Very efficient heat exchanger for cryogen free mri magnet

0001The present application relates to the cooling of superconducting magnets, and specifically to heat exchangers and the like.

0002Magnetic resonance (MR) scanners use superconducting magnets, which are cooled to a superconducting temperature, e.g. less than 5.2⁰ Kelvin. Traditionally liquid helium has been used to cool superconductive magnets because of its thermal properties. However, liquid helium is expensive. Many areas of the world do not have a ready supply of liquid helium or replacement liquid helium.

0003Typically superconducting magnets are bathed in liquid helium which as it cools the magnet changes from a liquid to a gas. The gas is then re-condensed and/or cooled to a liquid state by a refrigerator or heat exchanger before re-circulating back to the magnet. Alternatives to using liquid helium require an efficient method of heat exchange in order to keep the coolant and in turn the magnet below the critical temperature. Physical space also places limitations on the size and placement of the cooling apparatus.

0004There are also operational complexities in the start-up of the system. A room temperature of 21° C is approximately 294⁰K, while the normal operating temperature of the superconducting magnet is typically below 4.8⁰K. Any device or method employed must accommodate a start-up of the system from normal room temperatures down to superconducting temperatures. This change in temperature may involve a change in pressure. A magnet quench causes the magnet temperature to rise above 70⁰K. During a quench or another resulting rise in temperature, costly coolant may be lost as the coolant expands with the temperature increases before the system can be re-cooled. Escaping helium can displace oxygen in the magnet room causing potential health risks to persons adjacent to the magnet.

0005United States Patent Application Publication <patcit id="pcit0001" dnum="US20060048522A1"><text>US 2006/0048522 A1</text></patcit> disclsoes a heat exchanger for a refrigerant gas that is condensed by a cooling stage. A bottom end of a heat pipe is connected to a common heat conducting member and gas liquefied therein exchanges heat with a helium gas circulating through pipes.

0006The present application provides a new and improved efficient heat exchanger for a cryogen free MR magnet which overcomes the above-referenced problems and others.

0007The invention provides a heat exchanger according to claim 1.

0008One advantage is that helium which is circulated to the MR magnet and between the coil heat exchanger and the system heat exchanger is in a single gaseous phase.

0009Another advantage is that the helium circulated to the MR magnet is in a closed system, which prevents loss during start-up or a quench.

0010Another advantage is the system holds helium in high pressure at room temperature.

0011Another advantage is that the system is fully passive requiring no external intervention during all cooling modes: cooldown, quench and normal operation.

0012Another advantage is that the system has low flow friction losses which are easily fabricated with hermetically sealed joints.

0013Another advantage is that the heat exchanger accommodates a simple start-up process from room temperature.

0014Another advantage is that the heat exchanger has no moving parts.

0015Another advantage is that the heat exchanger is compact and transfers heat very efficiently.

0016Another advantage is that the system has a large heat transfer area with a wound tube on a highly conductive element.

0017Still further advantages of the present invention will be appreciated to those of ordinary skill in the art upon reading and understand the following detailed description.

0018The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIGURE 1</figref> is schematic of one embodiment of the heat exchanger in a magnet system.</li><li><figref idref="f0002">FIGURE 2</figref> is perspective view of one embodiment of the heat exchanger.</li><li><figref idref="f0003">FIGURE 3</figref> is perspective view of one embodiment of the heat exchanger sleeve using cooling plates.</li><li><figref idref="f0004">FIGURE 4</figref> is a flowchart illustrating system operation.</li></ul>

0019With reference to <figref idref="f0001">FIGURE 1</figref>, one embodiment of the heat exchanger <b>5</b> in a system is shown. A magnet coil <b>10</b> during operation generates heat. The heat is efficiently transferred away from the magnet coil <b>10</b> to circulating helium vapor through an efficient coil heat exchanger <b>20</b> in a thermosiphon circuit. The helium vapor circulates between the coil heat exchanger <b>20</b> and the system heat exchanger <b>5</b> in tubing <b>30.</b> The tubing <b>30</b> is a hermetically sealed closed loop system. At one end of the loop, advantageously a lower end, the tubing and the gaseous helium absorbs heat from the magnet <b>10</b> via the coil heat exchanger <b>20.</b> At the other end of the loop, advantageously the upper end, the gaseous helium loses the heat via the system heat exchanger <b>5</b> to liquid helium contained within. The tubing <b>30</b> wraps around a thermally conductive sleeve or container <b>40</b> of the heat exchanger <b>5.</b> In the illustrated embodiment, denser cold helium gas flows by gravity from the system heat exchanger to the coil heat exchanger where it is warmed. The warm gas, being less dense, rises to the system heat exchanger where it is cooled.

0020The thermally conductive container <b>40</b> in contact with the wound tubing <b>30</b> provides enough surface area to transfers the heat from the warm circulating helium vapor to the liquid helium inside the container <b>40.</b> The transfer cools the helium vapor to recirculate to and cool the coil components. As the helium warms inside the container <b>40,</b> the helium rises above a septum or baffle plate <b>60.</b> The baffle plate <b>60</b> divides the sleeve or cylindrical shape container <b>40.</b> The upper portion of the sleeve or container <b>40</b> is connected to an outflow <b>70,</b> and the lower portion is connected to an inflow <b>80.</b> As the helium warms in the container <b>40</b> it rises and flows out the outflow to a cooling column 90. In one embodiment the cooling column includes a 1^st stage coldhead, wetsock, and a 2^nd stage coldhead. As the warm helium gas rises in the cooling column <b>90,</b> it encounters the cryogen coldhead <b>100,</b> wet sock, or other stage coldhead which cools the helium. As the helium cools it condenses into a cool denser liquid. The cool denser liquid sinks to the bottom of the column. The cool denser liquid flows with gravity out the bottom of the cooling column <b>90</b> and returns to the inflow <b>80</b> of the container <b>40.</b> The cool liquid helium flows around the bottom of the sleeve or container <b>40</b> and begins the process again of absorbing heat from the helium vapor circulating in the tubing <b>30.</b> The coldhead <b>100</b> will cool liquid helium to approximately 4.2⁰K.

0021Helium vapor is sealed in the tubing <b>30</b> under pressure. In the embodiment, helium gas at critical temperature exerts .75 - .83 bar (11-12 PSI) while the same volume of helium at room temperature exerts 68-105 bar (1000-1500 PSI). The tubing <b>30</b> which is thermally efficient in heat transfer also maintains the pressure 68-102 atmospheres of helium at room temperature such that when cooled to 4.5⁰K it flows at 75-.83 bar in a siphoning action. The helium vapor flows in the tubing to the bottom of the magnet coil heat exchanger <b>20,</b> around the container <b>40</b> of the system heat exchanger <b>50 not defined,</b> and back to the magnet component <b>20.</b> As the helium cools, the helium contracts, and creates a siphoning action. No moving parts are involved. A minimum of temperature difference is required. The tubing <b>30</b> is hermetically sealed for the useful life of the MR magnet once helium vapor is placed in the tube <b>30.</b> Stainless steel is example material used for the tubing <b>30</b> which provides strength under pressure and efficient heat transfer. The tubing can be one piece or can start out as separate sections in the system heat exchanger, the coil heat exchanger, and connecting tubing for simplicity of manufacture.

0022The container or sleeve <b>40</b> contains helium in both a gaseous and a liquid state. During initial start-up, the magnet assembly can be initially cooled to about 70⁰K with a nitrogen cooling system (not shown). The coldhead <b>100</b> is hermetically sealed with the cooling column <b>90.</b> The cooling column <b>90</b> and container <b>40</b> initially contain only gaseous helium. As the coldhead <b>100</b> cools the gaseous helium, the helium contracts, and more helium is introduced into the system from a helium expansion tank <b>130.</b> As the helium cools and becomes denser, it sinks to the bottom of the cooling column <b>90</b> and flows into inflow <b>80</b> of the container <b>40</b> and into the bottom of the container <b>40.</b> Once the cooler, denser helium enters the container <b>40,</b> it cools the container <b>40</b> and begins to cool the vapor in the tubing <b>30.</b> As the helium cools the heat exchanger <b>5,</b> the helium absorbs the heat, expands and rises to the top of the container <b>40.</b> Once the warmer, less dense helium rises to the top of the container <b>40,</b> it flows out the outflow <b>70</b> and back to the cooling column <b>90.</b> Once in the cooling column <b>90,</b> it rises and encounters the cryogen coldhead <b>100</b> and begins the process anew. As the helium is re-circulated between the container <b>40</b> and the cooling column <b>90 and</b> the coldhead <b>100,</b> it eventually reaches a liquid state in the lower portion of the column and the system heat exchanger <b>5.</b> When the magnet components <b>20</b> in turn drop in temperature below critical temperature, the magnet can be operated.

0023The process for re-starting the system following a quench is similar. During a quench, the operating temperature of the magnet rises, e.g. above 70⁰K. The heating of the helium gas in the coil heat exchanger disrupts the thermosiphon slowing the transfer of heat to the system heat exchanger. The system must be re-cooled before restarting operation. When the temperature begins to rise, and helium expands in the container <b>40</b> and cooling column <b>90,</b> helium transfers to the expansion tank <b>130</b> preventing loss. As the system is restarted and the temperature begins to drop, helium is transferred back from the expansion tank <b>130</b> and into the cooling column <b>90.</b> A simple and orderly process of cooling is maintained both during start-up and when a re-start occurs.

0024With reference to <figref idref="f0002">FIGURE 2</figref>, an embodiment of the heat exchanger <b>5</b> is shown with nine closed loops of tubing <b>30</b> which cool various magnet components. Six loops cool magnet coils <b>10,</b> two loops cool structural components, and one loop cools electronic and other magnet components. To achieve the surface area for heat transfer each tube <b>30</b> is wound about the container <b>40</b> approximately 2.75 times from a top connector <b>110</b> to a bottom connector <b>120.</b> The tubing <b>30</b> is thermally connected with the container <b>40</b> e.g. by brazing. The top tubing coil connector <b>110</b> receives the inflow of warmer gaseous helium. The thermally conductive container or sleeve <b>40</b> is manufactured from a thermally conductive material such as stainless steel or copper. After the appropriate number of revolutions about the container <b>40</b> to cool the gaseous helium, the tubing <b>30</b> exits contact with the container at a lower connector <b>120,</b> lower than the starting point <b>110</b> or about 3 o'clock in the illustrated embodiment.

0025With reference to <figref idref="f0003">FIGURE 3</figref>, another embodiment of the system heat exchanger <b>5</b> with cooling plates <b>200</b> is shown. The thermally conductive plates <b>200</b> are attached with a thermally conductive bond to the container <b>40.</b> A thermally conductive weld such as braze is used for example. The circular plates <b>200</b> extend out from the exterior of the container <b>40</b> perpendicular to the axis of the container <b>40.</b> The tubing <b>30</b> is affixed in a thermally conductive bond, e.g. brazing to the plate <b>200.</b> In order to achieve more surface area for heat transfer, the tubing <b>30</b> is wrapped concentrically or spirally on the plate <b>200.</b> The tubing can be brazed or soldered to the plate first and the assembly can be brazed to the container or vice versa. The warm inflow connectors <b>110</b> are at the outer most edge and progress in smaller windings about the container <b>40</b> with the smallest winding nearest to the surface of the container. The cold return <b>120</b> of the tubing <b>30</b> exits in a plane offset from the windings of the tubing about the container <b>40.</b> In the illustrated embodiment, there is one plate <b>200</b> for each cooling circuit of tubing <b>30.</b> The plates <b>200</b> are manufactured from a thermally efficient material such as copper or stainless steel. A manifold <b>150</b> can be used to equalize pressure between tubing circuits <b>30.</b>

0026The cooling column <b>90</b> is affixed directly to the sleeve <b>40</b> eliminating piping for a separate inflow and outflow. The cooling column <b>90</b> is located at the center of the sleeve or container <b>40.</b> Four tubing <b>30</b> circuits, <figref idref="f0001">Fig. 1</figref>, are located around the container on one side of where the cooling column <b>90</b> joins with the container <b>40</b> and five on the other side. A mounting bracket <b>210</b> is used to mount the heat exchanger <b>50</b> to associated structures.

0027Another embodiment deploys the container <b>40</b> in a vertical position. When the container is in a vertical position, plates <b>200</b> are mounted horizontally, and no baffle plate is used.

0028The efficiency of the heat exchanger is defined by the number of net heat transfer units (NTU), and NTU = h A/M C where h is the heat transfer coefficient, A is the heat transfer area inside the tubes, M is the mass flow due to the siphoning action, and C is the heat capacity of the helium vapor. A is a function of the length and diameter of the tube. Thermally conductive materials such as stainless steel, copper, aluminum or the like are contemplated. These heat exchanger embodiments achieve a high NTU in a compact space.

0029With reference to <figref idref="f0004">FIGURE 4</figref>, a flowchart illustrates the method of cooling a MR magnet. Beginning at room temperature <b>300</b> the magnet is cooled to about 70⁰K by a nitrogen or other cooling system, as described in <patcit id="pcit0002" dnum="US61290270B"><text>US 61/290,270</text></patcit> (<patcit id="pcit0003" dnum="WO2011080630A"><text>WO/2011/080630</text></patcit>). The cryogen coldhead <b>310</b> cools helium in the cooling column. The helium condenses as it cools and more helium is added <b>320</b> to the cooling column. As the helium condenses, it flows into the container <b>40.</b> Heat transfers <b>330</b> from the helium gas in the thermally conductive tubing <b>30</b> to the colder helium in the cavity of the container <b>40.</b> The helium in the tubing contracts as the heat is transferred and causes circulation in the tubing <b>30</b> to begin. As circulation continues the colder helium reaches the coil heat exchanger of the magnet. As the colder helium gas circulates around the magnet, heat transfers from the magnet coil to the helium gas <b>340</b> via the coil heat exchanger. This process of cooling the magnet continues until the temperature of the magnet drops below the critical temperature.

0030Once the magnet drops below the critical temperature, the magnet is operated as a superconducting magnet <b>360.</b> Helium continues to circulate <b>370</b> in the tubing through the thermal siphoning action. Heat transfers <b>380</b> from the magnet coil to the gaseous helium. The gaseous helium transfers <b>390</b> that heat to the liquid helium via the system heat exchanger. The liquid helium warms to a gaseous state and rises in the container and flows into the cooling column. The cryogen coldhead cools and re-condenses the helium to a liquid state <b>400.</b> The liquid helium drops in the cooling column and flows into the cavity of the container where the cycle repeats.

0031During a quench <b>410,</b> the rapid temperature rise of the magnet causes an increase in heat transfer to the circulating helium. The circulating helium in turn will transfer the heat to the liquid helium <b>420.</b> With a rapid rise in temperature, the helium in the cooling column will expand. The system moves <b>430</b> the expanding helium from the cooling column of the system heat exchanger into the expansion tank to prevent loss. The process of re-cooling the magnet to superconductivity is the same as the process for the system start-up <b>300.</b>

0032The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims.