This application is directed, in general, to cooling apparatus and methods and more particularly to controllable cooling apparatus and methods.
Electronically operated devices generate heat. Removal of the heat generated from such devices is often required to ensure satisfactory operation of the devices. One example of a cooling technique is pumped refrigerant-based liquid-cooling systems for cooling high-heat-density data centers and telecommunications equipment.
Some embodiments of the disclosure provide a method for cooling a heat source using a refrigerant flow in a heat exchanger, the method including: (1) measuring a value corresponding to a property of the refrigerant flow indicative of an amount of heat being removed by the heat exchanger, (2) determining a flow distribution rate for the flow of the refrigerant in the heat exchanger using the measured value and (3) operating a valve to adjust the flow rate of the refrigerant in the heat exchanger according to the determined flow distribution rate.
Other embodiments of the disclosure provide an apparatus including a heat exchanger configured for cooling a heat source using a refrigerant, the apparatus including: (1) a flow property meter for measuring a value corresponding to a property of the refrigerant flow, (2) a processing unit coupled to the meter and configured to determine a flow distribution rate for the flow of the refrigerant in the heat exchanger using the measured value and (3) a valve coupled to the processing unit and configured to adjust the flow rate of the refrigerant in the heat exchanger according to the determined flow distribution rate.
Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
Pumped refrigerant-based liquid-cooling systems for cooling high-heat-density data centers and telecommunications equipment have a number of advantages over traditional air-based cooling approaches. Such typical advantages may include low energy use, high heat-carrying capacity and high reliability.
One cooling technique currently employed based on this technology involves modular cooling. In modular cooling, liquid refrigerant is pumped to multiple heat exchangers (HXs) typically in a parallel arrangement for the refrigerant plumbing. A heat exchanger typically removes the heat of a given shelf in an equipment rack. The amount of refrigerant flowing to the heat exchanger is typically determined by the pump and the flow resistance of the refrigerant distribution and return network, including the heat exchangers. However, in practice, the amount of heat dissipated by one device to be cooled (i.e., its heat load) may not be the same as that of another heat load in the same cooling network. Also a single heat load may have variations in the amount of heat dissipated thereby at different times during operation. Therefore, such passive determination of flow rate of the refrigerant as used in the known solution typically does not use any control in flow in response to variations and non-uniformities in one or more heat loads.
In the context of the present disclosure, the heat load of a device is understood to be the power (measured in Watts) that is dissipated by that device and which needs to be removed by the cooling system.
Some embodiments of the disclosure provide a method for cooling a heat source using a refrigerant flow in a heat exchanger, the method including: (1) measuring a value corresponding to a property of the refrigerant flow indicative of an amount of heat being removed by the heat exchanger, (2) determining a flow distribution rate for the flow of the refrigerant in the heat exchanger using the measured value; and (3) operating a valve to adjust the flow rate of the refrigerant in the heat exchanger according to the determined flow distribution rate.
According to some specific embodiments, the operation of the valve is controlled by a control unit or an operator. According to some specific embodiments, the value corresponding to a property of the refrigerant flow is determined by measuring a parameter selected from a group consisting of pressure, temperature, capacitance, thermal conductance, scattering of light, ultrasonic response, infrared or visible image, coolant vapor volume fraction, mass flow, power consumption, or a combination of such parameters. According to some specific embodiments, the value corresponding to a property of the refrigerant flow indicative of an amount of heat is determined by measuring a temperature difference across a reference element located between the heat source and the heat exchanger. According to some specific embodiments, a flow property value is determined by measuring a first differential pressure at a first location upstream of the heat exchanger and a second differential pressure measured at a second location downstream of the heat exchanger and determining a ratio between the differential pressure at the second location and the differential pressure at the first location. According to some specific embodiments, the method includes maintaining a refrigerant flow rate in a branch higher than an optimum flow rate wherein the optimum flow rate is a flow rate at which a heat load present in the branch is the exact heat load required to vaporize the entire refrigerant without superheat.
Some embodiments of the disclosure provide an apparatus including a heat exchanger configured for cooling a heat source using a refrigerant, the apparatus including a flow property meter for measuring a value corresponding to a property of the refrigerant flow, a processing unit for determining a flow distribution rate for the flow of the refrigerant in the heat exchanger using the measured value, and a valve for adjusting the flow rate of the refrigerant in the heat exchanger according to the determined flow distribution rate.
According to some specific embodiments, the apparatus further includes a control unit for controlling the operation of the valve. According to some specific embodiments, the apparatus includes a plurality of heat exchangers associated with respective heat sources included in a cooling cycle network. According to some specific embodiments, the apparatus includes a pump for pumping refrigerant through the heat exchanger. According to some specific embodiments, the cooling cycle network includes a plurality of branches wherein a branch includes at least one heat exchanger, a flow property meter and a valve. According to some specific embodiments, the apparatus further includes a valve for adjusting a flow of the refrigerant in more than one individual branch in the network. According to some specific embodiments, the flow property meter is configured for measuring a capacitance of a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring a pressure within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring a thermal conductance within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring the scattering of light within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring an ultrasonic response within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring an infrared or visible image response within a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the flow property meter is configured for measuring a volume of the refrigerant in a refrigerant conduit. According to some specific embodiments, the apparatus includes a temperature meter for measuring a temperature in a heat exchanger or in a heat source. According to some specific embodiments, the flow property meter includes a first differential pressure meter for measuring a differential pressure at a location upstream the heat exchanger and a second differential pressure meter for measuring a differential pressure at a location downstream of the heat exchanger. According to some specific embodiments, the apparatus is configured for maintaining a refrigerant flow rate in a branch higher than an optimum flow rate wherein the optimum flow rate is a flow rate at which a heat load present in the branch is the exact heat load required to vaporize the entire refrigerant without superheat.
It is to be noted that throughout the present disclosure including the claims the term, “valve,” is to be construed broadly such that the term may not only include conventional valves as commonly known, but also any other mass-flow controller device which is capable of adjusting the flow rate of a refrigerant in a heat exchanger.
Referring back to
A fan tray (not shown) may be used to push cold ambient air through the shelf 11, where air gains heat dissipated by the electronic components in the shelf 11. This flow of air is shown in
It is to be noted that fans may also operate to pull air from the shelf through a fan tray, as well as operate in “push/pull” mode in which two or more fan trays, on different sides of the heat source cooperate to move the air in a determined direction.
The heat assemblies 222-i may be installed in parallel configuration in the network 22 as shown in
In operation, the heat assemblies 222-i may be supplied with refrigerant, for example R134a, by the refrigerant pump 212 through an input branch 224-1. The refrigerant is then branched from the input branch 224-1 to individual heat assemblies 222-i through individual branches 226-1, . . . , 226-N (generally 226-i). The refrigerant is subsequently output from the individual heat assemblies 222-i and the individual branches 226-i and is returned to the pump 21 through output branch 224-2. Heat exchangers may be placed at the air exhausts of respective shelves in equipment racks (11 in
The term “superheat” as used in the context of the present disclosure refers to a status in which the temperature of the heated fluid increases above its saturation (evaporation) temperature and the liquid is totally evaporated. The superheat is a parameter to gauge system's performance. The solution proposed herein is aimed at preventing the working fluid from complete vaporization.
As shown in
Herein the refrigerant vapor quality is to be understood to refer to the mass fraction of the refrigerant that has been converted from liquid to vapor. Likewise, the void fraction as referred to herein is considered to refer to the fraction of the volume that is occupied by vapor bubbles.
At the same time, the partially- or totally-vaporized refrigerant has a higher kinematic viscosity than liquid refrigerant. This leads to higher flow impedance and thus to lower flow rate, degrading the thermal performance in a positive feedback loop. The decrease of flow rate associated to the increase of the heat load is shown in
Referring now to
It may be observed that as the heat load in the heat assembly increases, the vapor quality in the heat exchanger reaches level 1 (complete vaporization) sooner (i.e., further upstream) than at lower heat load. In addition to having higher flow impedance, the full-vapor condition downstream in the HX typically has lower heat transfer performance than the two-phase (vapor-liquid) flow in the upstream part of the HX because the heat capacity of the vapor is much less than that of the phase change. It is to be noted however that, for the portion of the curve representing the boiling heat transfer coefficient corresponding to vapor quality less than 1, the shape of the curve shown in
In addition to these reductions in heat-transfer efficiency, system operation at high vapor quality may in some occasions damage the refrigerant pump and reduce overall reliability and heat transfer performance.
In one known solution the refrigerant flow is controlled by placing bypass valves that allow a fraction of the refrigerant leaving the pump to bypass the refrigerant-distribution network, when needed, and instead to flow immediately into the refrigerant return network that resupplies the pump. This approach is typically attempted with the aim of compensating limitations of certain refrigerant pumps in accommodating a wide range of heat loads. However, this known solution does not address performance issues related to non-uniform heat loads in the network of heat exchangers which are supplied with refrigerant.
Another known solution uses thermal expansion valves (TEVs) which are a technology commonly used in refrigeration systems to control the amount of refrigerant superheat leaving a heat exchanger. This is to ensure that no liquid exits the HX, which can damage the compressor which receives it, and to maintain a proper amount of superheat for energy-efficient operation of the system.
Thermal expansion valves however are typically not adequate for pumped refrigerant applications because it is often desirable that the refrigerant exits the HX in a saturated state, e.g., with a portion of refrigerant still in the liquid phase.
As already discussed above, the heat transfer performance of a HX is determined by the refrigerant flow rate and the vapor quality in the HX, which is affected by the existence of different heat loads in the racks within a cooling network. Therefore, a dynamic control and feedback mechanism is desired for enhancing the overall thermal performance and reliability when using HX for cooling multiple equipment racks.
Some embodiments of the disclosure aim at providing a mechanism that is capable of dynamically redistributing the refrigerant flow in different HXs within a cooling network taking into account the individual shelf heat loads and variations thereof.
Such a mechanism allows for increasing the flow of the refrigerant in heavily loaded HXs while decreasing the flow in lightly loaded ones, or for performing flow adjustments in the various branches of the cooling network according to specific needs.
Such a redistribution of the refrigerant flow would be particularly beneficial in systems using a refrigerant working in the two-phase regime. In such systems, the removed heat causes a certain amount of refrigerant to vaporize, and the vapor bubbles thus produced increase the flow impedance. In the standard parallel configuration of cooling networks, as already discussed, the amount of refrigerant flowing in a given HX decreases as the heat load increases which leads to lower heat transfer capacity in precisely those circumstances where it needs to be higher.
According to some embodiments, a feedback control configuration is provided in the pumped-refrigerant cooling system.
In some specific embodiments, a flow property meter is provided in a feedback control configuration to detect the refrigerant vapor quality exiting each HX. Information on the vapor quality is used in the feedback control operation, e.g., using an algorithm to adjust the quantity of refrigerant to be pumped into each HX.
In other embodiments, other diagnostic signals, such as the pressure drop across each HX, the temperature increase across the HX, or the refrigerant mass flow rate through the HX, may be used in the feedback control to detect vapor quality. The use of such information in a properly designed feedback control configuration enables an optimal distribution of the refrigerant flow among heat exchangers so as to adapt to different heat load conditions.
It is to be noted that, although some embodiments are provided herein in the context of the exemplary pumped refrigerant system, the invention is not to be construed as limited to only such applications and may be applicable to any system having a refrigerant distribution network.
Referring now to
The cooling system 4 of
The heat assembly 422-i may be installed in parallel configuration in the network 42 as shown in
In operation, the heat assemblies 422-i may be supplied with refrigerant, for example R134a, by the refrigerant pump 412 through an input branch 424-1. The refrigerant is then branched from the input branch 424-1 to individual heat assemblies 422-i through individual branches 426-1, . . . , 426-N (generally 426-i). The refrigerant is subsequently output from the heat assemblies 422-i and the individual branches 426-i and is returned to the pump 41 through output branch 424-2. Heat exchangers may be placed at the air exhausts of respective shelves in equipment racks (11 in
As already mentioned above, in a typical application of this system, due to variation in heat loads from shelf to shelf (or from branch to branch), the heat exchangers may provide different levels of heat transfer.
According to the present disclosure, individual valves 428-1, . . . , 428-N (generally 428-i) for adjusting the refrigerant flow into the individual heat exchangers may be installed on the inlet side of the heat exchangers 422-i. Furthermore, use is made of flow property meters so as to infer the refrigerant quality output from the heat exchangers 422-i.
As used herein, the term, “flow property,” is to be understood to refer to at least one property of the refrigerant as it flows through the heat exchanger. By way of non-limiting examples, the property of the refrigerant to be measured as it flows through a heat exchanger may be selected from any one of the following properties or a combination thereof: pressure, temperature, capacitance, thermal conductance, scattering of light, ultrasonic response, infrared or visible image, mass flow rate, power consumption. These properties are equivalent in the sense that they can be measured and provide at least some information regarding the amount of heat in the heat exchanger. Therefore a value obtained as a result of the measurement of the flow property may be associated to, and thus indicative of, an amount of heat in the heat exchanger.
Referring back to
Optionally, additional valves may be installed in the cooling cycle network 42. For example a valve 440 may be installed on the main line to control flow in the entire network in case of need; or bypass valve 450 may be installed at a position where in case of need the flow of the refrigerant may be bypassed from the heat exchangers. In the parallel network configuration of
In one embodiment, a processing unit 460 may be used to collect data related to the measurements of the individual flow property meters 430-i and any other feedback signals and is configured to compute the optimum flow distribution among the heat exchangers. A control unit 470 may then modify the flow in the valves accordingly. The processing unit may be any device available and suitable for the intended use such as a personal computer, server or other devices with functions to store, compute and/or process, a central processing unit or an FPGA. Likewise a control unit may be any device available and suitable for the intended use such as a computer-based data acquisition and feedback system.
In another embodiment, individual feedback control configurations, i.e., combination of a processing unit and a control unit, may be implemented on all or some branches 426-i. In such cases, each valve on such branches is adjusted according to a corresponding individual measurement performed by the individual processing unit and thereby controls the flow in a corresponding heat assembly 422-i as desired.
The valve 52 may be any known continuously-adjustable control valve such as a metering valve or a needle valve or a refrigerant-grade control valve with stepper-motor actuation.
The flow property meter 53 may be one configured to measure the flow property present in the refrigerant output path by measuring parameters in the refrigerant flow such as pressure, temperature, thermal conductivity, capacitance, scattering of light, ultrasonic response, infrared or visible image, and the like.
An example of a capacitance or voltage operated flow property meter is shown in
In one example, the flow property meter 52 may include electrodes or pins that are inserted inside the refrigerant input path 54. In another example, the flow property meter 52 may include metal plates or foils on the surface of the path 54 in which case it does not need to enter in contact with the liquid inside the path 54. As the dielectric constant of liquid refrigerant is higher than that of the vapor, the capacitance of a fixed volume of refrigerant depends strongly on the vapor quality.
With this configuration, as already discussed in relation to
It is to be noted that the use of electrodes inserted in the tube 62 of the flow property meter for performing measurements in the particular embodiment illustrated in
Signals for feedback control may also be generated by measuring the mass flow at each branch (e.g., at the inlet of the heat-exchanger), or the temperatures at particular points in the HX or the shelf (e.g., the hottest component in the shelf), or the power consumption of the shelves, or the heat-load on each heat-exchanger, or an appropriate combination of pressure measurements on each line (e.g., the ratio between the differential pressures at the outlet and inlet of the HX).
In other embodiments, instead of estimating the power dissipation as mentioned above, such dissipation may be directly computed from the voltage drop across and the current through the device being cooled. In other embodiments, instead of measuring the power dissipated by a given device, its temperature may be measured directly.
In this manner, if the value of the differential measurement shows an increase from a reference value, it may be determined that the heat load is increasing—thereby affecting the flow. Therefore, by performing control actions—for example, by opening the respective individual valve—one may allow higher flow of refrigerant in the branch and thereby increasing the cooling rate.
The above configuration is one example of determining the heat load. If this process is performed locally, some degree of control becomes available related to a heat exchanger in a branch. It is also possible to include a flow property meter upstream the heat exchanger such that, by knowing the refrigerant flow and the amount of the heat load, one may be able to calculate the vapor quality at the outlet of the heat exchanger.
In case of a global control scheme, e.g., using a central unit to process all the data obtained in a network cooling cycle network such as that of
In another embodiment of the disclosure, the feedback control configuration (including the pump mechanism and the cooling cycle network) may be used to redistribute the refrigerant among different branches in a rack or different components on a branch. A similar technique as described with reference to
In addition to the various embodiments for dynamically adjusting the flow of the refrigerant as described above, a further safety measure may be employed in the cooling process which may include the following principles. The temperature difference between the input and output refrigerant for a heat exchanger is measured. If the measured output temperature is higher than the measured input temperature, it may be concluded that the refrigerant has vaporized completely, and the vapor is getting hot. In this condition, the control unit may be configured to increase the refrigerant flow rate to that heat exchanger until the temperature increase is reduced below a pre-determined threshold value.
In this manner, a pumped-refrigerant-based system may be provided that is capable of dynamically adjusting the flow of the refrigerant through one or more heat exchangers so as to provide only the necessary or desired amount of refrigerant for dissipating a given heat load associated with the respective heat exchanger. This allows non-uniform distributions of refrigerant among non-uniform heat loads within the system, and thereby overcomes a limitation of conventional implementations of pumped-refrigerant technology. The proposed solution also allows achieving either locally or globally optimal distributions of refrigerant as desired in each specific application so as to maximize or at least improve the heat removing capacity of the system.
Furthermore, the proposed solution allows one to incorporate margin into the design, which is effective for handling any potential power excursions. Maintaining a margin is useful for the following reasons. In principle, one may be able to adjust the refrigerant flow rate to each branch such that its present heat load is exactly what is required to vaporize all the refrigerant (i.e., achieve vapor quality 1.0) without superheat (i.e., with output temperature becoming equal to input temperature). However, operation under such optimum or extreme condition may not be preferred because any sudden increase in heat load will cause more superheat (i.e., raises the temperature of the output refrigerant vapor). By the time the system detects the superheat increase, responds so as to increase the heat removal and the excess power is finally cooled, the devices being cooled may have heated too much. Therefore, it may be preferable to maintain a degree of margin in the operation by running the system at a refrigerant flow rate slightly higher than the above optimum flow rate. By doing so, a sudden increase in heat load may only vaporize more refrigerant without raising its temperature to a degree that the refrigerant has vaporized completely. As the cooling process takes time the proposed margin allows the system to respond and adjust the flow rate before temperatures has increased too much.
Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.
This application was made with government support under Department of Energy Grant No. DE-EE0002B95. The United States government may have certain rights in the invention.