JP2012102967A - Screw refrigerating machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a screw refrigerating machine equipped with a two-stage screw compressor capable of exerting cooling capacity matching to a load even under a condition that high pressure is low and low pressure is high and capable of avoiding the occurrence of motor overheating abnormality and the seizure of a screw rotor and a casing in a high stage compression section.SOLUTION: A high pressure detector 61 detects the pressure (high pressure) of a refrigerant discharged from the high stage compression section 23, an intermediate pressure detector 62 detects the pressure (intermediate pressure) of the refrigerant discharged from a low stage compression section 22, and a low pressure detector 63 detects suction pressure (low pressure) of the low stage compression section 22. The low stage compression section 22 and the high stage compression section 23 are provided with pistons, rods and volume control devices formed of slide valves for controlling the volume of the compressed refrigerant, respectively. A control section 64, based on the output of the high pressure detector 61 and the output of the intermediate pressure detector 62, calculates a pressure ratio of the high pressure to the intermediate pressure and controls the low stage volume control device 66 to vary an operation load of the low stage compression section 22 according to the magnitude of the pressure ratio.

Description

  The present invention relates to a screw refrigerator having a two-stage screw compressor that performs two-stage refrigerant compression by two or two pairs of screw rotors, and in particular, a two-stage having a capacity control device in each compression section. The present invention relates to capacity control of a screw compressor.

A conventional two-stage screw compressor (hereinafter sometimes simply referred to as a screw compressor or a compressor) has a capacity control device driven by a piston in each of a low-stage compression section and a high-stage compression section, By opening and closing the solenoid valve provided outside, the piston is driven by applying differential pressure to each of the low-stage and high-stage pistons, and the slide valve that adjusts the flow rate of the refrigerant gas is moved to adjust the capacity. Is known (for example, see Patent Document 1).
In addition, the low-stage compression unit has a capacity control device using a piston, and the piston is driven by applying a differential pressure to these low-stage pistons by opening and closing an electromagnetic valve provided outside. There is known one that adjusts the volume by adjusting the flow rate of (see, for example, Patent Document 2).
A general configuration of a screw refrigerator equipped with the two-stage screw compressor of Patent Document 2 is shown in FIGS. The screw refrigerator 1 includes a compressor 2, an oil separator 12, a condenser 13, an expansion valve 14, an evaporator 15, an oil cooler 25, and a refrigerant pipe that connects these components and circulates a refrigerant. Yes. The compressor 2 is a two-stage type having a low-stage compression unit 22 and a high-stage compression unit 23 driven by a single electric motor, and the discharge port of the low-stage compression unit 22 and the suction port of the high-stage compression unit 23 communicate with each other. is doing. The oil 24 separated by the oil separator 12 is supplied to the respective bearings of the low-stage compression section 22 and the high-stage compression section 23 of the screw compressor 2 through the oil cooler 25. A through-hole 39 is formed in the piston 38, which is a low-stage capacity control device, and the number of through-holes that bypass the refrigerant gas to the low-pressure portion changes as the piston moves. For this reason, capacity control becomes possible by controlling the movement of the piston and adjusting the number of through holes.
In addition, the two-stage screw compressor 2 shown in Patent Document 2 is driven by an inverter and does not have a capacity control device on the higher stage side.

The capacity control at the time of starting the conventional screw refrigerator 1 is shown in FIG. This is a case of star delta start, and is an example of a refrigerator having a constant operation frequency in which variable frequency operation by an inverter is not performed. When starting the screw refrigerator 1, first, the capacity of the screw compressor 2 is held at the minimum capacity, for example, 12%, for both the low stage and the high stage. After operation with a star connection for a certain time, it switches to the delta connection. After switching to the delta connection, when the set time elapses, the high stage is on-loaded to 100%. This state is called 20% operation for convenience. Thereafter, after the set time has elapsed, the lower stage is on-loaded to 50%. This state is called 60% operation for convenience. Furthermore, after this 60% operation has passed for a certain period of time, depending on the load on the refrigerator, both the high and low stages are 100% 100% operation (the high stage is kept at 100% and the low stage is On-load to 100%), previous 60% operation (operating with the high stage held at 100%, operating with the low stage kept at 60%), 20% operation (high stage at 100% The operation is carried out in a state in which the motor is held at a low level and the low stage is unloaded up to 20%).
In steady operation, the load on the refrigerator, i.e., the evaporator, regardless of the magnitude of the three pressures: high pressure, ie high stage discharge pressure, intermediate pressure, ie high stage suction pressure, and low pressure, ie low stage suction pressure. The operation capacity is controlled by the brine inlet temperature flowing through 15, the brine outlet temperature, the evaporation temperature, or a capacity control signal from the outside.

Japanese Patent Publication No. 7-59951 Japanese Patent No. 3837298 (first page and third page, FIGS. 1 and 2)

  As described above, the capacity control of the conventional screw refrigerator is, in steady operation, the magnitude of three pressures: high pressure, that is, high stage discharge pressure, intermediate pressure, that is, high stage suction pressure, and low pressure, that is, low stage suction pressure. Regardless, the operating capacity is controlled by the load of the refrigerator, that is, the brine inlet temperature flowing through the evaporator 15, or the brine outlet temperature, the evaporation temperature, and an external capacity control signal. The high pressure, that is, the condensing temperature of the refrigerant varies depending on the air and water that exchange heat with the condenser 13. The low-pressure pressure, that is, the evaporation temperature of the refrigerant varies depending on the load side, that is, the brine or air that exchanges heat with the evaporator 15.

  From autumn to spring, the temperature of air and water exchanging heat with the condenser 13 is low in an environment where the outside air temperature is low throughout the year, such as an environment where the outside air temperature is generally low and cold regions. The absolute value of pressure decreases. Further, when the load is high and the temperature of air or brine that exchanges heat with the evaporator 15 is high, the evaporation temperature of the refrigerant is high, and the absolute value of the low pressure is high. Under such conditions, the low-stage discharge pressure, that is, the intermediate pressure may be higher than the high pressure.

As described above, when the intermediate pressure becomes higher than the high pressure, in the conventional example shown in Patent Document 1, the high-stage capacity control device cannot be driven, and as a result, the high-stage compression unit 23 cannot perform on-road operation. The problem that the cooling capacity corresponding to the load cannot be exhibited occurs.
The reason will be specifically described with reference to FIG. As shown in FIG. 14, the high stage capacity control device 67 of the screw compressor 2 includes a capacity control valve called a slide valve 71 and a capacity control piston 74, a casing 51, a piston 74 and a casing 51 connected to the capacity control valve by a rod 72. The piston chamber 76, the spring 73, and the electromagnetic valve 75 formed by

  The piston chamber 76 is divided into a slide valve side chamber (right chamber in FIG. 14) 761 and an anti-slide valve side chamber (left chamber in FIG. 14) 762 by the piston 74. The slide valve side chamber 761 presses the piston 74 to the left side. The high-pressure pressure HP is always given from the high stage compression part 23. On the other hand, the anti-slide valve side chamber 762 is given the highest high pressure HP + α1 (referred to as HP1) in this refrigeration cycle immediately before discharge of the high-stage compression section by opening / closing the electromagnetic valve 75, or an intermediate pressure. Given. Further, as shown in FIG. 14, the discharge pressure of the high stage compression unit 23, that is, the high pressure, acts on the slide valve 71 on one side, and the discharge pressure of the low stage compression unit 22, that is, the high stage compression unit 23, on the opposite side. The intermediate pressure, which is the suction pressure, is acting.

In such a situation, when the intermediate pressure becomes higher than the high pressure, the control unit sends a capacity control signal to the high stage capacity control device 67 when trying to onload the high stage compression unit 23. Thus, the electromagnetic valve drive device keeps the inside of the anti-slide valve side chamber 762 of the piston chamber 76 at the high pressure HP1 by turning off the electromagnetic valve 75. As a result, the pressure of the anti-slide valve side chamber (left chamber in FIG. 14) 762 is higher by α1 than the slide valve side chamber (right chamber in FIG. 14) 761 of the piston 74. On the other hand, since the high pressure HP acts on one side of the slide valve 71 and the intermediate pressure MP higher than the high pressure HP acts on the opposite side, the total differential pressure acts in the unload direction (leftward in FIG. 14), Since the spring force acting on the urging force in the unloading direction is also urged, it moves in the unloading direction. That is, the slide valve 71 cannot move in the on-road direction (right side in FIG. 14).
As a result, the high-stage compression unit 23 cannot perform on-road operation and cannot exhibit a cooling capacity commensurate with the load. This is the reason.

  On the other hand, the motor cooling of the screw compressor 2 is realized by reducing the pressure of the high-pressure refrigerant liquid by adiabatic expansion and injecting the cooling refrigerant obtained by adiabatic expansion into the motor chamber which is an intermediate pressure chamber. Further, in order to prevent seizure of the screw rotor and the casing 51, oil injection is injected into the high-stage compression section 23 using the high pressure in the oil separator 12 as a conveying force. When the intermediate pressure becomes higher than the high pressure, the motor cooling refrigerant cannot flow, and the motor 21 is overheated and abnormally stops. Further, oil injection cannot be injected, and the screw rotor of the high-stage compression unit 23 and the casing 51 are seized, and compression cannot be performed.

  As described above, in a screw refrigerator equipped with a conventional two-stage screw compressor, on-load operation of the high-stage compression section 23 becomes impossible under the condition that the high-pressure pressure is low and the low-pressure pressure is high, and cooling corresponding to the load is performed. There was a problem that the ability could not be demonstrated. Further, there is a problem that the motor cooling refrigerant cannot flow and motor overheating abnormality occurs. Furthermore, there is a problem that oil injection cannot be injected, and the screw rotor of the high-stage compression section 23 and the casing 51 are seized and cannot be compressed.

  In such a condition that the high pressure is low and the low pressure is high, in the screw refrigerator shown in Patent Document 2, the discharge pressure of the second stage compression unit in which the differential pressure regulator is a high stage compression unit. That is, the pressure difference between the high pressure and the discharge pressure of the first stage compression section, which is the low stage compression section, that is, the intermediate pressure is detected, so-called differential pressure, and the first stage compression section (low stage compression) is detected based on this differential pressure. Can be continued by reducing the discharge pressure, that is, the intermediate pressure of the first stage compression unit 22 and preventing the on-load operation from occurring. I have to.

  However, since the capacity control of the two-stage screw compressor of Patent Document 2 is based on the differential pressure, on-road operation is not possible against a sudden change in the outside air temperature condition under the condition that the high pressure is low and the low pressure is high. There is anxiety about preventing the occurrence of a situation that becomes possible, and further, capacity control with improved preventive ability is desired.

  The present invention has been made to solve the above-described problems. The first object is to perform on-load operation of a high-stage compression section under the condition that the high pressure is low and the low pressure is high. It is possible to obtain a screw refrigerator equipped with a two-stage screw compressor that can be surely made than the conventional one and can exhibit a cooling capacity corresponding to a load. The second object is to allow the motor cooling refrigerant to flow, to avoid the occurrence of abnormal motor overheating, and to avoid the seizure of the screw rotor and casing of the high-stage compression section and perform the compression. The object is to obtain a screw refrigerator equipped with a stage screw compressor.

  The screw refrigerator according to the present invention includes a low-stage compression unit that sucks and compresses the first refrigerant gas and discharges the second refrigerant gas having a temperature higher than that of the first refrigerant gas to a first compression chamber. The high-stage compression section that sucks and compresses the second refrigerant gas discharged from the low-stage compression section and discharges the third refrigerant gas having a temperature higher and higher than that of the second refrigerant gas to the second compression chamber. A first pressure detector that detects the pressure of the second refrigerant discharged from the low-stage compression section, that is, an intermediate pressure, and a third pressure discharged from the high-stage compression section. And a second pressure detector for detecting the pressure of the refrigerant, that is, the high pressure, and the screw type two-stage compressor controls the capacity of the refrigerant in the first compression chamber of the low-stage compression section. Apparatus and second capacity control for controlling the capacity of refrigerant in the second compression chamber of the high-stage compression section And calculating the pressure ratio between the high pressure and the intermediate pressure based on the output of the first pressure detector and the output of the second pressure detector, and performing low-stage compression according to the calculated pressure ratio The control part which controls the 1st capacity | capacitance control apparatus of a low stage compression part so that the driving | operation load of a part may be changed is provided.

  According to this invention, the control unit calculates the pressure ratio by dividing the absolute pressure of the high pressure detected by the first pressure detector by the absolute pressure of the intermediate pressure detected by the second pressure detector. If the actual pressure ratio calculated is higher than the pressure ratio set with sufficient margin in advance, the low-stage compression section is allowed to be on-load (full load) operation to almost 100% and set in advance. When the actual pressure ratio is lower than the measured pressure ratio, the capacity control device is adjusted so that the low-stage compression section is partially loaded, that is, 60%, so that the high pressure is low and the low pressure is high. Enables on-load operation of the high-stage compression section, allows the cooling capacity suitable for the load to be exhibited, allows the motor cooling refrigerant to flow, prevents the occurrence of motor overheating abnormalities, and performs compression. It is possible. Furthermore, seizure of the screw rotor and casing of the high-stage compression section can be avoided.

It is a figure which shows the whole structure of the screw refrigerator which concerns on Embodiment 1 of this invention. It is a sectional side view of the two-stage screw compressor of the screw refrigerator which concerns on Embodiment 1 of this invention. It is a figure showing the capacity | capacitance control mechanism by the slide valve of a general screw compressor. It is a low stage capacity | capacitance control apparatus figure of the two stage screw compressor of the screw refrigerator which concerns on Embodiment 1 of this invention. In FIG. 3, it is a figure showing unloading operation. In FIG. 3, it is a figure showing on-load operation | movement. It is a figure showing the state of a high stage capacity | capacitance control apparatus when an intermediate pressure is lower than a high pressure. It is the figure which showed the pressure conditions which perform capacity | capacitance control of the low stage compression part 22 by Embodiment 1 of this invention. It is a block diagram which shows the structure of the control system of the screw refrigerator which concerns on Embodiment 1 of this invention. It is a capacity | capacitance control flowchart of the control part of the screw refrigerator which concerns on Embodiment 1 of this invention. It is the figure which showed the pressure condition which performs capacity | capacitance control of the low stage compression part 22 by Embodiment 1 of this invention, and the pressure condition which performs capacity | capacitance control of the low stage compression part 22 by patent document 1. FIG. It is a figure which shows the whole two-stage screw refrigerator 1 whole structure. 1 is a cross-sectional view of a two-stage screw compressor 2 disclosed in Patent Document 1. FIG. It is a figure showing capacity control at the time of starting of the conventional two-stage screw compressor 2. It is a figure showing the state of a high stage capacity | capacitance control apparatus when intermediate pressure is higher than high pressure.

Embodiment 1 FIG.
(1.1) Detailed Description of Configuration FIG. 1 is an overall configuration diagram of a screw refrigerator that is Embodiment 1 of the present invention.
In FIG. 1, a screw refrigerator 1 includes a two-stage screw compressor (hereinafter sometimes simply referred to as a screw compressor or a compressor) 2, an oil separator 12, a condenser 13, expansion valves 14a and 14b, and an evaporator. 15, the oil cooler 25, the electromagnetic valve 68, and these components are connected, and the refrigerant | coolant piping which circulates a refrigerant | coolant is provided. The compressor 2 is a two-stage compressor having a low-stage compressor 22 and a high-stage compressor 23 driven by a single motor 21, and has a discharge port for the low-stage compressor 22 and a suction port for the high-stage compressor 23. Are communicating. A pressure detector 61 that detects a high pressure that is the discharge pressure of the compressor 2, a pressure detector 62 that detects an intermediate pressure that is a high stage suction pressure inside the compressor 2, that is, a low stage discharge pressure, and a suction of the compressor 2 Control which calculates a pressure ratio from the pressure signal obtained from the low pressure detector 63 which detects a pressure (low pressure), and the pressure detectors 61 and 62, and outputs a capacity | capacitance control signal to the high stage capacity | capacitance control device 67 of the compressor 2. A portion 64 is provided. The expansion valve 14a is for depressurizing the refrigerant (main liquid) flowing through the main flow of the refrigeration cycle, and the expansion valve 14b is provided for cooling the oil cooler 25, and depressurizes the refrigerant. Thus, a low-temperature refrigerant is poured into the oil cooler 25. Further, when the two-stage screw compressor 2 is stopped, the electromagnetic valve 68 can simultaneously close all the electromagnetic valves 68 to collect all the refrigerant in the evaporator 15 in the condenser 13 and keep it away. When the operation is resumed, the refrigerant accumulated in the condenser 13 can be returned to the refrigeration cycle by opening the electromagnetic valve 68.

FIG. 2 is a side sectional view of the two-stage screw compressor of the screw refrigerator according to Embodiment 1 of the present invention. As described above, the compressor 2 is a two-stage compressor having the low-stage compression unit 22 and the high-stage compression unit 23 driven by the single motor 21, and the discharge port and the high-stage compression of the low-stage compression unit 22. The suction port of the part 23 is in communication. The compression part is a single screw type consisting of one screw rotor and two gate rotors for each of the low and high stages, but the low stage is composed of two gate rotors and the high stage is composed of one gate rotor. Even if it is a type, the first embodiment is similarly established. Further, the present embodiment 1 is established even with a twin screw type that is constituted by a pair of screw rotors, each not shown. The low-stage compression unit 22 and the high-stage compression unit 23 include capacity control devices 66 and 67, respectively, and are driven by using the magnitude relationship among the high pressure, the intermediate pressure, and the low pressure in the compressor 2. The low stage screw rotor and the high stage screw rotor are connected to the same shaft. In order to prevent eccentricity of the low-stage and high-stage screw rotors, a pair of low-stage and high-stage gate rotors are disposed at positions 180 degrees symmetrical to each other in the circumferential direction of the shaft. In the case of one type of high-stage gate rotor, the high-stage gate rotor 86 is arranged 90 degrees away from the low-stage gate rotor 87 in the circumferential direction of the shaft.
The low stage capacity control device 66 includes a slide valve 84, a rod 91, and a piston 85, and the high stage capacity control device 67 includes a slide valve 71, a rod 72, a spring 73, and a piston 74.
FIG. 2A is a diagram showing a capacity control mechanism using a general slide valve. Since both the low-stage compression section and the high-stage compression section have the same configuration, the high-stage compression section will be described for the sake of convenience.
The right side of FIG. 2A is an enlarged view of the left side, and shows a state where the slide valve 71 is unloaded. During on-road (100%) operation, the slide valve 71 moves to the position of the broken line in the figure and closes the bypass, so that the refrigerant gas compressed by the engagement of the screw rotor 52 and the high-stage gate rotor 86 has the maximum discharge pressure. It becomes the gas which has, and is discharged to a refrigerant circuit. On the other hand, in the case of unloading operation, a part of the refrigerant is bypassed, so that the refrigerant gas compressed by the engagement of the screw rotor 52 and the high stage gate rotor 86 is discharged to the refrigerant circuit in a state where the pressure is lowered by that amount. .

  FIG. 3 is a diagram showing the low stage capacity controller 66 of the low stage compressor 22. As shown in FIG. 3, the low stage capacity control device 66 of the low stage compression unit 22 includes a slide valve 84, a piston 85, a rod 91 that connects the piston 85 and the slide valve 84, and a connecting arm 96. Yes. The piston 85 is accommodated in the piston chamber 93 and divides the inside of the piston chamber 93 into a slide valve side chamber (right chamber in FIG. 3) 931 and an anti-slide valve side chamber (left chamber in FIG. 3) 932. Further, the rod 91 is provided with a spring 92 that applies a biasing force to the piston 85 on the unload side (leftward in FIG. 3). An intermediate pressure MP is constantly applied to the slide valve side chamber 931 from the discharge side of the low-stage compression unit 22. In this figure, in order to simplify the description, the description of the other slide valve 84 that is in a symmetrical position with the piston 85 about the center in the longitudinal direction of the connecting arm 96 is omitted.

Next, the operation will be described. The capacity control is performed by opening and closing a port for bypassing the refrigerant gas by moving the slide valve 84. As shown in FIG. 3, an intermediate pressure MP and a low pressure LP are applied to both ends of the slide valve 84. When the intermediate pressure is higher than the high pressure, the control unit 64 determines the intermediate pressure MP based on the pressure ratio. When it is detected that the pressure is higher than the high pressure HP, a capacity control signal is output to the capacity controller 66 so as to be unloaded. Thereby, unloading is executed. In this case, the solenoid valve 94 is opened (ON) as shown in FIG. 4 by driving a solenoid valve drive device (not shown), and the high stage acting on the anti-slide valve side chamber (left chamber in FIG. 3) 932 of the piston chamber 93. Intermediate pressure MP + α2 (referred to as MP1) that is higher by α2 than the intermediate pressure after the start of compression is released to the low-pressure part. Thereby, a low pressure pressure that pushes the piston 85 to the right acts on the anti-slide valve side chamber (the left chamber in FIG. 3) 932 of the piston chamber 93. On the other hand, a relatively weak spring force that pushes leftward is applied to the anti-slide valve side chamber (right chamber in FIG. 3) 932 through the connecting arm 96 together with an intermediate pressure MP that pushes the piston 85 leftward. Further, an intermediate pressure MP that pushes right and a low pressure LP that pushes leftward act on the slide valve 84. The leftward differential pressure between the intermediate pressure MP and the low pressure LP acting on the piston 85 and the rightward differential pressure between the intermediate pressure MP and the low pressure LP acting on the slide valve 84 are substantially equal, but the pressure receiving area of the piston 85 is the slide valve. The piston 85 is much larger than the pressure receiving area 84, and a spring force that pushes it to the left acts on the piston 85, so that the slide valve 84 moves to the unload side (leftward) as shown in FIG.
Accordingly, the capacity of the low-stage compression unit 22 is controlled and the refrigerant gas is bypassed, so that the intermediate pressure of the discharged refrigerant is reduced. Therefore, when the intermediate pressure becomes lower than the high pressure, the on-load operation at the high stage compression unit 23 becomes possible.

On the other hand, when the intermediate pressure is lower than the high pressure, the control unit 64 detects that the intermediate pressure is lower than the high pressure based on the pressure ratio, and the capacity control signal so that the low stage capacity controller 66 is on-loaded. Is output. As a result, a solenoid valve drive device (not shown) closes (OFF) the solenoid valves 94 and 95 as shown in FIG. 5, and the inside of the anti-slide valve side chamber 932 of the piston chamber 93 is only α2 from the intermediate pressure immediately after the start of high-stage compression. High intermediate pressure MP + α2 (MP1). Thus, an intermediate pressure MP1 that pushes the piston 85 to the right acts on the anti-slide valve side chamber (the left chamber in FIG. 3) 932 of the piston chamber 93. On the other hand, an intermediate pressure MP that pushes the piston 85 leftward and a relatively weak spring force that pushes leftward act on the slide valve side chamber (right chamber in FIG. 3) 931. Further, an intermediate pressure MP that pushes right and a low pressure LP that pushes leftward act on the slide valve 84. Since the differential pressure α2 between the intermediate pressure MP1 and the intermediate pressure MP acting on the piston 85 and the rightward differential pressure due to the intermediate pressure MP and the low pressure LP acting on the slide valve 84 act on the rod and a rightward force is applied, The slide valve 84 moves in the on-road direction by overcoming the relatively weak spring force leftward from the force balance.
In this case, since the intermediate pressure is lower than the high pressure, the high-stage compression unit 23 can perform on-road operation.
In order to perform almost 100% full load operation in the high stage compression unit 23, it is necessary to perform an on-load operation by the high stage capacity control device 67 of the high stage compression unit 23. This will be described below.
When the high stage side is onloaded, the control unit 64 outputs a capacity control signal to the high stage capacity control device 67 so as to be onloaded. As a result, a solenoid valve driving device (not shown) closes (OFF) the solenoid valve 75 as shown in FIG. 6, and sets the inside of the anti-slide valve side chamber 762 of the piston chamber 76 to the high pressure HP + α1. As a result, a high pressure HP1 that pushes the piston 74 to the right acts on the anti-slide valve side chamber (left chamber in FIG. 6) 762 of the piston chamber 76. On the other hand, a high pressure HP pushing the piston 74 leftward and a relatively weak spring force pushing leftward act on the slide valve side chamber (right chamber in FIG. 6) 761. Further, a high pressure HP that pushes right and an intermediate pressure MP that pushes leftward act on the slide valve 71. In this case, the intermediate pressure MP is lower than the high pressure HP due to low stage side capacity control. The high pressure HP1 acting on the piston 74 and the right differential pressure α1 due to the high pressure HP and the right differential pressure α1 due to the high pressure HP acting on the slide valve 71 and the intermediate pressure MP act on the rod 72 and a rightward force is applied. Therefore, the spring force that is relatively weak to the left than the force balance is overcome, and the slide valve 71 moves in the on-road direction.

In order to move the high stage side in the on-road direction, the following conditions must be satisfied.
High pressure is HP, intermediate pressure is MP, piston pressure receiving area is A, slide valve pressure receiving area is a, rod pressure receiving area is b, spring constant is k, spring displacement is x,

(HP + α1) × A−k × x−HP × (A−b) + HP × (ab) −MP × a> 0 (Equation 1)
If k × x = F, and rearranging Equation 1, HP> MP + (F−α1 × A) / a (Equation 2)

The above contents are summarized as follows.
In the screw refrigerator 1, when the intermediate pressure becomes higher than the high pressure, as shown in FIG. 14, even if the control unit 64 tries to load the high stage capacity control device 67, the slide valve 71 is loaded on the basis of the balance of force. There are cases where it cannot be moved in the direction.
If the high-stage compression unit 23 cannot be on-loaded, there arises a problem that the cooling capacity corresponding to the load cannot be exhibited. Further, there is a problem that the motor cooling refrigerant cannot flow and the motor 21 is abnormally overheated. Furthermore, there is a problem that oil injection cannot be injected, and the screw rotor of the high-stage compression section 23 and the casing 51 are seized and cannot be compressed. In order to prevent such a state, the control unit 64 always detects the high pressure and the intermediate pressure with the pressure detectors 61 and 62, and the pressure signal obtained from the pressure detectors 61 and 62 determines the pressure. When the ratio is calculated and the pressure ratio falls below a preset value, a capacity control signal is output to the low stage capacity control device 66 of the compressor so as to unload the low stage compressor 22. When the low-stage compression unit 22 unloads to, for example, 60% capacity based on the capacity control signal, the discharge pressure, that is, the intermediate pressure of the low-stage compression unit 22 decreases. As the intermediate pressure decreases, the pressure ratio with the high pressure exceeds a preset value, and the slide valve 71 of the high-stage compression unit 23 can be driven.

FIG. 7 shows an example of an operation region in which operation is possible by this control. A line where the pressure ratio is 1.5 is indicated by a broken line, and a line where the high pressure and the intermediate pressure are equal is indicated by a solid line. In FIG. 7, a region 1 is a region where high pressure> intermediate pressure × pressure ratio set value, and is a region where the high-stage compression unit 23 can be stably on-loaded.
Region 2 is a region where intermediate pressure <high pressure ≦ intermediate pressure × pressure ratio set value. In this region, the on-load operation of the high-stage compression unit 23 becomes unstable, and the above-described functional aspects and reliability aspects Problems occur. For this reason, it is an area | region which performs control which unloads the low stage compression part 22 and reduces an intermediate pressure, and prevents generation | occurrence | production of these problems.
The region 3 is a region where the high pressure ≦ the intermediate pressure. In this region, the on-load operation of the high-stage compression unit 23 becomes impossible, and the above-described problems in terms of function and reliability occur. For this reason, it is an area | region which performs control which unloads the low stage compression part 22 and reduces an intermediate pressure, and prevents generation | occurrence | production of these problems.

FIG. 8 is a block diagram showing the configuration of the control system of the screw refrigerator according to Embodiment 1 of the present invention. In FIG. 8, the control unit 64 includes, for example, a microcomputer and a DSP, and is connected to a high pressure detector 61, an intermediate pressure detector 62, and a low pressure detector 63 via an input / output bus 65. From these, a detection signal is input. The low stage capacity control device 66 and the high stage capacity control device 67 are also connected, and the low stage capacity control device 66 is controlled to open and close the solenoid valves 94 and 95 via the solenoid valve driving device 94c. For the high stage capacity control device 67, the opening and closing of the electromagnetic valve 75 is controlled via the electromagnetic valve driving device 75c.
FIG. 9 is a flowchart showing the operation of capacity control by the control unit 64 of the screw refrigerator 1. In order to explain the flow in a simple manner, the restrictions of protection control and electronic expansion valve control associated with these are omitted, but electronic control for securing control and functions for protecting such a refrigerator to this control is omitted. It is acceptable to add the control of the refrigerator components such as the expansion valve as a condition.

  Next, the operation of the control unit 64 will be described with reference to FIGS. The controller 64 always obtains the outputs of the pressure detectors 61 and 62 for the high pressure and the intermediate pressure (step S1), and calculates the ratio of the high pressure to the intermediate pressure as the pressure ratio (step S2). This pressure ratio is compared with a value set with a sufficient margin in advance (step S3). If the pressure ratio is lower than the set value, the intermediate pressure is larger than the high pressure, so the high stage cannot be on-loaded. Since there is a risk of overheating of the motor, the low-stage compression unit 22 is unloaded (capacity down) to, for example, a compressor capacity of 60% (step S4), and the same processing is repeated from step S1. Due to the unloading of the low-stage compression unit 22, the discharge pressure, that is, the intermediate pressure of the low-stage compression unit 22 decreases. Then, in the comparison of step S3, if the pressure ratio is larger than the set value, the control unit 64 is able to perform the on-load operation in a safe state even if the low-stage compression unit 22 is on-road operated. Then, the on-load of the low-stage compression unit 22 is permitted (step S5), and the same processing is repeated from step S1.

  FIG. 10 shows a comparison between a region where the low-stage compression unit 22 is unloaded by the control based on the high pressure and the pressure ratio and a region where the control is performed based on the differential pressure between the high pressure and the intermediate pressure disclosed in Patent Document 1. Show. As an example, a line with a pressure ratio of 1.5 is indicated by a broken line, a line with a differential pressure of 0.15 MPa is indicated by a two-dot chain line, a line of 0.3 MPa is indicated by a one-dot chain line, and a line of high pressure = intermediate pressure is indicated by a solid line. Region A is an example of a region where conventional control acts in a region where the differential pressure between the high pressure and the intermediate pressure is 0.15 MPa or less. In this region, control is not triggered unless the intermediate pressure is higher than the same high pressure as in the region where the pressure ratio is 1.5. Therefore, when the intermediate pressure gradually increases during the operation, the control starts. However, in terms of reliability, problems are likely to occur. Region B is an example of a region where conventional control acts in a region where the differential pressure between the high pressure and the intermediate pressure is 0.3 MPa or less. In this case, if the high pressure is high, the control will not be triggered unless the intermediate pressure is higher than the case where capacity control is performed at the pressure ratio of 1.5 at the same high pressure. Therefore, the intermediate pressure gradually increases during the operation. At this time, the start of control is delayed, and problems are likely to occur in terms of reliability. When the high pressure is low, the control is not activated unless the intermediate pressure is lower than the capacity control at the same high pressure with a pressure ratio of 1.5. Therefore, the intermediate pressure gradually increases during the operation. Sometimes, the start of control becomes early, and the capacity control by the frequency control on the low-stage compression side operates quickly in terms of function, and is inferior in terms of ensuring the cooling capacity.

Thus, by performing capacity control with the pressure ratio between the high pressure and the intermediate pressure, it is possible to maintain the function and ensure the reliability more appropriately than in Patent Document 2.
In the above example, the capacity of the low-stage compression unit is controlled based on the pressure ratio. However, if the pressure is less than a predetermined pressure, the capacity is based on the differential pressure between the high pressure and the intermediate pressure, which is a conventional technique. The capacity control of the low-stage compression unit may be performed, and the capacity control may be performed based on the pressure ratio above a predetermined pressure. That is, in FIG. 10, when the intermediate pressure is less than 0.6 MPa (abs), the dash-dot line (HP = MP + 0.3 line) of the differential pressure is larger than the pressure ratio broken line (HP = MP × 1.5 line). Since the high pressure is higher, it is safer to judge by the differential pressure. On the other hand, when the intermediate pressure is 0.6 MPa (abs) or higher, the high pressure is higher on the broken line of the pressure ratio than on the alternate long and short dash line, so it is safer to judge based on the pressure ratio. Therefore, in such a case, the control unit performs capacity control by the differential pressure when the intermediate pressure is less than 0.6 MPa (abs), and performs capacity control by the pressure ratio when the intermediate pressure is 0.6 MPa (abs) or more.

  DESCRIPTION OF SYMBOLS 1 Screw refrigerator, 2 Screw compressor, 12 Oil separator, 13 Condenser, 14, 14a Expansion valve (main liquid), 14b Expansion valve (oil cooler), 15 Evaporator, 21 Motor, 22 First stage compression Part (low stage compression part), 23 second stage compression part (high stage compression part), 24 oil, 25 oil cooler, 51 casing, 52 screw rotor, 61 high pressure detector, 62 intermediate pressure detector, 63 low pressure Pressure detector, 64 control unit, 65 input / output bus, 66 low stage capacity controller, 67 high stage capacity controller, 68 solenoid valve, 71 slide valve, 72 high stage capacity control rod, 73 spring, 74 piston, 75 Solenoid valve, 75C Solenoid valve drive device, 76 piston chamber, 761 slide valve side chamber, 762 anti slide valve side chamber, 84 slide valve, 85 piston, 86 Stage gate rotor, 87 Low stage gate rotor, 88 Main bearing, 89 Intermediate bearing, 90 Sub bearing, 91 Rod, 92 Spring, 93 Piston chamber, 931 Slide valve side chamber, 932 Anti slide valve side chamber, 94 Solenoid valve, 94C Solenoid valve Drive device, 95 solenoid valve, 96 connecting arm, HP high pressure, HP1 high pressure (immediately before discharge), α1 differential pressure between high pressure just before discharge and high pressure, MP intermediate pressure, MP1 intermediate pressure (after starting high-stage compression), α2 Differential pressure between intermediate pressure and intermediate pressure after starting high-stage compression, LP low pressure.

Claims (6)

A first refrigerant gas is sucked and compressed, and a second refrigerant gas having a temperature higher than that of the first refrigerant gas and discharged to the first compression chamber is discharged to the first compression chamber, and discharged from the lower stage compressor. A high-stage compression section that sucks and compresses the second refrigerant gas that has been discharged and discharges a third refrigerant gas having a temperature higher and higher than that of the second refrigerant gas into the second compression chamber. A stage compressor;
A first pressure detector that detects a pressure of the second refrigerant discharged from the low-stage compression unit, that is, an intermediate pressure;
A second pressure detector that detects the pressure of the third refrigerant discharged from the high-stage compression section, that is, a high pressure, and
The screw-type two-stage compressor controls a refrigerant capacity of a first compression chamber of the first compression chamber of the low-stage compression section and a refrigerant capacity of a second compression chamber of the high-stage compression section. A second capacity control device,
Based on the output of the first pressure detector and the output of the second pressure detector, a pressure ratio between the high pressure and the intermediate pressure is calculated, and the low-stage compression is performed according to the calculated pressure ratio. A screw refrigerator comprising a control unit that controls the first capacity control device of the low-stage compression unit so as to change an operation load of the unit. The pressure ratio is a value obtained by dividing the high pressure by the intermediate pressure,
The control unit controls the first capacity control device of the low-stage compression unit to operate with a first load when the pressure ratio is larger than a preset reference value, and the calculated pressure The capacity control device of the low-stage compression unit is controlled to operate with a second load smaller than the first load when the ratio is equal to or less than the reference value. Screw refrigerator.   The operation at the first load is a full load operation that operates at a load of almost 100%, and the operation at the second load is a partial load operation that operates at a predetermined load smaller than the 100%. The screw refrigerator according to claim 2, wherein the screw refrigerator is provided. The first capacity control device includes a slide valve that adjusts the capacity of the first compression chamber of the low-stage compression unit by moving in a predetermined direction;
A piston chamber;
A piston that is connected to the slide valve via a rod and divides the piston chamber into an anti-slide valve side chamber and a slide valve side chamber to which an intermediate pressure is always applied;
A solenoid valve for controlling whether the pressure of the refrigerant to be applied to the anti-slide valve side chamber is an intermediate pressure or a low pressure after the start of high-stage compression;
A spring disposed in the slide valve side chamber and pressing the piston toward the anti-slide valve side,
The control unit controls the opening and closing of the solenoid valve based on the pressure ratio to cause a differential pressure between the intermediate pressure and the low pressure pressure or a differential pressure between the intermediate pressure and the intermediate pressure after the start of the high-stage compression to the piston. The screw according to any one of claims 1 to 3, wherein the piston is driven in a predetermined direction or the opposite direction by moving the slide valve to move in the same direction as the piston. refrigerator.   When the control unit detects that the intermediate pressure is equal to or lower than the high pressure based on the pressure ratio, the control unit closes the electromagnetic valve to reduce the internal pressure of the anti-slide valve side chamber to an intermediate level after the start of high-stage compression. When the piston is moved to the on-load side by the differential pressure between the intermediate pressure acting on the slide valve and the low pressure, and the intermediate pressure is detected to be higher than the high pressure based on the pressure ratio The opening of the solenoid valve makes the internal pressure of the anti-slide valve side chamber a low pressure, and the piston is moved to the unload side by the differential pressure between the intermediate pressure and the low pressure. The screw refrigerator as described.   The control unit performs capacity control based on a differential pressure between the high pressure and the intermediate pressure when the intermediate pressure is lower than a predetermined pressure, and controls the capacity according to the magnitude of the pressure ratio when the intermediate pressure is equal to or higher than the predetermined pressure. The screw refrigerator as described in any one of Claims 1-5 characterized by performing.