US3734140A

US3734140A - Cross-rifled vapor generating tube

Info

Publication number: US3734140A
Application number: US00051434A
Authority: US
Inventors: H Nakamura; M Tanaka
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 1969-07-02
Filing date: 1970-07-01
Publication date: 1973-05-22
Anticipated expiration: 1990-05-22
Also published as: DE2032891B2; GB1290588A; ZA704552B; FR2050447A1; CH510847A; JPS4931863B1; FR2050447B1; DE2032891A1; SE358720B

Abstract

A vapor generating tube of the two-phase type which is operated at a pressure between sub-critical and critical pressure of the vapor has a tube-like body through which fluid is passed. The inner wall surface of the tube is crossed-rifled to provide a plurality of quadrilaterally shaped projections uniform and regularly spaced along the cross-rifled axes. The minor inside diameter d1, the projection height h of each projection, the projection pitch P, projection width b, and spiral lead angle Alpha and Beta have the following relationships: P/h is from 5 to 4 h/d1 is from 0.005 to 0.08, b/P is in the range of 0.2 to 0.8, and the spiral lead angles Alpha + Beta are from 20* to 80* .

Description

United States Patent 1 Nakamura et al.

[ 1 May 22, 1973 [54] CROSS-RIFLED VAPOR GENERATING TUBE [73] Assignee: Sumitomo Metal Industries Limited,

Osaka, Japan [22] Filed: July 1, 1970 [21] Appl. No.: 51,434

[30] Foreign Application Priority Data July 2, 1969 Japan ..44/52752 [52] US. Cl ..138/177 [51] Int. Cl ..F16l 9/00 [58] Field of Search..... 138/32-38, 177; 165/109 T, 165/179 [56] References Cited UNITED STATES PATENTS 2,537,797 l/l951 Simpelaar ..l65/l79 3,088,494 5/1963 Koch et al. ..l38/37 2,663,321 12/1953 Jantsch .4 2,080,626 5/1937 Mojonnier 138/38 Primary Examiner-Houston S. Bell, Jr. Att0rneyWatson, Cole, Grindle & Watson [57] ABSTRACT A vapor generating tube of the two-phase type which is operated at a pressure between sub-critical and critical pressure of the vapor has a tube-like body through which fluid is passed. The inner wall surface of the tube is crossed-rifled to provide a plurality of quadrilaterally shaped projections uniform and regularly spaced along the cross-rifled axes. The minor inside diameter (1,, the projection height h of each projection, the projection pitch P, projection width b, and spiral lead angle a and B have the following relationships: P/h is from 5 to 4 h/d is from 0.005 to 0.08, b/P is in the range of 0.2 to 0.8, and the spiral lead angles a+B are from 20 to 80 4 Claims, 4 Drawing Figures TEMPERATURE PATENTED 3,784,140

SHEET 2 BF 3 FIG.3

o smoom TUBE L we FLUX 4on0 kccfl/m -s Q2058 RLFLEU TUBE HEAT FLUX SOXK)4 kcai/m -hr 500 crzoss ram-:0 TUBE HEAT Fl ulr 1h 4on0 m r SMOOTH TUBE M CG HEAT FLUX X\0 kcd lm hr SMOOTH TUBE 400- t HEAT FLUX \0x\0 kcollm' -hr CROSS mFLED TUBE HEAT FLUX 3on0 kco! /m"-hr CROSS Yaw-LED TUBE HER FLUX QOX\O kc0\/m .hr

QUMJTY ENTHALPY i kcal/ kg INVENTORSI ///5,45H/ M/(flMU/(A Mas/47mm 74mm CROSS-RIFLED VAPOR GENERATING TUBE This invention relates to improvements in vapor generating tubes to be operated mostly under a pressure below the critical pressure and to be subjected to high heat flux, and more particularly to a cross-rifled vapor generating tube provided uniformly with many projections of a rhombic or parallelogramic shape made on the inside wall surface by cross-rifling the inside surface.

More particularly, the above mentioned uniform projections are regularly set by cross-rifling the inside surface of a tube to be used mostly as a wall tube for a high temperature-high pressure boiler using a fossil fuel (such as coal, heavy oil or natural gas), so that there is produced a remarkable improvement in the critical heat flux so as to be higher than in a smooth tube by promoting the maintenance of nucleate boiling of the fluid passing through the tube.

Further, not only in two-phase flow but also in singlephase flow, the heat transfer is remarkably promoted. Therefore, when the cross-rifled tubes of this invention are used for general heat-exchangers having no boiling or vapor generating tubes operated under super-critical pressure, a remarkable effect improvement is obtained.

The safety of the operation of vapor generating tubes of a boiler depends on the wall temperature of the tube. That is to say, it is related so closely with the coefficient of heat transfer on the inside surface of the tube, the temperature of the operating liquid (such as the mixture of steam and water) and the heat flux that it is necessary to always pay careful attention to the wall surface exposed to strong flame radiation during its operation. Today, due to the increase of the adoption of an oil burning system and the general use of a oncethrough boiler, there has increased the possibility of the local presence of a condition exceeding in some cases the critical heat flux causing physical burn-out (breakdown) of the vapor generating tube. As a countermeasure for increasing the critical heat flux, a tube called a ribbed tube made by forming spiral lands on the inside surface of a vapor generating tube is suggested, for example, in U.S. Pat. No. 3,088,494 (or Canadian Pat. No. 684,836).

An object of the present invention is to provide a cross-rifled vapor generating tube having the effect of remarkably improving the critical heat flux to be higher than in a smooth tube by promoting the maintenance of nucleate boiling of the fluid passing through the tube.

Generally, the burn-out phenomenon of a vapor generating tube subjected to high heat flux and used below the critical pressure is thought to consist of two factors of fast burn-out (also called heat flux burn-out) in a low quality region and slow burn-out (also called enthalpy burn-out) in a high quality region. The former is caused by the transition from nucleate boiling to fllm boiling but the latter is caused by the annular flow pattern of the gas-liquid mixture. Thus two kinds of burn-out of the vapor generating tube are caused by quite different mechanisms. Therefore, in order to make the critical heat flux in the vapor generating tube definite, it is an essential condition to measure the critical heat flux by heat transmission tests and to watch the flow pattern by flow tests.

1n the drawings:

FIG. 1 is a sectional view of an embodiment of a cross-rifled vapor generating tube according to the present invention;

FIG. 2 is a sectional view showing another embodiment of the tube shown in FIG. 1;

FIG. 3 is a graph showing a comparison of inside sur face temperatures of a smooth tube and a cross-rifled tube of the present invention in the case of a pressure of 210 atmo (Kg/cm and a mass velocity of 700 to 710 l(g./m see;

FIG. 4 is a graph showing a comparison of the maximum values of the inside surface temperatures to the heat fluxes of a smooth tube and a cross-rifled tube of the present invention in the case of a pressure of 210 atmo (Kg/cm);

The cross-rifled tube according to the present invention has many projections such as 18 or 20 formed on the inside surface of a tube as shown in the embodiment in either FIGS. 1 or 2.

Examples of the chemical compositions mechanical properties and dimensions of the test tubes are shown in the following Tables 1 and 2.

TABLE 1 Chemical compositions and mechanical properties of cross-rifled tubes of the present invention.

Chemical compositions (in percent by weight.)

Test tubes 0 Si M11 1 S Ni C1 Mo A 0. 0S 0. 57 1. T4 0. 024 0. 006 13. 40 10. 50 1.. 20 B 0. 07 0. 54. 1. 08 0. 026 0. 006 E). 60 18. 50 0. 07

Mechanical properties Test Tubes Tensile strength Yield point (in Kglmm (in Kg/mm) A 56.3 28.8 B 51.4 27.2

Mechanical properties Test Tubes Elongation (in TABLE 2:

Dimensional data of cross-rifled test tubes Test tubes A-l A2 B-l 8-2 Outside diameter D, (in mm) 20.27 20.16 20.09 20.06 Minor inside diameter d, (in mm) 9.66 9.54 13.27 13.18 Height h (in mm) of e projection 0.55 0.64 0.47 0.52 Number of spiral 12 12 12 12 (number per cross-section) Width 11 (in mm) of the projection 3.16 3.16 3.75 3.75 Lead angle 0: 2220 2220 2148 2148 Lead angle [3 1845 1845 1837 1837 Lead angle a 5 4105 4105 4025 4025 littlll p (in llllll.)[=l(:t(l (in lltight 7.32 7.32 11. .20 11.20 1n m.)Xnlu n|u-r of spisnls] [Left 1;.88 ti. 88 8.115 8. ()5 11:11.10 (.OIHHUOIIZ WI lltl'tll 13.30 11.42 23. 21.60 i (L011. 12. 50 10. 7X 18. 40 1G. (15 ll/ll1 4 0. 057 0. 007 0. 036 0. 0 10 b/IL iltlgllt U. 432 U. 432 1). 335 0. 335 ll. 0. 45'.) 0. 151) 0. 433 0. 433

The inside surface temperatures of the smooth tube and the cross-rifled tube of the present invention in the case of a pressure of 210 atmo (Kg/cm) and a mass velocity of 700 to 710 kg/m sec. are shown for bulk average specific enthalpy with the heat flux as a parameter in FIG. 3. In the same graph, the heat flux q is taken in the range of X 10' to 50 X 10 kcal/m hr., the test data of the inside surface of the smooth tube are shown with solid lines and those of the cross-rifled tube according to the present invention are shown with dotted lines to compare the relative effects and a quality scale is also given on the abscissa for reference. As is obvious from this graph, in both a single-phase region a twophase region, in the case of the cross-rifled tube, the inside surface temperature is much lower than in the case of the smooth tube and the heat transfer is promoted. That is to say, even in the region in which the wall temperature rises quickly in the case of the smooth tube, the nucleate boiling is kept up to the high quality region in the case of the cross-rifled tube. Therefore, the wall temperature of the cross-rifled tube is much lower than of the smooth tube.

Further, as can be seen from FIG. 3, the quick rise of the wall temperature in the case of the smooth tube begins below a quality of 50%. It is judged from these test results that the burn-out occurs substantially in the state of bubble flow and is so-called fast burn-out by the transition from nucleate boiling to film boiling.

The maximum value (Tw max.) of the wall temperature vs. the heat flux at that time as shown with the mass velocity as a parameter is as in FIG. 4. In this graph, the smooth tube and the cross-rifled tube of the present invention are compared with each other for the three conditions of mass velocities of 900, 700 and 400 kg/m sec. In the case of the cross-rifled tube of the present invention, the above mentioned maximum value (Tw max.) of the wall temperature is much lower than in the case of the smooth tube and, in this respect, too, the effect of the cross-rifled tube is shown to be remarkable.

It is shown that the cross-rifled tube the present invention can well endure physical burn-out at a heat flux of 60 X 10 kcallm hr. even under such severe condition as ofa mass velocity of 400 kg/m. sec. As the maximum local heat flux of an oil burning oiler is 50 to 60 X 10 kcal/m. hr., if such cross-rifled tube is used in high heat flux parts of an ordinary boiler using a fossil fuel, there is no danger of a physical burn-out and it contributes much to the design of a subcritical pressure boiler.

Further, the superiority of this cross-rifled tube can be proved even from the results of the two-phase airwater flow test at normal temperature and pressure. That is to say, according to the flow test, in the crossrifled tube, in the case of bubble flow, bubbles are more likely to concentrate in the center part of the tube than in the smooth tube and the ribbed tube of the prior art and, in the case of annular flow, the water film thickness becomes larger. Therefore, it is thought that the cross-rifled tube of this invention is superior to tubes of any other type and shape in both fast burn-out and slow burn-out and enables an increase in the critical heat flux. Further, what is to be specifically noted is that the pressure drop in the cross-rifled tube in a single-phase flow and two-phase flow is small. This a remarkable superiority to the ribbed tube having a large lead angle.

The cross-rifled tube of the present invention is made by using a cold-drawing process with a die and plug. First of all, as a first step working, a plug on which a plurality of spiral grooves are made in advance is inserted into a tube and a plurality of spiral lands are formed on the inside surface of a smooth tube which is a mother tube by the free rotation of this plug and then as a second step working, another plug on which a plurality of spiral grooves with the lead angle reversed are made or on which straight grooves are made is inserted into the above mentioned tube in which the spiral lands are made and a part of the spiral lands made in said tube is plastically pressed down by the free rotation or straight drawing of this plug so that many projections of a rhombic or any shape may be uniformly and discontinuously made on the inside surface of the tube.

As mentioned above, many projections are set on the inside surface of the tube. The shape of the projection is different depending on the shape of the plug to be used or the combination of such dimensional data as the percentage of the area reduction.

From the results of broad tests, it has been found that the shape and arrangement of the projections of the cross-rifled tube of this invention are very advantageous to the heat transfer effect. As a result, it has been found that, in order that the cross-rifled tube of the present invention may maintain nucleate boiling from a low quality region to a high quality region for such given conditions as pressure, heat flux and mass velocity, it should have projections (lands satisfying the following rations and arrangement conditions:

P/h 5 to 40, h/d, 0.005 to 0.08

b/p 0.2 to 0.8 and a B 20 to 80 wherein P represents a pitch of the projections deter mined by the number of spirals determined by the lead distances of the lead angles a and B of the spirals and one cross-section, h represents a height of the projection, d represents a minor inside diameter of the tube and b represents a width of the projection as projected in the radial direction of the tube. Further, the shape and arrangement of the projections selected to obtain the best results in maintaining nucleate boiling should satisfy the following conditions:

P/h 8 to 25,

h/d 0.01 to 0.07

b/P= 0.3 to 0.6 and a+B=30to In the above mentioned limitations of the respective magnitudes, the numbers of spirals counted in one cross-section and convenient to provide in the manufacture are 6, 12, 18 and 24 spirals. However, in the tube of the present invention, 12 and I8 spirals are used. In the above mentioned Table 2, the respective ratio conditions of four kinds of test tubes are shown. As shown in the table, the respective test tubes are satisfactory within the range of all the ratio and arrangement conditions. No great difference is recognized at all in the test results of these tubes. Among the above mentioned conditions, only the last mentioned arrangement condition of a B 20 to is a condition determined by the cold-drawing by the free rotation of the plug, that is,the limitation in the manufacture. The setting of the spiral lands on the inside surface of the tube by the free rotation of the plug is determined by the frictions between the plug and die and the tube and the maximum limit of each of a and B is 43.

The shape and arrangement of the projections in the cross-rifled tube bringing about the maximum critical heat flux are different depending on the pressure to be used. However, it has been confirmed that, in the above mentioned embodiment of a sub-critical pressure of 210 atmo (Kg/cm") in the case of a mass velocity of 700 Kg/m" sec. and a heat flux of 60 X kcal/m hr., nucleate boiling is maintained until a quality of 70 percent. Further, at a mass velocity of 400 kg/m sec,, it is possible to keep the allowable temperature of an ordinary boiler tube below 500C. under such severe condition as heat flux of 60 X 10 kcal/m hr. By using a cross-rifled tube of the present invention, almost all the problems in the design of the boiler tube are obviated.

What is claimed is:

1. A vapor generating tube of the two-phase type operated at a pressure above sub-critical pressure and below critical pressure of the vapor and sub-jected to high heat flux, comprising;

a tube-like body for passing fluid therethrough and having an inner wall surface, said inner wall surface is cross-rifled and has a plurality of projections uniformly and regularly spaced thereon along the cross-rifling axes, and each of said projections hav ing a quadrilateral shape.

2. A vapor generating tube as in claim 1 wherein said tube-like body has a minor inside diameter d and each of said projections have a projection height h, a projection pitch P, and a projection width b, wherein P/h is in the range of 5 to 40, h/d, is in the range of 0.005 to 0.08, and b/P is in the range of 0.2 to 0.8, the spiral lead angles of the cross-riflings are a and B and a B is in the range of 20 to 80.

3. A vapor generating tube as in claim 2 wherein P/h is in the range of 8 to 25, h/d is in the range of 0.01 to 0.07, b/P is in the range of 0.3 to 0.6 and a B is in the range of 30 to 4. A vapor generating tube as in claim 2 wherein 20 a 43 and B 0.

Claims

1. A vapor generating tube of the two-phase type operated at a pressure above sub-critical pressure and below critical pressure of the vapor and sub-jected to high heat flux, comprising; a tube-like body for passing fluid therethrough and having an inner wall surface, said inner wall surface is cross-rifled and has a plurality of projections uniformly and regularly spaced thereon along the cross-rifling axes, and each of said projections having a quadrilateral shape.

2. A vapor generating tube as in claim 1 wherein said tube-like body has a minor inside diameter d1, and each of saiD projections have a projection height h, a projection pitch P, and a projection width b, wherein P/h is in the range of 5 to 40, h/d1 is in the range of 0.005 to 0.08, and b/P is in the range of 0.2 to 0.8, the spiral lead angles of the cross-riflings are Alpha and Beta , and Alpha + Beta is in the range of 20* to 80*.

3. A vapor generating tube as in claim 2 wherein P/h is in the range of 8 to 25, h/d1 is in the range of 0.01 to 0.07, b/P is in the range of 0.3 to 0.6 and Alpha + Beta is in the range of 30* to 75*.

4. A vapor generating tube as in claim 2 wherein 20* Alpha <43* and Beta 0*.