US2219673A - Apparatus for operating cascade systems - Google Patents

Description

Oct. 29, 1940. G. H. zENNr-:R

lAPPARATUS FR OPERATING CAS-CADE SYSTEMS original Filed June 24', 153e 3 Sheets-She'et 1 INVENTOR- omn 29,v 1940.

Urginal Filed June 24.

G. H. zENNER APPARATUS FOR OPERATING CASCADE SYSTEMS Sheets-Sheet 2 ATTORNEYS Oct. 29, 1940. G, H, ZENNER 2,219,673

' APPARATUS FOR OPERATING CASCADE SYSTEMS v Original Filed June 24, 1936 3 Sheets-Sheet 3 INVENTOR Patented oct. 29, 1940 PATENT OFFICE APPARATUS FOR OPERATING CASCADE SYSTEMS George Hf Zenner, Kenmore, N. Y.,

The Linde Air Products Company,

assigner te New York.

N. Y., a corporation of Ohio Original application .June 24, 1936, Serial No.

86,954. Divided and this application October 27, 1938, Serial No. 237,199

4 claims. (ci. sz-1) This invention relates to apparatus for operating a cascade system having but one set of transfer vessels arranged in parallel for transferring material, volatile at normal atmospheric pressure, from a region of relatively low pressure to a region of relatively high pressure, and more particularly to an arrangement for operation using a single pair of transfer vessels for transferring a precious volatile liquid, such as liquid oxygen, liquid nitrogen, .certain liquefied hydrocarbons, and the like, from a storage vessel to a receiving device at a desired high pressure.

The invention has for its object generally an improved construction of operation for cascade systems of the character indicated, whereby a simplification of necessary parts is effected .and the number of operating steps reduced, in order to obtain lightness in weight and celerity of operation.

More specifically, it is an object of thefinvention to provide a cascade system with duplicate transfer vessels operating in parallel but simplified to avoid series and cross-equalization connections between transfer vessels and the steps incident to the eilecting of condensations of material from the gas phase into the liquid phase through such connections.

Another object is to provide a cascade systemof the character indicated with an improved cycle of operating events whereby a relatively large volume of a volatile liquid may be passed from a supply vessel to a receiver or vaporizer in a relatively short period of time with desired correlation of the flow of material in the gas and liquid phases without the provision of elaborate control apparatus.

Still another object is to provide an improved arrangement for systems of the character indicated whereby the blowdowns incident to the beginning of any cycle of charging and discharging a selected transfer vessel may be completed before final discharge of liquid from another such vessel when discharging liquid, thereby reducing the time required for operation of the system and increasing the capacity thereof.

Still another object is to provide a system of the character described with an improved heat exchanging device disposed in the liquid phase withdrawal connection and constructed to have a gas pass and heat storage arrangement Whereby the refrigeration of the liquid may be more efficiently utilized and retained in the system A in order to reduce the amount of losses incident to the blowdown.

This application is a division of my application,

Serial No. 86,954, filed Jime 24, 1936, now matured into Patent No. 2,157,103, issued May 9, 1939.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features o construction, combinations of elements and arrangement of parts which are adapted to eect such steps, all as exemplifled inthe following detailed disclosure, andthe scope of the invention will be indicated in the claims.

For a fuller understanding of the natine and objects of the invention reference should be had to the following detailed description takenin connection with the accompanying drawings, in which:

Fig. 1 is a view partly in section and partly in elevation showing a cascade system having duplicate transfer vessels adapted for the transfer of liquid oxygen provided in accordance with the invention.

Fig. 2 is a View similar to Fig. 1, but showing a modified arrangement of apparatus;

Fig. 3 is a view, mainly in vertical section, showing a regulator for accomplishing throttling in accordance with the invention;

Fig. 4 is a View partly in section and partly in elevation showing details of a heat exchanging or regenerative device of the character shown in Figs. 1 and 2;

Fig. 5 is a fragmentary view showing as a top plan the features of an end of the device shown in Fig. 4; and

`Fig. 6 is a vertical cross section through a modified form of heat exchanging device adapted for use in accordance with the invention A cascade system which is adapted to transfer a precious volatile liquid, such as certain liquefied hydrocarbons, liquid oxygen, liquid ni trogen, and the like, is described generically in U. S. Patent No. 2,035,399, issued March 24, 1936, in the name of J. J. Murphy, as a system of communicable vessels which receive and effect the countercurrent passage of such material in the gas and liquid phases by stages in such a manner that the liquid phase is advanced from a region of low pressure to a region of high pressure. Advantage of the countercurrent passage of material in the gas and liquid phases is taken to effect condensations of the material from the gas phase into the liquid phase so that losses incident to blowdown at the inception of a cycle of operation may be reduced to substantially any desired extent. Time is, of course, required for the gaseous material to be passed in heat exchanging relation with the liquid, the liquid being retained at some stage in a relatively stationary position While the condensation takes place, connections for the vessels being provided lto accommodate the liquid and the gas passage in such stage.

In the practice of the present invention, the apparatus is simplified by avoiding the use of a series of transfer vessels with the incident equalizing connections whereby a relatively light and inexpensive device is provided capable of operation at relatively high rates of speed.

'Ihe operation cycle of a transfer vessel as herein provided comprises two main steps or events, namely, filling with a desired charge and then discharging. During the discharge the gas material, which is at a relatively high pressure in the parallel vessel, is passed in heat exchang ing relation with the discharged liquid and conveyed to a storage space associated with the supply, this passage being made to take place with appropriate throttling, whereby a portion oi the gas phase may be conserved and returned in the liquid phase to augment the material being transferred. When a transfer vessel filled with gas at high pressure is thus blown down, it is ready again for the inception of a cycle 0f operation.

Simple apparatus for carrying out the present invention comprises duplicate transfer vessels arranged tov be operated in parallel to receive charges alternately from a common supply of liquefied gas, such as an' insulated storage container. The duplicate transfer vessels are arranged to discharge alternately into a common receiving device such as a vaporizer where the` material is converted into gas at a desired high pressure. For example, liquid oxygen may be vaporized in such a device and converted into gas supplied at 2000 psi. gauge.

Transfer vessels employed in cascade systems in accordance with the principles set forth in the above referred to patent to Murphy provide for the exclusion of heat of external origin from the material being transferred prior to a predetermined point, such as the vaporizer, in order to reduce the amount of material in the gas phase that must be blown down at the incep tion of a cycle. Heat exclusion is practiced in conjunction with the transfer vessels provided in accordance with the present invention, such vessels being provided either with interior baskets or with exterior insulating jackets or with both. It is desirable, however, to practice the heating of isolated portions of the liquid taken from a transfer vessel while being transferred in order to build pressure to accelerate the discharge. Convenient means for effecting this heating step in conjunction with a cascade system is disclosed in U. S. Letters Patent to Zenner, No. 2,037,673, dated April 14, 1936, such means being briefly described as a thermal leg.

The conservation in the system of the refrigeration of the liquid being discharged from a transfer vessel is conveniently effected by means of a heat exchanging or regenerative device, such as shown in U. S. Letters Patent to Dana, No. 2,037,679, dated April 14, 1936. Such device in general comprises countercurrent passes provided with intervening heat storage elements, the passages being associated with the gas and liquid withdrawal means of the transfer vessels, i. e.,

one pass is arranged to convey the gas phase supplied from a transfer vessel which has discharged its liquid and is filled With gas at relai tively high pressure, the heat of this gas being abstracted and transferred to the liquid passing in the other pass. In consequence, the input of heat incident to the ultimate conversion of the material into the gas phase is reduced. Here, the use of a regenerator with a relatively large heat storage effect is desirable because it avoids the necessity for relatively close control of the gas and liquid discharge rates from the respective transfer vessels.

In order to conserve the material in the gas phase thus being withdrawn, without the practice of equalizations, the gaseous material is led through the regenerator into a storage space associated with the supply, the material being deprived of heat so as to effect substantially complete condensation of the gas phase into the liquid. This last step of heat removal is effected by heat exchange with liquid being discharged and thev pressure reduced to that of the storage space. The preure reduction may be accomplished in any convenient manner, for example, by throttling. A convenient device for accomplishing this comprises a pressure reducing valve so placed as to discharge the cooled gas mate- Yrial that'has passed through the regenerator,

into the storage space of the storage vessel. The vessel in such case is formed to have dimensions adapted to this purpose. Any pressure added in consequence of such introduction into the storage vessel is seen to assist in building a pressure for expelling the liquid; excess pressure, of course, being blown down to the atmosphere. A suitable chamber, however, may be provided instead, to receive the cooled gas material, and whenso provided takes the place of the space in the storage vessel.

Referring now to the drawings, and particularly to Fig. 1, I0 denotes an insulated storage container for the volatile material which, in the case of liquid oxygen and the like, is relatively highly insulated. A Withdrawal connection II leads to a manifold I2 which has inlet connections I3 and I 3' leading respectively to transfer vessels I4 and I4', the inlets being controlled by suitable valves as indicated at I3rc1 and I3, respectively. The transfer vessels are also provided with blowdown connections I5 and I5' that are controlled respectively by valves I5a and The transfer vessels here provided are constructed to withstand considerable pressure, and, consequently, have relatively heavy outer walls. In order to exclude heat to the desired extent, these vessels, when oxygen is being transferred, have both inner baskets and exterior insulating jackets. Liquid withdrawal connections for the vessels are shown at I6 and I6 which are controlled respectively by valves shown at I6a and Ilb. These connections have a common outlet II which communicates with the inlet end of the liquid pass of the regenerative device shown generally at I8. From the outlet of the liquid pass a connection I9 leads to the heating coil of a receiving or vaporizing device shown generally at 20; the vaporizing device having a service connection or pipe line 2| which leads the vaporized material away to a suitable storage receiver or to a place of consumption at high pressure.

In order to practice the heating of isolated portions of material Withdrawn from the transfer vessels, a common thermal leg is provided comprising a heating element shown generally at 22, which has an inlet 23 at its lower end that communicates with an auxiliary withdrawal connectlon 24 that communicates with both transfer vessels, the communication being controlled respectively by valves as shown at 24"- and 24h. At the upper end of the heating element, a connection 25 communicates with the gas space of the transfer vessels through a common connection 2i, the communication with the two vessels being controlled respectively by valves 26 and 26h. To afford relief in the event undesirable high pressures obtain in the thermal-leg a by-pass 21 is provided leading from the upper end of the thermal leg directly to the connection t3, such by-pass being normally closed by a valve 21. Another by-pass 28 is provided leading from the withdrawal connections I8 and I6 which also communicates with the connection I9, this second by-pass being normally closed by a threeway valve 28 located at the junction and arranged to shut olf the branch of conduit I9 leading to the regenerator 18 when opening communication between conduits 28 and I9.

In order to provide for the blowdown of gas from a transfer vessel through a regenerator to the storage space, as proposed herein, a connection 36 is provided leading to the inlet of the gas pass of the regenerator I8, this inlet being adjacent the liquid outlet. The connection 30 is made to have selective communication with the transfer vessels through a manifold 3I whose communication is controlled respectively by valves 3la and 31h. From the outlet of the gas pass of the regenerator i8 a connection 32 is arranged to discharge into the space above the liquid level in the storage container Ill, the discharge into this space being at a temperature and pressure materially below that of the transfer vessel supplying the gaseous material, the pressure reduction being effected by throttling produced by a suitable means, such as the pressure regulator shown at 33 having an orifice 33' (Fig. 3) introduced at a point just beyond the point where regulation is accomplished. A suitable auxiliary control valve may be introduced in connection 32 as shown at 34, the container being also shown as provided with a filling connection 35 and a. relief discharge device 35X.

The mode of operation to be here practiced with the arrangement abovel described is as follows: A charge of predetermined volume of the material in the liquid phase is drawn from the container Il! and run into a vented transfer vessel, say vessel Il, by the opening of valve |31; the venting having been eected by the opening of valve l5=L ,inst prior to the lling event. Vessel Il' is, of course, lied with material in gas phase remaining from a previous operation. Valve I5L and valve I3E are closed at the conclusion of lling, after which discharging to the vaporizer 20 may be begun. This is effected by opening valve IGI. This discharge is accelerated, of course, by putting the thermal leg into operation, which is effected by opening valves 24e and 26B, the corresponding valves leading to vessel I4' being maintained closed.

As soon as the discharge is commenced, material in the gas phase at high pressure is led from vessel It' through the gas pass of regenerator I8. This is effected by opening valve 3|". The gas thus released passes into the gas space of container lll through reducing valve. 33, the valve shown at 3i being normally open.. This passage is continued until pressures in vessels I4' and lll are equalized, or until it is necessary to begin filling vessel I4', when it is vented by opening valve |511, the valve 3 Ib having been closed; With vessel I4 vented, the operating cycle is repeated by iilling and discharging this vessel. The liquid thus discharged through the regenerator countercurrent to the direction of the passage of gas causes a relatively complete liquefaction of the gas before exit from the regenerator 'and relatively continuous operation of the system is thus possible.

In the arrangement here shown, no special means for determining the charge admitted to a transfer vessel is shown since any convenient means may be employed; many suitable metering means being known in the prior art, for example, a venting connection may be used which depends into the transfer vessel a predetermined distance and seals off the escape of gas through the vent upon the attainment of a predetermined liquid level. Such means is shown in the patent to Murphy above referred to.

The metering means, whether it comprises the depth ltube arrangement or an independent metering device, is adjusted differently for the present system fromr that in the system of the aforesaid patent to Murphy since here it is not necessary to provide as much space for expansion of liquid caused by heating and for additions of liquid added during condensations prior to final discharge. The duplicate vessels, in consequence, may be more fully lled than heretofore, that is, the charge of liquid material introduced may be made to occupy substantially the full liquid holding space. In this way, the filling enlciencies for cascade systems are increased by the present invention.

While substantially any countercurrent regenerator having separate passes and the requisite heat capacity may be employed, it is found that regenerators which divide the passing gas stream into a. plurality of little streams surrounded by the body of liquid passing in the other direction accelerate the heat transfer and are highly desirable. A convenient arrangement of this character is illustrated in Figs. 4 and 5 and symbolically indicated as embodied in the systems of Figs. l and 2. Here a plurality of small tubes 36 are disposed between headers 31 and arranged to pass gas. About each tube 36 is a tube 38, the group of the latter being connected between headers 39 that are arranged parallel to headers 31 and pass liquid.

Another form of regenerator which may be employed is shown in Fig. 6 and comprises an outer shell of metal 38 for the liquid pass which encloses a plurality of small tubes 36 beginning and ending in headers disposed respectively at the ends of the shell, the tubes being preferably maintained in a desired spaced relation by Wrappings of wire as indicated at 31x. The greater the number of tubes of this character which are used the greater will be the amount of heat transferred since it multiplies the heat conducting surface which intervenes between the gas and liquid phases.

The regenerator of the present invention'is designed to exchange a relatively large part of the heat in the gas at high pressure remaining in a transfer Vessel at the conclusion of a liquid discharge period with the refrigeration in the liquid discharge from the other vessel, regardless of whether the temperature and pressure conditions for heat exchange remain an optimum in the gas and liquid passed in the exchanger throughout the period and regardless of whether the two are always in simultaneous passage. Accordingly, the mass of metal that comprises the tubing of the gas pass is an important factor since it is desired to have enough metal in the tubing to store the heat absorbed from the gas from the instant liquid ceases to ilow from one vessel until resumed from another, or to store refrigeration of the liquid from the .instant gas ceases to flow from one vessel until resumed from the other. While enough metal to provide the regenerator with the desired storage capacity to take care of these overlapping periods is desirable, it is also important that an excess of metal be avoided in order to preserve a desired degree of lightness.

Among the factors which determine the amount of material required in the regenerator are the magnitude of the blowdown loss permissible, the heat unbalance in the masses of the liquid and gas which are passed, and the magnitude in the temperature fluctuation from initial to nal conditions. While it is desirable to achieve lightness and reduce the mass of the regenerator to a relatively low value, each reduction in the amount of metal employed results in an increase in the losses resulting from uncondensed gas material. It is seen, in consequence, that economical operation determines a lower limit in the amount of vmetal employed in connection with the regenerator. In connection with the heat unbalance between the mass of the gas and liquid passed, no adjustment is readily available for maintaining optimum heat transfer conditions. The excess of heat to be supplied by the blowdown gas or the excess refrigeration to be taken out from the liquid passed must be compensated for under these conditions. The metal in the regenerator consequently should have suiiicient heat `capacity to take up this excess and act like a thermal ywheel taking over excess heat during one portion of the cycle and delivering it at another time. 'Ihe amount of unbalance in the heat transfer therefore largely determines the upper limit of the mass required for the regenerator.

With reference to the magnitude of the temperature iiuctuation it is seen that under the best countercurrent conditions Without heat storage in the heat exchanger the discharged iluid should issue from the regenerator at successively lower temperatures, the discharge at the end of the cycle being substantially as cold as the liquid which enters. When the mass of the regenerator is suiiicient, the outlet temperature of the discharged iiuid will remain nearly constant. This in short makes greater use of the lower temperatures available for refrigeration.

The diameter of the tubing while preferably small is not so small that the resulting frictional resistance offsets the effect of rapid heat transfer or the advantage had by the increased coeillcient of heat exchange resulting from the arrangement of the tubing here employed. Since length is the factor which determines the pressure drop. it substantially determines the limit of the smallness of the tubing that can be utilized. A balance of these factors within the range of reasonable economy is a matter that may be attained by skill in design.

The regulator provided between the regenerator and the gas receiving space is shown in detail in Fig. 3 and is of a character adapted to keep a constant pressure on the material passed to the blowdown orifice. The pressure drop through the regenerator is preferably as small as can be feasibly employed while that through the orifice and regulator is substantially the whole drop so that only liquid reaches the orifice. substantially all gas having been condensed. The constant head of pressure maintained by the regulator should cause a constant amount by weight of material to pass the orifice. Such an arrangement is found to facilitate the transfer of heat in the regenerator. 'I'he actual pressure drop thus produced varies from time to time in the operation of the system. For example, at the beginning of a blowdown period when the pressure is high, there is a relatively small pressure drop through the regenerator. Later during the period of this blowdown, as the velocity of the gas in the regenerator increases, the pressure drop in the regenerator increases, and that due to throttling decreases since only a small drop is now required of the regulator. There comes a time, however, when the regulator is wide open and ceases to maintain the passage of a constant amount of gas material by weight. After this, the rate of ow and pressure drop in the regenerator decrease. 'I'he maximum pressure drop occurs when the regulator has just reached the wide open position and before the weight-rate of flow has begun to drop olf.

While the supply vessel or container shown in Fig. 1 is indicated as constructed to provide not only the space desired to hold the quantity of material in the liquid phase but also to receive the condensed material blown down from a transfer vessel whereby there is provided a single source to supply the transfer vessels, it is contemplated employing containers adapted to accommodate only the liquid supply material, the storage space being externally provided in the form of aseparate chamber and associated to receive the condensate resulting in a manner such that it may be supplied to the transfer vessels in parallel with the liquid from the supplyV vessel the same as if it had been delivered originally from the supply vessel. Such separately provided chamber may be any suitable vessel that is insulated and connected to the transfer `vessels to receive blowdown gas material and supply condensate through connections arranged in parallel with the supply vessel. An arrangement of this character is shown in Fig. 2.

In Fig. 2, a supply vessel for liquid material is shown at 40 which has a liquid withdrawal connection 4I leading to a manifold 42 having inlet connections 43 and 43 for introducing liquid to the transfer vessels 44 and 44 selectively. Venting connections for the vessels 44 and 44 are shown at 45 and 45' respectively. Likewise, these vessels have liquid withdrawal connections 46 and 46' leading to the common connection 41 that communicates with the inlet end of the liquid pass of a heat exchanger I8, a connection 49 leading from the other end of the liquid pass to any suitable receiving means such as a vaporizer 50 that has a connection 5| leading to gas storage or consuming devices.

I'his modified form is also shown as provided with a thermal leg comprising a heating element 52 connected with the liquid space of the vessels 44 and 44 through connections 53 and 54 and with the gas space through connections 55 and 55. A by-pass 5l leads from the thermal leg directly to the connection 49. A second by-pass 58 is also shown leading from the liquid withdrawal connections 46 and 46 directly to the connection 49. By means of this second by-pass and three-way valve 58 the heat exchanger is cut out when desired. All of these connections are provided with control valves, the same as in Fig. 1.

In the arrangement shown in Flg. 2, a connection 60 is provided, which has selective communication through the manifold 6| with the gas space of the transfer vessels 44 and N', to withdraw gaseous material at high pressure from the transfer vessels to blow down the same. The connection 60 leads to the inlet end of the gas pass of the exchanger I8. From the outlet end of the gas pass a connection 62 leads to an insulated vessel 63 which receives the gas at a point external to the supply vessel 4U. 'I'he vessel 63 has connections 64 and 64 adapted to lead condensate from the lower portion of the vessel 63 and introduce the same in the liquid space of the transfer vessels, these connections being individually controlled by valves shown at 64a and 64b respectively, whereby the condensate from the vessel 63 may be introduced into a selected transfer vessel.

The vessel 63 is here arranged to receive the conv densate resulting from the temperature and pressure drop produced by a reducing valve 66 inthe connection 52 similar in character and position to that shown in Fig. 3. A normally open auxiliary valve B'l is also preferably inserted in the connection adjacent valve 6B; the insulated vessel being also provided with a suitable pressure relief device 68.

The mode -of operation of the modification shown in Fig. 2 is similar to that described in connection with Fig. l, since the parts described are similarly shown and situated with the exception of the arrangement of the means for receiving and storing condensate. A detailed description of the steps of filling and discharging the vessels 44 and 44 is therefore omitted. In this connection, it will be observed, however, that during the filling step when liquid is introduced in a transfer vessel from the supply vessel 40 and preferably after the step is completed, the liquid connection leading from the liquid space of the storage chamber to the transfer vessel is opened in order that liquid condensate from the vessel 63 may supplement that from the vessel 40.

Since certain changes in carrying out the above process and in the constructions set forth, which embody the invention, may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A heat exchanging device for use in conserving a gas phase generated from a liquid phase of a volatile material such as liquid oxygen when transferred in a cascade system, comprising, inv

combination, a liquid conveying means having a liquid inlet connection at one end and a liquid outlet connection at the other end, a pair of headers cooperatively disposed with respect to said means, one header having a connection serving as a gas inlet being disposed adjacent said liquid outlet, the other header having a connection serving as a gas outlet and disposed adjacent said liquid inlet, and a plurality of tubes extending between said headers for passing gas through said liquid conveying means, said means being constructed with walls of a total mass adapted to provide storage capacity for refrigeration, the total of lwhich is in excess of that inherent in said liquid conveying means in the portion through which said tubes are passed, whereby the gas normally passed is cooled to a temperature in the neighborhood of the dew point for a predetermined period when gas and liquid may not be passing simultaneously.

2. A heat exchanging device for use in conserving a gas phase generated from a liquid phase of a volatile material such as liquid oxygen when transferred in a-cascade system, comprising, in combination, a plurality of enlarged tubes provided at one end with a common connection having a liquid inlet and at the other end with a common connection having a liquid outlet, a pair of headers-disposed exteriorly of said tubes adjacent to said connections, the header adjacent said liquid outlet being arranged to serve as a gas inlet means, the other header adjacent the liquid inlet being arranged to serve as a gas outlet, and a plurality of tubes of relatively small diameter, each disposed concentrically within one of said enlarged tubes and communicating with said headers, said inner tubes being proportioned to have a length and mass adapted to provide storage capacity for refrigeration, the total of which is in excess of that inherent in said liquid conveying means in the portion through which said tubes are passed, whereby the gas normally passed is cooled to a temperature in the neighborhood of the dew point during relatively brief periods when liquid and gas may not be passing simultaneously.

3. A heat exchanging device for use in conserving a gas phase generated from a liquid phase of a volatile material such as liquid oxygen when transferred in a cascade system, comprising, in combination, an outer element consisting of a plurality of enlarged tubes having at one end a common connection provided with a liquid inlet and at the other end a common connection provided with a liquid outlet, a pair of headers cooperatively disposed with respect to said common connections, the header disposed adjacent said liquid outlet having a connection arranged to serve as a gas inlet, the other header adjacent said liquid inlet being arranged to serve as a gas outlet, and a set of relatively small tubes disposed within said first-named tubes and adapted to pass gas, said set of small tubes having a length and total mass sufcient to provide storage capacity for refrigeration, the total of which is in excess of that inherent in said liquid conveying means in the portion through which said tubes are passed, whereby the gas normally passed is cooled to fa temperature in the neighborhood of the dew point when gas and liquid may not be passing simultaneously and retains the temperature fluctuation between predetermined limits.

4. A heat exchanging device for use in conserving a gas phase generated from a liquid phase of a volatile material such as liquid oxygen when transferred in cascade systems, comprising, in combination, a liquid conveying element consistlng of a plurality of enlarged tubes having at one end a common connection provided with a liquid inlet and at the other end a common connection provided with a liquid outlet, said enlarged tubes excess oi' that inherent in said liquid conveying means in the portion throughwhich said tubes are passed, whereby the gas normally passed is cooled to a temperature in the neighborhood o1' the dew point during short periods when a predetermined 5 temperature uctuation may take place.

GEORGE H. ZENNER.

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