GB2274930A

GB2274930A - Refrigerating apparatus and control methods therefor

Info

Publication number: GB2274930A
Application number: GB9326573A
Authority: GB
Inventors: Tooru Kubo; Yoshinobu Fujita; Takayuki Kambe
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1993-01-27
Filing date: 1993-12-30
Publication date: 1994-08-10
Anticipated expiration: 2013-12-30
Also published as: GB9326573D0; GB2274930B; KR940018635A; CN1093792A; JPH06281234A; CN1071882C; JP3117339B2; TW228024B

Abstract

The degree (SH) of superheat of a refrigerant in evaporators (12 and 22) is sensed. The opening of expansion valves (11 and 21) is controlled so that the sensed each degree of superheat (SH) may equal a first preset value (SHs). The deviation ( DELTA SH) of the sensed each degree of superheat (SH) from the first preset value (SHs) is sensed, and the maximum value ( DELTA SHmax) of the deviations sensed is sensed one after another. When the latest maximum value ( DELTA SHmax(n)) sensed is decreased blow the preceding maximum value ( DELTA SHmax(n-1)) sensed, the amount of change is sensed. It is judged whether or not the amount of change D sensed is 2 DEG C or lower and the latest maximum value ( DELTA SHmax(n)) is 1 DEG C or higher. When the conditions in this judgment are met twice consecutively, the opening control value for the expansion valves (11 and 21) is corrected in a decreasing direction. <IMAGE>

Description

REFRIGERATING APPARATUS AND CONTROL METHODS THEREFOR This invention relates generally to refrigerating apparatus for use in an air conditioner and, more particularly, to refrigerating apparatus intended to keep the degree of superheat of a refrigerant in an evaporator at a preset value.

An air conditioner contains a refrigerating cycle.

The refrigerating cycle comprises a compressor, an outdoor heat exchanger, a pressure reducing unit, and an indoor heat exchanger, all piped with one after another in that order.

By causing a refrigerant discharged from the compressor to flow through the outdoor heat exchanger, the pressure reducing unit, and the indoor heat exchanger in that order, the outdoor heat exchanger functions as a condenser and the indoor heat exchanger acts as an evaporator, thereby effecting a cooling operation.

In a refrigerating cycle of the heat-pump type, a compressor, a four-way valve, an outdoor heat exchanger, a pressure reducing unit, and an indoor heat exchanger are piped with one another in that order. In this case, by switching the four-way valve so that a refrigerant may flow in the reverse direction to that in a cooling operation, the indoor heat exchanger serves as a condenser and the outdoor heat exchanger functions as an evaporator, thereby effecting a heating operation.

With a compressor of the variable capacity type, the optimum cooling and heating capacity for an air conditioning load can be achieved by sensing an air conditioning load based on indoor temperature and controlling the capacity of the compressor according to the air conditioning load.

Since in controlling the capacity, the quantity of flow of the refrigerant in the refrigerating cycle changes as the capacity changes, the degree (magnitude) of superheat of the refrigerant in the evaporator changes. The degree of superheat must be kept at a preset value in order to assure a stable operation.

To achieve this, an expansion valve of the variable opening type is used as a pressure reducing unit. The opening of the expansion valve is controlled so that the degree of superheat may be maintained at the preset value.

An air conditioner using such superheat degree control has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 60-263065. In this air conditioner, the degree of superheat of an refrigerant in the evaporator is sensed and the opening of the expansion valve is controlled so that the degree of superheat may be in the proper range.

Practical methods of superheat degree control include feedback control and fuzzy control.

In an example of using feedback control, the degree of superheat of an refrigerant in an evaporator is sensed and the degree of superheat is fed back to a PID control section. The PID control section senses the deviation of the superheat degree from the preset value and, on the basis of the deviation, computes the opening of the expansion valve to be controlled. The opening of the expansion valve is then regulated as much as the computed amount.

In an example of using fuzzy control as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-169755, the degree of superheat of an refrigerant in an evaporator is sensed and then the deviation of the superheat degree from the preset value is sensed. Fuzzy operations are carried out on the basis of the sensed deviation and the amount of change of the deviation to compute the amount of opening required to be changed with respect to the present opening of the expansion valve. The opening of the expansion valve is then regulated as much as the computed amount.

When the opening of the expansion valve has been regulated, the flow of an refrigerant to the evaporator first changes, leading to a change in the degree of superheat. Specifically, there is a time lag from when the opening of the expansion valve has been regulated until the superheat degree actually changes. Because of the effect of this time lag, the superheat degree does not converge on the preset value in a short time, but fluctuates above and below the preset value and gradually converge on the value.

The amplitude fluctuation of superheat degree is absorbed to some extent by feedback control or fuzzy control. When fluctuations in the air conditioning load are great, however, the superheat sometimes does not converge easily. Under some load conditions, far from converging, the superheat can diverge or remain stable at a point away from the preset value, thus sometimes taking a very long time to converge at the preset value. It is a common practice to set constants in feedback control or fuzzy control so that these problems may not take place. However, it is difficult to set suitable constants in systems where fluctu-ations in the load are very large as in a refrigerating cycle.

The object of the present invention is to provide a refrigerating apparatus and a control method thereof capable of causing the degree of superheat in an evaporator to rapidly converge on a preset value and assuring a constantly stable operation.

The foregoing object is accomplished by providing a refrigerating apparatus keeping the degree of superheat of a refrigerant in an evaporator at a preset value, comprising: a compressor for sucking and compressing a refrigerant and discharging the compressed refrigerant; a condenser for condensing the refrigerant discharged from the compressor; an expansion valve of the variable opening type for depressurizing the refrigerant passed through the condenser; at least one evaporator for evaporating the refrigerant passed through the expansion valve; first sensing means for sensing the degree of superheat of the refrigerant in the at least one evaporator; first control means for controlling the opening of the expansion valve so that the degree of superheat sensed at the first sensing means may equal a first preset value; second sensing means for sensing the deviation of the degree of superheat sensed at the first sensing means from the first preset value and further sensing the maximum value of the sensed deviations one after another; third sensing means for sensing the amount of change when the latest maximum value sensed at the second sensing means is decreased below the preceding maximum value sensed at the second sensing means; first judging means for judging whether or not the amount of change sensed at the third sensing means is equal to or smaller than a second preset value and the latest maximum value sensed at the second sensing means is equal to or larger than a third preset value; and first correction means for correcting the opening control value with the first control means in a decreasing direction, when the conditions judged by the first judging means are met a predetermined number of times consecutively.

The forgoing object is accomplished by providing a method of controlling a refrigerating apparatus comprising a compressor for compressing a refrigerant, a condenser for condensing the refrigerant discharged from the compressor, an expansion valve of the variable opening type for depressurizing the refrigerant passed through the condenser, and at least one evaporator for evaporating the refrigerant passed through the expansion valve, the method comprising: a first sensing step of sensing the degree of superheat of the refrigerant in the at least one evaporator; a first control step of controlling the opening of the expansion valve so that the degree of superheat sensed at the first sensing step may equal a first preset value; a second sensing step of sensing the deviation of the degree of superheat sensed at the first sensing step from said first preset value and further sensing the maximum value of the sensed deviations one after another; a third sensing step of sensing the amount of change when the latest maximum value sensed at the second sensing step is decreased below the preceding maximum value sensed at the second sensing step; a first judging step of judging whether or not the amount of- change-sensed at the third sensing step is equal to or smaller than a second preset value and the latest maximum value sensed at the second sensing step is equal to or larger than a third preset value; and a first correction step of correcting the opening control value at the first control step in a decreasing direction, when the conditions judged at the first judging step are met a predetermined number of times consecutively.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 shows the construction of refrigerating cycles according to a first and a second embodiment of the present invention; FIG. 2 is a block diagram of the control circuits of the first and the second embodiment; FIG. 3 is a flowchart for explaining the overall operation of the first and the second embodiment; FIG. 4 is a block diagram for explaining feedback control in the first embodiment; FIG. 5 is a time chart showing the change of the deviation of the degree of superheat SH from the preset value SHS in the first embodiment; FIGS. 6A, 6B and 6C are flowcharts of an opening control routine in the first embodiment;; FIGS. 7A, 7B, 7C and 7D are flowcharts of an opening control routine in the second embodiment; and FIG. 8 is a time chart showing the relationship between the change of superheat degree SH and prediction time t4.

Hereinafter, a first embodiment of the present invention will be explained with reference to the accompanying drawings.

In FIG. 1, reference symbol A indicates an outdoor unit, and B1 and B2 represent indoor units. These units are provided with the following refrigerating cycle.

An outdoor heat exchanger 3 is connected to the outlet of a compressor 1 of the variable capacity type via a four-way valve 2. A liquid-side main pipe w is connected to the outdoor heat exchanger 3. The liquidside main pipe branches into liquid-side branch pipes w and W2. A plurality of indoor heat exchanges 12 and 22 are connected to the liquid-side branch pipes W1 and W2.

The liquid-side branch pipes W1 and W2 are provided with expansion valves 11 and 21 of the variable opening type, respectively. These expansion valves 11 and 21 are pulse motor valves whose opening changes continuously according to the number of driving pulses supplied. Hereinafter, the expansion valves are referred to as PMVs. The expansion valves are not limited to these PMVs. For instance, expansion valves whose opening changes with the heating valve of an electric heater additionally provided may be used.

Gas-side branch pipes Z1 and Z2 are connected to the indoor heat exchangers 12 and 22, respectively. The gas-side branch pipes Z1 and Z2 are gathered into a gas-side main pipe Z. The gas-side main pipe Z is connected to the inlet of the compressor 1 via the four-way valve 2.

In the vicinity of the outdoor heat exchanger 3, an outdoor fan 4 is provided. One end of a bypass pipe 5 for defrosting is connected to the pipe between the outlet of the compressor 1 and the four-way valve 2.

The other end of the bypass pipe 5 is connected to the liquid-side main pipe W. A two-way valve 6 is provided in the bypass pipe 5. The outdoor heat exchanger 3 is provided with a temperature sensor 7. A temperature sensor 8 is provided on the pipe between the four-way valve 2 and the inlet of the compressor 1.

In the vicinity of the indoor heat exchangers 12 and 22, indoor fans 13 and 23 are provided, respectively. The indoor heat exchanger 12 is provided with a temperature sensor 14. The indoor heat exchanger 22 is provided with a temperature sensor 24. A temperature sensor 15 is provided on the gas-side branch pipe Z1. A temperature sensor 25 is provided on the gas-side branch pipe Z2.

A schematic diagram of the control circuit in the embodiment is shown in FIG. 2.

In the figure, an outdoor controller 40 of the outdoor unit A is connected to a commercial alternatingcurrent (a.c.) power supply 30. The outdoor controller 40 is composed of a microcomputer and its peripheral circuitry. Connected to the outdoor controller 40 are the PMVs 11 and 21, the two-way valve 6, the four-way valve 2, an outdoor fan motor 4M, the temperature sensors 7, 8, 15, and 25, and an inverter 41.

The inverter 41 rectifies the voltage of the power supply 30, converts it into a voltage of the frequency and the level according to instructions from the outdoor controller 40, and outputs the converted voltage. The output is used as a driving power for a compressor motor 1M.

Each of the indoor units B1 and B2 is provided with an indoor controller 50. The indoor controller 50 is made up of a microcomputer and its peripheral circuitry.

Connected to the indoor controller 50 are an indoor temperature sensor 51, the temperature sensor 14 (and 24), a remote-control operating unit 52, and an indoor fan motor 13M (and 23M).

The indoor controllers 50 and the outdoor controller 40 are connected to each other with power lines ACL and serial signal lines SL.

Each indoor controller 50 and the outdoor controller 40 perform the overall control of the air conditioner by exchanging data synchronized with the power-supply voltage between them using the serial signal lines SL.

Each indoor controller 50 primarily has the following functional means: [1] Means for informing the outdoor controller 40 of operating conditions (including a cooling mode instruction, a heating mode instruction, and a target value Ts of indoor temperature) set at the operating unit 52 [2] Means for sensing as an air conditioning load the difference between the sensed temperature Ta at the indoor temperature sensor 51 and the target value Ts of indoor temperature set at the operating unit 52, and informing the outdoor controller 40 of the air conditioning load [3] Means for informing the outdoor controller 40 of the sensed temperature Tc at the temperature 14 (and 24) and the sensed temperature Ta at the indoor temperature sensor 51 The outdoor controller 40 primarily has the following functional means: : [1] Means for performing a cooling operation by passing the refrigerant discharged from the compressor 1 through the four-way valve 2, the outdoor heat exchanger 3, the PMVs 11 and 21, the indoor heat exchangers 12 and 22, and again the four-way valve 2, and returning it to the compressor 1, on the basis of the cooling mode instruction informed by each indoor controller 50 [2] Means for performing a heating operation by switching the four-way valve 2, passing the refrigerant discharged from the compressor 1 through the four-way valve 2, the indoor heat exchangers 12 and 22, the PMVs 11 and 21, the outdoor heat exchanger 3, and again the four-way valve 2, and returning it to the compressor 1, on the basis of the heating mode instruction informed by each indoor controller 50 23] Means for controlling the operating frequency F of the compressor 1 (= the output frequency of inverter 41) according to the total of the air conditioning loads informed by each indoor controller 50 in a cooling and a heating operation [4] First cooling sensing means for, in a cooling operation, sensing the difference (= Tg - Tc) between the sensed temperature Tg at the temperature sensor 15 and the sensed temperature Tc at the temperature 14, as the degree of superheat SH of the refrigerant in the indoor heat exchanger (evaporator) 12, and sensing the difference between the sensed temperature Tg at the temperature sensor 25 and the sensed temperature Tc at the temperature 24, as the degree of superheat SH of the refrigerant in the indoor heat exchanger (evaporator) 22 [5] First cooling control means for, in a cooling operation, controlling the opening of each of PMVs 11 and 21 so that each degree of superheat SH sensed may equal a cooling preset value (a first preset value) SHS [6] First heating sensing means for, in a heating operation, sensing the difference (= Ts - Te) between the sensed temperature Te at the temperature sensor 7 and the sensed temperature Ts at the temperature sensor 8, as the degree of superheat SH of the refrigerant in the outdoor heat exchanger (evaporator) 3 [7] First heating control means for, in a heating operation, controlling the opening of each of PMVs 11 and 21 so that each degree of superheat SH sensed may equal a heating preset value (a first preset value) SHs [8] Means for, in a heating operation, periodically performing a defrosting operation on the outdoor heat exchanger 3 by opening the two-way valve 6. When the two-way valve 6 is opened, the high-temperature refrigerant discharged from the compressor 1 is injected into the outdoor heat exchanger 3.

[9] Second sensing means for, in a cooling and a heating operation, sensing the deviation ASH of the sensed superheat degree SH from the preset value SH5, and sensing the maximum value ASHmax of the sensed deviations ASH one after another. Here, the latest maximum value ASHmax sensed is stored in a memory as ASHmax(n). The preceding maximum value tSHmax sensed is stored likewise in the memory as ASHmax(n-1).

[10] Third sensing means for sensing the amount of change D in the decreasing direction when the latest maximum value ASHmax(n) sensed at the second sensing means is de creased below the preceding maximum value ASHmax(n-1) sensed at the second sensing means. The amount of change D in the decreasing direction is the difference between them (= ASHmax(n-l) - ASHmax(n)) when tSHmax(n-l) > ASHmax(n).

[11] First judging means for judging whether or not the amount of change D sensed at the third sensing means is at a preset value (a second preset value), e.g., 2"C or lower, and the latest maximum value aHmax(n) sensed at the second sensing means is at a preset value (a third preset value), e.g., 1"C or higher [12] First correction means for correcting the opening control value with the first control means in the decreasing direction, when the conditions judged by the first judging means are met a specified times (e.g., twice) consecutively [13] Second judging means for judging whether or not the deviation ASH sensed at the second sensing means has remained at a preset value (a fourth preset value), e.g., 1"C or higher, for a certain period of time t1 (e.g., five minutes) tl4] Second correction means for correcting the opening control value with the first control means in the increasing direction, when the condition judged by the second judging means is fulfilled Explained next will be the operation of the embodiment.

First, the overall operation will be described, referring to FIG. 3.

When the cooling mode is set for each operating unit 52, the refrigerant discharged from the compressor 1 flows in the direction of the solid-line arrows in FIG. 1, whereby the outdoor heat exchanger 3 functions as a condenser and the indoor heat exchangers 12 and 22 act as evaporators. This effects a cooling operation.

In the cooling operation, the indoor units B1 and B2 sense indoor temperature Ta (step 91). The difference between the indoor temperature Ta and the target value Ts of indoor temperature previously determined at each operating unit 52 is sensed as an air conditioning load (step 92). The indoor units B1 and B2 notify the outdoor unit A of the individual air conditioning loads sensed.

In the outdoor unit A, the operating frequency F of the compressor 1 (= the output frequency of inverter 41) is controlled according to the total of air conditioning loads informed by the indoor units B1 and B2 (step 93).

At the same time, the degree of superheat SH of the refrigerant in each of the indoor heat exchangers 12 and 22 of the indoor units B1 and B2 is sensed (step 94).

The indoor units B1 and B2 inform the outdoor unit A of each degree of superheat SH sensed.

In the outdoor unit A, the opening control routine is executed on the basis of each degree of superheat SH informed by the indoor units B1 and B2 (step 95).

Specifically, in the opening control routine, the opening of each of PMVs 11 and 21 is controlled so that each degree of superheat SH may be equal to the cooling preset value SHs. This opening control enables the proper amount of refrigerant to flow through the indoor heat exchangers 12 and 22.

When the heating mode is set for each operating unit 52, the four-way valve 2 is switched and the refrigerant discharged from the compressor 1 flows in the direction of the broken-line arrows in FIG. 1, whereby the indoor heat exchangers 12 and 22 function as condensers and the outdoor heat exchanger 3 serves as an evaporator. This effects a heating operation.

In the heating operation, the indoor units B1 and B2 sense indoor temperature Ta (step 91). The difference between the indoor temperature Ta and the target value Ts of indoor temperature previously determined at each operating unit 52 is sensed as an air conditioning load (step 92). The indoor units B1 and B2 inform the outdoor unit A of the individual air conditioning loads sensed.

At the same time, the degree of superheat SH of the refrigerant in the outdoor heat exchanger 3 of the outdoor unit A is sensed (step 94).

In the outdoor unit A, the opening control routine is executed on the basis of the degree of superheat SH sensed (step 95). Specifically, in the opening control routine, the opening of each of PMVs 11 and 21 is controlled so that the degree of superheat SH may be equal to the heating preset value SHs.

Actual control of the degree of superheat in the opening control routine is executed in the form of feedback control shown in FIG. 4.

Specifically, the sensed superheat degree SH is fed back to a PID controller 401. The PID controller 401 takes in the preset value SHs as well. In the PID controller 401, the deviation ASH of the sensed superheat degree SH from the preset value SHs is first obtained, and on the basis of the deviation ASH, the opening control value (the number of driving pulses) for PMVs 11 and 21 is calculated. The opening control value itself is not used for control, but is supplied to a correction section 402. On the other hand, the sensed superheat degree SH is supplied not only to the PID controller 401 but also to a gain G setting section 403.

The gain G setting section 403 monitors a state in which the superheat degree SH converges on the preset value SHs, and sets gain G on the basis of the converging state. The set gain G is supplied to the correction section 402. The correction section 402 make a correction of gain G in the opening control value supplied from the PID controller 401. Then, according to this corrected opening control value, the opening of each of PMVs 11 and 21 is actually controlled.

Referring to FIGS. 5, 6A, 6B and 6C, how the degree of superheat SH is monitored and gain G is set in the opening control routine will be described.

When an operation is started, when the number of indoor units B1 and B2 in operation has changed, when the opening of each of PMVs 11 and 21 is being set toward the initial opening, or when the operation is in the defrost mode (YES at steps 101, 102, 103, and 104), ASHmax(n), ASHmax(n-l), number-of-times count N, and time count t are all cleared, and gain G is set at "1" (step 105). The gain G = 1 corresponds to no correction, and means that the opening control value obtained at the PID controller 401 is used as it is for opening control of PMVs 11 and 21.

When the opening of each of PMVs 11 and 12 has been regulated according to the change of the state of the refrigerant, the quantity of flow of the refrigerant to the evaporator first changes, resulting in a change in the degree of superheat SH. Namely, there is a time lag from when the opening of PMVs 11 and 21 has been regulated until the degree of superheat SH actually changes.

Because of this time lag, the degree of superheat SH does not converge on the preset value SHs in a short time, but fluctuates above and below the preset value SHs and gradually converge on the value. The amplitude fluctuation of superheat degree SH is absorbed to some degree by PID control or the like. When fluctuations in the air conditioning load are great, however, the superheat degree SH sometimes does not converge easily.

Under some load conditions, far from converging, the superheat can diverge or remain stable at a point away from the preset value SHs, thus sometimes taking a very long time to converge at the preset value.

When a certain period of time has elapsed since the start of operation, there is no change in the number of indoor units B1 and B2 in operation, the opening of PMVs 11 and 21 is not being set toward the initial opening, and the operation is not in the defrost mode (NO at steps 101, 102, 103, and 104), the deviation ASH of the sensed superheat degree SH from the preset value SHs and the maximum value ASHmax of the deviations ASH are sensed one after another (step 106). At the same time, flag H is set at "0" (step 107).

It is judged whether or not the deviation ASH is equal to 1"C (a forth preset value) or higher (step 108). If the deviation ASH is equal to 10C or higher (YES at step 108) and if the operating frequency F remains unchanged (NO at step 110), on the basis of the fact that flag H is at "0" (NO at step 113), time count t is started (step 114).

When the deviation ASH is equal to or below 1C (NO at step 108), flag H is set at "1" (step 109). When the operating frequency F has changed (YES at step 110), ASHmax(n), ASHmax(n-1), and number-of-times count N are cleared (step 111), and flag H is set at "1" (step 105). In this-case, on the basis of the fact that flag H is at "1" (YES at step 113), time count t is cleared (step 115).

Time count t is compared with a certain period of time tl, e.g., five minutes (step 116). When time count t has reached five minutes, that is, when the deviation ASH has remained at 1"C or higher for five minutes (YES at step 116), gain G is increased by a specified value, e.g., 0.2 (step 117). With gain G thus set, the opening control value, i.e., the number of driving pulses, for PMVs 11 and 21 is corrected (step 133).

Specifically, under some load conditions, the degree of superheat SH can remain at 10C or higher or lower for five minutes or more. In this case, gain G is increased. When gain G has increased, the opening control value for PMVs 11 and 21 is corrected in the increasing direction, making the change of flow rate of the refrigerant to the evaporator greater. This stops the unnecessary stable state of the degree of superheat SH and allows the superheat degree SH to converge on the preset value SHs rapidly On the other hand, the degree of superheat SH fluctuates above and below the preset value SHs and converges on the value gradually.In this case, each time the deviation ASH lies on the positive side (the + side) (YES at step 118), the maximum value ASHmax of the deviations ASH is stored as the latest maximum value ASHmax(n) for updating (step 119).

When the deviation ASH falls from the positive side (the + side) to the negative side (the - side) (YES at step 120), the latest maximum value ASHmax(n) is stored as the preceding maximum value ASHmax(n-l) for updating (step 121).

The degree of superheat SH converges toward the preset value SHs, the maximum value ASHmax changes in the decreasing direction (YES at step 122). In this case, the relationship between the preceding maximum value ASHmax(n-l) and the latest maximum value ASHmax(n) is expressed by hSHmax(n-l) > ASHmax(n). The difference between them (= ÇSHmax(n-l) - ASHmax(n)) is sensed as the amount of change D in the decreasing direction (step 123).

It is judged whether or not the amount of change D sensed is 2"C (a second preset value) or lower and the latest maximum value ASHmax(n) is equal to a specified value, e.g., 1"C or higher (steps 124 and 125). This is done for monitoring a state where the amplitude fluctuation of the superheat degree SH converges toward the preset value HSs.

If the amount of change is 20C or lower (YES at step 124), and if the latest maximum value ASHmax(n) is equal to 1"C or higher (YES at step 125), that is, if the conditions in the above judgment are met, it is judged that hunting has taken place (step 126). If the amount of change D is equal to 2"C or higher, or if the latest maximum value ASHmax(n) is 1"C or lower, that is, if the conditions in the above judgment have not been met, it is judged that no hunting has occurred (step 127).

If hunting has taken place, number-of-times count N is counted up by 1 (step 128). If no hunting has occurred, number-of-times count N is cleared (step 129).

When number-of-times count N has reached preset value N1, e.g., twice (YES at step 130), number-of-times count N is cleared (step 131), and gain G is decreased by a specified value, e.g., 0.3 (step 132). With gain G thus set, the opening control value, i.e., the number of driving pulses, for PMVs 11 and 21, is corrected (step 133).

Specifically, the amplitude fluctuation of the superheat degree SH is absorbed to some extent by PID control or the like. When fluctuations in the air conditioning load are great, however, the superheat degree sometimes does not converge readily. Although the delay in convergence cannot be helped to some extent, too much a delay permits an inefficient operation to continue.

To overcome this problem, in.a situation where there is a delay in the convergence of the superheat degree SH or where the superheat degree SH diverges to the contrary, gain G is decreased.

When gain G has decreased, the opening control value for PMVs 11 and 21 is corrected in the decreasing direction, making the change of flow rate of the refrigerant to the evaporator smaller. This suppresses the amplitude fluctuations in the superheat degree SH, thereby enabling the superheat degree SH to converge on the preset value SHs rapidly. As a result, a constantly stable operation can be achieved.

A second embodiment of the present invention will be described.

After the opening control value for PMVs 11 and 21 has been corrected, the operating frequency F sometimes changes. In this case, even though the correction has been made, the change of the operating frequency F introduces the danger of eventually delaying the convergence of the superheat degree SH. To overcome this drawback, the second embodiment is designed.

For the functional means of the outdoor control section 40, the following items [15] to [18] are provided in addition to items [1] to [14) in the first embodiment.

[15] Means for, when the operating frequency F of the compressor 1 has changed, sensing as the amount of change of superheat degree E the difference between the maximum value ASHmax sensed at the second sensing means and the deviation ASH sensed at the second sensing means a specified time t3 after the previous sensing [16] Prediction means for predicting time t4 required for the superheat degree SH to reach preset value SHs on the basis of the amount of change of superheat degree E sensed and the deviation ASH sensed at the second sensing means [17] Third correction means for, when prediction time t4 is longer than preset value tup (a fifth preset value), correcting the opening control value for PMVs 11 and 21 in the increasing direction [18] Fourth correction means for, when prediction time t4 is shorter than preset value tdw (a sixth preset value), correcting the opening control value for PMVs 11 and 21 in the decreasing direction The remaining construction is the same as that of the first embodiment.

The second embodiment is characterized in that as shown in the flowcharts of FIGS. 7A, 7B, 7C, and 7D, steps 112a and 112b are added between steps 112 and 113, and steps 131 to 139 are added between steps 121 and 122. In addition, the clearing of time count t2 is added to step 105. The remaining operation is the same as that of the first embodiment.

Specifically, if the operating frequency F has changed (YES at step 110), flag H2 is set at "1" (step 112a), and time count t2 is cleared (step 112b).

If flag H2 is at "1" (YES at step 131) when the deviation ASH decreases from the positive side (the + side) to the negative side (the - side) (YES at step 120), it is judged that the operating frequency F has changed and time count t2 is started (step 132).

When time count t2 has reached a specified time t3, the difference (= ASHmax(n) - TOSH) between the latest maximum value ASHmax(n-l) stored at step 121 and the current deviation *SH is sensed as the amount of change of superheat degree E. Then, time t4 from when the deviation ASH starts to decrease (from the start of time count t2) until the superheat degree SH reaches preset value SHs is predicted (step 134) by obtaining a ratio of the current deviation ASH to the amount of change of superheat degree E and adding the specified time t3 to the ratio as shown in the following equation: t4 = ASH/(ASHmax(n-1) - ASH) + t3 The relationship between the change of superheat degree SH and the prediction time t4 is shown in FIG. 8.

The prediction time t4 is compared with preset value tupi e.g., five minutes (step 135).

When the prediction time t4 is longer than preset value tup (YES at step 135), gain G is increased by a specified value, e.g., 0.2 (step 136). When gain G has increased, the opening control value for PMVs 11 and 21 is corrected in the increasing direction, making the change of flow rate of the refrigerant to the evaporator larger. This enables the superheat degree SH to converge on preset value SHs rapidly.

Additionally, prediction time t4 is compared with preset value tdw, e.g., one minute (step 137).

When the perdition time t4 is shorter than preset value tdw (YES at step 137), there is a possibility that the superheat degree SH will continue amplitude fluctuation centering on preset value SHs. To avoid this problem, gain G is decreased by a specified value, e.g., 0.3 (step 138). When gain G has decreased, the opening control value for PMVs 11 and 21 is corrected in the decreasing direction, making the change of flow rate of the refrigerant to the evaporator smaller. This prevents the superheat degree SH from continuing amplitude fluctuation centering on preset value SHs, thereby allowing the superheat degree SH to converge on preset value SHs rapidly.

While in the above embodiments, the present invention is applied to the air conditioner, it may be applied to other apparatuses as long as they are provided with a refrigerating cycle.

Claims

Claims:

1. A refrigerating apparatus keeping the degree of superheat of a refrigerant in an evaporator at a preset value, comprising: a compressor for compressing a refrigerant; a condenser for condensing the refrigerant discharged from the compressor; an expansion valve of the variable opening type for depressurizing the refrigerant passed through the condenser; at least one evaporator for evaporating the refrigerant passed through the expansion valve; first sensing means for sensing the degree of superheat of the refrigerant in said at least one evaporator; first control means for controlling the opening of said expansion valve so that the degree of superheat sensed at the first sensing means may equal a first preset value;; second sensing means for sensing the deviation of the degree of superheat sensed at said first sensing means from said first preset value and further sensing the maximum value of the sensed deviations one after another; third sensing means for sensing the amount of change when the latest maximum value sensed at the second sensing means is decreased below the preceding maximum value sensed at the second sensing means; first judging means for judging whether or not the amount of change sensed at the third sensing means is equal to or smaller than a second preset value and the latest maximum value sensed at said second sensing means is equal to or larger than a third preset value; and first correction means for correcting the opening control value with said first control means in a decreasing direction, when the conditions judged by the first judging means are met a predetermined number of times consecutively.

2. An apparatus according to claim 1, further comprising: second judging means for judging whether or not the deviation sensed at said second sensing means has remained at a fourth preset value or more for a certain period of time; and second correction means for correcting the opening control value with said first control means in an increasing direction, when the condition judged by the second judging means is met.

3. An apparatus according to claim 2, further comprising an inverter capable of supplying a driving power for driving said compressor and changing the output frequency.

4. An apparatus according to claim 3, further comprising: setting means for setting a target value of indoor temperature; an indoor temperature sensor for sensing indoor temperature; fourth sensing means for sensing the difference between the temperature sensed by the indoor temperature sensor and the target value of indoor temperature set at said setting means; and second control means for controlling the output frequency of said inverter according to the difference sensed at said fourth sensing means.

5. An apparatus according to claim 4, further comprising: fifth sensing means for, when the output frequency of said inverter has changed, sensing the difference between the latest maximum value sensed at said second sensing means and the deviation sensed at the second sensing means a specified time after the preceding sensing; prediction means for predicting the time required for the degree of superheat in said evaporator to reach said first preset value on the basis of the difference sensed at the fifth sensing means and the deviation sensed at said second sensing means; third correction means for correcting the opening control value with said first control means in an increasing direction, when the time predicted at the prediction means is longer than a fifth preset value; and fourth correction means for correcting the opening control value with said first control means in a decreasing direction, when the time predicted at said prediction means is shorter than a sixth preset value.

6. A method of controlling a refrigerating apparatus comprising a compressor for compressing a refrigerant, a condenser for condensing the refrigerant discharged from the compressor, an expansion valve for depressurizing the refrigerant passed through the condenser, and at least one evaporator for evaporating the refrigerant passed through the expansion valve, said method comprising:: a first sensing step of sensing the degree of superheat of the refrigerant in said at least one evaporator; a first control step of controlling the opening of said expansion valve so that the degree of superheat sensed at the first sensing step may equal a first preset value; a second sensing step of sensing the deviation of the degree of superheat sensed at said first sensing step from said first preset value and further sensing the maximum value of the sensed deviations one after another; a third sensing step of sensing the amount of change when the latest maximum value sensed at the second sensing step is decreased below the preceding maximum value sensed at the second sensing step;; a first judging step of judging whether or not the amount of change sensed at the third sensing step is equal to or smaller than a second preset value and the latest maximum value sensed at said second sensing step is equal to or larger than a third preset value; and a first correction step of correcting the opening control value at said first control step in a decreasing direction, when the conditions judged at the first judging step are met a predetermined number of times consecutively.

7. A method according to claim 6, further comprising: a second judging step for judging whether or not the deviation sensed at said second sensing step has remained at a fourth preset value or more for a certain period of time; and a second correction step for correcting the opening control value with said first control step in an increasing direction, when the condition judged by the second judging step is met.

8. A method of controlling a refrigerating apparatus comprising a compressor for compressing a refrigerant, a condenser for condensing the refrigerant discharged from the compressor, an expansion valve for depressurizing the refrigerant passed through the condenser, and at least one evaporator for evaporating the refrigerant passed through the expansion valve, and an inverter capable of supplying a driving power for driving said compressor and changing the output frequency, said method comprising:: a first sensing step of sensing the degree of superheat of the refrigerant in said at least one evaporator; a first control step of controlling the opening of said expansion valve so that the degree of superheat sensed at the first sensing step may equal a first preset value; a second sensing step of sensing the deviation of the degree of superheat sensed at said first sensing step from said first preset value and further sensing the maximum value of the sensed deviations one after another; a third sensing step of sensing the amount of change when the latest maximum value sensed at the second sensing step is decreased below the preceding maximum value sensed at the second sensing step; ; a first judging step of judging whether or not the amount of change sensed at the third sensing step is equal to or smaller than a second preset value and the latest maximum value sensed at said second sensing step is equal to or larger than a third preset value; a first correction step of correcting the opening control value at said first control step in a decreasing direction, when the conditions judged at the first judging step are met a predetermined number of times consecutively; a second judging step for judging whether or not the deviation sensed at said second sensing step has remained at a fourth preset value or more for a certain period of time; and a second correction step for correcting the opening control value with said first control step in an increasing direction, when the condition judged by the second judging step is met.

a setting step for setting a target value of indoor temperature; an indoor temperature sensing step for sensing indoor temperature; a fourth sensing step for sensing the difference between the temperature sensed by the indoor temperature sensing step and-the target value of indoor temperature set at said setting step; and a second control step for controlling the output frequency of said inverter according to the difference sensed at said fourth sensing step.

9. A method according to claim 8, further comprising: a fifth sensing step for, when the output frequency of said inverter has changed, sensing the difference between the latest maximum value sensed at said second sensing step and the deviation sensed at the second sensing step a specified time after the preceding sensing; a prediction step for predicting the time required for the degree of superheat in said evaporator to reach said first preset value on the basis of the difference sensed at the fifth sensing step and the deviation sensed at said second sensing step; a third correction step for correcting the opening control value with said first control step in an increasing direction, when the time predicted at the prediction step is longer than a fifth preset value; and a fourth correction step for correcting the opening control value with said first control step in a decreasing direction, when the time predicted at said prediction step is shorter than a sixth preset value.

10. A refrigerating apparatus keeping the degree of superheat of a refrigerant in an evaporator at a preset value and a control method thereof, substantially as hereinbefore described with reference to the accompanying drawings.