GB2034355A

GB2034355A - Improved heat-transfer surface and method for producing such surface

Info

Publication number: GB2034355A
Application number: GB7845781A
Authority: GB
Original assignee: UOP LLC
Current assignee: Honeywell UOP LLC
Priority date: 1978-11-23
Filing date: 1978-11-23
Publication date: 1980-06-04
Also published as: GB2034355B

Abstract

A heat-transfer tube 10 or other member having a surface coated with a non-particulate reticulated metal structure which is in intimately bonded thermally conducting relationship thereto and is shell- like with the metal therein optionally surrounding a reticulated non- metallic core portion is obtained by applying a layer of reticulated organic foam material to the surface of the metal heat-transfer member and plating the exposed surface(s) of the reticulated foam with a metal (eg copper) first by electroless then by electrodeposition, optionally followed by pyrolysis of the organic foam material, to provide a porous nucleate-boiling surface 18. <IMAGE>

Description

SPECIFICATION Improved heat-transfer surface and method for producing such surface Improved heat-transfer tube technology in recent years has been highly dependent upon the improvement of two-phase heat transfer, that is the transfer of thermal energy due to the phase transformation from the liquid to the vapor phase. The methods to improve this two-phase heat transfer include both passive and active techniques. Passive techniques in clude surface treatments, roughening the surface, extending the surfaces, displaced enhancement, swirl flow techniques, alteration of surface tension, and the inclusion of additives to the coolant. Active techniques include mechanical aids, surface vibration, fluid vibration, and the addition of electrostatic fields.

In the area of treated surfaces, various materials are deposited on the heat-transfer tube surfaces to promote boiling. Such materials have included polytetrafluoroethylene, tube surface oxides, and the addition of high-surface copper powder. These surface treatments improve the wettability of the surface and result in a low wall super-heat which eliminates boiling curve hysteresis.

Surface roughening is a technique to provide a large number of nucleation sites on the tube surfaces. The technique involves the mechanical deformation of the surface to provide a large number of re-entrant cavities.

Extended surface tubes are produced by finning techniques which yield high external surface areas to the tube and allow very large heat transfer rates if the base temperature is in the film boiling range; however, nucleate boiling is not promoted with this type of heat transfer tube.

Displaced enhancement techniques promote boiling by taking advantage of hydrodynamic instability in the coolant when open structures are placed directly above the heat transfer surface.

Surface tension devices operate on the wicking principle which relies on capillary forces while the addition of additives to the coolant affects the wettability of the coolant to the heat transfer tube.

A number of mechanical boiling aids have been proposed including rotating of the boilers themselves, the introduction of rotating plates, and the introduction of bubbles into the vicinity of the heat-transfer surface.

The purpose of vibrating either the fluid or the surface is to form localized nucleate boiling sites due to pressure variations in the liquid. The use of electrostatic fields improves mixing within the coolant and is used principally with poorly conducting or dielectric fluids.

Of the above techniques, those that promote nucleate boiling are of principal interest.

from a technological viewpoint. The parameters of importance in a nucleate boiling tubecoolant system include the specific heat of the liquid, the specific heat of the tube material, the heat transfer coefficient, the latent heat of vaporization, the thermal conductivity of the liquid on the heater tube, the geometry of the nucleation site, the temperature of the coolant, vapor, and surface, the liquid Viscosity, the surface tension, and the densities of the liquid and vapor phases.

The nucleate boiling phenomenon involves two separate operations. The first of these is the nucleation of the vapor phase within the liquid while the second is the subsequent growth of the vapor phase to form bubbles within the liquid. It has been postulated that improved efficiency of heat transfer can be attained when the nucleation process does not have to be continuously redone. This nucleation process requires a large amount of superheating. Improved efficiency can be observed if the thermal energy is transferred by the growth of pre-existing vapor phase nuclei.

This approach has resulted in the specification of re-entrant cavities as highly effective nucleate boiling sites.

A number of patents have been issued whereby the surface of a heat transfer tube is mechanically altered to provide these re-entrant sites. These include U.S. Patent Numbers 3,326,283; 3,454,081; 3,566,514; 3,881,342 and 3,906,604. While all of the above patents propose the improvement of nucleation by the mechanical introduction of nucleation sites, they all suffer from the common characteristic of having a relatively few number of nucleation sites per given area of tubing surface. This limitation is imposed by the manufacturing tooling required to produce the tubes, and is an inherent limitation for any mechanically produced tube.

The demonstrated heat transfer capability of a tube produced with a much higher density of nucleation sites is covered in U.S. Patent No. 3,384,1 54. This tube is of the treated surface variety mentioned above where copper powder particles are sintered to the surface of the heat exchanger tube. This provides a very high density of nucleation sites on the tube surface and allows retention of the vapor phase throughout the open pore structure of the sintered surface. This sintered-surface tube, while an effective boiling-surface and heat-transfer tube, suffers from manufacturing difficulties. The copper powder is mixed with an organic binder and sprayed onto the tube surface for ease of handling. The coated tube is then subjected to a high-temperature exposure.This decomposes the organic binder and sinters the copper particles together as well as to the base tube. The sintering temperature is stated to be about 960"C which is about 100on below the melting point of copper.

This high-temperature treatment is not only difficult to do but can result in serious degradation of the mechanical properties of the base tube. The degradation problems can be minimized by utilizing alloys whose superior recrystallization and grain growth characteristics will reduce the amount of property degradation but such alloys introduce added cost and have lower thermal conductivity.

The present invention seeks to provide an improved heat-transfer surface on a member, e.g. a tube, and a method of making same, which will produce a very high density of nucleation sites at a relatively low cost and without affecting the properties of the base member, e.g. tube.

According to the present invention there is provided a method of providing a metal heattransfer member with a porous nucleate-boiling surface comprising the steps of applying a layer of reticulated organic foam material to the surface of the metal heat-transfer member and plating the exposed surface(s) of the reticulated foam material with a metal so as to form a reticulated metal surface which overlies the surface of the metal heat-transfer member and is firmly adhered thereto.

The invention further provides a metal heattransfer member having a surface which is exposed in use to a boiling medium, said surface being coated with a non-particulate reticulated metal structure which is in intimately bonded thermally conducting relationship to the surface, the reticulated metal structure being shell-like with the metal therein optionally surrounding a reticulated nonmetallic core portion.

The heat-transfer member may be a tube, plate or other member having a heat-transfer surface. The reticulated organic foam material may be an open-cell polyurethane or other foam. When the heat transfer member is a tube the foam can be in the form of a thin strip or tape that is spirally wound around the base tube or it can be in a tubular shape which could be slipped over the tube. The foam coating can also be directly applied to the tube surface if it is foamed in such a manner as to leave open cells rather than a closed cell skin in contact with the base tube.

The open-celled nature of the foam allows free and easy access of the coolant all the way to the tube surface.

The reticulated foam forms a substrate upon which a metal, preferably copper, is plated in a subsequent operation which may be a multistep operation. The initial step of such an operation is to electrolessly plate copper using well known technology as will be discussed in detail below. After the surface of the organic foam has been made electrically conductive with the copper electroless deposition, standard electroplating of copper can then be used to build this surface layer up to the point where it has structural integrity. After washing and drying of the plated foam, the organic foam acting as precursor can be pyrolyzed if desired.

In one experiment for coating tubes we have used a reticulated polyurethane foam of 97% void volume with a pore size controlled at 39 pores per linear cm. For coating the tubes, strips approximately 2.54 cm wide by 1.6 mm thickness were wrapped in a spiral fashion along the length of the base tube and held in place during the plating operations.

The foam was first cleaned using Enthone PC-452 cleaner at 60"C. After washing, the surface was neutralized with Enthone AD-480 at room temperature. Water washing was then followed by cleaning at room temperature with a 15% HCI solution. The pretreated foam was then washed and given a room temperature exposure to Enthone 432 sensitizer, washed, then activated at room temperature with Enthone 40 activator. The assembly was then double water-rinsed and dried.

Electroless plating was done at 21 to 24"C.

using Enthone solution CU-404. The electroless plating was allowed to build up until the foam was sufficiently electrically conductive to be measured with a VOM. After washing and drying the electroless tube was then electroplated in a standard copper sulfate electroplating solution using a copper electrode and a DC voltage. Electroplating was continued until a sufficiently thick copper electrodeposit was formed so that the foam had sufficient strength to allow normal handling.

Heat transfer testing of an as-plated tube in Refrigerant R-1 1 showed a considerable improvement in the surface nucleation boiling characteristics of this tube as compared to a standard fin tube. The boiling characteristics were also superior to a commercially available nucleate boiling tube produced by mechanical means in accordance with the aforementioned U.S. Patent No. 3,906,604. Observation of the surface boiling characteristics when compared with a length of tubing as produced in accordance with the aforementioned U.S. Patent No. 3,384,154, showed that nucleation on the foam surface was quite close to that produced by the sintered copper surface.

The effect of pyrolysis of the polyurethane foam on the surface structure and boiling characteristics was then determined. The plated foamed tube was held in a laboratory gas flame until pyrolysis of the polyurethane substrate was complete. Optical and scanning electron microscopy of the remaining copper foam showed a series of very small pores along the surfaces of the skeletal copper remaining after the pyrolysis of the substrate.

These pores varied in size with a maximum of 50 ym in their largest dimension. The pores were probably produced by the pressure created during the pyrolysis of the organic substrate.

Boiling tests of the pyrolyzed tube in the same R-1 1 coolant as used previously indi cated superior performance of the pyrolyzed tube as compared to the tube before pyrolysis.

'This is undoubtedly due to the large number of very tiny vapor phase nucleation sites re sulting from the porosity due to the pyrolysis.

Since the polyurethane can be pyrolyzed at temperatures in the range of 302 to 482"C, it is obvious that the degradation problems which can take place at temperatures closer to the melting point of copper are of little conse quence.

Of the accompanying drawings, Figure 1 is a perspective view showing a thin strip of reticulated foam being wound about a plain tube; Figure 2 is a front view of a tube which has been wrapped with foam and then plated being passed over a flame to pyrolyze the foam; Figure 3 is a photomicrograph showing a portion of the plated surface of Fig. 2 before it has been pyrolyzed; and Figure 4 is a photomicrograph showing a portion of the plated surface of Fig. 2 after it has been pyrolyzed.

Referring ta Fig. 1, a copper tube 10 which has been thoroughly cleaned has a strip 1 2 of reticulated polyurethane or other organic open celled foam material wound around it. The starting end can be anchored in place by a mechanical holding means such as an elastic band 14. The opposite end of the foam strip 1 2 would be similarly anchored. After the tube is wrapped with foam, it is electrolessly plated with copper and then electroplated as previously described. A plating thickness in the range of 6.35 to 63.5 ym would seem to be sufficient.

In Fig. 2, the tube 10 is shown after the foam layer 1 2 has been plated with a coating 1 8 of copper. Although the tube 10 can provide excellent performance in nucleate boil ing with the non-pyrolyzed surface portion 18a, tests have shown that substantial im provements in performance are achieved by pyrolyzing, such as with a burner 20, to provide a pyrolyzed surface 18b.

Fig. 3 is an approximately lOOX photomi crograph of the surface area 1 8a of Fig. 2.

The copper plating can be seen as completely coating the underlying network of foam which 'it surrounds.

Fig. 4 is an approximately 100X photomi crograph of the pyrolyzed surface area 1 8b of Fig. 2. In comparing Fig. 4 to Fig. 3, one can see that the copper plating tends to break away as shown at 1 8c and fissure as shown at 1 8d as the pressure of the gas produced by the pyrolysis of the foam substrate is released.

Thus, even smaller pores are provided than those shown in Fig. 3 which are produced by the reticulated nature of the foam. These additional pores greatly increase the number of nucleation sites and permit vapor to be trapped inside the copper skeleton, which is essentially hollow as a result of the pyrolysis of the foam substrate CLAIMS (23 Nov 1978) 1.A method af providing a metal heattransfer member with a porous nucleate-boiling surface comprising the steps of applying a layer of reticulated organic foam material to the surface of the metal heat-transfer member and plating the exposed surfaces of the reticulated foam material with a metal so as to form a reticulated metal surface which overlies the surface of the metal heat-transfer member and is firmly adhered thereto.

2. A method as claimed in claim 1 wherein the plating operation comprises an initial electroless deposit of metal followed by an electro-deposit deposit of metal.

3. A method as claimed in claim 1 or 2 wherein the metal heat-transfer member is cleaned before it is plated.

4. A method as claimed in any of claims 1 to 3 wherein the metal heat-transfer member is heated after it is plated to pyrolyze the organic foam material.

5. A method as claimed in claim 4 wherein the reticulated metal coating is made sufficiently thin that portions of it will be fractured by gas developed during the pyrolysis to produce pore openings therethrough.

6. A method as claimed in claim 4 or 5 wherein the heating for pyrolysis takes place at a temperature of less then 482"C.

7. A method as claimed in any of claims 1 to 6 wherein the heat-transfer member is a tube and the layer of foam material is applied by spirally wrapping a strip of reticulated organic foam material around the tube.

8. A method as claimed in any of claims 1 to 7 wherein the reticulated metal coating has a thickness in the range of 6.35 to 63.5 ym.

9. A method as claimed in claim 1 carried out substantially as hereinbefore specifically described or illustrated with reference to the accompanying drawings.

10. A metal heat-transfer member having a surface which is exposed in use to a boiling medium, said surface being coated with a non-particulate reticulated metal structure which is in intimately bonded thermally conducting relationship to the surface, the reticulated metal structure being shell-like with the metal therein optionally surrounding a reticulated non-metallic core portion.

11. A heat-transfer member as claimed in claim 10 wherein the reticulated metal structure surrounds a non-metallic core portion which comprises an organic foam.

1 2. A heat transfer member as claimed in claim 10 wherein the reticulated metal structure is hollow.

13. A heat-transfer member as claimed in claim 1.2 wherein the reticulated metal structurn includes a plurality of small openings in its surface in communication with its hollow

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Claims

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cated superior performance of the pyrolyzed tube as compared to the tube before pyrolysis.

'This is undoubtedly due to the large number of very tiny vapor phase nucleation sites re sulting from the porosity due to the pyrolysis.

Since the polyurethane can be pyrolyzed at temperatures in the range of 302 to 482"C, it is obvious that the degradation problems which can take place at temperatures closer to the melting point of copper are of little conse quence.

Of the accompanying drawings, Figure 1 is a perspective view showing a thin strip of reticulated foam being wound about a plain tube; Figure 2 is a front view of a tube which has been wrapped with foam and then plated being passed over a flame to pyrolyze the foam; Figure 3 is a photomicrograph showing a portion of the plated surface of Fig. 2 before it has been pyrolyzed; and Figure 4 is a photomicrograph showing a portion of the plated surface of Fig. 2 after it has been pyrolyzed.

Referring ta Fig. 1, a copper tube 10 which has been thoroughly cleaned has a strip 1 2 of reticulated polyurethane or other organic open celled foam material wound around it. The starting end can be anchored in place by a mechanical holding means such as an elastic band 14. The opposite end of the foam strip 1 2 would be similarly anchored. After the tube is wrapped with foam, it is electrolessly plated with copper and then electroplated as previously described. A plating thickness in the range of 6.35 to 63.5 ym would seem to be sufficient.

In Fig. 2, the tube 10 is shown after the foam layer 1 2 has been plated with a coating 1 8 of copper. Although the tube 10 can provide excellent performance in nucleate boil ing with the non-pyrolyzed surface portion 18a, tests have shown that substantial im provements in performance are achieved by pyrolyzing, such as with a burner 20, to provide a pyrolyzed surface 18b.

Fig. 3 is an approximately lOOX photomi crograph of the surface area 1 8a of Fig. 2.

The copper plating can be seen as completely coating the underlying network of foam which 'it surrounds.

Fig. 4 is an approximately 100X photomi crograph of the pyrolyzed surface area 1 8b of Fig. 2. In comparing Fig. 4 to Fig. 3, one can see that the copper plating tends to break away as shown at 1 8c and fissure as shown at 1 8d as the pressure of the gas produced by the pyrolysis of the foam substrate is released.

Thus, even smaller pores are provided than those shown in Fig. 3 which are produced by the reticulated nature of the foam. These additional pores greatly increase the number of nucleation sites and permit vapor to be trapped inside the copper skeleton, which is essentially hollow as a result of the pyrolysis of the foam substrate CLAIMS (23 Nov 1978) 1.A method af providing a metal heattransfer member with a porous nucleate-boiling surface comprising the steps of applying a layer of reticulated organic foam material to the surface of the metal heat-transfer member and plating the exposed surfaces of the reticulated foam material with a metal so as to form a reticulated metal surface which overlies the surface of the metal heat-transfer member and is firmly adhered thereto.
2. A method as claimed in claim 1 wherein the plating operation comprises an initial electroless deposit of metal followed by an electro-deposit deposit of metal.
3. A method as claimed in claim 1 or 2 wherein the metal heat-transfer member is cleaned before it is plated.
4. A method as claimed in any of claims 1 to 3 wherein the metal heat-transfer member is heated after it is plated to pyrolyze the organic foam material.
5. A method as claimed in claim 4 wherein the reticulated metal coating is made sufficiently thin that portions of it will be fractured by gas developed during the pyrolysis to produce pore openings therethrough.
6. A method as claimed in claim 4 or 5 wherein the heating for pyrolysis takes place at a temperature of less then 482"C.
7. A method as claimed in any of claims 1 to 6 wherein the heat-transfer member is a tube and the layer of foam material is applied by spirally wrapping a strip of reticulated organic foam material around the tube.
8. A method as claimed in any of claims 1 to 7 wherein the reticulated metal coating has a thickness in the range of 6.35 to 63.5 ym.
9. A method as claimed in claim 1 carried out substantially as hereinbefore specifically described or illustrated with reference to the accompanying drawings.
10. A metal heat-transfer member having a surface which is exposed in use to a boiling medium, said surface being coated with a non-particulate reticulated metal structure which is in intimately bonded thermally conducting relationship to the surface, the reticulated metal structure being shell-like with the metal therein optionally surrounding a reticulated non-metallic core portion.
11. A heat-transfer member when -made by a method as claimed in any of claims 1 to 10.

11. A heat-transfer member as claimed in claim 10 wherein the reticulated metal structure surrounds a non-metallic core portion which comprises an organic foam.

1 2. A heat transfer member as claimed in claim 10 wherein the reticulated metal structure is hollow.

13. A heat-transfer member as claimed in claim 1.2 wherein the reticulated metal structurn includes a plurality of small openings in its surface in communication with its hollow

interior.

14. A heat-transfer member as claimed in any of claims 10 to 1 3 wherein the reticulated metal structure has a metal thickness in the range of 6.35 to 63.5 ym.

1 5. A heat-transfer member as claimed in any of claims 10 to 14 wherein the reticulated metal structure is copper and the reticulated non-metallic core portion, if present, is open-cell polyurethane.

1 6. A heat-transfer member as claimed in claim 10 when made by a method as claimed in any of claims 1 to 9.

CLAIMS (4 July 1979)

1. A method of providing a metal heattransfer member with a porous nucleate-boiling surface comprising the steps of applying a layer of reticulated organic foam material to the surface of the metal heat-transfer member and plating the exposed surface(s) of the reticulated foam material with a metal so as to form a reticulated metal surface which overlies the surface of the metal heat-transfer member and is firmly adhered thereto, the plating operation comprising an initial electroless deposit of metal followed by an electro-deposit of metal.

2. The method of claim 1 wherein said heat transfer member is a tube and said layer of foam material surrounds the tube and is thin relative to the outside diameter of the tube.

3. The method of claim 1 or claim 2 wherein the layer of foam material applied to the transfer member is not more than about 1.66 mm thick.

4. The method of claim 1 or claim 2 wherein the layer of foam material applied to the transfer member is about 1.66 mm thick.