IL31978A - Process for transferring heat from heated surfaces to boiling liquids - Google Patents

Description

8»t¾*m α*·η**¾> a*8»iR¾ a^nease ο η Awn* -|·^ηΛ L-728J This invention relates to the art of improving heat transfer from heated surfaces to boiling liquids.

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 surfac 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 examplej 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 2000 Btu/hr. sq. ft. °F (0.271 gm-cal/sec-cm2-°c) , while the boiling heat transfer coefficient against the opposite of the heat transfer surface may be only 100 to 200 Btu/hr. sq.ft. °F (0.01355 to 0.0271 gm-cal/sec-cm -°C) . 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 : 1 . 1 + 1 U ϊζΓ hB and U - 4B • hB+hC where hg and he are the boiling and condensing heat transfer coefficients respectively. It is clear that if hfi is small compared to hp, then the value of U approaches 1¾ and most of the advantage of a high condensing coefficient is lost.

Principal objects of the present invention are: to provide a process for boiling liquids which produces boiling heat transfer coefficients many times as large as those obtained with conventional smooth or roughened surfaces, and to provide a process able to transfer to a boiling liquid large quantities of heat at much lower temperature differences than required in conventional boiling heat transfer processes.

This invention relates to a process for transferring heat from a heat source to a boiling liquid having a Kelvin parameter 2 CO of between 1 and 18 mils x °F (0.001411 and 0.0 54 cm x °K) where = Surface tension, lbs. force/ft. (dynes/cm) Tg = Saturation temperature of boiling liquid corresponding to the vapor pressure of the liquid, °R (°K) Py = Density of vapor, lbs. mass/ft3 (gm/cm3) of boiling liquid, Btu/lb.

C Conversion factor, 15.48 B¾t»uH x¾ ra ϊΐil £?.4 x 10-8 gm_ca) TtTlTTET frSrce dyne -cm) Kelvin parameters for representative fluids within this range are listea in Table I.

In this process, heat exchange apparatus having a thermally conductive wall with a porous boiling surface bonded to one side of the wall is provided. This porous boiling layer is constructed of thermally conductive discrete particles, at least some being sufficiently small to pass through 35 (0.05 cm) mesh screen (based on United States Standard screen series). The particles are bonded together and to the wall in random stacked relation to form a uniform" matrix with interstitial and interconnecting pores between adjacent particles having equivalent pore radii between 1.5 (0.003&1) and 4.5 (0.01143 cm) mils. as at least a liquid film, and the heat source is contacted with another side of the thermally conductive wall such that sufficient heat is transferred through the wall to form vapqr bubbles within the porous boiling layer. The vapor is dls-charged as bubbles emerging from the porous boiling layer into the liquid film portion.

Kelvin parameters for representative fluids within the 1 to 18 mils x °F (0.001411 and 0.0254 cm x °K) range are listed in Table I.

TABLE i' Fluid* Kelvin Parameter (103420 dynes/cm2 absolute) Water at 1.5 psia 18 (0.0254) (cm x °K) Water 2.3(0.00325) Hydrazine 2.2(0.0031) Hydrogen Peroxide 2.0(0.0028) Toluene 1.4(0.00198) Ethylene Glycol 1.3(0.00183) Benzonitrile 1.1(0.00155) Ammonia 1.0(0.00141) *at one atmosphere pressure unless otherwise noted.

In the accompanying drawings: Figure 1 is a graph which illustrates the relationship between equivalent pore radii for porous layers and the boiling side temperature difference for water, and Figure 2 is a series of graphs illustrating the relationship between heat flux and boiling side temperature difference for the instant process and processes employing smooth boiling surfaces.

According to the process of the invention, there is provided a porous boiling surface layer having a multitude of small pores bonded on a thermally conductive wall of a heat exchange apparatus for transferring heat to a liquid. The particles integrally and thermally bonded together to provide interconnected pores of capillary size within the porous boiling surface layer. The pores are provided in ^reat number per unit- area with little non-porous material between them. The porous boiling surface layer is characterized by pronounced capillarity which is described in detail subsequently.

Because of the high boiling heat transfer coefficients produced by the porous boiling surface layer, the temperature difference between a boiling liquid and its source of heat may be substantially reduced while nevertheless achieving a remarkably high rate of heat transfer. Porous boiling surface layers of this invention have been found particularly advantageous in systems requiring elevated pressures to achieve a necessary temperature difference. By using a porous boiling surface layer b according to the process of the present invention, the necessary pressure differential between the fluids may be reduced and power costs minimized.

An important function of the porous boiling surface layer of this invention is to Increase the boiling heat transfer 0 coefficient. In the well-known heat transfer equation Q/A = hAT, the porous boiling surface layer markedly increases the value of the coefficient "h". The effect of the increase in this coefficient can be utilized in several ways. The total quantity of heat transferred "Q" may be increased, the area "A" may be b decreased, the "^T" may be reduced, or various combinations of these benefits may be realized.

Since the invention improves the coefficient "h", it is distinct from prior art uses of porous elements and devices known as "ebullators" which are commonly suspended or immersed 0 in the boiling liquid to reduce superheat and to bring the bulk liquid temperature more nearly in equilibrium with the liquid's'*"" vapor pressure. Ebullators are often non-metallic materials such as pumice which contain relatively large random-size pores, only a few of which need be active. The task of the ebullator is relatively easy and simple because the sensible heat represented by a few degrees superheat in the liquid is comparatively small and can be dissipated effectively by a modest degree of boiling. The suspended ebullator material need not be thermally conductive since it does not transfer heat to the liquid. The 0 heat required for boiling on an ebullator surface is transported to that point by the liquid itself in the form of superheat.

Thus, while a suspended ebullator may reduce the bulk superheat of the liquid substantially, some degree of liquid superheat must exist in order for boiling to take place on the ebullator.

In contrast to the ebullator art, the porous boiling surface layer of this invention is disposed on the hot wall of the boiling device between the hot wall and the liquid to be boiled and its material must be thermally conductive. Its task is the more difficult one of transferring a very large quantity C of heat with a greatly reduced heat flow resistance, i.e. with a reduced temperature difference between the warm wall and the liquid. This temperature difference between the warm wall and the saturated liquid, which is a measure of the heat flow resistance, is the^T which appears in the standard heat transfer 5 equation. Its reduction is achieved herein by the increased heat transfer coefficient of the porous boiling surface layer.

Even if materials constructed according to accepted ebullator practice are disposed against the warm wall, they will not achieve the superior results of this invention. To be effec-0 tive, the porous surface must meet special limitations on the equivalent pore radius as defined hereinafter.

The extreme thinness of the liquid film within the ί a esis thought to account in large part for the striking improvement in heat transfer coefficient h achieved with this invention. It has been discovered that this is an effect which b assumes significant proportions only in very small pores, and it is not significant in ebullators designed primarily to reduce superheat. The performance of ebullators is both described and predicted by the combination of the well-known Gibbs and Clapeyron equations which relate the thermal potential required for growth of a bubble (in terms of the superheat of the liquid surrounding the bubble) to the size of the bubble: ( } Ts (PL-Py) rc = ( T - TS ) PL.Py. where rc = cavity radius (Interchangeable with the equivalent lb pore radius r of the porous boiling surface layers of the present invention, ft. (cm) Also the approximate radius of a bubble emerging from a cavity of radius rc) Surface tension, lbs. force/ft. (dynes/cm) T = Temperature of liquid surrounding a bubble, °R (°K) Ts = Saturation temperature of boiling liquid corresponding to the vapor pressure of the liquid, °R (°K) PL « Density of liquid, lbs. mass/ft3 (gm/cm3) Py B Density of vapor, lbs. mass/ft3 (gm/cm3) = Latent heat of boiling liquid, Btu/lb. (gm-cal/gm) C = Conversion factor,^ „ Bt^ gm~cal ) '77o ft x lb. force (dyne cm) The value of T must be greater than Ts by an amount sufficient to cause a bubble of radius rc to grow against' surface tension.

Hence T - o is the minimum superheat required to sustain the boiling process. According to the Gibbs-Clapeyron equation, the superheat necessary for bubble growth is' reduced, i.e. T - Tg is minimized, by increasing rc and hence an ebullator should be constructed with a porous surface having pores as large as vs. the superheat necessary to sustain bubble growth, i.e.

T - Tg , according to the Gibbs-Clapeyron equation, for water boiling in contact with surfaces of. various pcre size with a heat flux of 3000 Btu/hr. sq . ft. (0.2262 gm-cal/sec-cm2 ) .

In keeping with the objective of ebullators to reduce the required superheat, the performance of a good ebullator would fall on the portion of Curve A corresponding to low values of ΔΤ and high values of rc .

However, if one wishes to take a step beyond the phenomenon of the ebullator and to improve overall boiling performance, it will not be sufficient merely to reduce the superheat required to sustain bubble growth. When boiling proceeds by the formation of bubbles within pores or cavities of a surface which comprises a heat source, the superheat ΔΤ, T - Tg, correlated by the Gibbs-Clapeyron equation has been discovered to be only one of the resistances to the overall boiling process. A second ΔΤ exists across the liquid film between the wall temperature Tw and the superheated vapor-liquid interface temperature T, and in · effect this film Δ T is in series with the superheat ΔΤ of the Gibbs-Clapeyron equation. The total between the wall and the vapor is the sum of the superheat Δ and the film A T, and Curve B of Figure 1 plots r against this total h T. This film ΔΤ, i.e. w - T, has been unexpectedly found to increase as r Increases, an effect opposite to that observed for the superheat Δ.Τ, T - Ts. The horizontal difference between Curves A and B of Figure 1 depicts this film AT.

Stated otherwise, Figure 1 reveals that as the equivalent pore radius r becomes smaller, the film ΔΤ diminishes and the superheat AT as predicted by the Gibbs-Clapeyron equation dominates the total AT, Conversely, as the equivalent pore radius r increases, the superheat AT diminishes and the film ΔΤ dominates the total ΔΤ. These opposing influences on the total AT produce an inflection in Curve B corresponding to an optimum value of r For water at one atmosphere pressure and a representa- - tlve value of heat flux Q/A at 3000 Btu/hr/ft2 (0.2262 gm cal/ p sec-cm ) as depicted in Figure 1, the optimum r occurs between 2 and 4 mils (0.00503 and 0.01016 cm), within the broader range of between 1.5 and 4.5 mils (0.00381 and 0.01143 cm). While Figure 1 is based on performance data for water ac one atmosphere pressure, it is representative for fluid having a Kelvin parameter 2C<f TS^Py between 1 and 18 mils x °F (0.001411 and 0.0254 cm x °K). Tt should be noted that the Kelvin parameter C is approximately equal to rc (T - T5) as determined by the aforementioned Gibbs-Clapeyron equation and differs only in the assumption that PL - Py is equal to pL.

Liquid metals as for example liquid sodium and liquid mercury are characterized by Kelvin parameters above l8 (0.0254) 5 and thus require relatively huge equivalent pore radii r to promote bubble growth. For the extremely small pore radii required of this invention, vapor bubbles of the liquid metals would collapse rather than grow. Accordingly, a heat transfer process for boiling liquid metals and employing porous sur- i faces of 1.5 - 4.5 mil (0.00381 - 0.01143 cm) radii would not involve bubble growth within the pores but only on the exterior thereof In the manner of a roughened surface. Moreover, liquid metals possess extremely high thermal conductivities and processes for boiling same do not have the severe disadvantage of a long heat transfer path between the prior art roughened surface and the bubble growing In the bulk liquid.

Liquids characterized by Kelvin parameters below 1, (0.001411) e.g., common re rigerants, cryogens, and light hydrocarbons may be boiled in porous surfaces of 1.5 - 4.5 mils (0.00381 - 0.01143 !>C cm) pore radii with significantly higher boiling coefficients than prior art roughened surfaces. However, such heat transfer processes do not afford the optimum performance of the instant process because the upper portion of this pore radii' range, 3.0 to .5 mils (0.00762 - 0.01143 cm) is larger than best suiteu for these low Kelvin parameter liquids.

The equivalent pore radius of a porous boiling surface layer is most conveniently and accurately determined by vertically immersing one end of the porous boiling surface layer in a freely wetting liquid and measuring the capillary rise of the liquid t along the surface of the porous boiling surface layer. When determined in this manner, the equivalent pore radius, r, is equal to 2 Ci/ph where: p is the density in lbs. mass/cu. ft. (gm/cm ) of the liquid in which an end of the porous boiling surface layer is vertically immersed, ϋ" is the surface tension 5 in lbs, force/ft. (dynes/cm) of the liquid in which an end of the porous boiling surface layer Is vertically immersed, and h is the vertical capillary rise in ft. (cm) of the liquid along the surface of the porous boiling surface layer.

The advantage in choosing a freely wetting liquid to C determine the equivalent pore radius is that the liquid phase contact angle (©), which the liquid surfaces makes with the materials of which the porous boiling surface layer is composed, will be very small and, therefore, will not effect the determination. If a freely wetting liquid is not chosen, the expression 5 2 d/'ph must be equated to r/cos Q and the contact angle (β) will have to be accounted for in determining the value of the equivalent pore radius (r). Since the exact measurement of the contact angle is difficult and unnecessarily introduces risk of error, it is preferable to use a freely wetting liquid to deter-0 mine the equivalent pore radius. Liquids exhibiting a contact angle of less than 20° with the material of which the porous boiling surface layer is composed are defined as "freely wetting" the effect of the contact angle since cos 20° is 0.95 and the error resulting from neglecting the contact angle will be less than 52.

Examples of suitable liquids which freely wet aluminum and copper surfaces (two of the preferred materials for constructing porous boiling surface layers) are methanol, fluoro-trichloromethane , dichlorotetra luoroethane , acetone, ethyl chloride, liquid oxygen and liquid nitrogen. The particular liquid chosen for determination of the equivalent pore radius should preferably be a good solvent for oil and grease so that the effect of the presence of these common surface contaminants will be minimized. Pure water is not considered to be a freely wetting liquid inasmuch as its contact angle (Q) with an aluminum surface, for example, is about 66°.

The equivalent pore radius is independent of the properties of the material used to construct the porous boiling surface layer. It defines qualitatively the geometrical and dimensional characteristics of the porous boiling surface layer itself.

The equivalent pore radius should not be employed to predict the quantitative performance of any particular porous boiling surface layer inasmuch as such performance will depend, inter alia, on the material of which the porous boiling surface layer is constructed and the liquid which is to be boiled.

To illustrate the method of determining the equivalent pore radius, consider the following data and computation for the 100-120 mesh (0.01^9-0.0125 cm diameter) granular copper-nickel porous surface of Table II. A small strip of copper sheet with the porous surface applied thereon 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 O.lb? ft. (5.09 cm). Pluorotrichloromethane has a surface tension of 1.30 x 10~3 lbs. /ft. (18.98 dynes/cm) and a density of 91.4 lb./cu. fu. - 6 g/cmJ). Substituting the&a. values into the equivalent pore radius equation gives a value for r of 0.175 x 10~3 ft. or 2.1 mils (0.003334 cm).

Table TI, column 2, summarizes the values of the effective pore radius determined experimentally for a number of surfaces as well as porous boiling surface layers of the present invention in freely wetting liquids - specifically methanol, fluorotrichloro- methane, and dichlorotetrafluoroethane. Column 3, of Table II, shows the values of temperature differences required by the several surfaces to transfer 3000 Btu/hr. sq. ft. (0.2262 gm-cal/sec-cm2 ) while boiling water at one atmosphere pressure. Since Q/A is held constant at 3000 (0.2262), the only variable in the heat transfer equations are4T and h, and these variables are inversely proportional. Consequently, a decrease in the required ^T by a factor of 10 will increase the heat transfer coefficient h by a factor of 10. Thus, column 3 of Table II provides a means of comparing the heat transfer capabilities of the various surfaces in a common fluid.

TABLE II Equivalent Required Pore Radius for Q/A * 3000 Flat Plate Surface in mils (cm) R n/hr. sa. ft. (0.2262 gm-cal/sec-cm2) Porous Boiling Layers 1. Copper-Nickel, 100-120 mesh 2.1 (0.0053) 1.4 (0.78) (0.0125-0.0149 cm) Granular C 2. Copper, 35-60 mesh 3.4 (0.00864) 1.0 (0.56) (0.025-0.05 cm) G anular 3. Copper-Nickel, 50-60 mesh 3.0 (0.00762) 0.9 (0.50) (0.025-0.0297 cm) P Granular Aluminum, 140-270 mesh 1.8 (0.004o) 2.3 (1.28) (0.0053-0.0105 cm) Granular 6. Copper-Nickel, 50-325 mesh 2.0 (0.00508) 1.7 (0.9*0. ( 0.00^-0.0297 cm) Granular 7. Aluminum, 25-35 mesh 7.0 (0.0177b) 2.0 (1.11) (0.05-0.071 cm) Granular Smooth Surfaces ti. Copper* (non-porous) 15.0 (8.33) 9. Aluminum* (non-porous) 23.0 (12.78) A porous boiling surface layer, as described above, in operation provides a multitude of interconnecting partially liquid filled capillaries which act as nuclei for the growth of many bubbles of the boiling liquid. If the pores were not interconnected, their continued performance as nuclei for bubble growth would be critically dependent upon retaining entrapped air or vapor within the pores. However, with interconnected pores, vapor formed in one pore can activate one or several adjacent pores, so that the process continues without interruption and without dependence upon air or vapor entrapment. At C least some of the pores in the interconnected matrix are also believed to supply liquid to adjacent pores. As the bubbles grow, they finally emerge from the interconnected capillaries, due to continued generation of vapor within the capillaries, break away from the surface, and rise through the liquid film covering the porous boiling layer. The liquid continues its flow into the capillaries and maintains the capillary walls wet, thus giving increased surface evaporation. The high boiling coefficient results from the fact that the heat leaving the base metal surface does not have to travel through an appreciable liquid layer before meeting a vapor-liquid surface producing evaporation .

Within the porous boiling surface layer, a multitude of bubbles are grown so that the heat, in order to reach a vapor- liquid boundary, need travel only through an extremely thin liquid of the confini-ng pore. Vaporization of liquid takes place entirely within the pores and substantially no superheating of the bulk liquid is required or can occur.

With a smooth metal surface, however, only a few bubole points exist and the initiation of bubble growth requires a large degree of superheat due to the compressive force of liquid surface tension on a very small bubble. The heat for bubble growth must be transferred by convection and conduction from the smooth base metal to the distant vapor-liquid interface of a bubble v/hich is almost completely surrounded by bulk liquid.

The above described performance of a porous boiling surface layer is not merely the result of increasing the surface area by, for example, mechanically roughening the surface. This fact was shown by a test comprising immersing a porous boiling surface layer bonded to a copper block containing embedded heating coils to boil water at one atmosphere pressure. At very low heat fluxes insufficient to activate the pores with vapor, the boiling heat transfer coefficient and the visual phenomena of bubble points were quite similar to those obtained with a smooth surface copper block. However, at higher heat fluxes producing vapor activation of the pores, extremely high boiling coefficients were obtained which are impossible to achieve with the smooth block or with a block having thoroughly mechanically roughened surfaces. The following test results in boiling water illustrate the effect of porous boiling surfaces at three temperature differences, and at heat fluxes sufficiently large to produce vapor activation of the pores.

TABLE III Heat Flux* Heat Transfer Flat Plate Surface Equivalent T (0/A) Coefficient Porous Pore Radius °F 3tuu//hnr .. sLsqq .. fitu.. BLiti,u//hnrr ,, ssqq .. fι τ, .· Boiling Layer mils (cm) (°C) ( gm- cal/sec-em^) ( m-cal/sec-cm^-°C Copper-Nickel 1.5(0.83) 3,500(0.2639) 2,340(0.3171) 100-200 mesh 1.4 2.0(1.11) 17,500(1.3195) 8,750(1.1856) (0.0125-0.0149 cm) (0.00356) Granular Copper 1.5(0.83) 8,500(0.640) 5,650(0.7656) 35-60 mesh 3. 2.0(1.11) 17,500(1.3195) 8,750(1.1850) (0.025-0.05 cm) Granular (0.00864) 2.5(1.39) 31,000(2.3374) 12,400(1.6802) Copper-Nickel 1.5(0.83) 9,500(0.7163) 6,330(0.8577) 50-bO mesh 3.0 2.0(1.11) 17,500(1.3195) 8,750(1.1850) (0.025-0.0297 cm) Granular (0. ΟΟ762) 2.5(1.39) 2,800(0.2111) 1,120(0.1518) Aluminum 140-270 mesh 1.8 (0.0053-0.0105 cm) Granular (0.0046) 2.5(1.39) 5,800(0.4222) 2,200(0.2981) Aluminum 1.5(0.83) 9,000(0.6786) 6,000(0.813) -60 mesh 2.7 2.0(1.11) 15,000(1.1310) 7,500(1.0163) (0.025-0.05 cm) Granular (0.0069) 2.5(1.39) 22,500(1.6965) 9,000(1.220) Copper-Nickel 1.5(0.83) 2,200(0.1659) 1,470(0.1992) 0-3 mesh 2.0 2.0(1.11) 4,600(0.3468) 2,300(0.3117) (0.0044-0.02977 cm) Granular (O.OO508) 2.5(1.39) 8,300(0.6258 3,320(0.4499) Aluminum -35 mesh 7.0 2.0(1.11) 3,000(0.2262) 1,500(0.2033) (O.O5-O.O71 (0.01778) 2.5(1.39) 5,600(0.4222) 2,200(0.2981) Figure 2 shows the Table III data in the form of a graph with heat flux as the ordinate and temperature differenceAT °F (°C) as the abscissa. The same surfaces are numbered with the same identification numbers in Table IT, Table III and Figure 2. Comparison of the individual lines for porous surfaces prepared from different mesh particles reveals a wide variation in performance. This comparison may for example, be based on the A'? at the same level of heat flux, so that the most effective surface *Where values are omitted, the heat flux Z/A is outside the 3,000 Btu/hr. x ft2 (0.2662-2.639 gm-cal/sec-cm2 ) range of Figure 2. requires the smallest · T and the least effective surface demands the largest ΔΤ (temperature difference can be directly translated into power requirements). t ^ Metal particles used to construct the porous boiling layer may comprise a wide variety of mesh sizes. However, at least some of the particles should pass through a 35 mesh (0.050 cm opening) screen in order to produce pores of sufficiently small dimension to become active at low Α'ΐ. With reference to Figure 2, and beginning with smooth surface Nos. 8 and 9, per-formance improves with the largest pored surface layer (No. 7 having 25-35 mesh (0.050-0.071 cm) particles), continues improving with finer pores (Nos. 2, 5, 3 and 1 having 35-60,(0.025-0.05 cm) 35-60 (Ο.Ο25-Ο.Ο5 cm), 0- 0 (O.025-O.O297 cm) and 100-120 mesh (0.0125-0.01^9 cm) particles respectively), and subsequently diminishes at still finer pores (No. 4 having 1*10-270 mesh (O.OO53-O.OIO5 cm) particles and No. 6 having 50-325 mesh (0.0044-O.0297 cm) particles). Accordingly, the particles used to construct the porou¾ surface layer should be between 35 and 120 mesh (0.0125 and 0.05 cm) for optimum results, which means that substantially all of the metal particles pass through a 35 mesh (O.05 cm) screen (United States Standard Sieve Series) and are held on a 120 mesh (0,0125 cm opening) screen. Powder used to prepare this preferred surface may contain minor percentages of particles coarser or finer than the preferred range of 35-120 mesh (O.OI25-O.05 cm), and experience indicates that such particles in small quantities, e.g., 10 weight %t either larger or smaller than the preferred range, neither enhance nor detract from the performance.

It should be noted that whereas in general, smaller particles produce porous surface layers with smaller equivalent radii, there is no direct correlation between these two parameters. This is partly because the individual particles used to prepare a nor do these particles necessarily correspond in shape to the particles of different mesh size used to prepare other porous I surfaces. Moreover, the particles are stacked in random relation on the thermally conductive wall, and sizes of the interstitial and interconnecting pores may vary consiaerab ly . Particle shape will affect pore size since spneres,for example, will stack more compactly than irregular shapes and will produce smaller voids. The innumerable variations which these factors permit make it impractical to bracket all suitable powders within a single specification. For this reason, the equivalent pore radius determined by routine testing on specimens of the finished porous boiling surface layer as described previously is an -ttccurate method of identification.

In general, any metallic material is suitable for preparing the porous boiling surface, provided it has good thermal conductivity, is available as a fine powder, is bondable to itself and to the base metal, and Is easily wetted by the liquid to be boiled. The powder particles used In preparing the porous boiling surface are preferably either granular or spherical. Geometrical considerations suggest that granular or spherical particles are more effective than flakes or dendritic particles in producing a large number of approximately uniform sized pores. Very thin flakes are less desirable since they are difficult to bond as discrete particles, and because their extremely large surface area complicates the task of cleaning the powder thoroughly.

The particle material should preferably have high thermal conductivity as stated previously. Among others, nickel and copper porous boiling surface layer materials on copper base metal nave Deen tested under identical conditions; the copper-on-copper heat transfer coefficient being about three times as large as tne nickel-on-copper combination which is to be expected because of the higher thermal conductivity of copper. Copper these metals being 224 and 117 Btu/hr. sq. ft. °F/ft. (0.926 and 0.4b4 gra-cal/sec-cm2.°C/cm) , respectively. In corrosive service, alloys, such as stainless steel, resistant to chemical attack may be used.

The thicxness of the porous boiling surface layer may vary by at least a factor of 10 without severe detriment, and is only slightly affected by the physical properties of the boiling liquid. The thickness should be greater than the average particle diameter and preferably should be at least twice the average particle diameter. With fine particles such as 325 mesh (0.0044 cm), uniformity and complete continuity of the coating will usually govern the minimum thickness which is applied.

Functionally, the maximum thickness which can be used without detriment is controlled only by the capillarity of the surface and by the ability of the surface to discharge the vapor produced in boiling. In operation, the surface should be capable of drawing the liquid all the way through the thickness to the base metal so that the surface is completely wetted, while at the same time discharging and disengaging the vapor from the pores.

Excellent results have been obtained with relatively thick surfaces; for example, the porous boiling surface layer of item of Table II and of Figure 2 is about 42 mils (1.1 mm) thick. Λ suitable porous surface according to the invention may be produced by sintering 35-60 mesh (0.025-0.05 cm) particles of a heat conductive metal such as copper to the hot wall supplying heat to the boiling liquid. The particles are applied in such quantity to provide a porous layer thickness of about (0.11 cm). The interstices or voids between the particles should be essentially free of solid material and should be interconnected within the depth of the layer. The interconnected pores thus formed will vary widely in size and many will fall between 4 and 8 mils (0.010 and a great .number of pores pe unit area of surface.

Various methods for producing a sintered metal porous boiling surface on a base metal may be employed. One preferred method is to use a temporary binder such as a plastic material to establish and maintain a uniform coating on the base metal surface, the binder being such that it decomposes and varporizes during the heating and sintering process. One such plastic is an isobutylene polymer having a molecular weight of about 1^0,000 and known commercially as "Vistanex". 1C The plastic binder is dissolved in an appropriate solvent such as kerosene or carbon tetrachloride and sufficient metal powder is aaded to give a uniform viscous slurry with a metal- plastic weight ratio of about 92 to 1. The base metal surface must be free of grease, oil and oxide coating to obtain proper It) bonding of the porous coating. Just before applying the slurry, the surface may be flushed with the plastic solution to facilitate wetting by the slurry, thereby obtaining more even distribution.

A number of methods may be used for applying a slurry coating to the base metal. The object is to achieve a uniform coating, and the selection of the method will depend uponthe geometry and orientation of the surface. Spraying and dipping are two procedures which have been used successfully.

The coating is air-dried either during or after the application procedure. The bulk of the solvent is thus removed 2Lj by evaporation leaving a solid, self-supporting layer which is held in place by the binder. Then the base metal and the coating are blanketed with a mildly reducing atmosphere and the temperature is raised for a sufficient time to sinter the particles together and to the' base metal. The circulating reducing gas removes the thin oxide film and also purges the decomposition proaucts from tne surface materials. In the case of copper, the coating is sintered at about 180°P below its melting point, or about 1760°F.

In another successful method, the binder and solvent only are coated on the surface, and the metal powder is then dustea in dry form onto the tacky coating. This has the advantage that the solvent may be evaporated before the metal particles are applied and the plastic is less fluid and mobile. With careful uustintf procedure, a very uniform layer can be obtained which exhibits reduced tendency to run or slump. The coating-dusting steps can be repeated if desired to build up relatively thick layers using thin applications.

Another successfully employed binder is a methyl cellulose polymer having a viscosity of 4000 cps and known commercially lb as " ethocel". A preferred slurry comprises 32 grams of copper powuer in 100 cc. of a 2% water solution of such polymer. After application of a coating of the slurry on the base metal, it is. air dried at ordinary temperatures (below the boiling point of water), and then furnace dried at about 750°F in an atmosphere of water saturated annealing gas. The coating is then sintered at appropriate sintering temperatures. °till another plastic binder- which is successfully used in polystyrene which has a molecular weight . of about 90,000 and is soluble in toluene or xylene. 2 fls stated above, binders and slurries are used to facilitate distribution and to hold the powder temporarily in place until a permanent thermal bond can be achieved. When surface arrangement permits, the powder may be applied without binder and sintereu in ury form. _,u One arrangement consists of applying the porous boiling surface on the inner wall of heat exchanger tubes. For such arrangement, an excellent procedure is to preliminarily dis- tribute the slurry within the length of the tube, and then place the tube In a machine capable of rotating the tube about its own axis at a rate sufficient to produce a smootn coating, about 200 rp-i . '"he coating is air-dried during tr.e spinning operation and is then furnace sintered unaer tne conditions previously described, 'not her application procedure lends itself particularly to flat, corrugaged or outside-cylindrical surfaces. According to this method, a slurry of metal powder and plastic binder is fed from a hopper onto the outer surface of a polished roll in 0 a uniform layer. As the roll slowly rotates, the evaporation of the vehicle sets the plastic film and the latter is continously stripped off the roll as a sheet of the plastic material containing the embedded metal powder. The composite film is then placed in contact with the surface of a metal sheet which is conducted into the furnace wherein the plastic is vaporized and the metal powder sintered to the sheet. This process lends itself very readily to quantity production methods.

The above-mentioned plastic coating-metal dusting technique is preferred for preparing boiling surfaces on the outer wall 0 of heat exchanger tubes. The plastic coating is readily applied by spraying or brushing. The powder can be dropped from an overhead screen. Optionally, the plastic-coated tube can be rolled in metal powder, then tamped to dislodge excess particles which are not firmly adhered. j The process of this invention may be practiced in heat exchangers wherein at least two passageways are thermally associated, as for example by a common wall, so that the liquid to be boiled flows through a first passageway and a warmer fluid flows through the second passageway. In such apparatus, 0 the warmer fluid constitutes the warm source and the common wall forms tne thermally conductive wall receiving heat from the warmer fluid. The porous layer is bonded to the common Dynamic flow heat exchangers suitable for practicing this invention may for example, be the plate type in which a plurality of parallel, spaced parting sheets are housed in a core section with appropriate headers and manifolds for the fluid streams.

The heat exchanger may also be the shell-and-tube con iguration with one or more tubes positioned within a shell such that the tubes form a first fluid passageway ana the surrounding shell constitutes a second fluid passageway. The 1C porous layer is bonded to one side of the tube wall so as to contact the boiling liquid.

It should be appreciated that various means may be employed in heat exchangers to improve the condensing coefficient, in combination with the instant process. For example, fins or lb corrugations may be employed to increase the heat transfer area in the passageway through which the warmer fluid flows. '"his invention may also be practiced in pool boiling- type heat exchangers in which the porous surface layer is bonded to one side of a thermally conductive wall positioned in a con- 20 tainer. The other side of the wall is contacted with a heat source which may for example, be a warmer fluid or an electrical, nuclear or other solid state heat source. The porous layer is covereu with the liquid to be boiled and the resulting vapor is released from the porous layer and passes through the liquid pool into the overlying gas space. still another suitable heat exchanger configuration, the porous layer may be applied on the inside walls of a tube bundle vertically positioned in a shell. The colder liauid in the tubes is heatea and boiiea oy the warmer fluid within the liquid slugs is discharged from the tubes' upper end.

To practice the invention, the liquid to be boiled need only provide a thin film over tne porous layer as long as the latter is completely covered. Tn other embodimen s, the porous layer is immersed in a relatively deep liquid pool.

Tt is seen that the porous layer may be bonded to a wall positioned in any orientation ranging from horizontal to vertical. Tn either the inclined or vertical position, the liquid to be boilea may be introduced at the lower end as above described or, alternatively may oe introduced at the upper end for downward flow.

Claims (1)

1. A process for transferring heat from a warm source to a coiling liquid having a Kelvin parameter of between 1 and mils x where Surface tension Saturation temperature of boiling liquid corresponding to the vapor pressure of liquid Latent heat of boiling liquid Density of Vapor C Conversion x force comprising the steps of providing heat exchange apparatus having a thermally conductive wall with a porous boiling layer bonded to one side of such porous boiling layer being constructed of mally conductive discrete at least some of which are suf iciently small to pass through mesh said particles being bonded together and to said wall in random stacked relation to form a uniform matrix with interstitial and interconnection pores between adjacent particles having equivalent pore radii between 1 and completely covering said porous boiling layer with said liquid as at least a liquid and contacting said heat source with another side of said wall such that sufficient heat is transferred through the wall to form vapor bubbles within said porous boiling and discharging said vapor as bubbles emerging from said porous boiling layer into the liquid portion covering said porous boiling A process according to claim 1 in which water is said boiling A process according to claim i in said pore radii between 2 and and A process according to 1 in water is said liquid and radii are between 2 and mils and A process according to claim 1 in which said particles are sufficiently small to pass through a 35 mesh cm screen and sufficiently large to be retained on a 120 mesh A process according to claim i in which water is said boiling said particles are sufficiently small to pass through a 35 mesh cm screen and sufficiently large to be retained on a 120 mesh 5 cm and said pore radii are between 2 and and Attorney for Applicants insufficientOCRQuality