Liquid coolant circulation control system for immersion cooling systems

A liquid coolant circulation control system that minimizes the requisite feed pressure and keeps the flow rate of a liquid coolant constant includes a casing accommodating electronic circuits to be immersed in and cooled by a liquid coolant, a feed pump for feeding the liquid coolant to the casing, and a collecting pump for collecting the liquid coolant from the casing. A first measuring meter, interposed between the feed pump and the casing, measures a flow rate and/or a pressure of the liquid coolant flowing into the casing. A second measuring meter, interposed between the casing and the collecting pump, measures a flow rate and/or a pressure of the liquid coolant flowing out of the casing. A determining section calculates a difference or differences between the two flow rates and/or pressures. The determining section sends this information to a control section. The control section controls the output of one or both pumps, so that the difference or differences remain at a predetermined value. The liquid coolant circulation control system is also provided with a heat exchanger that cools the liquid coolant to a predetermined temperature, and a buffer tank that absorbs an increase or decrease in the volume of the liquid coolant.

BACKGROUND OF THE INVENTION 
The present invention relates to an immersion cooling system for cooling 
electronic sections included in a data processor or similar electronic 
apparatus by immersing them in a liquid coolant. More particularly, the 
present invention is concerned with a liquid coolant circulation control 
system for minimizing the required feed pressure and maintaining the flow 
rate of a liquid coolant constant while the coolant is circulated. 
An immersion cooling system mentioned above is disclosed in, for example, 
U.S. Pat. No. 4,590,538. This U.S. Patent discloses an immersion cooling 
system having a hollow cylindrical tank storing an inert liquid coolant 
therein. A plurality of frames are arranged radially in the tank and 
support a plurality of electronic circuit modules in a stack 
configuration. A plurality of coolant feeding members and a plurality of 
coolant collecting members are supported by nearby frames while 
alternating with each other. These members form passages for the coolant. 
A plurality of pumps cause the coolant to circulate. A plurality of heat 
exchangers cool the coolant having been heated by causing it to release 
heat. This type of conventional immersion cooling system has some problems 
left unsolved, as follows. 
To begin with, each pump has to feed the coolant under a pressure high 
enough to overcome losses ascribable to the resistance of the heat 
exchangers, tank and so forth to the flow of the coolant. As a result, the 
pressure acting on the coolant itself increases which raise the boiling 
point of the coolant in the tank, thereby suppressing nuclear boiling 
cooling. 
Another problem is that the tank and other members defining the liquid 
passages have to be provided with high resistivity to pressure since the 
pressure acting on the passages increases. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a liquid 
coolant circulation control system which maintains the pressure acting on 
a liquid coolant and liquid passages constant and low to thereby promote 
nuclear boiling cooling, i.e., to enhance efficient cooling while 
implementing members defining the passages with low resistivity to 
pressure. 
A liquid coolant circulation control system for an immersion cooling system 
of the present invention comprises a casing accommodating electronic 
circuits to be immersed in and cooled by a liquid coolant, a collecting 
pump for collecting the liquid coolant from the casing, first measuring 
means for measuring a flow rate of the liquid coolant flowing into the 
casing, second measuring means interposed between the casing and the 
collecting pump for measuring a flow rate of the liquid coolant flowing 
out of the casing, determining means for calculating a difference between 
the flow rates measured by the first and second measuring means, and 
control means for controlling the operation of the collecting pump such 
that the difference determined by the determining means remains at a 
predetermined value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To better understand the present invention, a brief reference will be made 
to a conventional immersion cooling system, shown in FIG. 8. As shown, the 
conventional system has a hollow cylindrical tank 81 storing an inert 
liquid coolant therein. A plurality of frames 82 are arranged radially in 
the tank 81 and support a plurality of electronic circuit modules 87 in a 
stack configuration. A plurality of coolant feeding members 83 and a 
plurality of coolant collecting members 84 are supported by nearby frames 
82 while alternating with each other. These members 83 and 84 form 
passages for the coolant. A plurality of pumps 85 cause the coolant to 
circulate. A plurality of heat exchangers 86 cool the coolant having been 
heated by causing it to release heat. The coolant cooled by the heat 
exchangers 86 is fed to the coolant feeding members 83 by first conduits 
801 and 803. The coolant heated by the electronic circuit modules 87 is 
returned to the pump 85 via the coolant collecting member 84 by second 
conduits 802 and 804. 
Referring to FIG. 1, a first embodiment of the present invention is shown 
and includes a feed pump 1 for feeding a liquid coolant, e.g., Fluorinert 
available from 3M , to a coolant passage. A collecting pump 2 collects the 
coolant. A constant flow valve 3 delivers the coolant while adjusting it 
to a desired flow rate. A casing 5 accommodates therein electronic 
circuits, not shown, to be cooled by the coolant by immersion. A detecting 
section 6-1 detects a difference in flow rate between the coolant entering 
the casing 5 and the coolant coming out of the casing 5. A control section 
7 controls the output of the collecting pump 2. A heat exchanger 8 cools 
the coolant, having been heated by the electronic circuits of the casing 
5, to a predetermined temperature. A buffer tank 9 is provided for 
absorbing the increase or decrease in the volume of the coolant due to 
temperature. Electronic flow meters 10-1 and 10-2 each measures the flow 
rate of the coolant in the associated coolant passage. The reference 
numeral 20 designates a piping through which the coolant circulates. In 
the figure, an arrow indicates the direction in which the coolant flows. 
In operation, while liquid coolant is fed from the feed pump 1 to the 
casing 5, the constant flow valve 3 provides it with a desired flow rate. 
The coolant from the casing 5 is collected by the collecting pump 2 under 
the following control. 
The flow meters 10-1 and 10-2 measure respectively the flow rate of the 
coolant flowing into the casing 5 and the flow rate of the coolant flowing 
out of the casing 5. The measured flow rates are sent from the flow meters 
10-1 and 10-2 to the detecting section 6-1. In response, the detecting 
section 6-1 calculates a difference .DELTA.L between the two flow rates 
and sends it to the control section 7. Then, the control section 7 
controls the output of the collecting pump 2 such that the difference 
.DELTA.L decreases to zero. For this purpose, the control section 7 may be 
provided with a linear control mechanism using an inverter. By so 
controlling the output of the collecting pump 2, it is possible to 
maintain the pressure in the casing 5 constant substantially at the 
atmospheric level. Therefore, the boiling point of the coolant in the 
casing 5 is prevented from rising. This is successful in promoting nuclear 
boiling cooling, i.e., enhancing the cooling efficiency. In addition, the 
pressure maintained substantially at the atmospheric level allows casing 5 
and associated passage members to be designed with low resistivity to 
pressure. 
FIG. 2 shows a second embodiment of the present invention which also 
includes the feed pump 1, collecting pump 2, constant flow valve 3, and 
casing 5. Pressure gauges 4-1 and 4-2 measure respectively the pressure 
acting on the coolant flowing into the casing 5 and the pressure acting on 
the coolant flowing out of the casing 5. A detecting section 6-2 receives 
the measured pressures from the pressure gauges 4-1 and 4-2 to determine 
their difference .DELTA.P. A control section 7 controls the output of the 
collecting pump 2 in response to the pressure difference .DELTA.P fed 
thereto from the detecting section 6-2. The heat exchanger 8, buffer tank 
9 and piping 20 are identical with those of the first embodiment. 
In operation, on receiving the measured pressures from the pressure gauges 
4-1 and 4-2, the detecting section 6-2 calculates a difference .DELTA.P 
between the pressures and sends it to the control section 7. In response, 
the control section 7 controls the output of the collecting pump 2 such 
that the pressure difference .DELTA.P goes to a predetermined value. The 
control section 7 may also be implemented by a linear control mechanism 
using an inverter. In this manner, the output of the pump 2 is so 
controlled as to control the difference in pressure between the coolant 
entering the casing 1 and the coolant coming out of the same. This is also 
successful in maintaining the pressure in the casing 5 constant 
substantially at the atmospheric level. This embodiment, therefore, 
achieves the same advantages as those of the first embodiment. 
Referring to FIG. 3, a third embodiment of the present invention includes 
the feed pump 1, collecting pump 2, and casing 5. Electronic flow meters 
10-1 and 10-2 are respectively responsive to the flow rate of the coolant 
flowing into the casing 5 and that of the coolant flowing out of the 
casing 5. The pressure gauges 4-1 and 4-2 are respectively responsive to 
the pressure acting on the coolant flowing into the casing 5 and the 
pressure acting on the coolant flowing out of the casing 5, as sated 
earlier. The detecting section 6-1 determines a difference .DELTA.L 
between the flow rates sent thereto from the flow meters 10-1 and 10-2, 
while the detecting section 6-2 determines a difference .DELTA.P between 
the pressures sent thereto from the pressure gauges 4-1 and 4-2. In 
response to the differences .DELTA.L and .DELTA.P, the control section 6-1 
controls the output of the collecting pump 2. The embodiment further 
includes the previously stated heat exchanger 8, buffer tank 9, and piping 
20. 
In operation, the flow rates measured by the flow meters 10-1 and 10-2 and 
the pressures measured by the pressure gauges 4-1 and 4-2 are sent to the 
detecting sections 6-1 and 6-2, respectively. In response, the detecting 
section 6-1 determines a difference .DELTA.L between the two flow rates 
and sends it to the control section 7. Likewise, the detecting section 6-2 
calculates a difference .DELTA.P between the two pressures and sends it to 
the control section 7. Then, the control section 7 so controls the output 
of the pump 2 as to reduce the differences .DELTA.L to zero and .DELTA.P 
to the predetermined value. Again, the control section 7 may be 
implemented by a linear control mechanism using an inverter. 
Since this embodiment controls the output of the collecting pump 2 such 
that both of the differences .DELTA.L decrease to zero and .DELTA.P to the 
predetermined value, it maintains the pressure in the casing 5 
substantially at the atmospheric level more positively than the first and 
second embodiments. 
FIG. 4 shows a fourth embodiment of the present invention. As shown, this 
embodiment has a setting section 11 for setting a desired flow rate, in 
addition to the feed pump 1, collecting pump 2, casing 5, flow meter 10-1, 
control section 7, heat exchanger 8, buffer tank 9 and piping 20. 
In operation, the flow meter 10-1 measures the flow rate of the coolant 
flowing into the casing 5 and sends it to the control section 7. The 
setting section 11 holds a preset flow rate of the coolant. In the 
illustrative embodiment, the pumps 1 and 2 have the same ability. By 
comparing the flow rate from the flow meter 10-1 with the preset flow rate 
from the setting section 11, the control section 7 controls the outputs of 
the pumps 1 and 2 at the same time such that the former coincides with the 
latter. Such control may also be effected by a linear control mechanism 
using an inverter. 
As stated above, this embodiment controls the outputs of the feed pump 1 
and collecting pump 2 at the same time such that the flow rate of the 
coolant entering the casing 1 coincides with the preset flow rate. As a 
result, the pressure in the casing 5 is maintained substantially at the 
atmospheric level, whereby the advantages stated in relation to the first 
embodiment are achieved. 
FIG. 5 shows a fifth embodiment of the present invention which is made up 
of the feed pump 1, collecting pump 2, casing 5, electronic flow meter 
10-2, setting section 11 holding a preset flow rate, control section 
responsive to the outputs of the flow meter 10-2 and setting section 11, 
heat exchanger 8, buffer tank 9, and piping 20. 
In operation, the flow meter 10-2 measures the flow rate of the coolant 
flowing out of the casing 5 and sends it to the control section 7. In the 
illustrative embodiment, the feed pump 1 and collecting pump 2 have the 
same output, as in the fourth embodiment. The control section 7 compares 
the flow rate from the flow meter 10-2 with the preset flow rate from the 
setting section 11 and controls the outputs of the pumps 1 and 2 such that 
the former coincides with the latter. For this purpose, use may be made of 
a linear control mechanism using an inverter, as in the previous 
embodiments. 
As described above, this embodiment so controls the outputs of the two 
pumps 1 and 2 as to equalize the flow rate of the coolant flowing out of 
the casing 5 and the preset flow rate. This is also successful in 
maintaining the pressure in the casing 5 constant substantially at the 
atmospheric level and, therefore, achieving the advantages stated in 
relation to the first embodiment. 
A sixth embodiment of the present invention is shown in FIG. 6 and includes 
the feed pump 1, collecting pump 2 constant flow valve 3, casing 5, heat 
exchanger 8, and buffer tank 9. A level gauge 12 measures the level of the 
coolant existing in the buffer tank 9. A detecting section 13 monitors the 
level gauge 12 to determine the liquid level in the buffer tank 9. A 
digitizing section 14 converts the liquid level detected by the detecting 
section 13 to a digital or numerical value. The control section 7 controls 
the output of the collecting pump 2 in response to the numerical data sent 
from the digitizing section 14. The constituents 1, 2, 3, 5, 8 and 9 are 
fluidly communicated by the piping 20. 
In operation, the coolant is feed from the fed pump 1 to the casing 5 at a 
predetermined flow rate via the constant flow valve 3. Implemented by a 
CCD (Charge Coupled Device) camera, for example, the detecting section 13 
constantly monitors the level gauge 12 to determine the liquid level in 
the buffer tank 9. As the detecting section 13 sends a signal 
representative of a liquid level to the digitizing section 14, the 
digitizing section 14 converts it to a numerical value and delivers the 
numerical value to the control section 7. In response, the control section 
7 controls the output of the pump 2 such that the data representative of 
the liquid level of the buffer tank 9 remains constant. This control may 
also be implemented with a linear control mechanism using an inverter. 
As stated above, this embodiment controls the output of the collecting pump 
2 in such a manner as to maintain the liquid level in the buffer tank 9 
constant. Hence, the pressure in the casing 5 is maintained constant, so 
that the advantages stated in relation to the first embodiment are 
achieved. 
FIG. 7 shows a seventh embodiment of the present invention also including 
the feed pump 1, collecting pump 2, constant flow valve 3, casing 5, heat 
exchanger 8, buffer tank 9, and electronic flow meter 10-1 responsive to 
the flow rate of the coolant between the valve 3 and the casing 5. A 
digitizing section 15 converts the flow rate measured by the flow meter 
10-1 to a numerical value. The output of the collecting pump 2 necessary 
to maintain the same flow rate as the digital flow rate sent from the 
digitizing section 15 is read out of a conversion table 16: The control 
section 7 determines requisite output of the collecting pump 2 on the 
basis of the conversion table 16. 
In operation, the coolant from the feed pump 1 flows into the casing 5 at a 
constant flow rate via the constant flow valve 3. The flow meter 10-1 
intervening between the valve 3 and the casing 5 measures the flow rate of 
the coolant flowing toward the casing 1 and sends it to the digitizing 
section 15. The digitizing section 15 converts the measured flow rate to a 
numerical value and delivers the numerical value to the control section 7. 
In response, the control section 7 scans the conversion table 16 on the 
basis of the received flow rate so as to find pump control data needed to 
maintain the same flow rate. Then, the control section 7 determines the 
output of the collecting pump 2 matching the flow rate found on the 
conversion table 16. 
As described above, this embodiment selects a particular output of the 
collecting pump 2 by referencing the conversion table 16 on the basis of 
the actual flow rate of the coolant to the casing 5. This maintains the 
pressure in the casing 5 constant substantially at the atmospheric level, 
thereby attaining the advantages described in relation to the first 
embodiment. 
While the present invention has been described in conjunction with the 
preferred embodiments thereof, it will now be readily possible for those 
skilled in the art to put the present invention into practice in various 
other manners.