GB2053444A

GB2053444A - Heat transfer tubes with heat flux limiters

Info

Publication number: GB2053444A
Application number: GB8018781A
Authority: GB
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1979-06-11
Filing date: 1980-06-09
Publication date: 1981-02-04
Also published as: FR2458783B1; FR2458783A1

Abstract

A heat exchanger having tubes with at least a portion of the inner or outer surface of the tubes overlaid with a thermo-resistive sheath. This reduces the temperature differential within the tubes in areas in which, otherwise, the thermal stresses could cause failure. <IMAGE>

Description

SPECIFICATION Heat transfer tubes with heat flux limiters This invention relates to heat transfer tubes and more particularly to limiting the heat flux across a portion of the tube walls.

In heat exchanger application systems, certain parameters may result in large temperature differences across the walls of the heat exchanger tubes. Large temperature differences are not a problem unless the wall heat transfer resistance drops and the heat flux increases greatly. This can occur in steam generators when the water side traverses from nucleate boiling to film boiling. If the other fluid is a liquid metal, the heat flux would be extremely high particularly in the areas between boiling and superheating. To provide acceptably long life, approximately 30 to 40 years, the heat flux level must be maintained at an acceptable value. To keep the heat flux at an acceptable value, the tube wall thickness could be increased to increase the resistance to heat flow, however, this is an expensive solution since the material of which the tubes consist is quite expensive.

It is therefore the principal object of the present invention to provide steam generator tubes which are not exposed to overly high heat flux rates to avoid tube failures.

With this object in view, the present invention resides in a heat exchanger tube having portions subjected to high differential temperatures and subjected to unstable boiling which vacillates between film and nucleate boiling, characterized in that said subjected portions have at least one of their inner and outer surfaces overlaid with a thermo-resistive sheath.

The invention wili become more readily apparent from the following description of preferred embodiments thereof shown, by way of example only, in the accompanying drawings, in which: Figure 1 is a perspective view of a heat exchanger tube made in accordance with this invention showing a portion of the outer tube removed; Fig. 2 is a sectional view taken on the line Il-Il of Fig. 1; Fig. 3 is a sectional view taken on the line Ill-Ill of Fig. 1; Fig. 4 is an elevational view of a heat exchanger tube made in accordance with this invention; Fig. 5 is an elevational view showing an alternate embodiment of this invention; Fig. 6 is an elevational view of another embodiment of this invention; and Fig. 7 is an elevational view of still another embodiment.

Referring now to the drawings in detail, and in particular to Figs. 1 through 3, there is shown a double-wall heat exchanger tube 1 with grooves 3 disposed at the interface of the tube walls. At the interface of at least one portion of the tube, there are additional grooves or the grooves 3' are enlarged, preferably by making them wider so as to substantially reduce the contact area at the interface. Preferably, a low conductivity gas such as nitrogen is utilized in the grooves 3', but it is understood that any gas or gaseous mixture could be utilized to increase the thermal resistivity in the portion of the tubes having the enlarged grooves 3' in order to reduce the heat flux in that portion of the tube.

Fig. 4 shows an alternate embodiment in which a portion of the heat exchanger tube 1 is coated on the outside with an overlay of ceramic or some other sheathing material 5 having a thickness of approximately 5 to 10 mils. A porcelain, nucerite glass or other thermal resistive high temperature material could be utilized as the sheathing material 5.

Fig. 5 shows a heat exchanger tube 1 having the inner side of a portion of the tube 1 overlaid with a ceramic sheathing or coating material 7 approximately 5 to 10 mils thick.

Fig. 6 shows a heat exchanger tube 1 having an inner liner 9 disposed inside a portion of the tube 1 to form a sheath to increase the thermal resistance of that portion of the tube. Preferably, the liner 9 is expanded into engagement with the inside of the tube 1 at each end and there is a gap 11 between the liner 9 and the tube 1 except at the ends of the liner 9. The gap 11 is preferably filled with inert gas or gaseous mixture and may be a low conductivity gas such as nitrogen.

As shown in Fig. 7, the gap 11 may be filled with a ceramic or other insulating material 13.

The thermo-resistive sheathing hereinbefore described extends over only a small portion of the length of the tube in order to reduce the heat flux over that portion of the tube and is particularly intended to overlay that portion of the tube which is subjected to vacillate between nucleate and film boiling and other portions which are potentially subjected to high heat flux and/or tube burnout.

1. A heat exchanger tube having portions subjected to high differential temperatures and subjected to unstable boiling which vacillates between film and nucleate boiling, characterized in that said subjected portions have at least one of their inner and outer surfaces overlaid with a thermo-resistive sheath.

2. A heat exchanger as claimed in claim 1, characterized in that said sheath consists of a ceramic material.

3. A heat exchanger as claimed in claim 2, characterized in that the ceramic material is from a group of material comprising porcelain and nucerite glass.

4. A heat exchanger tube as claimed in claim 1, 2 or 3, characterized in that the sheath is disposed on the inner surface of the tube.

5. A heat exchanger tube as claimed in any of claims 1 to 4, characterized in that there are gap areas between the sheath and the tube.

6. A heat exchanger as claimed in claim 1, wherein each heat exchange tube has a double

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Heat transfer tubes with heat flux limiters This invention relates to heat transfer tubes and more particularly to limiting the heat flux across a portion of the tube walls. In heat exchanger application systems, certain parameters may result in large temperature differences across the walls of the heat exchanger tubes. Large temperature differences are not a problem unless the wall heat transfer resistance drops and the heat flux increases greatly. This can occur in steam generators when the water side traverses from nucleate boiling to film boiling. If the other fluid is a liquid metal, the heat flux would be extremely high particularly in the areas between boiling and superheating. To provide acceptably long life, approximately 30 to 40 years, the heat flux level must be maintained at an acceptable value. To keep the heat flux at an acceptable value, the tube wall thickness could be increased to increase the resistance to heat flow, however, this is an expensive solution since the material of which the tubes consist is quite expensive. It is therefore the principal object of the present invention to provide steam generator tubes which are not exposed to overly high heat flux rates to avoid tube failures. With this object in view, the present invention resides in a heat exchanger tube having portions subjected to high differential temperatures and subjected to unstable boiling which vacillates between film and nucleate boiling, characterized in that said subjected portions have at least one of their inner and outer surfaces overlaid with a thermo-resistive sheath. The invention wili become more readily apparent from the following description of preferred embodiments thereof shown, by way of example only, in the accompanying drawings, in which: Figure 1 is a perspective view of a heat exchanger tube made in accordance with this invention showing a portion of the outer tube removed; Fig. 2 is a sectional view taken on the line Il-Il of Fig. 1; Fig. 3 is a sectional view taken on the line Ill-Ill of Fig. 1; Fig. 4 is an elevational view of a heat exchanger tube made in accordance with this invention; Fig. 5 is an elevational view showing an alternate embodiment of this invention; Fig. 6 is an elevational view of another embodiment of this invention; and Fig. 7 is an elevational view of still another embodiment. Referring now to the drawings in detail, and in particular to Figs. 1 through 3, there is shown a double-wall heat exchanger tube 1 with grooves 3 disposed at the interface of the tube walls. At the interface of at least one portion of the tube, there are additional grooves or the grooves 3' are enlarged, preferably by making them wider so as to substantially reduce the contact area at the interface. Preferably, a low conductivity gas such as nitrogen is utilized in the grooves 3', but it is understood that any gas or gaseous mixture could be utilized to increase the thermal resistivity in the portion of the tubes having the enlarged grooves 3' in order to reduce the heat flux in that portion of the tube. Fig. 4 shows an alternate embodiment in which a portion of the heat exchanger tube 1 is coated on the outside with an overlay of ceramic or some other sheathing material 5 having a thickness of approximately 5 to 10 mils. A porcelain, nucerite glass or other thermal resistive high temperature material could be utilized as the sheathing material 5. Fig. 5 shows a heat exchanger tube 1 having the inner side of a portion of the tube 1 overlaid with a ceramic sheathing or coating material 7 approximately 5 to 10 mils thick. Fig. 6 shows a heat exchanger tube 1 having an inner liner 9 disposed inside a portion of the tube 1 to form a sheath to increase the thermal resistance of that portion of the tube. Preferably, the liner 9 is expanded into engagement with the inside of the tube 1 at each end and there is a gap 11 between the liner 9 and the tube 1 except at the ends of the liner 9. The gap 11 is preferably filled with inert gas or gaseous mixture and may be a low conductivity gas such as nitrogen. As shown in Fig. 7, the gap 11 may be filled with a ceramic or other insulating material 13. The thermo-resistive sheathing hereinbefore described extends over only a small portion of the length of the tube in order to reduce the heat flux over that portion of the tube and is particularly intended to overlay that portion of the tube which is subjected to vacillate between nucleate and film boiling and other portions which are potentially subjected to high heat flux and/or tube burnout. CLAIMS

6. A heat exchanger as claimed in claim 1, wherein each heat exchange tube has a double wall structure with grooves formed in the wall structures at their interface, characterized in that said portions subjected to high differential temperatures and unstable boiling have substantially larger grooves so as to reduce the contact area between the inner and outer wall of the tube.

7. A heat exchanger tube as claimed in claim 6, characterized in that the grooves are filled with a gas.

8. A heat exchanger tube as claimed in claim 7, characterized in that the gas is nitrogen.