CA1310838C - Vapour injection system for refrigeration units - Google Patents

Abstract

IMPROVEMENTS IN
VAPOUR COMPRESSION CYCLE
REFRIGERATION SYSTEMS

ABSTRACT
The present invention relates to a method for combining at least two discrete flows of a refrigerant in respective substantially dissimilar thermodynamic states in a vapour compression cycle refrigeration system, including the step of imparting substantial turbulent mixing of the at least two flows to produce a generally thermodynamically uniform admixture thereof. The present invention also relates to an improved vapour compression cycle refrigeration apparatus including means for turbulent mixing of at least two discrete flows of a refrigerant in respective, substantially dissimilar thermodynamic states, which means is operable to produce a generally thermodynamically uniform admixture thereof. The means may be retrofitted to existing equipment and the present invention extends to kits useful to this end and to refrigeration sub-assemblies including such means.

Description

:~3~8~8 FIELD OF l~ INVENq~ION
The present inven~ion relates to improvements in vapour compression cycle refrigeration systems, and especially those utilizing hot gas by-pass systems for varying the re~rigeration capacity of the system.

BAC~GR~UND OF THE I~ENTION
In a simple cycle ~ie single stage) vapour compression cycle refrigeration system, the refrigerant ideally enters the evaporator as a mixture of saturated liquid and saturated vapour. Full utiliæati~n of the heat transfer surfaces in the evaporator circuits re~uires the presence of liquid refrigerant in or on all parts of the tubes that make up the various circuits o~ the evaporator. In the evaporator the liquid refrigerant changes, under relatively constant pressures, into a vapour and absorbs heat from the zone serviced by the evapor~tor. The refrigerant then leaves the evaporator preferably as a saturated vapour, or as a slightly superheated vapour.
The refrigerant next anters the compressor, where it is isentropically compressed to the condensers operating pressure. The refrigerant flows from the compressor and through the condenser under fairly constant pressure conditions, and dissipates heat to the atmosphare.
Finally the refrigerant leaves the condenser as a liquid and flows through t~e expansion valve and back to the 3~

evaporator, with part of the refrigerant flashing into vapour as the line pressure drops across the expansion valve.
Such a simplistic system only operates efficiently and safely within a narrow range of ambient heat loads. Since normal seasonal a~d even diurnal variations in ambient conditions impose loads outside of the range that can be handled by simplistic systems such as that described hereinabove, steps are usually taken in the design of modern refrigeration eq~ipment so as to provide for a broader range of operating conditions.
These steps typically include the use of thermostatic expa~sion valves in combination with capillary tubes, flat valves and automatic expansion valves. Even so such equipment is capable of dealing only with those variations in heat load that are imposed by a ~airly modest range of ambient operatlng conditions.
Even with such addition~, however, ambient load conditions can still exceed dasign li~itations, and further precaution~ have been found to be necessary. It is important, 20 for example, that no substantial amount of liquid refrigerant be carried out of the evaporator with the vapour that is returned to the compressor. This problem does arise, however, when the refrigerativn equipment is operated at ambient heat : loads below the lower limits of the de~igned heat transfer capacity ~or that e~uipment. In such cir~umstances the amount : o~ ambient heat available ~t a given flow rate of re~rigerant ; through the evaporator is insufficient to vapouri~e sub~tantially all of the liquid pre~ent in the evaporator.

3 ~
The liquid re~rigerant that does exit the evaporator must be trapped before it reaches ~he campressor, or serious loss of compressor lubrication is likely to result. In-line liqui~
traps ranging ~rom simple l'U~ tu~eæ, or ~w~n neck~, up to S ~omplex suc~ion gas/llquid heat exchangar and ~uction pots are used ~or this purpo~e, wlth th~ choice o~ apparatus dependin~
on the anticipated operating loads.
As has alr~ady been mentioned, the full utilization oP the ~eat ~ran~er s~rfaces in all evapora~or circuits requir~s the presen~e of liquid refrigerant in or on all parts o~ the tubes that ma~e up the heat transfer suraces i~ tl~e various evaporator circuits. Under very low load conditions ~he need to maintain suffi~i~nt liquid refrigerant in the evaporator, and the proportionatsly ~mall amount af hea~ taken up by that re~rigerant relative to th~ design capacity o~ the sy~tem, may result in too large an amount of liquid re~rigerant leaving the evaporator and exceeding the capacity of thæ a~ove-mentioned traps. ~e consequences Gf returning ; li~uid r~rigerant to ~he c~mpre3~0r nas a~so already been ~ 2~ mentioned~
O~her approaches a~e therefor use~ in t~nde~ with those mention~d hereinabove. In small sy~t~ms ~he compressor is merely shu~ dbwn when the cooling thermosta~ setting ~s ~en satisfie~. In lar~e systems that ~ptiQn i~ not as readily availa~le, beause of the wear that at~end~ ~ha onJof~
cyaling of the lar~e compres~o~æ us~d in these sys~e~s.

~3~3~

~ çcordingly, in large systems employing centrifugal compressors the capacity may be varied to match a change in ambient loading by: 1~ varying the speed at which the compressor is driven; 2) adjusting vanes at the inlet to the impellers; 3) th~ottling the suction gas; or, ~ varying the condenser pressure. Methods 1 and 2 require feedback controls with their attendant increased capital and maintenance C05tS.
Attempting to control the capacity by either throttling the suction gas or varying the condenser pressure results in reduced system efficiency.
In large systems u~ing the more common reciprocatin~
compressors (and in which the lu~rication problems are much more serious than with centrifugal compressors), capacity control can be accomplished through several means which are used in combination with one ~nother. The most common approach is to unload the co~pressor through a series of unloading sta~es until a final unloading stage, wh~reupon a controlled reErigerant bypass of the condenser and the expansion valve is employed to reroute hot-gas to the 20 evaporator, by directing gas from the compressor discharge into the low pressure side of the system, at a point either up or downstream of the e.vaporator. This approach is known to seriously reduce system efficiency since even though the reduc2d condenser pressures whiçh normally accompany a reduced 25 system load result ln a saving in compressor power, it may interfere with the flow of liquid re~rigerant through the expansion device and cause unsatisEactQry operation oE the system. This i8 ~ecau~e the expansion valve meters less refrigerant to the evaporator when the system is operated at reduced condenser pressures. In a typical installation equipped with such a hot-gas bypass system, the discharge bypass valve will attempt to compensate for the substantial reduction in suction pressure when the compressor is in its final unloading stage and maintain a given predetermined (ie design~ pressure. With the reduced demand for refrigerant and less volume o~ liquid throughput, the expanding liquid has less velocity in the evaporator~ It has now been found that this allows the hot gas, that has entered the auxilliary side connector upstream of the evaporator and has been mergad into the refrigerant flow leaving the expansion valve and entering the dis~ributorf to push the expanding liquid refrigerant away from some of the distributor tubes. This in turn causes an uneven distribution of vapour and liquid within the various evaporator circuits. The desuperheating that then taXes place within the evaporator not only renders some circuits inactive for cooling purposes, but actually results in localized heating of the ambient environment over certain portions of the evaporators heat exchange surface.
One example of the type of installatioll where these problems are particularly acute is in ship-board airconditioning syste~s. These l'mobilei' systems must have a design capacity which will deal with large ranges of sensi~le heat variation, particularly in the case of ocean-going vessels which often txaver~e both tropical and high latitudes.

; 5 .

~31l~38 31~RY OF T~ I~r~IO
The pres~nt inverltion rela~es to a metho~, ar ~ppara~us and a sul~-assembly for enhanoing the operating rang~
of vapour compre~ on ~ycle ~efrigeration systems.
According~y, there i~ provided a method of operating a vapour c:omp~e~slon cycle re~rig~ration sys tem comE~rising an evaporator, co~pressor, con~lens6~r a~ expansion valve, and ~url~her including compressor unloading means in combination wit~ hot-gas b~pas3 rQeans op~rable during the ~inal compr~r wlloading stage ~or compen~ating for imbalances b2tween the e~apc)rator's and the compre~sor's respeo~iYe cooling capacities under low-load operating conditiQns ~ w~erein the meth~d comprises the step o meter~ ng a ~low o:e ho~as through ~aid by-pas$ means while the compressor is still ~ub~tantially loaded, wherel3y re6ulting vapour injeCtion into the di~'crib~tor increases th~3 re~ri~e~an~ velocity through the ~aporator to thereby a~sis~ in ~aturning ~il tQ ~he c:ompressor .
In addl~ion to iTnproving oi~ ~sturn, ~his method has tne Purther adv~ntag~3 of helpiny to ~3nsure more e~ual di~tribution o~ the hot ga~ to each circuit o~ a mul~i-~ ircuited evaporator. Morec~Yer this lnethod al~3o help~; to increa~e the amoun~ o~ e~EIporator sur~ace that is ac~iv~, an~
th~r~by ~ids in air de~umidif ic~tion ev*n ~hile the system i5 operatirlg under low-~oading ct~r~dition~. Pr~ra1~ly the hot-is metered throug~ the by-pass means in accordance with the above method, while the con~presso~ is ~t~ 11 fully lo~ded~

:

~-~s~

Additionally, there is provided a method for combining at least two discrete flows of a refrigerant in substantially dissimilar thermodynamic states in a vapour compression cycle r~frigeration system, including the step of imparting substantial turbulent mixing of the at least two flows to produce a generally thermodynamically uniform admixture thereof.
In one aspect, ~he method i5 intended for use in a gas/liquid refrigerant mixing stage of a vapour compression cycle refrigeratlon system, and comprises the steps of imparting a substantial helical motion to a first flow of fluid xefrigerant in one thermodynamic state, and mer~ing tha first flow with a second flow of fluid refrigerant in a dissimilar thermodynamic state. q'he helical m~tion of the first flow results in sub~tantial turbulent mixing of the first and second flows upon merging thereof, to produce a generally thermodynamically uniform admixture. In practice ; these refrigerant flows may be discrete coaxial flows at the point of mixing. Accordingly the present invention includes a method substantially as set forth hereinabove, comprising the steps of impartin~ a substantial helical motion to the fixst, axial flow of fluid refrigerant, and merging it with the second, coaxial flow.
Preferably the first/ axial flow of fluid re~rigerant has a substantial gaseous component, and the second, aoaxial flow o~ ~luid refrigerant has a substantial liquid co~ponent.

3 ~
The method of the present invent.ion finds application in the so-called hot-gas condenser by-pass systems ment.ioned hereinabove. In one such embodiment the first axial flow comprises a hot-gas condenser bypass flow betwe~n high and low pressure sides of the vapour compression cycle refrigeration system, which flow is directed through an outer annular channel in a multicircuited evaporator distributor.
The second coaxial flow comprises an expanding liquid flow exiting from a thermo~tatic expansion valve located upstream of the distributor, whi~h second flow is dirscted through a cylindrical channel located centrally witnin the outer annular channel of the distri~utor. In accord~nce ~ith this embodiment of the invention, the first flow is passed through flow redixecting means arranged within the annular channel.
The first flow is thereby imparted with a sub~tantial helical motion, and eXit~ the annular channel and merge~ with the ~econd flow as the two flows exit their respective cnannels into the distribution manifold of the distributor.
Substantial turbulen~ mixing of the two flows takes place and results in the formation of a subs~antially thermodynamicalIy uniform mixture thereof.
Preferably the flow redir~cting means imparts a helical ~otion to the first flow that is substantially normal to the outl~ts of the ~istributor.
Also prefer~bly, the helical motion of the ~irst flow comprises a plurality of coaxial helical paths.
The pres~nt invention also relate6 to an apparatus comprising in-line re~rigerant flow redirecting means having a 3~

plurality of vanes adapted ko be di~posad in the path of a generally linear refri~erant flow in a vapour compression cycle refrigeration ~ystem. The ~low redirsctin~ means is pre~erably a static de~ic~, operable in situ to r~irect the linear.flow into a non-linear ~low, where~y down~tream thermodynamic uniformity of ~he ~low is incr~a~ed.
As with ~he ~o~e ~escribed method, the a~paratus of the present invention has applicatio~ where two flows of ~efrigerant are to b~ commingle~. ~n this a~pect o~ the pre~ent inve~tion the flow redirecting mean~ is di~po~ed in the path o~ a ~ir~t generally linear ~low in a first th~rmodynamie state, at a location generally upstrea~ o~ a point at which a second gene~ally linear flow of refrigerant in a second t~ermo~ynamic state, is introduced ~hereto. trhe ~5 ~low redirecting means is operable in si~u ~o ~edlrect the irst linea~ ~low into a non~ ear ~low, ~o thereby pro~ucs turbulent ad~ixin~ o~ ~he first and second flows ~t the po}nt wher~ the two ~lows ar~ brought ~o~eth~r.
Pre~erably tha vanes are arr~nged so as to impart a s~betantially helical, ~ie non-linear) ~l~w to the first flow.
In a pr~ferred em~odiment of the present invention the f3OW redirecting msans comp~ises ~ di~c adapt~d to be arra~ge~ w~th ~ha plane of said disc n~r~al to ~he dirsc~ion : o~ tha ~irst ~low. A plurality o~ radi~lly ~xtending slots in : 25 the disc de~ine r~spective sur~ace portion~ o~ the di~c betwe~n adjacen~ pairs o~ the ~lots and an edge o~ the disc.
~ch s~ch sur~ace porti~n has a ~oot end attachad to t~
balanc~ of the disc at an an~e adjacent that ro~t ~nd and ~ 3 ~

relative to the plane o~ the disc so as to provide vanes adapted to i~part substantially helical, non-linear flow to the first flow.
In one aspect of ~he pre6ent invention it is contemplated that the disc incl~de resilient root portions, and that the vanes are resiliently biased at said angle, in a first position, and are deflectable into a plurality of other positions on flexion of the root portion caused by the flow of refrigerant past the disc. ~his has the advantage of maintaining a more constant helical flow velocity, by creating what i6 in effect a variable venturi between respective leading and tralling edgea of adjacent pairs of vanes.
Where the first and second flows are coaxial, the flow redirecting means is preferably adapted to accommodate said second flow through an aperture i~ the center o said disc. In one embodiment of this aspect of the present invention, the aperture is adapted to receive a tube for conducting the second flow thereth~ough.
In accordance with yet another aspect of the present invention, there is provided a refrigeration sub-assembly ~ including a multicircuited evaporator distributor adapted to : be arranged in a refrigerant flow ~nd flow redirecting means positioned upstream of the distributor. The flow redirecting means is adapted to introduce a non-linear flow of refrigerant into the distributor to improve the uniformity of di~tribution of refrigerant exiting through the outlet of the distri~utor.
In one e~bodiment this aspect of the in~ention includes a side connector for receiving a first flow of hot gas condenser 1~

, ~ 3 ~

~ypass refrigerant and entraining within the first flow a second, coaxial flow of refrigerant from an expansion valve located upstream of the side connector. In this embodiment the flow redirecting means is disposed intermediate the distributor and the side connector, and is o]perable therebetween to produce a non-linear flow of the hot gas condenser bypass rerigerant around the seco:nd coaxial ~low.
~he flow redirecting means in the sub assembly preferably comprises a plurality of vanes adapted to be disposed in the path of the first flow of said hot gas condenser bypass refrigerant. The vanes are preferably arranged so as to impart a substantially helical, non-linear flow to said first flow~ As with the above described apparatus, the f low redirecting means preferably comprises a : 15 disc adapted to be arranged with the plane of the disc normal to the direction of flow of the firs~ flow. The disc has a plurality of radially extending slots defining respective ~urface portions of the disc between adjacent pairs of the slots and ~n edge of the disc~ Each such surface portion has a root end attached to the ~alance of the disc, and each surface portion is angled adjacent the root end and relative to ~ne plane of the disc so a6 to provide vanes adapted to impart substantial helical, (ie non-linear), motion to the fir~ flow.
; 25 As before the disc is preferably adapted to accommodate the seaond flow through an aperture in the center of the disc. That aperture is, in one embodiment, adapted to receive a side connector tube for conducting the second flow.

3 ~

In any case, a pre~erred su~-a~sembly of the present invention i6 adapted to produc~e a helical flow which is substantially n~rmal to the outlets of -the distri}~u~or, and in particular a heliaal flow which compri~es ~ plurality of 5 coaxial helical paths is espec~ally pre~erred. In one embodiment, tne helical flow compr.ises seven such helic~l paths .

~l~eA~ D DE~ lOlR OE~ F~15D ~l~I~NT
1~ Intxc~ducti~2n ~;:o thç ~awings Figure 1 o~ the drawlngs appended h~re~o is a schema'cic cross-section throu~h a vapour compression cycle r~rigeratlon ~yst~m.
Fi~re 2 o~` ~he drawing~ is ~ explode~l p~rspective 15 ~riew o~ a subas~e~bly including means ror impartin~ helica:L
ms:~tion to a f irst axial f low ln R hot-~a~ ~pass sy~tem .
F t gure 3 shows ~n alterna~ive em:~odiment o~ the hellc~l motion i~parting me~ns depict~d in Flqure 2.

Re~erring now tc~ Ure 1 of the ~Iraw.ing~;, ther~ is shown in partial cro6s-æ~c~ion, a ~chematic: repre~n~ation of a v~pour compress~on refri~eratiorl system which i~ equipped wi~h ~ hot-ga~ bypas~ line, 1, connec:~ed at one end thereof la, to the high pr~isure side o~ the ~ystem, between the Gcsmpressc~r 2~ and ltne cc~ndenser 3. In keeping with known prac~ics~, the connection o end la to the hi~h pre~:sure side o~ tne sy~tem is p~a:Eera~ly as close to the compres~;or as pos~;ibleO ~e hot-gi3~; lbypass line i~3 aonnected at itæ oth~r ~ 3~3~

end, lb, to the low pressure side oE the system, through a side connector to a thermostatic expansion valve/ distributor sub-assembly, 4. A metering valve 5, including an external pressure equalizing line 6 arranged between valve 5 and the vapour collection manifold 7 of evaporator ~, is connected intermedlate the two ends~ la and lb, of byE~ass line 1, preferably as close to end la as possible, in keeping with known practice in the art. Valve 5 is operable, to control the flow of hot-gas that bypasses condenser 3 in response to pressure drops across the system.
Subassembly 4 includes a thermostatic expansion valve 9 and a side connector/distributor subassembly 12.
Expansion valve 9 is connected downstream of the condenser 3 and is adapted to receive and meter refrigerant flowing from condenser 3 to evaporator 8. Subassembly 4 is connected in known manner to the vapour collection ~ani~old 7, througn an equalizer line 10 and through a temperature sensing element/bulb subassembly 11.
Referring now to Figure 2 of the drawin~s, there is shown an exploded, partially cut-away perspective view of side connector/distributor sub-assembly 12, which is shown in cross-section in Figure 1, and which is connected to receive hot-gas metered through valve 5. In operation, the hot-gas flow, indicated by arrow ~ in Figure 2, exits tube end lb, and enters an annular chamber 13 whase only exits are per~orations ~ 14 ~ormed in the septum plate 15 that is located at one end o~
:~ chamber 13, downstream of tube lb. The hot-gas leaves the chamber 13 through the per~orations 14 and enters an outer ~3~3~
.
annular channel 16. Disc 17 is adapted to be arranged with the plane of the disc normal to the direction Qf the axial flow of the hot-gas w.ithin ~he outer annular channel 16. A
plurality oE radially extending slots 21 in the disc 17 define respective surface portions of the diæc between adjacent pairs of the slots and an edge 22 of the disc. E~ch slot 21 is generally "~" shaped, having a first/ radially extending portion and a second portion extending substantially normal to the first and generally parallel to the outer edge 23 of the disc. Each such surface portion has a root end attached to the balance of the disc. The surface portions are angled adjacent their respective root ends and relative to the plane of the disc so as to form vanes 18 adapted to impart a ~ubstantially helical flow pattern to the first flow~ Di~c 18 is located within passage 16 in in-line refrigerant flow-diracting relation in the pa~h of the g~.nerally linear hot-gas ~eXrigerant flow exiting from the septum plate 15. ~he disc redir~cts the linear flow o~ hot-gas refrigerant into a helical (ie non-linear) flow pattern. The disc i5 formed to receive tube 1g through a hole in the center o~ the disc.
~ he flow of saturated liquidJvapour re~rigerant, indicated by Arrow B, exits the thermostatic expansion valve 9, enters tube 19 at the base thereof, and travels coaxially relative to the flow of hot-gas in outer annular channel 16, upwardly int~ t~e interior of the distributor body 20. In the distributor ~ody 20 the generally linear flow ~'B" mixes with the helical fl~w of the hot-gas as the two flows exit tube 19 and chamber 16, respectively. Th~ ~ixing of the two flows in ~3~3~

their respec~ive, di~ferent thermodynamic states, due to the helical flow pattern imparted to the f irst f low o~ hot-gas by disc 17, improves ~he uni~or~ity o~ the admixtur~. Thi5 in turn helps to reduce the risk of locali2atio~ of unevap~rated refrigerant within the various evapQrator circuits and tha various pro~lems that ensue under such undesirable conditions.
Moreover, ~he ~aten~ heat o~ the hot-gas is w~ll distributed and this helps to avoid li~uîd re~rigerant ~eing returned to the compr~s~or.
Figure 3 of the drawing~ illuetra~es ~n al~ernakiv~
embodimQnt of the disc 17 shown in Figure 2. D.i~c 24 in Figure 3 c~mprises a disc in which adjac~nt pairs o~ sl~ts extend radially inwardly ~rom th~ outer edge 26 of ~he disc 24, and defina batween them, surface portions eaah havinq a root end at~ach~d to the ~alance of the disc. These sur~ace p~tions are angled ~d~acent their respecti~e root cnds and relative to the plan~ of the disc so as to form vanes 27 a~apted to impart a s~bst~ntially helical flow pattern to ~he ~irs~ f~ow. In ~his em~ nt a p~rtion o~ ~he outer edCJe of eacll surface portion, indicated at referenc~ numeral 28, i.
rem~ved to i~cr~a~e ~he æiz~ of the passag~ ~ormed betwe~n respectiYe le~ding and trailing adge~ o~ adjacent vanes 270 In accordance ~i~h one c~ntempla~ed arrange~ent o~ the present invention ~he dis~ 17 and 24 are used in a tandem mutually ~pacQd-apar~ arrang~ment w.i~hin t~e ohannel 16, pre~erably ; ~ith di~c l~ po~itioned ups~ream in the re~rig~rant ~low, relative to disa ~4,

Claims (28)

1. A method for use in a gas/liquid mixing stage of a vapour compression cycle refrigeration system, comprising the steps of imparting a substantial helical motion to a first flow of fluid refrigerant in one thermodynamic state, and merging said first flow with a second flow of fluid refrigerant in another dissimilar thermodynamic state, whereby the helical motion of the first flow results in substantial turbulent mixing of the first and second flows upon merging thereof, to produce a generally thermodynamically uniform admixture.

2. The method of claim 1 comprising the steps of imparting a substantial helical motion to a first, axial flow of fluid refrigerant in one thermodynamic state, and merging said first flow with a second, coaxial flow of fluid refrigerant in another dissimilar thermodynamic state.

3. The method of claim 2 wherein the first, axial flow of fluid refrigerant has a substantial gaseous component, and the second, coaxial flow of fluid refrigerant has a substantial liquid component.

4. The method of claim 3 wherein the first, axial flow comprises a hot gas, condenser bypass flow between high and low pressure sides of the vapour compression cycle refrigeration system, which flow is directed through an outer annular channel to a multicircuited evaporator distributor, and the second, coaxial flow comprises an expanding liquid flow exiting a thermostatic expansion valve located upstream of the distributor, which second flow is directed through a cylindrical tube located centrally within the outer annular channel of the distributor, and wherein the first flow passes through helical-flow-imparting flow redirecting means arranged within the annular channel and is thereby imparted with a substantial helical motion, and the first flow exits the annular channel and merges with the second flow as the first and second flows exit their respective channels into a distribution manifold of the distributor, where substantial turbulent mixing of the two flows takes place and results in a substantially thermodynamically uniform mixture thereof.

5. The method of claim 4 wherein the flow redirecting means imparts a helical motion substantially normal to the outlets of the distributor.

6. The method of claim 4 wherein the helical motion comprises a plurality of coaxial helical paths.

7. The method of claim 4 wherein the helical motion comprises seven helical paths.

8. An apparatus comprising in-line refrigerant flow directing means having a plurality of vanes adapted to be disposed in the path of a generally linear refrigerant flow in a refrigeration system, and operable in situ to redirect said linear flow into a non-linear flow whereby the thermodynamic uniformity of the flow is increased;

said means being disposed in the path of a first generally linear flow in a first thermodynamic state, at a location generally upstream of a point at which a second generally linear flow of refrigerant in a second thermal dynamic state, is introduced thereto, said means being operable and situ to redirect said first linear flow into a non-linear flow to thereby produce turbulent admixing of said first and second flows at said point;

said vanes being arranged to impart a substantially helical, non-linear flow to said first flow;

said means comprising a disc adapted to be arranged with the plane of said disc normal to the direction of flow of said first flow and having a plurality of radially extending slots in said disc defining respective vane surface portions of said disc between adjacent pairs of said slots and an edge of said disc, each such surface portion having a root end attached to the balance of said disc, at an angle adjacent said root end and relative to said plane of said disc so as to be adapted to impart said substantially helical common non-linear flow to said first flow.

9. A refrigeration subassembly including a distributor adapted to be arranged in a refrigerant flow and means positioned upstream of said distributor and being adapted to introduce a non-linear flow of refrigerant into said distributor to improve uniformity of distribution of refrigerant exiting through the outlet of said distributor and further including: a side connector for receiving a first flow of hot gas condenser bypass refrigerant and entraining within said first flow a second, co-axial flow of refrigerant from an expansion valve; and wherein said means is disposed intermediate said distributor and said side connector, and is operable therebetween to produce a non-linear flow of said hot gas condenser bypass refrigerant around said second co-axial flow.

10. A method of operating a vapour compression cycle refrigeration system comprising an evaporator, compressor, condenser, and expansion valve, and further including compressor unloading means including hot gas bypass means operable as a final compressor unloading step for compensating for imbalances between the evaporators and compressors respective cooling capacities under low load operating conditions, wherein the method comprises a step of metering a flow of hot gas through said bypass means while the compressor is still substantially loaded, whereby resulting vapour injection into the distributor increases the refrigerant velocity through the evaporator to thereby assist in returning oil to the compressor; and further including the step of imparting substantial turbulent mixing of the hot gas bypass flow with the flow of refrigerant from the condenser at a point downstream of the condenser to produce a generally thermodynamically uniform admixture thereof, wherein a substantial helical motion is imparted to the bypass flow of hot gas, which is then merged with the flow of refrigerant exiting the expansion valve, whereby the helical motion of the hot gas flow results in substantial turbulent mixing of the hot gas and expanding refrigerant flows upon merging thereof, to produce a generally thermodynamically uniform admixture.

11. The sub-assembly according to claim 9 wherein means comprises a plurality of vanes adapted to be disposed in the path of said first flow of said hot gas condenser bypass refrigerant.

12. The sub-assembly according to claim 11 wherein said vanes are arranged so as to impart a substantially helical, non-linear flow to said first flow.

13. The sub-assembly according to claim 12 wherein said means comprises a disc adapted to be arranged with the plane of said disc normal to the direction of flow of said first flow and having a plurality of radially extending slots in said disc defining respective surface portions of said disc between adjacent pairs of said slots and an edge of said disc, each such surface portion having a root end attached to the balance of said disc, and being angled adjacent said root end and relative to said plane of said disc so as to be adapted to impart substantially helical, non-linear flow to said first flow.

14. The sub-assembly according to claim 13 wherein said disc is adapted to accommodate said second flow through an aperture in the center of said disc.

15. The sub-assembly according to claim 14 wherein said aperture is adapted to receive a side connector tube for conducting the second flow.

16. The sub-assembly of claim 15 wherein said helical flow is substantially normal to the outlets of the distributor.

17. The sub-assembly of claim 16 wherein the helical flow comprises a plurality of coaxial helical paths.

18. The sub-assembly of claim 17 wherein the helical flow comprises seven helical paths.

19. The method of claim 10 comprising the steps of imparting a substantial helical motion to a first, axial bypass flow of hot gas, and merging said hot gas flow with a second, co-axial flow of expanding refrigerant exiting the expansion valve.

20. The method of claim 19 wherein the axial flow of hot gas has a substantial gaseous component, and the coaxial flow of refrigerant has a substantial liquid component.

21. The method of claim 20 wherein the flow redirecting means imparts a helical motion substantially normal to the outlets of the distributor.

22. The method of claim 20 wherein the helical motion comprises a plurality of coaxial helical paths.

23. The method of claim 20 wherein the helical motion comprises seven helical paths.

24. The apparatus according to claim 8 including resilient portions and wherein the vanes are resiliently biased at said angle in a first position, and are deflectable into a plurality of other positions on flexion of the root portion caused by the flow of refrigerant past the disc.

25. The apparatus according to claim 8 wherein the disc is adapted to accommodate said second flow through an aperture in the center of said disc.

26. The apparatus according to claim 25 wherein said aperture is adapted to receive a tube for conducting the second flow therethrough.

27. A method of operating a vapour compression cycle refrigeration system comprising: an evaporator, compressor, condenser, and expansion valve, and further including compressor unloading means, including hot gas bypass means operable as a final compressor unloading step for compensating for imbalances between the evaporators and the compressors respective cooling capacities under low load operating conditions, wherein the method comprises a step of metering a flow of hot gas through said bypass means while the compressor is still substantially loaded, whereby resulting vapour injection into the distributor increases the refrigerant velocity through the evaporator to thereby assist in returning oil to the compressor; wherein a first, axial hot gas condenser bypass flow between high and low pressure sides of the vapour compression cycle refrigeration system, is directed through an outer annular channel to a multi-circuited evaporator distributor, and a second co-axial flow comprises an expanding liquid flow exiting a thermostatic expansion valve located upstream of the distributor, which second flow is directed through a cylindrical tube located centrally within the outer annular channel of the distributor, and wherein the first flow passes through helical flow imparting flow redirecting means positioned within the annular channel and is thereby imparted with a substantial helical motion, and the first axial flow exits the annular channel and merges with the second flow as the first and second flows exit their respective channels into a distributor manifold of the distributor, where substantial turbulent mixing of the two flows takes place and results in a substantially thermodynamically uniform mixture thereof.

28. An apparatus comprising in-line refrigerant flow-directing means having a plurality of vanes adapted to be disposed in the path of a generally linear refrigerant flow in a refrigeration system, and operable in situ to redirect said linear flow into a non-linear flow whereby the thermodynamic uniformity of the flow is increased; wherein said means is disposed in the path of a first generally linear flow in a first thermodynamic state, at a location generally upstream of a point at which a second generally linear flow of refrigerant in a second thermodynamic state, is introduced thereto, said means being operable in situ to redirect said first linear flow into a non-linear flow to thereby produce turbulent admixing of said first and second flows at said point; and, wherein said vanes are arranged so as to impart a substantially helical, non-linear flow to said first flow.