2. 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
3. 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
4. Vapour Compression System
Low Pressure Side
High Pressure Side
Liquid receiver
Expansion
valve
Compressor
Evaporator
Liquid
Condenser
Heat in
Heat out
Hot Gas
Gas
4 numbers principle components :
(1) Evaporator
(2) Compressor
(3) Condenser
(4) Expansion Valve
5. Vapour Compression System
Low Pressure Side
High Pressure Side
Liquid receiver
Expansion
valve
Compressor
Evaporator
Liquid
Condenser
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.
6. Vapour Compression System
Low Pressure Side
High Pressure Side
Liquid receiver
Expansion
valve
Compressor
Evaporator
Liquid
Condenser
Heat in
Heat out
Hot Gas
Gas
COMPRESSOR :
1) Compresses the refrigerant
(gaseous state).
2) Raising its Temperature &
Pressure.
3) Discharges refrigerant to
Condenser.
7. Vapour Compression System
Low Pressure Side
High Pressure Side
Liquid receiver
Expansion
valve
Compressor
Evaporator
Liquid
Condenser
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.
8. Vapour Compression System
Low Pressure Side
High Pressure Side
Liquid receiver
Expansion
valve
Compressor
Evaporator
Liquid
Condenser
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.
9. 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
10. • 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
12. 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 )
Pressure(absolute)
1
4
2
Liquid 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
13. 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 )
Pressure(absolute)
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)
14. 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
15. • Pressure – Enthalpy chart , of a practical
cycle (refer to page 8)
• Effects of pressure loss resulting from
friction.
16. 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
17. 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)
18. Two systems employed:
• Direct Expansion System
• Indirect expansion system
aka Brine System
19. Direct Expansion System : Provisional Refrigeration System
Condenser
Cooling water in / out
Fan/blower
expansion valve
Solenoid
stop valve
Thermostat
Temperature
sensor
MEAT ROOM
LP pressure switch
HP pressure switch
Refrigerant
compressor
Sight glass
Drier
Evaporator
Capillarytube
: 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
20. 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
Evaporator
Capillarytube
: Refrigerant flow
Oil separator
Oil return to
compressor sump
Bulb
T1
T2
pump
Brine
header
tank
Secondary refrigerant
to various
compartment
Oil pressure switch
21. 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.
24. 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
25. 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
26. 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
31. 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.
