US2229438A - Method for controlling dry ice storage - Google Patents

Description

Jan- 21, 1941- w. T. BIRDSALL) 2,229,438

METHOD FOR CONTROLLING DRY ICEA STORAGE Filed Nov. 4 1957 2 Sheets-Sheet 1 @if Y Z Il P ll E c 6 Hl 4 lLl "J f D gl FII /l :l

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l/ f y /l/ l INVENTor? f f ,//fe/zzfdsaiz y dqceased 7M #WA M@ ATTORNEYS Jan. 2l, 1941. W T BIRDSALL 2,229,438

METHOD FOR CONTROLLING DRY ICE STORAGE Filed NOV. 4, 1937 2 Sheets-Sheet 2 Y umili,

Patented Jan. 21, 1941 UNITED STATES PATENT OFFICE METHOD FOR CONTBOLLING DRY ICE STORAGE Wilfred cr. irasau, deceased, me of Montclair,

N. J., by Alvin C. Blrllsall administrator, Washof New York Application November 4, 1937, Serial No. 172,720 10 Claims. (Cl. 6291.5)

'Ihis invention relates to methods for controlling and stabilizing storage conditions to prevent disintegration of low temperature refrigerants, such as dry ice and the like, whereby such materials can be stored for long periods of time.

In applicants copending application, Serial No. 172,715, led Nov. 4, 1937, it has been pointed out that processes for the storage of low temperature refrigerants in order to preserve the material in a useful form should embody three principal features, namely; protection of the refrigerant 'from the action of external energy so that it will not sublime, evaporate, or otherwise lose its usefulness, second, the material should l5 be enclosed so that dissipation of the vapor is prevented or restricted and third, the stored material should be maintained as nearly as possible in a state of thermodynamic equilibrium, or in other words, in a state in which changes in the relative amounts of solid orv liquid and vapor of the stored material do not change spontaneously.

' The present invention relates particularly to the latter of these three elements and embodies methods and means for controlling storage conditions to produce, maintain, or restore substantially constant relative amounts of the solid or liquid and the vapor phases of the stored material.

In said copendlng application it also is pointed 3 out that `there are six factors which must be stabilized in proper relations with respect to each other in 4order to maintain thermo-dynamic equilibrium in the material and preserve its usefulness substantiallyunchanged. Four of these factors, namely, the temperature, vapor pressure,

density and total heat of the material are propertles of every unit of the stored material while the remaining two factors, namely; the total weight of the material stored and the total volume in which the material is confined, are independent of the properties of the material and therefore may be arbitrarily fixed as desired for any particular installation, However all of the factors bear definite relations to each other and it is possible therefore to draw a diagram in which the simultaneous or corresponding values of the factors may be represented by ordinates and abscissa at a. given point. Such a diagram for mixtures of solid and gaseous carbon dioxide below the triple point, that is at absolute pressures belowapproximately l'l5 pounds per square inch and at temperatures below -70 F. is shown in diagrammatic form and not to exact scale as Figure 1 of the drawings. v

Each ot Figures 2 to 6 o! the drawings is a diagrammatic sectional view of an alternative form of means adapted for use in the practice of the present invention.

Referring to Figure -1, a scale O-X is established reading from left to right along which may be measured heat contents in any suitable scale, such as B. t. u. per pound of mixture of solid and vaporized dry ice. All points on the diagram lying on a vertical line, such as A-B, therefore represent conditions under which a body of carbon 10 dioxide may exist while having the same heat content. However, the diagram is intended for illustrative purposes only and no specific values have been applied to the scale. Moreover, the relations between the four factors referred to 15 above are not definitely known for the range wherein the values of the heat content and the vapor pressure approach absolute zero, and the relations of the factors at temperatures above the triple point in the case of carbon dioxide are 20 peculiar and are not ordinarily encountered in the storage of dry ice. Figure l, therefore, should be understood ite portray only those conditions existing between these limits and within the range of values usually encountered in the 25 practice of present invention in the storage of solid carbon dioxide.

On the left hand side of the horizontal scale O-X a vertical scale O -Y is erected having equal divisions representing vapor pressure in 3i* pounds per square inch absolute or in any other suitable scale.v Since the temperature of the material is so related to its vapor pressure that one increases as a function oi the other it is Yalso possible to erect a similar vertical scale X-Z at -on the right hand side of the diagram to represent temperature. 'I'he scales 0-Y and XZ. therefore, are so related that if the divisions of either represent uniform change then the divisions on the other represent a systematic butlo nonlinearscale of related changes.

Having erected scales along which to measure total heat, temperature and vapor pressure it is possible to draw a line M-N which represents the maximum yheat content which a pound of carbon dioxide can have while remaining entirely in the solid phase and within the limits of the diagram. Similarly it is possible to draw al1- other line such as P-Q which represents the minimum heat content which a pound of the, 5l material can contain while remaining entirely in' the form of vapor. With these lines located it will be evident that in passing from the solid line M--N to the vapor line P-Q along any line such asc-D, successive points on the line rep-.65

resent conditions in which more and more of the solid form. Moreover any selected point on the line C--D will represent specific conditions under which denite proportions of solid and vapor are present and incidentally will represent conditions' under which the mixture of solid and vapor When at that temperature Will occupy a denite volume.

The point so located may be indicated by the letter G and while this point can be determined, if the temperature and the vapor pressure are known, the proportion of the material that will be in the form of solid and vapor will varywith changes in pressure, temperature and total heat.

It is therefore necessary to describe a line such as F-G-H in order to represent all of the possible conditions under which one pound of a mixture of solid and vapor will occupy a predeterf mined volume, say 30 cubic inches. In like manner other constant volume lines, such as F'GH' and F"GH.' may be drawn on the diagram to represent conditions under which unit quantity of carbon dioxide will occupy other volumes.

Since the curves F-G-H represent constant volume per unit weight of material, they also represent constant density conditions. Thus the curves F-G-H may be considered as either constant volume or constant density curves.

Having completed the diagram by representing therein each of the factors on which equilibrium is dependent (in that range of importance in the storage of dry ice) it will be evident that the relation oi these factors for any particular state of thermodynamic equilibrium can be represented by a single point, such as G", on the diagram of Figure 1.

All changes in the position of point G may be considered as changes in a vertical or a horizontal component or both, 'and conversely by fixing both a vertical and a horizontal component of the factors on the diagram the point G will be xed and a-specilc equilibrium condition will be established. Thus it will be seen that the problem presented in controlling or maintaining equilibrium conditions to preserve the material in a useful form resolves itself into that of avoiding continued or uncontrolled shifting of a point such as G, on a diagram, such as that of Figure 1.

' The means normally available for storing dry ice inherently, impose restrictions on at least one of the factors represented in the diagram of Figure 1. For example, when the material is stored in a tank under constant pressure, changes in the condition of storage can take place only along the horizontal line C-D. Therefore, by fixing a factor having a vertical component, such as heat content, volume or density, it is possible to nx a point on the diagram and establish equilibrium conditions between all of the factors. Similarly a closed chamber of constant volume xes a line, such as F-G-H on the diagram and it is possible to establish equilibrium conditions between all of the factors'by fixing either a vertical or a horizontal component, such as heat content, pressure or temperature.

If the storage apparatus is such that changes in pressure and volume occur simultaneously, as when an elastic or variably opposed closure is provided for a container in which the material is stored, changes in the values of all of the factors take place along some such line as EE. However, while neither the pressure nor the volume is fixed, these factors are so related that it is possible to establish equilibrium between all of the factors by xing the value of either a. vertical or a horizontal component of one factor.

In accordance with the present invention change or variation in one or more of the factors on which equilibrium is dependent or in external conditions tending to influence one or more of said factors are utilized to initiate or control the operation of means for opposing such changes. In this way it is possible to preserve or to reestablish the original storage conditions or to produce equilibrium between the factors at values'other than those initially produced whereby loss of refrigerating value of the material during storage is prevented or materially reduced.

Typical mechanism for controlling the storage conditions is illustrated diagrammatically in Fig. 2 wherein a tank 2 is provided with means for absorbing or adding heat to the stored material. Such means are shown asa cooling coil 4 and a body of broken dry ice 6 immersed in a liquid 8 or otherwise maintained under variable pressure, although either cooling element or other suitable means may be used. A Weighted cover lll for the tank 2 is sealed about the edges by a liquid l2 and serves to maintain the material I4 under a constant pressure. The apparatus thus fixes a vertical .component so that changes in the conditions of storage can occur only along a line such as C-D of the diagram of Fig. V1. If heat flows into the stored material from external sources evaporation will take place and an increase in volume of the material will occur corresponding to movement of the point G on line FGH to the right along the line C-D of Fig.` 1 to the point G' on the line F'G'H. Evaporation will continue and the point G will move to the right along line CD past one constant volume line after another as long as heat ilows into the stored material or until the material is completely converted to vapor. In accordance with the present invention such changes in the storage conditions are opposed to check movement of the point G -at G' and thus maintain equilibrium with the factors assuming new and suitably related values. -If the opposing forces are suitably applied it is possible to reverse the operation and to extract heat from the material, condensing the vapor and reducing the volume' until the original storage conditions are restored.

. The means responsive to change in the storage conditions in Fig. 2 is shown to be responsive to changes in volume of the material and includes a rotatable 'gear IB meshing with a rack I8 on the vexterior of the cover l0. The gear I6 is secured to a shaft 20 by which the action of a compressor 22 is controlled to supply a volatile refrlgeraut to the coil I at a variable rate.

If broken dry.A ice i is used either by itself or or pump 2l afterl a predetermined volume change has taken place. The cooling means may function to restore the original conditions by extracting heat from the stored material `thus moving the point from G' back to G with condensation of vapor and resulting reduction in volume or they may function only to prevent transfer of in conjunction with the coil l a pump 24 may anism may be actuated in response to any change in volume whatever and operate to maintain substantially constant equilibrium at all times. In either case the material is preserved and loss of the useful refrigerating value thereof is avoided.

In that form of the invention illustrated in Figure 3 of the drawings, both the pressure and the volume of th-e stored material vary, whereas the total heat of the stored material is controlled to compensate for changes in the pressure and volume so as to reestablish or reproduce equilibriumJ conditions within the stored material.

4Changes in the storage conditions thus vary along the line EGE of the diagram of Figure 1.

The construction of Figure 3 includes a movable chamber 30, the lower extremities 32 of which are sealed by meansy of a liquid. The stored material 34 is thus enclosed by the chamber 30 and movement oi the chamber up and down with changes in pressure and volume is permitted by telescoping-movement of the chamber 30 within the casing 36. 'Ihe space between the top of the chamber 30 and the casing 36 is sealed and upon upward movement of the casing the pressure of air or gas within the space between the chamber 30 and the casing 36 varies, giving an indication of a change in volume and pressure within the storage chamber. The chamber is surrounded by a heat insulating jacket 38 which is spaced from the walls of the chamber 30 and has coils 40 for receiving the suitable refrigerant together with the broken dry ice 42 located in the space between the jacket 38 and the chamber 30. A diffusion-resisting and heatinsulating medium 44, such as a viscous liquid, preferably is located in the space between the jacket 38 and chamber 3U to` reduce the transfer of heat to the dry ice 42 and the chamber 30 from external sources.

The upper` portion of the space between the heat insulating jacket and the chamber is preferably closed by a member 46 so that pressure may be applied to the surface of the medium 44 by means of a pump 48. A compressor 50 serves to circulate a refrigerant or brine through lthe coils 40 for absorbing heat penetrating the insulated jacket 38 and tending to flow toward the chamber 30.

The broken dry ice 42 and the coils 40 serve tocontrol or alter the total heat content of the stored material by the0 absorption of heat from the stored material as well as that penetrating the insulated jacket 38. Thus a point is established on the line EGE of Figure 1 to prevent continued disturbance of the equilibrium conditions under which the material is stored.

In order to control the rate at which heat is absorbed by the broken dry ice and the coils 40, the operation of the pump 48 and compressor 50 is varied in response to change in the pressure of thel gas between the chamber 30 and the casing 36.. For this purpose a pressure responsive element 52 is employed for actuating a rheostat 54 or other suitable means for varying the operation of the pump and compressor. In this way continuous and accurate control of the storage conditions can be obtained in response to change in the volume and pressure oi.' the stored material. l

In the construction illustrated in Figure 4 the material is stored under constant volume in a closed chamber 56 surrounded by heat absorbvincrease the temperature of the ing means such as the broken dry ice 58 and the coils 00 through which a refrigerant or brine may be circulated. The storage conditions of the material 02 to be preserved therefore can vary only along a line such as FGH of the diagram of Figure 1. The total heat content of the stored material is varied or maintained constant by` means of the broken dry ice and coils 60 and the rate at which heat is absorbed by these elements is varied as in the construction of Figure 3 by means of a pressure responsive element 64 controlling electrical means 66 to vary the operation of a pump 68 and compressor 10. In this way a point such as G on the line FGH may be established so as to preserve the material without material loss thereof for extended periods of time.

'I'he construction of Figure 5 is similar to that of Figure 4 in that the material 12 is stored in a chamber 14 of fixed volume. 'I'he heat absorbing means employed are shown in the .form of a broken dry ice 16 and coils 'I8 controlled by a pump 80 and a compressor 82, respectively. Operation of the pump and compressor to vary the rate at which heat is absorbed by the heat absorbing means 16 and 'I8 is controlled in response to the density of the stored gaseous carbon dioxide and the weight.of the stored solid carbon dioxide. This is effected by supporting the solid material within the chamber 14 upon aplatform 84 which is raised and lowered in response to change in the weight thereon. As the solid carbon dioxide sublimes,the density of the gaseous carbon dioxide increases while the weight of the solid carbon dioxide decreases. Under these conditions the platform rises and an electrical element such as the rheostat 86 is actuated by movement of the platform 84 to increase the speed of operation of the pump and compressor and so stored material. As the temperature of the stored material is so .decreased the density of the gaseous carbon dioxide decreases and the weight of the solid carbon dioxide increases. As the weight of the solid carbon dioxide increases the platform 84 drops and the rheostat 86 is actuated to decrease the speed of operation of the pump and compressor.

Referring to Figure 6, a construction is shown wherein the material 80 is stored within a chamber 90 of ixed volume and the pressure to which the material within the chamber is subjected is varied by operation of a valve 92 controlling the ow of carbon dioxide gas under` high pressure from a cylinder 94 or other suitable source to the chamber 90. A temperature responsive element 96 located Within the chamber serves to actuate the servo-motor 98 to vary the operation of the valve $2 upon a change in pressure within the storage chamber. A relief valve |00 also actuated by a servo-motor is provided to relieve the pressure within the chamber upon the creation of excessive pressure therein.

From the foregoing description of the various forms of apparatus which may be employed in the practice of thepresent invention, it will be evident that the means used to establish thermodynamic equilibrium between those factors upon which the storage of dry ice is dependent can be fixed or controlled by suitable .selection and control of the values of a limited number of factors represented by the diagram of Figure 1. Moreover, various changes and modifications may be made in carrying out the process of4 the present invention to control the equilibrium conditions and to establish conditions represented by a single point on the equilibrium diagram. For this reason it should be understood that particular process steps and the relation thereof in operation of the apparatus herein described are intended to be illustrative of the invention and are not intotal heat of the material at such values that subi stantial thermodynamic equilibrium is produced between the solid and vapor phases of the materiall within said envelope.

2. The method ofpreserving solid or liquid materials which have vapor pressures at atmospheric temperature exceeding atmospheric pressure which comprises the steps of enclosing the material and its vapor in an envelope which substantially excludes air from the material, maintaining the material under an external pressure substantially equal to atmospheric pressure, insulating said material from its surroundings and absorbing heat penetrating said insulation at a rate which varies in accordance with changes in atmospheric pressure.

3. The method of preserving solid or liquid materials which have vapor pressures at atmospheric temperatures exceeding atmospheric pressure which comprises the steps of enclosing the material in an envelope, applying to the envelope an external pressure not materially greater than atmospheric pressure, maintaining the temperature of the inner surface of the envelope enclosing the material at substantially the same temperature as that of the material itself, and varying the volume and density of the stored material upon change in the storage conditions.

4. The method of preserving dry ice which comprises the steps of enclosing the material and its vapor in an envelope of variable volume, subjcting said envelope to an external pressure sub. stantially equal to the vapor pressure of the dry ice at the temperature at whichit is maintained, preventing transfer of heat to the dry ice through said envelope by absorbing heat owing toward said envelope, and -controlling the absorption thereof in response to change in atmospheric temperature to establish thermodynamic equilibrium in said'material.

5. The method of preserving solid or liquid ma- .terials 'which have vapor pressures higher than atmospheric pressure at atmospheric temperature which comprises the steps oi enclosing the material and its vapor in an envelope, substantially preventing the passage of heat through said envelope to said material, and varying the external pressure to which the material is subjected in response to change in atmospheric temperature.

6. The method of storing materials which have higher vapor pressures than atmospheric pressure at atmospheric temperatures, which comprises the steps of placing the material in a chamber of variable volume, absorbing heat owing toward the stored material and varying the temperature of said material in response to change 'in' volume thereof.

7. The method of storing materials which have higher vapor pressures than atmospheric pressure at atmospheric temperatures, which comprises the steps of placing the material in a chamber of variable volume, absorbing heat flowing toward the stored material and varying the temperature of said material in response to change in pressure in said chamber.

8. The method of storing materials which have higher vapor pressures than atmospheric pressure at atmospheric temperatures, which com# prises the steps of placing the material in a chamber of variable volume, absorbing heat owing toward the stored material and varying the temperature of said material in response to a change in condition of said stored material.

9.y 'I'he method of storing dry ice, which comprises the steps of placing the dry ice in a chamber of fixed volume, introducing gaseous carbon dioxide into and removing the same from said chamber in response to change in a condition Within said chamber to maintain the pressure in said chamber at a constant value and absorbing heat flowing toward said chamber from external sources to reduce to a minimum the number of times 'gaseous carbon dioxide is introduced into and removed from said chamber during the period of storage.

10. The method of storing dry ice, .which comprises the steps of placing the material in a chamber of fixed volume, introducing gaseous carbon dioxide into and removing the same from said chamber in response to change in temperature within said chamber to maintain the pressure in said chamber at a constant'value and absorbing heat flowing toward said chamber from external sources to reduce to a minimum the number of times gaseous carbon dioxide is introduced into and removed from said chamber durying the period of storage.

ALVIN C. BIRDSALL, Admimstrator of the Estate of Wilfred T. Brdsall,

Deceased.

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