scroll moves in an orbital motion,

squeezing the refrigerant vapor into a smaller

and smaller space.

Compressor Design

Scroll Compressor

•The scroll compressor employs a COMPLIANT

design that gives it superior tolerance to liquid

refrigerant.

•In the presence of liquid refrigerant the scrolls

are allowed to separate, passing the liquid

refrigerant.

•This compliant design improves the durability

of the compressor, reducing the need for

accumulators or crankcase heaters.

Scroll Compressor

Scroll

Scroll

Scroll

Scroll

Scroll

Hermetic Scroll Design

Compliant

Non-Positive Displacement

•Centrifugal compressors are non-positive

displacement compressors.

•This design utilizes an impeller to spin the

fluid at high speed and expel it into the volute.

•As the high speed vapor enters the volute

most of the velocity pressure is converted into

static pressure, forcing the vapor out of the

volute.

Non-Positive Displacement

•Centrifugal compressors are of the non-

positive displacement type, they do not

produce pulsations in refrigerant flow.

•The spinning impeller increases the

momentum of the vapor, thereby increasing

it’s velocity and pressure before it’s discharged

into the condenser.

•Each impeller inside a centrifugal compressor

represents one stage of compression.

Centrifugal Compressor

Centrifugal Design

Centrifugal Design

Two Stage Centrifugal

Open Drive Centrifugal

Centrifugal Compressors

•Compressors employing two, three and four

impellers is common, but additional impellers

may be used to meet the design pressures.

•The impeller is the only moving part inside

the centrifugal compressor.

•The operating principle of the centrifugal

compressor are similar to those of centrifugal

fan or pump.

Centrifugal Compressors

•The leakage between multistage compressors

is minimized by the use of labyrinth-type seals.

•The labyrinth seal is located between the

impeller’s rotor shaft, and the stationary

partitions.

•A labyrinth seal is a series of grooves

positioned side-by-side minimizing the

pressure differential of adjoining grooves,

effectively limiting transfer of vapour.

Centrifugal Compressors

•The labyrinth seal consists of several thin

strips of steel that are fastened to the rotor.

•Capacity control in centrifugal compressors is

generally accomplished by adjustable inlet

guide vanes, called pre-rotation vanes.

•There is a lower limit to the available capacity

control of centrifugal compressor.

•Lowering the capacity bellow this limit causes

surging.

Centrifugal Compressors

•Surging occurs when the energy contained in

the high side vapor exceeds the energy that

can be added to the low side vapor by the

spinning impeller.

•This condition causes the refrigerant flow to

actually reverse and flow back into the

impeller from the high side of the system.

•Surging generates abnormal noise, heat and

vibration that can damage the impeller.

Compressors

Questions

Causes of Compressor Failure

Compressor Design

Compressor Motor Cooling

Lubrication System

Compressor Capacity

Compressor Design

Compressor Design

Compressor Design