GB2180673A - Refrigeration circuit pressure control - Google Patents

Abstract

In a method of regulating the operating pressure of a refrigeration circuit (1) in a multi-compressor refrigerating plant in which the compressors (2) are sequentially activated or deactivated with delays for the attainment of an optimum operating pressure (Po), the cut-in delay (K, H) of each compressor (2) is calculated in dependence on the difference between the actual pressure (P) of the circuit (1) and the optimum pressure (Po) to be achieved. The delay increases as the actual pressure (P) approaches the optimum pressure (Po). Such a method reduces hunting. When the actual pressure (P) is outside a band from 2Po to 1 DIVIDED 2Po the delay is held constant. If the actual pressure (P) differs from the optimum pressure (Po) by less than a tolerance DELTA then the number of compressors in operation remains unchanged. If all the compressors are found to be in a state of maximum utilization and the pressure P>/=(Po) + DELTA , then an alarm is given and one or more evaporators (3) in the refrigeration circuit are disconnected to reduce the thermal load. <IMAGE>

Description

SPECIFICATION Method of regulating the operating pressure of a refrigeration circuit in a multi-compressor refrigerating plant The present invention relates to a method of regulating the operating pressure of a refrigeration circuit in a multi-compressor refrigerating plant in which the compressors are sequentially activated or deactivated in the circu it with predetermined delaysforthe attainment of an optimum working pressure.

In the following context, the term "multi-compressor refrigerating plant" is understood to mean a plant in whose refrigeration circuit it is possible to effect a partialization ofthe circulating refrigerantfluid,for example by providing a plurality of compressors connected together in parallel and equipped with single-speed or multispeed electric motors.

In refrigerating plants of this type it is found necessary to maintain the pressure of the refrigerantfluid circulating in the refrigeration circuit within a predetermined range oftolerance of an optimum pressure value.

In orderto do this, the prior art provides forthe sequential activation or deactivation of the compressors in accordance with the pressure occurring in the refrigeration circuit.

In this sequential operation the activation or deactivation of a compressor is effected with a constant delay with respect to the activation or deactivation of the preceding compressor.

A normal activation delay is about 4 minutes, while the deactivation delay is usually much shorter, each delay being calculated from the activation or deactivation of the preceding compressor.

The delaytimes can be reduced to a shorter constant delay in the event of particular pressure unbalance in the circuit.

The sequential activation or deactivation of the compressors is effected until the operating pressure ofthe refrigeration circuit reaches the tolerance range accepted for the desired optimum pressure value.

The purpose of introducing these delays in the sequence of operation ofthecompressors is that limiting the effects of the transient state conditions which occur on the activation or deactivation of a compressor.

This criterion of delayed sequential activation of the compressors nevertheless gives rise to some disadvantages, which are found particularly when the tolerance range accepted for the operating pressure of the refrigeration circuit is narrow.

What happens in fact is that when the numberofcompressors in operation is increased the pressure level of the refrigeration circuitfalls so rapidly as to become difficultto control closetothe tolerance range ofthe predetermined optimum pressure. This gives rise to the possibility ofthe hunting of the effective pressure in the circuit around the aforesaid tolerance range, thus causing an undesirable number of successive activations and deactivations of the compressors every time the values ofthis range are exceeded for a length oftime greaterthan the delay applied.

On the other hand, this delay must be sufficiently restrained to restrict as much as possible the transient state of the refrigerating plant when the value ofthe effective pressure in the refrigeration circuitvaries substantially from the desired optimum value.

The problem dealtwith by the present invention isthat of providing a method of regulating the operating pressure of a refrigeration circuit so as to obviatethe abovementioned disadvantages encountered in the priorart.

This problem is solved by a method of the aforesaid type which is characterized by the fact that it consists in increasing the sequential activation or deactivation delays of the compressors in accordance with the reduction of the divergence between the effective pressure in the refrigeration circuit and the optimum pressure to be achieved.

The characteristics and advantages ofthe method according to the present invention will emerge more clearly from the detailed description of a preferred embodiment thereof which is illustrated, by way of indication and without limitation, in the accompanying drawings, inwhich: Figure 1 shows a diagram corresponding to the curve of operating pressure plotted againsttime, in a refrigeration circuit controlled by the method according to the invention (continuous line) and according to the prior art (broken line); Figure2shows schematically some of the components in a refrigeration circuit; Figure 3 is a schematic view of apparatus for controlling the operating pressure of a refrigeration circuit by the method ofthe present invention;; Figures4to Gareflow diagrams illustrating the sequences of operating phases in the method according to the present invention.

In Figure 2, 1 denotes generally a refrigeration circuit comprising a plurality of compressors 2 and a plurality of evaporators 3, all connected hydraulically in parallel to a condenser 5. The plurality of compressors 2 may be entirely or partly replaced by one or more compressors driven byvariable-speed electric motors.

Asolenoid valve 6 is associated with each evaporatorto interceptthe refrigerantfluid circulating inthe latter.

Figure 1 shows in theform of a diagram the curve ofthe operating pressurep ofthe refrigeration circuit1 (shown on the ordinate) plotted against time t(shown on the abscissa).

The pressurep is measured on the suction side ofthe compressors, that is to say in that partofthecircuit which is included between the compressors 2 and the evaporators 3.

In order to optimize thethermodynamic efficiency ofthe circuit 1, it is necessary to keep the operating pressurep ofthe refrigerant fluid close to a value pO.

In practice it is acceptableforthevaluep ofthe pressure to be maintained within a predetermined tolerance range, the limit values of which are indicated in Figure 1 by pO + A and pO - A. The pressurep is controlled by activating or deactivating, in accordance with requirements, one or more compressors 2 of the circuit 1. It is also necessary to space apart sufficiently, in terms of time, two successive activations or changes of speed of a given compressor (this delay being imposed by design limitations), and also to provide a delay in the sequence of successive activations or deactivations of compressors, so as to attenuate the transient effects on the pressurep whenever the number of compressors operating the circuit 1 is varied.

A curve representing the variation of the operating pressure in a refrigeration circuitwhich is controlled by traditional methods is designated A in Figure 1. The sequence of activation or deactivation of the compressors is controlled by a constant delay, which prevents the variation of the number of compressors in operation in the circuit 1 for a predetermined period oftime.

A starting phase of the circuit 1 corresponds to the abscissa 0 in the diagram in Figure 1. The pressurepin the circuit is at its maximum because all the refrigerant fluid contained in it is in the gaseous state.

The value ofthe pressure in the circuit decreases rapidly because the number of compressors in action rises asthetime increases.

The number of compressors is increased when the effective pressure value in the circuit 1 is higher than pO + A, is reduced when this valuefalls below - A, and is held unchanged in the range of values included between these limit values.

Sincethe delay imposed between two successive activations ofthe compressors is constant, the curveA intersects the line representing the pressure pO with an appreciable inclination. The example illustrated refers to the case where a compressor is activated immediately beforethe pressurep in the refrigeration circuit reachesthevalue pO + A. This gives rise to a sudden fall ofthe pressurep belowthe Iowerlimitvalue pO - A. When the pressure rises above this lower limit value, one of the compressors is deactivated,thus giving rise to another rise ofthe pressure to a value above pO + A.

It will be noted howthe stabilization of the operating pressurep of the refrigeration circuit is subjectto "hunting" around the value pO and the predetermined tolerance is exceeded.

The method of regulating a refrigeration circuit according to the present invention provides forthe compressors 2 to be activated and deactivated sequentially with predetermined delays for the purpose of stabilizing, in the branch included between the evaporators 3 and the compressors 2 ofthe circuit 1, an optimum operating pressure pO. Atolerance A ofthevalue pO is accepted.

According to the method of the present invention the duration ofthe abovementioned delays is increased in proportion as the effective pressure value in the circuitapproachesthe optimum pressure value pO.

The variation of the pressurep in a refrigeration circuit regulated by this method is represented bythecurve B in the form of a continuous line in Figure 1.

The diagram in Figure 1 also shows on the ordinate two new pressure values S1 and S2,which are respectively the upper and lower limits of a larger range oftolerance acceptable forthe pressures pO, pO + A andpO -A.

The value of S1 istwicethe value of pO, whilethevalue of S2 is equal to half thevalue of pO.

In a first example of application ofthe method ofthe present invention, when the value ofthe effective pressurep in the refrigeration circuit is outside the range S1 - S2, the sequence of activation or deactivation of the compressors is regulated by constant delays. The delay in the sequence of activation of two successive compressors is for example fixed at 10 seconds, while the deactivation delay is fixed at one second.

The point of intersection ofthecurve B and the straight line representing the pressure S1 being designated 8, the curve B shows in its initial portion, upstream ofthe point 8, a gradual diminution of pressure asthe number of compressors in action in the refrigeration circuit increases.

Downstream ofthe point 8, in the direction of increasing times, the curve B has an inflection and changes its concavity. This is explained by the fact that when the pressurep ofthe circuit 1 reachesthe value S1 (S2),the sequential activation of the compressors is controlled with delays proportional to the ratio between the effective value ofthe pressurep in the circuit and the optimum pressure value pO. Consequently, the curve B representing the pressurep approaches asymptotically the straight line representing the pressure pO.

The law of variation of delays in the range S1 - S2 can be obtained from the following table, in which K indicates the value, expressed in seconds, ofthe delay applied in the sequence of activation or deactivation of the compressors when the value of the effective pressurep in the refrigeration circuit is contained in the range S1 - pO, and in which H indicates the same delay when the value ofthe pressurep is contained intherange pO - S2.

plPO K plPO H 1.0 50 sec. 1.0 5 sec.

1.1 42 sec. 0.95 5 sec.

1.2 35 sec. 0.9 5 sec.

1.3 29 sec. 0.85 4 sec.

1.4 24 sec. 0.8 4sec.

1.5 20 sec. 075 4sec.

1.6 16 sec. 0.7 3 sec.

1.7 13 sec. 0.65 3sec.

1.8 10 sec. 0.6 2 sec.

1.9 7sec. 0.55 1 sec.

2.0 Ssec. 0.5 1 sec.

In the range of pressure values included between pO + A and pO Athe number of compressors in action in the refrigeration circuit is not changed.

Figure 1 indicates the effect ofthermal load unbalance in the refrigerating plant with which the circuit 1 is associated.

This thermal unbalance is manifested as an increase ofthethermal load ofthe plant occurring in the period of time included between the time and thetime t2 to which the points 9-10 on curve B correspond.

If thins overload can be completely absorbed by the number of compressors in action in the refrigeration circuit in a time shorterthan the delay fixed forthe activation ofafurther com pressor, that isto say ifthe effective pressure in the refrigeration circuit is maintained at a level higherthan po + A for a period oftime shorter than the corresponding delay k, the number of compressors in action in the circuitwill not be modified and the pressurep in the refrigeration circuit will gradually return tothevalue pO.

In practice this avoids the occurrence of pressure hunting in the refrigeration circuit wheneverthethermal load of the refrigerating plant associated with it undergoes slight disturbances.

The regulation ofthe pressurep in the refrigeration circuit 1 by the method ofthe present invention is effected with the aid ofan apparatus given the general reference 30 in Figure 3.

The apparatus 30 comprises a control panel 31, a pressure pickup 32 with its associated transducer 33, and also a first and a second alarm device designated respectively 34 and 35.

The control panel 31 has a first and a second terminal strip respectively designated 36 and 37 To each pairof terminals 38 ofthe terminal strip 36 are connected the cables (not shown) forthe electricity supply ofthe compressors 2 of the circuit 1. A series of microswitches 39 control the number of compressors connected in the circuit 1 and the number of operating speeds of each compressor 2. To a first pairofterminals40 ofthe terminal strip 37 is connected a line Lforthe electricity supply ofthe control panel 31. Thefirstandsecond alarm devices 34,35 and also the transducer 33 ofthe pressure pickup 32 are likewise connected to the terminals 40.

The pressure pickup 32 is contained in that branch of the refrigeration circuitwhich is included between the plurality of evaporators 3 and the plurality of compressors 2.

The alarm devices 34,35comprise optical and/or acoustic indicators known per se for indicating any anomalies in the operation ofthe circuit 1, as will be seen more clearly later on. They are also connected by means of respective leads 34a and 35a to a corresponding solenoid cutoffvalve for one or more evaporators 3, in such a manner as to permit the isolation of some ofthe evaporators3from the refrigeration circuit 1 in the mannerto be described later on.

On the control panel 31 are also mounted a display 41 showing the pressure measured by the pickup 32, a first and a second indicator light 42,43, a control knob 45forsetting the tolerance value permitted in relation tothe optimum pressure pO, and a pushbutton 46forchanging the indication of the display 41 from thevalue ofthe effective pressurep ofthe refrigeration circuit 1 to the value of the optimum pressure pO set by means ofthecontrol knob45.

Referring to Figures4to 6, the flow ofthe operations contemplated in an example of application ofthe method ofthe present invention will now be described in detail.

These operations are subdivided into two main groups, the first group relating to operations of short periodicity, that is to say repeated at high frequency (ofthe order of one second), and the second group relating to operations of higher periodicity (repeated every 5 seconds).

The diagrams shown in Figures 4 and 5 belong to the first group of operations, while the diagram shown in Figure 6 belongs to the second group.

Thefirstoperation included in the diagram in Figure 4, indicated bythe block 50, comprises the storing of the value of the effective pressurep existing in the refrigeration circuit and measured in the form of an electric signal by the pickup 32, the storing ofthe optimum pressure pO to be reached, which is set intheform of an electric signal by means ofthe control knob 45, and the storing ofthevalue ofthe acceptabletolerance A setin the form of an electric signal by means of the regulator44.

The reference 51 in Figure 4 designates generally a block of operations for checking whether or not a state of alarm exists in the refrigeration circuit 1. The operations of this block are illustrated in detail in Figure 5 and will be described later on.

The value ofthe pressurep measured in the block 50 is compared in a comparator node 52 with the upper limit pressure value S1 which was discussed in connection with Figure 1.

The value of S1 is predetermined and equal, in the example referred to, to twice the value of pO. lfthevalue ofthe pressurep is lowerthan the value of Si this value ofp is recompared in a second comparator node 53 with the lower limit pressure values S2.Thevalue of S2 is likewise predetermined and equal, in this example, to halfthevalueofpo.

If value ofp is greaterthan S2, that is to say ifp is between S1 and S2, it will then be checked whetherthe value ofthe pressurep is between pO + A and pO - A. This verification is made in two comparator nodes 54,55 connected in cascade downstream of the comparator node 53, the first of which compares the value ofp with the value of pO + A, while the second compares the value ofp with the value of pO - A.

All these verifications are made by comparison of corresponding electric signals.

If the value ofp is found to be within the predetermined tolerance rangeforthevalue pO, that isto say if comparisons in the comparator nodes 52-55 give a negative reply, the next operation scheduled, indicated schematically bythe block 56 in Figure 4, comprisesthetransmission of a stop signal to a programmable timer, of perse conventional type, provided for counting the activation-deactivation delays of the compressors 2. The number of compressors in action inthe refrigeration circuit 1 remains unchanged.

The sequence of operations in the method of the present invention therefore provides a block 57 in which the indicator lights 42,43 are reset.

These indicators 42,43, when lit, indicate on the control panel a state of pressure in the circuit 1 respectively below or above the tolerance acceptableforthe optimum pressure pO. In Figure 4,58 indicates a phase of resetting of a second timer which periodically, at constant intervals (ofthe value of one second), brings about the repetition of all the operations ofthe first group.

On the zeroization of the timer of phase 58, the cycle restarts with the operation 50.

Ifthevalue ofthe effective pressurep ofthe refrigeration circuit 1 is found in the comparator node 55to be below or equal tothevalue pO - A, i.e. lowerthan the acceptabletolerancevalue in relation to the optimum pressure pO, the method according to the invention provides for the sequential deactivation, with variable delays H, ofthe compressors 2. The law of delay variation is shown in the table above.

The sequence of phases described so far is interrupted in the comparator node 55 and continues according to the following plan. A comparator node 60 checks whether a count is being made of a delay H due to the deactivation of a preceding compressor; this delay is counted with the aid of the timer associated with the block 56.

If timer is zeroized, that is to say if a time has passed which is longer than the delay H calculated forthe specific pressure conditions existing in the circuit 1 from the deactivation ofthe last compressor2, another compressor2 is deactivated in a phase 61 and, in a phase represented by the block 62, the timer associated with the block 56 is brought into action for the counting of a new delay H proportional to the pressurep stored in the block 50.In a successive phase 63 the indicator light 42 is operated to indicate on the control panel 31 the existence of pressure conditions in the circuit 1 lower than the tolerance acceptable for the optimum pressure value pO, and the indicator light 43 is reset. If thecomparator node 60 finds that a count is being made of a delay H duetothe deactivation of a preceding compressor2,the phases 61 and 62 are omitted and onlythe phase 63 is proceeded with.

The cycle thus continues with the performance of the phase 58 for the zeroization of the second timer.

If the comparator node 54 finds the value ofthe effective pressurep in the refrigeration circuit 1 to be greater than or equal to the value pO + A ofthetolerance acceptable in relation to the optimum pressure pO,the method according to the invention provides for the sequential activation, with variable delays K, offurther compressors. The law of variation of the delay K is shown in the table above.

The sequence of phases 50 - 58 is interrupted in the comparator node 54 and continued in accordance with the following plan.

A comparator node 65 verifies whether a count is being made of a delay K due to the activation of a preceding compressor; this delay is counted by means ofthe timer associated with the block 56 If thistimer is zeroized,that is to say if a time has passed which is longerthan the delay programmed forthe effective pressure value p in the circuit 1 from the activation of the last compressor, a further compressor 2 is activated in a phase 66 and, in a successive phase represented by the block 67, the timer associated with the block 56 is programmed and activated forthe counting of a new delay K proportional to the valuep ofthe pressure stored in the block 50.

In a successive phase 68 the indicator 43 is activated to indicate on the control panel 31 the existence of a pressure condition in the circuit 1 higherthan the tolerance acceptable in relation to the optimum pressure value pO, and the indicator light 42 is reset If the comparator node 65 finds that a count is being made of a delay Kduetothe activation of a preceding compressor 2, the phases 66 and 67 are omitted and onlythe phase 68 is proceeded with.

Ifthe comparator node 52 finds the value of the effective pressurep in the refrigeration circuit 1 higherthan or equal to the limit value S1, i.e. twice the value of pO, the method according to the invention provides forthe sequential activation, with constant delays, of further compressors. The preceding sequence of phases 5058 is interrupted in the comparator 52 and continued in accordance with the following plan. A comparator node 70 verifies whether a count is being made of a constant delay (10 seconds) due to the activation of a preceding compressor; this delay is counted by means of the timer associated with the block 56.

If timer is zeroized, that is to say if a time has passed which is longer than the predetermined delay of 10 seconds from the operation ofthe last compressor put into action in the circuit 1, a further compressor 2 is activated in a phase 71 and, in a successive phase represented by the block 72, the timer associated with the block 56 is activated for the counting of a new constant delay of seconds. The reference numeral 73 indicates an interconnection inside the diagram shown in Figure 4. On completion ofthe phase 72, the phase 68 and the following phases arse carried out.

If comparator node 70 finds that a count is being made of a constant delay of 10 seconds due to the activation of a preceding compressor 2, the phases 71 and 72 are omitted and only phase 68 is proceeded with.

If comparator node 53 finds the value ofthe effective pressurep in the refrigeration circuit 1 to be smallerthan or equal to the limit value S2, that is is to say halfthe value of pO, the method according to the invention provides for the sequential deactivation, with constant delays, of further compressors.

The preceding sequence of phases is interrupted in the comparator node 53, and in a phase 75 a compressor is deactivated. In this case it is not necessary to check the delay in the deactivation ofthe compressors, because this delay, forthese pressure conditions in the refrigeration circuit 1, is predetermined in a second time, and therefore equal to the delay controlled by the second timer associated with the phase 58.

The reference numeral 76 indicates an interconnection inside the diagram shown in Figure 4. On completion of the phase 75, the phases 63 and 58 are carried out.

The operations associated with the block 51 also entail dealing with alarm situations within the refrigeration circuit 12. In the block 51 the values ofthe effective pressurep ofthe refrigeration circuit stored in the block 50 are compared in a comparator node 80 with the value of the tolerance A acceptable abovethe optimum pressure value pO.

lfthevalue ofthe pressurep is greater than or equal tothevalue of p0 + A, a check is made ofthe availabilityofthe refrigeration circuit 1 for an increase ofthe numberofcompressors in action oran increase ofthe operating speed ofthe compressors.

This check is made in a comparator node 81, in which the effective utilization ofthe compressors 2 in action in the circuit 1 is established and compared with a compressor availability value fixed by means ofthe microswitches 39.

If all the compressors 2 are already in a state of maximum utilization, the refrigeration circuit 1 is considered to be in a state of alarm.

The intervention ofthe alarm devices 34 and 35 is sequential and delayed by a predetermined period of time, both with regard to the appearance ofthestate ofalarm-forexamplewith a delay of one hourforthe intervention of the first alarm device 34, and with regard to the intervention ofthe devices 34 and 35,for example with a delay of half an hour from the intervention of the first device 34, for the activation ofthe second device 35. These delays are controlled by respective timers of perse conventional type.

If therefore in the comparator node 81 the said state of alarm is found, it will be checked whether the first alarm device 34 is in action and, if so, whether the second alarm device is also in action. On the intervention of one or both ofthe alarm devices 34 and 35, the respective solenoid valves 6connected to them by means of the leads 34a and 35a are operated to interruptthe supply to the corresponding evaporators 3, cutting them out of the refrigeration circuit.

With the disconnection of the evaporators 3 cut off by the solenoid valves 6, the cooling ofthe corresponding refrigeration compartments will cease, these compartments obviously being selected on the basis of the perishability of their content, so thatthe thermal load of the refrigeration circuit 1 is reduced.

The verification of the state of activity of the alarm devices 34 and 35 is effected sequentially in respective comparator nodes 82,83.

If in the comparator8othevaluep ofthe effective pressure in the refrigeration circuit 1 comes backwithin the accepted tolerances, that isto say is below pO + A, or if no state of alarm is found in the comparator81, two new verifications will be made respectively in comparator nodes 84 and 86.

The comparator nodes 84 and 86 are in sequence and they respectively verify whether the respective delay timers of the first alarm device 34 and of the second alarm device 35 are in the counting phase.

In both cases, if this check gives a positive reply, the delay timers ofthe alarm devices are deactivated in respective phases 85,87. Following the phases 85 and 87, if the check in the comparator node 86 gives a negative reply, the previously described operation 52 is repeated.

If the check in the comparator nodes 82,83 gives a negative result, that isto say if the alarm devices 34,35 are not switched on, respective comparator nodes 88 and 90 whether the respective delay timers are or are not in the counting phase.

In the affirmative case, the alarm devices 34,35 remains deactivated until the end ofthe preset delay; in the negative case the delay timers are activated in respective phases 89 and 91. In both cases the sequence of phases therefore continues with the comparison made in the comparator node 52.

At lower frequencies, that is to say once every 5 seconds, a further series of checks is interposed between the series of operations 51 and the operation 52. For reasons of clarity the flow diagram ofthesechecking operations is shown separately in Figure 6. Logisticallythese operations start fro a node R in the diagram shown in Figure 5.

A comparator node 100 checks whether the delay timer of the first alarm device has completed the counting of the preset delay (1 hour). lf so, the first alarm device is activated, together with a fourth timer independent ofthe previously mentioned timers, and at the end of a predetermined period oftime (24 hours) from the activation ofthe first alarm device this fourth timer effects the deactivation and resetting of both the alarm devices 34,35. This operation is represented by the block 101 in Figure 6.

If the delaytimerofthefirstalarm device shows non-activity in the check in the comparator node 100, a comparator node 102 verifies whetherthe count made by the delay timer of the second alarm device 35 has been completed.

If the reply to this check is positive, that is to say if this count has been completed, a phase 103 of activation ofthe second alarm device 35 follows; in the contrary case, the operation 52 is repeated. The same operation 52 is also repeated if the check in the comparator node 102 gives a negative reply.

The method of regulation ofthe operating pressure of a refrigeration circuit according to the present invention provides numerous advantages overtraditional methods of regulation. In particular, it permits better utilization ofthe compressors through the small number of activations and deactivations made (substantial reduction of hunting of the effective pressure of the circuit around the desired optimum pressure value), and an appreciable increase of the total thermodynamic efficiency of the refrigeration circuit.

Itwill of course be understoodthatthe present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Claims (6)

1. Method of regulating the operating pressure of a refrigeration circuit (1) in a multicompressor(2) refrigerating plant in which the compressors (2) are sequentially activated or deactivated in the circuit with predetermined delays for the attainment of an optimum working pressure, characterized by the fact that it consists in increasing the sequential activation or deactivation delays (K, H) of the compressors (2) in accordance with the reduction ofthe divergence between the effective pressure (p) in the refrigeration circuit and the optimum pressure (pO) to be achieved.

2. Method according to Claim 1,characterized by the fact that said activation or deactivation delays (K, H) of the compressors (2) are increased when the effective pressure (p) of the refrigeration circuit is contained in a predetermined range of values (S1-S2) including the optimum pressure (pO)to be attained, and that said delays (K, H) are kept constant when the effective pressure (p) ofthe refrigeration circuit (1) is outside said range ofvalues (51-52).

3. Method according to Claim 1, characterized by the factthat it includes a phase of comparison ofthe effective thermal load of the refrigeration circu it with the thermal potentiality of said circuit, a phase of activation of alarm means (34,35) when said effective thermal load exceeds the thermal potentiality of said refrigeration circuit (1), and a phase of interception of the compressed refrigerantfluid delivered to one or more evaporators (3) of said refrigeration circuit (1) in orderto balance said effective thermal load with said thermal potentiality ofthe refrigeration circuit(1).

4. Method according to Claim 3, characterized by the fact that said alarm means (34,35) are activated with a predetermined delay with respect two said comparison phase.

5. Method according to Claim 4, characterized by the fact that said alarm means comprise a plurality of alarm devices (34,35) each acting on a solenoid valve (6) isolating a corresponding evaporator (3), said alarm devices (34,35) being activated by one another sequentially with predetermined delays.

6. A method of regulating the operating pressure of a refrigerating circuit in a multicompressor refrigerating plant substantially as hereinbefore described with reference to the accompanying drawings.