1. Field of the Invention This invention relates to a gas-balanced Brayton cycle engine and specifically to gas-balanced Brayton cycle engines designed to operate at about 150 K having input power in the range of 5 to 30 kW.
2. Background of the Invention
A Brayton-type or Brayton cycle engine includes three essential components: a gas compressor, a counter-flow heat exchanger, and an expander.
Four recent patent applications assigned to SHI Cryogenics describe gas-balanced Brayton cycle expansion engines and two adaptations, one to minimize cool down time to cryogenic temperatures, the other to cool a cryopump for pumping water vapor. A system that operates on the Brayton cycle to produce refrigeration consists of a compressor that supplies gas at a discharge pressure to a counterflow heat exchanger, which admits gas to an expansion space through a cold inlet valve, expands the gas adiabatically, exhausts the expanded gas (which is colder) through in outlet valve, circulates the cold gas through a load being cooled, then returns the gas through the counterflow heat exchanger to the compressor.
U.S. Patent Application Publication 2011/0219810 dated Sep. 15, 2011 by R. C. Longsworth describes a reciprocating expansion engine operating on a Brayton cycle in which the piston has a drive stem at the warm end that is driven by a mechanical drive, or gas pressure that alternates between high and low pressures, and the pressure at the warm end of the piston in the area around the drive stem is essentially the same as the pressure at the cold end of the piston while the piston is moving. U.S. Patent Application Publication 2012/0085121 dated Apr. 12, 2012 by R. C. Longsworth describes the control of a reciprocating expansion engine operating on a Brayton cycle, as described in the previous application, which enables it to minimize the time to cool a mass to cryogenic temperatures. U.S. Ser. No. 13/106,218 dated May 12, 2011 by S. Dunn, et al., describes alternate means of actuating the expander piston. U.S. Ser. No. 61/504,810 dated Jul. 6, 2011 by R. C. Longsworth describes the application of a Brayton cycle engine to cooling coils for cryopumping water vapor. The engines described in published patent application 2011/0219810 and U.S. Ser. No. 13/106,218 are referred to as “Gas-balanced Brayton cycle engines”. A compressor system that can be used to supply gas to these engines is described in U.S. Patent Application Publication 2007/0253854 titled “Compressor With Oil Bypass” by S. Dunn filed on Apr. 28, 2006. The engine of this present invention incorporates a cold rotary valve which has some features in common with U.S. Pat. No. 3,205,668 dated Sep. 14, 1965 by W. E. Gifford, and U.S. Pat. No. 4,987,743 dated Jan. 29, 1991 by A. J. Lobb. It also incorporates a vibration absorbing double bumper as described in U.S. Pat. No. 6,256,997 dated Jul. 10, 2001 by R. C. Longsworth and an anti-abrasion coating on the piston as described in U.S. Pat. No. 5,590,533 dated Jan. 7, 1997 by H. Asami et al.
A cryopump for pumping water vapor requires a cryopanel that is cooled to a temperature between 120 K and 170 K. This is a lot warmer than the temperature range of 10 K to 20 K needed to cryopump air. A paper by C. B. Hood, et al., titled “Helium Refrigerators for Operation in the 10-30 K Range” in Advances in Cryogenic Engineering, Vol. 9, Plenum Press, New York (1964), pp 496-506, describes a large Brayton cycle refrigerator having a reciprocating expansion engine capable of producing more than 1.0 kW of refrigeration at 20 K.
This refrigerator was developed to cryopump air in a large space chamber. Starting in the early 1970's cryopumping water vapor at temperatures in the range of 120 K to 170 K and capacities of 500 to 3,000 W have been dominated by refrigerators that use mixed gases as described in U.S. Pat. No. 3,768,273 dated Oct. 30, 1973 by Missimer. A more recent patent, U.S. Pat. No. 6,574,978 dated Jun. 10, 2003 by Flynn, et al., describes means of controlling the rate of cooling and heating a refrigerator of this type which produces about 500 to 3,000 W at about 150 K to pump water vapor
The refrigerants used in mixed gas refrigerators include some that are being phased out because of their impact on global warming It is thus desirable to use a Brayton cycle engine which uses helium, argon, or nitrogen, all environmentally friendly. The present invention is based on the recognition that a Brayton cycle engine that operates at about 150 K can be a lot simpler than one that is designed for lower temperatures. These simplifications make it practical to design an engine that can produce over 3,000 W of refrigeration and thus compete with present mixed gas refrigerators.
A particular feature of the invention is the design of a light weight reciprocating piston that provides a high displacement rate with low vibration. This is preferably accomplished by a reciprocating cup shaped piston having a bottom and a cylindrical side wall, the bottom separating a space near room temperature and an expansion space below 200 K, and the side wall sliding within a cylinder having a temperature gradient between room temperature and below 200 K. A drive stem is attached to the piston which can produce a reciprocating motion by pneumatic or mechanical forces. The engine that is described herein operates on a gas-balanced Brayton cycle as described in U.S. Ser. No. 13/106,218. Reciprocating motion is further minimized by using a cold rotary valve to cycle gas in and out of the cold expansion space.
The warm end of cylinder 8 is surrounded by cylinder sleeve 9 which has a high thermal conductivity in order to keep cylinder 8 near room temperature in the region where piston seal 5 reciprocates. Cylinder 8 is shown welded into warm flange 10 to which drive housing 14 is bolted.
Drive stem 28 has seal 13 that separates low pressure gas in 28 from the gas in displaced volume 29. Drive stem 12 engages double bumper 15 which has elastomer seals, for example, “O” rings that absorb the impact before piston 1 hits drive housing 14 or valve base 25. The gas porting at the warm end of engine 100 is shown for gas-balanced operation. Drive stem volume 28 is connected to low pressure through gas line 51. Gas lines 48, 49, and 50 are all connected to high pressure.
After piston 1 reaches the warm end rotary valve disc 16 turns to the position shown in
Rotary valve disc 16 has an extended shaft 17 that is coupled to valve motor shaft 21 by drive pin 19 through coupling 18. Valve motor 20 can operate at a fixed or variable speed. Valve disc 16 may be made of an aluminum alloy that has a low thermal conductivity and can be hard-coated. In the design shown it rotates on valve seat 26 which is a low friction polymer that is bonded to valve base 25. In
System pressures are controlled by valves 39, which puts excess gas from high pressure line 35 into storage tank 38, and valve 40, which puts gas from storage tank 38 into low pressure line 36.
The speed at which piston 1 moves is controlled by valves 45 and 46. Gas flows into displaced volume 29 at room temperature through valve 45 and flows out at an elevated temperature through after-cooler 41 and valve 46. Because operation is well above the temperature where air will liquefy it is practical to insulate the cold components with foam insulation, 47.
While the light weight piston which is the subject of this invention has been illustrated for a gas-balanced Brayton cycle engine it can be applied to other drive and control mechanisms. Several of these options are described in U.S. Patent Application Publication 2011/0219810 and U.S. Ser. No. 13/106,218.
Table 1 provides an example of the design and performance of engine 100 as shown in
All patents, published patent applications, and pending applications mentioned in this application are hereby incorporated by reference in their entirety for all purposes.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US2012/048321 | 7/26/2012 | WO | 00 | 12/10/2014 |