Patent Application
20070261416
1. Technical Field of the Invention
This invention is in the field of cryocoolers, and more particularly in the field of regenerative cryocoolers.
2. Background of the Related Art
Multi-stage cryocoolers are of fundamental interest for many applications in which cryogenic cooling is required. For example, some applications require the simultaneous cooling of two or more objects to cryogenic, but different, temperatures. In the case of a long wave infrared sensor, for instance, the focal plane assembly may require an operating temperature of around 40 K, while the optics may need to be maintained at a different temperature, such as about 100 K. One approach for such situations is to use a single-stage cooler and extract all of the refrigeration at the coldest temperature. However, this is thermodynamically inefficient. Another approach is to use two single-stage cryocoolers with one each at the two temperature reservoirs. This approach has the disadvantage of being expensive and large in size. A better approach that has been done in the past is to use a two-stage cryocooler with the first-stage cooling the higher operating temperature component, and the second stage cooling the lower operating temperature component. Multi-stage cryocoolers are generally more efficient than single-stage coolers, because a portion of the internal parasitic thermal losses can be removed from the system at higher temperatures, thus producing less entropy generation.
In installation of the prior art cryocooler 10, the cold cylinder 25, the first-stage manifold 28, and the second-stage pulse tube expander 30 (collectively a cold head 50) are often required to be supported only at the expander housing 26. This leaves the second-stage pulse tube expander 30, the second-stage manifold 41, the first-stage manifold 28, and much of the cold cylinder 25, cantilevered off of the housing 26. This has caused difficulties, particularly in space flight applications, where the cooling system must be able to withstand loads and random vibrations generated during launch.
In order to provide cooling at very low temperatures, for instance at about 10 K or below, multi-stage coolers with three or more stages have been utilized. Improvements are desirable in multi-stage cryocoolers capable of providing cooling at these very low temperatures.
According to an aspect of the invention, a cryocooler has three or more stages, at least one of which is an active stage, defined as a stage driven by a moving piston, and at least another of which is a passive stage.
According to another aspect of the invention, a cryocooler has an active first stage, and two or more passive stages coupled to the active stage. The active stage may be a Stirling expander, and the passive stages may include pulse tubes.
According to yet another aspect of the invention, a cryocooler has three or more stages, all of which utilize the same working fluid.
According to still another aspect of the invention, a cryocooler has three or more stages, including an active stage that operates at at least about 20 Hertz.
According to a still further aspect of the invention, a cryocooler has an active first stage that includes a Stirling expander, passive second and third stages that include pulse tubes, and a pair of surge volumes which are maintained at substantially ambient temperature.
According to another aspect of the invention, a multi-stage cryocooler includes: an active first stage; and plural passive stages operatively coupled to the active first stage.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
In the accompanying drawings, which are not necessarily to scale:
A multi-stage cryocooler has three or more stages, including an active first stage and passive second and third stages. The active stage may include a Stirling expander, and the passive second and third stages may be pulse tube coolers. The cryocooler may provide cooling at three different temperatures. The coldest cooling temperature may be at or below 10 K, and may be at or below 5 K. The system may provide cooling at such low temperatures while still operating at a relatively high frequency, for example, at a frequency of at least about 20 Hertz.
With reference initially to
The cold cylinder 125 (and its contents) and the second-stage pulse-tube expander 130 are parts of a cold head. The cold head is mechanically coupled to the expander housing 126.
The second-stage pulse tube expander 130 is in many respects similar to the pulse tube expander 30 shown in
The second-stage manifold also contains a flow plug 152, shown in detail in
It will be appreciated that a wide variety of alternative configurations are possible for the flow plug 152. The flow plug 152 may divert a relatively fixed amount of flow away from the third stage 150 of the cryocooler 100. Alternatively, the amount of flow diverted by the flow plug may be controllable.
A flow passage 158 in a third-stage manifold 161 is used to turn the flow from a downstream end of the third-stage regenerator 151. Within the third-stage manifold 161, the flow passes through a third-stage heat exchanger 164, and into a third-stage pulse tube 172. Return flow of the working fluid passes from the third-stage pulse tube 172 into the second-stage manifold 141 and the second-stage heat exchanger 144.
The outlet from the second-stage pulse tube 132 may be coupled to a surge volume 176, via an inertance tube 178 coupled to the first-stage manifold 128. In addition, the outlet from the second-stage pulse tube 172 may be coupled to a second surge volume 179 via an inertance tube 180 coupled to the second-stage manifold 141. The surge volumes 176 and 179 may be maintained at an ambient warm temperature, or alternatively one or both of the surge volumes may be maintained at a cooler temperature. Further details regarding configuration and use of an ambient-temperature surge volume may be found in commonly-assigned U.S. application Ser. No. 10/762,867, titled “Cryocooler With Ambient Temperature Surge Volume” filed Jan. 22, 2004, the description and figures of which are incorporated herein by reference.
The cryocooler 100 provides an efficient means for getting to very low cryogenic temperatures. In one example, the Stirling first stage may cool to about 80 K, the pulse tube second stage may cool to about 20 K, and the pulse tube third stage may cool to about 10 K. For lower objective temperatures, alternative temperature arrangements are achievable, such as a first stage cooling to about 50 K, the second stage cooling to about 12 K, and the third stage cooling to about 5 K. These lower temperatures may be desirable for some installations, such as cooling of niobium superconducting electronics. It will be appreciated that a wide variety of other cooling temperatures may be produced using various embodiments of the cryocooler 100. For example, temperatures in ranges between the above specific examples may be achievable. Also, temperatures warmer or colder than the above temperatures may be achievable.
The temperatures described above may be achievable with operation at higher frequencies than have been used in previous cryocooler systems for achieving such low temperatures. For example, the cryocooler 100 may operate at a frequency of at least about 20 Hertz. Systems for achieving such low temperatures have previously operated at much lower frequencies (5-10 Hertz), which requires a large moving assembly for an active device such as a Stirling expander. By operating at low frequencies, such expanders must have a moving portion that is relatively large. In contrast, the high-frequency operation of the cryocooler 100 allows reduction of the physical size and weight of the cryocooler. The cryocooler 100 advantageously is able to operate at a higher frequency at least because the active-passive-passive configuration of the stages of the cryocooler 100 is more efficient. Control of the phase angle in the active first stage allows optimization in the amount of refrigeration provided by each stage.
Overall refrigeration capacity may be shifted between the active first stage and the passive second and third stages simply by changing the phase angle of the active stage displacer relative to the pressure wave generated by a mated compressor.
One advantage of the cryocooler 100 is that use of passive second and third stages reduces the need for moving parts and cryogenic moving seals, relative to three-stage Stirling or other active cryocoolers. This simplifies the system and reduces cost. Relative to systems involving only three or more pulse tubes, the efficiency of the active first stage has the advantage of reducing the cooling burden required from the passive second and third stages. Efficient single-stage Stirling coolers may achieve cooling temperatures as low as 35 K, for instance. In contrast, single-stage pulse tubes often may provide competitive cooling only down to substantially higher temperatures, such as about 60 K. By combining an active first stage with passive or pulse-tube second and third stages, the cooling burden on the passive stages is reduced, and the need to add additional stages may be avoided.
Another advantage of the cryocooler 100 is that the same working fluid is used for all three stages.
Microelectromechanical systems (MEMS) devices may be incorporated at various positions within the cryocooler 100 to control flow within the cryocooler 100. Such MEMS devices may be used to control any of a variety of flow parameters, for example, providing suitable flow rate, dynamic response, power efficiency, and/or other operating characteristics for MEMS devices or flow controllers. A schematic illustration of use of MEMS devices is shown in
Features of the embodiment shown in
The use of concentric pulse tube stages may have the advantage of providing better mechanical integrity. In addition, use of the vacuum gap may advantageously reduce heat transfer between parts of the same pulse tube expander stage. Further details regarding concentric pulse tube expanders may be found in U.S. Pat. No. 5,680,768, and in U.S. patent application Ser. No. 11/038,833, titled “MULTI-STAGE CRYOCOOLER WITH CONCENTRIC SECOND STAGE,” filed Jan. 19, 2005. The description and figures of the above patent and patent application are herein incorporated by reference.
Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.
