Note: Descriptions are shown in the official language in which they were submitted.
~ CA 02208~36 1997-06-20
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VAPOUR COMPRESSION S~STEM
The present invention relates to vapour compression systems such as might be used in, for
example, air conditioners, refrigerators and heat pumps, and to components of vapour
compression systems such as condensers, evaporators and expansion devices. The
invention addresses issues of control of such systems and components. The systems of the
invention are suitable for use with mixtures of mutually soluble refrigerant substances with
different boiling points (such that the mixture boils or condenses through a temperature
range), and can enable power savings identified through the use of such mixtures to be
achieved.
Conventional vapour compression systems comprise an evaporator, a condenser, and a
compressor for raising the pressure of refrigerant vapour from that which prevails in the
evaporator (where the refrigerant takes in heat) to that which prevails in the condenser
(where the refrigerant loses heat). Condensed liquid refrigerant is supplied from the
condenser to the evaporator through an expansion device which m~int~in~ the pressure
difference between the condenser and the evaporator and regulates the flow of refrigerant
through the system. In many applications, the components of such systems are assembled
together into integrated sealed units.
Patent specification US-A-1884186 (Peltier) describes such a system, in which a quantity
of a refrigerant circulates between at least two pressure levels in a condenser and an
evaporator respectively, comprising
a compressor for increasing the pressure of refrigerant vapour;
a condenser for high pressure refrigerant vapour received from the compressor,
an expansion device across which the pressure differential between the condenserand the evaporator is m~int:~ined, to control the withdrawal of liquid refrigerant from the
condenser according to the volume of liquid refrigerant that is within or behind it;
an evaporator for liquid refrigerant received from the condenser;
a receiver into which refrigerant is discharged from the evaporator, the receiver
including a reservoir into whi ~h~liquid refrigerant discharged from the evaporator collects;
A~,ENDE~ S~t~T
~ CA 02208~36 1997-06-20
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- la-
a vapour withdrawal conduit through which vapour is withdrawn from the receiver
for supply to the compressor; and
a liquid withdrawal conduit through which liquid refrigerant is withdrawn from the
reservoir and supplied at a venturi into the vapour withdrawal conduit.
Particularly when a vapour compression system is required to cool a fluid through a
temperature range while rejecting heat to another fluid which warms up through atemperature range, the efficiency of the system can be increased by using a
refrigerant which consists of two or more mutually soluble substances which do not
form an azeotrope, and can therefore condense or boil over a range of temperatures.
The normal boiling points of the two substances are separated by about 15 to 60~C.
By appropriate selection of substances for the mixed refrigerant, the ch~n~in~
boiling point ofthe rnixed refrigerant as it contlçn~çs canbearrangedtofollowclosely
AMEI'~DE~ SffEET
CA 02208~36 1997-06-20
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the temperature of the ~luid being heated in the con~pnser
throughout the length of the co~ncer with the refrigerant and
heat transfer fluid ~lowing in countercurrent relationship with
each other. Similar considerations apply to the evaporator.
As a result, less power is required in order to drive the
compressor because the pressure ratio required of the
compressor is reduced.
It is appropriate for effective operation of a system with a
m;~r~A refrigerant for the relative proportions of the various
components of the mixture to remain substantially constant
throughout the system. It is also preferred that the two
phases of the re~rigerant flow cocurrently at least through the
evaporator and the condenser, so that the separate phases are
each well mixed and there is effective mixing between the
phases. This condition can be referred to as equilibrium
evaporation or c~n~n.~ation. It can arise for example when
liquid and vapour flow cocurrently with vapour flowing down the
bore of the ch~nnel, and liquid flowing along the walls of the
evaporator or the con~Pn~er, effectively as a varying thickness
film around the flowing vapour. Preferably, the equilibrium
conditions of evaporation or co~n-~ation are sust~ne~
throughout su~stantially the entire length of the evaporator or
co~Pn~er (as the case may ~e). This can be difficult to
achieve because the change in phase is accompanied by a large
change in volume, which affects the flow condition of the two
phases.
A vapour compression system is disclosed in WO-A-92/06339 which
incorporates a two-section evaporator which discharges
refrigerant into a low pressure receiver. Subject matter
disclosed in that document is incorporated in the specification
of the present application by this reference. The first (or
major) section of the evaporator receives liquid ~rom the
con~n~er through an expansion device, and discharges
refrigerant vapour together with a small quantity of li~uid
into the low pressure receiver, from which vapour is supplied
, . ~
~ =
CA 02208~36 1997-06-20
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to the compressor. Liquid from the receiver is supplied to the
second section of the evaporator and ensures that, under steady
state operating conditions of the system, the discharge from
the first section of the evaporator r~mA~n~ wet. The system
includes a modulating float valve as the expansion device,
which opens when the quantity of liquid within or behind it
exceeds a pre-determined level, the force required to open the
valve being substantially independent of the pressure drop
across it. The valve ensures that liquid does not accumulate
in the co~enser and is supplied steadily to the evaporator.
The system disclosed in WO-A-92/06339 operates satisfactorily,
enabling the heat exchange surfaces in both evaporator and
con~Pn~er to be optimally employed independent of the duty
required of the system. This enables the power consumption of
the compressor to be reduced. It has demonstrated that the
power saving advantages of mixed refrigerants (that have long
been identified as possible) can be realised.
A two-section evaporator such as is incorporated in the system
disclosed in WO-A-92/06339 can be complicated, especially when
liquid refrigerant must be distributed amongst an array of
separate tubes fed from the receiver, to maintain an
appropriate flow rate of refrigerant through the second section
of the evaporator. The distribution should be such that each
tube in the array is maintained active, even when the load on
the system is light.
According to the present invention, it has been found that, in
a closed system, the wetness of refrigerant discharged from an
evaporator into a receiver can be ensured by the controlled
steady removal of a small quantity of liquid refrigerant from
the receiver in proportion to the amount of refrigerant that is
removed from the receiver as vapour, so as to control the
wetness of the refrigerant discharged into the receiver from
the evaporator to ensure that it is wet under normal operating
conditions of the system.
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F,\137\336\W00lSPEC\AM~7ill 16
Accordingly, in one aspect, the invention provides a method of operating a vapour
compression system in which a quantity of a refrigerant circulates between at least two
pressure levels in a condenser and an evaporator respectively, comprising:
(a) a compressor for increasing the pressure of refrigerant vapour;
(b) a condenser for high pressure refrigerant vapour received from the
compressor;
(c) an expansion device across which the pressure differential between the
condenser and the evaporator is m~int~ined, to control the withdrawal of liquid
refrigerant from the condenser according to the volume of liquid refrigerant that is
within or behind the expansion device, i.e. waiting to pass the expansion device,
i.e. (as stated) from the condenser;
(d) an evaporator for liquid refrigerant received from the condenser; and
(e) a receiver, independent of the expansion device, into which receiver
refrigerant is discharged from the evaporator, the receiver including:
a reservoir for liquid refrigerant,
~ a vapour withdrawal conduit through which refrigerant vapour is withdrawn
from the receiver for supply to the compressor, and
a liquid withdrawal conduit through which liquid refrigerant is withdrawn
from the reservoir and supplied into the vapour withdrawal conduit for supply
to the compressor,
the method comprising cor~rolling the rate of removal of liquid refrigerant from the
receiver in proportion to the amount of refrigerant that is removed from the receiver
as vapour~ so as to control the wetness of the refrigerant discharged into the
i'" ,1~ ~ T
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receiver from the evaporator to ensure that it is wet under
normal operating conditions of the system.
It has been demonstrated by the present invention that removal
of a controlled small quantity of liquid refrigerant from the
receiver at a steady rate can lead to a controlled small degree
of wetness in the discharge from the evaporator, which in turn
can lead to substantially the entire heat exchange surface of
the evaporator rPm~;n;ng wet. Accordingly, a high heat trans-
fer coefficient can be maintained as a result of heat transfer
along the entire length of the evaporator, substantially
independent of the loading placed on the system and on the
composition of the refrigerant. Preferably, the rate of
removal of liquid from the receiver is such that the wetness of
the refrigerant discharged from the evaporator is not more than
about 5~ by weight, more preferably not more than about 3.5~ by
weight, especially not more than about 2.5~, for example
between about 1 and 2~ by weight.
The invention can also ensure that the liquid content in the
refrigerant that is supplied to the compressor is controlled so
that the amount of liquid is kept steady without significant
fluctuations. The wetness of the refrigerant supplied to the
compressor can be similar to the wetness of the refrigerant
that is discharged from the evaporator to the receiver.
However, in many circumstances, the two wetnesses will be
different. The differences between the two wetnesses can be
accounted for by, for example, evaporation of some of the
liquid refrigerant that is removed from the receiver. The
differences can also be balanced by the receiver.
In another aspect, the invention provides a vapour compression
system in which a quantity of a refrigerant circulates between
at least two pressure levels in a co~Pn~er and an evaporator
respectively, comprising:
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(a) a compressor for increasing the pressure of refrigerant vapour;
(b) a condenser for high pressure refrigerant vapour received from the compressor;
(c) an expansion device across which the pressure differential between the
condenser and the evaporator is m~int~ined, to control the withdrawal of liquid
refrigerant from the condenser according to the volume of liquid refrigerant that is
within or behind the expansion device, i.e. waiting to pass the expansion device,
i.e. (as stated) from the condenser;
(d) an evaporator for liquid refrigerant received from the condenser;
(e) a receiver, independent of the expansion device, into which refrigerant is
discharged from the evaporator, the receiver including a reservoir into which liquid
refrigerant discharged from the evaporator collects, to conkol supply of liquid
refrigerant to the compressor,
(f) a vapour withdrawal conduit through which vapour is withdrawn from the
receiver for supply to the compressor,
(g) a liquid withdrawal conduit through which liquid refrigerant is withdrawn from
the reservoir into the vapour withdrawal conduit for supply to the compressor; and
(h) means for conkolling the rate of removal of liquid refrigerant from the
reservoir in proportion to the amount of refrigerant that is removed from the
receiver as vapour,
so that refrigerant from the means (h) supplied to the compressor (a) is of such composition
that the wetness of the refrigerant subsequently discharged into the receiver from the
evaporator is controlled to e ~sllre that it is wet under normal operating conditions of the
system.
~MEI~JDF~J '''."'-T
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Preferably, the receiver is arranged so that liquid refrigerant
contained in the reservoir is retained in the reservoir at
shut-down of the system.
The control of the flow of liquid refrigerant from the receiver
can be achieved using a liquid withdrawal conduit through which
liquid is supplied to the conduit for vapour feed from the
receiver to the compressor, due to a pressure drop along the
vapour withdrawal conduit downstream of the reservoir. The
control can be achieved through use of a receiver into which
refrigerant is discharged from the evaporator, which includes:
a reservoir for li~uid refrigerant,
~ a vapour withdrawal conduit through which refrigerant
vapour is supplied from the receiver to the compressor,
and
~ a liquid withdrawal conduit through which liquid
refrigerant is supplied from the reservoir into the
vapour withdrawal conduit,
the vapour withdrawal conduit being arranged so that the
pressure of vapour flowing in it is reduced at a point
downstream of the reservoir relative to the pressure in the
reservoir, so that liquid refrigerant in the reservoir is drawn
into the vapour withdrawal conduit through the liquid
withdrawal conduit.
A system which includes such a receiver with means for removing
a controlled quantity of liquid refrigerant has the advantage
that appropriate control of the wetness of the refrigerant
supply to the receiver can be achieved without having to
include a two-section evaporator. This enables the power
consumptions available from use of mixed refrigerants to be
obtained, while also m;n;m;sing equipment costs by avoiding the
use of certain complicated multi-tube heat exchanger
CA 02208~36 1997-06-20
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constructions. By appropriate design of the flow resistance of
the vapour and liquid withdrawal conduits including their
disposition relative to the reservoir, it is possible to ensure
that, at steady state operation of the system, the liquid
supplied from the receiver to the vapour withdrawal conduit
(for supply to the compressor) is such that the refrigerant
discharged from the evaporator to the receiver has an
appropriate low degree of wetness. Such operation of the
system involves optimum use of the heat exchange surfaces of
the evaporator, and can allow the advantages of reduced power
consumption from the use of mixed refrigerants to be realised.
A further advantage that arises from the use of the receiver
referred to above is that the optimised use of the heat
~h~nge surfaces of the evaporator is achieved without
deterioration of the control due to accumulation of compressor
oil. This is in contrast to the system disclosed in
WO-A-92/06339 in which compressor oil can tend to accumulate
excessively in the second evaporator section, especially if the
velocity of the refrigerant in the second section drops too low
as can happen if the tubes in the second section are not
appropriately manifolded. Such accumulation of oil can give
rise to operational instability, especially when the duty
required of the system is reduced or when the system is
restarted after a temporary shut-down.
The quantity of liquid refrigerant that is removed from the
reservoir is controlled so that it is removed at a substan-
tially steady rate. The rate at which liquid refrigerant is
removed from the reservoir is preferably determined in relation
to the quantity of refrigerant that is removed as vapour; this
can be achieved by means of so-called proportionating devices.
Details of such devices are set out below.
Provided that the rate of flow of liquid refrigerant is
substantially steady, it has been found that the quantity of
liquid, required to be removed from the reservoir and supplied
CA 02208~36 1997-06-20
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to the compressor to promote appropriate wet discharge from the
evaporator, need not give rise to mechanical difficulties in
operation of the system, or affect adversely the efficiency of
the compressor.
Preferably, the receiver is arranged such that not more than
about 4~ by weight of the compressor throughput of refrigerant
passes through the liquid withdrawal conduit, the r~; n~er
passing through the vapour withdrawal conduit. More pref-
erably, the li~uid withdrawal conduit carries not more than
about 3~ by weight of the compressor throughput. Preferably,
the liquid withdrawal conduit carries at least about 0.5~ of
the compressor throughput, more preferably at least about 1~.
For example, the receiver can be arranged so that about 2~ by
weight of the compressor throughput of refrlge~ant passes
through the liquid withdrawal conduit.
Preferably, the junction between the vapour and liquid
withdrawal conduits is at a level that is about or slightly
above the level of liquid refrigerant in the reservoir when the
system is at steady state operation. This has the advantage
that the tendency of li~uid refrigerant to drain into the
vapour withdrawal conduit during temporary shut down of the
system is reduced. The level of the said junction should
preferably be only slightly above the steady state liquid level
so that the system provides about the same proportion of liquid
injected into the liquid withdrawal conduit over a range of
duties.
Preferably, the opening for vapour to enter the vapour
withdrawal conduit for supply to the compressor is located
above the level of the refrigerant liquid in the reservoir, and
is preferably at or towards the top of the reservoir. This
arrangement has the advantage that it reduces the tendency for
liquid refrigerant to be drawn with refrigerant vapour from the
reservoir or the discharge from the evaporator or both, and
transferred to the compressor suction. Generally, in this
CA 02208~36 1997-06-20
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--10--
arrangement, the vapour withdrawal conduit will include a
section which extends downwardly to a level below the level of
liquid in the reservoir when the system is in operation.
Preferably, the opening for liquid to enter the liquid
withdrawal conduit to flow to the vapour feed line is located
close to the bottom of the reservoir, more preferably in the
base of the reservoir, so that liquid will continue to be drawn
from the reservoir, even when the level of liquid in the
reservoir is low.
Preferably, the opening from the liquid withdrawal conduit into
the vapour withdrawal conduit discharges liquid refrigerant
into the vapour withdrawal conduit at a point at least about
one quarter of the distance across the vapour withdrawal
conduit, more preferably at least about one third of that
distance. This has the advantage that it encourages the
dispersion of the liquid refrigerant into the vapour in droplet
form.
The cross-sectional area of the vapour withdrawal conduit can
be greater at a point downstream of the junction with the
liquid withdrawal conduit than at a point upstream of that
junction, so that the overall pressure drop in the compressor
suction is m; n;m; sed.
The change in cross-sectional area of the vapour withdrawal
conduit can be associated with a constriction in the conduit.
The constriction can be such that a venturi is provided in the
vapour withdrawal conduit. Preferably, the venturi is mounted
horizontally, with its centre-line at about the normal level of
liquid in the reservoir when the system is operating in a
steady state condition~ It has been found that injection of a
small quantity of liquid refrigerant into the stream of
refrigerant vapour at the throat of a venturi constriction does
not significantly affect pressure recovery adversely.
Consequently, the pressure drop in the vapour conduit between
=~:
CA 02208~36 1997-06-20
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the receiver and the compressor suction r~m~;nq low, providing
energy efficient performance. An arrangement using a venturi
can lead to liquid refrigerant being removed from the reservoir
in proportion to the amount of refrigerant removed as vapour.
.
The liquid withdrawal conduit can include an n-shaped portion
with two limbs and a connecting portion extending between them,
in which liquid is drawn from the reservoir and made to flow
initially upwardly from the reservoir along a first one of the
limbs, and downwardly to the junction with the vapour flow
conduit along the other of the limbs, thus acting as a syphon.
The height of the n-shaped portion of the liquid withdrawal
conduit above the normal level of liquid refrigerant in the
reservoir will be selected so that the n-shaped portion is at
least as high as the highest anticipated level of liquid that
will be contained in the reservoir at any time during operation
of the system, to ensure that liquid refrigerant will not drain
from the reservoir to the compressor, particularly at shut
down. The second down-flow limb will preferably include the
capillary flow resistance conduit. An arrangement in which the
liquid withdrawal conduit includes an n-shaped portion can lead
to liquid refrigerant being removed from the reservoir in
proportion to the amount of refrigerant removed as vapour.
The reservoir, into which refrigerant is discharged from the
first evaporator section, will generally be arranged so that
refrigerant collected within it has a large surface area. For
example, the surface area of liquid refrigerant may be at least
about twice the square of the height of the reservoir,
preferably, at least about three times the square of that
height. This has the advantage that variation in the amount of
liquid refrigerant contained in the reservoir does not affect
significantly the depth of the liquid and frothing of the
refrigerant in the reservoir is less likely to lead to liquid
refrigerant being supplied to the compressor. This allows a
significant gap to be maintained between the upper surface of
collected liquid refrigerant, and the outlet through which
CA 02208~36 l997-06-20
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-12-
vapour is supplied to the compressor, thus m;n;m; sing and
preferably avoiding the possibility of liquid refrigerant being
supplied in bulk to the compressor under any possible operating
conditions.
It is particularly preferred to use a vapour withdrawal conduit
with a venturi provided in it by a constriction, together with
a liquid withdrawal conduit which includes an n-shaped portion
as described above. The venturi can give rise to a significant
pressure difference between the reservoir and the exit from the
vapour withdrawal conduit at the junction with the liquid
withdrawal conduit. The pressure difference can be arranged
such that liquid refrigerant is drawn up the first limb of the
n-shaped portion of the liquid withdrawal conduit, at a rate
appropriate to maintain the discharge from the evaporator wet
as discussed above. ~'his arrangement has the advantage of
reduced power loss due to frictional effects in the vapour
withdrawal conduit can be reduced, because the pressure loss
involved in accelerating the vapour through the throat of the
venturi is largely recovered in the divergent diffuser section.
The liquid withdrawal conduit is designed to have an overall
pressure drop, when supplying liquid refrigerant at the desired
flow rate, which equals the overall pressure drop in the exit
vapour conduit between the receiver and the point of liquid
injection into the liquid conduit. This can be achieved by
selection of the configurations of the vapour and liquid
withdrawal conduits such that the pressure drop in the vapour
withdrawal conduit between the reservoir and the junction with
the liquid withdrawal conduit provides a controlled flow of
liquid along the liquid withdrawal conduit from the reservoir
to the said junction in proportion with the suction flow rate
of vapour to the compressor. The selection involves parameters
such as cross-sectional areas and of lengths the conduits
between the junction between them and the reservoir.
Accordingly, the conduit may be in the form of a capillary tube
having a small cross-section, along at least a part of its
CA 02208~36 1997-06-20
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-13-
length. Alternatively or in addition, the cross-sectional
configuration of the vapour withdrawal conduit can differ
between the portions upstream and downstream respectively of
the junction with the liquid withdrawal conduit.
The vapour withdrawal conduit can include a U-shaped portion
with two limbs and a connecting portion extending between them.
The upstream limb of the U-shaped portion can then provide a
downwardly extending section of the vapour withdrawal conduit,
from the opening for vapour to enter the vapour feed line for
supply to the compressor, above the level of the refrigerant
liquid. In this arrangement, it will generally be preferred
for the junction between the vapour withdrawal conduit and the
liquid withdrawal conduit is located in the downstream limb of
the U-shaped portion of the vapour withdrawal conduit.
The drop in pressure in the vapour withdrawal conduit between
the reservoir and the junction with the liquid withdrawal
conduit preferably corresponds to a head of liquid refrigerant
of between 45 and 200 mm, more preferably between 65 and
160 mm, especially between 80 and 130 mm.
The liquid withdrawal conduit may be configured so that liquid
contained in it is placed in heat exchange relationship with
liquid refrigerant discharged from the condenser so that it is
heated by the said liquid refrigerant, between discharge into
the conduit from the reservoir and discharge from the conduit
into the vapour withdrawal conduit. In this construction, the
liquid withdrawal conduit includes a constriction in it, by
which flow of refrigerant along the conduit is controlled. It
will be preferred for this refrigerant stream to flow generally
upwardly while in heat exchange relationship with the
con~Pn~ate.
Examples of materials which are suitable for use as
refrigerants in a single refrigerant system include those
designated by the marks R22 and R134a. A particular advantage
~ CA 02208~36 1997-06-20
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- 14 -
of the system of the invention is that it is well suited to the use of a wide boiling
non-azeotropic mixed refrigerant in which it is particularly desirable that, at all places
within the condenser and the evaporator, liquid and vapour refrigerant flow together
co-currently and are in equilibrium, whilst the refrigerant mixture flows essentially
counter-currently with the fluid with which it is exch~n~ing heat. This objective can be
achieved by the system of the invention, particularly when it includes both an expansion
valve where the force required to open it is substantially independent of the pressure drop
across it. The vapour compression system of the invention therefore makes possible the
power saving which is available from the use of a wide boiling mixed refrigerant. In
addition, further power saving can be achieved because of the ability of the system of the
invention to adapt to varying duty, start-up conditions, varying ambient conditions and so
on, while operating at optimum efficiency.
Thus, the refrigerant preferably consists of two or more m~ ly soluble refrigerant
substances which do not form an azeotrope. Examples of suitable mixed refrigerants
include those ~lesi~7~tecl by the marks R23/R134a and R32/R227. Suitable mixtures of
refrigerant substances can have boiling points separated by at least about 10 ~C, for example
at least about 20~C. The dirrelellce in boiling points will often be less than about 70~C,
preferably less than about 60~C, for exarnple less than about 50~C.
It will be understood that the term "refrigerant", used in this document to denote the fluid
circulating in the vapour coll~plession system, is applicable to the fluid which circulates in
systems which function as air conditioners or heat pumps.
The duty performed by the vapour compression system is determined by appropriateadjustment of the flow rate of the refrigerant vapour through the system. This can be
achieved in a number of ways: for example, the throughput of the compressor can be
adjusted, for example by adjustment of its speed or by unloading one or more cylinders, or
more than one compressor may be provided of which some or all may be used according
to the quantity of refrigerant required to be circulated.
Al\fiEi~DcD ~ T
CA 02208~36 l997-06-20
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-15-
Alternatively, the desired duty may be o~tained by selectively
switching the compressor on and off as necessary.
The control of the compressor through-put may be in response to
a detected change in temperature in the medium required to be
heated or cooled by the system. For example, in a
refrigeration system, a temperature sensor may be used to cause
the through-put of a compressor to increase on detecting an
increase in temperature of a cold chamber.
When air is used as the heat transfer medium in the con~n~er
or the evaporator, and in cases where the duty of the unit
varies widely, variable output fans may be used to modulate air
flow and to conserve power.
The present invention will now be described, by way of example
only, with reference to the accompanying drawings, in which:
Figure 1 is a schematic illustration of a vapour
compression system in accordance with the present
invention;
Figure 2 is a schematic illustration of a receiver
suitable for use in the vapour compression system shown
in Figure 1;
Figure 3 is a schematic illustration of another
embodiment of receiver;
Figure 4 is a schematic illustration of another vapour
compression system in accordance with the present
invention; and
Figure 5 is a schematic illustration of a vapour
compression system in which condensed refrigerant is
split into two streams which are split between two
evaporators.
~ CA 02208~36 1997-06-20
F \137\336\WOOISPEC\Ab~9701 16
- 16-
Referring to the drawings, Figure l shows a vapour compression system which comprises
a compressor 1 for increasing the pressure of refrigerant vapour, and for forcing the vapour
through a first conduit 3 to a condenser 5. The condenser 5 comprises an array of
condenser tubes 7, connected both in series and in parallel, which are attached to a plurality
of fins which facilitate heat transfer between a cooling medium which flows over the fins
and the refrigerant contained within the condenser tube. The medium might be for example
air when the system forms part of an air conditioning unit or a refrigerator. The flow
directions of the two fluids are essentially countercurrent so this design is suitable for
mixed refrigerants as well as pure refrigerants.
Refrigerant is discharged from the condenser 5 into a second conduit 1 1 through a valve 13.
A vapour return tube 14 is provided to ensure that the inlet to the valve 13 does not become
vapour locked. The valve 13 is aTranged to open when the quantitv of the condensed liquid
refrigerant waiting to pass it, i.e. behind or within it, exceeds a predetermined level. The
construction of an ~lu~liate valve is disclosed in WO-A-92/06339. An a~l~liate valve
will be one in which the force required to open it is substantially independent of the
pressure drop across it.
The refrigerant from the condenser passes to an evaporator 15 through the valve 13 and the
second conduit 11. The evaporator 15 comprises an array of tubes connected in series and
in parallel. It further comprises evaporator fins over which a fluid flows so as to transfer
heat and to cause the refrigerant to evaporate. The fluid is cooled as a result. The fluid
might be, for example, air when the refrigeration system forms part of an air conditioning
unit or a refrigerator.
The refrigerant is discharged from the evaporator 15 into a receiver 21. Liquid refrigerant
discharged from the evaporator collects in the reservoir 23 of the receiver which can provide
~R~ ,r! ~ r~ t' J~~-T
CA 02208~36 1997-06-20
W096/20378 PCTIGB95/02982
-17-
buffer storage of the liquid refrigerant. A vapour withdrawal
conduit 25 extends from the top of the reservoir 23, to convey
the major part of the refrigerant as vapour (that is,
essentially liquid-free vapour) from the reservoir to the
compressor. It will therefore be understood that refrigerant
can be separated in the receiver, into liquid and vapour
phases.
The receiver includes a liquid withdrawal conduit 27 through
which liquid refrigerant is supplied from the reservoir into
the vapour withdrawal conduit 25. By selection of its diameter
and length, taking into account restrictions to flow such as
are provided'by bends, the vapour withdrawal conduit is
arranged so that the pressure of vapour flowing in is reduced
at a point downstream of the reservoir related to the pressure
in the reservoir, so that liquid refrigerant in the reservoir
is drawn into the vapour withdrawal conduit 25 through the
liquid withdrawal conduit 27 at a rate proportional to the
vapour flow. The pressure drop corresponds approximately to a
head of liquid refrigerant of about 100 mm.
The evaporator, receiver and flow proportioning means in
combination ensure that all of the evaporator surface is
employed for heat transfer, irrespective of the duty required
of the system. When the system is running, the arrangement of
vapour and liquid withdrawal conduits in the receiver will
ensure that liquid is drawn from the reservoir at a controlled
rate. Consequently, the level of liquid in the reservoir will
tend to go down. The use of an expansion device which opens
when the quantity of condensed liquid refrigerant within or
behind it reaches a pre-determined level ensures that liquid
cannot accumulate anywhere in the system other than in the
reservoir, because the expansion device ensures that liquid
does not accumulate in the condenser. Liquid is admitted to
the evaporator from the con~pn~er at the rate at which it is
produced in the condenser. The system will therefore tend
towards a steady state condition in which the liquid
CA 02208~36 l997-06-20
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refrigerant removed from the reservoir by means of the liquid
withdrawal conduit is exactly compensated by the liquid
component of the two phase refrigerant discharged into the
reservoir from the evaporator. The pressure in the evaporator
will adjust itself automatically to achieve this balance. This
means that all of the evaporator surface must be wet during
such steady state operation.
Figures 2 and 3 show constructions of receivers in more detail.
Referring first to Figure 2, in which the disclosed
construction comprises a reservoir 31, into which refrigerant
is discharged from the evaporator through a discharge conduit
33. The outlet from the discharge conduit is located towards
the top of the reservoir 31. The vapour withdrawal conduit 25
from which the refrigerant vapour is supplied from the
reservoir 31 to the compressor is located towards the top of
the reservoir.
The vapour withdrawal conduit 25 has a U-shaped portion 35
;mm~;ately downstream of the reservoir 31. The U-shaped
portion comprises first and second limbs 37, 39 and a
connecting base portion. The base portion is located at a
level well below the normal level 41 of liquid refrigerant
contained in the reservoir 31 when the system is running in a
steady state condition.
The second limb 39 of the U-shaped portion of the vapour
withdrawal conduit is flared.
A liquid withdrawal conduit 43 extends from the base of the
reservoir 31 (well below the normal liquid level 41) and joins
the second limb 39 of the U-shaped portion of the vapour
withdrawal conduit. The junction between the liquid and vapour
withdrawal conduits 25, 43 is at a point just upstream of the
flare in the vapour withdrawal conduit. The opening from the
liquid withdrawal conduit into the vapour withdrawal conduit
discharges liquid refrigerant into the vapour withdrawal
CA 02208~36 1997-06-20
W096/20378 PCTIGB95/02982
--19--
conduit at a point about one third of the distance across the
vapour withdrawal conduit, so that the liquid refrigerant
discharged into the vapour will tend to atomise as it is
discharged.
The liquid withdrawal conduit 43 is provided by a capillary
tube. The diameter and length of the capillary tube are
selected so that, for a rate of liquid injection into the
vapour withdrawal conduit 25 at the desired ratio (for example
about 2~ by weight of the throughput of refrigerant through the
compressor), the pressure drop across the liquid withdrawal
conduit is equal to the pressure drop in the vapour withdrawal
conduit 25. The junction between the vapour and liquid
withdrawal conduits is at a level that is about or slightly
above the level of liquid refrigerant in the reservoir when the
system is at a steady stage operation.
Figure 3 shows an alternative construction of receiver 51. It
comprises a reservoir 53 into which refrigerant is discharged
from the evaporator through a conduit 55.
A vapour withdrawal conduit 57 has an opening towards the top
of the reservoir for entry of vapour for supply to the
compressor. The vapour withdrawal conduit includes a
downwardly extending portion and a portion which extends
approximately parallel to the surface of liquid refrigerant
contained in the reservoir, at about the level of liquid when
the system is running in a steady state condition.
A constriction in the vapour withdrawal conduit provides a
venturi 59, by which the pressure of the vapour in the vapour
withdrawal conduit is decreased and then increased.
A liquid withdrawal conduit 61 is provided in the form of an
n-shaped tube, with its opening 63 for entry of liquid located
towards the base of the reservoir 53.
CA 02208~36 1997-06-20
W096/20378 PCT/GB95/02982
-20-
The opening 65 for discharge of liquid refrigerant into the
vapour withdrawal conduit 57 is located relative to the venturi
59 such that liquid refrigerant is drawn from the reservoir 53
into the vapour withdrawal conduit through the liquid
withdrawal conduit as a result of the pressure changes imposed
on vapour in the vapour withdrawal conduit by the venturi.
The quantity of liquid that is drawn into the vapour withdrawal
conduit is controlled at least partially by the ~;mPn~ions of
the venturi.
Figure 4 shows a vapour compression system which comprises a
compressor 81 'for increasing the pressure of refrigerant
vapour, and for forcing the vapour through a first conduit 83
to a con~n~er 85. Refrigerant is discharged from the
condenser into a second conduit 91 through a valve 93. A
vapour return tube can be provided to ensure that the inlet to
the valve does not become vapour locked. The valve is arranged
to open when the quantity of the condensed liquid refrigerant
behind or within it exceeds a pre-determined level.
The refrigerant from the condenser passes through an evaporator
95 through the valve 93 and the second conduit 91. The
refrigerant is discharged from the evaporator 95 into a
receiver 101. Liquid refrigerant discharged from the
evaporator collects in the reservoir 103 of the receiver. A
vapour withdrawal conduit 105 extends from the top of the
reservoir, and to convey the major part of the refrigerant as
vapour from the reservoir to the compressor.
The receiver includes a liquid withdrawal conduit 107 through
which liquid refrigerant is supplied from the reservoir into
the vapour withdrawal conduit. The liquid conduit includes a
constriction 108 which provides a resistance to flow. Liquid
refrigerant in the conduit 107 is exposed to heat imparted by
liquid refrigerant that is discharged from the condenser as it
flows generally upwardly through a heat exchanger 109 (such
CA 02208~36 1997-06-20
W09612037~ PCTlGB95102982
-21-
that the point of entry to the heat exchanger is lower than the
point of exit) so that the liquid refrigerant is evaporated, at
least partially. The refrigerant from the heat exchanger is
then injected into the vapour withdrawal conduit 105 for supply
to the compressor.
The valve 93 comprises a float 111 which is exposed to
saturated liquid refrigerant from the condenser, and valve
orifices 113 which are exposed to liquid refrigerant that has
been sub-cooled by passage through the heat exchanger 109. The
float 111 and the needles by which the valve orifices are
closed are connected by an elongate rod 115 in a close fitting
tube which are such that the flow of liquid that is permitted
between the float chamber and the valve orifices through the
resulting annular passage is negligible. Accordingly,
saturated refrigerant con~nsate is caused to flow from the
base of the float chamber, through the heat exchanger, into the
valve body, where it expands through the orifices of the valve.
The evaporator, receiver and flow proportioning means in
combination ensure that all of the evaporator surface is
effectively employed for heat transfer, irrespective of the
duty required of the system. When the system is running, the
arrangement of vapour and liquid withdrawal conduits in the
receiver will ensure that liquid is drawn from the reservoir at
a controlled rate. Consequently, the level of liquid in the
reservoir will tend to go down. The use of an expansion device
which opens when the quantity of condensed liquid refrigerant
within or behind it reaches a pre-determined level ensures that
liquid cannot accumulate anywhere in the system other than in
the reservoir, because the expansion device ensures that liquid
does not accumulate in the condenser. The system will
therefore tend towards a steady state condition in which the
liquid refrigerant removed from the reservoir by means of the
liquid withdrawal conduit is exactly compensated by the liquid
component of two phase refrigerant discharged into the
reservoir from the evaporator. The pressure in the evaporator
CA 02208~36 1997-06-20
W 096/20378 PCT/GB95/02982
will adjust itself automatically to achieve this balance. This
means that all of the evaporator surface must be wet during
such steady state operation.
Figure 5 shows a system which can accommodate a temperature
change in the evaporator which is significantly greater than
that in the coP~Pn~er, for example by as much as a factor of
two or more. For example, the temperature change across the
con~Pn~er might be about 10~C (between say 19 and 29~C), while
the temperature change across the evaporator might be about
22~C (in two stages from say 27 to 16~C and 16 to 5~C).
The system includes a receiver 120 into which liquid is
discharged from the condenser 122. Liquid from the reservoir
is split between two streams, each of which supplies
refrigerant into first and second evaporators 124, 126. Flow
of refrigerant into the evaporators is controlled by means of
valves 128, 130.
Refrigerant is discharged from the evaporators into respective
reservoirs 132, 134, in liquid and vapour phases, from which
refrigerant vapour is withdrawn for supply to the compressor
assembly 136. The reservoirs also supply liquid refrigerant in
small controlled quantities to the compressor through liquid
withdrawal conduits 138, 140 which join the vapour withdrawal
conduits, in the manner described above with reference to
Figure 2 or Figure 3.
The valves 128, 130 are controlled by level sensors for liquid
in the reservoirs.
The compressor assembly 136 comprises two separate compressors,
which operate at high and low pressures respectively. The use
of two compressors in this way facilitates operation of the two
evaporators of the system over different temperature profiles.
CA 02208~36 1997-06-20
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-23-
The components are arranged so that the receiver can hold all
of the free refrigerant in the system when the reservoirs do
not hold any. The reservoirs are sufficiently large that they
can hold liquid refrigerant without frothing into the
compressor.
