1.refrigeration

Refrigeration and

Air conditioning

Lesson Plan• Vapour compression refrigeration cycle• Components of a refrigeration system• Pressure enthalpy chart• superheating & sub-cooling• Heat exchanger• Coefficient of performance ( COP )• System Capacity• Direct and Indirect Expansion System• Back Pressure Regulating valve• Types of compressor• Oil Separator• Filter/Drier• Throttling device• Capacity control Method

Basic refrigeration cycle:• Heat energy flows from a hot region to a cooler region.

• Vapour Compression Refrigeration System uses a circulating refrigerant as a medium which

1) absorbs & removes heat from space to be cooled

2) rejects the heat elsewhere (cooler)

Heat energy

Refrigerant flow

cooler

Cooling water

Cold room

Vapour Compression System

Low Pressure Side

High Pressure Side

Liquid receiver

Expansion

valve

Compressor

Evaporator

LiquidCondenser

Heat in

Heat out

Hot Gas

Gas

4 numbers principle components :

(1) Evaporator

(2) Compressor

(3) Condenser

(4) Expansion Valve

Vapour Compression System

Low Pressure Side

High Pressure Side

Liquid receiver

Expansion

valve

Compressor

Evaporator

LiquidCondenser

Heat in

Heat out

Hot Gas

Gas

EVAPORATOR:

1) The evaporator coils are located in the compartment to be cooled.

2) The low pressure liquid refrigerant ,after passing through the expansion valve, expands.

3) Takes in heat from the surrounding and evaporates.

4) The gas is then sucked up by the compressor.

Vapour Compression System

Low Pressure Side

High Pressure Side

Liquid receiver

Expansion

valve

Compressor

Evaporator

LiquidCondenser

Heat in

Heat out

Hot Gas

Gas

COMPRESSOR :

1) Compresses the refrigerant (gaseous state).

2) Raising its Temperature & Pressure.

3) Discharges refrigerant to Condenser.

Vapour Compression System

Low Pressure Side

High Pressure Side

Liquid receiver

Expansion

valve

Compressor

Evaporator

LiquidCondenser

Heat in

Heat out

Hot Gas

Gas

LIQUEFACTION:

1) Hot refrigerant gas cooled in the condenser.

2) Condensed liquid refrigerant flows into a receiver.

3) Then liquid refrigerant flows to the expansion valve.

Vapour Compression System

Low Pressure Side

High Pressure Side

Liquid receiver

Expansion

valve

Compressor

Evaporator

LiquidCondenser

Heat in

Heat out

Hot Gas

Gas

EXPANSION:

1) The expansion valve acting as a regulating valve, limits the amount of refrigerant flowing through.

2) Resulting in reduction of pressure of the liquid and expansion takes place.

P-H chart ( Pressure – Enthalpy chart )

Sub cooled liquid

• Pressure – Absolute pressure

Unit : bar , psi

• Enthalpy – Total amount of energy per unit weight of substance.

Unit : BTU / Lb or kJ / kg

• The lines ,saturated liquid & vapour respectively are plots of pressure vs enthalpy for the saturated state of a given refrigerant.

• This chart is used to understand the property changes that takes place in each phase of the cycle.

Saturated liquid line

Saturated vapour line

Superheated region

Sub-cooled region

Liquid – vapour

mixture

• Enthalpy – Total amount of energy per unit weight of substance.Unit : BTU / Lb or kJ / kg

• Entropy – Measure of heat dispersion in a system divided by temperature.

Unit : BTU / Lb / deg change or kJ / kg / deg change for a

substance.

Refrigeration Cycle : Pressure-Enthalpy graph

Ideal Refrigeration Cycle : Pressure-Enthalpy chart

Superheated vapour

Sub cooled liquid Liquid vapour mixture

Ideal Refrigeration Cycle : Pressure – Enthalpy chart

Enthalpy ( BTU / lbs or KJ / kg )

Pre

ss

ure

(ab

so

lute

)

P1

P2

1 2

34

Refrigeration Cycle : Pressure-Enthalpy chart

Non ideal Refrigeration Cycle : Pressure – Enthalpy chart , showing superheating & sub cooling

Sub cooled liquid Superheated vapour

Liquid vapour mixture

Enthalpy ( BTU / lbs or KJ / kg )

Pre

ss

ure

(a

bs

olu

te)

1

4

2Liquid to Vapour

Transformation in

EVAPORATOR

Throttling at expansion

valve

Vapour to Liquid

transformation in

CONDENSER

Work done in the compressor

Liquid vapour

mixture

superheated

Superheated

subcooling 3

Non ideal Refrigeration Cycle : Pressure – Enthalpy chart , showing superheating & sub cooling

Sub cooled liquid Superheated vapour

Liquid vapour mixture

Enthalpy ( BTU / lbs or KJ / kg )

Pre

ss

ure

(a

bs

olu

te)

1

34

2Liquid to Vapour

Transformation in

EVAPORATOR

Throttling at expansion

valve

Vapour to Liquid

transformation in

CONDENSER

Work done in the compressor

H1 H2 H3

The amount of heat that the refrigerant absorb must equal the cooling load.

(1) Refrigerant cooling load ( F ) = cooling load / ( H2 – H1)

(2) Work done by compressor = F x ( H3 – H2 )

(3) Heat rejected by condenser = F x ( H3 – H1 )

(4) Heat absorbed by evaporator = F x ( H2 – H1 )

Coefficient of Performance (COP) = heat absorbed by refrigerant / Energy required driving compressor

= ( H2 – H1) / ( H3 – H2)

Pressure-Enthalpy chart

Pressure

( bar )

Enthalpy ( KJ / kg of refrigerant )

Liquid to Vapour

Transformation in

EVAPORATOR at -13 deg C

Vapour to Liquid

transformation in

CONDENSER at 42 deg C

Work done in the compressor

Throttling at expansion valve

1

4

1-2 : EVAPORATOR – extraction of heat from room

2-3 : COMPRESSOR – compression work

3-4 : CONDENSER – energy thrown to sea

4-1 : EXPANSION VALVE – throttling at the expansion valve

3

2

16

3.2

150 304 365

For each kg of refrigerant flow ,

Energy extracted from meat room : 304 - 150 = 154 KJ / kg

Work spent on compressor = 365 – 304 = 61 KJ / Kg

Coefficient of Performance ( COP ) =

Energy extracted from room / Energy spent

= 154 / 61 or 2.52

• Pressure – Enthalpy chart , of a practical cycle (refer to page 8)

• Effects of pressure loss resulting from friction.

Superheating & Sub-cooling

compressor

condenser

receiver

refrigerant control (expansion valve)

evaporator

Heat exchanger

Saturated liquid

Superheated suction vapour

Saturated suction vapour

Sub cooled liquid

Improvement in cycle efficiency with a heat exchanger – as compared to another cycle where vapour is superheated without producing any useful cooling

Page 7

Refrigeration system capacity

• Rate at which system removes heat from.• Rate depends :

(1) mass of refrigerant circulated per unit time

(2) refrigerating effect per unit mass circulated (undercooling increases the refrigerating effect)

Two systems employed:

• Direct Expansion System

• Indirect expansion system

aka Brine System

Direct Expansion System : Provisional Refrigeration System

Condenser

Cooling water in / out

Fan

/ b

low

er

expansion valveSolenoid

stop valve

Thermostat

Temperature

sensor

MEAT ROOM

LP pressure switch

HP pressure switch

Refrigerant compressor

Sight glass

Drier

Eva

po

rato

r

Cap

illar

y t

ub

e

: Refrigerant flow

From FISH ROOM

From VEGETABLE ROOM

To FISH ROOM

To VEGETABLE ROOM

Oil separator

Oil return to compressor sump

Bulb

T1

T2

receiver

Oil pressure switch

Purging line

LP pressure gauge

Oil pressure gauge

HP pressure gauge

Back pressure regulating valve

Indirect Expansion (Brine System)

Condenser / Receiver

Cooling water in / out

expansion valve

Solenoid

stop valve

Thermostat

Temperature

sensor

LP pressure switch

HP pressure switch

Refrigerant compressor

Sight glass

Drier

Eva

po

rato

r

Cap

illar

y t

ub

e

: Refrigerant flow

Oil separator

Oil return to compressor sump

Bulb

T1

T2

pump

Brine header

tank

Secondary refrigerant to various compartment

Oil pressure switch

Back pressure regulating valve

• Normally fitted to higher temperature rooms, ie the vegetable room

not for the fish room or meat room.

Purpose :• Act as system balancing diverters –

a) When all solenoid valves are opened, the valve restrict liquid flowing into the vegetable room &

therefore deliver the bulk to the colder rooms.

b) Limits the pressure drops across the expansion valve by giving a set minimum pressure in the

evaporator coil. Prevents cold air blowing directly onto delicate vegetables.

Refrigerant Compressor types:

• Reciprocating

• Rotary

• Centrifugal

• Screw

Oil SeparatorGas from compressor

Float

Oil to compressor crankcase

Oil

Gas to condenser

Internal baffles

Page 12

Liquid-line Filter / Drier

Desiccant(dehydrating material)

Refrigerant in Clean,dry refrigerant

Fine filter to remove small particles

Course filter to remove large particles

Felt pad

Drying agent : silica gel or activated alumina

Page 13

Condenser:

• Air cooled type – up to 5 hp

• Large capacity – shell & tube type , SW cool

• Tubes – aluminium brass (option ext. fins)

• Water velocity < 2.5 m/s minimise erosion

• Anodes – avoid corrosion non ferrous metals

Throttling device:

• Metering of refrigerant – rate suitable to maintain designed operating pressures at different load.

• Maintain pressure differential between HP & LP side.

The pressure of the refrigerant is reduced as it passes through the small orifice

of the throttling device. With the reduction in pressure, the corresponding

boiling point of the liquid is reduced.

Types of throttling devices:• Hand expansion valves

• Automatic constant pressure expansion valve

• Thermostatic expansion valve

• Externally equalized expansion valve

• Pressure balancing expansion valve

• Expansion valves with centrifugal type distributors

• Flow control device for flooded evaporators

Expansion valve

• automatic expansion valve

• thermostatic expansion valve

• externally equalised thermostatic expansion valve

Automatic expansion valve

Liquid Refrigerant IN

Refrigerant OUT

Evaporator

Throttling Orifice

Adjusting screw for altering spring pressure

Spring

diaphram

Spring pressure – Opening

action

Refrigeration pressure –

Closing action

Page 18

Thermostatic expansion valve (TEV)

Liquid Refrigerant IN

Refrigerant OUT(slightly superheated gas)

Evaporator

diaphram

Refrigerant

Throttling orifice

spring

Adjusting screw to alter spring tension

bulb

Page 19

External equalised thermostatic expansion valve

Liquid Refrigerant IN

Refrigerant OUT(slightly superheated gas)

Evaporator

diaphram

Refrigerant

Throttling orifice

spring

Adjusting screw to alter spring tension

bulb

Equalising line

120

– 15

0 m

m

Page 20

Capacity control methods

• Manual start/stop

• Speed variation

• Cylinder unloading reciprocating compressor

• Suction side throttling centrifugal compressor

• Inlet guide vane centrifugal compressor

• Hot gas bypass

• Compressor in parallel

• Slide valve Screw compressor - control effective working length of rotor.

To maintain constant temperature, a constant pressure must be present in the EVAPORATOR.

Ideally, the compressor should remove from the EVAPORATOR exactly the volume of refrigerant that boils off in it. Change in loading : change in quantity of boiling off the refrigerant.

Unloading device

Attached pump

Stop valve

Filter

Solenoid valve

Stop valve with orifice plate

Oil strainer

To internal oil passages in crankshaft

Oil pressure switch

Suction pressure

Oil Pressure gauge

To crankcase

Capacity regulator

Oil separator

Cylinder for unloading mechanism

Cylinder cover

Safety spring

Delivery valve

Suction valve

Liner

Piston

Unloader

Cylinder for unloading mechanism

Spring

Piston

Unloader

Ball joint

Spring

Ring

PinSuction valve

Lubricating oil pressure

No oil pressure

Refrigerant flowRefrigerant flow

Screw compressor

LOBES

DRIVE SHAFT

Min

BYPASS GAS OUTLET

DISCHARGE PORT

INLET

UNLOADING PISTON

Max

SLIDE VALVECYLINDER

NORMAL LOADING

Page 12

Screw compressor

LOBES

DRIVE SHAFT

Min

BYPASS GAS OUTLET

DISCHARGE PORT

INLET

UNLOADING PISTON

Max

SLIDE VALVECYLINDER

REDUCE LOADING