CA1156060A - Control of vapour compression cycle refrigeration systems - Google Patents

Abstract

ABSTRACT

A vapour compression cycle refrigeration system of the type having the expansion valve (11) controlled by a temperature sensing bulb (12) at the downstream end of the evaporator (5). A by-pass line (15) is provided from the expansion valve outlet to the evaporator (5) to inject wet vapour upstream from the temperature sensing bulb (12) and provide proportioned negative feedback to the expansion valve (11) to reduce hunting or oscillating of the system.
Variations of the by-pass line are also described to give positive feedback and combinations giving integral and derivative action.

Description

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~

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~

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.

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.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A refrigeration system including an evaporator controlled by an expansion valve having means for sensing the temperature or vapour dryness at the downstream end of the evaporator, characterized by means for injecting wet vapour at a rate which is a function of the rate of flow of refrigerant through the expansion valve into said evaporator upstream of said temperature sensing means.

2. A refrigeration system as claimed in claim 1 wherein said means for injecing wet vapour comprise a by-pass line between the outlet from the expansion valve and a position upstream of said temperature sensing means.

3. A refrigeration system as claimed in claim 2 wherein said position is directly upstream of said temperature sensing means.

4. A refrigeration system as claimed in claim 2 wherein said position is spaced upstream from said temperature sensing means by a predetermined distance within the heat absorbing part of the evaporator.

5. A refrigeration system as claimed in any one of claims 1 to 3 wherein said system includes a pressure equalizer line from said expansion valve to the downstream end of said evaporator upstream from said temperature sensing means and wherein said by-pass line comprises in series a conduit communicating between the outlet from said expansion valve and the expansion valve end of said pressure equalizer line and the pressure equalizer line.

6. A refrigeration system as claimed in any one of claims 1 to 3 wherein said system includes a pressure equalizer line from said expansion valve to the downstream end of said evaporator upstream from said temperature sensing means and wherein said by-pass line comprises in series a conduit incorporated within said expansion valve and communicating between the outlet from said expansion valve and the expansion valve end of said pressure equalizer line and the pressure equalizer line.

7. A refrigeration system as claimed in any one of claims 2 to 4 wherein a second by-pass line is provided in parallel with the first said by-pass line, said second by-pass line incorporating in series restrictors and a capacity.

8. A refrigeration system as claimed in any one of claims 2 to 4 wherein a second by-pass line is provided in parallel with the first said by-pass line, said second by-pass line incorporating a heater, in series, restrictors and a capacity,

9. A refrigeration system as claimed in any one of claims 2 to 4 wherein second and third by-pass lines are provided in parallel with said first by-pass line, said second by pass line incorporating in series restrictors and a capacity and said third by-pass line incorporating in series a restrictor, a heater and a capacity.

10. A refrigeration system as claimed in claim 2 wherein said by-pass line incorporates a heater.