Vapour Compression Cycle

Front 1

VCC

Back 1

Imp for installation, servicing and troubleshooting

Front 2

Two refrigeration cycles

Back 2

Vapour compression cycle

and Absorbtion cycle

Front 3

Absorption cycle

Back 3

Specialized fields - large buildings and recreational vehicles use absorption cycle

Front 4

Pressure enthalpy (PE) charts

Back 4

Show the characteristics of refrigerants

Shows various refrigerant conditions throughout the operating region system: as liquid , vapour and as saturated

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Vapour

Back 5

Form of gas that easily converts to to vapours.

Vapours exist below the critical point for all refrigerants

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The critical point

Back 6

On a PE chart is a specific temp and pressure relationship for each refrigerant

Above critical point a gas cannot be liquified and a liquid cannot be changed into a gas.

Refrigerant become unpredictable above the critical point.

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Below the critical point

Back 7

Refrigerant gases are called vapours

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Vapour compression

Back 8

Squeezing action that attempts to make something smaller

1. Positive displacement

2. Centrifugal compression

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Positive displacement

Back 9

Process using a moving piston to squeeze vapour in a cylinder.

Piston rings seal the vapours in cylinder

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Reciprocating compressor

Back 10

Positive displacement compressor

Vapour compression occurs, when the piston positively displaces any vapour within the cylinder

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Centrifugal compression

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Non- positive displacement compressor with a radial wheel that moves vapours.

Non positive displacement compressors has no cylinders or pistons

They has impeller wheels inside a chamber called a volute.

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Centrifugal compressor

Back 12

The increase in velocity of the refrigerant vapour passing through a centrifugal compressor increases the pressure of the refrigerant.

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Front (Term)

Back 13

Vapour flow centrifugal compressor

Front 14

Heat of compression ( HoC)

Back 14

Heat energy added to refrigerant vapour by the work of compressor.

The amount of pressure increases with a compressor relates directly to the amount of heat produced during compression

Front 15

Heat of compression ( HoC)

Back 15

Heat energy added to refrigerant vapour by the work of compressor.

The amount of pressure increases with a compressor relates directly to the amount of heat produced during compression

Front 16

HoC

Back 16

The higher the HoC, the lower the refrigeration systems efficiency, or Coefficient of Performance (CoP)

Front 17

Heat of compression ( HoC)

Back 17

Heat energy added to refrigerant vapour by the work of compressor.

The amount of pressure increases with a compressor relates directly to the amount of heat produced during compression

Front 18

HoC

Back 18

The higher the HoC, the lower the refrigeration systems efficiency, or Coefficient of Performance (CoP)

Front 19

Coefficient of Performance (CoP)

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Coefficient of Performance (CoP) is the amount of heat gain in the evaporator divided by the HoC

Front 20

Stages of the Vapour Compression Refrigeration Cycle

Back 20

Back (Definition)

Front 21

Refrigeration Cycle

Back 21

Back (Definition)

Front 22

In a perfect refrigeration system, how much does the pressure change in the high (condenser) side of the refrigeration system?

Back 22

0%

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Refrigeration system components

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Back (Definition)

Front 24

Five compressor types

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Positive displacement compressor

1. Reciprocating

2. Rotary ( stationary and rotating blade type) Rotating blade are number one vacuum pump.

3. Scroll - In AC industry and now in oil and gas. One of the most efficient. Must turn in one direction.

4. Screw- lateral space compression. ( twin flooded, single flooded and twin dry)

Hint 24

Front 25

Five compressor types

Back 25

Positive displacement compressor

1. Reciprocating

2. Rotary ( stationary and rotating blade type) Rotating blade are number one vacuum pump.

3. Scroll - In AC industry and now in oil and gas. One of the most efficient. Must turn in one direction.

4. Screw- lateral space compression. ( twin flooded, single flooded and twin dry)

Hint 25

Front 26

Non- positive displacement

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Cetrifugal

Use velocity to turn vapours into high velocity vapours. ( High pressure)

Front 27

Compressor liquid tolerance

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Some compressor can tolerate a little liquid passing through them. Compressors should never pump liquid refrigerant or oil, since liquids resist compression.

This is especially true for reciprocating compressors

Front 28

Types of reciprocating compressors

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Hermetic - resiprocating

Semihermatic - reciprocating

Open

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Condenser

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HPHT vapours entering the condenser continues to lose heat throughout the first pass or two of the condenser.

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Deauperheating

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The initial heat loss is called desuperheating

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Phase change in condenser

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After initial heat loss in condenser, refrigerant begins a phase change by starting to condenser into a liquid.

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Phase change in condenser

Back 32

After initial heat loss in condenser, refrigerant begins a phase change by starting to condenser into a liquid.

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Saturated pressure in condenser

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Saturated pressure in condenser sets a constant pressure throughout the high pressure side of the vapour compression cycle.

The condenser gives off heat while condensing the refrigerant and sub cooling the liquid refrigerant that collect near the end of the refrigerant’s flow through the condenser.

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Subcooling

Back 34

The liquid remains at the condensing pressure to the end of the condenser, but just before leaving the condenser, the temp of the liquid drops.

When liquid’s temp drops while its saturated pressure remains, it is known as subcooling.

Once the ref vapour turned to liquid, the ref leaves the condenser.

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Metering device

Back 35

Is a pressure reducer.

Metering device receives liquid red from the condensing unit.

No vapour should enter the metering device.

High pressure sub-cooled liquid pass through the metering device( metering or control point) - expansion of the refrigerant takes place and pressure dropped to low side or evaporator pressure.

Front 36

Metering device

Back 36

Is a pressure reducer.

Metering device receives liquid ref from the condensing unit.

No vapour should enter the metering device.

High pressure sub-cooled liquid pass through the metering device( metering or control point) - expansion of the refrigerant takes place and pressure dropped to low side or evaporator pressure.

Front 37

Front (Term)

Back 37

Capillary tube

Automatic expansion valve

Small thermostatic expansion valve

Large thermostatic expansion valve ( TX valve)

Front 38

Boiling

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Process of adding heat to a liquid at atm pressure

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Refrigeration

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Process of moving heat by means of evaporation or expansion. It takes heat away from air, water or brines and move the heat to a refrigerant,

Usually at a pressure below atmospheric ( in a vacuum)

Front 40

Refrigeration

Back 40

Refrigeration utilizes low boing points, but you should use the term boiling only with hot water or steam heating systems.

Front 41

Evaporator

Back 41

Is a heat exchanger that draws heat into vapour compression cycle.

LTLP vapours enters into evaporator and draws heat from surrounding and coverts into HPHT bubbling vapours.

Rate of vapour leaving the evaporator changes only if the compressor’s pumping rate or its capacity changes.

Running compressor refuses the vapour pressure above the saturated mixture of liquid and vapour ref. The reduction in vapour reaches the desired saturation point and maintains a steady flow of refrigerant vapour out of evaporator

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Energy

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Ability or capacity to do work.

Potential energy

Kinetic energy

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Latent Heat

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The heat added or removed to cause state change is called latent heat.

It is a hidden heat cause state change.

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Latent Heat

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The heat added or removed to cause state change is called latent heat.

It is a hidden heat cause state change.

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Sensible heat

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Causes a change in temp.

Sensible heat is important in refrigeration when liquids are Subcooled or vapours are superheated.

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Saturation

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Is a condition in which one substance holds as much as it can of a different substance.

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Saturation

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Is a condition in which one substance holds as much as it can of a different substance.

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Saturation point

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Is a condition in which one substance has reached 100% saturation with a second substance, or with a different phase of the same substance.

The different phase of the same substance is a critical component of the refrigeration cycle.

Front 49

Pressure and saturation point

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When you raise the pressure of water, you also raise the saturation point or boiling point.

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Two saturation points in refrigeration cycle

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One at low pressure and one at

High pressure.

First saturation point during evaporation process in evaporator.

A Pressure temp chart helps to find the corresponding pressure to maintain the temp.

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Law of thermodynamics

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1. Energy cannot be created or destroyed. Also called law of conservation of energy.

2. Energy moves or transfer from high energy position to low energy position.

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Second saturation point

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In condenser.Each condensing pressure corresponds to one condensing temp.

Both first and second saturation points are used for refrigeration design and troubleshooting.

Saturation points are valuable troubleshooting data and are a major feature of your work as ref mechanic.

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Superheating

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Is the process of adding sensible heat to the vapour of a substance while that substance remains at the same pressure.

When the temp of a vapour is raised above its boiling point( while at the boiling pressure), the sensible heat added is caked superheat.

Front 54

Superheating

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Is the process of adding sensible heat to the vapour of a substance while that substance remains at the same pressure.

When the temp of a vapour is raised above its boiling point( while at the boiling pressure), the sensible heat added is caked superheat.

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Negative implication of superheat

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Negative implication of superheat is the increase in vapour volume as the superheat increases. A higher vapour volume means a reduction in amount of refrigerant the compressor is able to pump.

Front 56

Subcooling

Back 56

Subcooling takes place in a liquid when it’s temp is lowered below its boiling point.

An increase in subcooling of liquid refrigerant means a reduction in sensible heat in the refrigerant.

A decrease in sensible heat means a reduction in the amount of heat contained by the refrigerant.

A lower amount of heat, or enthalpy, results in increased capacity in the cooling section.

Front 57

Subcooling

Back 57

Too much Subcooling ~ too much refrigerate in lines after condenser => system malfunction

Too less subcooling - bubbles going to metering device

Front 58

Stages of Refrigeration cycle

Back 58

Back (Definition)

Front 59

Stages of Refrigeration cycle

Back 59

Back (Definition)

Front 60

Stages of Refrigeration cycle

Back 60

Back (Definition)

Front 61

Stages of Refrigeration cycle

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2 super heated vapours

One vapour is LPLT superheated vapour

Second is HPHT superheated vapour

1- subcooled liquid ( High Pressure)

Front 62

Evaporation

Back 62

Evaporator absorb heat from cooling air or brine

Evaporator receive its supply of saturated liquid refrigerant from the metering device.

Evaporator has refrigerant mixture of low pressure, low temp, saturated liquid and vapour.

The refrigerant mixture is mostly liquid at the begging of evaporating state, half liquid half vapour in middle and complete vapour at the end of evaporating stage.

Front 63

Evaporation

Back 63

Back (Definition)

Front 64

Metering device

Back 64

Metering device feeds saturated liquid to evaporator

Front 65

Compression

Back 65

Compressor receive low pressure vapour refrigerant from the evaporator

Adds more superheat to the refrigerant vapour

Front 66

Desuperheating

Back 66

Takes place when sensible heat leaves the vapour, but not at the rate high enough to reduce the vapour’s temp to the saturation point.

The final desuperheating should not take place until the refrigerant is in the condensing stage.

Front 67

Compressor flow

Back 67

Back (Definition)

Front 68

Compressor flow

Back 68

Back (Definition)

Front 69

Condensing

Back 69

Desuperheating that started in discharge line. Once ref vapour reaches saturation point within the confines of a condenser, it begins to condense into liquid refrigerant.

At this point we have a predictable pressure and temperature. The pressure is called condensing pressure and is same from the outlet of the compressor to the inlet of metering device.

Front 70

Condenser

Back 70

Last areas of the condenser, entire refrigerant is liquid.

The liquid ref continues to lose heat to the surrounding media, so it becomes subcooled.

Subcooling continues as the refrigerant leaves the condenser and enters the liquid line.

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Subcooling two major purposes

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1. Increase in refrigerant’s ability to absorb heat later in the evaporator; the lower the temp the more it can attract heat.

2. Formation of a liquid seal at the entrance of the metering stage. Which make sure no vapour enter the metering device.

Front 72

Condensing

Back 72

Back (Definition)

Front 73

Metering device

Back 73

Metering device is a restriction or orifice that reduce the refrigerant pressure from condensing pressure to evaporating pressure.

If the office is too small, then evaporator strayed of refrigerant, if it is too large then evaporator floods with liquid refrigerant that can spill into suction line and damage the compressor.

Liquid line delivers the liquid refrigerant to the metering device.

Metering device used is based on the application and size of refrigeration unit.

Front 74

Capillary tube or cap tube

Back 74

Small refrigeration systems, such as refrigerators, small ice machines and window air conditioners have simple metering device called capillary tube or cap tube

Front 75

Capillary tube or cap tube

Back 75

Cap tube must be sized to exactly match the capacity of the system.

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Flash gas

Back 76

Once the refrigerant’s pressure drops due to passing through the metering device,s orifice, some of the refrigerant flashes or becomes a vapour. This vapour is called flash gas.

The flash gas is at a very low temp because it experience a drastic reduction in pressure.

Flash gas cools the rest of the liquid refrigerant, which is now moving towards the entrance of the evaporator.

Front 77

Capillary tube or cap tube

Back 77

When high pressure subcooled liquid refrigerant enters the cap tube, the pressure drops due to the friction produced by the narrow cap tube passage being applied to liquid refrigerant.

Toward the far end of the entering point of the capillary tube the refrigerant pressure drops below the saturation point and it begins to evaporate.