Electric resistance heating devices – Heating devices – Continuous flow type fluid heater
Reexamination Certificate
1998-05-22
2001-01-23
Walberg, Teresa (Department: 3742)
Electric resistance heating devices
Heating devices
Continuous flow type fluid heater
C165S104290
Reexamination Certificate
active
06178293
ABSTRACT:
The present invention relates to a method and an apparatus for improving heat transfer in a heat exchanger, in which a heating medium flows through heat exchange elements which transfer their heat to a liquid with which they are in contact and which is located in the jacket region of the heat exchanger. The invention relates to a method for improving heat transfer on the surface of heating elements (pool boiling) in processes where the heat flow density and wall overheating is low and no bubble vaporization takes place on the surface of the heating elements. In such cases the heat on the surface of the heating elements is only transferred by free convection and heat transfer is usually poor.
In evaporator boilers the vaporization of liquids on heating surfaces takes place on flat horizontal or vertical heating walls or on the surfaces of tube bundles. The heating elements can be heated either by a fluid or by means of electric or nuclear energy (cf. for example the VDI heat atlas, 4th Edition, Section Ha). Such apparatuses are designed using known calculation methods or on the basis of experiments. Where the overheating of the walls is sufficiently high, bubble vaporization takes place on the surfaces of the heating walls. In such cases the heat transfer coefficient is usually relatively high and the required heating area relatively small for a given apparatus capacity. Where the temperature differences between the surfaces of the heat exchange elements and the boiling temperature of the surrounding liquid are less than 20° C., and particularly less than 2 to 10° C., no bubble vaporization usually takes place on the heating walls. In such cases the heat is only transferred via heat conduction and convection. The heat transfer coefficient on the heat exchange surfaces is in such cases relatively low, as a result of which the dimensions of the required heating surface area and the volume of the apparatus have to be relatively high.
In refrigeration technology, horizontal tube bundles are used where the differences between the temperature of the heating walls and the boiling temperature of the fluid are small. These horizontal tube bundles are sprinkled with the refrigerating medium to be evaporated [cf. for example R. Billet: “Verdampfung und ihre technischen Anwendungen” (Evaporation and its technological applications), Verlag Chemie (1983)]. In order to avoid dry areas on the tubes, the quantity of refrigerant sprinkled must be larger than the quantity of liquid evaporated. In general the refrigerant is circulated several times with the aid of a pump. Usually pumps are subject to wear. Also, they have to have a corresponding inlet height when conveying boiling liquids.
The invention is based on the problem of considerably improving heat transfer where small differences in temperature exist between the surfaces of the heat exchange elements and the boiling temperature of the adjacent liquid (low wall overheating), thus resulting in low heat flow densities on the heat exchange surface. In the present instance (i.e. in the context of the present invention) “small differences in temperature” are understood to refer to such low overheating of the walls that bubble vaporization does not occur on the surfaces of the heat exchange elements.
According to the invention this problem is solved by heating the liquid between the heat exchange elements in the jacket region locally to such a high temperature by means of additional heating elements that nucleate boiling occurs on the surfaces of these additional heating elements and the vapour bubbles formed ascend between the main heat exchange elements in the jacket region.
The bubbles formed thus flow upwards between the other heating elements on which no bubble vaporization takes place. Due to the ascending bubbles an increase in convection takes place and the heat transfer on all of the heat exchange elements is consequently improved. The vapour bubbles escape from the surface of the liquid. The bubble-free liquid, which has a higher density than the bubble-containing liquid between the heating elements flows downwards outside the region of the heating elements. Circulating flow is thus produced in the evaporator.
Preferably the difference in temperature &Dgr;T
H
between the surfaces of the additional heating elements and the boiling temperature of the surrounding liquid is more than 10° C., and in particular more than 20° C.
The additional heating elements can be advantageously heated electrically or with the aid of fluids, gases or condensing vapours as the heating media. In particular, the heating medium used can be gaseous chlorine, chlorine condensed in the additional heating elements, or another refrigerant.
According to a special embodiment the method according to the invention is advantageously carried out in a heat exchanger for condensing chlorine vapours containing inert gases, in which the liquid present in the jacket region also consists of liquid chlorine and the combined heat exchange and jacket region is operated in the form of a chlorine recuperator. Liquefied chlorine is continuously introduced into the jacket region, evaporated as a result of the thermal energy introduced via the heat exchanger and is then continuously discharged once again in the form of chlorine gas, while gaseous chlorine is introduced into the heat exchange elements as the heating medium which at least partially condenses on passing through the heat exchange elements acting as evaporative condensers for the gaseous chlorine and is discharged continuously in the form of liquefied chlorine.
The apparatus for carrying out the abovementioned method is based on an evaporative condenser containing vertically superimposed heat exchange elements extending in a horizontal direction and a shell enclosing the jacket region of the heat exchange elements and is characterized according to the invention by additional heating elements which are arranged below or between the heat exchange elements, preferably in the bottom third of the vertical arrangement, and are connected to a heating medium circuit which is separate from the heat exchange elements.
Preferably the heat exchange elements consist of tubes arranged parallel to the axis of the shell.
Alternatively the heat exchange elements can also consist of hollow plates having vertically orientated heat exchange surfaces and arranged parallel to the axis of the shell.
The additional heating elements used are appropriately in the form of electrically heatable heating elements arranged parallel to the axis of the shell or in the form of tubes through which a heating medium flows.
In one embodiment which has proven to be particularly advantageous, guiding plates are arranged on both sides of the heat exchange elements in order to promote circulating flow within the vessel similar to a loop reactor. The guiding plates form a dividing line between a region of upward flow containing the heat exchange elements and a region of downward flow formed outside the region of the heat exchange elements.
In evaporators in which no bubble vaporization occurs on the surfaces of the heating elements as a result of a small difference in temperature between the surfaces and the boiling point of the adjacent liquid, the heat transfer can be considerably improved in the entire apparatus as a result of the abovementioned additional heating elements.
It has been found that, given identical conditions, even a small number of additional heating elements are sufficient to obtain a multifold improvement in the average heat exchange coefficient of a tube bundle compared with that obtained by free convection without any bubble formation. The required heating surface area and volume of the apparatus can be considerably reduced as a result of this feature. The abovementioned additional heating elements are suitable both for evaporators containing horizontal tube bundles and for vertically arranged plate elements.
REFERENCES:
patent: 4050507 (1977-09-01), Che et al.
patent: 4471833 (1984-09-01), Yabe et al.
patent: 5030327 (1991-07-
Brockhaus Hans-Joachim
Casper Clemens
Clasen Peter
Jonen Karl-Heinz
Bayer Aktiengesellschaft
Campbell Thor
Norris McLaughlin & Marcus P.A.
Walberg Teresa
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