Abstract:
A method of apparatus for immersion cooling electronic equipment including immersing the electronic equipment in a pressure-sealed tank containing a heat transfer fluid and including a vapor space fluidicly coupled to a condenser; operating the electronic equipment to generate heat and evaporate some of the heat transfer fluid, causing heat transfer fluid vapor to enter the condenser; condensing the heat transfer fluid vapor in the condenser to produce a condensate; returning the condensate to the tank; and increasing power consumption to increase heat generated by the electronic equipment and develop an increased pressure of the heat transfer fluid vapor to bring the apparatus into an equilibrium condition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.K. Application No. GB1607662.2 filed May 3, 2016, which is hereby incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to immersion cooling and more particularly to immersion cooling of electronic equipment. 
       BACKGROUND 
       [0003]    Immersion cooling is a technology used to cool electronic processing equipment, particularly for high speed and high data capacity processing equipment, where conventional air cooling has been found inadequate. Immersion cooling involves immersing the equipment (e.g. electronic data processing modules) in a tank containing a liquid. Convection and evaporation of the liquid can provide very high cooling rates to keep the modules within their normal operating temperature range. Water is not an ideal liquid to use for these applications because (a) it is electrically conductive meaning that some type of insulation would need to be used between the water and the electronics, thereby reducing the cooling effectiveness, and (b) because water evaporates at too high a temperature. Instead more effective, non-conductive cooling fluids that evaporate at lower temperatures have been developed. An example of this is the NOVEC™ 649 engineered fluid manufactured by 3M™ of St. Paul, Minn. However, even with the use of these fluids problems can arise if moisture gets into the liquid. Even small amounts of water can cause shorting of electrical connections in the electronic equipment. Another problem is that because the specialized cooling liquids are expensive, it is important to retain all, or as much as possible of the material in the vessel and to minimize any loss as a result of the evaporation. 
         [0004]    Various modifications and improvements in immersion cooling are presented below. 
       SUMMARY 
       [0005]    In one aspect the present invention provides a method of immersion cooling of electronic equipment. The electronic equipment is immersed in a pressure-sealed tank holding heat transfer fluid in liquid form. The tank includes a vapor space above a surface of the liquid, and the vapor space is connected to a condenser. The electronic equipment is operated to generate heat so as to evaporate some of the heat transfer fluid and cause heat transfer fluid vapor to enter the condenser. The heat transfer fluid vapor is condensed in the condenser, such that gaseous heat transfer fluid returns to its liquid state. The heat transfer fluid condensate is returned to the tank. The power consumption by the electronic equipment is increased to increase the heat generated and develop an increased pressure of the heat transfer fluid vapor as the gaseous form of the heat transfer fluid uses more volume that its liquid form. The increased pressure increases effectiveness of the condenser and changing back voluminous heat transfer gas to its more compact liquid phase brings the system into an equilibrium condition. 
         [0006]    The method may further comprise reducing power consumption by the electronic equipment to reduce the heat generated and cause the pressure of the heat transfer fluid vapor to drop, the lower pressure decreasing the boiling point of the heat transfer fluid to bring the system back to equilibrium. The reduced power consumption may cause a degree of vacuum to be developed inside of the tank. The power consumption by the electronic equipment may be reduced substantially to zero, so that the pressure of the heat transfer fluid vapor in the tank is reduced to about 0.4 bar (absolute). 
         [0007]    The method may further comprise filtering the condensate to remove any moisture and/or metal particles before returning the condensate to the tank. 
         [0008]    The heat transfer fluid is preferably a dielectric fluid. 
         [0009]    In some embodiments the method may include features more commonly associated with other methods that involve a non-pressurized tank. For example, the method may further comprise condensing some of the heat transfer fluid vapor by means of condenser tubes in the tank. 
         [0010]    As another example, the method may further comprise removing moisture from air in the vapor space of the tank by means of a desiccant. 
         [0011]    In another aspect the present invention provides an apparatus for immersion cooling of electronic equipment. A pressure-sealed tank holds heat transfer fluid in liquid form into which the electronic equipment can be immersed. The pressure-sealed tank includes a vapor space above a surface of the liquid. A condenser having an inlet connected to the vapor space receives heat transfer fluid vapor, and has a sealable vapor outlet and a condensate outlet. Condensed heat transfer fluid can return from the condensate outlet to the tank through a condensate return line. 
         [0012]    The apparatus may further comprise a filter in the condensate return line for removing moisture and/or metallic particles. 
         [0013]    In some embodiments the apparatus may include features more commonly associated with a non-pressurized tank system. For example, the apparatus may further comprise means inside the tank for condensing heat transfer fluid vapor. The means for condensing may comprise a bank of condenser tubes. 
         [0014]    As another example, the apparatus may further comprise a desiccant for aiding removal of moisture from air in the vapor space of the tank. 
         [0015]    In another aspect the invention provides a method of commencing immersion cooling of electronic equipment. The electronic equipment is immersed in a pressure-sealed tank holding heat transfer fluid in liquid form. The tank includes a vapor space above a surface of the liquid. The vapor space is connected to a condenser, which is disposed above the tank and includes a vapor outlet with a valve operable for sealing the vapor outlet. Operation of the electronic equipment is commenced to generate heat so as to evaporate some of the heat transfer fluid and produce heat transfer fluid vapor in the vapor space. The valve is opened to allow the heat transfer fluid vapor generated to drive air from the vapor space through the condenser and out through the vapor outlet until substantially all of the air has been driven out of the vapor space. The valve is then closed and operation of the electronic equipment continued to generate heat so as to produce heat transfer fluid vapor. The heat transfer fluid vapor is condensed in the condenser to produce heat transfer fluid condensate, which is returned to the tank. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic illustration of an immersion cooling system in accordance with embodiments of the invention. 
           [0017]      FIG. 2  is a schematic illustration of another immersion cooling system. 
           [0018]      FIG. 3  is a flow chart illustrating the principal method steps in an immersion cooling method in accordance with embodiments of the invention. 
           [0019]      FIG. 4  is a flow chart illustrating the principal method steps in a method of commencing immersion cooling in accordance with embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  is a schematic illustration of an embodiment of an immersion cooling system. In this system a condenser is mounted as a separate component outside the evaporation tank. The tank is sealed from the surrounding atmosphere and permitted to operate at a variable pressure. After the initial start-up procedure is completed there is no air in the tank atmosphere, only vapor of the heat transfer fluid. 
         [0021]    As shown in  FIG. 1 , the electronic components  10  are immersed in the heat transfer fluid  12  in liquid form in a pressure-sealed tank  30 . The liquid heat transfer fluid  12  has a surface  13  in the tank  30 , above which is a vapor space  15 . The liquid heat transfer fluid  12  removes heat from the components by convection and evaporation within the tank  30 . The vapor of the heat transfer fluid  12  is heavier than air. 
         [0022]    A riser tube  32  extends from an opening  31  in the roof of the tank  30  to an inlet  33  of a condenser  34 . The condenser  34  includes internal tubes (not shown) through which a coolant (e.g. water) is circulated. The vapor passes through the condenser in passages between the tubes towards a vapor outlet  35 . A valve  36  can be opened to allow air or vapor to pass from the vapor outlet  35  to the atmosphere, and can be closed to seal the vapor outlet  35 . The condenser also has a condensate outlet  37  into a return line  39 , through which condensate can flow back to the tank  30  and which includes an in-line filter  38 . The coolant that is circulated through the condenser  34  is returned to an air cooler  40  for removal of heat that is taken from the vapor in the condenser. 
         [0023]    To ensure that the heat transfer liquid and vapor within the tank is completely dry the air in the vapor space  15  is removed at start up. During the start-up procedure when power is provided to the electronic equipment  10  and the heat transfer liquid  12  starts to evaporate the pressure in the tank  30  starts to rise. At this moment the valve  36  is opened. Because the heat transfer fluid vapor is heavier then air, most of the air is located in the condenser  34  or in the top part of the vapor space  15 . When the valve  36  is opened and some amount of pressure is built up within the tank  30 , air quickly leaves the system through the valve  36 . Air and any vapor leaving through valve  36  may be directed to a purger system (not shown), which condenses the heat transfer fluid vapor to liquid form and releases only air. This way it is possible to save some additional heat transfer fluid during the start-up sequence. After that the valve  36  is closed and the tank  30  is ready for long term cooling operation. 
         [0024]    The heat transfer fluid vapor rises through the riser tube  32  into the condenser  34 , where it condenses to form a liquid condensate. The condensate returns to the tank  30  through the return line  39  and filter  38 . The filter  38  is provided to remove any moisture remaining in the condensate and may also remove metallic (e.g. copper) particles that may have been picked up by the liquid from the condenser tubes. 
         [0025]    There is an equilibrium point in the tank. For a certain power consumption by the electronic equipment  10  (say 250 kW), a certain water flow through the condenser  34  (say 500 liters per minute) and a certain input water temperature (say 48 C) then the pressure in the tank  30  is about normal (e.g. atmospheric). The pressure in the tank  30  will change with changing power consumption by the electronic equipment  10 . If power consumption stops, then pressure in the tank will typically drop to about 0.4 bar (absolute). If power is consumed, but at a lower rate than normal, then some degree of vacuum is developed inside the tank  30 . Lower pressure decreases the boiling point of the heat transfer liquid bringing the system back to a stable equilibrium. If there is higher than normal power consumption, then the tank  30  develops a somewhat increased pressure. Because of this increased pressure the condenser  34  becomes more effective bringing the system back to a stable equilibrium again. 
         [0026]      FIG. 2  illustrates schematically another immersion cooling system. Equivalent features are designated with the same reference numerals as used in  FIG. 1 . In the system of  FIG. 2  a tank  14 , which contains the heat transfer fluid  12 , is designed to operate at a substantially constant pressure (e.g. atmospheric pressure). As with the system of  FIG. 1 , the electronic equipment  10  is immersed in the heat transfer fluid liquid  12 , which removes heat from the electronic equipment  10  by convection and evaporation within the tank  14 . 
         [0027]    Banks of water-cooled tubes  16  act as condensers to condense the evaporated liquid inside the tank  14 . To ensure that the heat transfer fluid liquid  12  and the atmosphere within the tank  14  is completely dry, the atmosphere (evaporated fluid in air) inside the tank  14  is circulated through a desiccant  18  (e.g. silica gel). A small fan (not shown) may be used to aid the circulation through the desiccant  18 . 
         [0028]    An auxiliary condenser  22  is disposed above the tank  14  and connected to an opening in the roof of the tank  14  by a riser tube  20 . The auxiliary condenser  22  also has an outlet tube  24  that is connected to an expansion chamber or volume, which in the system depicted in  FIG. 1  is a balloon  26 . Other forms of expansion chamber, such as a bellows could also be used. 
         [0029]    Both the main condenser tubes  16  in the tank  14  and the auxiliary condenser  22  are supplied with cooling water, which is circulated through air-coolers  28  to remove the heat picked up from the condensing heat transfer fluid. 
         [0030]    The atmosphere inside the tank  14  is maintained at a substantially constant pressure, which may typically be atmospheric pressure. This helps to ensure that the heat transfer fluid liquid  12  continues to evaporate to draw heat away from the electronic components  10  at a constant, optimal rate. In order to maintain the pressure at a substantially constant level any small increase in volume of the atmosphere in the tank  14  (e.g. due to a rise in temperature) results in an upward flow of air/vapor in the riser tube  20  and into the auxiliary condenser  22 . Vapor from the heat transfer fluid is condensed in the auxiliary condenser  22  and runs back under gravity down the riser tube  20  and into the tank  14 . The balloon  26  maintains a seal against the external atmosphere (to ensure that moisture cannot enter the system). The air/vapor in the riser tube  20 , after passing through the condenser  22 , passes into the outlet tube  24  causing the balloon  26  to expand. A reduction in the volume of the tank  14  atmosphere will have the opposite effect, causing the balloon  26  to contract. 
         [0031]    Although the two systems of  FIGS. 1 and 2  operate under different pressure control regimes, it will be appreciated that there many features of the system of  FIG. 2  that could be employed in the system of  FIG. 1 . For example, the tank  30  in the system of  FIG. 1  could include additional banks of condenser tubes similar to the main condenser tubes  16  of  FIG. 2 . Also a desiccant could be used to aid removal of moisture from air in the vapor space  15  of the tank of  FIG. 1 . 
         [0032]    Similarly there may be features of the system of  FIG. 1  that could be employed in the system of  FIG. 2 . For example, a separate condensate return pipe could be employed from the auxiliary condenser  22  to include a filter for filtering the condensate. Also, the system of  FIG. 2  could employ a method of removing air from the tank  14  at start up similar to the method described for the  FIG. 1  system. In that case a valve could be provided that could be opened to allow the outlet tube  24  to vent to atmosphere during the start-up procedure. 
         [0033]    Referring to  FIG. 3 , a method of immersion cooling of electronic equipment begins at step  101  by immersing the electronic equipment in a tank, such as the pressure-sealed tank  30  of  FIG. 1 , holding heat transfer fluid (h.t. fluid) in liquid form. At step  102  the electronic equipment is operated to generate heat and evaporate some of the heat transfer fluid so as to cause heat transfer fluid vapor to enter the condenser. At step  103  the heat transfer fluid vapor is condensed in a condenser such as the condenser  34  of  FIG. 1 , to produce heat transfer fluid condensate. At step  104 , the heat transfer fluid condensate is returned to the tank. The system adopts an equilibrium condition in which the tank operates at a steady pressure (e.g. atmospheric pressure). 
         [0034]    After step  104  further operation of the electronic equipment may incur either an increase or a decrease in power to the electronic equipment. Increasing power consumption by the electronic equipment, as indicated at step  106   a  leads to an increase in the heat generated and, at step  107   a  develops an increased pressure of the heat transfer fluid vapor. The increased pressure results, at step  108   a , in an increase of the effectiveness of the condenser to bring the system back into an equilibrium condition (step  105 ). 
         [0035]    Decreasing power consumption by the electronic equipment, as indicated at step  106   b  leads to a decrease in the heat generated and, at step  107   b , has the opposite effect, causing a decrease in pressure of the heat transfer fluid vapor. 
         [0036]    Referring to  FIG. 4 , a method of commencing immersion cooling of electronic equipment commences at step  201  by immersing the electronic equipment in a tank, such as the pressure-sealed tank  30  of  FIG. 1 , holding heat transfer fluid (h.t. fluid) in liquid form. At step  202  operation of the electronic equipment is commenced to start generating heat so as to evaporate some of the heat transfer fluid and produce heat transfer fluid vapor. At step  203 , a vent valve (e.g. vent valve  36  of  FIG. 1 ) is opened so that, at step  204 , the heat transfer fluid vapor generated drives air through the condenser and out through the vent valve until substantially all of the air has been driven out. At step  205  the vent valve is closed. At step  206 , operation of the electronic equipment is continued, generating heat and producing heat transfer fluid vapor. At step  207 , the heat transfer fluid vapor is condensed in the condenser to produce heat transfer fluid condensate, which is returned to the tank at step  208 . 
         [0037]    Having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. The features and functions of the various embodiments may be arranged in various combinations and permutations, and all are considered to be within the scope of the disclosed invention. Accordingly, the described embodiments are to be considered in all respects as illustrative and not restrictive. The configurations, materials, and dimensions described herein are also intended as illustrative and in no way limiting. Similarly, although physical explanations have been provided for explanatory purposes, there is no intent to be bound by any particular theory or mechanism, or to limit the claims in accordance therewith.