Surface for boiling liquids

Abstract

A boiling surface layer is formed on a thermally conductive wall comprising a plurality of ridges separated by grooves provided at microscopic density, with outer sections of the ridges partly deformed into adjacent grooves to provide sub-surface cavities with restricted openings to the outer surface and sub-surface, openings between some of the cavities.

Description

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 634,403 filed Apr. 7, 1967 and entitled, "Surface for Boiling Liquids," and now abandoned which in turn is a continuation-in-part of application Ser. No. 414,755 filed Nov. 30, 1964 and entitled, "Heat Exchange System," and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the art of improving heat transfer from heated surfaces to boiling liquids, and particularly to surfaces which enhance the phenomenon of nucleate boiling. The invention also relates to a method for forming a layer containing the surfaces.

The transfer of heat at effective rates from a heated surface to a boiling liquid in contact therewith ordinarily requires a substantial temperature difference between the surface and the liquid which greatly affects the efficiency of heat transfer. One important factor controlling this efficiency is the nature of the heated surface in contact with the liquid; it being known, for example, that smooth boiling surfaces produce low heat transfer coefficients on the boiling side. Low boiling heat transfer coefficients often severely restrict the heat transfer capacity of boiling apparatus. For example, when the heat for boiling is supplied by a vapor condensing on a smooth-walled heat transfer surface, the condensing heat transfer coefficient may easily be on the order of 2,000 B.t.u./hr./sq. ft./°F., while the boiling heat transfer coefficient against the opposite side of the heat transfer surface may be only 100 to 200 B.t.u./hr./sq. ft./°F. According to the familiar method of summing heat transfer resistances when the boiling and condensing heat transfer surfaces are of equal area, the overall heat transfer coefficient U is obtained approximately as follows: ##EQU1## where h_B and h_C are the boiling and condensing heat transfer coefficients respectively. It is clear that if h_B is small compared to h_C, then the value of U approaches h_B and most of the advantage of a high condensing coefficient is lost.

Principal objects of this invention are: to provide a thermally conductive wall for transferring heat to a boiling liquid in a heat exchange apparatus having a boiling surface layer containing a plurality of cavities adapted to provide boiling nucleation sites within the surface layer; to provide a thermally conductive wall with a grooved boiling surface layer of a character which produces boiling heat transfer coefficients many times as large as those obtained with conventional smooth or roughened surfaces; and to provide a cross-grooved boiling surface layer of a character that is able to transfer to a boiling liquid large quantities of heat at much lower temperature differences than required in conventional heat exchange apparatus.

These and other objects and novel features of the invention will become apparent from the following description and accompanying drawing.

SUMMARY

According to this invention, there is provided a heat exchange wall having a boiling surface layer formed thereon with a plurality of cavities within the boiling surface layer. These cavities are sub-surface cavities adapted to entrap vapor bubbles within the boiling surface layer to provide boiling nucleation sites. The cavities open to the outer surface of the boiling surface layer through restricted openings which have cross-sectional area smaller than the largest cross-sectional areas in the cavity interiors and which provide communication between the interiors of the cavities and the surface of the boiling surface layer for vapor egress during boiling and liquid ingress. The cavities also have sub-surface openings providing communication between the interiors of the cavities for liquid ingress to sustain growth of the entrapped vapor bubbles during the boiling process as vapor is expelled from the restricted openings.

In one embodiment of this invention, the boiling surface layer of a heat exchange wall is formed by providing a plurality of ridges in the surface of the wall, each ridge being separated from adjacent ridges by grooves at microscopic density. The outer sections of the ridges remote from the wall are partially deformed into adjacent grooves such that the aforementioned cavities are formed in the grooves. As used herein the term "microscopic" refers to objects so small or fine as to be not clearly distinguished without the use of a microscope. The individual grooves, cavities and ridges of the low groove density boiling surface layers (e.g. 20 grooves per inch) are visible to the naked eye. However the restricted openings from the cavities to the outer surface of even these layers cannot be readily distinguished without aid of a microscope. Since the cross-sectional area relationship between the cavity and the restricted opening is essential to the growth of vapor bubbles, the layers of this invention may not be clearly identified by the naked eye. It is in this sense that the grooves are provided at microscopic density.

In another, and preferred, embodiment of this invention the boiling surface layer of a heat exchange wall is formed by providing a plurality of ridges in the surface of the wall, each ridge being separated from adjacent ridges by grooves, and by providing a second plurality of depressions or grooves superimposed on the ridges at an angle to the orientation of the ridges. By superimposition of depressions or grooves on the ridges, the ridges are segmented into sections and the extent of segmentation depends in part on the relative depths of the two sets of grooves. For example, if the superimposed grooves have the same depth as the first-formed grooves, the ridge sections will tend to be completely isolated from adjacent ridge sections. The outer sections of the ridges are partially deformed into adjacent grooves such that the aforementioned cavities are formed in the grooves. This embodiment has a cross-grooved appearance.

In both of the embodiments described above, the grooves perferably extend substantially completely across the surface of the heat exchange wall and are preferably of uniform density. These two preferred conditions enhance the likelihood of uniform boiling performance across the boiling surface layer. In addition, the density of the grooves is preferably relatively high, being greater than 20 grooves per inch, for reasons that will be discussed subsequently.

Another aspect of this invention relates to a method for forming a boiling surface layer from a thermally conductive metal wall. The grooved boiling surface layer embodiments described above are preferably formed by scoring the surface of the heat exchange wall such that the wall material is substantially displaced into adjacent ridges rather than removed. When a scoring tool is used to form the boiling surface layer, the tool will tend to displace the wall material upward from the wall surface and outward away from the tool as the tool moves across the wall surface such that grooves separated by ridges are formed in the wall material. In forming the preferred cross-grooved boiling surface embodiment, a second set of grooves is scored across the first-formed grooves and ridges, at an angle--preferably 90°--to the orientation of the latter, such that the first-formed ridges are segmented into sections. This cross-scoring further displaces the wall material.

If needed, according to the method of this invention, cutting techniques other than scoring may be used to form the first set of grooves as for example milling. This novel method also contemplates forming a second set of depressions or grooves using other metal displacement techniques such as rolling or knurling.

BRIEF DESCIPTION OF THE DRAWING

FIG. 1 is a photomicrograph, magnification -20 fold, of the top surface of a cross-grooved boiling surface layer embodiment of this invention.

FIG. 2 is a photomicrograph, magnification -75 fold, of a cross-section of the boiling surface layer of FIG. 1 taken in a vertical plane approximately along the lines 2--2 in FIG. 1.

FIG. 3 is a photomicrograph, magnification -40 fold, of a cross-section of a boiling surface layer similar to that shown in FIGS. 1 and 2 taken in a vertical plane in the same manner as FIG. 2.

FIG. 4 is a photomicrograph, magnification -20 fold, of the top surface of another cross-grooved boiling surface layer embodiment of this invention.

FIG. 5 is a photomicrograph, magnification -40 fold, of a cross-section of still another boiling surface layer embodiment taken in a vertical plane.

FIG. 6 is a photomicrograph, magnification -20 fold, of the top surface of a single direction grooved boiling surface layer embodiment of this invention.

FIG. 7 is a photomicrograph, magnification -40 fold, of a cross-section of the boiling surface layer of FIG. 6 taken in a vertical plane approximately along the lines 7--7 in FIG. 6.

FIG. 8 is a photomicrograph, magnification -40 fold of a cross-section of a boiling surface layer similar to that shown in FIGS. 6 and 7 taken in a vertical plane in the same manner as FIG. 7.

FIG. 9 is a graph showing pool boiling performance data in water for a smooth aluminum surface, dashed line, and for aluminum surfaces of this invention.

FIG. 10 is a graph showing pool boiling performance data in liquid nitrogen for a smooth aluminum surface, dashed line, and for aluminum surfaces of this invention.

FIG. 11 is a schematic view taken in cross-sectional elevation, of single-direction scoring apparatus suitable for practicing the method of this invention for forming boiling surface layers.

FIG. 12 is a schematic view taken in cross-sectional elevation of two-direction scoring apparatus.

FIG. 13 is a schematic view taken in cross-sectional elevation of apparatus suitable for simultaneously scoring several grooves using a circular tool.

FIG. 14 is an end view of the FIG. 13 apparatus.

FIG. 15 is a schematic view taken in cross-sectional elevation of single-direction milling apparatus.

FIG. 16 is a schematic view taken in cross-sectional elevation fold, apparatus suitable for milling the boiling surface layer according to the method of this invention using a rotary cutter.

FIG. 17 is a schematic view taken in cross-sectional elevation of apparatus suitable for cross knurling the first groove set.

FIG. 18 is an end view of the FIG. 17 apparatus.

greater than 20, i.e. between .Badd.The performance advantages of this boiling surface layer as compared to smooth surfaces are also demonstrated in the FIGS. 19 and 20 graphs for single direction scored surfaces in boiling water (FIG. 19) and boiling liquid nitrogen (FIG. 20) for a wide range of groove densities between 29 and 230 grooves per inch. These particular fluids are selected as representing those characterized by low surface tension (liquid nitrogen) and high surface tension (water) spanning a wide temperature range of between -196°C and 100°C

The boiling surface layers were all formed from aluminum sheeting using scoring tools similar to those illustrated in FIGS. 11 and 12, and all parameters were identical except the groove spacing. The groove depth was nominally 8 mils, that is, the scoring tool and the wall were positioned to cut grooves at this depth. The included angle of the scoring tool tip was 30 degrees and the scoring tool inclined angle to the wall was +10 degrees. The number of grooves per inch was nominal, in that the scoring assembly was set to cut the designated number of grooves. This is the same criteria used to designate the groove density throughout the disclosure and claims. The single-direction scored aluminum surfaces used in the water and liquid nitrogen pool boiling test summarized in FIGS. 19 and 20 are follows.

Table III
______________________________________
Surface No.:    Grooves per inch
______________________________________
1                29
2                45
3                70
4               100
5               140
6               230
______________________________________

FIG. 19 demonstrates that with even the relatively low groove densities of surfaces 1 and 2, a very significant improvement is afforded over smooth surfaces. For example if a heat flux of 5×10³ B.t.u./hr.-ft.² is required in a given system, this level may be achieved with a ΔT of about 6° F. whereas the smooth surface requires a ΔT of about 13.5° F. The improvement is even greater with higher heat fluxes, primarily due to the steep slopes of the boiling surface layers as contrasted with the lower slope of the smooth surface. It should be noted that although the highest groove density (surface 6) affords the lowest ΔT values for a given heat flux, the slope of this surface is appreciably lower than for the lower groove densities (surfaces 1-5). At relatively high heat fluxes these curves demonstrate that surface 6 is only marginally superior to surfaces 4 and 5, and the latter would be preferred for water boiling due to their lower fabricating costs and higher durability. A probable explanation for this phenomenon is the relatively high surface tension of water, whereby vapor bubbles may be retained in relatively large cavities characteristic of relatively low groove densities.

FIG. 20 demonstrates a very substantial improvement for the single-direction scored aluminum surfaces in boiling liquid nitrogen. For example at a heat flux of 1×10⁴ B.t.u./hr.-ft.², the smooth surface requires a ΔT of about 15° F. whereas the ΔT values for surfaces 1, 4 and 6 are respectively as follows: 5.8, 4.1, and 1.8. It will be apparent from FIG. 20 that the slope for surface 6 (highest groove density) is about the same as for surface 1 (lowest groove density) along with the former's substantially superior performance, reflecting a preference for smaller reentrant cavities for relatively low surface tension liquids such as nitrogen.

FIGS. 19 and 20 (in addition to FIGS. 9 and 10) demonstrate that the boiling surface layer affords outstanding performance for boilable liquids of any surface tension, whether relatively low or high.

FIG. 21 19 illustrates the performance of ten different cross-score surfaces with between 45 and 225 grooves per inch for boiling liquid nitrogen at -196°C water. The surfaces were formed from six different metals as follows: copper (surface 1), a low copper-high nickel alloy with 3% Fe, Mn and trace elements (surfaces 2 and 3), nickel (surface 4), 70% Cu-30% Ni (surface 5), 90% Cu-10% Ni (surface 6) and aluminum (surfaces 8, 9 and 10). The surfaces were all scored using tools very similar to those illustrated in FIGS. 11 and 12, and the second set of grooves were cut at 90 degrees orientation to the first set of grooves and ridges. The parameters used in preparing surfaces 1-10 by the method of this invention are summarized in Table IV III as follows:

TABLEIVIII
______________________________________
Depth of
Depth of
first- second-
Included
Grooves  formed formed angle of
per      grooves
grooves
scoring
Surface
Material  inch     (mils) (mils) tool1
______________________________________
1      Copper     60      23     8      30
2      30% Cu,2
83      8      6
67% Ni.
3      30% Cu, 2
200      11     8
67% Ni.
4      Nickel    140      8      8
5      70% Cu,   100      8      4(3)
3
30        % Ni.
6      90% Cu,   140      8      4(3)
3
10% Ni. -7
Aluminum  45    8      8 30
8      "         100      8      8      30
9      "         120      8      4      49
10    "         225      8      8      30
______________________________________
1 Angle of scoring tool inclination is +10° for all surfaces
except as noted.
2 Balance of 3% is Fe, Mn and trace elements.
3 Angle of scoring tool inclination is 0°.

Inspection of FIG. 21 19 reveals that in general the higher groove density surfaces performed more efficiently, compare for example surfaces 5-6 and surfaces 7-8. This is the same conclusion drawn with the single-direction scored surfaces of FIG. 20, and again is attributed to the relatively low surface tension of liquid nitrogen. Another conclusion from FIG. 21 19 is that remarkable improvement in boiling heat transfer efficiency can be achieved using virtually any type of metal with the surface of this invention. That is, all surfaces 1-10 demonstrated far higher effectiveness than the smooth surfaces.

It has been demonstrated that the boiling surface layers of this invention afford remarkably high heat transfer coefficients in sea water brine under brine-scaling conditions. Moreover it was found that when scale-forming conditions were established such that the boiling surface layers became fouled and the coefficent dropped, the original heat transfer performance of the surface was completely restored by adding 0.5% HCl to the sea water. This hydrochloric acid dissolved the scale on the grooved layer.

More particularly, the cross-scored copper surface illustrated in FIG. 5 and identified in Table IV III as surface 1 having 60 grooves per inch was used in a continuously circulatory system to boil three different simulated sea water solutions at a constant heat flux of 5,000 B.t.u./hr.-ft.². The alkalinity was about 100 p.p.m. and the concentration factor was 2.5 to 3∅ Concentration factor is defined as the ratio of the solution's salinity to that of normal sea water. The first solution was selected to contain sufficient CaCO₃ scale for precipitation, the pH value being about 0.5 pH units above the saturation pH of 6.9. Under these conditions the boiling coefficient deteriorated from about 3,000 to about 500 B.t.u./hr.-ft.² -° F. Addition of 0.5% HCl caused the CaCO₃ scale to dissolve with the evolution of CO₂, and the coefficient returned to about 3,000. Based on the amount of HCl added, it was estimated that 2- 5 grams of CaCO₃ deposited on the boiling surface layer (50-100 grams per sq. ft. of grooved surface boiling layer) when complete fouling had occurred.

Next the concentration factor was increased to 3.0 where both CaSO₄ and CaSO₄.1/2H₂ O were above saturation. The pH of the feed in this test was reduced to prevent the other scale former from precipitating. The heat transfer characteristics of the grooved surface boiling layer again deteriorated to a coefficient of about 750 indicating scaling. Hydrochloric acid was again added in the same concentration as before, but no carbon dioxide evolved in this case indicating absence of CaCO₃. The boiling coefficient again returned to about 3,000 B.t.u./hr.-ft.² -° F., showing that CaSO₄ scale can be removed by washing with acid. The system was then operated for several days under conditions where all of the major scale-forming compounds (CaCO₃, CaSO₄, and Mg(OH)₂) were below their solubility limits at the boiling point. The boiling coefficient remained constant at about 3,000, indicating that scaling had caused no permanent deterioration of the grooved surface layer. This coefficient is about 10 times greater than achievable with an equivalent smooth metal surface under the same operating conditions.

In the foregoing discussion, reference has been made to liquids such as oxygen and nitrogen comprising a class of fluids characterized by low surface tension which are preferably boiled on surface layers having high groove density between 140 and 200 grooves per inch. Similarly, reference is made to liquids such as water comprising a class of fluids characterized by high surface tension which are preferably boiled on surface layers having low groove density of greater than 20, i.e. between .Badd.20.Baddend. 45 and 120 grooves per inch. In general, as the groove density is creased, the size of the cavities and the limiting dimension of the restricted openings therefrom to the outer surface is decreased.

FIG. 22 20 correlates the limiting dimension in mils of the restricted opening and the heat transfer coefficient in B.t.u./hr.-ft.² -° F. measured at a heat flux of 20,000 B.t.u./hr.-ft.². The curves represent data accumulated from a number of boiling surface layers of different groove densities and restricted opening limiting dimensions. For each boiling surface layer, the limiting dimension of the restricted opening was measured by visually scaling directly from a microscopic enlargement of the boiling surface layer cross-sections. As used herein, the "limiting dimension" represents the largest diameter vapor bubble which may emerge from the cavity to the outer surface of the boiling layer. For example, the limiting dimension may be the minor dimension of an ovoid or elliptically-shaped restricted opening. Limiting dimensions for certain of the boiling layers illustrated in the photomicrograph figures may be directly sealed as follows in Table V IV.

TABLEVIV
______________________________________
Limiting dimension
of restricted open-
FIG. NO.  Grooves per inch
ing(mils)
______________________________________
1 and 2   140            0.5
3         208            0.5
5          60            3.1
6 and 7   230            1.0
______________________________________

Three different fluids, liquid nitrogen (B.P. -320° F.), water (B.P. 212° F.), and a 30% by weight ethylene glycol in water (B.P. 218° F.) were boiled at one atmosphere pressure in contact with both single and cross-grooved surface layers to obtain the data for FIG. 22 20. The data for each fluid is plotted separately. It is evident that the surface exhibiting the highest heat transfer coefficients are rather precisely defined as having limiting dimensions for restricted openings of less than about 5 mils. Such surfaces represent a preferred embodiment of this invention.

Also based on the data summarized in FIG. 22 20, the surface layers best suited to boiling low surface tension fluids such as liquid nitrogen are those having limiting dimensions of restricted openings below about 1.75 mils (most suitably at the previously discussed 140-200 grooves per inch), while those best suited to the high surface tension fluids such as water have limiting dimensions between about 1.75 and 4 mils (most suitably at the previously discussed .Badd.20-120.Baddend. greater than 20, i.e. 45-120 grooves per inch). Such boiling surface layers represent still more preferred embodiments of this invention.

It is to be expected that for a particular boiling surface layer, the heat transfer coefficients for the 30% ethylene glycol solution will be consistently well below those for pure water. This is because liquid mixtures are characterized by lower coefficients than their pure constitutents.

Claims

1. A thermally conductive metal wall for transferring heat to a boiling liquid in a heat exchange apparatus which comprises a boiling surface layer formed from the wall having a plurality of ridges in said wall separated by grooves provided at microscopic density, with outer sections of said ridges partly deformed into said grooves such that a plurality of sub-surface cavities are formed therein with at least some of said cavities adapted to entrap vapor bubbles to provide boiling nucleation sites, the nucleation site cavities opening to the outer surface of said boiling surface layer through restricted openings having smaller cross-sectional area than the largest cross-sectional area of the cavity interiors with said restricted openings providing communication between the interiors of said cavities and the outer surface of said boiling surface layer for vapor egress; and said grooves and cavities being formed to provide sub-surface openings between at least some adjacent cavities providing fluid communication with the outer surface of said boiling surface layer for liquid ingress to sustain growth of entrapped vapor bubbles as vapor is expelled from said

restricted openings. 2. A thermally conductive wall according to claim 1 17 in which said ridges and grooves are parallel to each other.

3. A thermally conductive wall according to claim 1 in which the grooves are provided at density of greater than about 20 grooves

per inch. 4. A thermally conductive wall according to claim 1 17 in which the grooves are provided at density of greater than about 80 grooves per inch.

5. A thermally conductive wall according to claim 1 in which said ridges are formed by metal displaced from said grooves.

6. A thermally conductive wall according to claim 1 in which a plurality of second grooves in spaced relation to each other and at microscopic density are superimposed on said ridges in intersecting relation therewith.

7. A thermally conductive wall according to claim 1 in which a plurality of depressions in spaced relation to each other and at microscopic density are superimposed on said

ridges in intersecting relation therewith. 8. A thermally conductive metal wall for transferring heat to a boiling liquid in a heat exchange apparatus which comprises a boiling surface layer formed from the wall having a plurality of ridges in said wall separated by first grooves provided at microscopic density of from about 45 to about 225 grooves per inch with outer sections of said ridges partly deformed into said first grooves, and a plurality of second depressions in rows spaced from each other provided at microscopic density of from at least about 45 to about 225 depressions per inch and superimposed on said ridges in intersecting relation therewith, said ridges, first grooves and second depressions being shaped such that a plurality of sub-surface cavities are formed in said first grooves with at least some of said cavities adapted to entrap vapor bubbles to provide boiling nucleation sites, the nucleation site cavities opening to the outer surface of said boiling surface layer through restricted openings having smaller cross-sectional area than the largest cross-sectional area of the cavity interiors with said openings providing communication between the interiors of said cavities and the outer surface of said boiling surface layer for vapor egress, and said first grooves, second depressions and cavities being formed to provide sub-surface openings between said at least some adjacent cavities providing fluid communication with the outer surface of said boiling surface layer for liquid ingress to sustain growth of entrapped vapor

bubbles as vapor is expelled from said restricted openings. 9. A thermally conductive wall according to claim 8 in which said ridges and first

grooves are parallel to each other. 10. A thermally conductive wall according to claim 8 in which said ridges are formed by metal displaced

from said first grooves and second depressions. 11. A thermally conductive wall according to claim 8 in which said second depressions are grooves

oriented parallel to each other. 12. A thermally conductive wall according to claim 8 in which the depth of said first grooves is greater than the

depth of said second depressions. 13. A thermally conductive wall according to claim 8 formed of aluminum in which said ridges and first grooves are parallel to each other with said ridges formed by metal displaced from said first grooves and second depressions, said second depressions are grooves oriented parallel to each other and 90 degrees from said first grooves, and said first grooves and second depressions are

provided at density of 140- 200 per inch. 14. A thermally conductive wall according to claim 8 in which said ridges and first grooves are parallel to each other, said second depressions are oriented parallel to each other and 90 degrees from said first grooves, and said first grooves are provided at density of .Badd.20-120.Baddend. 45- 120

per inch. 15. A thermally conductive wall according to claim 8 in which said ridges and first grooves are parallel to each other, said second depressions are parallel to each other and oriented 90 degrees from said first grooves.

16. A thermally conductive metal wall for transferring heat to a boiling liquid in a heat exchange apparatus which comprises a boiling surface layer formed from the wall having a plurality of ridges in said wall separated by grooves provided at density of greater than about 80 grooves per inch, with outer sections of said ridges partially deformed into said grooves such that a plurality of sub-surface cavities are formed therein with at least some of the cavities adapted to entrap vapor bubbles to provide boiling nucleation sites, the nucleation site cavities opening to the outer surface of said boiling surface layer through restricted openings having smaller cross-sectional area than the largest cross-sectional area of the cavity interiors providing communication between the interiors of said cavities and the surface of said boiling surface layer for vapor egress, and said grooves and cavities being formed to provide sub-surface openings between at least some adjacent cavities for communication between the interiors of said adjacent cavities and the outer surface of said boiling surface layer for liquid ingress to sustain growth of entrapped vapor bubbles as vapor is expelled

from said restricted openings. 17. A thermally conductive metal wall for transferring heat to a boiling liquid in a heat exchange apparatus which comprises a boiling surface layer formed from the wall having a plurality of ridges in said wall separated by first grooves with outer sections of said ridges partially deformed into said first grooves, and a plurality of second grooves superimposed on said ridges at an angle to the orientation of said ridges, said ridges and first and second grooves being shaped such that a plurality of sub-surface cavities are formed in said first grooves with at least some of the cavities adapted to entrap vapor bubbles to provide boiling nucleation sites, the nucleation site cavities opening to the outer surface of said boiling surface layer through restricted openings having smaller cross-sectional areas than the largest cross-sectional area of the cavity interiors providing communication between the interiors of said cavities and the surface of said boiling surface layer for vapor egress, and said first and second grooves and cavities being formed to provide sub-surface openings between said at least some adjacent cavities for communication between the interiors of said adjacent cavities and the outer surface of said boiling surface layer for liquid ingress to sustain growth of entrapped vapor bubbles as vapor is expelled from said restricted openings , with said first and second grooves each provided at density of from about 45 to about 225 grooves per inch.

18. A thermally conductive metal wall for transferring heat to a boiling liquid in a heat exchange apparatus which comprises a boiling surface layer having a pluraity of ridges separated by grooves provided at microscopic density with said ridges shaped to form a plurality of sub-surface cavities with at least some of said cavities adapted to entrap vapor bubbles and constitute boiling nucleation sites, the nucleation site cavities communicating with the outer surface of said boiling layer through restricted openings between outer sections of said ridges and having smaller cross-sectional area than the largest cross-sectional area of the cavity interiors for vapor egress, and said grooves and cavities being formed to provide sub-surface openings between at least some adjacent cavities providing fluid communication with the outer surface of said boiling surface layer for liquid ingress to sustain growth of entrapped vapor bubbles as vapor

is expelled from said restricted openings. 19. A thermally conductive wall according to claim 18 17 in which the limiting dimension of said restricted openings is less than about 5 mils.

20. A thermally conductive wall according to claim 1 in which the limiting

dimension of said restricted opening is less than about 5 mils. 21. A thermally conductive wall according to claim 8 in which the limiting

dimension of said restricted openings is less than about 5 mils. 22. A thermally conductive wall according to claim 8 in which said ridges and first grooves are parallel to each other with said ridges formed by metal displaced from said first grooves and second depressions, said second depressions are grooves oriented parallel to each other, said first grooves and second depressions are provided at density of 140- 200 per inch, and in which the limiting dimension of said restricted openings is

less than 1.75 mils. 23. A thermally conductive wall according to claim 8 in which said ridges and first grooves are parallel to each other, said second depressions are oriented parallel to each other, said first grooves are provided at density of 45- 120 per inch, and in which the limiting dimension of said restricted openings is 1.75- 4 mils.