GB2062207A

GB2062207A - Boiling Surface for Heat Exchangers

Info

Publication number: GB2062207A
Application number: GB8028473A
Authority: GB
Original assignee: Sulzer Escher Wyss GmbH; Escher Wyss GmbH
Current assignee: Sulzer Escher Wyss GmbH
Priority date: 1979-09-08
Filing date: 1980-09-03
Publication date: 1981-05-20
Also published as: ATA428580A; BR8005602A; ES494685A0; IT8024448A0; ZA805109B; ES8200765A1; DE2936406C2; AR220863A1; BE885100A; GB2062207B; CH648651A5; AT366493B; NL8004997A; FR2465180A1; DE2936406A1; FR2465180B1; IT1132642B

Abstract

The invention provides a boiling surface for heat exchangers comprising on a metal base a metal layer applied thereto, in which the layer comprises individual partly- cohering metal particles of various shapes, the layer is substantially free from continuous capillaries and closed pores, and the depth of the voids left between individual metal particles and open towards the boiling surface is on average more than 40% of the thickness of the layer and the thickness of the layer is from 30 to 300 mu m as measured over the highest protuberance. The invention also provides a process for the preparation of such a metal layer in which the particles are applied to the metal base by means of a powder flame spray technique in which the metal particles are injected into a gas flow heated by combustion of two gases and cooled and accelerated by means of a cooling gas, the mass balance of all the component flows responsible for such gas flow having a stoichiometric ratio of combustible gas and oxygen. <IMAGE>

Description

SPECIFICATION Boiling Surface for Heat Exchangers The invention relates to a boiling surface for heat exchanger elements comprising a metal layer applied to a metal base.

It is desirable to improve heat transfer from the heated surface of a heat exchanger element to a liquid to be boiled by a suitable surface structure.

It is known, for instance, to apply a smooth metal heat exchange surface, a porous metal layer having capillaries whose exit orifices are disposed in the layer surface and which serve as nucleation sites for the growth of vapour bubbles. Layers of this kind are built up from individual members joined together on the contact surfaces, so that in the layer there are voids which are interconnected in a statistical distribution, a capillary structure being formed which has a large number of capillaries open towards the surface.

The resulting structure of a large number of individual members joined together only by way of the contact surfaces is that heat flow inside the layer can proceed only by way of the heat bridges formed by the contact zones and is therefore impaired considerably. This in turn leads to resistance of thermal conduction which impairs the heat exchange during boiling.

Also, the fine capillary structure is very susceptible to pollution arising in manufacture and further processing and, when in use as a boiling layer, by foreign matter in the boiling liquid.

Layers of this kind can be produced, for instance, by elaborate soldering or sintering processes such as described in United States Patent Specification 3,384,154; in this process metal additives or organic process materials are used, some of which remain in the layer.

Most of the metal additives enter into compounds with the material of the individual members of the layer and the base substance while some are left in their pure metal form, and so the resulting layer consists of a large number of materials and their compounds.

Relatively small quantities of the organic process materials necessary for sintering are also left in the layer.

Heat exchangers having boiling layers devised as described are used more particularly for evaporating organic liquids serving as working media in cyclic processes. Media of this kind which have been used for a prolonged period of time are very sensitive to contact with foreign organic and metal substances. For instance, it is common for the working media in heat pump and refrigeration systems to be fluorinated hydrocarbons which contain small admixtures of lubricating oil from the refrigerating machinery. Organic foreign substances still present in the boiling layer enter into often harmful chemical compounds with the mixture of working medium and lubricating oil, while foreign metal substances may destroy the working agent or medium.

United States Patent Specification 3,990,862 discloses another possible procedure for producing capillary structures in boiling layers wherein metal particles are applied to the base substance by means of a special flame spray process and a boiling layer having the required capillary structure is formed from a large number of mostly very deformed metal particles. An important feature of this process lies in the partial oxidation of the metal particles in a flame operated with an excess of oxygen.

To ensure that oxidation occurs, the metals used must be of the kind which rapidly form a substantial oxide skin having a melting point differing considerably from the melting point of the starting material.

Copper, for instance, is not a metal which would meet this requirement. The metal oxides must be such as to ensure that the capillary boiling layer is of adequate strength, yet, as just described, they are chemical compounds which may destroy the working medium.

It is the object of the invention to devise a boiling surface for heat exchangers which ensures a very good heat transfer from the boiling surface to the boiling liquid, is free from harmful impurities and has low thermal resistance.

Accordingly the present invention provides a boiling surface for heat exchangers comprising on a metal base a metal layer applied thereto, in which the layer comprises individual partly-cohering metal members of various shapes, the layer is substantially free from continuous capillaries and closed pores, and the depth of the voids left between individual metal members and open towards the boiling surface is on average more than 40% of the thickness of the layer, and the thickness of the layer is from 30 to 300 lim as measured over the highest protuberance.

In another aspect, the invention provides a process for the preparing of such a metal layer in which the particles forming the individual members are applied to the base substance by means of a powder flame spray technique in which the particles of powdered metal are injected into a gas flow heated by combustion of two gases and cooled and accelerated by means of a cooling gas, the mass balance of all the component flow responsible for such gas flow having a stoichiometric ratio of combustible gas and oxygen, and in which the particles are heated by the gas flow to melt on their surface and are carried to the metal base.

The layer in accordance with the invention therefore comprises single substance and is therefore free from the disadvantages of the known capillary structures so far as foreign agents are concerned.

Since the layer is built up from a large number of individual members of various shapes, the members have an enlarged surface area near the boundary layer of the boiling liquid, and so a large liquid volume in the boundary layer zone can be heated quickly. The resulting bubbles of vapour therefore remove large quantities of heated boundary layer liquid away from the boiling surface, thus ensuring a very satisfactory transfer of heat from the surface to the boiling liquid.

A large number of notches, cut-back parts and similar geometrical shapes arise at the junctions between the individual members and act as bubble nucleation sites. There is therefore intensive bubbling, a feature to promote removal of the heated boundary layer liquid.

For a better understanding of the effects provided by the invention, reference will be made to the following facts.

Boiling on surfaces follows laws such as described in the DKV Paper No. 18,1964 "Beitrag zur Thermodynamik des Wärmeüberganges beim Sieden" by K. Stephan; small vapour bubbles arise at bubble nucleation sites distributed statistically over the surface, increase in size until reaching their detachment diameter, then become detached from the surface and rise in the boiling liquid. A new vapour bubble arises and the process repeats at what is known as the bubble frequency. As shown, e.g.

by Han and Griffith in the article 'The Mechanism of Heat Transfer in Nucleate Pool Boiling" Int. J. Heat Mass Transfer, Volume 8, 1 965, pp 887-914 and by Beer and Durst in the article "Mechanismen der Wärmeübertragung beim Blasensieden und ihre Simulation," Chemie-lng.-Tech. 40 (1968), 13, pp.

632-638, the time referred to as the bubble frequency can be broken down into two parts-a waiting time and the actual bubble growth time, the waiting time amounting to from 60 to 80% of the total time. During the waiting time the liquid boundary layer adhering to the boiling surface is heated by heat conduction. Once a particular temperature profile has arisen in the boundary layer, the vapour bubble starts to arise, grows to the bubble detachment diameter and detaches from the boiling surface. In this detachment the vapour bubble causes the heated liquid boundary layer around it away from the boiling surface by "mass convection" or by its indexed "drift flow", the range of influence which the drift flow has on the boundary layer corresponding substantially to bubble detachment diameter.After the boundary layer mass convection, "cold liquid" flows to the boiling surface and is heated thereby. In other words, the heat is removed from the boiling surface mainly by unsteady heat conduction.

As can be gathered from the publications referred to, the values can be estimated with the help of the following equations. The bubble detachment diameter dA is:

Bubble frequency f can be approximately determined by the equation: f. do'/2=1.75 (2) The boundary layer thickness at bubble detachment aA is given by:

where: in equations (1), (2) and (3):: g (m/s2) denotes the acceleration of gravity; dA (m) denotes the bubble diameter at detachment; p (o) denotes the edge angle of the adhering vapour bubble; p (N/m) denotes the surface tension of the boiling liquid; v (m3/kg) denotes the specific volume of the liquid; v (m3/kg) denotes the specific volume of the evolving vapour; f (I/s) denotes bubble frequency; a (m2/s) denotes the temperature conductivity of the liquid; TWZ (s) denotes the waiting time; Th (s) denotes the bubble growth time; and SA (m) denotes the boundary layer thickness at detachment.

Table 1 below gives the calculated values for a number of boiling liquids and boiling temperature ranges customary in refrigeration engineering.

The physical phenomena during boiling as hereinbefore described lead to the underlying basis of the invention that heat exchange can be improved by considerably increasing the boiling surface in the range of influence of the boundary layer and of the bubbles.

Consequently, the time taken for the boundary layer temperature profile to form-i.e. the waiting time-can be reduced considerably while the volume of the boundary layer liquid carried away by a bubble from the boiling surface can be increased considerably.

The invention stems from the recognition that to improve heat transfer the structure serving to increase the superficial area must have an overall height, i.e. a layer thickness measured over the highest protuberance, of the same order of magnitude as boundary layer thickness but not more than 5 times the same.

In order to promote a fuller understanding of the above and other aspects of the invention some embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagram showing how various shapes of individual members can increase the superficial area of the structure.

Figure 2 is a diagrammatic view in longitudinal section of a part of a metal layer embodying the invention.

Figures 3 and 4 are photographs taken using a scanning microscope or surface structures enlarged 100 times (Figure 3) and 500 times (Figure 4).

Figure 5 is a simplified diagrammatic view of a flame sprayer for producing a metal layer while in operation, and Figure 6 is a diagram showing curves denoting measured heat flux plotted against overtemperature.

To estimate the possible surface-area increase F/Fo as compared to a smooth basic surface F0 which can be provided by a finely structured surface embodying the invention, calculations were made for individual members of simple geometric shape, such as oblong blocks, cylinders, pyramids and so on. It was assumed that the individual members were in a dense arrangement on the smooth parent surface, with a very small ratio V of < 0.2 of the shortest distance between adjacent individual members to the edge length or diameter of their basic surface.

The increases F/Fo thus found are plotted in Figure 1 against the relationship V for various geometric shapes. Curve I is for parallepipeds having a ratio h/s of their height h to their edge length s of unity, curve II is for cylinders having a ratio h/d of their height h to their diameter d of unity, curve III is for cones having an apex angle y of 300, curve IV for pyramids having an apex angle y of 450, curve V is for cones having an apex angle y of 450 and curve VI is for hemisphere.

As can be gathered from Figure 1, considerable surface area increases are possible and they are independent of the absolute height of the individual members. Even greater increases can be provided when a variety of shapes are superimposed on one another in the applied layer.

Figure 2 is a diagrammatic view in longitudinal section through a layer applied to a metal wall 1.

The individual members are of different shapes produced mainly in preparation, as will be described hereinafter. The layer consists of substantially spherical and very flattened individual members 4, 5.

Some of the members adhere to the wall 1 and adjacent members adhere to one another by way of their contact surfaces 2, 3, contact surfaces 6 shaped like hooks arising. The resulting layer is strong and a good heat conductor. The transition between the surfaces of contiguous members is often in the form of notches as at 7.The differently shaped members provide a layer 11 of completely heterogeneous structure, with the result that there arises orifices 8 extending between the discrete members as far as the base substance, craters 9, elongated gaps 10 and similar shapes which make excellent bubble nucleation sites and thus ensure complete covering of the boiling surface with vapour bubbles even when the over-temperature T-T9 of such surface is very low as compared with the saturation temperature T9 of the boiling liquid.

The layer thickness As is measured between the highest protuberance and the surface of the heatyielding wall and is, in accordance with the invention, from 30 to 300 ym. There remain between the members forming the layer and their defined height open voids which can be of a depth varying from a few percent to one hundred percent of layer thickness but whose depth is on the average more than 40% of layer thickness. The surface structure provided by the invention is apparent from the layer photographs of Figures 3 and 4 where the references are the same as in Figure 2.

As previously stated, a metal layer structure embodying the invention can provide a considerable enlargement, for instance, of the order of magnitude of from 2 to 10 of the surface area of the base substance.

An advantageous process for the production of the layer comprises a modified form of a known powder flame spray technique in which metal particles of the starting material are sprayed on to the base substance in a molten or doughy state. The geometric shape of the metal particles before spraying can vary; for instance, spherical, cubic, prismatic, ployhedric, dendritic and spatter shapes can be used. The metal particles are injected into a hot stream of gas and uniformly distributed therein. The gas stream serves both as a vehicle to convey the metal particles to the base substance and as a heat source to warm the metal particles.The metal particles are heated from their surface and the dwell time of the particles in the gas stream and therefore the extent of heating or melting can be controlled in known manner by control of the temperature and speed of the gas stream and on the flight path of the particles. The particles will deform in dependence upon how far they have melted and the speed at which they strike the base substance and, in the manner visible in Figures 2-4, will become anchored or welded to the base substance and, by way of their contact zones, to one another. The dimensions of the members forming the layer can be so controlled by appropriate choice of particle dimensions as to correspond in order of magnitude to the values given for boundary layer thickness in Table 1.

Advantageously, the starting material is in the form of spherical metal particles whose diameters dare from 30 to 150 cm.

Figure 5 is a diagrammatic view of an apparatus 20 for performing the modified powder flame spraying process.

Combustible gas and oxygen are fed to the apparatus through supply lines 21,22. The mixture flows through a nozzle ring 23 and is then burnt in a flame. The hot flame gases are mixed with and, therefore cooled by, a cooling gas which is injected through a line 24 and goes through an external nozzle ring 25 into the flame. Some of the cooling gas also serves as a vehicle for metal powder fed in through a line 26. The powder is distributed in the gas flow in accordance with flow conditions, is conveyed to a heat exchange element 27 made of the base metal and impinges thereon as a substantially circular surface hereinafter called the spray spot. The spray spot spreads over the surface of the metal base so that the layer is applied in a manner leading to a large superficial area.The relative speed of traverse of the spray spot over the base metal is a factor which can be used to influence layer quality.

If the speed is too low, the dwell time of the applied layer in the sprayed stream is excessive, the metal particles of the layer melt and a relatively homogeneous layer without a variety of body shapes and without enough bubble nucleation sites is formed. Relative speed, layer thickness and powder throughput through the spray nozzle are inter-connected for the production of the layer. Other important factors affecting layer quality are the gas flows before the spray nozzle and the distance between the same and the metal base which, as previously stated, determines together with the gas flow after the spray nozzle the dwell time of the metal particles in the latter gas flow.Since a metallically pure layer is required, all the gases flowing through the spray nozzle must together ensure a stoichiometric balance between the combustible gas and the oxygen, to preclude oxidation and other chemical changes of the metal particles.

As examples of the combustible gas, acetylene or hydrogen can be used and as an example of the cooling gas, dried and cleaned air or nitrogen can be used. The material for the metal particles is preferably copper or copper alloys, although other metals, such as iron and iron alloys, can be used.

In an experiment, a layer, of the kind shown by way of example in Figures 3 and 4, comprising copper members was applied to a base metal copper tube of 18 mm outside diameter. The heat flux q8 during boiling was measured in an appropriate apparatus in conditions corresponding to those found in flooded evaporators of turbo refrigerating machines, with the use of the refrigerant CF2Cl2 and at an evaporation temperature of 000.

In Figure 6 the heat fluxes qa measured are plotted against the temperature difference AT (see curve W); for purposes of comparison the heat fluxes for tubes having a smooth surface (see curve Z) are also plotted.

Table 2 below gives in a numerical example the throughputs of the gas streams and of the metal powder, the corresponding spraying interval and the relative speed of the spray spot over the metal base, a process for producing a layer embodying an aspect of the invention.

Table 1 Liquid t("cl f(l/sJ d(um) SA (urn) CF2Cl.CF2Cl(R114) 60 72 589 40 0 64 745 51 CF2Cl2(R12) 30 72 569 50 -30 65 732 59 CHF2CI (R22) 20 68 672 53 -40 59 877 64 CF3Br(R13B1) -20 75 549 47 -60 68 655 54 CF3CI (R13) -40 72 592 47 -80 65 734 54 Table 2 Range of Preferred rei'e supply rate of supply Combustible gas: Acetylene C2H2 at 1 bar 0.5-1.0 0.74 (m3/h) Oxidation gas: Oxygen O2 at 1 bar 0.6-1.2 0.85 (m3/h) Cooling gas: Air at 1 bar 3.1-6.2 5.3 (m3/h) Powder (spherical copper): : throughput 2.5-10 3.0 (Kg/h) grain size 30-150 40--80 (Fcm) Spraying distance: 5-1 5 10 (cm) Relative speed of spray spot to base: 0.02-0.1 0.05 (m/s)

Claims

1. A boiling surface for heat exchangers comprising on a metal base a metal layer applied thereto, in which the layer comprises individual partly-cohering metal members of various shapes, the layer is substantially free from continuous capillaries and closed pores, and the depth of the voids left between individual metal members and open towards the boiling surface is on average more than 40% of the thickness of the layer and the thickness of the layer is from 30 to 300 pm as measured over the highest protuberance.

2. A surface as claimed in Claim 1, in which the grain size of the starting material forming the individual members is between 30 and 1 50 pom.

3. A process for the preparation of a metal layer according to Claims 1 and 2 in which the particles forming the individual members are applied to the base substance by means of a powder flame spray technique in which the particles of powdered metal are injected into a gas flow heated by combustion of two gases and cooled and accelerated by means of a cooling gas, the mass balance of all the component flows responsible for such gas flow having a stoichiometric ratio of combustible gas and oxygen, and in which the particles are heated by the gas flow to melt on their surface and are carried to the metal base.

4. A process as claimed in Claim 3, in which the powdered metal comprises starting copper particles having a grain size of from 30 to 150 pom and its mass throughput is from 2.

5 to 10 0 kg/h; acetylene in volume flow of from 0.5 to 1.0 m3/h at 1 bar is used as a combustible gas; pure oxygen in a volume flow of from 0.6 to 1.2 m3/h at 1 bar is used as an oxidation gas; and air at ambient temperature and in a volume flow of from 3.1 to 6.2 m3/h at 1 bar is used as a cooling gas, the spraying distance being from 5 to 1 5 cm and the relative speed of traverse of the spray spot over the metal base being from 0.02 to 0.1 m/s.

6. A process as claimed in Claim 5, in which said particles have a grain size of 40 to 80 pm at a mass throughput of 3.0 kg/h, said acetylene flow is 0.74 m3/h, said oxygen flow is 0.85 m3/h, said air flow is 5.3 m3/h, said spraying distance is 10 cm, and said relative speed of the spray spot is 0.05 m/s.

7. A boiling surface for heat exchangers substantially as hereindescribed with reference to the accompanying drawings.

8. A process for applying a metal layer boiling surface to a metal base substantially as herein described with reference to the accompanying drawings.