Abstract:
A magnetic refrigeration system provides flow-balanced channels between fluid control valves and the magnetocaloric beds to eliminate inefficiencies caused by unequal utilization of the magnetic beds from flow variations.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. provisional application Ser. No. 61/917,025, filed Dec. 17, 2013, the entire contents of which is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
       [0002]    Magnetic refrigeration (MR) is an emerging cooling technology that is based on the magnetocaloric effect, a property exhibited by certain materials which heat up when placed in a magnetic field and cool down when the field is removed. Magnetic refrigeration offers a number of distinct advantages over vapor compression, which is currently the most widely used method for cooling. First, MR uses no hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), nor any other gaseous materials; the refrigerant in the MR system is in the form of a porous solid. The absence of any gases greatly reduces the potential for leaks, a common problem in vapor compression systems. As a result, MR systems can have greater reliability with reduced maintenance and downtime. The elimination of HFCs and CFCs has benefits for the environment, as these gases are ozone-depleting and contribute to global warming. Finally, theoretical studies demonstrate that MR systems can be more energy-efficient than vapor compression systems, particularly under off-peak load conditions. 
         [0003]    General background on magnetic refrigeration may be found at K. Gschneidner and V. Pecharsky, “Thirty years of near room temperature magnetic cooling: Where we are today and future prospects”, Int. J. of Refrig. 31: 945-961, 2008 and K. Engelbrecht, G. Nellis, S. Klein, and C. Zimm, “Recent Developments in Room Temperature Active Magnetic Regenerative Refrigeration”, HVAC&amp;R Research, 13(4): 525-542, 2007. Modern room temperature MR systems implement the so-called Active Magnetic Regenerator (AMR) cycle to perform cooling, as disclosed in U.S. Pat. No. 4,332,135, hereby incorporated by reference. This cycle has four stages, as shown schematically in  FIGS. 1A ,  1 B,  1 C, and  1 D. The MR system in these figures consists of a porous bed of magnetocaloric material (MCM)  190  and a heat transfer fluid which exchanges heat with the MCM as it flows through the bed  190 . The left side of the bed  190  is the cold side, while the hot side is on the right side. The timing and direction (hot-to-cold or cold-to-hot) of the fluid flow is coordinated with the application and removal of the magnetic field  192 . In the first stage of the cycle (“magnetization”),  FIG. 1A , while the fluid in the bed  190  is stagnant, a magnetic field  192  is applied to the MCM causing it to heat. In the next stage (the “hot blow”),  FIG. 1B , while the magnetic field  192  over the bed  190  is maintained, fluid at a temperature T Ci  (the cold inlet temperature) is pumped through the bed from the cold side to the hot side though the cold inlet  182 . This fluid pulls heat from the MCM in the bed and rises in temperature as it passes through the bed  190 . During the hot blow, the fluid exits the bed  190  at the temperature T Ho  (the hot outlet temperature) through the hot outlet  186  and is circulated through a hot-side heat exchanger  194 , where it gives up heat to the ambient environment and returns to the temperature T Hi  (the hot inlet temperature)&lt;T Ho . In the next stage (“demagnetization”),  FIG. 1C , the fluid flow is terminated and the magnetic field is removed. This causes the bed  190  to cool further. In the final stage (the “cold blow”),  FIG. 1D , fluid at a temperature T Hi  is pumped through the bed  190  from the hot side via the hot inlet  188  to the cold side in the continued absence of the magnetic field. The fluid is cooled as it passes through the MCM in the bed  190 , reaching a temperature T Co  (the cold outlet temperature)&lt;T Ci . The colder fluid exiting the bed  190  during the cold blow via the cold outlet  184  is circulated through a cold-side heat exchanger  196 , picking up heat from the refrigerated environment. The fluid exits the cold-side heat exchanger  196  at temperature T Ci  and completes the AMR cycle. The heat absorbed by the cold fluid in the cold-side heat exchanger  196  during the cold blow allows the refrigerated environment to maintain its colder temperature. 
         [0004]    Although  FIGS. 1A ,  1 B,  1 C and  1 D illustrate the operation of a single-bed MR system, one of ordinary skill in the art would see that multiple beds, each undergoing the same AMR cycle, may be combined in a single system to increase the cooling power, reduce the system size, or otherwise improve the performance of the cycle. 
         [0005]    To implement the AMR cycle, a magnetic refrigerator needs one or more porous beds of magnetocaloric material, a heat transfer fluid, a pump to drive the fluid through the beds, a means for applying and removing a magnetic field to the beds, and a flow control system which coordinates the timing and direction of the fluid flow through a bed with the application and removal of the magnetic field over the bed. In one implementation of the AMR cycle in a magnetic refrigerator, a magnet assembly with a gap, such as that disclosed in U.S. Pat. No. 7,148,777, hereby incorporated by reference, rotates over fixed beds of magnetocaloric material. The fixed beds fit into the gap of the magnet assembly and the magnetic field is applied to a given bed when the magnet assembly gap rotates over it. The field is maintained over the bed as it remains within the magnet gap. As the magnet rotates away from the given bed, the magnetic field is removed. This implementation, referred to as a “rotating magnet” magnetic refrigerator or RMMR, is described in U.S. Pat. No. 6,668,560, hereby incorporated by reference. 
         [0006]    Each bed in an RMMR has four fluid ports, as shown in  FIGS. 1A ,  1 B,  1 C and  1 D. Two of these ports, the hot inlet port  188  and the hot outlet port  186 , are located on the hot side of the bed  190 , while two other ports, the cold inlet port  182  and cold outlet port  184 , are located on the cold side of the bed  190 . The inlet ports  188  and  182  deliver fluid to the magnetocaloric material in the bed  190 , while the outlet ports  186  and  184  collect fluid emerging from the magnetocaloric material. By using separate inlet and outlet ports, the mixing of inlet and outlet fluid streams, which are generally at different temperatures, is minimized. This improves MR system performance by preventing the thermal loss associated with mixing. 
         [0007]    Generally, to control the fluid flow, the RMMR uses four valves, referred to as the hot inlet (Hi) valve, the hot outlet (Ho) valve, the cold inlet (Ci) valve, and the cold outlet (Co) valve. When a bed is within the gap of the rotating magnet assembly, the cold inlet valve delivers flow to the cold inlet port of the bed; simultaneously, the hot outlet valve collects fluid from the hot outlet port of the bed. The hot inlet valve blocks flow to the hot inlet port of the bed, while the cold outlet valve blocks flow from the cold outlet port. In this manner, flow can only proceed through the bed from the cold inlet port to the hot outlet port, the desired flow path for a magnetized bed undergoing the hot blow stage of the AMR cycle. When the magnet rotates away from the bed, so that the bed is now demagnetized, the cold inlet valve now blocks flow from entering the cold inlet port, while the hot outlet valve blocks flow from emerging through the hot outlet port. The hot inlet valve opens and directs hot inlet fluid to the hot inlet port of the bed, while the cold outlet valve opens, allowing fluid to exit the bed through the cold outlet port. In this manner, flow can only proceed through the bed from the hot inlet port to the cold outlet port, the desired flow path for a demagnetized bed undergoing the cold blow stage of the AMR cycle. It is clear that for the proper functioning of the MR system, the opening and closing of the valves must be coordinated with the angular position of the magnet assembly relative to a bed. 
         [0008]    Rotary valves, such as those disclosed in U.S. Pat. No. 6,668,560, hereby incorporated by reference, may be used for implementing the flow control described above. Generally, rotary valves employ two elements, a stator containing an annular arrangement of holes and a rotor containing a slot, extending over a certain angular distance. The rotor slot is centered over the same path as the holes in the stator, so that the slot of the rotor overlaps one or more of the holes in the stator. When the rotor slot overlaps a stator hole, a continuous fluid path is established through the valve; when the rotor slot does not overlap a stator hole, flow cannot proceed through the valve and flow is blocked. The contact faces of the rotor and stator are typically highly polished, so that no fluid can leak between them. In the valve, the stator has a plurality of ports. Each of these valve ports is connected to a fluid conduit (e.g., a pipe), the other end of which is connected to a bed port. Each hole in the stator is connected to one of these valve ports. Another end of the chamber contains a single axial port, which is connected to a fluid conduit (e.g., a pipe). The other end of this conduit is connected to a heat exchanger. The rotor is attached to a rotary shaft which rotates the rotor with respect to the stator. When the rotor is positioned so that its slot overlaps a stator hole, then a continuous fluid path is provided between a bed port on one side of the valve and the heat exchanger on the other side; otherwise, flow to or from the bed port is blocked. As the rotor rotates, the slot alternately allows and blocks flow from or to the bed port. The position of the rotor in the cold inlet valve is set so that when a bed is within the gap of the magnet assembly, the rotor slot overlaps the hole connected to the cold inlet port of the bed (through the associated cold inlet valve port). The position of the rotor in the hot outlet valve is set so that at this same time, its rotor slot overlaps the hole connected to the hot outlet bed port (through the associated hot outlet valve port). In this manner, a continuous fluid path from the cold-side heat exchanger, through the bed from its cold inlet port to its hot outlet port, to the hot-side heat exchanger, is established. The angular extent of the rotor slots is chosen so that holes in the cold inlet and hot outlet valves remain uncovered as long as the bed remains within the gap of the magnet assembly. The positions of the rotors in the hot inlet and cold outlet valves are set so that the holes connecting to the hot inlet and cold outlet ports of the magnetized bed are blocked. 
         [0009]    With the valves and magnet assembly driven off the same motor, the rotors will rotate in exact coordination with the magnet assembly. In particular, as the magnet assembly rotates away from a given bed so that the bed becomes demagnetized, the rotors in the cold inlet and hot outlet valves will now block the holes connected to the cold inlet and hot outlet ports of the bed. The rotors in the hot inlet and cold outlet valves rotate so that the rotor slots uncover the holes connected to the hot inlet and cold outlet ports of the now demagnetized bed. Thus, flow is established from the hot-side heat exchanger, through the demagnetized bed from its hot inlet to its cold outlet, to the cold-side heat exchanger. 
         [0010]    In existing RMMRs, and as described in U.S. Pat. No. 6,668,560, hereby incorporated by reference, the four valves are placed at four positions outside of the sweep of the magnet assembly, and the valve shafts are driven by the magnet assembly shall through belts and pulleys which connect the valve shafts to the magnet assembly shaft, which is in turn driven by a motor. In contrast, in the current invention, the valves are located coaxial with the magnet assembly shaft on each side of the magnet assembly, so that the valves can be directly driven by the magnet assembly shaft. 
       SUMMARY OF THE INVENTION 
       [0011]    The present inventor has determined that substantial inefficiencies can arise in conventional magnetic refrigeration systems as a result of variations in the length, configuration and construction of the inter-communicating conduits used to conduct fluid within the complex circuits of the device. These variations can significantly underutilize the magnetocaloric beds reducing efficiency. Accordingly, the present invention provides a magnetic refrigeration system in which the conduits between the valve system and the magnetocaloric beds are balanced with respect to flow either when multiple conduits are active or over successive intervals of conduit activation. A rotary design with concentric positioning of the valves facilitates this balancing which considers not only steady-state resistance to flow but also dynamic effects caused by variations in conduit volume and/or elasticity. 
         [0012]    In one embodiment, the invention provides a magnetic refrigeration system having at least a first and second bed of magnetocaloric material, each bed having a first and second opposed side between which fluid may flow. At least one manifold communicates a hot inlet conduit and a hot outlet conduit to the first side of each bed and communicates a cold inlet conduit and a cold outlet conduit to the second side of each bed. A magnet assembly is movable to apply a greater magnetic field to the first bed than the second bed in a first state and a greater magnetic field to the second bed than the first bed in a second state, and a valve system communicates with the conduits and synchronizes to the magnet assembly to permit circulation of fluid through the first and second beds to remove heat from the first bed by providing flow through at least one first conduit pair (each pair being a series-connected cold inlet conduit and hot outlet conduit) and to add heat to the second bed in the first state by providing flow through at least one second conduit pair (each pair being a series-connected hot inlet conduit and cold outlet conduit). Each of the first and second conduit pairs are adapted to provide substantially equal fluid flow through each first conduit pair when connected for flow by the valve system. 
         [0013]    It is thus a feature of at least one embodiment of the invention to address cooling inefficiencies that can result from relatively minor flow imbalances. 
         [0014]    Each first conduit pair may have substantially equal flow resistance and each second conduit pair has substantially equal flow resistance. In this respect, each first and second conduit pair may have a substantially identical length. 
         [0015]    It is thus a feature of at least one embodiment of the invention to balance flow resistances in the conduit such as affects steady-state flow. 
         [0016]    The conduit pairs carrying greater flow may be made shorter than conduit pairs carrying lesser flow. 
         [0017]    It is thus a feature of at least one embodiment of the invention to provide a system that may be better tailored to permitting an equal flow in the hot and cold cycle portions. 
         [0018]    Alternatively or in addition, each first and second conduit pair may have substantially equal internal volume. 
         [0019]    It is thus a feature of at least one embodiment of the invention to address flow imbalances caused by dynamic “inductive” effects related to the inertial mass of flowing material in the conduit pairs. 
         [0020]    Alternatively or in addition, each conduit pair has substantially equal change in internal volume as a function of change in pressure. 
         [0021]    It is thus a feature of at least one embodiment of the invention to compensate for flow imbalances caused by dynamic “capacitive” effects related to the elasticity of the conduit. 
         [0022]    The change in internal volume of each conduit pair to a bed of magnetocaloric material, when subjected to the increase from a minimum to a maximum fluid pressure during the operation of the magnetic refrigeration system, may be less than 5% of the total fluid volume delivered to a single bed during the time interval in one AMR cycle that the conduit pair is delivering flow to that bed. 
         [0023]    It is thus a feature of at least one embodiment of the invention to limit potential backflow and inefficiencies caused by stored pressure in possibly elastic conduits. 
         [0024]    Each of the hot inlet conduits, hot outlet conduits, cold inlet conduits, and cold outlet conduits may be adapted to provide substantially equal resistance to fluid flow. 
         [0025]    It is thus a feature of at least one embodiment of the invention to provide balanced resistance according to the function of the conduit. 
         [0026]    The valve system may provide four valves including a hot outlet valve, a hot inlet valve, a cold outlet valve and a cold inlet valve, wherein in the first state, the hot outlet valve connects the hot outlet conduit of the first bed to the inlet of a hot heat exchanger and the cold inlet valve connects the cold inlet conduit of the first bed to an outlet of a cold heat exchanger and the hot inlet valve connects the hot inlet conduit of the second bed to an outlet of the hot heat exchanger and the cold outlet valve connects the cold outlet conduit of the second bed to an inlet of the cold heat exchanger. And further wherein in the second state the hot outlet valve connects the hot outlet conduit of the second bed to the inlet of the hot heat exchanger and the cold inlet valve connects the cold inlet conduit of the second bed to the outlet of the cold heat exchanger and the hot inlet valve connects the hot inlet conduit of the first bed to the outlet of the hot heat exchanger and the cold outlet valve connects the cold outlet conduit of the first bed to the inlet of the cold heat exchanger. 
         [0027]    It is thus a feature of at least one embodiment of the invention to provide for balanced flow in a system that preserves unidirectional flow through each conduit to eliminate losses from backflow. 
         [0028]    The hot outlet valve and the hot inlet valve may include movable elements opening and closing the valves and in mechanical communication with the magnet assembly, and wherein the cold inlet valve and cold outlet valve are one-way valves actuated by fluid flow. Alternatively, the cold outlet valve and the cold inlet valve may include movable elements opening and closing the valves and in mechanical communication with the magnet assembly, and wherein the hot inlet valve and hot outlet valve may be one-way valves actuated by fluid flow 
         [0029]    It is thus a feature of at least one embodiment of the invention to simplify the valve structures by using some one-way type valves. 
         [0030]    The first and second bed may be arranged around a central axis and the magnet assembly may be attached to a shaft rotatable with respect to the first and second bed along the central axis and the hot outlet valve and hot inlet valve may be disk valves having rotor portions attached coaxially about the shaft to move with respect to stationary stator portions positioned coaxially about the shaft. 
         [0031]    It is thus a feature of at least one embodiment of the invention to employ an axially balanced rotating architecture to facilitate balancing of the conduit structure. 
         [0032]    The hot outlet valve and hot inlet valve may have stator portions fixed with respect to the beds and rotor portions fixed with respect to the magnet wherein the stator portions are mounted between the rotor portions. 
         [0033]    It is thus a feature of at least one embodiment of the invention to adopt a valve orientation and inherent sealing between the valve rotor and stator to balance the forces necessary to seal the rotors to the stators. 
         [0034]    The magnetic refrigeration system may include a plurality of magnetic beds arranged about the central axis, each having a manifold communicating a hot inlet conduit and a hot outlet conduit to the first side of each bed and communicating a cold inlet conduit and cold outlet conduit to the second side of each bed wherein the valve assembly provides valves attached to the shaft communicating with either inlet conduits or outlet conduits. 
         [0035]    It is thus a feature of at least one embodiment of the invention to provide balanced flow in a multibed system where inefficiencies from unbalanced flow may be aggravated. 
         [0036]    The valves may provide substantially unobstructed communication with multiple inlet conduits or outlet conduits at one or more positions of the shaft. 
         [0037]    It is thus a feature of at least one embodiment of the invention to ensure equal flow sharing among conduits when multiple conduits are operated in parallel. 
         [0038]    The magnetic refrigeration system may further include a positive displacement pump circulating the fluid through the valve system and inlet and outlet conduits. 
         [0039]    It is thus a feature of at least one embodiment of the invention to provide a pump that can handle quick changes in flow rate necessary for switching among multiple beds and to provide a conduit system compatible with this rapid switching. 
         [0040]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]      FIGS. 1   a - 1   d  are schematics illustrating an Active Magnetic Regenerator (AMR) cycle to perform cooling; 
           [0042]      FIG. 2  shows a first embodiment of the invention with four disk valves; 
           [0043]      FIG. 3  shows a second embodiment of the invention with stators for the hot inlet valve and the hot outlet valve mounted to a common assembly; 
           [0044]      FIG. 4  shows a third embodiment of the invention with the magnet at a larger radius; 
           [0045]      FIG. 5  shows a fourth embodiment of the invention with stators of the hot inlet and cold inlet valves mounted to a common assembly; 
           [0046]      FIG. 6  shows a fifth embodiment of the invention with check valves on the cold side; 
           [0047]      FIG. 7  shows an enlarged view of a flow connection at one side of a bed; 
           [0048]      FIG. 8  shows an end view of an eight bed configuration of the second embodiment shown in  FIG. 3 ; 
           [0049]      FIG. 9  is a figure similar to  FIG. 8  but showing a system providing unequal blows; 
           [0050]      FIG. 10  is a figure similar to that of  FIG. 5 , but showing a system providing unequal blows; and 
           [0051]      FIG. 11  is a figure similar to  FIG. 5  showing a system for unbalanced flow. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0052]    The invention comprises a “rotating magnet” magnetic refrigerator (RMMR) which uses rotary disk valves to control flow to and from the beds where these valves are located coaxially with the shaft rotating the magnet assembly. A first embodiment of this invention is shown in  FIG. 2 .  FIG. 2  shows a cross section of a two-bed system  1 , where a first bed  2  (magnetized) is within the gap  8  of the magnet assembly  6  while a second bed  4  (demagnetized) is outside the gap  8  of the assembly. A motor  10  (which may be an electric motor) rotates the central shaft  12 , which is mounted to bearings  102 ,  104 ,  106  and  108 , and passes through rotary seals  122 ,  124 ,  126  and  128 . This central shaft  12  also drives the rotors  14 ,  16 ,  18 ,  20  in each of the coaxial valves  22 ,  24 ,  26 ,  28 . A pump  30  drives fluid flow through the system  1 . 
         [0053]    In the configuration shown in  FIG. 2 , the rotor  14  in the hot inlet (Hi) valve  22  uncovers the hole  32  connected to the hot inlet port  42  of the demagnetized (lower) bed  4 . At the same time, the rotor  16  in the cold outlet (Co) valve  24  uncovers the hole  34  connected to the cold outlet port  44  of the bed  4 . Thus, pressurized fluid emerging from the hot-side heat exchanger (HHEX)  40  at temperature T Hi  is carried by a pipe  62  into a chamber  52  at one end of the hot inlet valve  22 , through the uncovered hole  32  in stator  88  of the hot inlet valve  22  and is driven into a hot inlet pipe  64   b  and through the bed  4  from its hot inlet port  42  to its cold outlet port  44 . After passing through the cold (demagnetized bed)  4 , this fluid, now at temperature T Co , is carried by a cold outlet pipe  66   b  and collected by the open cold outlet valve  24  through the hole  34  in the stator  90 , and directed via the chamber  54  at one end of the valve  24  through pipe  92  to the cold-side heat exchanger (CHEX)  60  where the fluid absorbs heat from the refrigerated environment and rises in temperature to T Ci . The cold inlet port  68  and cold inlet pipe  72   b  of the demagnetized bed  4  are blocked by the rotor  16  position in the cold inlet (Ci) valve  26  covering the hole  38   b,  and the hot outlet port  70  and hot outlet pipe  82   b  of the demagnetized bed  4  are also blocked by the rotor  20  position in the hot outlet (Ho) valve  28  covering the hole  94   b.  Fluid at temperature T Ci  emerging from the other end of the cold-side heat exchanger  60  enters the single port  36  in the chamber  56  at one end of the cold inlet valve  26 . This fluid is directed through the cold inlet rotor  18  and through the hole  38   a  in the stator  86  into a cold inlet pipe  72   a  and to the cold inlet port  74  of the magnetized (upper) bed  2 . The fluid passes through the magnetized bed  2  from the cold inlet port  74  to the hot outlet port  78  and rises in temperature to T Ho . Flow through the cold outlet port  76  and cold outlet pipe  66   a  of the bed  2  is blocked by the cold outlet valve  24 . Flow through the hot inlet port  80  and hot inlet pipe  64   a  of the bed  2  is blocked by the hot inlet valve  22 . Hot outlet fluid at temperature T Ho  from the port  78  of the bed  2  is carried by a hot outlet pipe  82   a  through a hole  94   a  in the stator  96  into the hot outlet valve  28 , exits the valve  28  via the chamber  58  and returns via a pipe  84  to the pump  30 , where it gets directed through the other end of the HHEX  40 , completing the flow circuit. 
         [0054]    Although the figures show pipes that carry the fluid flow between components of the invention, any suitable conduits that carry the fluid between the components might be used. For example, the conduits might be fluid passages in an injection-molded assembly, or the conduits might be fluid passages in an assembly made by additive manufacturing, or the conduits could be pipes as shown in the drawings. 
         [0055]    A second embodiment of this invention is shown in  FIG. 3 . The second embodiment has the same components as the first embodiment, and the components perform the same functions in the same manner as the first embodiment. The difference is that the stator  86  and rotor  18  of the cold inlet valve  26  are inverted left to right, and the stator  88  and rotor  14  of the hot inlet valve  22  are inverted left to right, allowing the stator  88  for the hot inlet valve  22  and the stator  96  for the hot outlet valve  28  to be mounted to a common assembly  98 ; the stator  86  for the cold inlet valve  26  and the stator  90  for the cold outlet valve  24  also can be mounted to a common assembly  100 . The magnet assembly  6 , the beds  2 ,  4 , and the pump  30  are in similar positions in the first and second embodiments. 
         [0056]    By mounting the stators  88  and  96  on opposed walls, the forces needed to compress the rotors  14  and  20  to their stators  88  and  96  are counter-acting, and the forces needed to compress the rotors  16  and  18  to their stators  90  and  85  are counteracting, thus reducing loads on the shaft  12  and simplifying the design. 
         [0057]    A third embodiment of this invention is shown in  FIG. 4 . The third embodiment has the same components as the second embodiment, and the components such as the motor  10  perform the same functions in the same manner as the second embodiment. The difference is that magnet assembly  6  and beds  2 ,  4  in the first and second embodiments are located between the hot inlet valve  22  and cold inlet valve  26  at a similar radius, while the magnet assembly  6  and beds  2  and  4  of the third embodiment are located outside the valves  22 ,  26  at a larger radius, allowing the length of the assembly  1  to be reduced. Note that in  FIG. 4 , the hot outlet pipes  82   a,    82   b  are each the same length and shape, and the cold inlet pipes  72   a,    72   b  are also each the same length and shape, although the hot outlet pipe  82   a  is a different length and shape from the cold inlet pipe  72   a.    
         [0058]    In  FIGS. 2 ,  3 , and  4 , all the pipes of the same function, such as hot outlet, are the same length, although pipes of different function, such as hot outlet and cold inlet, may be of different length. More generally, conduit pairs, such as cold inlet pipe  72   a  in series with and hot outlet pipe  82   a,  and cold inlet pipe  72   b  in series with hot outlet pipe  82   b  (first conduit pairs), or being hot inlet pipe  64   a  in series with cold outlet pipe  66   a  and hot inlet pipe  64   b  in series with cold outlet pipe  66   b  (second conduit pairs), are configured for equal or balanced flow among all similar conduit pairs. This is provided by ensuring that the conduit pairs provide equal steady-state flow resistance, but also by addressing dynamic factors such as flow inductance by setting equal the total internal volume of the conduit pairs, and flow capacitance by ensuring that the change in internal volume with changes in pressure is equal for the conduit pairs. These values may also be identical but need not be identical when the first conduit pairs are compared to the second conduit pairs. 
         [0059]    A fourth embodiment of this invention is shown in  FIG. 5 . The fourth embodiment has the same components as the third embodiment, and the components such as the motor  10  perform the same functions in the same manner as the third embodiment. The difference is that the stators  88 ,  86  of the hot inlet  22  and cold inlet  26  valves are mounted to a common assembly  101 , allowing for shorter hot inlet piping  64   a,    64   b  and cold inlet piping  72   a,    72   b  to the beds  2 ,  4  than is possible for the first three embodiments. 
         [0060]    Additional variants for the above four embodiments may be created by replacing the cold side inlet and outlet valves by one-way valves. Examples of one-way valves that might be used in the invention are check valves and reed valves. A one-way valve, also known as a check valve, allows fluid flow in only one direction and blocks fluid flow in the opposite direction. For example, a ball cheek valve uses a spherical ball to block the flow of fluid in one direction. A conically tapered seat will place the ball within the valve opening to prevent flow in one direction, but allow flow in the opposite direction when the ball is displaced from its seat. Placement of the ball within the seat may be aided by a spring. Other types of one-way valves include diaphragm check valves, swing check valves, tilting disc check valves, stop-cheek valves, lift-check valves, in-line check valves, duckbill valves, pneumatic non-return valves, etc. One-way valves can be smaller and less expensive than rotary disk valves. 
         [0061]    An example of a fifth embodiment using one-way valves is shown in  FIG. 6 , where the cold side valves  24 ,  26  of embodiment 2 in  FIG. 3  have been replaced by check valves  120 , 121 ,  125 , and  127  in  FIG. 6 . Additional variants for the first four embodiments may be created by replacing the hot side inlet and outlet valves by one-way valves and moving the pump to the cold side. For example, if the pump  30  of embodiment 2 is moved to the cold side, the hot side inlet  22  and outlet  28  valves of embodiment 2 can be replaced by one-way valves, while retaining cold side disk valves  24  and  26 . 
         [0062]      FIG. 7  shows details on how the connection might be made between one end of a bed and the inlet and outlet pipes coming from a valve. The cold inlet pipe  72   b  and cold outlet pipe  66   b  come in from the top of the figure and enter a bed plenum assembly  110 . The cold inlet pipe  72   b  terminates at a cold inlet port  68  and the cold outlet pipe  66   b  terminates in a cold outlet port  44  that connect at a rectangular opening  112  that can be attached to one side of a bed, such as the bed  4  of  FIG. 2 . The bed is not shown in  FIG. 7 . 
         [0063]    Although two-bed embodiments are shown in  FIGS. 2 through 6 , it is usually advantageous to fit additional beds in the path swept by the magnet gap. The additional beds increase the cooling power and can make more efficient use of the magnet assembly. The valves may be designed to allow flow in a given direction to multiple beds at the same time. For example, an eight-bed version of the first embodiment is shown as an end view from the cold end in  FIG. 8 . Not shown are the cold inlet pipes, the hot inlet and outlet pipes, the valve housings and seals, the HEX&#39;s, the pump, the motor, and the bearings. The magnet assembly  6  and the cold outlet valve rotor  16  are connected to the shaft  12  and rotate with it. The magnet assembly is shown over two magnetized beds  2   a,    2   b,  which are both under flow from their cold ends to their hot ends. Two demagnetized beds  4   a,    4   b  are in the lowest field region and both are under flow from their hot ends to their cold ends, and four remaining beds  3   a,    3   b,    3   c,  and  3   d  at intermediate fields are not under flow. Each bed is attached to a cold side plenum assembly  110  and a hot side plenum assembly  111 . Together these plenums create a manifold about the bed. The cold outlet valve rotor  16  is shown exposing two holes  34   a,    34   b  in the cold outlet valve stator  90 , allowing flow to leave the demagnetized beds  4   a,    4   b  through the cold outlet ports  44   a,    44   b  and the cold outlet pipes  66   a,    66   b  which are attached to the cold side plenum assemblies  110   a,    110   b.  Meanwhile, the cold outlet valve rotor  16  is blocking the holes  34   c,    34   d,    34   e,    34   f,    34   g  and  34   h,  thereby blocking flow from the cold outlet ports of beds  2   a,    2   b,    3   a,    3   b,    3   c,  and  3   d.    
         [0064]    Note that the flow situation of  FIG. 8  can be implemented using cold inlet, cold outlet, hot inlet and hot outlet valve rotors that each expose two holes in their matching stator at a time. 
         [0065]    Although  FIG. 8  shows a situation where two beds are simultaneously under flow from cold to hot and two beds are under flow from hot to cold, there are four beds that are not under flow and thus are not contributing to the cooling of the device. If the cold outlet and hot inlet valve rotors expose more holes in their matching stators than the cold inlet and hot outlet valves, then more beds will be subjected to hot to cold flow than will be subjected to cold to hot flow.  FIG. 9  shows such an arrangement, where the cold outlet valve rotor  16  exposes four holes in its stator  90 , thereby allowing simultaneous hot to cold flow for the four beds  3   a,    4   b,    4   a  and  3   d  provided that the hot inlet valve also exposes four holes in its corresponding stator to allow the hot inlet flow to enter beds  3   a,    4   b,    4   a  and  3   d.  Meanwhile, if the cold inlet and hot outlet valve rotors still expose only two holes in their corresponding stators, only two beds will simultaneously undergo cold to hot flow. The additional beds  3   a  and  3   b  under hot to cold flow share some of the flow that was formerly carried only by beds  4   a  and  4   b,  thereby reducing system pressure drop and system heat transfer losses. 
         [0066]    The magnet assemblies shown in the above embodiments are a single lobe design, with one high field region, and an opposite low field region. However, it may be advantageous to employ magnet assemblies with multiple high field regions and multiple low field regions. For such cases, co-axial disk valves could be implemented with additional slots that direct cold to hot flow simultaneously to beds in multiple high field regions, and direct hot to cold flow simultaneously to beds in multiple low field regions. 
         [0067]    By placing the valves coaxially with the main drive shaft, the need for connecting belts and pulleys between this shaft and the valve shafts is eliminated. These belts and pulleys waste energy provided by the motor, so their elimination improves the energy-efficiency of the MR system. The belts and pulleys take up space, so their elimination also results in a smaller, more compact system. 
         [0068]    Moreover, the coaxial valve placement reduces the length of the fluid conduits (commonly called pipes) connecting the valves and the fixed beds. Note that this invention allows the use of separate inlet and outlet pipes on both the cold and hot sides for each bed. By using separate inlet and outlet pipes with unidirectional flow in each pipe, all the fluid that enters the pipe eventually will reach the destination bed or destination heat exchanger. Thus the fluid contained in the pipes will contribute to the operation of the AMR cycle and not represent “dead volume”. However, even with separate inlet and outlet pipes, the shorter pipe lengths possible with the coaxial valves still offer two advantages. First, the shorter length reduces the pressure drop experienced by the fluid as it flows through the pipe through the conduit, that is, the fluid resistance of the pipe to steady flow is reduced. This reduces the load on the pump and further improves the energy efficiency of the system. Second, the shorter pipe lengths reduce the magnitude of bypass flow, a phenomenon in which fluid bypasses the beds and proceeds directly from the hot inlet valve to the hot outlet valve. Bypass flow does not contribute to refrigeration and therefore wastes energy provided by the pump; its reduction therefore improves the energy efficiency of the MR system. 
         [0069]    Bypass flow is caused, in part, by periodic expansion of a deformable plumbing element under pressurization, followed by fluid expulsion under depressurization, a form of fluid capacitance for the plumbing element. To explain this bypass flow mechanism, we refer to  FIG. 2 . The hot inlet fluid is at the highest pressure in the fluid circuit. Under this pressure, the pipe  64   b  connecting the hot inlet valve  22  to the hot inlet port  42  of the demagnetized bed  4  will expand slightly, storing some fluid that would otherwise pass through the bed  4 . After the cold blow is completed, the Hi and Co valves  22 ,  24  seal off the hot inlet pipe  64   b  of this bed  2 , preventing the stored fluid from leaving the hot inlet pipe  64   b.  When the valves rotate for the hot blow, the hot outlet pipe  82   b  connected to the bed  4  can now carry flow, so the pressurized fluid stored in the hot inlet pipe  66   b  can be expelled through the hot outlet pipe  82   b  and into the hot outlet valve  28 , allowing the hot inlet pipe  64   b  to return to its original shape. This cyclical process of pressurization, expansion, and fluid storage during the cold blow, followed by fluid expulsion and depressurization during the following hot blow, produces bypass flow. The amount of fluid that can be stored during the cold blow increases with the length of pipe connecting the hot inlet valve to the hot inlet port of a bed. The coaxial valve placement minimizes this conduit length, minimizing the increase in fluid volume during pressurization, thus minimizing bypass flow and improving system performance. For best operation of an AMR system, the change in internal fluid volume of a conduit to a bed when subjected to the increase from the minimum to the maximum fluid pressures during the AMR cycle should be less than 5% of the total fluid volume delivered to a single bed during the time interval in one AMR cycle that the conduit pair is delivering flow to that bed. 
         [0070]    An additional advantage of the coaxial valve arrangement is that it allows the conduits of a similar flow function connecting the beds to the valves to be symmetrically placed around the shaft axis and to be of identical shape and length. There are four flow functions for conduits connecting the beds to the valves: hot inlet, hot outlet, cold inlet, and cold outlet. Two pipes that each conduct hot inlet flow both have a similar function, although they might be connected to different beds. For an example of symmetrical placement and identical shape, in  FIG. 2 , if the two beds  2  and  4  shown in the figure are located at a 180 degree rotational angle from each other around the axis of the shaft  12 , and the ports  38   a  and  38   b  in the cold inlet valve are also located at a 180 degree angle from each other around the same axis, then the two cold inlet pipes  72   a  and  72   b  can be identical components of identical shape and length, but mounted at a 180 degree angle from each other around the axis of the shaft  12 . In addition to saving fabrication cost, the identical shape and length of conduits of a similar flow function ensures that the resistance of the conduits to steady flow will be equal. In addition, if the conduits of a similar flow function are of identical shape and length and wall thickness, then the conduits of similar function will have equal change in internal fluid volume when subjected to the increase from the minimum to the maximum fluid pressures during the AMR cycle. Finally, if the conduits of a similar flow function have the same internal cross section as well as identical shape and length, the conduits will have equal internal fluid volume, the mass of fluid stored in the conduits will be identical, and thus the dynamic pressure drop needed to accelerate fluid flow at the start of the fluid blow will be equal. The equivalent characteristics of conduits of a similar flow function thus ensure that the pressure drop due to flew friction, and the flow transient effects due to conduit expansion and fluid inertia, will be identical for all the beds. This helps ensure that all the beds get similar flow versus time profiles during an AMR cycle, which can improve efficiency and temperature span. 
         [0071]    The flow from the hot outlet valve to the pump in the first five embodiments ( FIGS. 2 ,  3 ,  4 ,  5  and  6 ) only occurs in one direction, from the valve to the pump, and is thus unidirectional flow. 
         [0072]    Although this invention enables conduits of a similar flow function to be of equal length, conduits of dissimilar flow function, such as hot outlet and hot inlet, may be of different length. In the case where the flows in conduits of dissimilar flow function are not of the same magnitude, it may be advantageous to adopt a design where the conduits of functions that carry the highest flow rates are made the shortest. For example, in the case that was described in connection with  FIG. 9 , where four beds at a time undergo cold to hot flow while only two beds at a time undergo hot to cold flow, it could be advantageous to make the conduits carrying hot to cold flow shorter than the pipes carrying cold to hot flow. Note that the total hot to cold flow carried by all the beds is the same as the total cold to hot flow carried by all the beds, but because fewer beds carry cold to hot flow than carry hot to cold flow, the rate of flow in each conduit that carries cold to hot flow will be greater than the rate of flow in each conduit that carries hot to cold flow. In the sixth embodiment shown in  FIG. 10 , the hot outlet valve  28  and cold inlet valve  26  are mounted adjacent to one another. The hot outlet stator  96  and cold inlet stator  86  are mounted to a common assembly  101  and are positioned between the hot outlet rotor  20  and the cold inlet rotor  18 . The hot inlet valve  22  and the cold outlet valve  24  are mounted outside the hot outlet and cold inlet valves, closer to the outer ends of the shaft  12 . This valve arrangement allows the hot outlet conduit  82   a  and cold inlet conduit  72   a  that carry flow to the magnetized bed  2  to be much shorter than the hot inlet pipe  64   b  and cold outlet pipe  66   b  that carry flow to the demagnetized bed  4 . Thus when two magnetized beds are undergoing cold to hot flow, and thus only two sets of cold inlet and hot outlet conduits must carry the flow, the conduits are short, reducing what otherwise might be a large pressure drop. Meanwhile, four demagnetized beds are undergoing hot to cold flow, and thus four sets of hot inlet, and cold outlet conduits are sharing the hot to cold flow, and thus the longer length of these conduits will not produce a large increase in pressure drop. Note that although flow conduits of different function are of different lengths, conduits of the same function can still be of the same length, so all the beds can get similar flow versus time profiles during the AMR cycle. 
         [0073]    Turning now to  FIG. 11 , another magnetic refrigeration system being used as a fluid chiller is shown, in accordance with at least some embodiments of the present disclosure. Specifically, in some cooling applications (e.g. ventilation air conditioning or cooling water generation), what is desired is not the pumping of heat from a cold reservoir at T c  to a hot one at T h , but the cooling of an air or fluid stream from T h  to T c  (e.g., a “fluid chiller”). If the fluid has a temperature-independent heat capacity C, the total amount of heat Q C  to be removed from the fluid is Q C =C (T H −T C ). Additionally, the minimum theoretical work W required to move a given amount of heat Q c  from a cold absolute temperature T c  to a hot absolute temperature T h  via a reversible refrigerator is W=Q C (T H −T C )/T C  where the coefficient of performance (COP) may be defined as Q c /W. The theoretical minimum amount of work required to cool a fluid using a single stage refrigerator that pumps all the heat from absolute temperatures T c  to T h  is: 
         [0000]        W=C ( T   H   −T   C ) 2   /T   C    Equation 1
 
         [0000]    and the related COP 
         [0000]      COP= Q   C   /W =( T   C /( T   H   −T   C ).   Equation 2
 
         [0074]    Actual refrigerators may be relatively less efficient, with major losses occurring due to viscous losses in the compression and expansion of the refrigerant. 
         [0075]    Less work may be needed if the fluid were cooled by a large number of separate refrigerators with the first cooling the fluid from T H  to T H-d  and pumping heat to T H , and the next cooling the fluid from T H-d  to T H-2d  and pumping heat to T H , etc., where d&lt;&lt;(T H −T C ). This occurs because much of the cooling of the fluid is accomplished by refrigerators acting through a small temperature difference, and hence acting at high efficiency. For the ideal fluid chiller comprised of an infinite number of successive refrigerators, each of ideal efficiency, the work required would be: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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         [0000]    with the resulting COP: 
         [0000]      COP= Q   C   /W   C =( T   H /( T   H   −T   C ) ln( T   H   /T   C )−1) −1 .   Equation 4
 
         [0076]    The work input is lower than the single stage refrigerator because the generation of entropy that occurs when the initially warm fluid stream contacts the cold heat exchanger is no longer present. When T C  is close to T H , the best single stage refrigerator may require twice as much work input as a multi-stage ideal chiller. As the ratio of T H /T C  gets larger, the efficiency penalty may increase slightly; for example, for T H =100° F. and T C =45° F., the best single-stage refrigerator may consume 2.07 times more input work than an ideal multi-stage chiller. 
         [0077]    An AMR-type magnetic refrigerator may be set up to act as a fluid chiller by relaxing the requirement of equal hot to cold and cold to hot total flows, and instead send more AMR beat transfer fluid from the hot to the cold ends of the demagnetized bed(s) than is returned from the cold to the hot ends of the magnetized bed(s), which is a case of unbalanced flow in the beds. The excess heat transfer fluid that accumulates at the cold end may be chilled in a nearly reversible manner from T H  to T C . This excess heat transfer fluid may be re-warmed in a counter-flow heat exchanger that chills an external fluid stream, such as water for a chilled water loop, or ventilation air for air conditioning a building. The warm excess heat transfer fluid may be returned to the hot end of the demagnetized AMR bed, once again becoming the excess heat transfer fluid flowing from the hot to cold ends of the AMR bed. 
         [0078]    In the case of unbalanced flow in the beds, the hot to cold flow rate through each of the beds can be higher than the cold to hot flow, so it may be advantageous to make the hot inlet and cold outlet conduits shorter than the cold inlet and outlet conduits, as is shown in  FIG. 11 . In  FIG. 11 , fluid from the pump  30  passes through the hot heat exchanger  40 , enters the hot inlet valve  22  and passes through the short conduit  64   b  into the demagnetized bed  4 . Fluid leaves the bed  4  and passes through the short conduit  66   b  into the cold outlet valve  24 . The fluid leaving the cold outlet valve  24  passes through the conduit  134  and is divided, with part of the fluid entering the first cold heat exchanger  60 , and part of the fluid entering the second cold heat exchanger  129 . The fluid leaving the first cold heat exchanger  60  enters the cold inlet valve  26  and is passed through the long conduit  72   a  into the magnetized bed  2 , and then passes through the long conduit  82   a  into the hot outlet valve  28  and is returned to the inlet of the pump  30 . The fluid leaving the second cold heat exchanger  129  enters the flow-proportioning valve  132  and is returned to the inlet of the pump  130 . The first cold heat exchanger can be used to cool an external refrigeration load at a cold temperature near that of the temperature of the fluid leaving the cold outlet valve, while the second heat exchanger, with flow rate adjusted using flow proportioning valve  132 , can be used to cool an external fluid stream over a large temperature range from a temperature near that of the fluid leaving the hot outlet valve to near a temperature of the fluid leaving the cold outlet valve. Because all of the fluid leaving the pump must pass through the hot inlet conduit  64   b  and cold inlet conduit  66   b,  but only some of the fluid leaving the pump must pass through the cold inlet conduit  72   a  and the hot outlet conduit  82   a,  making the conduits  64   b  and  66   b  shorter than the conduits  72   a  and  82   a  may reduce the overall pressure drop in the system. Note that although flow conduits of different function are of different lengths, conduits of the same function can still be of the same length, so all the beds can get similar flow versus time profiles during the AMR cycle. 
         [0079]    The flow from an outlet valve to the pump in the embodiments described above only occurs in one direction, from the valve to the pump, and is thus unidirectional flow. This means that the fluid contained in the pipe  84  between the hot outlet valve  28  and the pump  30  in  FIG. 2 , for example, does not contribute to dead volume losses, and thus the pump  30  can be located outside the coaxial valve and bed assembly. This allows the use of any convenient type of pump. In particular, positive displacement pumps, such as gear pumps, screw pumps, piston pumps, diaphragm pumps, rotary vane pumps and scroll pumps, can be used. Positive displacement pumps produce a flow that is nearly constant over a wide range of operating pressures. The use of a positive displacement pump allows the flow rate to quickly reach intended levels as the flow is switched between different AMR beds. In addition, efficient positive displacement pumps can be made over a wide range of flow capacity and pressure capacity, while centrifugal pumps, a common form of non-positive displacement pump, are only efficient at relatively large flow capacity or low pressure capacity. Efficient heat transfer in AMR beds requires a large internal heat transfer area, which tends to lead to high operating pressures, which are not well suited to efficient operation of centrifugal pumps for small to medium scale systems. 
         [0080]    If hot to cold flow or cold to hot flow occurs to only one bed at a time, the use of a positive displacement pump may require either precise valve timing to ensure flow is not blocked for a period of time, or alternately, the use of a fluid accumulator at the pump outlet. 
         [0081]    Although the description of the present invention above has been based on the use of rotary disk valves, it is clear that other valve types that also rely on rotary motion to open and close desired fluid paths could be used and fall within the scope of the present invention. 
         [0082]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0083]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0084]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.