JP2581622B2 - Method and apparatus for controlling capacity of screw compressor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the operation of a screw compressor in a large multi-type air conditioner.

[0002]

2. Description of the Related Art A conventional multi-type air conditioner has a relatively small capacity, and the capacity of a compressor is controlled by using an inverter to control the capacity according to the number of load air conditioners in operation or by using a bypass valve. A method of reducing the load on the load side is adopted. However, in a large multi-type air conditioner with multiple load-side air conditioners, it is difficult to immediately control the capacity of the screw compressor to the required capacity according to the operation of the load-side air conditioner. The control is limited to a step-by-step rough control of the capacity by a solenoid valve or the like. A known example of this type is described in Japanese Patent Application Laid-Open No. 50-37013.

Japanese Unexamined Patent Publication No. 55-17027 discloses a current upper limit limiting operation method using a compressor and a bypass valve for protecting electric wires. It is not intended to always control the required capacity between the compressors.The compressor itself operates under a certain fixed capacity and forms a bypass circuit using a solenoid valve between the heat exchangers of the refrigeration cycle. It is intended only to reduce the current below the maximum current allowable in terms of the life of the compressor, the life of the wiring, and the like. Therefore, a plurality of load-side air conditioners, such as a multi-type air conditioner, frequently start and stop individually,
When it is required to control the capacity of the air conditioner in order to follow the load fluctuation, it is necessary to control the compressor capacity itself to stabilize the fluctuation of the operating pressure and the fluctuation of the current at the same time.

In the technique disclosed in Japanese Patent Application Laid-Open No. 55-17027, current value detection is performed. However, in the case of a multi-type air conditioner in which a plurality of air conditioners, that is, heat exchangers are connected to the load side, the same current condition depends on the combination of the unload state of the compressor and the number of operating heat exchangers. And the above-mentioned JP-A-55-17.
The technology disclosed in Japanese Patent No. 027 is simply for the purpose of preventing overload, and does not control the refrigeration cycle for the purpose of capacity control in order to follow the load. Instead, the capacity of the compressor is determined based on the detected current value. Is not intended to determine how much is output. In contrast, with conventional compressor capacity control using an inverter, it is possible to control the capacity of the compressor in accordance with the number of operating load-side air conditioners, and the output of the inverter is arbitrarily output to perform smooth capacity control. .

[0005]

When the compressor is used as a compressor for a multi-type air conditioner, the capacity of the reciprocating compressor is controlled stepwise by the fluctuation of the load. In such a case, the capacity is insufficient or the capacity is excessive, and the start / stop of the compressor is repeatedly controlled, which causes problems such as unstable behavior of the refrigeration cycle and a reduction in the reliability of the compressor due to these effects.

On the other hand, the screw compressor is operated at a rotation speed higher than a predetermined value so as to reduce the ratio of the amount of leakage from the seal portion because the screw compressor compresses the refrigerant by confining the refrigerant in the screw-type rotating portion of the compression mechanism. Need to be done. When the rotation speed control is performed by the inverter or the like described above, a predetermined capacity cannot be obtained due to a decrease in compression efficiency during the capacity control at a low speed rotation equal to or less than the predetermined value. Therefore, the capacity of the compressor, that is, the output of the compressor is controlled by increasing or decreasing the capacity of the compression chamber by a slide valve constituting a part of the wall of the compression section. The capacity can be controlled continuously from the capacity to the maximum capacity of 100%. However, the current screw compressor does not have a structure in which the position of the slide valve can be controlled from the outside to the position of the required compressor capacity, and the position of the slide valve is not easily detected and there is no inexpensive practical means. For example, a plurality of load side air conditioners are operated for cooling, and
When several air conditioners on the load side are thermo-stopped while operating with the% output balanced, the compressor capacity must be reduced. , And the load side and the non-load side are in an unbalanced operation state, and it often happens that the operation such as the operation of the protection device cannot be continued.

Further, in the step control system in which the position of the slide valve is stopped mechanically at a position where the displacement is predetermined, there is a limit to the stage in which the displacement can be controlled, and the matching capacity with the load is set in the middle of the set stage. In some cases, the capacity becomes too large or small, and the capacity becomes too large or too small, which degrades comfort and is incompatible with the operation of multiple load-side air conditioners. In order to adapt to different loads, continuous capacity control by a slide valve is absolutely necessary.

An object of the present invention is to provide a multi-type air conditioner equipped with a screw compressor, in which a plurality of load-side air conditioners are started and stopped irregularly during operation, and the compressor is commensurate therewith. To control the compressor capacity.

[0009]

In order to achieve the above object , a pressure detector for detecting a discharge pressure during operation of a compressor and a current detector for detecting a compressor operating current are provided and taken out as electric signals, respectively. The relationship between the discharge pressure and the compressor operating current, which varies according to the number of operating load-side air conditioners in operation, is stored in advance as reference data, and the extracted electric signal is compared with the reference data to determine the relationship. It is determined whether or not the compressor capacity at the time does not exceed the operable compressor capacity. If the current is excessive with respect to the detected pressure and the number of operating load side air conditioners at that time, it is determined that the compressor is operating with a capacity equal to or greater than the appropriate capacity, and the compressor slide valve is controlled. To reduce the capacity,
Capacity control is performed while feeding back so as to fall within an appropriate current value range, and compressor capacity control is continuously performed according to the load.

In order to achieve the above object , an outdoor unit for compressing refrigerant by a screw compressor having a slide valve for capacity control, and a plurality of units connected to the outdoor unit and circulating the refrigerant for air conditioning. A multi-type air conditioner comprising a combination of load side air conditioners, a deviation detecting means for detecting a deviation amount between a set temperature of each load side air conditioner and a suction air temperature of the air conditioner; Means for calculating the control amount of the slide valve of the compressor from the deviation amount, means for moving the slide valve steplessly based on the calculated control amount, a current detector for detecting the screw compressor operating current, and refrigeration. A pressure detector for detecting the cycle pressure, an operating number detecting means for detecting the operating number of the load side air conditioners, and the detected screw compressor operating current and the refrigeration cycle pressure at that time. And an arithmetic unit for calculating a compressor output of time, the calculation unit, the actual compressor output is calculated,
Means are provided for generating a signal for steplessly controlling the position of the slide valve so as not to exceed the operable compressor capacity determined from the detected number of operating load-side air conditioners.

[0011]

The operation unit does not control the compressor to a given compressor capacity (absolute output value), but moves the slide valve in a direction in which the input deviation becomes smaller, and determines how the deviation as a result of the movement is. Correct the position of the slide valve according to the condition. That is, the slide valve is always moved with reference to the current position. Therefore, it is not necessary to move the slide valve to a specific position to achieve a certain compressor capacity,
No means for specifying the position of the slide valve is required.

Further, since the compressor output has a correlation between the compressor operating current and the discharge pressure, the current compressor capacity (output) can be known by detecting these at the same time. Continuous (stepless) while detecting the state of load change
The position of the slide valve is changed, and the compressor output (capacity) changes accordingly. However, the number of operating load side air conditioners is constantly checked, and the operable compressor capacity determined from this number and the compressor capacity are determined. The compressor capacity (output) calculated from the machine operating current and the discharge pressure is compared, and the slide valve is controlled so that the calculated compressor capacity (output) does not exceed the operable compressor capacity. Therefore, the compressor output does not become excessive.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. FIG. 1 shows a main configuration of an air conditioner to which the present invention is applied. The illustrated air conditioner includes an outdoor unit 1 and a refrigerant branch pipe 3 connected to the outdoor unit 1 via a refrigerant pipe 19.
A plurality of load-side air conditioners 2 each having one end connected in parallel to the refrigerant branch pipe 3; a refrigerant branch pipe 4 connected to the other end of each of the plurality of load-side air conditioners 2; And a refrigerant pipe 20 connecting the pipe 4 to the outdoor unit 1, and a power supply 18 is connected to the outdoor unit 1. The outdoor unit 1 includes a screw compressor (hereinafter, simply referred to as a compressor) driven by a motor to compress refrigerant gas, a high-pressure detector 1a that detects a discharge pressure of the compressor, and the motor that drives the compressor. Current detector 1b for detecting the input current to
And a processing unit 1c for processing the data and controlling the compressor.

A part of a casing accommodating a rotor of the compressor is constituted by a slide valve. The slide valve is stepless in an axial direction by a hydraulic piston moving in a cylinder and a solenoid valve for putting and receiving pressure oil into and out of the cylinder. Is to be moved. When the position of the slide valve changes,
The bypass amount of the refrigerant gas changes, and the discharge amount of the refrigerant gas, that is, the output (capacity) of the compressor changes.

FIG. 6 shows the concept of the moving operation section of the slide valve. A part of a casing accommodating a rotor 61 of the compressor is constituted by a slide valve 62 movable in the axial direction, and the slide valve 62 has a piston rod 6 parallel to the rotor axial direction.
3 is attached. A piston 64 attached to the tip of the piston rod 63 slides in a cylinder 65. A high-pressure side hydraulic pipe 70 is provided at an end of the cylinder 65 at the piston rod 63 side, and at an opposite end. Is connected to a low-pressure side hydraulic pipe 66. The high-pressure-side hydraulic pipe 70 branches into three, and electromagnetic valves 67, 68, and 69 are provided, respectively. Have been. The low pressure side hydraulic pipe 66 is also connected to the low pressure side of the hydraulic pressure generator. Also, the cylinder 64
A spring 71 for pulling the piston 64 toward the piston rod is provided between the piston 71 and the piston 64. Solenoid valve 6
When the solenoid valve 67 is opened and the solenoid valve 67 is opened, the high-pressure oil in the cylinder 65 is discharged to the low-pressure side, the piston 64 is attracted to the piston rod side by the spring 71, and the slide valve 62
Returns to the position where the capacity of the compressor is maximized. Solenoid valve 6
7 and closing the solenoid valves 68 and / or 69 for a short time,
High-pressure oil flows into the piston rod side of the piston 64, and the piston 64 moves to the opposite side of the piston rod. Piston 64
The slide valve 62 also moves in the axial direction with the movement of the compressor, and the bypass amount of the refrigerant gas increases, thereby reducing the capacity of the compressor.
The amount of movement of the piston 64 is proportional to the time during which the solenoid valves 68 and / or 69 are open, and the arithmetic unit 1c controls the combination of opening and closing of the solenoid valves and the opening time to perform capacity control.

The plurality of load-side air conditioners 2 are provided with a heat exchanger 2a for performing heat exchange between the refrigerant supplied from the outdoor unit 1 and the indoor air, a blower 2b, the set temperatures of the respective load-side air conditioners, and the like. Deviation detecting means 2c for detecting a deviation amount of the intake air temperature of the air conditioner. The deviation detecting means 2c is connected to the calculation section 1c, outputs the deviation amount, and also transmits whether the load-side air conditioner is in an operating state.

FIG. 2 shows time in the horizontal axis, temperature and compressor capacity (%) in the vertical axis, and operates the air conditioner of this embodiment in a section where the load changes as shown by a load fluctuation curve 5. And the change in the capacity of the continuous displacement control compressor used in the air conditioner of this embodiment at that time, and the change in the room temperature when the conventional step displacement control type air conditioning is performed under the same conditions. And the change (capacity (output)) 9 of the stage control compressor at that time.

FIG. 3 shows the compressor discharge pressure on the horizontal axis and the compressor capacity (output) (%) on the vertical axis, the input current value to the motor for driving the compressor, the compressor discharge pressure and the compressor. The relationship between capacity (output) (%) is shown. Straight lines 10A, 10
B and 10C are the compressor discharge pressure and the compressor capacity (output) (%) when the input current value to the motor for driving the compressor is a current value a, b, c (c>a> b), respectively. Shows the relationship.

FIG. 4 shows the number of operated load-side air conditioners on the horizontal axis and the operable capacity (output) (%) of the compressor on the vertical axis. The operable compressor capacity (output) (%) is indicated by a bar 11. FIG. 4 shows the operable compressor capacity at the designed discharge pressure, and the number changes when the compressor is operated at a different pressure.

Next, a capacity control method for the case where the air conditioner shown in FIG. 1 performs a cooling operation will be described. After the start of operation, a high-temperature and high-pressure gas refrigerant is discharged from the compressor of the outdoor unit 1, condensed in the outdoor unit heat exchanger, and distributed to the load-side air conditioner 2 via the refrigerant branch pipe 3. The liquid refrigerant distributed to each load-side air conditioner 2 exchanges heat with the indoor load to remove heat from the indoor air to cool the indoor air and, after evaporating and vaporizing itself, collects in the refrigerant branch pipe 4. And returns to the outdoor unit 1. At this time, the pressure of the refrigerant to be condensed is determined by the ambient temperature of the outdoor unit 1 and the operating capacity of the compressor, and the input current value to the motor for driving the compressor (hereinafter referred to as the compressor operating current) is also in its operating state ( (Load condition). The compressor operating current and the compressor discharge pressure, which is one of the refrigeration cycle pressures, are constantly detected by the current detector 1b and the high-pressure pressure detector 1a attached to the outdoor unit 1, and are calculated by the calculation unit 1c from the characteristics shown in FIG. The compressor operating capacity (output%) at that time is calculated.

FIG. 2 shows the operating state of the air conditioner in a section where the load changes as indicated by a load fluctuation curve 5. After the compressor is started, a deviation signal indicating the difference between the set temperature of the load-side air conditioner and the suction air temperature of the load-side air conditioner is transmitted from the deviation detection means 2c of each load-side air conditioner 2 to the calculation unit 1c. Entered,
The arithmetic unit 1c performs arithmetic processing on a plurality of input deviation signals to convert the deviation signals into slide valve control signals. If the deviation is large, a control signal in the direction of increasing the compressor capacity is output, and the control signal opens and closes the solenoid valve of the slide valve drive unit.

As the suction air temperature gradually approaches the set value, the compressor capacity is reduced accordingly. As shown in the compressor capacity change 9 of this embodiment, which is continuous capacity control, and the compressor capacity change 10 in the case of stepwise capacity control, in the continuous capacity control of this embodiment, capacity can be smoothly controlled in accordance with a load change. The room temperature also follows the set value, but in the step control, the compressor output does not match the load in the middle of the step, so that the compressor must be operated with an output larger than the load. For this reason, start / stop control is performed after the temperature is too cold, and the room temperature fluctuates greatly, reducing comfort and causing a drop in efficiency due to start / stop.

On the other hand, in a multipackage in which an arbitrary load variation is given to each of a plurality of n load-side air conditioners, the load-side air conditioners individually start and stop according to the degree of indoor load. Therefore, if one or more load-side air conditioners suddenly stop at the next point in time even when operating at 100% capacity, the balance of the refrigeration cycle cannot be maintained, and the compressor capacity becomes excessive. Become. If the capacity of the compressor becomes excessive, the low-pressure pressure decreases, and the operation of the protection device becomes abnormal. For example, the capacity of the compressor must follow the number of operating units on the load side and be properly maintained.

One embodiment of the control method shown in the flowchart of FIG. 5 will be described below. The straight line 10 in FIG.
Data such as A, 10B, and 10C are created at fine current value intervals and stored in the memory of the arithmetic unit 1c.
Predetermined time interval after starting operation (sampling time)
Then, the discharge pressure and the compressor operating current are detected, and the calculation unit 1c
(Step 52). Next, a signal indicating whether or not each load-side air conditioner is operating is similarly input to the calculation unit 1c (step 53). The calculation unit 1c further includes a deviation signal (a plus-side deviation when the intake air temperature is higher than the set temperature) indicating the difference between the set temperature and the intake air temperature of the load side air conditioner from each load side air conditioner. Is input, and the magnitude of the load is determined from the input deviation signal. The deviation signal is individually input from a plurality of load-side air conditioners, and large and small deviations are mixed. Therefore, the arithmetic unit 1c detects the load by averaging or weighting these signals (Step 5).
4).

The arithmetic unit 1c then determines whether or not the compressor capacity (output) should be increased based on the detected load (step 55). If the deviation (load) is positive, the routine proceeds to step 58, where the capacity (output) is increased by the slide valve control. If the deviation (load) is not positive, the process proceeds to step 56, where it is determined whether the deviation is negative. If the deviation is negative, the routine proceeds to step 60, where the capacity (output) is reduced by the slide valve control. If the deviation is not minus, the procedure proceeds to step 57, and the compressor operation is continued with the same capacity. The arithmetic unit 1c includes a unit that receives the load (deviation) as input and generates the solenoid valve opening / closing signal by control logic such as proportional control, proportional integral control, and PID control.
The capacity increase / decrease due to the slide valve control in steps 58 and 60 is determined by the solenoid valves 67, 68 and 69 output from the calculation unit 1c.
This is performed by a solenoid valve opening / closing signal to the controller.

When the compressor output is increased in step 58, the process proceeds to step 59. In step 59, the operation unit 1c executes step 52
Calculates the compressor output at that time by referring to the data stored in the memory from the discharge pressure and compressor operation current input in the above, and also operates the air conditioner from the input load side air conditioner operation signal. The number is calculated, and the operable compressor capacity is calculated with reference to FIG. The operation unit 1c compares the calculated compressor output at that time with the operable compressor capacity. If the compressor output is low, the operation is continued, and the procedure proceeds to step 52. If the compressor output is high, the procedure proceeds to step 60 where the capacity (output) is reduced by the slide valve control. In this case, the slide valve control signal is generated by a simple proportional control method using a deviation between the compressor output and the operable compressor capacity as an input. When the output of the compressor is reduced by the control signal output from the arithmetic unit 1c in step 60, the process returns to step 52 again, and the previous steps are repeated.

In the stage control, the compressor capacity can be controlled stepwise according to the number of units, but the start / stop control in that range has a limit in following the capacity. In the control of the present embodiment in which the position of the slide valve is changed continuously (steplessly), it is possible to finely follow the load, and good control having the characteristics shown in the characteristic 8 in FIG. 2 is performed.

It should be noted that the current aging 10A, 10A shown in FIG.
B and 10C have, for example, compressor operating current values a
The values b, c, etc. are shown. In the illustrated current characteristics, for example, if the current value is a and the discharge pressure is P1, the compressor output at that time is 100%, and if the current value becomes b even if the discharge pressure is maintained at the same P1, The compressor output drops to m1%. Further, from the state where the pressure is P1 and the current value is a,
If an attempt is made to increase the pressure to P2 while maintaining the current value at a, the pressure characteristic will be changed on the current characteristic 10A, and the slide valve will be moved to increase the refrigerant bypass amount and decrease the refrigerant discharge amount. . Due to the decrease in refrigerant discharge,
The capacity of the compressor drops to m2%. This control is established and executed by formulating or storing these relationships in a mathematical expression or a numerical table, and inputting and storing them in the arithmetic unit of the outdoor unit.

According to this embodiment, in the multi-type air conditioner using the screw compressor, the compressor capacity can continuously follow the load fluctuation of the load side air conditioner, and the load side air conditioner unique to the multi type. Even during individual operation of the machine, it is possible to monitor and control the compression function so as not to be excessive, so that continuous capacity control from a minimum capacity to an appropriate capacity can be realized. In addition, since the capacity control is performed in a stepless manner, the number of start / stop of the compressor caused by the stepwise control can be reduced, which is effective in reducing excessive cooling, excessive rush current at the time of starting the compressor, and alleviating the generation of electrical noise. In addition, reliability is improved and energy consumption is reduced. By eliminating the start / stop of the step control, it is possible to save about 9% of energy when the COP is assumed to be constant.

[0030]

According to the present invention, in a multi-type air conditioner using a screw compressor, the compressor capacity can continuously follow the load fluctuation of the load-side air conditioner, and the load side unique to the multi-type air conditioner can be changed. Even during individual operation of the air conditioners, monitoring and control can be performed so that the compression function is not excessive, so that continuous capacity control from a minimum capacity to an appropriate capacity can be realized. Furthermore, continuous capacity control can reduce the number of compressor start / stops caused by step control, which is effective in alleviating excessive cooling, excessive rush current when starting the compressor, and reducing the occurrence of electrical noise. Energy can be achieved. Energy saving is about 9% when COP is assumed to be constant by eliminating start / stop of step control.

[Brief description of the drawings]

FIG. 1 is a system diagram of a multi-type air conditioner according to an embodiment of the present invention.

FIG. 2 is a conceptual diagram showing a compressor capacity control state with respect to a load change in comparison with an embodiment of the present invention and a conventional technique.

FIG. 3 is a conceptual diagram showing the relationship between compressor operating pressure and capacity via compressor operating current.

FIG. 4 is a conceptual diagram showing the operable compressor capacity with respect to the number of load-side air conditioners.

FIG. 5 is a control flowchart showing an embodiment of the present invention.

FIG. 6 is a conceptual diagram showing an example of a mechanism for operating a slide valve of the screw compressor in the embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Outdoor unit 1a High-pressure detector 1b Current detector 1c Operation part 2 Load side air conditioner 2a Heat exchanger 2b Blower 2c Deviation detecting means 3, 4 Refrigerant branch pipe 5 Load fluctuation 10A, 10B, 10C Current characteristic curve 11 Operable Compressor capacity 62 Capacity control slide valve 63, 64, 65, 67, 68, 69, 70, 71 Means for moving slide valve steplessly

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Kenji Kawasaki 390 Muramatsu, Shimizu-shi, Shizuoka Prefecture Inside the Shimizu Plant, Hitachi, Ltd.

Claims (6)

(57) [Claims] 1. A combination of an outdoor unit for compressing refrigerant by a screw compressor having a slide valve for capacity control and a plurality of load-side air conditioners connected to the outdoor unit and circulating the refrigerant for air conditioning. In the compressor capacity control method in the multi-type air conditioner, the screw compressor operating current, the refrigeration cycle pressure, and the number of operating load side air conditioners are detected, and the detected screw compressor operating current and the refrigeration cycle pressure are used. The actual compressor output at that time is calculated, and the slide valve is adjusted so that the calculated actual compressor output does not exceed the operable compressor capacity determined from the detected number of operating load-side air conditioners. A capacity control method for a screw compressor, wherein the capacity control of the screw compressor is performed steplessly by operation. 2. A combination of an outdoor unit that compresses a refrigerant by a screw compressor having a slide valve for capacity control and a plurality of load-side air conditioners connected to the outdoor unit and circulating the refrigerant to perform air conditioning. In the capacity control method of the compressor in the multi-type air conditioner, the operating current of the screw compressor, the refrigeration cycle pressure and the number of operating load-side air conditioners are detected, and the detected operating current of the screw compressor is:
A capacity control method for a screw compressor, comprising: operating the slide valve so as to be equal to or less than a maximum allowable current value determined from a detected refrigeration cycle pressure and the number of operating load-side air conditioners. 3. A method for controlling the capacity of a compressor in a multi-type air conditioner comprising a combination of an outdoor unit incorporating a screw compressor having a slide valve structure capable of continuously controlling the capacity and a plurality of load side air conditioners. Simultaneously detect the refrigeration cycle pressure and the compressor operating current at the time, operate the slide valve so that the operating current of the compressor becomes an operating current that matches the number of operating load side air conditioners and the refrigeration cycle pressure, A capacity control method for a screw compressor, wherein the capacity control of the screw compressor is performed steplessly. 4. A method for detecting a deviation between a set temperature of each load-side air conditioner and a suction air temperature of the air-conditioner, and calculating a control amount of a slide valve of the compressor from the detected deviation of each load-side air conditioner. 4. The capacity control method for a screw compressor according to claim 1, further comprising a step of moving the slide valve steplessly based on the calculated control amount. 5. A combination of an outdoor unit for compressing refrigerant by a screw compressor having a slide valve for capacity control and a plurality of load-side air conditioners connected to the outdoor unit and circulating the refrigerant for air conditioning. In the multi-type air conditioner, a current detector for detecting a screw compressor operating current, a pressure detector for detecting a refrigeration cycle pressure, an operating number detecting means for detecting the operating number of the load side air conditioners, A calculating unit for calculating the actual compressor output at that time from the screw compressor operating current and the refrigeration cycle pressure, the calculating unit comprising: A control signal generating means for continuously operating the slide valve position so as not to exceed the operable compressor capacity determined from the number of side air conditioners operated. Clew compressor capacity control device. 6. An outdoor unit for compressing refrigerant by a screw compressor having a slide valve for capacity control, and a plurality of load-side air conditioners connected to the outdoor unit and circulating the refrigerant for air conditioning. In a multi-type air conditioner, a deviation detecting means for detecting a deviation between a set temperature of each load side air conditioner and a suction air temperature of the air conditioner, and a compressor slide based on the detected deviation amount of each load side air conditioner. Means for calculating the control amount of the valve, means for continuously moving the slide valve based on the calculated control amount, a current detector for detecting the operating current of the screw compressor, and a pressure detection for detecting the refrigeration cycle pressure And an operating unit detecting means for detecting the operating number of the load side air conditioners, and calculating an actual compressor output at that time from the detected screw compressor operating current and refrigeration cycle pressure. And a computing unit for controlling the position of the slide valve so that the calculated actual compressor output does not exceed the operable compressor capacity determined from the detected number of operating load-side air conditioners. And a control signal generating means for continuously operating the screw compressor.