Vaporizer for low temperature liquid

Details Vaporizer for low temperature liquid Vaporizer for low temperature liquid

1996-05-24

1998-09-15

Bennett, Henry A.

Liquid heaters and vaporizers

Heat transmitter

1223673, 165175, F22B 2306, F22B 3710

Patent

active

058064702

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND ART

This invention relates to an open rack type vaporizer for vaporizing low temperature liquid such as liquefied natural gas, liquefied nitrogen.
Conventionally, vaporizers of open rack type have been known as a vaporizer for vaporizing such low temperature liquid. The open rack type vaporizer generally includes a lower header in which low temperature liquid is flowed, an upper header arranged in parallel with the lower header, and a great number of vertical heat exchange tubes connecting the lower and upper headers. Outside of the heat exchange tubes is flowed heating medium such as seawater so that the liquid flowing in the lower header is heated and vaporized by heat of the heating medium in each heat exchange tube. Thus obtained natural gas is recovered by the way of the upper header.
In this arrangement, the lower header and a lower portion of the heat exchange tube come into direct contact with the lower temperature liquid. Consequently, the temperature of these portions is very low. The formation of ice is liable to occur on an outside surface of these portions. The ice becomes a heat insulation barrier, and hinders the heat exchange at the lower portion of the heat exchange tube, thereby lowering the temperature of these portions further and resulting in an exceedingly low temperature condition. In the state that the lower portion of the heat exchange tube is cooled to an exceedingly low temperature, the contraction rate of the heat exchange tube becomes large. Accordingly, there is a likelihood that a slight deviation in the flow of seawater causes a great contraction difference among heat exchange tubes, and finally results in local deformations in heat exchange tubes.
Japanese Unexamined Patent Publication No. 4-217788 discloses provision of a heat insulator on a portion of a heat exchange tube except an outflow portion near an upper header to suppress heat transfer between the heat exchange tube and natural gas flowing inside the heat exchange tube. The heat insulator is formed by providing a cylindrical insulating member on an inner surface of a heat exchange tube, or by forming an insulating space in a heat exchange tube in which vaporized natural gas is contained. The heat insulator prevents an outside surface of the heat exchange tube from being cooled to a terribly low temperature, thereby preventing a large contraction. It is certain that the heat insulator hinders the heat exchange between low temperature liquid and heating medium. However, the vaporization efficiency of the heat exchange tube provided with the heat insulator may be substantially identical to that of the heat exchange tube which is not provided with the heat insulator but is likely to be formed with heat insulating ice on an outside thereof.
However, the heat insulator disclosed in the above publication provides a very low heat transfer rate. Accordingly, it is required to set the thickness of the heat insulator at a very small value. A slight difference in the thickness of the heat insulator causes a great difference in the heat transfer of the heat exchange tube. Specifically, if having a thickness slightly greater than a specified thickness, the heat insulator exceedingly insulates heat, resulting in an insufficient heat transfer between an outside and an inside of the heat exchange tube, consequently making it difficult to sufficiently heat and vaporize the low temperature liquid. Conversely, if having a thickness slightly smaller than a specified thickness, the heat insulator cannot insulate heat sufficiently, consequently resulting in ice formation as the conventional heat exchange tubes. For example, FIG. 25 shows a relationship between the thickness of the heat insulator (i.e., a radial size of an insulating space) and the thickness of ice providing a heat transfer rate equivalent to the heat insulator in the case where the heat insulator is formed by drawing natural gas in the insulating space as mentioned above. It will be seen in FIG. 25 that a variation of 0.1 mm in the radial size is equal to a

REFERENCES:
patent: 3133528 (1964-05-01), Litwinoff
patent: 5341769 (1994-08-01), Ueno et al.
JP 6-48146, B2, Jun. 22, 1994.
JP 6-221499, Aug. 9, 1994.

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