US2760356A

US2760356A - Method of liquefying gases

Info

Publication number: US2760356A
Application number: US347513A
Authority: US
Inventors: Sixsmith Herbert
Original assignee: Nat Res Dev
Current assignee: National Research Development Corp UK
Priority date: 1952-04-22
Filing date: 1953-04-08
Publication date: 1956-08-28
Anticipated expiration: 1973-08-28
Also published as: FR1076409A; GB727904A; DE928954C; BE519364A; CH339635A

Description

1956 H. SIXSMITH 2,760,356

METHOD OF LIQUEFYING GASES Filed April 8, 1953 3 Sheets-Sheet l INVENTOQ v HI mseaeer s/x SMITH 1956 H. SIXSMITH METHOD OF LIQUEFYING GASES 3 Sheets-Sheet 2 Filed April 8, 1955 mm mm INVENTO? HEEBERT S/XSM/TH A Tree vs YS Aug. 28, 1956 H. SIXSMITH 2,760,356

METHOD OF LIQUEFYING GASES Filed April 8, 1953 3 Sheets-Sheet 3 IN VEN 70? #525527 SIXSM/Th AMMLK.

A TTOENEVS United States Patent 2,760,356 METHOD OF rroonuvnsn AsEs Herbert Sixsmith, Reading, England, assignor National Research Development Gorp'oratiini, Boudoir, England, a corporafion of Great Brit'ain Application April 8, 1953, Serial No. 347,513 Claim s priority, application GreatBi'itainAprilfZZ,1952 7' c ai or, 61-175 This invention relates to a method of and apparatus for' liquefying' gases, for example air. 1

It is'kriown to liquefy gases by compressing. the gas, expanding a part of it adiabatically, for exampl athi bin'e", and effecting heat exchange in a condenser between. the expanded and the unexpanded parts so as to condense the latter, the whole of the compressed gas being first cooled by heat exchange with the expanded gas exhausted, from: the condenser. This method has the drawback that the. mass flow of warm gas through the first stage heatex changer or cooler is greater than that of the cold expanded gas exhausted from the condenser, so that the temperature difference between the expanded part and the compressed part at'the cold end of the said cooler is undesirably large, resulting in a loss of efiiciency.

Broadly stated, the invention consists ofarranging for the net mass flow of compressed gas through the first stage heat exchanger or cooler to be substantially equalto the mass flow of expanded gas therethrough. In this way, maximum efiiciency of heat exchange is obtainable, and the temperature difference between the incoming expanded gas and the efiluent compressed gas is Since, however, the flow of expanded gas is equal to the total available compressed gas less the fraction liquefied (neglecting any losses due to condensation of water vapour and sublimation of CO2), the above-mentioned balance of mass flow through the first stage cooler can; only be achieved if a proportion, equal to the fraction to be liquefied, is divided off from the total available compressed gas flow and either caused to by-passthe cooleror, having passed through the cooler with the; remainder, is recirculated therethrough in the reverse sense. In the latter case, the recirculating proportion isre-heated by the incoming gas, so that the net mass flow of compressed gas is substantially equal to the mass flow of expanded gas.

In the former case, the by-passing proportion may actually constitute the fraction to be liquefied, being-supplied direct to the condenser; alternatively, the fraction tobe liquefied is split off after the first stage cooler, and the equivalent proportion which by-passes the cooler is re united with-the balance of the compressed gas after this splitting oil and before expansion. In the recirculat'ory system mentioned above, the fraction to be liquefied is also split off from the cooled compressed gas, but after division therefrom of the recirculatory proportion, and this pro portion now re-heated, is reunited with the residual cold compressed eflluent from the cooler before expansion.

Where the gas to be liquefied isa permanent gas, for example air, it is desirable to remove, at as early a stage as possible, those constituents, such as water and/ or carbon dioxide, which solidify at the low temperatures involved. It is therefore convenient to arrange that the temperature at the cold end of the first stage cooler is suchthat the said constituents solidify in the cooler, and at least thecompress'ed' efiluent from the cooler is Where, however, the fraction to be liquefied, or an ec uivalent proportion thereto, by-pa'sse's the cooler, it is preferable to 2,760,356 Eatent d u 1956.

ensure that the gas is dry either at the input tothe compressor or at the point of division of the fraction to. be liquefied; or its equivalent proportion. Thisprevents the deposition of ice: or solid CO in the condenser on the one hand or in the turbine or other expansion device on the other. The recirculatory system mentioned above is thus seen to have an advantage over the other two alternatives according to the present invention, in that no special; preliminary precautions are necessary to dry the gas.

Where the gas issuing from the cooler is expandediin a turbine, the temperature of this cooled gas is in, certain cases" lower'than that at which maximum thermal efiiciency can be obtained from the expansion stage. In such cases, it is preferred to adopt a system in which the; balancing of the mass flows is achieved by splitting oil? a proportion of the compressed gas equivalent to the fraction to be liquefied, this equivalent proportion being already at, or then raised to, substantially the initial temperature and recombined with the cooled residual part of the gas which isfto be expahded' as this latter part passes tov the expansion stage. The effect of this splitting oh and recombination' of an equivalent proportion is to raise thetemperature of the. gas entering the expansion stage.

methods of the invention are efiiciently operable at eoin'par'ativelylow pressures and the compressed gas may, forexample, be at a pressure of between 8 and 15, at rn'ospheres. l

' The heat exchanger used in the invention is preferably of the're'g'enerative type, in which a number of chambers or compartments; are successively opened to gas flow in appropriate directions bya' valve device moving relatively to the said chambers or'com'partinents (hereinafter called; chambers for convenience) or to ducts serving them. At least one chamber is provided for the or each stream oi compressed gas which is to fiow through the. regen-v erator; and for the stream of expanded gas. It is desirable that the total regenerator volumes provided for the expanded and the compressed gas should be in inverse r'atio' to the respective gas" pressures. This would, however, involve providing an unduly large number of chambers, for the expanded gas. A convenient compromise where, for example, the compressed gas is at a pressure of 8 atmospheres, is to provide two chambers for the expanded gas and one for the compressed gas being. cooled, Where the mode of operation is such that the by-p'assing part of the gas is passed through the regenorator in reverse direction to the main stream of con pressed gas, a furthercharnber is provided for this part, In addition, as described below, it is convenient to provide an additional chamber, making five in all, in which, durin'g' any one regenerative step, the requisite pressure is built up to enable the reverse flow of by-passing gas to take place therein in the next succeeding regenerative step. 7

Where the adiabatic expansion is eficcted in a single stage turbine it is necessary that this shall rotate at extremely high speeds. In certain types of turbine the force of the air jets impinging on the rotor vanes has a component capable of exciting radial oscillations of the rotor when adequate provision for the damping of such oscillations has not been made. The invention comprises a turbine adapted for use in the above-described methods, in which at least one main bearing. for the rotor shaft is mounted ouFa member which is freeto carry out-small ing component of force of the air jets. It is furthermore an advantage of this arrangement that the amplitudes of oscillation of the rotor at the critical speeds are greatly reduced. The rotor shaft may be made long and slender, thus having a comparatively low heat conductivity. The bearings can thus be'maintained at a temperature sufficiently high to enable them to be lubricated. The turbine is preferably of the axial flow impulse type.

The invention as applied to the liquefaction of air'will now be particularly described with reference to the ,accompanying drawings, in which:

Figure 1 is a diagram illustrating the method of the invention, and

Figures 2 and 3' are diagrams illustrating modifications; J

Figure 4 shows, in diagrammatic form, an apparatus for carrying out the method of the invention as shown in Figure3; and

Figure 5 is a part sectional, part elevational view of a part of the axial flow impulse turbine used in the apparatus of Figure 4.

In Figure 1, an input stream of air A from a compressor and pre-cooler (not shown) (at a pressure of, for example, 8 atmospheres) is divided into first and second fractions or streams a, b respectively, and the first fraction a is passed through a first heat exchange stage or cooler E1, from which it passes to an expansion turbine T provided with an air brake T1. After leaving the expansion turbine, the expanded and cooled fraction enters a condenser E2 from which it flows in reverse direction through the first stage cooler E1 and thenceto atmosphere. Since the expanded gas stream is wholly constituted by the compressed fraction 11 (neglecting'losses due to condensation of water or sublimation of CO2) the etfective'mass flows in opposite directions through the first stage cooler E1 are substantially equal. The second fraction or stream b enters the condenser'Ez at atmospheric temperature. The temperature of the first fraction a leaving the turbine may be approximately 80 absolute, a temperature low enough to cause liquefaction of the compressed second fraction b in the condenser E2. The liquid is expanded in a valve V1 to atmospheric pressure and collected in a reservoir R from which it is tapped oif at intervals through a valve V2.

' In Figure 2, a subsidiary stream or proportion b, equivalent in mass flow to the fraction 0 to be liquefied, is

a split off from the input stream A before the latter enters the cooler E1 and passes to the turbine T. After leaving the first stage 'cooler E1, the cooled compressed gas splits into the residual stream a and the fraction 0 respectively. The proportion b is then recombined with the residual stream a. Thus, carbon dioxide and water vapour are removed in E1 from the residual stream a and the fraction c before passage through the turbine T and the con.- denser E2 respectively. Since the equivalent proportion b by-passes the first stage cooler E1, it serves to warm the fraction a so as to increase the expansion efliciency of the turbine.

In Figure 3, the equivalent proportion b is not split 01f from the input stream A until this has passed through the first stage cooler E1. Carbon dioxide and water vapour are thus removed from it, and it is passed in reverse direction back through E1 to be reheated before joining the residual stream a entering the turbine T. The fraction 6 be liquefied passes to the receiver R through the condenser B2.

In each of Figures 1-3 the massflow of the stream b is substantially equal to that of the air withdrawn as a Connection to the 4 v in plan although it is shown for convenience in a schematic quasi-developed form.

The main stream of air from a compressor 2, in which the air is compressed to a pressure of, for example, 8 atmospheres, passes through a pre-cooling and cleaning stage 3 in which it is cooled to atmospheric temperature. In the position of the rotary valve 1 shown in Figure 4, the cooled compressed air passes by way of this valve, through the valve port P1, in to the regenerator chamber A1 in which it transfers its heat to the packing therein and is cooled nearly to the temperature of the expanded air stream leaving the condenser in the manner to 'be later described.

The stream of cooled compressed air now passes via the valve Vs, the manifold 4, and a restriction R1 to the expansion turbine 5 in which it is cooled by adiabatic expansion. At D and B, respectively, an equivalent pro portion b and the fraction 0 to be liquefied leave the main stream and accordingly, between points B and C the stream constitutes the residual stream a referred to in the description of Figures 2 and 3. The cold exhaust air from the turbine 5 passes through the pipe 6 to a condenser 7, in which it absorbs heat from the fraction 0 which reaches the condenser via pipe 8 and valve 9. The expanded fraction flows via the valves V2 and V3 to the regenerator chambers Az and A2, and thence to atmosphere via the ports P2 and P3, The fraction 0 is liquefied by heat exchange with the expanded fraction in the con denser 7..

The equivalent proportion b, after leaving the main stream at D, flows through valve V15 into the regenerator chamber A5, through which it flows in reverse direction to the stream through the chamber A1. the chamber A5 through the port P5, the stream b splits at X. The main part b1 flows through a valve VM and pipe 10 to join the stream a at C. A subsidiary part b2, controlled by a constant flow regulator 11, flows by way of a valve P4 into the regenerator chamber A4 in order to build up the pressure in this chamber for a purpose described below. I

The rotary valve I slowly rotates so as,- for example, to bring the port P1 successively into line with theinlets' 0f the re'generator chambers A1 (as shown), A5, A4, As, An, A1 etc. Thus the infiowing air from the pre-cooler 3 is passed, for a predetermined interval of time, through the chamber A1, then for the same interval through the chamber A5, and so on until finally it is again'passed through the chamber A1. I

The function of the stream b2 can now be explained. In the position of the rotary valve 1 next succeed'mg that shown in Figure 4, it will be necessary for the stream b to pass through the valve V14 into the'chamber A4. In 'order to ensure that this valve is already opened, the

. regenerator chamber A4 is slowly filled with dry air provided by the stream b2. When the pressure in. A4

almost reaches the manifold pressure, the valve V14 drops 7 V13, V14 and V15 is provided with a cylindrical exten v sion 32 constituting a piston which is slightly smaller in diameter than the tube 33 in which it is free to move;

The annular space between the tube and the piston constitutes a restriction, and the pressure drop across the restriction tends to raise the piston, thus closing the valve. The value of the restriction is such that the valve remains open against the by-passing stream b, but closes when the flow slightly exceeds this value, as will happen in the caseof V14 when the port P2 reaches A4 and air from the manifold 4 rushes out to the atmosphere via V14, A4 and P2. 1 1

Condensed liquid and a slight excess of gas are continually and automatically drawn off from the condenser 7 by means of an automatic expansion valve 12 which is adjusted to give an output pressure slightly above atmospheric pressure, so that the flow rate can be con- After leaving trolled by means of an orifice 12a of normal dimensions. The liquid air is delivered to an automatic float trap 13 of conventional design, which discharges at intervals into a suitable receptacle. Excess air escaping with the liquid is returned to the turbine exhaust pipe 6 by way of a pipe 14.

It has previously been mentioned that a feature of the invention resides in the construction and design of the expansion turbine 5. In the apparatus of Figure 4, the

turbine is an axial flow impulse turbine, the rotor, stator, shafting and one bearing of which are shown in Figure 5. A shaft 15 upon which is mounted the rotor 16 also carries a centrifugal blower 17 which acts as the brake 18 shown in Figure 4. The blower is shown as a schematic block in Figure 5, since its design forms no part of the present invention. It may have the form of a drum having radial holes for air flow. High pressure air is fed via the inlet 19 to the stator blades 20 having a shroud ring 21 shrunk thereon to avoid tip leakage. A turbine according to the invention has been constructed, in which the turbine rotor is machined from a solid piece of brass and having a tip diameter of about the rotor blades being about A long and having a chord width of A". The shroud ring is of brass and the rotor is mounted on the shaft 15 which is of silver steel having a diameter of This shaft is about 4" long and is mounted in cup bearings at each end. The shaft 15 is machined conically at its end which seats in balls 22 located in the cup 23; the stalk 24 of the cup being hollow to allow for the passage of oil to the bearing.

An annular shoulder of the cup 23 bears on a radial compression spring 25, which in turn bears on the bottom of a cup 26. This spring 25 takes up the normal amount of differential expansion and effectively maintains a substantially constant load on the ball bearings thereby preventing play between the conical end of the shaft and the outer case of the hearing. A stem 27 integral with the cup 26 passes through an externally threaded sleeve 28 as shown, which sleeve is adapted to pass through a hole in the outer case (not shown) and to have a nut screwed thereon to support the shaft and bearing assembly within the casing. The stem 27 is hollow, its bore communicating with the bore through the hollow stalk 24 for the supply of oil thereto.

A small clearance is allowed between the cup 26 and the internal wall of the sleeve 28. Thus the hearing as a whole is capable of radial displacement, which is damped by means of two oil-filled dashpot mechanisms of which only one (29) is shown in the drawing and which are mutually at right angles. These dashpots are of conventional design, and the pistonrods 30 are connected to the cup 26 through a close-fitting ring 31. The bearing at the other end of the shaft 15 may be similarly constructed or, alternatively, may be similar except that no allowance is made for radial displacement.

In an alternative construction of the turbine, the cup 26 may be replaced by a sliding collar to which are attached the piston rods of three-dashpots at 120 to each other. In another construction, the piston rods are secured to the stem 27 instead of being attached to the cup 26.

If the regenerator chambers A1 to A are to be of the small size necessary to keep the bulk of the machine as small as possible, they require a packing material which is eflicient and simple to construct and has a high conductivity in the direction transverse to the direction of flow therethrough. A low thermal conductivity in the direction of the flow is desirable. In the present invention, it is preferred to use a packing consisting of discs of fine wire gauze, which discs are separated by discs of thermally insulating material such as cloth of open weave, for example, rayon net.

What I claim is:

l. The method of liquefying a gas comprising compressing the gas, dividing the compressed gas into two parts, a major part and a minor part, passing at least the major part through a first stage cooler to cool it, dividing the major part, after such cooling into two parts, a small part and a large part, after such division passing the small part through a condenser to liquefy it and after such division combining the large part with the minor part which is uncooled, expanding the combined parts adiabatically accompanied by the expenditure of energy to reduce their temperature, and passing the expanded combined parts first through the condenser in counterfiow with respect to the small part liquefied therein and secondly through the first stage cooler in counterfiow with respect to the compressed gas flowing through it, the mass flow per unit time of compressed gas flowing in one direction through the first stage cooler being substantially equal to the mass flow per unit time of gases passing through the first stage cooler in the other direction.

2. The method according to claim 1 wherein the division of the compressed gas into the major part and the minor part takes place before passing through the first stage cooler, the minor part being expanded as aforesaid along with the large part of the major part, Without the minor part passing through the first stage cooler.

3. The method according to claim 1 wherein the whole of the compressed gas is passed through the first stage cooler before division into the major part and the minor part, the minor part being then passed through the first stage cooler in counterflow with respect to the said whole of the compressed gas where the minor part is restored to the uncooled condition before expansion along with the large part.

4. The method according to claim 1 wherein the said expanded combined parts are combined with vapour rising from the surface of the liquefied gas before passing through the condenser in counterflow as aforesaid.

5. The method according to claim 2 wherein the said expanded combined parts are combined with vapour rising from the surface of the liquefied gas before passing through the condenser in counterflow as aforesaid and wherein the mass flow per unit time of the expanded combined parts together with the said vapour, passing through the first stage cooler in counterfiow as aforesaid, is substantially equal to the mass flow per unit time of the major part which passes through the first stage cooler.

6. The method according to claim 3 wherein the said expanded combined parts are combined with vapour rising from the surface of the liquefied gas before passing through the condenser in counterflow as aforesaid and wherein the mass flow per unit time of the expanded combined parts together with the said vapour, passing through the first stage cooler in counterflow as aforesaid, is substantially equal to the mass flow per unit time of the whole of the compressed gas passed through the first stage cooler as aforesaid minus the mass flow per unit time of the minor part.

7. The method according to claim 1 wherein the expansion of the said combined parts is effected in an impulse turbine from which the expanded combined parts emerge at a temperature at least a few degrees above their liquefaction point.

References Cited in the file of this patent UNITED STATES PATENTS 1,027,863 Linde May 28, 1912 1,264,845 Norton Apr. 30, 1918 1,901,389 Hazard-Flamand Mar. 14, 1933 2,165,994 Zerkowitz July 11, 1939 2,529,880 McClure Nov. 14, 1950 FOREIGN PATENTS 1,086 Great Britain Jan. 24, 1916 of 1915

Claims

1. THE METHOD OF LIQUEFYING A GAS COMPRISING COMPRESSING THE GAS, DIVIDING THE COMPRESSED GAS INTO TWO