Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FLASH DEFROST SYSTEM
TECHNICAL FIELD OF THE INVENTION
This invention relates to a flash defrost system for defrosting
evaporators in vapour compression refrigeration systems. As
will be explained more fully herein, the invention is applicable
to direct expansion, flooded evaporator and liquid overfeed
refrigeration systems.
BACKGROUND
In many applications of vapour compression refrigeration
systems an evaporator is used to cool air, inter alia, in chiller
rooms, supermarket chilled display cabinets, domestic freezers
and air source heat pumps. In such applications the external
surfaces of the evaporator become covered in ice during
operation due to condensation and freezing of water vapour in
the atmosphere. Ice formation adversely affects the heat
transfer performance, and the power consumption of the
compressor rises to compensate for loss of evaporator
efficiency. All such systems are therefore designed to
periodically defrost the evaporator in order to restore
performance and minimise running costs.
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Common methods of defrost include, in order of defrost speed:
discontinuation of the refrigeration process whilst electrical
heaters attached to the evaporator are used to nnelt and
release the accumulated ice; discontinuation of the refrigeration
effect but, with the compressor still running, diversion of the
hot gas output along an extra line to the evaporator for a time
sufficient to nnelt and release the ice; discontinuation of the
refrigeration effect and the use of ambient air to nnelt the ice.
To minimise temperature rises in the refrigerated products the
time of defrost needs to be short, so that electrical defrost is
most commonly used in food applications. However, electrical
defrost and hot gas defrost also incur a cost penalty in terms of
extra energy used.
WO 2009 034 300 Al discloses an ice maker which includes a
vapour compression refrigeration system having multiple
evaporators. Relatively hot refrigerant from a condenser flows
through a defrost receiver before passing through the
evaporators. Individual evaporators can be defrosted by means
of a valve system which connects the evaporator to the defrost
receiver to allow hot fluid to pass thernnosyphonically from the
defrost receiver to the evaporator and liquid refrigerant in the
evaporator to return by gravity to the defrost receiver.
However, in such a system the length of the defrost period is
relatively unimportant since the remaining evaporators will
continue to operate.
The present invention seeks to provide a new and inventive
form of defrost system which is capable of providing more rapid
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and energy-efficient defrosting of the evaporator than has
hitherto been possible.
SUMMARY OF THE INVENTION
The present invention proposes a vapour compression
refrigeration system including a compressor arranged to re-
circulate refrigerant through a condenser, an expansion device
and an evaporator, in which relatively hot refrigerant from the
condenser flows through a defrost receiver before passing
through the expansion device, and, in a defrost phase, a valve
arrangement connects the evaporator to the defrost receiver to
create a defrost circuit which allows hot fluid to pass from the
defrost receiver to the evaporator and liquid refrigerant in the
evaporator to flow to the defrost receiver,
characterised in that the refrigeration system is
constructed and operated such that, in a pre-defrost phase, the
valve arrangement closes the fluid input to the evaporator and
the compressor operates to partially evacuate the evaporator
before the evaporator is connected to the defrost receiver.
By isolating the input to the evaporator prior to commencement
of the defrost phase and allowing the compressor to remove
refrigerant from the evaporator, the commencement of the
defrost phase causes the hot refrigerant to boil and results in
immediate flash flooding of the evaporator with hot refrigerant
vapour. The invention therefore provides a means of defrosting
the evaporator which uses a minimum amount of net energy
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from the system and which also enables a significant reduction
in the defrost period. In food applications therefore, the
invention minimises excursions from the ideal storage
temperature of the product.
BRIEF DESCRIPTION OF THE DRAWINGS
The following description and the accompanying drawings
referred to therein are included by way of non-limiting example
in order to illustrate how the invention may be put into
practice. In the drawings:
Figure 1 is a diagram of a known form of vapour
compression refrigeration circuit upon which the
present invention is based;
Figure 2 is a diagram of a first such refrigeration
circuit incorporating a defrost system in accordance
with the invention;
Figure 3 is a diagram of a second such refrigeration
circuit incorporating a defrost system in accordance
with the invention;
Figure 4 is a modified form of the refrigeration circuit
shown in Fig. 3;
Figure 5 is a modified form of the refrigeration circuit
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shown in Fig. 2 which can be used with multiple
evaporators; and
Figure 6 shows a further modification as applied to the
refrigeration circuit of Fig.5.
DETAILED DESCRIPTION OF THE DRAWINGS
Fig. 1, shows a widely used direct expansion arrangement to
which the present invention may be applied, comprising a
closed refrigerant circuit in which a compressor 1 pressurises
vapour phase refrigerant. The hot superheated gas leaving the
compressor passes to a condenser 2 in which desuperheating
and subcooling occurs. The warm high pressure liquid
refrigerant then passes to a liquid receiver vessel 3 acting as a
refrigerant reservoir. Liquid from the reservoir supplies an
expansion device 4 where a rapid drop in pressure produces a
two phase stream of cold vapour and liquid which then enters
the bottom of evaporator 5. Evaporation of the liquid phase is
completed in the evaporator so that the required cooling effect
is achieved. Cold sub-cooled vapour from a top exit of the
evaporator 5 then returns to the inlet of the compressor 1 via
the suction line of the compressor and the cycle is repeated.
Various embodiments of the invention will now be described
which achieve rapid energy-efficient defrosting of the
evaporator in such a refrigeration system. In the following
description and drawings the reference numbers used in Fig. 1
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are applied to corresponding items within the refrigeration
system.
In the first embodiment which is shown in Fig. 2 a defrost
receiver 6 is inserted into the liquid stream between the main
liquid reservoir 3 and the expansion device 4, which may be an
expansion valve. A shut-off valve 7 is inserted into the flow
path between the receiver 3 and the defrost receiver 6, and an
isolation valve 8 is inserted between the exit of the evaporator
and the inlet of the compressor 1. A drain valve 9 is
connected in parallel with the expansion valve 4, and a defrost
valve 10 is connected between the top of the defrost receiver 6
and the exit of the evaporator 5. During normal operation the
expansion valve 4 and valves 7 and 8 are open and valves 9
and 10 are closed resulting in a refrigerant flow circuit which is
essentially the same as that shown in Fig. 1. As previously
explained however, normal operation of the circuit will result in
ice formation on the outside of the evaporator due to
condensation of atmospheric water vapour.
When defrosting of the evaporator is required the expansion
valve 4 is firstly closed to close off the fluid inlet of the
evaporator while the compressor 1 continues to run. The
suction line to the compressor continues to draw refrigerant
vapour from the evaporator 5, causing partial evacuation of the
evaporator. After a sufficient period of time, valves 7 and 8 are
closed and valve 10 is opened allowing high pressure liquid
refrigerant in the defrost receiver 6 to flash over into the
evaporator 5, which is at a very low pressure. (The compressor
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may be turned off during this phase.) Refrigerant vapour
condenses in the evaporator releasing latent heat and
transferring it at high heat transfer efficiency until the
pressures in the evaporator 5 and the defrost receiver 6
equalise, at which point drain valve 9 is opened to allow liquid
refrigerant in the evaporator to drain back into the receiver 6
under the action of gravity. When the temperature of the liquid
in the receiver 6 falls to a predetermined level indicating that
defrost is complete, valves 9 and 10 are closed and valves 4, 7
and 8 are opened and the normal operation of the refrigeration
circuit resumes.
In a further improvement of the defrost system in accordance
with the invention the heat energy extracted from the hot liquid
refrigerant and made available for defrost may be augmented
by means of a phase-change unit 11 contained within the
defrost receiver 6. A suitable phase-change medium is
encapsulated within the phase-change unit 11 so that during
normal operation the hot liquid refrigerant flows in contact with
the phase-change unit melting the phase-change material and
storing enthalpy from the liquid refrigerant stream as latent
heat. During the defrost stage the stored heat energy is
released into the refrigerant stream circulating in the closed
loop thereby accelerating the defrost process. The result of
such extraction of heat from the hot liquid refrigerant stream is
to increase the thermodynamic efficiency of the overall
refrigeration circuit through a more effective expansion
process, which largely compensates for the extra energy
needed to re-cool the evaporator after a defrost. The energy
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cost of the defrost process is thereby minimised.
In a second embodiment of the invention which is shown in Fig.
3 the liquid reservoir 3 is arranged to act as a defrost receiver.
The evaporator is at a higher level than the receiver, and the
expansion device 4 is of a type which can be fully opened to
remove the restriction, for example an expansion valve driven
by a stepper motor. An isolation valve 12 in the compressor
suction line is open when the compressor is running and closed
at other times. A defrost valve 13 connects the exit of the
evaporator to the top of the receiver 3 and is shut in normal
operation. When defrost is initiated the expansion valve 4 is
fully closed for a period to allow the evaporator to empty via
the suction line. The compressor 1 is then switched off and
valve 12 is shut. The expansion valve 4 is fully opened
allowing hot liquid to drain back to the liquid receiver, and
valve 13 opens allowing vapour from the top of the receiver 3
to flash over into the partially evacuated evaporator. As the
evaporator is above the receiver and the line from the receiver
3 through the expansion valve 4 is full of liquid a flow will be
established from the evaporator through the expansion valve
back to the receiver 3. Vapour will continue to flow from the
receiver 3 through the defrost valve 13 to the evaporator 5
where it will condense, and the condensed liquid will then flow
back to the receiver 3 via the expansion valve 4.
In a variation of this embodiment a heat exchanger 14
containing a phase change medium may be added between the
receiver 3 and the expansion valve 4. This increases the
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energy storage capacity while minimising the refrigerant
charge. Alternatively, as shown in Fig. 4, a heat exchanger 15
of the fluid-to-fluid type can be used. The secondary of the
heat exchanger is connected to a pump 16 which circulates an
antifreeze fluid from a separate tank 17 in a closed circuit, thus
acting to increase the thermal storage capacity of the defrost
system.
In refrigeration installations with multiple evaporators fed from
common liquid supply and suction manifolds, such as those
used in supermarket display cabinets or cold storage facilities,
the embodiment of the invention shown in Fig. 5 may be used.
The individual evaporators 5 and associated defrost circuitry
constructed and operated as previously described in relation to
Fig. 2 are each connected to the common liquid manifold 18
and suction manifold 19. It will be noted that in this case each
evaporator 5 is associated with its own defrost receiver 6 so
that flash defrosting of the individual evaporators may again
take place as described.
In the embodiments described above the evaporator 5 should
be higher than the heat store module formed by the defrost
receiver 6 and the phase-change unit 11 (if provided) so that
liquid refrigerant can return to the receiver 6 under the action
of gravity. Fig. 6 shows how this requirement can be obviated
by adding a pump 20 in series with the valve 9 between the
liquid outlet from the evaporator 5 and the defrost receiver 6.
The pump 20 will return cold liquid refrigerant from the
evaporator 5 to the heat store 6, 11 where it can evaporate and
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return to the evaporator as vapour. It should also be noted
that with such an arrangement the valve 9 could be replaced
with a non-return valve, removing the requirement for
actuation by the refrigeration control system.
Although the specific embodiments described above are applied
to refrigeration systems of the direct expansion type which
maintain a constant superheat at the evaporator exit, the
invention can also be applied to flooded evaporator and liquid
overfeed refrigeration systems. In such systems the
evaporator is fed with liquid refrigerant and filled with boiling
refrigerant so that a mixture of liquid refrigerant and refrigerant
vapour exits from the evaporator. This requires the addition of
a low pressure accumulator in the suction line so that the liquid
can be separated from the vapour which is returned to the
compressor. Provided the return to the accumulator is above
the fluid level in the evaporator all of the liquid in the
evaporator should evaporate when the liquid feed to the
evaporator is turned off during the pre-defrost phase. The
valve arrangement may need to be modified, but the basic
principle of partial evacuation of the evaporator followed by
flash flooding with hot refrigerant from the liquid supply line
would still apply.
In each embodiment of the invention the heat energy extracted
from the hot liquid refrigerant can be augmented by means of
electrical power supplied by a resistance heater located in or
around the defrost receiver with the purpose of accelerating the
defrost process.
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The timing and sequencing of the valve operation, the sizing
and positioning of the defrost receiver relative to the
evaporator, and the use of thermal capacity enhancement by
means of phase change materials, secondary fluid circuit or
electrical power can be optimised for maximum overall system
efficiency.
The type of valves which may be employed in the refrigeration
units described above include, inter alia, check valves, solenoid
valves, expansion valves and three-way valves.
The control system employed to manage the operation of the
refrigeration systems described above will initiate and
terminate the defrost process based on information supplied by
temperature and pressure sensors fitted at strategic points
around the refrigerant circuits.
Whilst the above description places emphasis on the areas
which are believed to be new and addresses specific problems
which have been identified, it is intended that the features
disclosed herein may be used in any combination which is
capable of providing a new and useful advance in the are.
