1.refrigeration
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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