Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2 -
1 ~5~06~3
This invention rela~e~ to improvements in the
control of vapour compression cycle refrigeration
systems.
The problem of lack of stability in refrigeration
systems controlled by a t-hermal expansion valve (TX valve)
ha~ been the subject of many papers and experiments since
this form of control was introduced. For exa~ple in
"The Journal of Refrig~ration" Vol. 6 No. 3, the following
statement is made:
"In the development of automatic refrigeration the
thermostatic expansion valve has playled a vital part in
the past and continues to do so still. As a means
of regulating the flow of refrigerant into an evaporator
to equal the rate at which vapour is pumPed out by the
compressor without demanding a large evaporator charge
as does the low-side float control and without being
unduly sensitive to total charge as is the high-side
float control, it is still the preferred method for
commercial and much industrial plant. Recent years have
seen the adoption-of the thermostatic valve in larger
sizes and it is possible that this trend will continue.
Nevertheless it must be admitted that the TX valve
is not always the most efficient method of using evaporator
surface. In principle it can be and often is efficient
but there are many examples of its use in which th'Ls is
not so~ Under ideal operating conditions the valve
should admit juat the right amount of refrigerant which
can be evaporated and slightly superheated, then the -
evaporator should be wetted to the maximum extent with a
correspondingly good heat transfer rate. (Though even under
these ideal conditions it is not always realized how much
evaporator surface is needed to provide the n~rmal suparheat.)
~t tha ot~er e~treme when the valve is limit-cycling nr
huntin~ ~etween its ~ully open and ~ully clo~ed position~
~he evaporator i~ completely wetted for pa~t of ~he time and
~tarved ~rom the remainder. The period of ~ull Wetting
~oe~ not compensate for the pexiod ~ starvation and poor
overall heat transfer is ~he re~ult. ~ a tima when
~.
1 156~0
intensive efforts are being rnade to improve the rate of
heat transfer to boiling refrigerant it seems that means -
of improving the evaporator feed should be investigated
al~o, 6ince any improvement obtained in one might be
nullified by carelessness about the other".
The article from which the above two paragraphs
were taken was written in 19~3 but the same problems
still exist. (See "Refrigeration and Air Conditioning"
February 1979 at Page 42 and ~nore recently "Transactions
of the A.S.M.E." Volume 102 June 1980 at Page 130.
This latter article proposes a mathematical model to
describe the hunting of evaporators controlled by a
thermostatic expansion valve but ~oes not propose any
solution other than the technicians field solution of
insulating the temperature sensing bulb fxom the
evaporator tube wall by one or more layer~ of insulation
tape. This solution ~ ds to negate ~h~ advantages the TX valve has
Qver other si~pler devioe. This is despite a considerable amDu~t
of research aimed at determining the criteria governing
the stability of vapour compression cycles (V.C.C.)
systems and particularly those-systems controll~d by the
widely used Thermostatic Expansion Valve (TX valve).
Stoecker, Danig and others have analysed the
stability problem using control theory techniques ~see
(1) "Journal of Refrigeration" Volume 6 No. 3, May/June
1963 pp 52-55;
(2) "ASHRAE Transactions" Volume 72 Part 11 pp lV3~1 to
3.7;
(3) "ASHRAE Transactions" 1971-72 pp 80 to 87).
Stoecker also looked at the behaviour o~ the refrigerant
inside the evaporator and at the motion of the transition
point. (see (4)"ASHRAE Transactions" Volume 72, Part 11,
pp lV2.1 to 2.15; and
(3) I'A~HRAE Transactions" 1971-7~ pp 80 to 87)~
Thi~ he define a~ being ~he p~itlon in the evaporAtor
where ~he last of the liquld is vapoxised and i~ the
boundary between the two-phase xegion and the supe~heated
reyion. The conclusions reached usin~ contx~l theory are
115 130~0
not numerically precise but nevertheless they show what
con~ination of characteristics is mo6t likely to give
stability, for example, the effect of time lags is
demonstrated as is the effect of varying the gain of
the TX valve.
Heulle (I'Proceedings ~f Industrial~
Congress of Refrigeration" 1967 Volume 3.32, 3.33 pp 985
to 1010~ and others have taken a different more
empirical approach. Like Stoec~er, Heulle investigated
the motion of the transition point but he formed the
conclusion that stability can be achieved by sizing and
adjusting the TX valve so that the transition point never
reaches the position where the bulb is located.
The following observations can be made based on
work leading to the present invention:
1. The importance of preventing the transition point
~rom going past the exit of the evaporator and reaching
the location of the bulb can, in practice, be seen.
However, as shown by Stoecker and Danig, this is not the
only criteria for stability and therefore, a system with
the TX valve sized accordingly to Heulles' recommend-
ations may not always be stable.
2. If a system is controllable and is thus within the
limits defined by Stoecker and ~anig, then Heulles'
methods for sizing the TX v~lve appears applicable.
3. Hunting results in wide variations in evaporator
saturation pressure/temperature (see Figure 10) but this
has been ignored to simplify the system~ for the purposes
of analysis, in all the ~or~ carried out in the references.
This m~y have resulted in a considerable underestimation
of the problem as when the system i9 huntiny (i~e.
un~able~ variations in ~aturation ~evaporator) temperature~
pressure can be ~hown to add appxoximately 5~% to the
total amplitude of the ~uperheat oscillation~.
It is therefore an object of the pxes~nt invention
~o provide a refrigeration sy~tem which will obvia~e ox
minimize the h~ltLng or oscillatin~ problems descrihed
l ~ s~o~o
above and will improve the stability and controllability
of the system or which will at least provide the public
with a useful choice.
The invention is primarily for use in V.C.C.
systems controlled by the "Thermal Expansion Valve"
(TX valve). It is however of equal ~se in systems
controlled by any form of expansion valve in which one
of the measured variables is the temperatureor vapour ~ess at
~e ~stream end,of the evaporation zone. ~lerefo~ in the
following description the term "TX val~e" should be
und~stood to include any expansion valve.
Accordingly the invention consists in a
refrigeration system including an evaporator controlled
by an expansion valve having means for sensing the
temperature at the downstream end of the evaporator,
characterized by means for injecting wet vapour at a
rate which is a function of the rate ~f flow of refri~erant
through the expansion valve into said evaporator upstream
of said temperature sensing means.
20 ~ In one embodiment of the invention a wet vapour
by-pass line is connected to the evaporator between a
position immediately downstream of the expansion valve
and a position immediately upstream of the thermal
sensor. In another embodiment a similar wet vapour
by-pass line is provided between a position immediately
downstream of the expansion valve and ~ position a
predetermined distance upstream of the thermal ~ènsor
so that the wet vapour entering ~he evaporator from the
by-pass line is heated by the evaporator sur~ace
before reaching the thermal sensor.
Notwithstanding any other forms that may fall
wlthin i~ scope onq preferred form of ~he invention and
vaxiatlonq thereof will now be described with re~erence
t~ the accompanying drawings in which:
Fi~. 1 i8 a dia~ramatic vi~w o a 8tandard
vapour compr~s~lon cycle refrig~ration ~ystem,
Fig, 2 is ~ diagramatic vi~w of a TX valve
and evaporator with ~ wet vapour by-pass lin~ according
1 ~60~0
to one preferred fo~n of the invention,
Fig. 3 is a diagramatic view similar to Fig. 2
showing wet vapour injection into the evap~rator., some
distanc~ upstr~am of the temp~ra.ture sensor.
Fig. 4 is a diagramatic view of a TX valve and
an evaporator according to the invention showing a
modification using the pressure equaliser line
as the wet vapour injection lin~,
Fig. 5 is a partially cut away cross-sectional
view of a TX valve having a bullt in by-pass to enabl.e
the equaliser line to be used in the configuration shown
in ~ig. 4,
Fig. 6 shows a wet vapour injection system used
to obtain proportional and derivative control of the
15 TX valve,
Fig. 7 shows an.evaporator and TX valve with
positive ~eed back, (hot gas injection)
Fig. 8 shows a hot gas injection system used to
obtain proportional and integral action,
2n Fig. 9 a system with modifications giving
proportional, integral and derivativE. action,
Fig. 10 is a chart showing the hunting action
of a normal TX valve controlled refrigeration system
and,
2~ Fig. 11 is a chart showing the performance of a
system having the wet vapour injection shown in ~ig. ~.
In a normal refrigexation system controlled
by a thermostatically controlled expansion valve ~TX valve)
the system comprises a compressor 1 driven by a motor
2 for example an electric~motor provided with power through
wires -3 from a control box 4. The compressor draws
refrigerant from an evaporator 5 through a suction line
6 and pumps the refrigerant at increased pressure thxough
a conden~or 7 to a liquid xeceiver 8 ~rom where it passes
~5 through line 9 to a ~ilter dryer 10~ The xe~rigerant
thcn p~e9 at a controlled ra~e through a TX valve
11 lnto the ~vaporator 5. The TX valve is controlled
by ~vaporator pre~sure ~which is ~.r~kl-y r~lativ~ ~o
-the evapora~.iQn temperature) and al~o by .the temperature at
, 115~0~;0
the evaporator outlet sensed by temperature sensing bulb
12 and fed as a p~ssure si~nal to the_TX valve through line 13. The
motor 2 may also be controlled by a thermal element
14. As this system is well known the modifications thereto
which comprise the invention will be de~cribed below with
reference solely to the components comprising the TX valve
ll the evaporator 5 and the temperature sensing bulb 12.
The basis of the invention is the utilization of
a TX valve sensor and in particular the bulb 12 ~s a
summing device, the temperature which the sensor detects
having been increased or decreased by a controlled amount
which is dependent on the flow through the TX valve.
Thus the temperature which, say the bulh detects, i6 altered
such that it becomes the evaporator exit temperature -
some alteration "A". (See Fig. 11)
The magnitude of the Alteration "A" i5 arranged
to be a function of the flow throughthe TX valve, "F"
and therefore A = f(F~. If this is done a closed loop
is created and the input signal to the TX valve is now the
original input signal - f(F). As Flow "F" is the output
- from the TX val~e then the input signal can be described
as: the original 'true' input signal - feedback. If "A"
is also made a function of time 't' i.e. A = f(F,t) then
we have time dependant feedback.
Thus control of a refrigeration system can be improved by
m~ng ~he buIb's signal to the TX valve ~ he im~ified ~ignalplus A and~
1. Incorporating negative feedback i.e. A ~ -~(F);
2. Incorporating positive time depend~nt feedback,
i.e. A = f(F,t), arranged to give integxal action;
or 3. Incorporating negative time dependant feedback,
i.e. A = -f(F,t), arrany~d to give derivative
action.
or a combination of tha three.
Ba~ic~lly ~1) improve~ linearity and enables the galn to
b~ e~sily adjusted ~2) works to eliminate th~ o~set
inherent in proportional-only controllers, and (3) gives
increased xesponse tQ rapid chang~s in input. Strai~ht
positive eedback (A~ is not likely to be used a~ the
maxlm~n gain can be a~nged to be ~xn~ the anticipa~d~ra~
-- 8 --
1 15~0f~)
~aximum by the correct selection of controller (TX valve)
components.
In a TX valve controlled system a negative "A"
applied to the exit of the evaporator will ~,ive negative
feedback as the measured degree of superheat will be
reduced by "A" which i5 a function of th~ flow. Thus
the opening of the TX valve, and~ therefore the flow,
~ill be reduced by an amount proportional to the flow.
In the first and simplest embodiment of the
invention as shown in Fig. 2 wet vapour injection is
used to provide negative feedback to contol the gain
of the TX valve. This is achieved by providing
a wet vapour by-pass line 15 between the inlet to the
evaporator at a point 16 just downstream o the TX valve
11 and a point 17 at the downstream end of the evaporator
5 and just upstream of the TX valve sensor bulb 12. The
flow rate through the by-pass line 15 can be controlled
by a regulating valve 18. A restrictor 19 is preferably
placed just downstream of the junction 1~ to make the pressure
in the by-pass injection line 15 respond primarily to
the flow through the TX valve itself. In many systems
a suitable restrictor is present in the form of the distributor.
Alternatively a "pitot tube" or upstream facing type of
pick-up may be used at junction 16.
Thus wet vapour is injected just upstream of
the bulb 12 and the temperature at this point is altered
accordingly. The amount of vapour injected is a
~unction of the flow through the TX valve and therefore
A = -f(F) which, as stated previously, gives a form ofnegative
feedback. The volume enclosed by the restrictor, the
TX valve and the injection control valve should be kept
~o a minimum, ~o keep time lags as small as possible.
The injected wet vapour ~s th~ benefi~ial ~lde
~ffec-t ~ reducing 1uctuations in, and lowerin~, the
~uction (rom the evaporator) gas superheat. The point
of injection should be f~r enou~h upstream o~ the bulb to
~llow complete mi~in~ and maximis~ the ef~ect~ di~cussed abova.
If ~he inj~ction point is close to the bulb, only a minute
1 1 5 ~
amount of injection is required as there is considerable
local chilling of the tube walls near the injection point,
although the gas temperature after mixing will be hardly
altered.
As the amount of heat needed to change the
fiuperheat of a refrigerant is relatively much smaller than
the latent heat of vapourisation, only a very small
amount of refrigerant need be injected to alter the
evaporator exit temperature.
The injection of wet vapour into the superheated
gas leaving the evaporator chill5 the walls of the pipe
work to well below the temperature attained after mixing
is complete. This phenomenon which is cau~ed by the evaporating
wet vapour being forced by the gas out into the tube walls
(annular flow) increases the ability of the region immediately
downstream of the in~ected point to pick up heat from the
heat source. Therefore by injecting into ~he latter part of
the evaporator itself,preferably downstream of the wet vapour t~
superheat ~r~sition poin~ changes in heat input to the
evaporator are quickly detected by the bulb which is located
at the downstream end of this zone. This is achieved as
shown in Fig. 3 by joining the wet vapour by-pass line
20 with the evaporator 5 at a junction point 21 in the
evaporator which is upstream from the downstream end of
the evaporator but is ~ally downstrea~ frcm the tranfii~ion point
which may fox example be located in the region 2~. The
- 1OW rate through the by-pass line 20 i~ again controlled
by a valve 23. As in the configuration shown in ~ig. 2 the
bulb 12 is effectively being used as a s~unming device. If
the modificakion shown in Fiy. 3 is used then the
temp~xature detected by the ~ulb is the evaporator exit
temperatur~ plus a f~edback component, plUB a heat input
~c~mpoTIen~ (Xrom the portion of the e~rapoxator between ~he
jun~ion point 21 and the bulb 1~).
r~'hi~ modifi~ation also ~eeks to counteract the
'inversed' si~nal which is re~eived by the T~ v~lv~
immediately a~ter a rapid change in hea~ input. ~hi~
-- 10 --
t l 5~06~
effect is caused by the s~turation temperature/pressure
chanc3ing much faster th~n the tempe.rature at the exit
of the evaporator. Thus after, say, an increase in heat
input, the saturation temperature/pressure (detected through
the equaliser line ~4) rises before the evaporator exit
temperatùre (detected by the bulb) and the TX valve sees
a fall in superheat. Initially~ there~ore, until
.the evaporator exit temperature also rises, the TX valve
closes instead of opening. By making the levaporator exit
temperature more responsive to heat input, the configuration
shown in Fig. 3 can be seen as to oppose t'his effect and
reduce it to a more acceptable level.
Although the invention described with reference
to Figs. 2 and 3 has shown a separate wet vapour by-pass
line (15 or 20) it is possible to achieve the same effect
by using the equalizer line 24 to feed wet vapour at a rate which
is a func~lon of the flow rate through the TX valve,.into
the evaporator upstream from the sensing bulb 12. This
configuration can be seen in Fig. 4 where the equalizer line
25 has been rerouted to enter the evaporator at a junction
poin,' 26 just upstream of the bulb 12 (rather than downstream
frol,i the ~ulb ~.as sh~wn in~Figs..2 and 3). ' :
The TX valve ll.is provided with an ex~ernal by-pass line 27
controlled by a flow rate valve 28 to by-pass wet
vapour from a junction point 29 immediately downstream :Erom th~
TX valve ,(shcwn for clari'ty ~n Fig. 4 as back through the ~ber-30 in the
valve,.),to the;~izer li~2~ and thence to ~ jun~iQn,point
~6~ In this manner the pressure equalizer line 25 can he
used as the wet vapour injection line and 80 obviate the
necessity to provide a separate line as shown in Figs~ 2 and 3.
In a further embodiment of the invention the by-pass line
27 and valve 28 may be incorporated into the TX valve
as ~hown in Fig. 5. In this configuration the outlet 31
~rom the rrx v~lve is prnvided with an internal by-paas
3~ controlled by neadle valve 33 to the equalizer 1ine ~utlet
34. The pasaa~e 3~ i~ the ~quivalent of the external by-pass
line ~7 and the needle valve 33 the ~quivalent of the ~low
rate control valve ~ shown ~n ~ig~ 4.
l ls~n~o
In situations where it is desired to provide
even further control over the TX va~ve than the v~ri~ble sensitivqty
"propo~on~ ac~on" cantrol described so far it is possible to
take the concept further and provide integral and derivative
action by adaption of the principles described above. Fig.
6 shows a 6ystem modifie~ in such a way as to incorporate
derivative action as well as the wet vapour injection system
described above. Negative time dependent feedback is
required and a second wet vapour injection system has been~
added, modified so that injection increases with time as
well as flow. This is achieved by providing a second by-
pass line 35 in parallel with the original by-pass line 15
and providing the line 35 with a restrictor valves
36 and 38 and a volume capacity 37. Although the time
lS lag in this case has been achieved using a capacity
and restrictors this is not mandatory and other
methods such as using thermal inertia to generate the
time lag by delaying the effects of the injected wet
vapour are applicable.
In some cases it may be desirable to use positive
feedback to the TX valve and this is achieved by the
configuration shown in Fig. 7. This is identical to the
configuration used to pro~ide negative feedback (as shown in
Fig. 2) except that in this case the vapour passing
through the by-pass line 39 is heated in a heater 40 until
it becomes highly superheated. The heating stage can be
arranged so that heat is obtained from the same source as the
evaporator. Alternatively the heat may be drawn from the casing
or the sump of the compr~ssor. Any hea~ source will achieve
the desired result and the ~inal choice must be made
on t~ermodynamic/practical grounds. The injection of hot
yas in~o the suction line is undesirable ~rom the point o~
vieW ~ r~duGin~ æuction yas te~perature. To keep the actual
~moun~ of ~as to a minimum the injection point should be
right next to the bulh,
The p~sitlve feedback system can al~o be modified
as was done with the negative feedback sy~tem when dexiva~iv
action wa9 obtained. In this case integral action is obtained
0 ~ ~
and A = +f(~,t). Thi~ configuration using a proportional
and integral control is shown in Fig. 8 where the time delay
is once again shown as being obtained by a capacity and
restrictors. In Fig, B the normal proportional control is
achieved through the wet vapour by~pass line 15 and the
positive feedback with integral control is provided through
by-pass line 41 which incorporates ~es~rictors 42 a heater 43
and a capacity 44.
In a similar manner a system may be provided with
variable sensi~vity, i~tegral acti~n, and derivati~ve action
as shown in Fig. 9. In this configuration the
no~al wet vapour injection line is provided at ~S
in parallel with a wet vapour/time function injection
(derivative) line 46 incorporating a capacity 47 and
valves/restrictors 48.The by-pass line 45 joins the
evaporator at junction 49 just downstream from the transition
point in the evaporator and the line 46 joins the
evaporator just upstream from the temperature sensing bulb
12. A further hot gas~time function (integral) by-pass
line 50 is also provided in parallel with the by-pass line
46 and incorporating valves/restrictors 51, a heater 52/ and
a capacity 53. The by-pass line 50 also joins the evaporator
at junction 54 just upstream of the temperature sensing bulb
12.
The systems described above enable a feedback
cont~ol system for a TX valve to be provided which enables
hunting of the valve to be reduced or eliminated in anumber
of different ways. The simple negative feedback proportional
control may be achieved in the configuration showm in Figs. 2
3Q and 3 and whexe further control of the TX valve is re~uired
thls may be provided using the modi~ications shown in Figs.
7 to 101
~ach o~ the systems described above is particularly
suitable for use with hea~ pumps of the solar assisted
~5 ~pe fox example a~ descrlbed in our Australian Pa~ent No.
S09,~01. In this application ma~imum ef~iciency is di~icult
to attain due to wide vaxia~ions in heat input and the low
thermal inextia o~ the evaporation plate.
- 13 -
1 1 56Q~)
The invention of course has wider applications to air
conditioning and refrigeration systems generally.
The effect of the invention may be readily
seen with reference to Figures 10 and 11 wherein Fig. 10
is a graph of temperature against time for an experimental
solar assisted heat pump of the prior art type with unstable
con~rol and Fig. 11 is the same graph of a similar heat pump
using a control system according to the invention. It will
be seen that the invention considerably reduces the
huntiny effect of the TX valve resultiny in a much more
stable and efficient system.
