Patent Application
20070193300
1. Field of the Invention
This invention relates generally to two-phase liquid cooling systems, and more particularly to two-phase liquid cooling systems that have an active venting system for regulating the pressure within the system by removing gases such as non-condensable gases from the cooling system.
2. Background of the Invention
Liquid cooling is well known in the art of cooling electronics. As air cooling heat sinks continue to be pushed to new performance levels, so has their cost, complexity, and weight. Because computer power consumptions will continue to increase, liquid cooling systems will provide significant advantages to computer manufacturers and electronic system providers.
Liquid cooling technologies use a cooling fluid for removing heat from an electronic component. Liquids can hold more heat and transfer heat at a rate many times that of air. Single-phase liquid cooling systems place a liquid in thermal contact with the component to be cooled. With these systems, the cooling fluid absorbs heat as sensible energy. Other liquid cooling systems, such as spray cooling, are two-phase processes. In the two-phase cooling systems, heat is absorbed by the cooling fluid primarily through latent energy gains. Two-phase cooling, commonly referred to as evaporative cooling, allows for more efficient, more compact, and higher performing liquid cooling systems than systems based on single-phase cooling.
An example two-phase cooling method is spray cooling. Spray cooling uses a pump to supply fluid to one or more nozzles, which transform the coolant supply into droplets. These droplets impinge the surface of the component to be cooled and can create a thin coolant film. Energy is transferred from the surface of the component to the thin-film of coolant. Because the fluid is dispensed at or near its saturation point, the absorbed heat causes the thin-film to turn to vapor. This vapor is then removed from the component, condensed (often by means of a heat exchanger or condenser), and returned to the pump.
Significant efforts have been expended in the development and optimization of spray cooling. A doctorial dissertation by Tilton entitled “Spray Cooling” (1989), available through the University of Kentucky library system, describes how optimization of spray cooling system parameters, such as droplet size, distribution, and momentum can create a thin coolant film capable of absorbing high heat fluxes. In addition to the system parameters described by the Tilton dissertation, U.S. Pat. No. 5,220,804 provides a method of increasing a spray cooling system's ability to remove heat. The '804 patent describes a method of managing system vapor that further thins the coolant film, which increases evaporation, improves convective heat transfer, and improves liquid and vapor reclaim.
Dielectric fluids such as FLUORINERT® (a trademark of 3M Company) are well-suited for use in electronic cooling systems, as they are safe for electronic components and systems. The fluids have boiling points close to atmospheric conditions and have latent heat of vaporization values that provide efficient two-phase cooling.
A significant challenge in the use of some two-phase cooling systems is presented by non-condensable gases. Dielectric fluids like FLUORINERT® can contain significant amounts of air and other non-condensable gases in solution. When the dielectric fluid is placed into a system at atmospheric conditions, the fluid may thus contain a significant amount of air dissolved in the fluid. During use within a thermal management system, according to Henry's Law, as the fluid approaches its saturation temperature the amount of air in solution decreases. The air that was previously in solution occupies a volume within the system. According to the ideal gas law, the partial pressure of the air will raise the boiling point of the cooling fluid above the natural saturation curve. This, in turn, reduces the cooling efficiency of the system because it raises the boiling point of the fluid to a level that renders less than ideal cooling performance. But some amount of air is useful for some cooling systems, for example, to avoid pump cavitation. The actual amount of air in the system can vary as air seeps into the system during operation, so it can be difficult to maintain the amount of air within the system at an optimal level.
For the foregoing reasons, there is a need for a two-phase liquid cooling solution that can maintain an ideal amount of air or other non-condensable gas within the system. With changing conditions inside a cooling system, there is a need for a method of regulating the non-condensable gases in the cooling system. Such a cooling system would result in significant improvements in both the performance and reliability of the two-phase liquid cooling process.
To avoid at least some of the problems encountered with existing two-phase liquid cooling systems, as described above, a cooling system with active venting is provided. An active venting system actively regulates the pressure within the cooling system, for example, by regulating the amount of non-condensable gases in the cooling system. With appropriate control of the active venting system, the performance and reliability of the system can be increased and maintained over long and continuous periods of operation.
Embodiments of the invention include liquid cooling systems and methods that can provide thermal management for one or more electronic components. In one embodiment, a cooling system includes a cooling liquid, or coolant, that is circulated through a closed loop by one or more pumps. The cooling fluid enters one or more cooling modules as a liquid or saturated liquid, and changes phase in the cooling module by means of latent energy gains. The resulting liquid and vapor mixture is then removed from the cooling module and condensed so that it can be returned to the pump and circulated back through the system. An active venting system is coupled to a volume in the cooling system to regulate the pressure in the cooling system. A control system is coupled to the active venting system to activate the venting based on any of a number of criteria, such as process conditions within the cooling system.
The active cooling system can exhaust gases out of the system using various mechanisms. In one embodiment, a vent is located between the cooling module and the pump, and an auxiliary pump is coupled to the vent to pump a desired amount of gas out of the system. The control system is coupled to the auxiliary pump and vent to provide the ability to regulate the amount of gas removed from the system. The active venting system may also be capable of adding gases into the cooling system (e.g., by pumping air into the system) when a pressure increase is desired. In this way, the cooling system can regulate the pressure in the cooling system to achieve a desired overall cooling efficiency. Adding air to the cooling system may also help avoid cavitation in the pumps.
Within the cooling system there may be one or more non-condensable gases. Non-condensable gases may include any gases or mixtures thereof that do not condense into liquid form under conditions experienced during normal operation of the two-phase liquid-cooling system. Air is a common non-condensable gas in cooling systems, since they are typically run at pressures below atmospheric so that air tends to seep in slowly through points in the system that are not completely sealed or otherwise allow air permeation into the system. The non-condensable gases cause a partial pressure within the closed volume of the cooling system, which alters the boiling point of the cooling fluid and thus affects the operation of the cooling system. While removing the gases from the system, it is often desirable to remove the non-condensable gases while minimizing the removal of the coolant in vapor phase. Otherwise, over time the cooling system would lose coolant and would need to have the coolant replaced. By removing non-condensable gases rather than vapor-phase coolant from the cooling system, the need to replace coolant is reduced. Accordingly, the active venting system may be configured to remove an amount of the non-condensable gases from the system.
Various embodiments of the system include mechanisms in the venting system for separating the coolant vapor from the non-condensable gases to be removed. By separating the coolant vapor from the non-condensable gases, the active venting system can remove only the non-condensable gases and allow the coolant vapor to recycle through the cooling system. In one embodiment, the active cooling system includes using a semi-permeable membrane separator coupled between the cooling module and the return line, allowing only coolant vapor to recycle through the system. In other embodiments, the active venting system comprises a condensing separator, a centrifugal gas separator, or a semi-permeable membrane separator (such as a permeable tube vacuum system) to separate the vapor cooling fluid from the non-condensable gases to be removed.
In one embodiment, the control system measures process conditions such as the temperature and pressure within a volume of the cooling system. Based on the measured temperature and pressure, the control system determines whether the process conditions within the system are inside a desired range. In one embodiment, the control system determines that removal of non-condensable gases is needed based on the saturation curve of the coolant. For example, the control system may detect when the pressure and temperature inside the system deviate from the saturation curve of the coolant by a predetermined amount. When the control system determines that venting is needed, it activates the venting system, for example, causing the active venting system to open the vent and turn on the auxiliary pump to remove gases in the system.
These and other features, aspects, and advantages of various embodiments of the invention will become better understood with regard to the following description and accompanying drawings.
In the course of the detailed description to follow, reference will be made to the attached drawings. These drawings show different aspects of embodiments of the present invention and, where appropriate, reference numerals illustrating like structures, components, and/or elements in different figures are labeled similarly. It is understood that various combinations of the structures, components, and/or elements other than those specifically shown are contemplated and within the scope of the present invention:
Two-Phase Cooling System with Active Venting
Although the two-phase liquid cooling system 100 is shown with only the main components, the system 100 may include other well known components, such as filters, heaters, manifolds, coolers, and other components of fluid systems. In addition, the system 100 is described as just one example of a system in which the active venting techniques described herein can be applied. The system 100 may be a modular cold plate type system or a global cooling system where the cooling fluid comes directly in contact with the electronics to be cooled. Moreover, the cooling system 100 is not limited to any particular type of two-phase liquid cooling system. Rather, the techniques described herein can be applied to any type of two-phase liquid cooling system, such as, but not limited to, spray cooling, micro-channels, mini-channels, pool boiling, immersion cooling, or jet impingement. Examples of liquid cooling systems and their components that can be used with embodiments of the invention are described in the following, each of which is incorporated by reference in its entirety: U.S. Pat. No. 6,889,515, which describes a spray cooling system; U.S. Pat. No. 6,955,062, which describes a spray cooling system for transverse thin-film evaporative spray cooling; and U.S. Pat. No. 5,220,804, which describes a high heat flux evaporative spray cooling; and U.S. Pat. No. 5,880,931 which describes a spray cooled circuit card cage.
Coupled to the cooling system 100 is an active venting system 125 for removing gases and/or adding gases to the liquid cooling system 100. As shown in
A control system 140 is coupled to the venting system 125 to provide for selective activation of the venting of gases by the venting system 125. Using control signals (illustrated as dotted lines in
In one embodiment, the non-condensable gases removed by the active venting system 125 are released into the surrounding environment. In some applications, however, it is undesirable to allow the non-condensable gases to be released. To address this need, in another embodiment, the active venting system 125 vents, pumps, or otherwise directs the non-condensable gases removed from the cooling system 100 into a sealed chamber 160 for storage therein. The sealed chamber allows the cooling system 100 to be used in very sensitive areas where the non-condensable gases cannot be introduced.
In another embodiment, the gas storage chamber 160 houses a condenser unit 162, which may comprise condensing fins that aid in condensing any vapor in the chamber 160. The chamber is further coupled to a relief valve 164. The relief valve 164 is designed to relieve the stored or collected non-condensable gases once a certain pressure inside the storage chamber 160 is reached. In one embodiment, the pressure relief valve 164 comprises a spring-loaded valve that automatically opens at a 10 psi differential between the inside of the chamber and the atmosphere. With the chamber 160 at room temperature, the added pressure helps to ensure that only air escapes from the system.
The control system 140 activates the venting system 125 based on process conditions within the cooling system. In this way, the control system 140 can achieve certain desired operating conditions in the cooling system 100. Although a variety of process conditions can be used to describe the cooling system, in one embodiment the process conditions include the pressure and temperature of the gases above the liquid coolant in the reservoir 115. Accordingly, a pressure transducer 145 and temperature sensor 150 (which may comprise a thermocouple, thermistor, resistance temperature detector (RTD), thermopile, infrared sensor, or any other suitable temperature sensor) coupled to the reservoir 115 provide readings of these process conditions. The control system 140 uses these pressure and temperature readings to determine whether and when to activate the venting system 125. Various embodiments of algorithms that the control system 140 can use to activate the venting system 125 are described in more detail below; however, it can be appreciated that the control system 140 can receive additional types of inputs and can be programmed to perform any number of algorithms to achieve a desired effect in the cooling system 100. Moreover, the pressure transducer 145 and temperature sensor 150 may be located at other parts of the system, such as a return manifold (see
Although the control system 140 is illustrated as a separate system in
The active venting techniques described herein can be implemented in various types of two-phase liquid cooling systems. For example,
An active venting system 260 is coupled to the return manifold 240, where it has access to gases in the flow path of the cooling system. As described above, the active venting system 260 may remove gases from and/or add gases to the flow path of the cooling system to adjust the pressure therein and thus affect the operation of the cooling system. Rather than being coupled to the return manifold 240, the active venting system 260 may alternatively be fluidly coupled to a volume of gas in the thermal management unit 250 for exchanging gases therewith. In a rack-mounted cooling system, the active venting system 260 and the thermal management unit 250 may also be rack-mounted devices.
One problem with the startup of a rack-mounted spray cooling system, where the supply manifold 230 and the return manifold 240 are mounted in the rack vertically, is that air can become trapped in the supply manifold 230 above the uppermost connection that leads to the uppermost cooling module 220. The trapped air undesirably increases system pressure, and because there is no fluid flow above the uppermost connection, the non-condensable gases must dissolve back into the coolant to be removed. It has been shown to take several days for the non-condensable gases to be removed fully with this configuration. After a system is shut down, moreover, a substantial amount of non-condensable gas may collect in the supply manifold 230, which again takes significant time to remove.
To address this problem, in one embodiment, a bypass flow path 260 is placed between the supply manifold 230 and the return manifold 240 near the tops thereof. The flow path 260 allows a small flow (e.g., around 1% of the full flow) of gas to pass from the top of the supply manifold 230 to the return manifold 240. The flow path 260 may comprises a tube, and a chemical filter 265 may be installed in the flow path 260, since this provides an ideal service location. The bypass flow path 260 with chemical filter 265 could replace a bypass filtration line that is often used within the thermal management unit 250. In an alternative embodiment, the bypass path 260 can be separate from the filter 265, although it is typically desired to reduce number of fluid joints in the system.
Active Venting System Embodiments
As described above, many coolants used in two-phase fluid cooling applications may absorb a significant amount of air or other non-condensable gases. Because the non-condensable gases remain in gas form throughout the cooling system, they impart a partial pressure that adds to the pressure within the cooling system. Although a slightly increased pressure may be useful to avoid cavitation in the pumps, it can also have detrimental effects on the cooling efficiency of the system by increasing the boiling point of the coolant. Accordingly, it is often preferable to control the amount of non-condensable gases that are present in the cooling system. When removing gases from the system, therefore, it is generally preferable to remove the non-condensable gases while leaving the coolant vapor in the system. Various embodiments of the active venting system designed to achieve this purpose are described below.
During operation of the venting system, the side of the membrane 310 that includes the coolant and non-condensable gas mixture is increased in pressure (e.g., by a pump, not shown). In this way, the coolant is allowed to pass through the membrane 310 and return to the thermal management unit 250, while the non-condensable gas remains in the manifold 240 (or another volume from which the venting system can extract gas). This increases the concentration of the non-condensable gas versus the coolant vapor in the manifold 240. If the venting system takes gases from the manifold 240, the gas mixture taken by the venting system will thus have a relatively higher concentration of non-condensable gas versus coolant vapor than in the rest of the system. In another embodiment, the membrane 310 can be configured in the reverse manner (such as in the embodiment described below in connection with
In operation, the venting system 410 receives a mixture of the coolant vapor and non-condensable gases from the return manifold 240. A valve 425 may be provided on the gas input line 420 to control when the venting system can take in the gases. The input gases are received in a chamber of the venting system 410, where a condenser 430 reduces the temperature of the gases until the coolant vapor condenses and collects as a liquid in the venting system. When a control system determines that the venting system should be activated to expel non-condensable gas from the system, the control system activates an auxiliary pump (as shown in
At various times, such as when the venting system has a predetermined amount of liquid coolant collected (e.g., as measured by a level sensor, not shown), a liquid return pump 440 is activated. The liquid return pump 440 passes the condensed liquid coolant from the venting system 410 back to the return manifold 240 by way of a liquid return line 450. A liquid return valve 455 may be provided in the liquid return line 450 to prevent liquid coolant from backing up into the venting system 410. In this way, the coolant vapor from the cooling modules is condensed so that it can be recycled through the system, rather than being venting from it. The pump 440 may be optional, e.g., the coolant may be gravity drained from the reservoir and reintroduced into the cooling system as well.
The centrifugal vapor pump 520 is activated by the control system when it is determined that the venting system 510 should remove gas from the cooling system. The centrifugal vapor pump 520 removes dissolved non-condensable gas from the coolant vapor by passing the mixed gas stream through a series of rapidly spinning disks. As the rotational motion is imparted to the gas stream, the more dense gases (e.g., FLUORINERT®, in a mixture of FLUORINERT® and air) are forced to the perimeter, while the less dense gases continue down the center of the device and exit the centrifugal pump. The centrifugal vapor pump 520 can be controlled by manipulating the rotation speed of the spinning disks by an ordinary brushless DC controller, and by the flow rate of the vacuum pump that pulls the mixed vapor through the device and vents to the atmosphere. Alternatively, where the coolant vapor is less dense than the non-condensable gases, the configuration may be changed to allow the denser gases to be removed.
In the embodiment shown in
In one embodiment, the tubing 620 comprises a co-extrusion having at least two layers. An exterior layer of the co-extruded tubing 620 may comprise ether or ester-based polyurethane, which is appropriate due to its high air and low PFC permeation properties. An interior layer of the co-extruded tubing 620 may comprise polyethylene, which has excellent fluid compatibility properties. The tubing 620 is preferably a semi-permeable membrane. This is in contrast to the tubing used in other parts of embodiments of the cooling system, in which a coextruded tubing that prevents permeation and provides good fluid compatibility while remaining flexible is used. This coextruded tubing may be used for all fluid connections in the system where rigid tubing is impractical, such as to connect pumps to the supply manifold, the supply manifold to the spray modules, the spray modules to the return manifold, and the return manifold to the condenser. The coextruded tubing may also connect the active venting system to the return manifold. The selection of materials for this and any other tubing may depend, in part, on the type of coolant used.
On one embodiment, the tubing used for some or all flexible connections within the system is a co-extruded tubing that comprises:
Alternatively, the venting system could be designed using a tubing that is permeable to the coolant but not to the non-condensable gas. In such a case, the tubing could comprise polyvinylidine fluoride (PVDF), or KYNAR®, which is permeable to FLUORINERT® but not to air. The coolant would be collected outside of the tubing and returned to the system, while the non-condensable gas left in the tubing would be exhausted from the system.
Operation
Controlling the pressure inside of the cooling system may be vitally important for many applications, as demonstrated by the saturation curve plotted in
In one embodiment, the cooling system can regulate the amount of non-condensable gases in the cooling system using a control algorithm implemented by the control module described above.
In one embodiment, the control system activates 830 the venting system according to a predetermined profile, which specifies an amount of time on and off for the venting system. The on period of the profile allows the system to exhaust a non-condensable gas for a period of time, while the off period allows the venting system to separate the coolant vapor from the non-condensable gas. The off period also allows the system as a whole to come into equilibrium, while other entrained non-condensable gases are moved to the venting system so they can be extracted. Although the particular profile used may depend on the system parameters, in one embodiment the profile is 10 seconds of venting followed by 3 minutes off. The venting system runs (e.g., according to the profile) until the control system determines 840 that the system pressure is within a predetermined differential (e.g., 2.5 PSI) of the saturation curve at the system temperature. Once this condition is met, the control system turns 850 the venting system off, and the control cycle repeats.
In one embodiment, the control system may check the pressure difference between the system and the saturation curve so that it can maintain the system above a minimum differential (e.g., a 1.8 PSI). This checking may occur, for example, continually during the running of a profile for the venting system. If the cooling system does come within the predetermined minimum differential of the saturation curve, the control system automatically shuts the venting system off. This helps to prevent the pumps from cavitating due to too low of a pressure in the cooling system.
During startup of the cooling system there may be different venting needs than during normal operation. For example, there is typically more need for venting since there is more air that has seeped into the cooling system. Moreover, the system can tolerate faster venting because the system is stagnant at startup; therefore, the vapor and air are more separated from one another. Once fluid is pumped through the system, the air and vapor tend to mix and extraction has to be done more slowly. Accordingly, a startup profile may be run until the cooling system reaches a desired point from the saturation curve, where the startup profile has more aggressive venting than the regular profile. In one embodiment, the startup profile runs the venting system for 55 seconds on and 5 seconds off, for up to 5 minutes or until the cooling system reaches 5 PSI above the saturation curve. As with the regular venting profile, various other startup profiles may be defined based on other system parameters and needs.
Rather than trying to maintain the cooling system at an ideal operating curve, the control system can also be used to maintain the cooling system at a given temperature. This may be useful, for example, as a tool for the testing or burn-in of semiconductors. Because non-condensable gases within the working fluid of the system affect the component temperatures, adding the gases to the system or allowing the gases to remain in the system can raise the temperature of the components being cooled by the system. The control system may therefore receive additional inputs, such as the temperature of a particular component attached to the cooling system. By adjusting the gases within the cooling system, the control system can maintain these inputs at desired values.
Summary
The foregoing description of the embodiments of the invention has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. For example, many of the fastening, connection, manufacturing, and other means and components that are described in various embodiments are widely known in the relevant field, and their exact nature or type is not necessary for a person of ordinary skill in the art or science to understand the invention. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above teachings. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
This application claims the benefit of U.S. Provisional Application No. 60/775,496, filed Feb. 21, 2006, which is incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60775496 | Feb 2006 | US |
