Patent Application
20100275620
In one respect, the present invention is directed to systems, for delivering or spraying cryogenic coolants, and in particular, to providing frost-free or ice-free surfaces in such systems by utilizing a gas filled microporous envelope that encases selected plumbing with ambient or room-temperature inert gas. In another respect, the present invention is directed to systems for providing a purge gas to cryogenically-cooled sensors to cool and/or prevent frost thereon.
Surfaces that are chilled to cryogenic temperatures and exposed to ambient air quickly accumulate frost and ice because moisture in the ambient air condenses and freezes on the chilled surfaces. Surface frost and/or ice can create operational problems and safety hazards in cryogenic coolant systems.
There have been attempts to reduce or eliminate the buildup of frost and ice, including the use of insulating jackets, foams, and electrically heated surface pads. Such devices increase cost, size and weight of the cryogenic delivery system, reduce thermal efficiency, and make it difficult to retrofit existing apparatus. In addition, the frost reduction devices of the prior art have been unsuccessful in preventing frost/ice formation around the discharge nozzles of cryogenic spray devices.
In one respect, the invention comprises an apparatus including a cryogenic fixture that is adapted carry a cryogenic fluid and a casing that envelopes at least a portion of the cryogenic fixture, thereby defining a purge volume between the cryogenic fixture and the casing, the casing comprising at least one purge gas inlet and a membrane, the at least one purge gas inlet being adapted to be connected to a pressurized supply of a purge gas. The casing is configured so that when the purge gas is introduced into the purge volume through the at least one purge inlet at a pressure equal to or greater than a minimum operating pressure, at least some of the purge gas diffuses outwardly from the purge volume through the membrane.
In another respect, the invention comprises a method for preventing frost formation on a cryogenic fixture, the method comprising: supplying a cryogenic fluid to the cryogenic fixture; supplying a purge gas to a purge volume, the purge volume at least partially enveloping the cryogenic fixture, the purge volume being defined by a casing comprising a membrane that is gas-permeable; and diffusing the purge gas through the membrane.
In yet another respect, the invention comprises a method for operating a cryogenic fixture having a first portion that is exposed to an atmosphere having a relative humidity of at least 30%, the cryogenic fixture having a discharge nozzle located within the first portion, the method comprising continuously discharging a cryogenic fluid through the discharge nozzle for a period of 30 minutes without any significant frost formation on the first portion of the cryogenic fixture.
In yet another respect, the invention comprises an apparatus comprising a casing having a first opening formed thereon and at least one porous wall that extends into the first opening; a cryogenic spray bar located within the casing, the cryogenic spray bar having a discharge nozzle that is aligned with the first opening, the discharge nozzle having at least one opening, the cryogenic spray bar being adapted to discharge a cryogenic fluid through the discharge nozzle; a plenum contained within the casing, the plenum being in flow communication with the at least one porous wall. The at least one porous wall extends into the first opening and is located between the casing and the cryogenic spray bar, the at least one porous wall having a first and second portions which are located on opposing sides of each of the at least one opening of the discharge nozzle. The plenum and the at least one porous wall are configured so that at least some of the purge gas diffuses outwardly from the plenum through the at least one porous wall when the purge gas is supplied to the plenum at a pressure equal to or greater than a minimum operating pressure.
The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the invention. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention, as set forth in the appended claims.
To aid in describing the invention, directional terms may be used in the specification and claims to describe portions of the present invention (e.g., upper, lower, left, right, etc.). These directional terms are merely intended to assist in describing and claiming the invention and are not intended to limit the invention in any way. In addition, reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features. In the drawings, cross-hatching in a sectional view indicates a microporous material and hatching in a sectional view indicates non-microporous material.
As used herein, the term “cryogenic fluid” is intended to mean a liquid, gas or mixed-phase fluid having a temperature less than −70 degrees C (203 degrees K.). Examples of cryogenic fluids include liquid nitrogen (LIN), liquid oxygen (LOX), and liquid argon (LAR), liquid carbon dioxide and pressurized, mixed phase cryogens (e.g., a mixture of LIN and gaseous nitrogen).
As used herein, the term “cryogenic fixture” is intended to mean any type of apparatus or device which is designed to deliver a cryogenic fluid. Examples of cryogenic fixtures include, but are not limited to, supply pipes, tubes, flanges, elbows, tees, vapor vents, phase separators, vaporizers, spray heads, spray nozzles, valves, gauges and regulators. Although the present invention was developed for the purpose of preventing frost build-up on cryogenic fixtures, it could be advantageously used on any low-temperature device on which undesirable frost or excessive condensation forms.
As used herein, the term “discharge nozzle” is intended to mean one or more openings located on a cryogenic spray device through which the cryogenic fluid is discharged. The opening(s) may vary in shape from a circular hole to an elongated slot.
As used herein, the term “microporous membrane” refers to a material or membrane having open, and preferably uniformly distributed, micro-channels extending between the external surfaces of the membrane. In the context of this invention, a microporous material can have an average pore size up to about 500 micrometers.
Referring to the drawings,
In this embodiment, the membrane 13 is a microporous, woven polyester fabric. There are, however, many alternate materials that could be used, including but not limited to, porous plastics (e.g., polyethylene and high-density polyethylene having an open pore size of about 10 micrometers), polymer, ceramic, and metallic foams having pore sizes as large as 500 micrometers and a pore fraction as high as 95% (by volume), and woven fabrics (e.g., cotton, nylon, glass felt, and ceramic fibers). In most applications a microporous material is preferred. In some applications, less thermally conductive metals, such as titanium alloy Ti-6V-4Al, could be used. In addition, other materials having a thermal conductivity of about 25 W/m K or less could be used. The preferred material will depend upon the specific application in which the membrane 13 is being used.
The membrane 13 is also preferably a hydrophobic material and/or has been treated with a hydrophobic agent, in order to prevent pore icing if the flow of purge gas Gp is or falls below a critical value and to reduce the likelihood of condensation on the outer surface 14a of the wall 14, which can lead to surface frosting.
Another important parameter in selecting the membrane material or membrane laminates is the velocity and the volume of the purge gas Gp passing through the membrane under normal operating conditions. It has been determined that, under typical North American office or manufacturing plant conditions (i.e., relatively stagnant air, typical North American moisture levels), a “face” velocity, i.e. the velocity of the purge gas Gp crossing the membrane 13, of at least 1 cm/second and a volumetric flow rate of the purge gas Gp of at least 1 cm/second/per square centimeter of membrane surface area are preferred. This means that each square centimeter of the membrane 13 may exude at least 1 cubic centimeter of purge gas Gp per second. Under these conditions, the flow of dry purge gas Gp prevents the ambient, moisture carrying air on the discharge side of the membrane 13 from coming into contact with the outer surface 14a, which prevents frost from forming on the outer surface 14a even if the temperature of the outer surface 14a was to drop to cryogenic temperatures. The preferred purge gas Gp face velocity and volumetric flow rates would be higher for more humid ambient air and/or windy conditions.
A purge gas Gp is introduced at an elevated pressure into the purge volume 15 through the T-connection 17 (also called a gas inlet). The inlet pressure of the purge gas Gp is sufficient to overcome frictional losses in delivery tubing and fittings, fill the purge volume 15 and to cause the purge gas Gp to flow out of the purge volume 15 through the membrane 13. In this embodiment, the casing is configured so that the purge gas Gp will diffuse through the membrane 13 at the inlet pressure of 345 millibar (5 psig) or higher. It should be understood that the internal pressure required for crossing the membrane will be less than the inlet pressure due to the aforementioned friction losses.
To be effective, the membrane 13 should act as a pressure barrier. In this embodiment, the pressure drop across the membrane 13 is preferably at least 2.5 millibars, assuming that the cryogenic fluid supply line 10 is carrying cryogenically cold nitrogen or argon and the purge gas Gp is supplied at roughly room temperature (e.g., 20-25 degrees C.). The minimum preferred pressure drop is determined by the desire to maintain a relatively uniform outward flow of the purge gas Gp throughout the membrane 13, regardless of orientation, gravitational forces, winds, or air currents in the immediate area surrounding the membrane 13. Although there is no absolute upper limit for the pressure drop across the membrane 13, it is desirable to use membrane materials and membrane thicknesses that do not require unnecessarily high pressure differentials, which would require higher purge gas Gp pressures to achieve desirable flow characteristics. Increased gas pressure results in increased thermal conductivity, which would be counter-productive to the objectives of the present invention. In most applications, a pressure drop (positive gauge pressure) in the range of 2.5 to 500 millibars (0.25 to 50 kPa) is preferable.
As used herein, the term “minimum operating pressure” means the minimum pressure at which the purge gas Gp must be supplied to cause the purge gas Gp to diffuse through the membrane 13. As explained herein, the minimum' operating pressure will vary from embodiment to embodiment of the invention due to, among other variables, differences in casing configuration, wall thickness, surface area and the size and volume of the interconnected pores in the membrane.
Preferably the composition of the purge gas Gp, including any minor components or impurities it carries, is selected so that it will not condense on the surface of a cryogenic fixture (e.g., cryogenic fluid supply line 10) in the normal operating pressure range of the purge volume 15. For this reason, it is preferable not to use carbon dioxide as a purge gas or purge gas additive with cryogenic fixtures carrying LIN or LAR. Similarly, it is preferable not to use Argon as a purge gas or purge gas additive with cryogenic members carrying LIN. For example, in this embodiment, gaseous nitrogen would be a suitable purge gas Gp for a tube 11 carrying LIN.
In addition, the purge gas Gp preferably has a low relative humidity (e.g., a relatively humidity corresponding to a dew point, of less than −60 degrees C/213 degrees K) so that water vapor from the purge gas Gp does not condense and/or freeze on the outer surface 14a of the membrane 13 or inside pore channels.
As discussed above, the flow rate of purge gas Gp through the gas-permeable membrane 13 is preferably sufficient to prevent ambient air from coming in contact with the outer surface 14a of the membrane 13, which prevents the formation of frost on the outer surface 14a of the membrane 13. The preferred flow rate may vary substantially in other embodiments of the present invention. Although small amounts of leakage (for example through the plug 16 and/or T-connection 17) may occur, it is preferable that substantially all of the purge gas that is supplied to the purge volume 15 flows out of the purge volume 15 through the membrane 13.
Spray bar 400 uses a cryogenic fluid delivery system that is very similar to the that used in U.S. patent application Ser. No. 11/846,116, which is hereby incorporated by reference as if fully set forth. Cryogenic fluid is supplied to the spray bar 400 through two cryogenic fluid feed lines 411a, 411b. Throttling gas is supplied to the spray bar 400 through two throttling gas feed lines 423a, 423b. Cryogenic fluid flows from the cryogenic fluid feed lines 411a, 411b into an inner tube 449, through openings 451 formed in the inner tube 449, into a mixing zone 450, which is a volume located between inner tube 449 and an outer tube 448, then exits the spray bar 400 through an elongated slot 420 (discharge nozzle) in the outer tube 448. Alternatively, a row of small discharge holes may be provided in the outer tube 448, instead of the elongated slot 420. Similarly, the throttling gas flows from the throttling gas feed lines 423a, 423b, into the mixing zone 450, then exits the spray bar 400 through the elongated slot 420. As is more fully explained in U.S. patent application Ser. No. 11/846,116, the characteristics of the cryogenic fluid discharge 421 that exits the spray bar 400 through the elongated slot 420 can be controlled and manipulated by varying the pressure of the throttling gas provided to the throttling gas feed lines 423a, 423b.
The spray bar 400 includes a casing 413 which houses the throttling gas feed lines 423a, 423b, the cryogenic fluid feed lines 411a, 411b, and purge gas feed lines 446a,446b. The casing 413 comprises a nonporous structure 424 (such as aluminum or copper or stainless steel channel) having a top 425 and bottom 426, a first end, 427 and second end 428, and a discharge section 429 opposite a manifold section 430. The discharge section 429, includes an elongated opening 418, which is preferably larger than the elongated slot 420 (discharge nozzle) of the outer tube 448. Angle members 431a and 431b are fixed to the casing 413 and positioned inboard from the discharge section 429 to provide a gap or space 434a and 434b there between.
The casing 413 also includes a first wall or microporous lip 432a, is fixed within and extends downward from space 434a so that lip 432a overlaps or extends into elongated opening 418. A second wall or microporous lip 432b, is fixed within and extends upward from space 434b so that lip 432b overlaps or extends into elongated opening 418. In alternative embodiments, the microporous lips 432a,432b could be bonded or otherwise fastened to the casing 413 and angle members 431a and 431b could be omitted (as shown in
Each lip 432a and 432b includes a tapered end 433a and 433b that engages the outside surface of outer tube 448, and seals chamber 415 (purge volume) adjacent the elongated opening 418. In this embodiment, the tapered ends 433a and 433b are angled to provide an opening that expands in the direction of flow of the cryogenic fluid discharge 421 (from left to right in
The manifold section 430 includes a baffle 444 within casing 413 that forms a plenum 445. The plenum 445 provides an evenly-dispersed flow of purge gas Gp from feed lines 446a, 446b, to the discharge section 429 of the casing 413 and through the microporous lips 432a, 432b. The baffle 444 may comprise any suitable microporous material (as shown in
In this embodiment, the microporous lips (walls) 432a, 432b are located above and below the elongated slot 420, respectively, which provides a flow of purge gas both above and below the cryogenic fluid discharge 421. In general, it is preferable to have microporous walls located on at least two opposing sides of each opening of the discharge nozzle. In embodiments (such as this embodiment), where the discharge nozzle comprises an elongated slot, positioning the microporous walls above and below the slot is sufficient. In embodiments where the discharge nozzle comprises multiple round openings, it may be desirable to have a microporous wall encircle each opening.
Applicants performed test on spray bar 400 having a nozzle with an active spraying length (the slot length or the length of the row of round orifice holes) of 0.75 meters. At LIN spray rates of up to 218 kg/hour and a nitrogen purge gas flow rates ranging from 22.6 cubic meters per hour (800 scfh) to 51 cubic meters per hour (1,800 scfh), no frost formed on the outer tube, including the areas immediately surrounding the nozzle. In fact, under these test conditions, no frost formed on any part of the spray bar. During tests using a reduced nitrogen purge gas flow rate of 12.3 cubic meters per hour (434 scfh), about 10% of the outer tube developed frost around nozzle exits after 20 minutes of continuous LIN flow. During tests without any nitrogen purge gas flowing, the entire front side of the outer tube develops heavy frosting and icicles around the LIN nozzles within the first few minutes of operation. Therefore, cryogenic spray devices, such as the spray bar 400, are capable of operating for significant periods of time (e.g., well in excess of 30 minutes) in ambient air environments having significant relatively humidity (e.g., 30% and above) without any significant frost formation on the discharge nozzle or on the surfaces surrounding the discharge nozzle.
The interior chamber or purge volume 115 of the casing 113 is purged with an inflow of room-temperature inert gas Gp at an elevated pressure. The chamber or purge volume 115 is preferably sealed at both ends in a manner similar to that shown in
The flow of purge gas Gp through openings 118 prevents frost formation in same manner as the lips 432a, 432b of the spray bar 400. In addition, the purge gas Gp permeates the purge volume 115, diffuses through the wall 114 of the casing 113 and disperse into the atmosphere and to prevent a build-up of frost or ice on the exterior surface of the casing 113 in the same manner as the supply line 10.
The cryogenic spray devices 200, 300 shown in
In order to reduce measurement errors of low-temperature, reflective target surfaces due to radiation from the gas-permeable portions 532a, 532b, the purge gas Gp is preferably supplied in this embodiment at a temperature that is lower than the temperature of the surface being measured. For example, if the IR sensor 574 is being used to measure temperature on a reflective, i.e. shiny metal strip having a surface temperature of 0 degrees C, it would be preferable to supply the purge gas Gp at a temperature below 0 degrees C (e.g., −30 degrees C.). Further, in order to reduce measurement errors due to penetration of ambient light, the IR sensor assembly 500 is preferably placed as close as possible to the target surface being measured.
Adjustment of the throttling gas proportional valves 772 and 773 modifies the spray pattern dispersed from the discharge section 729 of spray bar 700. For example, when the liquid cryogen control valve 770 and throttling gas proportional valves 772 and 773 are at the full open position, no cryogen fluid 721 is dispersed from spray bar 700, only inert gas flows from the discharge section of the spray bar. On the other hand, when the throttling gas proportional valves 772 and 773 are completely closed with the cryogen valve 770 fully open, only cryogen fluid 721 is dispersed from discharge section 729, and the cryogen fluid is dispersed in an even or flat spray pattern. Accordingly, the throttling gas proportional valves 772 and 773 may be individually or uniformly adjusted to provide a different mixtures of cryogen fluid 721 within spray bar 700 that produce various spray patterns, for example spray pattern (profile) 761.
The PLC 762 is preferably adapted to control the cryogenic fluid valve 770, the purge gas proportional valve 771, and the throttling gas proportional valves 772 and 773. The PLC 762 could adjust valves 770 through 773 based on user input, a pre-programmed setting, and/or input from one or more of the process sensors 760a, 760b, through 760z.
As such, an invention has been disclosed in terms of preferred embodiments and alternate embodiments thereof, which fulfills each one of the objects of the present invention as set forth above and provides frost-free surfaces in cryogenic delivery systems. Of course, various changes, modifications, and alterations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention only be limited by the terms of the appended claims.
This application is the National Stage of International Application No. PCT/US08/74462, filed Aug. 27, 2008, which claims the benefit of U.S. Provisional Application No. 60/968,479, filed on Aug. 28, 2007, which is incorporated herein by reference in its entirety as if fully set forth.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US08/74462 | 8/27/2008 | WO | 00 | 7/23/2010 |
| Number | Date | Country | |
|---|---|---|---|
| 60968479 | Aug 2007 | US |
