SCREW COMPRESSOR

Abstract

A screw compressor is for use of a screw chiller, and comprises a pair of screw rotors and a casing housing the screw rotors, a capacity control valve for varying a ratio of volume, a motor for driving the screw rotors and an inverter for varying the rotational speed of the motor. The screw compressor is controlled using rotational speed control means by the inverter and mechanical capacity control means by the capacity control valve independently or combined together according to loads. The maximum efficient point in a capacity control performed solely by the inverter is set to a rotational speed side lower than the rated operation point. In a region where the rotational speed is higher than the maximum efficient point, the inverter solely takes control from a rated rotational speed to a high rotational speed side.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a screw compressor, and in particular, relates to a screw compressor suitable for a screw chiller.

As a screw compressor used for a conventional refrigerating apparatus, there is the one disclosed in JP-A-59-211790. A scroll wrap compressor of JP-A-59-211790 operates to vary a ratio of volume by performing an unload control with a capacity control valve (slide valve) in the capacity control range of 100% to 75% of a refrigerating capacity. Further, in the capacity control range of 75% to 37.5% of the refrigerating capacity, the scroll wrap compressor increases the rotational speed of the screw compressor by 1.5 times by an inverter, and by using the capacity control valve in the range of 50% to 25% of the refrigerating capacity, can operate to vary the ratio of volume. Such operation can provide efficiency better than when operated under a 100% load.

Further, as another conventional screw compressor, there is the one disclosed in JP-A-2004-137934. The screw compressor of JP-A-2004-137934 concurrently uses a rotating speed control by means of an inverter and a compression ratio control for varying the ratio of volume by changing the end of the compression stroke by means of a variable VI valve at the time of adjusting the capacity so that the optimum compressor efficiency is attained depending on the operating condition of the screw compressor.

Because the screw compressor in JP-A-59-211790 adopts an operating method in which the rotational speed is increased to a constant rotational speed and a capacity control valve is used in the range of 50% to 25%, there is a problem that a highly improved performance cannot be expected because of the increase in mechanical loss due to the increased speed and a bypass imposed by the capacity control valve.

In the screw compressors in JP-A-59-211790 and JP-A-2004-137934, no disclosure has been made regarding the effective use of the screw compressors as one for a screw chiller. That is, a compressor for the screw chiller is often operated at a refrigerating capacity ratio lower than the 100% rated refrigerating capacity ratio, that is, in the vicinity of 80% of the rated ratio, whereas, the screw compressors in JP-A-59-211790 and JP-A-2004-137934 are controlled such that the maximum efficient point is attained at rated operation, and therefore, effective operation has not been provided as the compressor for the screw chiller.

An object of the present invention is to obtain a screw compressor capable of operation performing an effective operation for a screw chiller.

SUMMARY OF THE INVENTION

To achieve the above described object, a screw compressor for a screw chiller according to the present invention comprises a pair of screw rotors, a casing housing the screw rotors, a capacity control valve for varying a ratio of volume, a motor for driving the screw rotors, and an inverter for varying rotational speed of the motor, wherein the screw compressor is controlled using rotational speed control means by the inverter and mechanical capacity control means by the capacity control valve either independently or combined together according to a load, and wherein the maximum efficient point in a capacity control performed solely by the inverter is set to a rotational speed side lower than rated operation point, and in an area where the rotational speed is higher than the maximum efficient point, the inverter solely takes control from a rated rotational speed to a high rotational speed side.

Specific preferable configuration examples according to the present invention are as follows.

(1) When an operation in the rotational speed range at or below the maximum efficient point is demanded, the rotational speed control means by the inverter and the mechanical capacity control means by the capacity control valve are used concurrently according to the capacities, thereby operating such that an efficiency becomes the maximum.

(2) The capacity control valve varies a compression starting position provided in the casing.

(3) The maximum efficient point is set around 80% of the rated refrigerating capacity when the capacity control is performed independently by the inverter.

(4) The position of the capacity control valve is controlled based on the rotational speed of the motor according to a suction pressure, a discharge pressure, and the load of the pair of screw rotors.

(5) When an anomaly occurs on the inverter, so that the continuous operation of the motor by the inverter is made impossible, the motor is directly connected to a commercial power in an emergency manner, thereby the capacity controlled operation is continued by the capacity control valve same as before.

According to the present invention, a screw compressor capable of performing an effective operation as a screw chiller can be obtained.

Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a screw compressor showing one embodiment of the present invention;

FIG. 2 is an operation explanatory view of a capacity control valve in FIG. 1; and

FIG. 3 is a view showing a characteristic curve of compressor efficiency relative to refrigerating capacity ratio in the screw compressor of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, one embodiment of the present invention will be described with reference to FIGS. 1 to 3. FIG. 1 is a sectional view of a screw compressor showing an embodiment of the present invention, FIG. 2 is an operation explanatory view of a capacity control valve in FIG. 1, and FIG. 3 is a view showing a characteristic curve of compressor efficiency relative to refrigerating capacity ratio in the screw compressor of FIG. 1.

A screw compressor 50 is constituted by a screw compressor for a screw chiller comprising a compressor portion 17, a motor portion 18 and a control device 23. Refrigerant gas to be compressed flows to the compressor portion 17 through the motor portion 18, and after being compressed by the compressor portion 17, is discharged outside the compressor. While the screw compressor 50 performing a capacity control operates in such a manner that the rotational speed and the position of a capacity control valve are changed so that pressure becomes constant, the control pressure is set to an arbitrary pressure.

The compressor portion 17 comprises a main casing 1, screw rotors 2, a capacity control valve 11, a rod 12, a hydraulic piston 13, a coil spring 14, a discharge casing 21, roller bearings 6 and 7, a ball bearing 8, and the like.

The main casing 1 forms a suction port 9, a discharge port 10, a gas outlet 19, and the like. The suction port 9 forms a suction flow passage toward the screw rotors 2, and the discharge port 10 forms a discharge passage from the screw rotors 2, and the gas outlet 19 forms a discharge flow passage toward the outside. The discharge casing 21 is disposed opposite to the motor of the main casing 1, and is fixed to the main casing 1.

The screw rotors 2 are constituted by a pair of a male rotor 2A and a female rotor (not shown) which are engaged with each other, and is housed in a pair of cylindrical bores (not shown). Shaft portion provided at both sides of the male rotor 2A are respectively supported by the roller bearing 6 disposed in the main casing 1 and the roller bearing 7 and the ball bearing 8 disposed in the discharge casing 21.

The capacity control valve 11 is for performing a capacity control by bypassing a part of refrigerant gas sucked into the engaged portion of the screw rotors 2 toward the suction side, and is movably housed in a concave portion 1b extending laterally. According to motor frequency controlled by the inverter 5, a position of the capacity control valve 11 attaining the best efficiency is controlled. The capacity control to bypass a part of the sucked refrigerant gas to the suction side can be efficiently performed as compared with capacity control to bypass a part of discharge gas to the discharge side. The hydraulic piston 13 is a piston for driving the capacity control valve 11 left and right through the rod 12, and is slidably housed in a cylinder 15 which extends laterally. The coil spring 14 is disposed at a capacity control valve chamber side of the cylinder 15, and always applies a force to press the hydraulic piston 13 toward a side opposite to the capacity control valve. A capacity control mechanism (mechanical capacity control means) is constituted by the capacity control valve 11, the rod 12, the hydraulic piston 13, and the coil spring 14.

The motor portion 18 comprises a motor casing 16, a driving motor 22 and the like, and is disposed such that driving force of the motor portion 18 is transmitted to the compressor portion 17. The motor casing 16 and the main casing 1 are fixed to each other with both end surfaces thereof in closely contact with each other, and at the same time, the interiors thereof are communicated with each other. A side surface opposite to the compressor side of the motor casing 16 is formed with a gas inlet 20 for sucking the refrigerant gas to be compressed.

The driving motor 22 is constituted by a motor stator 3 and a motor rotor 4, and is disposed inside the motor casing 16. The motor stator 3 is arranged on the inner peripheral surface of the motor casing 16. The motor rotor 4 is fixed to the shaft portion formed at one side of the male rotor 2A, and is rotatably disposed inside the motor stator 3. By such structure, the driving force of the driving motor 22 is transmitted to the male rotor 2A. Note that the female rotor is driven by the male rotor 2A.

The control device 23 comprises an inverter 5 for controlling a rotational speed of the driving motor 22, and a valve control portion 26 for controlling the position of the capacity control valve 11.

The inverter 5 controls the rotational frequency of the motor portion 22 according to loads. The control device 23 is connected with a power supply, a suction pressure sensor 24, and a discharge pressure sensor 25. The suction pressure sensor 24 is a sensor for detecting a suction pressure of the compressor, for example, a pressure of the gas inlet 20 to input to the control device 23. The discharge pressure sensor 25 is for detecting the discharge pressure of the compressor, for example, a pressure of the gas outlet 20 to input to the control device 23.

The screw compressor 50 is constituted to be controlled using rotational speed control means by the inverter 5 and mechanical capacity control means by the capacity control valve 11 either independently or combined together according to loads. Further, the screw compressor 50 is structured such that the maximum efficient point when an independent capacity control is performed by the inverter 5 is set in a rotational speed side lower than the rated operation point and the region in which the rotational speed is higher than the maximum efficient point is controlled by the inverter alone from the rated rotational speed to the high rotational speed side. Further, the screw compressor 50 is structured such that, when the operation in a rotational speed range at or below the best efficiency point is required, both of the rotational speed control means by the inverter 5 and the mechanical capacity control means by the capacity control valve 11 are used according to the capacities so as to perform the operation in which the efficiency becomes the maximum.

The valve control portion 26, even when an anomaly occurs on the inverter 5 and the continuous operation by the inverter 5 is made impossible, directly connects the motor 22 to a commercial power in an emergency manner so as to effect a capacity control by the capacity control valve 11, and performs a control such that a capacity controlled operation by the capacity control valve 11 is continued same as before. In this manner, the operational reliability of the screw compressor 50 can be improved.

In the screw compressor 50 thus structured, by supplying the power to the driving motor 22 through the inverter 5, the driving motor 22 is rotated by the predetermined rotational speed by the inverter 5, and further, the compressor portion 17 is rotated. As a result, the refrigerant gas to be compressed is sucked into the motor casing 16 through the gas inlet 20, and cools the driving motor 22, and thereafter is sucked into the screw rotors 2 through the suction port 9, and after being compressed by the screw rotor 2, is discharged to the discharge flow passage from the discharge port 10, and further, is discharged to an outside flow passage from the gas outlet 19.

The refrigerating cycle of the screw chiller, as shown in FIG. 1, is structured by connecting the screw compressor 50, a condenser 27, an expansion valve 28, and an evaporator 29 in order in annular manner. High temperature and high pressure refrigerant discharged from the screw compressor 50 is condensed by the condenser 27 by heat exchange with the air by a fan 30 so as to become a low temperature and high pressure liquid refrigerant to be supplied to the expansion valve 28. The low temperature and low pressure liquid refrigerant depressurized by the expansion valve 28 is evaporated by heat exchange with water of a cold water piping 29b by a refrigerant piping 29a of the evaporator 29 so as to become a low pressure gas to be returned to the screw compressor 50. The cold water cooled by the cold water piping 29b is used for cooling control.

A temperature sensor 31 is attached to the cold water piping 29b of the evaporator 29, and a detection signal representing a cooling water temperature from the temperature sensor 31 is input to the control device 23. So, the control device 23, taking the cooling water temperature based on the input detection signal as information from a load side, controls the inverter 5 and the capacity control valve 11 to perform capacity adjustment for the load.

In the screw compressor 50, capacity adjustment for the load is executed by inputting the signal from the suction pressure sensor 24 and the signal from the discharge pressure sensor 25 into the control device 23, and at the same time, by inputting the signal from the temperature sensor 31 to the control device 23, and based on these signals, by performing the rotational speed control of the driving motor 22 by the inverter 5 and the position control of the capacity control valve 11 by the valve control portion 26.

The capacity adjustment for the load, as described above, is performed by the control of the inverter 5 alone from the rated rotational speed to the high rotational speed side in the region where the rotational speed is higher than the maximum efficient point when the independent capacity control is performed by the inverter 5, and by a combination of the rotational speed control by the inverter 5 and the mechanical capacity control by the capacity control valve 11 so that the efficiency becomes the maximum according to the capacity in the region where the rotational speed is at or below the maximum efficient point.

Here, in the region where the refrigerating capacity ratio is higher than the refrigerating capacity ratio serving as the maximum efficient point (refrigerating capacity ratio of 80% of the rating in the present embodiment), as shown in FIG. 2(a), the capacity control valve 11 is moved to the motor side in the shaft direction so as not to bypass the refrigerant gas, and the rotational speed of the driving motor 22 is controlled by the inverter 5. Further, in the region where the rotational speed is at or below the maximum efficient point, as shown in FIG. 2(b), the capacity control valve 11 is moved opposite to the motor side in the shaft direction so as to bypass the refrigerant gas to the suction side, and at the same time, the rotational speed of the driving motor 22 is controlled by the inverter 5.

Overall adiabatic efficiency and cooling capacity of the screw compressor 50 will be described with reference to FIG. 3. In FIG. 3, an axis of ordinate shows compressor efficiency and an axis of abscissas shows refrigerating capacity ratio. An alternate long and short dash line in the drawing indicates an efficiency curve by the rotation control by the inverter, and a broken line indicates an efficiency curve, in case where the rotational speed is changed by the inverter and is fixed to respective rotational speeds and at the same time, the capacity control is performed by the capacity control valve. Note that a sold line in the drawing indicates an efficiency curve in case where a control is performed such that the rotational speed control by the inverter and the capacity control by the capacity control valve are combined so as to reach the best efficiency.

As apparent from FIG. 3, in the case of the screw compressor 50, in the present embodiment, combining the control by the inverter 5 and the control by the capacity control valve 11, it is possible to increase the compressor efficiency in the refrigerating capacity ratio not more than 80%, and particularly in the region where the refrigerating capacity ratio is low, it is possible to sharply increase the compressor efficiency.

According to the invention, a screw compressor efficiently operated as a screw chiller can be obtained.

It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.

Claims

1. A screw compressor for a screw chiller, comprising:

a pair of screw rotors;

a casing housing the screw rotors;

a capacity control valve for varying a ratio of volume;

a motor for driving the screw rotors; and

an inverter for varying a rotational speed of the motor,

wherein the screw compressor is controlled using rotational speed control means by the inverter and mechanical capacity control means by the capacity control valve either independently or combined together according to loads, and

wherein the maximum efficient point in the capacity control performed solely by the inverter is set to a rotational speed side lower than a rated operation point, and in a region where the rotational speed is higher than the maximum efficient point, the inverter solely takes control from a rated rotational speed to a high rotational speed side.

2. The screw compressor according to claim 1, wherein when an operation at a rotational speed region at or below the maximum efficient point is demanded, the rotational speed control means by the inverter and the mechanical capacity control means by the capacity control valve are used concurrently according to capacities so that efficiency becomes the maximum.

3. The screw compressor according to claim 1, wherein the capacity control valve varies a compression starting position provided in the casing.

4. The screw compressor according to claim 1, wherein the maximum efficient point is set to around 80% of a rated refrigerating capacity when an independent capacity control by the inverter is performed.

5. The screw compressor according to claim 2, wherein a position of the capacity control valve is controlled based on the rotational speed of the motor according to a suction pressure, a discharge pressure and a load of the pair of the screw rotors.

6. The screw compressor according to claim 1, wherein when an anomaly occurs on the inverter and a continuous operation of the motor by the inverter is made impossible, the motor is directly connected to a commercial power in an emergency manner and the capacity controlled operation by the capacity control valve is continued same as before.