DK143421B - METHOD OF TRANSFER OF HEAT - Google Patents

Description

(Sn (19) DENMARK Vly

É | (ni IMPLEMENTATION SCRIPTURE OR 143421 B

DIRECTORATE OF THE PATENT AND TRADEMARKET SYSTEM

(21) Application No. 2685/69 (51) Int.CI.3 F £ 8 F 13/00 (22) Filing Day 19 · May 1 969 (24) Race Day 19 · May 1969 (41) Aim. available 21. November 1 969 (44) Posted Aug 17 1981 (86) International Application No. - (86) International Filing Day - (85) Continuation Day - (62) Master Application No. -

(30) Priority May 20, 1968; 730322, US

(71) Applicant UNION CARBIDE CORPORATION, New York, US.

(72) Inventor Alfred Michael Czikk, US: Patrick Scott O'Neill, US.

(74) Associate Engineer Lehmann & Ree.

(54) Heat transfer method.

The present invention relates to a method of transferring heat from a heat source to a cooking liquid through a heat-transferring partition using a porous cooking layer which is attached to one side of the wall and is composed of heat-conducting single particles, which cooking liquid has a Kelvin parameter 2 Οσ Τ_ / λΡ of between 0.001411 and 0.0254 cm x ° K, where: σ = Surface tension, dynes / cm, 0 = Saturation temperature for boiling liquid corresponding to the upward pressure of the liquid, ° K, 7} λ - Latent heat for the boiling liquid, gc & l / g, · * 3 OP „= Density of the steam, g / cm, i * ^ -8 __ C = Turnover factor, 2.4 x 10 gcal / dyn cm.

Transfer of heat at sufficiently high velocities from a heated surface to a cooking liquid in contact therewith generally requires a substantial temperature difference between the surface and the liquid, which temperature effect greatly affects the efficiency of heat transfer. An important factor controlling this efficiency is the nature of the heated surface which is in contact with the liquid, e.g. It is known that smooth cooking surfaces cause low heat transfer rates on the cooking side. Low boiler heat transfer rates often greatly limit the boiler's heat transfer capacity. For example, when heat to boiling is supplied by steam condensing on a smooth-walled heat transfer surface, the condensation heat transition number can easily be of the order of 0.271 gcal / sec. cm2 ° C, while the boiling heat transfer rate on the opposite side of the heat transfer surface can be only 0.01355 to 0.0271 gcal / sec. cm2 ° C. In accordance with the known method of summing heat transfer resistances when the boiling and condensing heat transfer surfaces have the same area, the total heat transfer number U is obtained approximately as follows: i. _i + _i 0 hc ^ and

hB + hC

Where h_. and h_ are the boiling and condensing heat transition numbers - B L, respectively.

lene. It is clear that if hQ is small compared to h „, approach

B C

the value for U itself, and most of the benefit of a large condensation

ISLAND

ring transition numbers are lost.

The object of the present invention is to provide a process for boiling liquids, thereby providing boiling-heat transfer rates many times greater than those obtained with ordinary smooth or rough surfaces, so as to allow the transfer of large amounts of heat to a cooking liquid at much smaller temperature differences than those required by the conventional cooking heat transfer methods.

This is achieved by a method of the kind referred to in the preamble of claim 1, characterized in that the cooking layer is made up of particles, at least some of which are sufficiently small to pass through a screen with a mesh width of 0.050 cm, where the particles are interconnected. and to the wall in a random context to form a uniform matrix of intermediate pores and interconnected pores between adjacent particles having equivalent pore radii of 0.00381 to 0.01143 cm, covering the porous boiling layer completely with the liquid. at least as a fluid membrane, and contacting the heat source with the other side of the wall so that sufficient heat is transmitted through the wall to form vapor bubbles in the porous cooking layer and to release the steam as bubbles resulting from the porous cooking layer, to the fluid retaining portion covering the porous cooking layer.

Kelvin parameters for representative liquids within a range of from 0.001411 to 0.0254 cm x ° K are listed in Table I.

Table I

X) o

Liquid Kelvin parameter, cm x K

Water at 103420 dynes / cm 2 (absolute) 0.0254

Water 0.00325

Hydrazine 0.0031

Hydrogen Peroxide 0.0028

Toluene 0.00198

Ethylene glycol 0.00183

Benzonitrile 0.00155

Ammonia 0.00141 y) at 1 atmospheric pressure unless otherwise indicated.

In the drawing: FIG. 1 is a graph showing the relationship between equivalent porous layers for porous layers and the boiling-side temperature difference for water; and FIG. Figure 2 is a series of graphs depicting the relationship between heat flow and boiling temperature difference in the method of the invention and in methods using smooth cooking surfaces.

In accordance with the method of the invention, a porous cooking layer having a plurality of small pores is provided, which layer is attached to the heat conducting wall of a heat exchanger for transferring heat to a liquid. The porous cooking layer is provided by heat-conducting particles so that they form a unit and are thermally connected to form interconnected pores of capillary size in the porous cooking layer. The pores are provided in a large number per minute. area unit with slightly intermediate non-porous material. The porous cooking layer provides a pronounced capillary action, which will be described in more detail below.

Due to the high heat transfer rates produced by the porous cooking layer, the temperature difference between a cooking liquid and its heat source can be substantially reduced, while nonetheless a remarkably high rate of heat transfer is obtained. Porous boilers have been found particularly useful in systems requiring elevated pressure to obtain a necessary temperature difference. By using a porous cooking layer in accordance with the method according to the invention, the necessary pressure difference between the liquids can be reduced and the energy costs can be minimized.

An important function of the porous cooking layer is to increase the boiler heat transfer rate. In the well-known heat transfer equation Q / A = hAT, the porous boiling layer significantly increases the value of the coefficient "h". The effect of increasing this coefficient can be used in a number of ways. The total amount of heat transferred "Q" can be increased, the area "A" can be reduced, "ΔΤ" can be reduced or various combinations of these advantages can be implemented.

Since by the process of the invention an increase in the coefficient "h" is obtained, the invention is different from the known use of porous components and devices known as "overheating inhibitors", which are usually suspended or immersed in the boiling liquid to reduce overheating. and to bring the liquid temperature closer to the vapor pressure of the liquid. Overheating agents are often non-metallic materials, such as pumice, which contain relatively large random-sized pores, of which only a few need to be active. The task of overheating agent is relatively easy and simple, as the free heat represented by a few degrees of overheating in the liquid is relatively small and can be effectively dissipated by a modest degree of boiling. The suspended slurry does not need to be heat conducting as it does not transfer heat to the liquid. The heat needed to boil on the surface of an overheating agent is supplied to it by the liquid itself in the form of overheating. Thus, while a slurried overheating agent can substantially reduce most of the overheating of the liquid, liquid overheating must exist to some extent for boiling to take place on the overheating.

Contrary to the known use of superheating agents, the porous boiling layer used in the process of the present invention is placed on the hot wall of the boiler between the hot wall and the liquid to be boiled and the material must be heat conducting. Its task is more difficult in that it consists in transferring a very large amount of heat with a greatly reduced heat transfer resistance, i.e. with a reduced temperature difference between the hot wall and the liquid. This temperature difference between the hot wall and the saturated liquid, which is a measure of the heat transfer resistance, is ΔΤ, as shown by the ordinary heat transfer equation. Its reduction is achieved according to the invention with the increased heat transfer rate of the porous cooking layer.

Although materials constructed in accordance with the known overheating means are placed against the hot wall, the improved results obtained according to the present invention will not be obtained. To be effective, the porous surface must meet certain special constraints with respect to the equivalent pore radius as discussed further below.

The extremely small thickness of the liquid membrane of the pores is believed to be largely due to the striking improvement in the heat transfer number h obtained by the present invention. It has been found that this is an effect which assumes only considerable extent in very small pores and is not significant in overheating agents designed primarily to reduce overheating. The effect of overheating agents is both described and envisaged by the combination of the well-known Gibbs and Clapeyronic equations relating to the thermal potential needed for the growth of a bubble (expressed in the overheating of the liquid surrounding the bubble) relative to size of the bubble:, I) TS Λ ~ PV)

rc lT - Ts' PL. Pv. X

where r = cavity radius (corresponding to the equivalent pore radius r of the porous boiling layers of the present invention in cm. Also the approximate radius of a bubble emitted from a cavity of radius r).

σ = surface tension, dyn / cm.

T = the temperature of the liquid surrounding a bubble, ° K.

Tg = saturation temperature for boiling liquid corresponding to the vapor pressure of the liquid, oK> 6 143421 3 P = density of the liquid, g / cm.

“3 P = density of steam g / cm.

λ = latent heat for the cooking liquid, gcal / g.

C = turnover factor 2.39 · 10 “® 3? Pa -! -.

dyn cm The value of T must be greater than Tc in a sufficient range to cause a bubble of radius rQ to grow against the surface tension. As a result, T - Tg is the least overheating required to maintain boiling. According to the Gibbs-Clapeyron lignin gene, the overheating required for bubble growth decreases, i.e. T - Tc is minimized with increasing r, and as a result, an overheating agent should be designed with a porous surface having as large pores as possible, which can still hold the enveloping gas or vapor needed to activate the aforementioned formation, back. In FIG. 1, curve A is produced by plotting r as a function of the overheating ΔΤ necessary to

C

keep bubble growth, i.e. T - Tg corresponding to the Gibbs-Clapeyron equation for water boiling in contact with different pore size surfaces with a heat flow of 0.2262 gcal / sec. cm. In accordance with the purpose of overheating agents, which aim to reduce the necessary overheating, the effect of a good overheating agent will lie on the part of curve A which corresponds to low values of ΔΤ and high values of r.

However, if one wishes to go beyond the phenomenon of the over-heating agent and, as a whole, to improve the boiling effect, it will not be sufficient merely to reduce the over-heating necessary to maintain bubble growth. When boiling occurs through the formation of bubbles in pores or voids in a surface comprising a heat source, it has been found that the overheating ΔΤ, i.e. T - Tg according to the Gibbs-Clapeyron equation, is only part of the resistance to total boiling. Another ΔΤ exists over the fluid membrane between the wall temperature T and the superheated steam-liquid contact surface temperature T, and in fact this membrane ΔΤ is associated with the overheat ΔΤ according to the Gibbs-Clapeyron equation. The total ΔΤ between the wall and steam is the sum of the overheating ΔΤ and the membrane ΔΤ, and the curve B in fig. 1 is made by plotting r as a function of this total ΔΤ. It has been unexpectedly found that this membrane ΔΤ, i.e. Tw - T, increases as r increases, which is an effect opposite to that observed with respect to the overheating ΔΤ, i.e. T - Tg.

The horizontal difference between the curves A and B in FIG. 1, it produces 7 143421 ne membrane ΔΤ.

In another way, FIG. 1, that as the equivalent pore radius r becomes smaller, the membrane ΔΤ decreases and the overheating ΔΤ according to the Gibbs-Clapeyron equation dominates the total ΔΤ. Conversely, as the equivalent pore radius r increases, the overheat ΔΤ decreases and the membrane ΔΤ dominates the overall ΔΤ. These opposite effects of total ΔΤ provide a maximum on curve B corresponding to an optimal value for r and a minimum value for total ΔΤ.

For water at 1 atmospheric pressure and a representative value for 2 heat flow Q / A of 0.2262 gcal / sec. cm as shown in FIG. In the optimum r between 0.00508 and 0.01016 cm, in the wider range between1 0.00381 and 0.01143 cm. Although FIG. 1 is based on test results for water at 1 atmospheric pressure, it is representative of liquids having a Kelvin parameter 2 Ca Tg / XPv between 0.001411 and 0.0254 cm x ° K. It should be noted that the Kelvin parameter is approximately equal to rc (T - Tg) determined by the Gibbs-Clapeyron equation ag above, differing only in the assumption that PL - Py is equal to P ^.

Liquid metals such as e.g. liquid sodium and liquid mercury, are peculiar to having Kelvin parameters 1 above 0.0254 and thus require relatively large equivalent pore radii to promote bubble growth. At the very small pore radii required by the present invention, vapor bubbles of liquid metals would collapse rather than grow. Accordingly, a heat transfer method for boiling liquid metals and using porous surfaces with radii of 0.00381 and 0.01143 cm would not include bubble growth in the pores but only on their exterior in the same way as a rough surface. Moreover, liquid metals have very high thermal conductivity, and methods of boiling them are not associated with the major disadvantage of a long heat transfer path between the known rough surface and the bubble growing in the liquid.

Liquids peculiar to having Kelvin parameter below 0.001411, e.g. ordinary refrigerants, cryogens and light hydrocarbons, can be boiled in porous surfaces with 0.00381-0.01143 cm pore radii with significantly greater heat transfer rates than with the previously known rough surfaces. However, such heat transfer modes do not present the optimum utilization of the present method, as the upper portion of this pore radius range 0.00762-0.01143 cm is larger than is best suited for these low Kelvin parameter fluids.

The equivalent pore radius of a porous cooking layer is most conveniently and accurately determined by vertically immersing one end of the porous cooking layer in a free-wetting liquid and determining the capillary rise of the liquid along the surface of the porous cooking layer. Determined in this way, the equivalent pore radius r is equal to 2 σ / ph, where p 3 is the density in g / cm of the liquid, wherein one end of the porous cooking layer is immersed vertically, σ is the surface tension in dyn / cm for the liquid in which one end of the porous cooking layer is immersed vertically and h is the vertical capillary rise in cm for the liquid along the surface of the porous cooking layer.

The advantage of choosing a free wetting fluid to determine the equivalent pore radius is that the liquid phase contact angle (Θ) provided by liquid surfaces with the materials of which the porous cooking layer is composed will be very small and therefore will not affect the determination. If a free wetting fluid is not selected, the term 2 σ / ph must be set equal to r / cos Θ and the contact angle (Θ) must be taken into account when determining the value of the equivalent pore radius (s). Since the precise determination of the contact angle is difficult and unnecessarily entails the risk of error, it is preferable to use a freely humidifying liquid to determine the equivalent pore radius. Liquids exhibiting a contact angle of less than 20 ° with the material of which the porous cooking layer is composed are defined as "free wetting" for this purpose and can be used without regard to the effect of the contact angle since cos 20 ° is 0, 95, and the error resulting from ignoring the contact angle will be less than 5%.

Examples of suitable liquids that freely moisten aluminum and copper surfaces (two of the preferred materials for building porous boilers) are methanol, fluorotrichloromethane, dichlorotetrafluoroethane, acetone, ethyl chloride, liquid oxygen and liquid nitrogen.

The liquid selected to determine the equivalent pore radius should preferably be a good solvent for oil and grease, so as to minimize the effect of the presence of these common surface pollutants. Pure water is not considered a free-wetting fluid since its contact angle (Θ), e.g. with an aluminum surface, is about 66 °.

The equivalent pore radius is independent of the properties of the material used to build the porous cooking layer. It is determined qualitatively the geometric and target properties of the porous boiling layer itself. The equivalent pore radius should not be used to express the quantitative effect of any particular porous boiling layer, as will depend on the material of which the porous cooking layer is made up and the liquid to be boiled.

9 143421

To illustrate the method for determining the equivalent pore radius, the following results and calculation for a porous copper nickel surface of grains having a diameter of 0.0149-0.0125 cm in diameter are given in Table II. A narrow strip of copper plate to which the porous surface was applied was suspended vertically with one end immersed in fluorotrichloromethane. The liquid wetted the surface by capillary action to a height h above the liquid surface of 5.09 cm. Fluorotrichloromethane has a surface tension of 18.98 3 dynes / cm and a density of 1.46 g / cm. If these values are inserted into the equivalent pore radius equation, a value for r of 0.005334 cm is obtained.

In Table II, column 2, the values of the effective pore radius determined experimentally for a variety of surfaces, as well as porous boiling layers of the present invention, in free wetting liquids, especially methanol, fluorotrichloromethane and dichlorotetrafluoroethane, are listed. Column 3 of Table II shows the magnitudes of temperature differences required by the different surfaces to transmit 0.2262 2 gcal / sec. cm by boiling water at 1 atmospheric pressure. Since Q / A is kept constant at 0.2262, the only variables in the heat transfer equations are ΔΤ and h, and these variables are inversely proportional. Accordingly, a decrease in the required ΔΤ by a factor of 10 will increase the heat transition number h by a factor of 10. Column 3 of Table II thus provides a means of comparing the heat transfer ability of different surfaces in a common liquid.

10 143421

Table II

ΔΤ ° C required for Equivalent pore Q / A = 0.2262 goal /

Flat plate surface radius in cm sec. cm2

Porous cooking layers 1. Copper-nickel, 100-120 mesh material, (0.0125-0 0.0149 cm), granular 0.0053 0.78 2. Copper, 35-60 mesh material (0.025-0.05 cm), granular 0 , 00864 0.56 3. Copper-nickel, 50-60 mesh material, (0.025-0.0297 cm), granular 0.00762 0.50 4. Aluminum, 140-270 mesh material, (0.0053-0, 0105 cm), granular 0.0046 1.28 5. Aluminum, 35-60 mesh material, (0.025-0.05 cm), granular 0.0069 0.44 6. Copper-nickel, 50-325 mesh material, ( 0.0044-0.0297 cm), granular 0.00508 0.94 7. Aluminum, 25-35 mesh material, (0.05-0.071 cm), granular 0.01778 1.11

Smooth surfaces 8 in Copper (non-porous) * 8.33 9. Aluminum (non-porous) 12.78

A porous cooking layer, as described above, provides during use a large number of interconnected partially liquid filled capillary tubes which act as the core for the growth of many bubbles in the boiling liquid. If the pores were not interconnected, their continued effect as the core of bubble growth would critically depend on retention of enclosed air or vapor in the pores. However, with interconnected pores, steam formed in a pore can activate one or more adjacent pores, so that the process proceeds without interruption and without dependence on air or vapor enclosure. It is also believed that at least some of the pores in the interconnected n 1A3421 basin add liquid to adjacent pores. As the bubbles grow, they eventually emerge from the interconnected capillary tubes due to the continued formation of vapor in the capillary tubes, tearing themselves free from the surface and rising through the liquid membrane covering the porous cooking layer. The liquid continues its flow into the capillary tubes and keeps the capillary walls moistened, thus providing increased surface evaporation. The large heat transfer is due to the fact that the heat leaving the basic metal surface does not have to move through a significant fluid layer until it encounters a vapor liquid surface providing evaporation.

Thus, in the porous boiling layer, a large number of bubbles grow, so that the heat to reach a vapor-liquid boundary need only move through an extremely thin liquid layer having a thickness substantially smaller than the very small diameter of the surrounding pore. Evaporation of fluid takes place exclusively in the pores, and by and large, overheating of the bulk of fluid is not required or can occur.

With a smooth metal surface, only a few bubble points exist, and the implementation of bubble wading requires a high degree of overheating due to the compressive force of the liquid surface tension on a very small bubble. The heat of bubble growth must be transferred by convection and conduction from the smooth base metal to the distant vapor-liquid contact surface in a bubble that is almost completely surrounded by the bulk of liquid.

The above-mentioned effect of a porous boiling layer is not merely the result of an increase in surface area, e.g. by mechanically making the surface rough. This was demonstrated by an immersion test of a porous boiler layer mounted on a copper block containing embedded heating coils to boil water at 1 atmospheric pressure. At very small heat fluxes insufficient to activate the pores with steam, the boiling point transition rate and a visible phenomenon of bubble points were quite similar to those obtained with a smooth surface copper block. However, at larger heat currents providing steam activation of the pores, much larger boiling was obtained. heat transfer rates that were impossible to achieve with the smooth block or with a block whose surfaces had been completely roughened by mechanical means. The following experimental results in boiling water show the effect of porous boiling surfaces at three temperature differences and at heat flows sufficiently large to provide vapor activation of the pores.

12 143421

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Η Ο · β s · β0 '(Ug · β0 · β Ο · β0 · β 0 rs η, Pc Η g ο cm g 44 0440' Τ g 44 in g 44 vog44 ^ Εί 44 · «13 143421 In Fig. 2 The results shown in Table III are shown in the form of a graph showing the heat flow as the ordinate and the temperature difference ΔΤ ° C as the abscissa. The same surfaces are indicated by the same identification numbers in Table II, Table III and Figure 2. Comparison of the individual lines for porous surfaces made from particles corresponding to different mesh sizes shows a great variation in effect, for example, this comparison can be based on the ΔΤ value at the same heat flow, so that the most efficient surface requires the least ΔΤ, and the least efficient surface requires the largest ΔΤ (the temperature difference can be directly transferred to energy requirements).

Metal particles used to build the porous cooking layer can include products with a wide variety of mesh sizes. However, at least some of the particles must be able to pass through a 35 mesh screen (0.05 cm mesh) to provide pores of sufficiently small size to become active at low ΔΤ. Referring to FIG. 2 and beginning with smooth surface nos. 8 and 9, the effect is enhanced with the largest pore surface layer (n. 7 having 25-35 mesh particles (mesh width 0.050-0.071 cm)), the improvement continues with finer pores (nos. 2, 5 , 3 and 1 with particle sizes corresponding to 35-60 (mesh width 0.025-0.05 cm), 35-60 (mesh width 0.025-0.05 cm), 50-60 (mesh width 0.025-0.0297 cm) and 100- 120 meshes (mesh width 0.0125-0.0149 cm), then decreases with even finer pores (No. 4 having a particle size corresponding to 140-270 meshes (mesh size 0.0053-0.0105 cm), and No. 6, which has a particle size corresponding to 50-325 meshes (mesh size 0.0044-0.0297).) Accordingly, the particles used to construct the porous surface layer must have a size corresponding to between 35 and 120 meshes ( 0.0125 and 0.05 cm) to achieve the best possible results, meaning that virtually all metal particles pass through a 35 mesh screen (0.05 cm mesh) (United States Standa rd targets) and retained on a 120 mesh screen (mesh width 0.0125 cm). Powders used to prepare this preferred surface may contain smaller amounts (percent) of particles coarser or finer than the preferred range of 35-120 meshes (mesh width 0.0125-0.05 cm), and experiments show such particles in small quantities, e.g. 10% by weight, which is either larger or smaller than the preferred range, neither increases nor degrades the effect.

It should be noted that although smaller particles generally provide porous surface layers with smaller equivalence radii, there is no direct correlation between these two parameters. This is partly due to the fact that the individual particles used in the manufacture of a given porous surface layer do not necessarily have the same shape, or that these particles necessarily correspond in shape to the particles of differing mesh sizes used for preparation. of other porous surfaces. In addition, the particles accumulate randomly on the heat conducting wall, and the sizes of the intermediate and interconnected pores may vary considerably. The particle shape will affect the pore size, e.g. For example, spheres will be more compacted than irregularly shaped particles and will provide less cavity. The myriad variations that these factors allow make it impractical to summarize all suitable powders under a single specification. For this reason, the equivalent pore radius is determined by routine testing of specimens of the finished porous cooking layer, as described above, an accurate method of identification.

Generally, any metal material is suitable for making the porous cooking layer, provided it has good thermal conductivity, is available as a fine powder, can be bonded by itself and with the base metal, and easily wetted by the liquid to be cooked. The powder particles used in the preparation of the porous cooking layer are preferably either grainy or spherical. Geometric considerations show that grainy or spherical particles are more effective than flakes or dendritic particles to obtain a large number of pores of approximately equal size. Very thin flakes are less desirable as they are difficult to bond together as separate particles and because their exceedingly large surface area complicates the work of cleaning the powder carefully.

Preferably, the particulate material must have a high thermal conductivity as indicated above. Among other things. For example, nickel- and copper-containing cooking layers have been tested under similar conditions. This showed that the copper-on-copper heat transfer rate is about three times that of the nickel-on-copper combination, which would be expected due to the greater thermal conductivity of copper. Copper also exhibits some advantages over aluminum, with the conductivity of these metals being 0.926 and 0.484 gcal / sec respectively. cm * 2 ° C / cm. For corrosive applications, alloys such as stainless steel that are resistant to chemical attack can be used.

The thickness of the porous cooking layer can vary by at least a factor of 10 without serious damage and is only slightly affected by the physical properties of the cooking liquid. The thickness should be greater than the average particle diameter and preferably be at least twice the average particle diameter. With particles as fine as 325 mesh (mesh size 0.0044 cm), the requirements for uniformity and complete consistency will usually determine the minimum thickness to be applied.

Functionally, the maximum thickness that can be used without damage is regulated only by the capillary action of the surface and by the surface's ability to emit the steam provided by boiling. During use, the surface must be capable of passing the liquid all the way through the thickness to the base metal so that the surface is completely wetted while simultaneously releasing or releasing the steam from the pores. Excellent results have been obtained with relatively thick surface layers; eg. is the porous boiling layer on sample # 5 in Table II and in FIG. 2 about 1.1 mm thick.

A suitable porous surface according to the invention can be produced by sintering 35-60 mesh (mesh width 0.025-0.05 cm) particles of a heat-conducting metal such as copper on the hot wall which feeds heat to the cooking liquid. The particles are applied in such an amount as to provide a porous layer thickness of approx. 0.11 cm. The gaps or voids between the particles must be substantially free of solid material and interconnected throughout the thickness of the layer. The interconnected pores provided will vary greatly in size, and many will be between 0.010 and 0.020 cm in diameter, as demonstrated microscopically. There will be a large number of pores per area unit of the surface.

Various methods can be used to prepare a sintered porous metal cooking layer on a base metal. A preferred method is to use a temporary binder, such as a plastic material, to provide and maintain a uniform coating on the base metal surface, the binder being of such a nature as to decompose and evaporate during the heating and sintering treatment.

One such resin is an isobutylene polymer having a molecular weight of approx. 140,000.

The resin binder is dissolved in a suitable solvent, such as petroleum or carbon tetrachloride, and sufficient metal powder is added to obtain a uniform viscous slurry with a metal shape's two-weight content of approx. 92 to 1. The base metal surface must be free of grease, oil and oxide coating to obtain the appropriate adhesion of the porous coating. Just before the slurry is applied, the surface can be rinsed with the resin solution to facilitate wetting of the slurry material so as to achieve a more uniform distribution.

A variety of methods can be used to apply a slurry coating to the base metal. The purpose is to obtain a uniform coating and the choice of method will depend on the shape and orientation of the surface. Spraying and immersion are two methods that have been successfully used.

The coating is air-dried either during or after application. Thus, the majority of solvent is removed by evaporation, leaving a solid self-supporting layer held in place by the binder. Then, the base metal and coating are covered by a slightly reducing atmosphere and the temperature is increased for a sufficient period of time to sinter the particles between themselves and to the base metal. The circulating reducing gas removes the thin oxide membrane and also removes the decomposition products from the surface materials. In the case of copper, the coating is sintered at about 100 ° C below its melting point or about 960 ° C.

According to another successful method, only the binder and solvent are applied to the surface and the metal powder is then sprinkled in a dry state over the sticky coating. This has the advantage that the solvent can be stripped off before the metal particles are applied and the resin is less liquid and movable. With a carefully carried out sprinkling method, a very uniform layer can be obtained which has a reduced tendency to run or collapse. The sprinkler can be repeated if desired to build relatively thick layers using thin applications. Another successfully used binder is a methyl cellulose polymer having a viscosity of 4000 cps. A preferred slurry comprises 32 g of copper powder in 100 cm of a 2% aqueous solution of this polymer. After applying a coating of the slurry to the base metal, it is air-dried at ordinary temperatures (below the boiling point of water) and then oven-dried at about 399 ° C in an atmosphere of water-saturated annealing gas. The coating is then sintered at appropriate sintering temperatures.

A third plastic binder that has been used successfully is polystyrene having a molecular weight of about 90,000 and which is soluble in toluene or xylene.

As stated above, binders and slurries are used to facilitate distribution and to temporarily hold the powder in place until a permanent thermal bond can be obtained. When allowed to surface, the powder can be applied without binder and sintered in a dry state.

The porous cooking layer can be applied to the inside of heat exchanger tubes. In such application, it is preferable to start by distributing the slurry throughout the length of the tube and then placing the tube in a machine capable of turning the tube about its own axis at a rate sufficient to provide a smooth coating. ie about 200 rpm. minute. The coating is air dried under the rotation and then sintered to heat the conditions mentioned above.

Another application method is particularly suitable for flat, ribbed surfaces or on the outside of cylindrical surfaces. According to this method, a slurry of metal powder and plastic binder is fed from a hopper to the outer surface of a polished roller in a uniform layer. As the roller rotates slowly, the evaporation of the binder causes the plastic membrane to stiffen and it is continuously pulled by the roller as a sheet of plastic material containing the embedded metal powder. The composite film is then contacted with the surface of a metal plate which is inserted into the furnace where the plastic material is expelled and the metal powder sintered to the plate. This method is particularly suitable for mass production.

The aforementioned sprinkling method using plastic material is preferred for making boiling layers on the outside of heat exchanger tubes. The plastic coating is easily applied by spraying or ironing. The powder can be delivered from an overlying sieve. Scan another option, the plastic-coated tube can be rolled into metal powder, which is then clogged to remove excess particles that are not firmly attached.

The process of the present invention can be practiced in heat exchangers where at least two passages are thermally connected, such as e.g. by means of a common wall so that the liquid to be boiled flows through a first pass and a warmer liquid flows through the second pass. In such a plant, the hotter liquid constitutes the hot spring and the common wall constitutes the heat conductive wall which receives heat from the hotter liquid. The porous layer is adhered to the common wall on one side thereof and forms part of the first passage.

Dynamic flow heat exchangers can e.g. be of the plate type, wherein a plurality of parallel spatially spaced separating plates are disposed in a core section provided with appropriate junction boxes and branch pipes for the fluid streams.

The heat exchanger may also be of the casing tube type with one or more tubes disposed in a casing so that the tubes form a first fluid passage and the surrounding casing constitutes a second fluid passage.

The porous layer is attached to one side of the pipe wall so that it can touch the cooking liquid.

It is to be understood that various agents may be used in the heat exchangers to improve the coefficient of condensation in the present process. Eg. ribs can be used or

Claims (2)

18 143421 grooves to increase the heat transfer area in the passage through which the warmer fluid flows. The present invention may also be used in conjunction with the carcass type heat exchanger, wherein the porous surface layer is adhered to one side of a heat conducting wall disposed in a container, the other side of the wall being contacted with a heat source that e.g. may be a hotter liquid or an electric, nuclear or other solid state heat source. The porous layer is covered with the liquid to be boiled and the resulting vapor is released from the porous layer and passes through the liquid into the overlying gas space. The porous layer may also be applied to the interior walls of a tube bundle vertically housed in a casing. The colder liquid in the pipes is heated and boiled by the warmer liquid in the casing which is in contact with the pipe surfaces. The liquid boils with a antifouling effect so that vapor bubbles arising from the porous layer pass through the relatively thin liquid membrane which completely covers the porous layer. The rising vapor causing liquid particles is emitted from the upper ends of the tubes. In carrying out the procedure, the liquid to be boiled need only provide a thin film over the porous layer as long as the latter is completely covered. In other embodiments, the porous layer is immersed in a relatively deep liquid container. It can be seen that the porous layer can be adhered to a wall positioned in any direction, from horizontal to vertical. In either the oblique or vertical position, the liquid to be boiled may be introduced at the lower end, as described above, or as another option be introduced at the upper end for downward flow. P a t e n t k r a v. 1, A method of transferring heat from a heat source to a cooking liquid through a heat transferring partition using a porous cooking layer which is attached to one side of the wall and is composed of heat-conducting single particles, which cooking liquid has a Kelvin parameter 2 Ca Tg / XPv of between 0.001411 and 0.0254 cm x ° K where: