Refrigerating system

A refrigerating system comprises screw compressor means which is adapted to control the capacity thereof by means of a slide valve provided in the compressor means and operatively associated with hydraulic actuator means. The hydraulic actuator means has hydraulic fluid supplying means and hydraulic fluid discharging means, both of which are connected thereto and provided with stop valve means, respectively, for controlling the supply and the discharge of a hydraulic fluid to and from the hydraulic actuator means to permit the latter to hydraulically actuate the slide valve. The refrigerating system further comprises means for selectively switching the operation mode of the system, means for detecting the outlet water temperature of an indoor heat exchanger of the system, and temperature control means for setting a water temperature for service. The temperature control means has a timer built therein and an arithmetic and logic circuit adapted to determine the operation period for the timer in proportion to the difference between the set water temperature and the outlet water temperature inputted from the detecting means. The slide valve is hydraulically actuated to effect the capacity control in proportion to the temperature difference under the control of control circuit means provided for interconnecting the timer with the stop valve means.

BACKGROUND OF THE INVENTION 
The present invention relates to a refrigerating system employing a screw 
compressor and, more particularly, to a refrigerating system having a 
function of quickly controlling the capacity in response to a change in 
the refrigeration load. In general, the screw compressor for refrigerating 
purpose is equiped with a hydraulically operated slide valve. The capacity 
of the compressor can be controlled linearly and continuously by a 
suitable control of the slide valve. 
Such a linear and continuous capacity control of the screw compressor by 
the slide valve is shown in, for example, Japanese Utility Model 
Publication No. 4564/1977. More specifically, this literature shows a 
screw compressor having a slide valve, which has additionally a suction 
block type capacity controller provided at the suction side thereof to 
widen a controllable range of the capacity of the compressor. In 
operation, when there is a drastic reduction in the load, the suction 
block type capacity controller having quick response characteristics 
responds first to the reduction and then the slide valve is operated, so 
that the capacity of the compressor can be varied over a wide range. In 
the refrigerators such as chilling units, however, there is a demand for 
compressors having a simpler capacity controller capable of quickly 
responding to a change in the load linearly and continuously. 
Generally, the capacity control in refrigerators such as chilling units in 
response to the load demand is conducted in the following manner. Namely, 
the set temperature of a chilled water temperature controller or warmed 
water temperature controller is periodically compared with the outlet 
water temperature of an indoor heat exchanger, and when any difference 
between the set temperature and the outlet water temperature exists, the 
slide valve of the compressor is operated for a fixed predetermined time. 
Since the duration of operation of the slide valve is fixed regardless of 
the magnitude of temperature difference, it is often experienced that the 
capacity control fails in adequately responding to the load demand due to 
insufficiency of the control time, particularly when the difference 
between the set temperature and the water temperature is large. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a refrigerating system 
incorporating a screw compressor having a capacity controller which can 
quickly and adequately accomodate the compressor to changes in the load 
while having a simple construction. 
To this end, according to the invention, there is provided a refrigerating 
system comprising screw compressor means which is adapted to increase or 
decrease the flow rate of a refrigerant fluid flowing therethrough by 
means of a slide valve provided in the compressor means and operatively 
associated with hydraulic actuator means. The hydraulic actuator means has 
hydraulic fluid supplying means and hydraulic fluid discharging means 
which are connected thereto and provided with stop valve means, 
respectively, for controlling the supply and the discharge of a hydraulic 
fluid to and from the actuator means to permit the latter to hydraulically 
actuate the slide valve. The refrigerating system further comprises means 
for selectively switching the operation mode of the system, means for 
detecting the outlet water temperature of an indoor heat exchanger of the 
system, and temperature control means for setting a water temperature for 
service. The temperature control means has a timer built therein and an 
arithmetic and logic circuit adapted to determine the operation period for 
the timer in proportion to the difference between the set water 
temperature and the outlet water temperature inputted from the detecting 
means. Control circuit means is also provided for connecting the timer 
with the stop valve means to control the operation of the latter in 
proportion to the temperature difference. 
Thus, according to the invention, the duration of operation of the slide 
valve, which inherently offers a linear capacity control of the screw 
compressor means, is changed in accordance with the difference between the 
outlet water temperature of the indoor heat exchanger and the set 
temperature of the chilled or warmed water temperature control means, such 
that the operation duration is increased as the temperature difference 
becomes greater and decreased as the temperature difference becomes 
smaller, thereby to attain a quick capacity control matching for the load 
of the refrigerating system. 
According to the above-described arrangement, the time duration of opening 
or closing of the stop valve means disposed in the hydraulic fluid 
supplying and the hydraulic fluid discharging means is controlled in 
proportion to the temperature difference, so that the supply and discharge 
of the hydraulic fluid to and from the hydraulic actuator means for 
actuating the slide valve is quickly controlled within this time duration. 
Since the slide valve can open or close adequately, it is possible to 
effect the capacity control of the screw compressor means quickly in 
response to the change of the load.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the invention will be described hereinunder with reference 
to the accompanying drawings. 
Referring to FIG. 1 which is a system diagram of a refrigerator 
incorporating a screw compressor, the screw compressor is generally 
designated at a numeral 10. The compressor 10 has major parts such as a 
pair of screw rotors 1 one of which is directly connected to an electric 
motor (not shown), a slide valve 2 disposed on the pair of screw rotors 1, 
a hydraulic piston 4 connected to the slide valve 2 and slidably received 
by a hydraulic cylinder 5, a refrigerant gas suction port 6, and a 
refrigerant gas discharge port 7. 
The screw rotors 1 are so formed and arranged in the conventional manner 
that they mesh with each other to successively define plural compression 
spaces as they rotate in opposite directions by means of the driving of 
the electric motor. 
The refrigerant gas suction port 6 communicates with an inlet 8 of each 
compression space formed around the meshing point of the pair of screw 
rotors 1 and also with a space 11 which is formed above the screw rotors 1 
and on one end side of the slive valve 2 when the latter moves to the 
right as viewed in FIG. 1, through a passage 9. 
The refrigerant gas discharge port 7 communicates through a passage 13 with 
an outlet 12 of each compression space formed in the area around the 
separating point of the pair of screw rotors 1. A reference numeral 14 
designates a spring mounted between the side wall 15 of the hydraulic 
cylinder 5 and one end surface of the hydraulic piston 4 to normally urge 
the slide valve 2 in the opening direction. However, the biasing force of 
the spring 14 is set to be weaker than that applied to the other end of 
the slide valve by the pressure of a compressed refrigerant gas so as to 
force the slide valve under the predetermined conditions as will be 
described hereinafter. A reference numeral 20 denotes a four-way valve 
connected through a suction pipe 21 to the refrigeration gas suction port 
6 of the screw compressor 10 and also to the refrigerant discharge port 7 
through a discharge pipe 22. A heat exchanger 30 on the outdoor side of 
the system is connected at its one end to the four-way valve 20 through a 
pipe 23 and at its other end to another heat exchanger 50 on the indoor 
side through a pipe 24 having an expansion valve 40. The heat exchanger 50 
is connected at its other end to the four-way valve 20 through a pipe 25. 
Water is introduced through a water inlet pipe 51 into the heat exchanging 
region of the heat exchanger 50 for exchanging heat with the refrigerant 
gas which is also introduced into this heat exchanger 50, and is 
discharged from the heat exchanging region through a water outlet pipe 52. 
A solenoid valve 60 is connected at its inlet side to a pressurized oil 
tank 62 through an oil pipe 61 and at its outlet side to a space 65 formed 
in the hydraulic cylinder 5 through oil pipes 63 and 64 for supplying oil 
in the space 65. A reference numeral 66 designates an oil supplying pipe 
for supplying the oil under high pressure to the tank 62. Another solenoid 
valve 70 is connected at its inlet side to the oil pipe 64 through an oil 
pipe 71 and at its outlet side to the suction pipe 21 through an oil pipe 
72 and a low-pressure pipe 73 for discharging the oil from the space 65. 
The other end of the low-pressure pipe 73 is connected to a space 74 in 
the hydraulic cylinder 5 to maintain the pressure in the space 74 at a low 
level. 
A temperature controller 80 with a built-in timer, adapted for free 
adjustment of set temperature, is electrically connected through a signal 
line 82 to a temperature sensor 81 serving as temperature detecting means, 
which is provided in contact with the water outlet pipe 52 of the heat 
exchanger 50. The temperature controller 80 is connected at its output 
side to the solenoid valve 60 and the solenoid valve 70 through signal 
lines 83 and 84, respectively. 
Referring now to FIG. 2 schematically showing the temperature controller 
80, the coils 60a and 70a of both solenoid valves 60 and 70 are connected, 
respectively, in series to an oil supplying timer contact 85 and an oil 
discharging timer contact 86. 
A reference numeral 90 denotes a computing circuit. The computing circuit 
is adapted to compare the water temperature sensed by the water 
temperature sensor 81 with the set temperature which is beforehand set in 
the temperature controller 80. On the basis of the result of the 
comparison, the computing circuit computes opening durations for the 
contacts 85 and 86 and delivers signals for representing adequate opening 
and closing durations to the solenoid valves. 
The operation of the embodiment will be described hereinunder. 
First, the operation for producing chilled water will be described. The 
refrigerant gas compressed to a high pressure and temperature by the screw 
compressor 10 is discharged from the discharge port 7. The refrigerant gas 
flows into the heat exchanger 30 serving as a condenser, through the 
discharge pipe 22, a passage formed in the four-way valve 20 as indicated 
by a full line in FIG. 1 and the pipe 23. The gas is then condensed and 
liquefied as a result of the heat exchange with air or water in the heat 
exchanger 30. The condensate is then introduced through the pipe 24 into 
the expansion valve 40 in which the refrigerant is expanded to be a low 
pressure, and is introduced into the heat exchanger 50 serving as an 
evaporator. The refrigerant under low pressure makes a heat exchange with 
water which is introduced into the heat exchanger 50 through the inlet 
pipe 51, so that the water is chilled to a low temperature while the 
refrigerant is evaporated into gaseous phase. The thus chilled water is 
led to the outside of the heat exchanger 50 through the outlet pipe 52 to 
serve as, for example, the heat source for air conditioning. On the other 
hand, the refrigerant gas after cooling the water is delivered through the 
pipe 25, a passage of the four-way valve 20 as illustrated by a full line 
and then through the suction pipe 21 into the suction port 6 of the screw 
compressor 10. The refrigerant gas is again compressed by the screw 
compressor 10 and is circulated through the refrigerating system described 
hereinbefore. 
For producing warm water, the four-way valve 20 is switched to open 
passages as indicated by broken lines in FIG. 1 so that the heat exchanger 
30 and the heat exchanger 50 perform their heat exchange in a manner 
reverse to that described hereinbefore on the operation for producing 
chilled water, thereby to permit the refrigerator to produce warm water. 
The capacity of the screw compressor is minimized to about 25 to 35% of the 
full capacity thereof, when the slide valve 2 is fully moved to the right 
as viewed in FIG. 1 to release the refrigerant through the passage 9. For 
conducting a continuous control of the capacity, therefore the slide valve 
2 is progressively moved to the right as viewed in FIG. 1, by supplying 
the pressurized oil to the space 65 in the hydraulic cylinder 5. To the 
contrary, for continuously increasing the capacity of the compressor, the 
oil is discharged from the space 65 to permit the slide valve 2 to 
progressively move towards the starting position. The pressure in the 
space 74 of the hydraulic cylinder 5 is maintained at a sufficiently low 
level. The pressurized oil is supplied from the pressurized oil tank 62 
through the solenoid valve 60, while the discharge of the oil is made 
through the solenoid valve 70 to the low-pressure side of the 
refrigeration system. FIG. 1 shows the compressor in a state in which the 
spring 14 is not fully compressed, i.e. in the state of partly unloading 
the compression operation to permit a part of the compressed gas to be 
relieved to the suction side of the compressor. After the compressor has 
been fully stopped, the solenoid valve 60 is opened to permit the supply 
of the oil to the space 65 and, at the sametime, the hydraulic piston 4 is 
fully moved to the right by the force of the spring 14, so as to set the 
slide valve 2 at the maximum open position, thereby to allow the 
compressor to operate with the minimum load when the same is started 
again. 
The normal operation of the compressor after the start up is controlled in 
accordance with the program flow chart shown in FIG. 4. 
The temperature of the water flowing through the water outlet pipe 52 from 
the heat exchanger 50 is sensed by the temperature sensor 81 at a suitable 
period of, for example, 60 seconds. The temperature signal is inputted to 
the computing circuit 90 of the temperature controller 80, and is compared 
with that of the set temperature in the temperature controller 80. If the 
set temperature and the outlet water temperature are equal, the computing 
circuit 90 does not make any further computation to stand by for the 
following 60 seconds. However, when the outlet water temperature is higher 
or lower than the set temperature, the computing circuit 90 commences 
computing a duration for operating the timer contacts. 
For instance, in the operation for producing chilled water, assuming that 
the set temperature in the temperature controller 80 is 7.degree. C. while 
the outlet water temperature takes a level above the set temperature due 
to an increment of load, e.g. 11.degree. C., so that there is a demand for 
a further chilling of water. In such case, the computing circuit computes 
the operation duration of the slide valve 2 corresponding to the 
temperature difference which is in this case 4 degrees. More specifically, 
the computing circuit determines the operation duration of the discharge 
timer contact 86 of this case to be 10 seconds on the basis of the 
operation characteristics shown in the diagram of FIG. 3. 
In the conventional capacity controller, this operation duration is fixed 
to be about 5 seconds. According to the invention, however, the time 
duration of operation of the slide valve is increased or decreased in 
proportion to the magnitude of the difference between the set temperature 
and the outlet water temperature, thereby to permit a continuous linear 
capacity control of the compressor. Consequently, the compressor can be 
quickly accomodated with changes in the load and can be operated under an 
adequate capacity control. 
After 10 seconds has elapsed, the solenoid valve 70 is closed and stands by 
for a next signal. The operation characteristics shown in the diagram of 
FIG. 3 is not fixed but may be varied depending on various factors such as 
the capacity of a screw compressor and the capacity of a service heat 
exchanger. 
To the contrary, when the outlet water temperature has come down below 
5.degree. C., i.e. when the outlet water temperature is below the set 
temperature, the capacity control of the compressor is conducted in 
accordance with the operation mode which is shown into the right part of 
the flow chart of FIG. 4. Namely, in this case, the computing circuit 
computes the duration of operation of the slide valve 2, i.e. the 
operating duration of the oil supplying solenoid valve 85 corresponding to 
the temperature difference of 2 degrees, to be 5 seconds as will be seen 
from the diagram of FIG. 3. Thus, the solenoid valve 60 is kept opened for 
5 seconds to permit the supply of the pressurized oil to the space 65 to 
increase the unloaded amount of the refrigerant. 
Although the above description has been made with specific reference to the 
chilling operation, the invention can be applied also to the case of a 
warming operation for producing warm water. Namely, the computation of 
operation durations for the oil discharging timer contact 86 and the oil 
supplying timer contact 85 is conducted in accordance with the result of 
comparison between the set temperature and the outlet water temperature, 
after switching the operation mode of the system.