Apparatus and method for pumping liquefied gaseous fluids

June 8, 1954

Filed Dec. 29, 1950

Η. O. MCMAHON

APPARATUS AND METHOD FOR PUMPING LIQUEFIED GASEOUS FLUIDS

2,680,352

Sheets-Sheet 1

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INVENTOR HOWARD O. MCMAHON

AGENT

June 8, 1954

Filed Dec. 29, 1950

Η. O. MCMAHON

APPARATUS AND METHOD FOR PUMPING LIQUEFIED GASEOUS FLUIDS

2,680,352

Sheets-Sheet 2

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FIG. S

INVENTOR

HOWARD O. MO MAHON

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Patented June 8, 1954

2_?680_?352

UNITED STATES PATENT OFFICE

2,680,352

APPARATUS AND METHOD FOR PUMPING LIQUEFIED GASEOUS FLUIDS

Howard O. McMahon, Lexington, Mass., assignor to Arthur D. Little, Inc., Cambridge, Mass., a corporation of Massachusetts

Application December 29, 1950, Serial No. 203,403

Claims.

This invention relates to means and method for pumping a liquefied gaseous fluid from a relatively low pressure to a relatively higher pressure without the expenditure of a corresponding amount of mechanical work. More particularly, 5 this invention relates to a pump means whereby the aforesaid pumping is performed primarily by thermal rather than by mechanical action.

It is well known that this objective can be accomplished by any one of a number of devices 10 known as warm converters. One such device may be visualized as a cylinder filled with a liquefied gaseous fluid and fitted with suitable check valves, into which a warm plunger is immersed. The heat from the plunger serves to ir, vaporize a sufficient amount of the liquefied gaseous fluid to elevate the pressure to any desired level. Such a device is irreversible in the thermodynamic sense and also in the operational sense because after one stroke the plunger is cooled off 20 and requires reheating before it can be used again. From the thermodynamic point of view, heat is transferred irreversibly (i. e. through a large temperature difference) from the heating member (the plunger) to the fluid receiving the heat. 25 Heat flowing across such a temperature difference occasions a larger entropy increase, which of course means that the device is very inefficient in the thermodynamic sense.

The pump of this invention is superficially similar to a warm converter of the type just described. ³⁰to the extent that a plunger is used as a source of sensible heat which serves to raise the pressure of the liquefied fluid. It differs, however, from the warm converter type of pump in the fundamental respect that the heat interchange ³⁵which occurs at every temperature level between liquefied fluid temperature and room temperature does so with a minimum temperature difference. In this respect it resembles a cold accumulator or regenerator, its function being ⁴⁰to minimize entropy changes. Stated in another way, it permits a close approach to reversibility both in the thermodynamic and in the operational sense.

At the present time it is understood that sue- ⁴⁵cessful pumps have been developed for pumping small quantities of liquid oxygen, or other liquefied gaseous fluid, to relatively high pressure by means of a fairly standard reciprocating mechanism, and for pumping relatively large volumes ° to low pressures by fairly standard centrifugal mechanisms; however no suitable pump exists for transferring large quantities of such a liquefied fluid to high pressures. A reciprocating pump 65 (CL 62—1) designed to deliver large volumes at high pressures would be extremely massive and cumbersome because of the very powerful forces which must be transmitted to a piston of large crosssection. Problems would also be encountered in designing a closely fitting piston and cylinder of large diameter and of sufficiently small clearance to prevent excessive leakage. Very heavy structural members would also have to be used in the crosshead and in the cylinder support to carry the high thrusts. These considerations all lead to the conclusion that a rotary pump would be more satisfactory than a reciprocating pump if a suitable design for the former could be worked out. In the pump described in the present invention, however, the action is reciprocating rather than rotary. Nevertheless the need for a closely fitting piston and cylinder has been eliminated and, since no appreciable amount of mechanical work need be done, no heavy forces need be applied. It follows that the chief objections to a reciprocating pump for raising a liquefied gaseous fluid to a high pressure are eliminated by mechanism of the present invention.

In order to pump a fluid from a lower pressure to a higher pressure, which is a primary object of this invention, work must be done on the fluid. In the case of the standard reciprocating pump this work is supplied mechanically directly to the fluid. In the present invention work is done indirectly, as in any heat engine, by absorbing heat at a higher temperature and discharging a portion of it at a lower temperature.

The present invention is useful in the treatment of any liquefied gaseous fluid, and while it is primarily useful for treating liquefied normally gaseous fluids such as oxygen, nitrogen, carbon dioxide, argon, hydrogen, “Freon,” sulfur dioxide, etc., which are gaseous at normal temperatures and pressures and are liquid at various subatmospheric temperatures, it is also applicable to the treatment of fluids which are normally in liquid form (e. g. water, ethyl alcohol, and various petroleum fractions) when there is present in the apparatus a source of heat sufficient to convert a part of said fluids to gaseous form. However, for convenience, the term “liquefied gas” will generally be used hereinafter, for convenience, in designating all such fluids.

This invention will now be described in more detail by reference to the accompanying drawings, wherein

Fig. 1 represents a side elevational view of a preferred form of the pump of the present invention, partly in section;

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Fig. 2 is a view similar to that of Fig. 1 but with the plunger at the other end of the stroke;

Fig. 3 is a side elevational view of the upper end of the pump showing one type of provision for increased heat exchange at the upper (warm) end;

Fig. 4 is a top view, somewhat enlarged, of one form of upper disk for the plunger of the pump; and

Fig. 5 is a schematic diagram showing the pump of the present invention in association with other apparatus suitable for transforming relatively low-pressure liquefied gas to relatively high-pressure liquefied gas and then vaporizing the latter.

The pump of this invention, as shown in detail in Figs. 1, 2, and 3, is indicated generally by the numeral 10. It comprises a pressure-resistant chamber ί I, which will ordinarily be in the form of a cylinder but which may, if desired, have a cross-section other than strictly circular. For convenience, however, this chamber will be generally referred to herein as a cylinder. This cylinder is entirely closed except for openings presently to be described, and is built to withstand the top pressures to which it is desired to compress the liquefied gas. Within this cylinder is a “piston” or, more properly, a plunger, 13, which is hollow and is filled with a. heat-conductive packing 15, such as metal turnings, chips, or pellets which are of such character and so packed as to provide high heat capacity, low resistance for fluid flow, small free volume, and low longitudinal heat conduction. The ends of the plunger are closed by foraminous disks 17, 19, which may be made of punched metal or of wire cloth, for example, in such a way as to permit ready passage of the fluid being pumped (either in liquid or gaseous form) while still retaining the packing IS within the plunger. To upper disk 17 is attached a “piston” rod or plunger rod 21, which actuates plunger 13 and which passes through hole 23 in top 25 of cylinder 11; this hole 23 is sealed by stuffing box 27. Also positioned at the top of cylinder 11 are the safety valve 29 and the blow-down line 31, both communicating with the interior of cylinder I ί. Line 31 is provided with pressure gauge 33 and controlled by valve 35. Rod 21 may be reciprocated to actuate plunger 13 by any conventional means, such as a manually operated handle 37 (Fig. 2) or any suitable crank means 39 (Fig. 3).

The bottom end of cylinder 11 is provided with an intake port 4! and an outlet port 43, controlled by valves 45 and 47 respectively.

If the top disk 17 is made of wire cloth, reinforcement means is desirable (unless the cloth is very heavy) to provide proper rigidity between rod 21 and plunger 13. One form of reinforcement is shown in Fig. 4, wherein axial braces 51 are fixed firmly at their ends to the upper edge of the walls of plunger 13, said braces crossing at point 53 at which rod 21 is attached; wire cloth 55 is positioned either over or under said braces and is attached at its periphery to the upper edge of the walls of plunger 13. It is not necessary to attach the wire cloth to the braces, although that may be done if desired.

Both cylinder 11 and plunger 13 are quite long in proportion to their diameters; the plunger is shorter than the cylinder so that the former is free to move longitudinally. The plunger has substantially the same cross-sectional area and shape as that of the inside of the cylinder but fits sufficiently loosely therein that there is no wall friction, or substantially none, in moving it up and down within the cylinder.

The lower end of the cylinder 11 and the fluid therein are maintained at substantially the temperature of the liquefied gas being pumped, while the upper end and the fluid therein are made relatively much warmer, say at room temperature, although this temperature may be appreciably higher or lower depending upon the nature of the liquefied gas being handled and the conditions of operation and results desired. The temperature at the lower end of cylinder 11 may be controlled by immersing the same in a bath of the same kind of liquefied gas as that to be pumped or by 15 any other suitable means. Temperature at the upper end may be controlled by any suitable means, such as by exposing the same to air at a suitable temperature; the effects of this air may be promoted by providing fins 57 around 20 the upper end of cylinder 11 and blowing air of the desired temperature over the fins by means of fan 59. On the other hand, if the liquefied gas is normally a liquid and becomes a gas only at higher temperatures than normal room tem25 perature, a still higher temperature may be employed for heating the upper end of the cylinder, e. g. a blast of highly heated air.

In the flow diagram shown in Fig. 5, the numeral 61 represents a hollow-walled vessel 30 (Dewar) filled with a body 63 of the same kind of liquefied gas as that to be pumped. Within this liquid body 63 is immersed the lower end of the pump 18. Safety valve 29, line 31, gauge 33, and valve 35 are provided, as already described, 35 at the top of cylinder 11; and 21 connects with the plunger within cylinder 11, also as already described.

A source 65 of liquefied gas to be pumped is provided, and leading from it is pipe 67 passing through supercooling coil 69 and vapor trap 71, both immersed in the liquid body 63. The vapor tran is provided, if desired, to minimize the possibility of “vapor lock.” Pipe 67 then enters the bottom of pump (6 through inlet 41. Also adjacent the bottom of pump 10 is outlet 43; both this and inlet 41 are valved as already described. From outlet 43 another pipe 73 leads out of the liquid body 63 to a vaporizer 75, which may be any conventional type of coil or the like capable of proriding adequate heat-exchange surface to cause the high-pressure liquefied gas therein to vaporize. The resulting gas, under the same high pressure, passes thereafter through pipe 77 to receiving bottles or cylinders 79, which after filling are removed for storage, shipment, and use, as desired. Pipe 77 is preferably provided with pressure gauge 81 and also with a blow-down valve 83.

Briefly stated, the operation of the pump of this invention comprises moving the plunger 13 repeatedly down and then up throughout its free stroke within cylinder 11, whereby the fluid being pressurized is caused to flow respectively upward and then downward through the plunger packing thereby becoming alternately Iieatedand cooled. The plunger acts as a thermal accumulator, passing as it does back and forth be'tween the warm end and the cold end of the pump.

In the operation of the present invention, the liquefied gas from source 65 flows through pipe 67, coil 69 and trap 71 into cylinder 11 through inlet 41, under pressure sufficient to cause stich flow and to open valve 45. Plunger 13 is positioned at the top Of its stroke, and the space below it within cylinder II fills with the liquefied

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gas. The plunger 13 is then lowered by a downward motion of handle 37 or crank 39. This requires no power other than to overcome frictional forces. As the plunger is lowered, some of the liquefied gas moves upwardly through foraminous bottom disk 19 and into the voids between the pieces of packing 15. This liquefied gas, as it moves upward, encounters progressively warmer portions of the packing and tends to vaporize, finally emerging as a gaseous fluid in the space above foraminous top disk 17. In this area it will have substantially the same temperature as that of the top of cylinder ί i. As the plunger 13 approaches the bottom of its stroke, the liquefied gas within cylinder 11 enters into· progressively warmer zones of the plunger where its vapor pressure consequently rises and the¹pressure throughout the entire interior of the cylinder finally reaches a value which causes the liquefied gas in the bottom part to be ejected through valve 47. Valves 45 and 47 are so set and arranged that the former closes when the pressure within cylinder 11 exceeds that in pipe 67, and the latter (47) opens when the pressure of liquefied gas has been raised to the desired high»!· level.

At the completion of the discharge stroke the plunger 13 has moved fully to the bottom of cylinder 11, and the free volume thus created in the upper part of the cylinder is filled with warmed gas at the pressure of the discharge through valve 47. The plunger is then raised to the top of the cylinder, by an upward pull on rod 21, whereupon the warmed high-pressure gas flows back downwardly through foraminous disk (7 and packing 15. In so doing, it reaches progressively colder parts of the plunger and packing, and thereby a substantial part thereof (in particular that which passes into and through the lower parts of the plunger and packing) becomes liquefied again. Accordingly, the pressure throughout cylinder I ί drops as the upstroke proceeds, valve 47 closes when the pressure drops below the desired output level, and finally the pressure reaches suction level, whereupon valve 45 opens and the free volume at the bottom of the cylinder again fills with liquefied gas from source 65, as before. This cycle is repeated as often as necessary to obtain the desired amount of high pressure product.

It is here again emphasized that the pressure is at all times the same both above and below plunger 13; hence the only force required to move the plunger is that needed to overcome the inevitable slight amount of friction. This slight friction is due almost entirely to stuffing box 27 and to the movement of liquid and gas through the foraminous disks and the packing. There is substantially no wall friction between plunger 13 and the interior of cylinder 11, as these are so constructed as to fit rather loosely.

The following will serve as an example of a specific operation of the present invention: Cylinder 11 was constructed of stainless steel, 2 inches i. e. and % inch thick wall. Plunger 13 consisted of a thin brass shell closed at both ends by perforated disks 17, 19 and filled with aluminum pellets so shaped and packed as to provide a void volume of 32%. The plunger length was 30 inches and its free stroke 6% inches. Clearance between interior cylinder wall and plunger was about 0.02 inch. The plunger was caused to move up and down by means of a thin stainless steel rod 21 entering through stuffing box 27 and actuated by hand. Simple ball check valves were used as valves 45 and 47, the balls being 0.025 inch in diameter and their seats 0.187 inch in diameter. The pump was operated as already described above, with associated apparatus as 5 shown in Fig. 5. The supply of liquefied gas was liquefied nitrogen, at about 10 p. s. i. g.; the body of liquid 63 was also liquefied nitrogen, but at atmospheric pressure. The pressure of the gas delivered to cylinders 79 was 1750 p. s. i., and was 10 prevented from being higher by the operation of safety valve 29. A complete up and down stroke of the plunger required a time of about V₂ minute. This could be speeded up by increasing the size of the ball check valves and their orifices, and 15 by warming the upper end of cylinder 11 (as shown in Fig. 3, for example)—since that end became appreciably colder than room temperature during the continuance of the operation described above. Another way of increasing the 20 frequency of stroke is to vent a small amount of the warmed gas from cylinder f through valve 35, while the cylinder is filling with liquid from source 65.

Various modifications of the above-described 25 invention may be made within the scope of the present invention. For example, the plunger may be any permeable mass of heat-conducting metal which will permit ready fiow through it of the fluid being treated, in both liquid and gaseous 30 form. And while the plunger preferably conforms generally to the inside walls of the cylinder, for best operation and efficiency, it may be of different shape and still permit operation in the manner herein described.