multi pressure refrigeration cycles
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Multi -pressure Systems
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Industrial refrigeration plants operate with a large
difference between evaporating and condensing
temperatures
50 ~ 80 C
Opportunities: Multi-stage compression, Flash gas,
intercooling, number of evaporators at different
temperatures
Multipressure systems - classified as compound
systemsor cascade systems
Multi-stage compression used when ratio of suction to
discharge pressure is high enough to cause a serious
drop in efficiency or an unacceptably high discharge
temperature
Introduction
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A multi-pressure system is a refrigeration system that
has two or more low side pressures
The low side pressure is the pressure of the refrigerantbetween the expansion valve and the intake of
compressor
Different than the single pressure system which has but
one low side pressure
In Dairy where one evaporator operates at -35 C to
harden ice cream while another evaporator operates at 2
C to cool milk
Two functions often integral to multi-pressure systems
are; Removal of flash gas
Inter-cooling
Several combinations of multiple evaporators and
compressors May have same or different refrigerants
Introduction
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Flash gas - the mass of vaporized refrigerant per kg of
total refrigerant which is just leaving the throttling valve Flash chamber/tank is that portion of the expansion
valve system where liquid-vapour mixture of refrigerant
gets accumulated before entering into the evaporator
Saving in the power requirement results if the flash gasthat develops in the throttling process between the
condenser and evaporator is removed and recompressed
before complete expansion
When saturated liquid expands through an expansion
valve, the fraction of vapour or flash gas progressively
increases
Removal of F lash Gas
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Removal of F lash Gas
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The expansion process shown on the pressure enthalpy
diagram takes place from 1 to 2
The state point as the expansion proceeds moves into aregion of a greater fraction of vapour
The end point of expansion, 2, could have been achieved
by interrupting the expansion at 3 and separating the
liquid vapour phases which are 4 and 6 respectively The expansion could then continued by expanding the
liquid at 4 and the vapour at 6 to the final pressure, giving
5 and 7 respectively
The combination of refrigerant at 5 and 7 gives point 2
The expansion process from 6 to 7 is wasteful because of
the following two reasons;
The refrigerant at 7 can do no refrigerating
Work will be required to compress the vapour back to
the pressure it had at 6
Removal of F lash Gas
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Why not perform the part of expansion, separate the liquid
from the vapour, continue expanding the liquid, and
recompress the vapour without further expansion The equipment to achieve this separation is called a flash
tank
Removal of F lash Gas
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The expansion from 1 to 3 takes place through a float
valve, which serves the further purpose of
maintaining a constant level in the flash tank
To recompress the vapour at 6, a compressor must be
available with a suction pressure of 6
Thus two compressors are needed in the system
Removal of F lash Gas
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Intercooling between two stages of compression reduces
the work of compression per kg of vapour
Intercooling
In two stage compression of air, for example an
intercooler from point 2 to 4 on PV diagram saves some
work
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This figure shows how compression with Intercooling
appears on the ph diagram of a refrigerant Processes 1- 2 - 3 & 4 - 5 are on lines of constant entropy
Between the same two pressures, therefore, process 4 5
shows a smaller increase in enthalpy, which indicates that
less work is required than in 2
3
Intercooling
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Between the two given pressures, the work of
compression is proportional to the specific volume ofentering gas
Sp. Volume at 2 is greater than it is at 4 so work required
for compressing from 2 to 3 is greater than in
compressing from 4 to 5
Intercooling in a refrigerating system can be
accomplished by two ways:
Using a water cooled heat exchanger
Using a refrigerant
The water-cooled intercooler may be satisfactory for two-
stage air compression, but for refrigerant compression,
the water is usually not cold enough
Multipurpose Refr igeration Systems
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Another way of Intercooling uses liquid refrigerant fromthe condenser to do the Intercooling
Discharge gas from the low-stage compressor bubbles
through the liquid in the intercooler
Refrigerant leaves the intercooler at 4 as saturated vapour
IntercoolingMultipurpose Refr igeration Systems
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Intercooling
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Intercooling
Example: Calculate the power needed to compress 1.2 kg/s
of ammonia from saturated vapour at 80 kPa to 1000 kPa (a)
by single-stage compression and (b) by two stage
compression with intercooling by liquid refrigerant at 300
kPa.
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Heat & Mass Balance Around Intercooler
&
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Heat & Mass Balance Around Intercooler
O E t & O C
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One Evaporator & One Compressor
With one compressor and one evaporator the flash tank may function as shown below
A pressure reducing valve throttles the flash gas from the intermediate pressure to the
evaporator pressure
Throttling is necessary because there is no compressor available with a high suction
pressure
Calculations show that the flash tank does not improve the performance of the system and
the system is used infrequently
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T E t & O C
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Two Evaporators & One Compressor
A pressure reducing valve is installed after the high-temperature evaporator
regulates the pressure and maintains a temperature in the air-conditioning
evaporator of 5 C
T C A d O E t
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Two Compressors And One Evaporator
Two stage compression with Intercooling and removal of flash gas is often the ideal
way to serve one low-temperature evaporator
The system requires less power than with a single compressor
T C A d O E t
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Two Compressors And One Evaporator
T C & T E t
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Two Compressors & Two Evaporators
The system which has two evaporators operating at
different temperatures is common in industrial
refrigeration Example of dairy cooling milk & manufacturing ice cream
Frozen food plant requires two evaporators at different
temperatures
Evaporators at two different temperatures can be handled
efficiently by a two-stage system which employs
Intercooling and removal of flash gas
Example
In an ammonia system one evaporator is to provide 180 kW ofrefrigeration at -300C and another evaporator is to provide 200kW
at 50C. The system uses two-stage compression with intercooling
and is arranged as in Fig. Below. The condensing temperature is
400C. Calculate the power required by the compressors.
T C & T E t
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Two Compressors & Two Evaporators
Press re Enthalp Diagram
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Pressure Enthalpy Diagram
Solution: Sketch the pressure-enthalpy diagram as shown
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Two Compressors & Two Evaporators
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Two Compressors & Two EvaporatorsThe discharge pressure of the low-stage compressor and the suction pressure of the
high-stage compressor are the same as the pressure in the 5 C evaporator.
Next determine the enthalpies at the state points.
The simplest way to calculate the mass rate of flow handled by the high-stage
compressor is to make a heat and mass balance about the high-temperatureevaporator and the intercooler, as shown
Two Compressors & Two Evaporators
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Two Compressors & Two Evaporators
Two Compressors & Two Evaporators
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Two Compressors & Two Evaporators
Multi Stage Systems
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a. Compound
Two low compression ratios = 1 high
First stage compressor meets cooling load
Second stage compressor meets load evaporator and
flash gas
Single refrigerant
b. Cascade
Preferred for -46 C to -101 C
Two systems with different refrigerants
Multi -Stage Systems
Cascade Refr igeration Systems
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Very low temperatures can be achieved by operating two or more vapor-compression
systems in series, called cascading.
Cascade Refr igeration Systems
65,21,
inin
LR
WW
QCOP
Advantages:Different refrigerants, equipment, and oils can
be used for the higher and the lower systems. Also, COP ofthe system increases.
Disadvantage:High first cost; More complicated.
Cascade Refr igeration Systems
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Consider a two-stage cascade refrigeration system operating between the pressure
limits of 0.8 and 0.14 MPa. Each stage operates on an ideal vapor compression cycle
with R134a as the working fluid. Heat rejection from the lower cycle to the upper
cycle takes place in an adiabatic counterflow heat exchanger where both streamsenter at about 0.32 MPa. If the mass flow rate of the refrigerant is 0.05kg/s,
determine
(a) The mass flow rate of the refrigerant through the lower cycle,
(b) The rate of heat removal from the refrigerated space and the power input to the
compressor,
(c) The COP of the refrigerator.
Cascade Refr igeration Systems
Multistage Compression with F lash Chamber
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Multistage Compression with F lash Chamber
I ntercooling (Compound System)
Optimum Intermediate pressure?
Multistage Compression with F lash Chamber
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Multistage Compression with F lash Chamber
I ntercooling (Compound System)
The optimum intermediate pressure is given by:
The optimum intermediate pressure in a two-stage refrigeration system is quite close,
but not equal, to that shown in above equation. The reason for the deviation is that the
cooling process in air compression is without cost, but in the refrigeration system there
is a power cost associated. with compressing the refrigerant vapor that performs thecooling.
Interstage pressure is usually set so that the compression ratio at each stage is nearly
the same for higher COPs.
Multistage Compression with F lash Chamber
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Consider a two-stage cascade refrigeration system operating between the pressure
limits of 0.8 and 0.14 MPa as before but, this time, with a flash chamber operating at
0.32 MPa. Determine
(a) The fraction of the refrigerant that evaporates as it is throttled to the flash
chamber,
(b) The amount of heat removed from the refrigerated space and the compressor
work per unit mass of refrigerant flowing through the condenser,
(c) The COP of the refrigerator.
Multistage Compression with F lash Chamber
I ntercooling (Compound System)
Multipurpose Refr igeration Systems
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A refrigerator with a single compressor can provide refrigeration at several
temperatures by throttling the refrigerant in stages.
Multipurpose Refr igeration Systems
Assignment:Whats the COP and heat transfer amounts for conditions used in
the previous problem (Check for both paths)? Assume QL,Ris a third of QL,F.
Also called
pressure-regulating
valve