EDMUND THELEN
ROBERT A. ERB
PROMOTING PERMANENT DROPWISE CONDENSATION Thin noble metal coatings increase condensation rates by more than half, last 70,000 hours or more, and look economically feasible t The Franklin Institute a solution has been found
A to the problem of producing permanent dropwise condensation on condenser surfaces. This solution involves the use of noble metal coatings on the condenser surfaces and may represent a significant contribution to condenser technology. The large increase in condensation rate thus obtained could lead to substantial savings in the cost of future distillation plants for saline water conversion, as well as to more compact design of condensers for power plants and distillation processes. That a great increase in heat transfer occurs in condensers when dropwise (as opposed to filmwise) condensation is present has been well known since the pioneer work of Schmidt, Schurig, and Sellschopp (5) in 1930. The problem has been to find a substrate with high thermal conductivity which will remain hydrophobic indefinitely during the continuous condensation of water on the surface. The approach most used by other investigators has been to treat the metallic surfaces with a material which will be chemisorbed by the surface. For copper alloys, long-chain fatty acids, such as oleic acid ( 3 ) , and sulfur-bonded materials, such as glycerol tri- [ 1I-ethoxy(thiocarbony1) thioundecanoate] (7), have been studied. Lives range from a few hours for the former to several thousand hours for the latter. Thin Teflon coatings have also been investigated (Z), despite the polymer's low thermal conductivity. Dropwise Condensation with Sulfldes and Selenides
Our initial program was based on considerations of the hydrophobic nature and extreme insolubility in water of copper and silver sulfides. In the first phase of study we formed sulfide films on the surfaces of various copper alloys (90-10 Cu-Ni, aluminum brass, admiralty), generally by reaction with moist hydrogen sulfide in air under controlled conditions of concentration, temperature, and exposure time. Thin films of sulfide on the surface did indeed change the wettability and promoted dropwise condensation for about 700 hours. The sulfided surfaces, however, always reverted to a filmwise condition. A possible explanation for this, because the sulfide did not appear to be removed, is that the mobile copper atoms diffused to the surface and'formed a wettable oxide. T o study the behavior of silver sulfide, both sterling silver and silver coatings electroplated on copper alloy
substrates were sulfided by H2S exposure. Condensation studies showed these sometimes to have dropwise condensation for longer periods of time than the copper sulfides. For example, a sample of sulfided sterling silver produced dropwise condensation for slightly over 1000 hours before filming out. A sample of sulfided silver on mild steel continues to show excellent dropwise condensation after more than 10,000 hours of exposure to continuous condensation. There appears to be an effect of the iron substrate material here because sulfided solid silver or silver electroplated over copper alloys, with or without a nickel undercoating, are not usually dropwise for more than 1000 hours. Selenides of copper and silver were also studied and behaved similarly to the sulfides in condensation tests. Dropwise Condensation with Noble Metals
I n the early condensation study program with sulfides and selenides, we were surprised to find that the unsulfided silver controls had even better dropwise condensation characteristics than did the sulfided silver samples. This would be contrary to the widely held belief that any clean, high energy surface is by nature wettable. The complete wettability of glass and titanium samples in the same apparatus helped to confirm that the behavior of the silver was not a phenomenon caused by organic contamination. By consideration of the position of silver in the periodic table (Cu-Ag-Au) we predicted that gold also would produce dropwise condensation. This was tested and verified with various solid and electroplated gold specimens. The inherently hydrophobic nature of clean gold has also been determined by White (6) with condensation under different experimental conditions, A general explanation for the observed nonwettability of silver and gold under the continuous condensing conditions is that there is a lack of surface oxides present and that water is therefore only weakly bonded to the surface by dipole-induced dipole forces and dispersiontype forces. O n this basis, we extended our consideration of metallic surfaces which should inherently produce dropwise condensation to include all the noble metals. Condensation studies were made on rhodium, palladium, and platinum, each of which exhibited dropwise condensation under long-term conditions with ultrapure water. Contact angle measurements of water were VOL. 5 7
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made on these surfaces during condensation ( 4 ) : some average advancing angles: gold, 55-85' : silver, 6889" ; rhodium. 65-82" ; palladium, 74' ; platinum, 50". Some of these surfaces have exhibited dropwise condensation for more than 10,000 hours of continuous exposure. Figure 1 shows dropwise condensation on gold, rhodium, and palladium surfaces after more than a year of condensation exposure. Results with more metals tend to confirm our thinking with respect to oxides and wettability-; the following exhibited filmwise condensation: 90-10 Cu-h-i; T i ; Type 316 stainless steel; 65-35 Sn-Ni; C r ; C d ; X i ; Mo; T a ; W ; and Nb. Condensation Rate and Heat Transfer
A refluxing condenser arrangement with eight X 5-inch vertical bayonet tubes has been used in studJing condensation rate and heat transfer with steam from distilled water and sea water.
Figure 7. Condensation on noble metals. Gold, palladium, and rhodium ( f r o m l e f t ) afcer e,tposure for 7 ) e a r to continuous condensation
T h e appearance of the first set of tubes after 3000 hours is shown in Figure 2, a to d . T h e noble metals are dropwise and the 90-10 Cu-Ni, stainless steel, and bright-electroplated chromium samples are filmwise. T h e only difference between the effects 011 the samples by sea water steam and by distilled water steam \vas a roughening of the silver surface when the steam was generated from sea water. This \vas accompanied also by a lowering of the condensation rate. This suggests that gold or one of the other noble metals would be more appropriate than silver for condensing tube surfaces in saline water distillation. Figures 3 and 4 show at 2550 hours total exposure time the condensation rates with the eight samples i n sea water steam as a function of cooling-water flow velocity. At 10 feet per second, the condensation rate 0 1 1 a 90-10 Cu-Ni tube coated with 50 microinches of gold over 500 microinches of nickel is about 537; higher than the same tube bare or chromium-plated. The stainless steel tube plated with silver similarly sho\\,s a 5076 higher condensation rate than the bare stainless tube. Increases of greater than 9074 have been obtained u.ith a more recent sample of 90-10 Cti-Si alloy plated \vitli 50 microinches of palladium. Similar increases in overall heat transfer coefiicient ( U ) have also been obtained. \\:it11 a cooling-water flow rate of 9.75 feet per second, the values of L'uith the noble metal coated specimens have been typically over 1000 B.t.u. per square foot-OF.-hour (using log mean temperature differences). For the above mentioned palladium-plated.sample after 33 days of condensation in steam at 114' C., U at 9.75 feet per second \vas 1430 B.t.u. per square foot-' F.-hour as compared with 785 B.t.u. per square foot-OF.-hour for the bare 90-10 Cu-Ni tube. Table I lists the overall heat-transfer coefIicients and condensation rates for a niimbcr of samples. Economic Considerations
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Figure 2. Tubes after 3000 hours of continuous condensation: ( a ) (left) 300 pin. of Au ot'er 200 pin. of 'Vi ouer 90-70 Cu-,Vi base and (right) 7 0 pin. of Rh ouer 7 0 0 pin. of A u over 9 0 - 7 0 Cu-.Vi base; base ( b ) (left) 50 pin. of Au oder 300 pin. of AVi over 9 0 - 7 0 CU-IV~ and (right) 500 pin. of A g over 500 pin. of 'Vi over 9 0 - 7 0 Cu--Vi base; ( c ) (left) 500 pin. of A g over AIS1 376 stainless steel base and (right) 500 pin. of Cr over 7 0 0 0 pin. of AYiover 9 0 - 7 0 Cu-Ni base; ( d ) (left) 90-70 Cu-iVi and (right) AIS1 376 stainless steel 50
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Emphasis i n the more recent part of our program has been on obtaining high pcrformance bvith cconomically feasible coating systems. Merely putting on extremely thin films-e.g., 5 or 10 microinches--of noble metals by conventional plating over a base coating of electroplated nickel \vi11 not produce a satisfactory dropwise surface; apparently enough base metal can get to the surface-e.g., through pores--to cause the surface to condense water i n a filmwise fashion. The followi~ig five approaches are currently being studied in our program to optimize the performance per unit thickness of noble metal used (gold and palladium primarily) : -the use of p l a t i n g systems whz'cfi produce noble m e t a l j l m s of relatively l o w porosity; -the use of silver a s the coating immediately below the thinner noble metal topcoat-e.g., 70 microinches of g o l d ocer 500 microinches of siluer over 500 microinches of nickel; -the use of an electrodeposited gold-silver alloy for a topcoate.g., a 72-25 A u - A g alloy, w h i c h i s approximately 2 / a the cost of p u r e gold for a given thickness; -the use of multiple depositions f r o m dzferent electrolytes to obtain the noble metal topcoat;
-the use of an oxide film--c.g., aluminum oxide, titanium dioxidcbeneafh fhe noble metal topcoat lo minimize d@tsion of base-metal ions info the noble mefalfilm; this mig& require fhe use of a process ofhcr fhan elecfrodeposifion to form the fojxoa--c.g., uacuum deposifion ar chemical deposifion.
&-we 3. Condnrrofionraft us. cooling wow wlocilyfor 90-7OCu-Ni con&nsation rates arc higncr than flmivisC rates (2ooo hours in disfilled wafcr sfeam plus 550 hours in sea wafa stcam, including 0 . a O2 for 2 days bcforc bleeding) ~ubcs. Drop-
Figure 5 shows graphically some estimated savings in condenser costs for a given installed condenser cost (in dollars per square foot of condenser surface) and a given cost of dropwise metal coating system, assuming a 50% increase in condensation rate per unit area; a linear decrease in condenser cost as a function of condensing area. For an installed condenser cost of $3.00 per square foot, the "break-even" cost of the coating system would be $1.50 per square foot. For lower-cost coating systems substantial savings in overall cost of condensers could be realized. Present results with systems consisting of 5 microinches of gold over silver and 10 microinches of
TABLE I.
HEAT TRANSFER COEFFICIENTS AND CONDENSATION RATE
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