Oil recovery system for closed type centrifugal refrigerating machine

An oil recovery system for a closed type centrifugal refrigerating machine. The refrigerating machine includes a compressor that has an oil sump below the suction side thereof, a condenser that liquefies compressed refrigerant gas by cooling, an evaporator that evaporates the liquefied refrigerant, a suction volume control valve that controls the volumetric flow of refrigerant gas sucked into the compressor from the evaporator, recovery means for recovering oil from the oil sump to an oil tank in a lubricating system, and control means for starting or stopping the compressor in accordance with the magnitude of refrigeration load. The oil recovery system comprises valve opening control means for controlling the opening of the suction volume control valve such that a decrease in the amount of oil recovered to the oil tank is compensated for when the number of times of start or stop of the compressor per unit of time exceeds a set number.

BACKGROUND OF THE INVENTION 
The present invention relates to an oil recovery system for a closed type 
centrifugal refrigerating machine which is capable of effectively 
recovering lubricating oil that has leaked in a refrigerant even when the 
refrigeration load is small. 
The refrigerating cycle of the closed type centrifugal refrigerating 
machine will be explained below with reference to FIG. 3. 
Refrigerant gas A that evaporates in an evaporator 1 is sucked into a 
compressor 2. The volumetric flow of refrigerant gas sucked into the 
compressor 2 is controlled by a suction volume control valve 3, for 
example, a suction vane, in accordance with the magnitude of refrigeration 
load. The refrigerant gas compressed in the compressor 2 is discharged to 
a condenser 4 where it is cooled to condense into its liquid phase. The 
now liquid refrigerant returns to the evaporator 1 through an expansion 
mechanism 5, for example, an orifice. In this system, a bearing for an 
impeller 6 in the compressor 2, a speed increasing gear 7, etc. are 
lubricated by a lubricating system (not shown), which is separate from the 
refrigerant system. 
In the closed type centrifugal refrigerating machine, the refrigerant 
system and the lubricating system are shut off from the outside as a 
whole, and these two systems are separated from each other by a sealing 
device. However, there is unavoidable communication between the two 
systems, although it is only a little. In general, oil leaks into the 
refrigerant system from a shaft extending portion of the compressor 2, as 
shown by B in FIG. 3. The leakage oil reaches the evaporator 1 through the 
condenser 4, together with the refrigerant gas. Since the refrigerant 
alone evaporates in the evaporator 1, the oil remains therein. 
Usually, however, refrigerant mist is sucked into the compressor 2, 
together with the refrigerant gas that evaporates in the evaporator 1, so 
that the leakage oil is eventually dissolved in the mist and carried out 
of the evaporator 1. An oil sump 8 is provided at the suction side of the 
compressor 2, as shown in FIGS. 4 and 5. Oil that gathers in the oil sump 
8 is sucked and recovered to an oil tank 13 by an ejector 11 through an 
ejector filter 12. The ejector 11 is disposed in the intermediate portion 
of a piping that connects together a scroll portion of a discharge gas 
passage for the gas compressed in the compressor 2 and the oil tank 13 
that is equalized in pressure with the upstream side of the suction volume 
control valve 3. The ejector 11 uses a pressure difference between the 
discharge pressure and the cooler pressure as driving force to return the 
oil in the oil sump 8 to the oil tank 13 through the ejector filter 12 by 
the ejector effect caused by the refrigerant gas. 
In FIG. 3, reference numeral 24 denotes a main motor for the compressor 2, 
and 9 a bypass line that bypasses part of the refrigerant gas flowing into 
the condenser 4 from the compressor 2 to the evaporator 1, the bypass line 
9 being provided with a hot gas bypass valve 10. Reference numerals 14 and 
15 denote a chilled water inlet and a chilled water outlet of the 
evaporator 1, and 16 and 17 a cooling water inlet and a cooling water 
outlet of the condenser 4. In addition, reference numeral 18 denotes an 
oil cooler, 19 a purge condenser, 20 a refrigerant pump, 21 a strainer, 
and 22 a refrigerant cooler. 
When the conventional closed type centrifugal refrigerating machine runs in 
a full-load condition or a load condition which is close to it, the flow 
velocity of the refrigerant gas is sufficiently high that an adequate 
amount of refrigerant mist can accompany the refrigerant gas to effect 
satisfactory recovery of the leakage oil. 
However, when the machine is in a partial-load condition, the amount of 
refrigerant mist accompanying the refrigerant gas is small, so that the 
amount of oil recovered becomes smaller than the amount of leakage oil, 
resulting in an increase in the amount of oil remaining in the evaporator 
1, being dissolved in the refrigerant. In consequence, the amount of oil 
in the lubricating system decreases, and a low oil pressure tripping 
device is eventually activated to stop the refrigerating machine. 
Thus, it has heretofore been necessary to supply oil to the lubricating 
system in order to continue the operation of the refrigerating machine 
without interruption. 
When the load increases to a level at which a full-load running is 
available, the oil recovery function by the accompaniment of refrigerant 
gas is restored. As a result, the oil level in the oil tank rises, so that 
an excess of oil must be taken out of the oil tank, which burdens the 
operator with a very troublesome task. 
There is another problem that, when the concentration of oil dissolved in 
the liquid refrigerant increases, the contamination of the tube in the 
evaporator 1 is promoted to check the heat transfer. 
To prevent the occurrence of the above-described problem, one type of 
centrifugal refrigerating machine makes use of a differential pressure 
which is produced across the suction volume control valve 3 that is 
automatically closed during a partial-load running, to lead the liquid 
refrigerant having the oil dissolved therein from the evaporator 1 to the 
downstream side of the suction volume control valve 3, thereby recovering 
the leakage oil. More specifically, when the refrigerating machine is in a 
partial-load condition, the opening of the suction volume control valve 3 
is small, so that the differential pressure across the control valve 3 
becomes sufficiently large to enable the liquid refrigerant having the 
leakage oil dissolved therein to be sucked into the oil sump 8 by virtue 
of a difference between the cooler pressure and the pressure at the 
downstream side of the control valve 3, thus recovering the leakage oil. 
However, even in a centrifugal refrigerating machine with such an oil 
recovery function, if the compressor 2 turns on/off under a small 
refrigeration load condition, the temperature of chilled water rises when 
the compressor 2 is off, and when it turns on, the suction volume control 
valve 3 in the prior art is fully opened because the chilled water 
temperature is relatively high for the moment. However, since the load is 
small, the chilled water temperature lowers within a short time, thus 
causing the compressor 2 to turn off. Accordingly, if the compressor 2 
starts and stops frequently, it is impossible to obtain sufficient time to 
remove the liquid refrigerant having the leakage oil dissolved therein by 
making use of a differential pressure across the suction volume control 
valve 3. 
In general, the oil pump in the lubricating system performs the residual 
running for a predetermined period of time after suspension of the 
compressor 2, and the leakage of oil into the refrigerant system occurs 
even during the suspension of the compressor 2. For this reason, it has 
heretofore been impossible to solve completely the problem that the 
leakage oil remains in the evaporator 1 and cannot be recovered to the oil 
tank. 
SUMMARY OF THE INVENTION 
In view of the above-described circumstances, it is an object of the 
present invention to provide an oil recovery system for a closed type 
centrifugal refrigerating machine which is capable of effectively 
recovering lubricating oil that has leaked in a refrigerant even when the 
refrigeration load is small. 
To attain the above-described object, the present invention provides an oil 
recovery system for a closed type centrifugal refrigerating machine 
including a compressor with a suction volume control valve, an oil sump 
provided at the suction side of the compressor, an oil recovery mechanism 
which recovers the oil gathered in the oil sump to an oil tank by a 
suction device, and a mechanism which automatically starts and stops the 
compressor in accordance with the magnitude of the load, wherein the 
improvement comprises a means for forcibly maintaining the opening of the 
suction volume control valve of the compressor above a predetermined value 
for a predetermined time when the number of times of automatic start or 
stop of the compressor per unit of time exceeds a predetermined set 
number. 
In addition, according to the present invention, when the opening of the 
suction volume control valve of the compressor is maintained above a 
predetermined value for a predetermined time, a hot gas bypass valve that 
is provided in a bypass line for bypassing part of the refrigerant gas 
flowing into a condenser from the compressor is opened. 
In the centrifugal refrigerating machine, the compressor is turned on/off 
by an automatic start/stop device, for example, a thermostat, which is 
installed at a chilled water outlet of the evaporator since satisfactory 
control cannot be available only with a control mechanism for the suction 
volume control valve, for example, a suction vane, by which the 
refrigeration load is reduced. When the refrigeration load is small, 
chilled water is cooled even more easily and returns with the lowered 
temperature, and the chilled water outlet temperature is therefore lowered 
by running the compressor for a relatively short time. For this reason, 
the frequency at which the compressor turns on/off rises. 
According to the present invention, the number of times of start or stop of 
the compressor per predetermined period of time is detected, and when the 
detected number exceeds a set value, the opening of the suction volume 
control valve of the compressor is maintained above a predetermined value 
for a predetermined time. The term "predetermined value" herein means a 
degree of opening of the suction volume control valve at which the 
volumetric flow of the refrigerant gas is sufficient for the refrigerant 
liquid to accompany the refrigerant gas, and it can be determined by a 
trial run. By doing so, since the refrigeration load temporarily increases 
to a level which is higher than is necessary, the running time of the 
compressor shortens; however, since the volumetric flow of the refrigerant 
gas becomes sufficient for the refrigerant liquid to accompany the 
refrigerant gas, the oil gathered in the evaporator is recovered. If the 
compressor is run until a low suction pressure tripping device or a low 
refrigerant temperature tipping device or a low chilled water temperature 
tripping device is almost activated, the running time of the compressor 
also becomes sufficient for the oil recovery. 
If the hot gas bypass valve for bypassing the refrigerant gas from the 
condenser to the evaporator is opened to a predetermined value, the load 
on the compressor becomes larger than the partial load in a normal 
running, which is against the energy saving running of the compressor. 
However, even when the suction volume control valve is forcibly opened to 
a predetermined value, there is no increase in the number of times of 
start and stop of the compressor per unit of time. 
According to another aspect of the present invention, there is provided an 
oil recovery system for a closed type centrifugal refrigerating machine, 
which comprises a suction volume control valve opening control means for 
controlling the opening of the suction volume control valve for the 
compressor below a predetermined value when the number of times of 
automatic start or stop of the compressor per unit of time exceeds a 
predetermined set number, thereby reducing the frequency of start and stop 
of the compressor. 
If the opening of the suction volume control valve increases, the suction 
gas quantity of the compressor increases and the refrigerating capacity 
increases. At the same time, the main motor power increases, and the main 
motor current value also increases. 
Accordingly, it is possible to lengthen the time for the chilled water 
outlet temperature to lower to a set level when the refrigeration load is 
small, by controlling the main motor current value at a predetermined 
value so as to limit the degree of opening of the volume control valve to 
thereby prevent the increase in the refrigerating capacity. That is, the 
suction volume control valve opening control means may be a means for 
effecting control such that the current value of the main motor for the 
compressor will not exceed a predetermined value, by a mechanism for 
limiting the current of the main motor, thereby controlling the opening of 
the suction volume control valve below a predetermined value. 
If the chilled water control temperature (i.e., the chilled water outlet 
set temperature) is temporarily raised above the required set temperature, 
the refrigeration load apparently decreases relative of the raised set 
temperature, so that a low-opening running is available without any 
increase in the opening of the suction volume control valve. Accordingly, 
it is possible to prevent the refrigerating capacity from increasing and 
lengthen the running time and hence possible to recover the leakage oil 
satisfactorily. That is, the suction volume control valve opening control 
means may be a means for controlling the opening of the suction volume 
control valve below a predetermined level by raising the chilled water 
control temperature in the refrigerating machine, thereby reducing the 
frequency of start and stop of the compressor. 
According to the present invention, the number of times of start or stop of 
the compressor per predetermined time is detected, and when the detected 
number exceeds a set value, the opening of the suction volume control 
valve for the compressor is controlled below a predetermined value. The 
opening of the control valve is determined to be a value with which a 
sufficient differential pressure to suck the liquid refrigerant from the 
evaporator is obtained across the suction volume control valve. The value 
for the valve opening can be determined by a trial run. Thus, the 
differential pressure across the section volume control valve is 
sufficiently high to suck the liquid refrigerant, and the running time 
lengthens. Therefore, the oil gathered in the evaporator can be recovered 
satisfactorily. 
In general, the main motor for driving the compressor is provided with a 
current-limiting mechanism for protecting the motor. By effecting control 
such that the current value of the main motor will not exceed a 
predetermined value, the opening of the suction volume control valve can 
be readily controlled at the desired level. 
It is also possible to control the opening of the suction volume control 
valve by temporarily raising the chilled water control temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One embodiment of the present invention will be described below with 
reference to the accompanying drawings. 
FIG. 1 is a flowchart showing the operation of one embodiment of the oil 
recovery system for a closed type centrifugal refrigerating machine 
according to the present invention. It should be noted that the 
arrangement of the closed type centrifugal refrigerating machine in this 
embodiment is the same as that shown in FIG. 3. 
Referring to FIG. 1, first, the closed type centrifugal refrigerating 
machine is run, and the number n of times of starting of the compressor 2, 
that is, the number n of times of ON of the compressor 2 per unit of time 
during an automatic running, is counted (Step 101). Next, a comparison is 
made between the number n of times of ON per unit of time and a reference 
number N of times of ON of the compressor 2 per unit of time (Step 102). 
Subsequently, it is judged whether or not n&gt;N (Step 103). If YES, the 
suction volume control valve 3 is opened to a predetermined value (Step 
104). This state is maintained for a predetermined time T, that is, for 
the minimum time T required for maintaining the oil recovery function 
(Step 105). Next, it is judged whether or not the time T has elapsed (Step 
106). If YES, the suction volume control valve 3 is closed (Step 107), and 
the process then returns to Step 101. If NO is the answer in Step 103, 
that is, if n is not greater than N, the process returns to Step 101 from 
Step 103. 
When YES is the answer in Step 103, the hot gas bypass valve 10 may be 
opened to a predetermined degree (Step 108). In such a case, the hot gas 
bypass valve 10 is closed (Step 109) after Step 107. 
The above-described operation can be realized by a control means that 
employs a microcomputer, for example. 
Another embodiment of the present invention will next be explained. 
FIG. 2 is a flowchart showing the operation of the second embodiment of the 
oil recovery system for a closed type centrifugal refrigerating machine 
according to the present invention. It should be noted that the 
arrangement of the closed type centrifugal refrigerating machine in this 
embodiment is the same as that shown in FIG. 3. 
Referring to FIG. 2, first, the closed type centrifugal refrigerating 
machine is run, and the number n of times of starting of the compressor 2, 
that is, the number n of times of ON of the compressor 2 per unit of time 
during an automatic running, is counted (Step 201). Next, a comparison is 
made between the number n of times of ON per unit of time and a reference 
number N of times of ON of the compressor 2 per unit of time (Step 202). 
Subsequently, it is judged whether or not n&gt;N (Step 203). If YES, the 
opening of the suction volume control valve 3 is limited to a 
predetermined value (Step 204). This state is maintained for a 
predetermined time T, that is, for the time T required for maintaining the 
oil recovery function (Step 205). Next, it is judged whether or not the 
time T has elapsed (Step 206). If YES, the limitation on the suction 
volume control valve 3 is canceled (Step 207), and the process then 
returns to Step 201. If NO is the answer in Step 203, that is, if n is not 
greater than N, the process returns to Step 201 from Step 203. 
When YES is the answer in Step 203, the current value of the main motor 9 
may be controlled at a predetermined level (Step 208), or the chilled 
water control temperature may be raised (Step 209). 
The above-described operation can be realized by a control means that 
employs a microcomputer, for example. 
As has been described above, the present invention provides the following 
advantageous effects: 
(1) Even if a partial-load running continues with a low oil recovery, the 
closed type centrifugal refrigerating machine can be continuously run 
without the low oil pressure tripping device being activated due to lack 
of oil. 
(2) The present invention eliminates the need for such a troublesome 
operation that oil is supplied and then taken out in order to run the 
machine continuously. 
(3) Since it is possible to check the increase in the concentration of oil 
dissolved in the liquid refrigerant in the evaporator, it is possible to 
prevent contamination of the heat transfer tube and maintain excellent 
heat transfer condition. Thus, an energy saving running can be attained.