Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DEFROST REFRIGERATION SYSTEM
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to refrigeration
systems, and more particularly to defrost configurations for
evaporators of industrial and commercial refrigeration
cabinets, by which hot refrigerant is circulated in the
evaporators for defrost.
2. Background Art
In refrigeration systems found in the food
industry to refrigerate fresh and frozen foods, it is
necessary to defrost the refrigeration coils of the
evaporators periodically, as the refrigeration systems
working below the freezing point of water are gradually
covered by a layer of frost which reduces the efficiency of
evaporators. The evaporators become clogged up by the
build-up of ice thereon during the refrigeration cycle,
whereby the passage of air maintaining the foodstuff
refrigerated is obstructed. Exposing foodstuff to warm
temperatures during long defrost cycles may have adverse
effects on their freshness and quality.
According to a method known in the art, gas is
taken from the top of the reservoir of refrigerant at a
temperature ranging from 80 F to 90 F and is passed through
the refrigeration coils, whereby the latent heat of the gas
is used to defrost the refrigeration coils. This also
results in a fairly lengthy defrost cycle.
U.S. Patent No. 5,673,567, issued on October 7,
1997 to the present inventor, discloses a system wherein hot
gas from the compressor discharge line is fed to the
refrigerant coil by a valve circuit and back into the liquid
manifold to mix with the refrigerant liquid. This method of
defrost usually takes about 12 minutes for defrosting
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evaporators associated with open display cases and about
22 minutes for defrosting frozen food enclosures. The
compressors are affected by hot gas coming back through the
suction header, thereby causing the compressors to overheat.
Furthermore, the energy costs increases with the compressor
head pressure increase.
U.S. Patent No. 6,089,033, published on July 18,
2000 to the present inventor, introduces an evaporator
defrost system operating at high speed (e.g., 1 to 2 minutes
for refrigerated display cases, 4 to 6 minutes for frozen
food enclosures) comprising a defrost conduit circuit
connected to the discharge line of the compressors and back
to the suction header through an auxiliary reservoir capable
of storing the entire refrigerant load of the refrigeration
system. The auxiliary reservoir is at low pressure and is
automatically flushed into the main reservoir when liquid
refrigerant accumulates to a predetermined level. The
pressure difference between the low pressure auxiliary
reservoir and the typical high pressure of the discharge of
the compressor creates a rapid flow of hot gas through the
evaporator coils, thereby ensuring a quick defrost of the
refrigeration coils. Furthermore, the suction header is fed
with low-pressure gas to prevent the adverse effects of hot
gas and high head pressure on the compressors.
Such defrost refrigeration systems are efficient
in defrosting evaporators. However, new compressor systems
are now available, which compressor systems operate
differently from existing compressors commonly used in the
refrigeration systems. It is therefore desirable to adapt
defrost configurations to such new compressor systems so as
to optimize the defrost of evaporators.
SUMMARY OF INVENTION
It is therefore an aim of the present invention to
provide a novel defrost refrigeration system.
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Therefore, in accordance with the present
invention, there is provided a defrost refrigeration system
of the type having a main refrigeration circuit in which a
refrigerant absorbs heat from an evaporator stage and
releases heat in a condensation stage, with a compression
stage sequentially between the evaporation stage and the
condensation stage during a refrigeration cycle, said
defrost refrigeration system comprising: at least a first
compressor and a second compressor serially positioned with
respect to one another such that refrigerant going through
the compression stage in the refrigeration cycle passes
sequentially through the first compressor and the second
compressor; a first line extending from an exit of one of
the first compressor and the second compressor of the
compression stage, and in fluid communication with the
evaporator stage in a defrost cycle and adapted to receive a
defrost portion of refrigerant compressed in the compression
stage; and valves for switching at least one evaporator
between the refrigeration cycle and the defrost cycle, by
stopping/allowing a flow of refrigerant from the
condensation stage to at least one evaporator of the
evaporation stage in the refrigeration cycle, and for
allowing/stopping a flow of said defrost portion of
refrigerant to defrost the at least one evaporator in the
defrost cycle.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of the
invention, reference will now be made to the accompanying
drawings, showing by way of illustration a preferred
embodiment thereof and in which:
Fig. 1 is a block diagram of a defrost
refrigeration system constructed in accordance with a
preferred embodiment of the present invention;
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Fig. 2 is a schematic view of a defrost
refrigeration system in accordance with a first embodiment
of the present invention;
Fig. 3 is a schematic view of a defrost
refrigeration system in accordance with a second embodiment
of the present invention; and
Fig. 4 is a schematic view of the defrost
refrigeration system of Fig. 3, with an alternative
configuration at the exit of refrigeration of a defrost
cycle.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, and more
particularly to Fig. 1, a defrost refrigeration system in
accordance with a preferred embodiment is generally shown at
10. The defrost refrigeration system 10 operates a
refrigeration cycle and has a compression stage 12, a
condensation/heat reclaim stage 14 and an evaporation stage
16.
In the refrigeration cycle, the compression stage
12 performs a compression of a refrigerant to a high-
pressure gas state. The compression stage 12 is in fluid
communication with the condensation/heat reclaim stage 14 by
way of line 13.
The condensation/heat reclaim stage 14 releases
heat from the high-pressure gas refrigerant received from
the compression stage 12. The heat is released to the
atmosphere, for instance using roof-top condensers.
Alternatively, heat may be recuperated using heat reclaim
systems in series or in parallel with condensers. Moreover,
the condensation/heat reclaim stage 14 may have refrigerant
tanks to accumulate refrigerant having released heat and
ready to be fed to the evaporation stage 16.
The condensed refrigerant is directed to the
evaporation stage 16 using line 15. The evaporation stage
16 typically has numerous evaporators in refrigeration
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cabinets, as well as the necessary expansion valves if
required to set the refrigerant to a suitable condition to
absorb heat. In some instances, the evaporators may be
flooded with liquid refrigerant such that expansion valves
are optional.
The refrigerant having absorbed heat is then
directed to the compression stage 12 using line 18 to
complete the refrigeration cycle.
The compression stage 12 uses high-efficiency
compressors. More specifically, the compressors used in the
compression stage 12 are magnetic-bearing, variable-speed
centrifugal compressors of the type manufactured by
Turbocor. These compressors operate at high efficiency, but
offer a compression ratio at a maximum of 4.5/5:1.
Accordingly, as shown in Fig. 1, these compressors
are cascaded in the compression stage 12 of Fig. 1. It is
required to cascade the compressors so as to provide the
required compression of refrigerant in view of the warmer
periods of the year, during which high pressures of
refrigerant must be reached for the effective release of
heat. More specifically, one or more first compressors 20
compress refrigerant that is fed through line 21 to an
accumulator 22. One or more second compressors 24 are
positioned downstream of the accumulator 22, and compress
refrigerant that is fed from the accumulator 22 through line
23. The refrigerant then exits the compression stage 12 to
be fed to the condensation/heat reclaim stage 14.
In order to proceed with the defrost of
evaporators from the evaporation stage 16, low pressure
refrigerant is directed from the compression stage 12 to the
at least one evaporator of the evaporation stage 16.
In a first embodiment, the low-pressure
refrigerant is produced by the first compressor 20 and a
portion of this refrigerant is fed directly to the
evaporator of the evaporation group 16 that is to be
defrosted. As the first compressors 20 compress the
refrigerant to a relatively low pressure, the refrigerant
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may be fed directly to the evaporators for defrost. As is
shown in Fig. 1, a line 25 directs a portion of refrigerant
from the first compressor 20 to the evaporation stage 16 for
defrost.
In the first embodiment, the refrigerant that has
released heat during defrost is returned to the compression
stage 12 for compression. Depending on its condition, the
defrost refrigerant uses either the lines 17 or 18, through
an appropriate network of valves, to be fed to the first
compressor 20 or to the accumulator 22. Moreover, the
defrost refrigerant may also be re-injected in the
evaporation stage 16 or directed to the condensation/heat
reclaim stage 14, depending on its state.
Referring to Fig. 2, the first embodiment of the
defrost refrigeration system 10 is illustrated in further
details. Like elements bear like reference numerals in the
Figs. 1 to 4. In Fig. 2, the defrost refrigeration system
10' has a roof-top condenser 14' and a heat-reclaim loop
14". The evaporation stage 16 is separated into a group of
low-temperature evaporators 16A (e.g., freezer
applications), and a group of medium-temperature evaporators
16B (e.g., refrigerator applications). Reference numerals
affixed with an A pertain to low-temperature refrigeration
in Figs. 2 to 4, whereas reference numerals affixed with a B
will relate to medium-temperature refrigeration in Figs. 2
to 4. Other evaporators 16C are typically provided in the
defrost refrigeration system 10', but are not illustrated to
simplify Fig. 2.
In the first embodiment illustrated in Fig. 1,
some refrigerant is directed by the line 25 from the output
of the first compressor/compressors 20 to the evaporator
stage 16 for defrost. As shown in Fig. 2, the line 25
diverges into lines 25A and 25B to respectively feed the
evaporators 16A and 16B, respectively, with defrost
refrigerant.
The lines 25A and 25B merge into the return lines
18A and 18B, using appropriate valves to prevent the defrost
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refrigerant to be sucked by the compressor stage 12. More
specifically, valves 30A and 30B are opened while valves 31A
and 31B are closed in the defrost sequence. These valves
are in opposite positions during a refrigeration cycle.
Defrost refrigerant is therefore directed to the
evaporators 16A and/or 16B in a defrost cycle. As is
illustrated in Fig. 2, a bypass 32A/32B is provided for the
defrost refrigerant to surround the expansion valves of the
evaporation stage 16.
The defrost refrigerant having released heat
during defrost in the evaporators 16A/16B is then directed
to the accumulator 22 using lines 17A/17B, respectively,
which merge into line 17. Valves 33A/33B are opened during
the defrost cycle, whereas the valves 34A/34B are closed.
These valves are in opposite positions during a
refrigeration cycle.
In a second embodiment, the low-pressure
refrigerant is produced by the second compressor 24 and a
portion of this refrigerant is fed to the evaporator of the
evaporation group 16 that is to be defrosted. As the first
compressors 24 compress the refrigerant to a relatively high
pressure, a pressure-reducing device 27 is provided to
ensure that the refrigerant fed to defrost evaporators is at
a suitable low pressure. As is shown in Fig. 1, a line 26
directs a portion of refrigerant from the second compressor
24 to the evaporation stage 16 for defrost, with the
pressure-reducing device 27 being positioned in the line 26.
In the second embodiment, the refrigerant that has
released heat during defrost is either returned to the
compression stage 12 for compression, or re-injected into
the evaporation stage 16 to be used in the refrigeration
cycle. Depending on its condition, the defrost refrigerant
uses either the lines 17 or 18, through an appropriate
network of valves, to be fed to the first compressor 20 or
to the accumulator 22.
Referring to Fig. 3, the second embodiment of the
defrost refrigeration system is illustrated in further
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detail. In Fig. 3, the defrost refrigeration system 10" has
stages similar to that of the defrost refrigeration system
10' of Fig. 2, whereby like elements will bear like
reference numerals.
In the second embodiment illustrated in Fig. 1,
some refrigerant is directed by the line 26 from the output
of the second compressor/compressors 24 to the evaporator
stage 16 for defrost, passing through a pressure-reducing
device 27 or a suitable solenoid valve. As shown in Fig. 3,
the line 26 diverges into lines 26A and 26B to respectively
feed the evaporators 16A and 16B, respectively, with defrost
refrigerant. Similarly to the defrost refrigeration system
10' of Fig. 2, the defrost refrigeration system 10" operates
a defrost cycle using the return lines 18A/18B, using a
network of valves for the defrost refrigerant to be directed
to the evaporators.
During the defrost of the evaporators, the valves
34A and 34B control the flow of defrost refrigerant in the
evaporators, by releasing refrigerant into the line 15.
During a refrigeration cycle, the valves 34A and 34B are
opened.
Referring to Fig. 3, a heat exchanger 40 is
provided between the lines 15 and 17'. The line 15 directs
refrigerant in the refrigeration cycle from the
condensation/heat reclaim stage 14 to the evaporator stage
16. The line 17' directs refrigerant from the
condensation/heat reclaim stage 14 to the accumulator 22, so
as to ensure that the refrigerant in the accumulator 22 is
in a suitable condition to be fed to the second compressor
24. As such that lines 15 and 17' converge downstream of
the heat exchanger 40. The heat exchanger 40, along with
expansion valve 41, controls the conditions of the
refrigerant being fed to the evaporator stage 16 for
refrigerating purposes and to the accumulator 22.
In an alternative configuration of the second
embodiment_ illustrated in Fig. 4, a defrost refrigeration
system is illustrated as 10" '. In Fig. 4, the defrost
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refrigeration system 10111 has stages similar to that of the
defrost refrigeration system 10' of Fig. 2- and 10" of
Fig. 4, whereby like elements will bear like reference
numerals.
A portion of refrigerant is directed by the line
26 from the output of the second compressor/compressors 24
to the evaporator stage 16 for defrost, passing through a
pressure-reducing device 27 or suitable solenoid valve. As
shown in Fig. 4, the line 26 diverges into lines 26A and 26B
to respectively feed the evaporators 16A and 16B,
respectively, with defrost refrigerant. Similarly to the
defrost refrigeration system 10' of Fig. 2 and 10" of
Fig. 3, the defrost refrigeration system 1011, operates a
defrost cycle using the return lines 18A/18B, using a
network of valves for the defrost refrigerant to be directed
to the evaporators. The defrost refrigerant is then
directed to the accumulator 22 at the compression stage 12
using line 17.
The lines 25A and 25B merge into the return lines
18A and 18B, using appropriate valves to prevent the defrost
refrigerant to be sucked by the compressor stage 12. More
specifically, valves 30A and 30B are opened while valves 31A
and 31B are closed in the defrost sequence. These valves
are in opposite positions during a refrigeration cycle.
Defrost refrigerant is therefore directed to the
evaporators 16A and/or 16B in a defrost cycle. As is
illustrated in Fig. 4, a bypass 32A/32B is provided for the
defrost refrigerant to surround the expansion valves of the
evaporation stage 16.
The defrost refrigerant having released heat
during defrost in the evaporators 16A/16B is then directed
to the accumulator 22 using lines 17A/17B, respectively,
which merge into line 17. Valves 33A/33B are opened during
the defrost cycle, whereas the valves 34A/34B are closed.
These valves are in opposite positions during a
refrigeration cycle.
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Although the choice of refrigerants has not been
described, it is pointed out that any suitable refrigerant
can be used taking into account the conditions at which the
refrigeration system will operate.
