US 3850714 A
A double-walled, hermetically sealed structure with the cavity between the walls containing a polyurethane foam blown with a fluorinated hydrocarbon. During fabrication the cavity is evacuated to remove air and water vapor and then refilled with a fluorinated hydrocarbon.
Description (OCR text may contain errors)
United States Patent 1191 Adorjan Nov. 26, 1974 1 1 THERMAL INSULATION STRUCTURE  References Cited  Inventor: Alexander S. Adorjan, Pearland, UNITED STATES PATENTS TEX. 2,939,811 6/1960 Dillon 62/D1G. 13 2,969,042 1/1961 1011 t n... 138/149  Asslgneei General Elem" COmPanY, New 3,091,946 6/1963 K6815; 62/DlG. 13 York, 3,559,660 2 1971 126111115 138/149 p 6 3,650,299 SCliiCl' Cl 211 138/149 1211 Appl. No.: 348,499 Primary Examiner-Edward 0. Whitby Related US. Application Data  Division of Ser. No. 167,258, July 29, 1971.  ABSTRACT A double-walled, hermetlcally sealed structure w1th 52 us. 01 156/77 62/268 62/DIG 13 the cavity between the walls Containing Polyure' 138/149 156/87 264/45 264/51 thane foam blown with a fluorinated hydrocarbon.  Int CL n B32b 5/18 During fabrication the cavity is evacuated to remove  Field 62/268 air and water vapor and then refilled with a fluori- 62/DIG. 13; l56/77,78,87; 53/79; hydrocarbon 285/47; 264/45, 5] 5 Claims, 2 Drawing Figures THERMAL INSULATION STRUCTURE This is a division, of application Ser. No. 167,258 filed July 29, 1971.
BACKGROUND OF THE INVENTION This invention relates generally to a thermal insulation structure of a double-walled type, and more particularly to such a structure where the cavity between the walls is hermetically sealed.
In considering the environment to which an oil well casing would be exposed in the Prudhoe Bay area of Alaska, certain new requirements were developed. In this vicinity there exists a layer of permafrost; i..e., a layer of subsurface soil which may extend down 2,000 feet and is permanently frozen. If a well is drilled utilizing the same approach followed in temperate climates the oil (which is approximately 180F) passing through the well casing will cause the permafrost about the well casing to melt. The melting of the permafrost causes it to subside, exerting downward drag on the well casing which may cause failure. Moreover, if the flow of oil is terminated, the soil surrounding the casing will eventually refreeze which may produce forces sufficient to cause the casing to collapse.
SUMMARY OF THE INVENTION In a preferred form of the invention, a double-walled thermal insulating structure is provided with the cavity between the walls hermetically sealed. The outer surface of the inner wall is covered with a reflective film. The cavity further contains a polyurethane foam which has been blown in an atmosphere of a fluorinated hydrocarbon such as trichloromonofluoromethane, CCI F, FREON-ll. The cavity also contains a gaseous fluorinated hydrocarbon such as difluorodichloromethane, CClgFg, FREON-12.
BRIEFDESCRIPTION OF THE DRAWINGS FIG. I is a schematic cross-section of one embodiment of the thermal insulating structure of this invention; and
FIG. 2 is a detail of a portion of FIG. 1 showing another embodiment of the invention.
DESCRIPTIONOF THE PREFERRED I EMBODIMENT Referring to FIG. 1, a double-walled, he'at insulating structure is illustrated having inner wall and outer wall 12 connected by end walls 14. Inner and outer walls 10 and 12 are fabricated of a material having sufficient structural strength for conveying liquids. End walls 14 are designed to have a low heat conductivity by utilization of a suitable material such as stainless steel, and hermetically seal the cavity between walls 10 and 12. Although not shown in FIG. 1, end walls 14 characteristics are achieved without reflective material 18. Filling the remaining space Illl the cavity is a polyurethane foam 20. Foam 20 may be of an opened cell or closed-cell type and is blown in; a fluorinated hydrocarbon atmosphere; e.g., trichlorofluoromethane, CCI F, FREON-l 1.
In some cases it may be desired to alternate layers of reflective material 18 with layers of foam 20, as shown in FIG. 2.
Film 18 and foam 20 are placed in the cavity between the double walls prior to the time end walls 14 are secured to inner and outer walls 10 and 12.
The space between the double walls is then evacuated to a pressure below 0.01 millimeters of mercury by removing plug 16. This ensures the removal of air and water vapor which would cause deterioration of the insulating qualities of foam 20 (also called aging of the foam). It is then refilled with a flluorinated hydrocarbon; e.g., difluorodichloromethane, CCl F FREON- 12. The FREON-IZ has the advantage of a low thermal conductivity (about one-third that of air) while not causing aging of the foam. The pressure of the FREON- 12 is a critical factor in forming the composite of the insulation of this invention. If the pressure is raised high enough to reach the dew point of the gas the resulting condensate will tend to circulate within the doublewalled cavity, condensing on the cooler wall and evaporating on the warmer wall. Thus: the temperature to which the inner and outer walls of the casing will be exposed must be considered. A pressure is consequently chosen which will preclude condensation within the operating temperature range of the structure while still being maximized. The FREON-l2 impedes any leakage of air into the cavity, and prevents the aging of foam 20. Polyurethane foam of the type described herein which has not aged has a thermal conductivity, K 0.] l7 Btu/hr ft F/in. at F mean temperature, while aged foam has a thermal conductivity, K 0.20 Btu/hr 'ft F/in. at the same mean temperature. Thus preventin'g aging of the foam preserves the insulation capabilities of the structure.
Preventing aging of the foam is also necessary prior to fabrication of the heat insulating structure. Thus unless the foam will be used within l or 2 days it should be kept in a hermetically sealed container which has been evacuated and backfilled with a fluorinated hydrocarbon as described above.
As is well known, heat may be transfered by conduction, convection, or radiation. As indicated above conduction is retarded through the use of low thermal conductivity foam and gas. The presence of the foam also prevents heat transfer by convection which would otherwise occur by circulation of the gas between the hot and cold walls of the structure. Finally the reflective film reduces heat transfer by radiation.
In the application of this heat insulating structure to a well casing, conduit, etc., the particular dimensions employed will vary. In order to provide some basis for comparison, however, for a 40 foot long well casing, with the inner steel pipe having an outside diameter of 5% inches, and the outer steel pipe having an inside diameter of 8 11/16 inches, a heat loss of about l,l Btu/hr. was calculated. This assumed the inside pipe contained oil at F, and the outside was exposed to air and kept at 20F.
While a particular embodiment of a heat insulating structure, and the process for making it, has been shown and described, it will be obvious that changes and modifications can be made without departing from the spirit of the invention and the scope of the appended claims.
I claim: 1. A method of producing a double-walled heat insulating structure comprising:
placing the outer wall concentrically about the inner wall; filling the cavity between the inner and outer walls with a polyurethane foam blown with a fluorinated hydrocarbon; hermetically sealing the ends of the inner and outer walls; evacuating the cavity to a pressure below 0.0l mm of mercury; and refilling the cavity with a gaseous fluorinated hydrocarbon to a pressure which is maximized without exceeding that at which it will condense in its operating temperature range. 2. A method of producing a double-walled heat insulating structure in accordance with claim 1 wherein:
the fluorinated hydrocarbon in said cavity is difluorodichloromethane.
3. A method of producing a double'walled heat insuthe fluorinated hydrocarbon with which said polyurethane foam is blown is trichloromonofluoromethane.
4. A method of producing a double-walled heat insulating structure in accordance with claim 3 wherein:
said polyurethane foam blown with trichloromonofluoromethane has been preserved against aging prior to use by keeping it in a hermetically sealed container which has been evacuated and refilled with a gaseous fluorinated hydrocarbon.
5. A method of producing a double-walled heat insulating structure comprising:
applying a heat reflecting film to the outer facing surface of the inner wall;
placing the outer wall about the inner wall; filling the cavity between the inner and outer walls with a polyurethane foam blown with trichloromonofluoromethane;
hermetically sealing the ends of the inner and outer walls together; evacuating the cavity; and
refilling the cavity with gaseous difluorodichloromethane to a pressure which is maximized without exceeding that at which it will condense in its operatingrange.