It is generally recognized that the cost of fresh water produced by means of sea water desalination using multi-stage flashing methods can be reduced by selecting low-cost heat transfer tube materials possesing high corrosion resistance. Therefore, the study of tube materials has been a chief item of the national research and development program. The main part of this research was conducted during the period from 1969 to 1973 (fiscal year) utilizing loop and field test plants at MITI's Chigasaki Test Facility. In loop tests, corrosion tests were conducted using three loops in which sea water, pre-treated with acid and cacuum-dearated, was circulated. Thousand-hour tests were conducted 17 runes for 106 kinds of specimens. In the field tests, a short-tube-type, 6-stage flashing plant (capacity 24m³/day) was operated, using the heat transfer tubes in each stage as test specimens, and 2, 000-hour tests were conducted five times, and a 5, 000-hr test was performed. The present paper is the first in the series composed of 6 reports entitled “Research and Development of Heat Transfer Tube Materials for a Multi-stage Flash Desalination Plant, ” and describes the concept of a multi-stage flash desalination and the methods of research and testing.

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A study was carried out to seek for cheaper copper alloys than those currently used in MSF desalting plant. 30 kinds of copper alloys were first evaluated by the loop test and then selected alloys by the field test. Finally, the long run field test was carried out on Al-brass as a representative of selected copper alloys to evaluate the applicability to heat transfer tubes for desalting plants. Test results obtained are as follows. 1) 70/30, 90/10 cupronickel and Al-brass currently used in MSF desalting plant exhibited very stable corrosion resistance, and its anti-corrosiveness ranks with the top class among tested alloys. 2) As new alloys, 6Ni-10Zn-2Al-Cu alloy and 35Zn-1Al-0.04As-Cu alloy exhibited good resistance to corrosion. The anti-corrosiveness of the former was comparable to 90/10 cupronickel and the latter to Al-brass. These alloys are cheaper than the comparable alloys. 3) 6% Al-bronze also exhibited good corrosion resistance, but its cold workability is not good. 4) Other alloys seemed to be hopeless because of some problems in the anti-corrosiveness and the material cost. 5) The amount of scale deposition on Al-brass was small and in the same level as Ti and stainless steel, and its heat transfer property seemed to be quite well over the long run. On the corrosion resisting alloys such class as Al-brass, it seems that the scale deposition depends largely on the brine and operational conditions. 6) The corrosion rate of Al-brass was saturated at about 4, 000hr in the long run field test, and was assumed to follow the parabolic law. From these rate of corrosion, Al-brass was recognized to have satisfactory anti-corrosiveness as the heat transfer tube for MSF desalting plant.

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The loop test, field test and some laboratory experiments were carried out for metallic titanium applied for the heat exchanger tube of MSF plant. In the field test, the brine heater and some parts of the condensers were furnished with extremely thin welded titanium tubes, and 2, 000-5, 000 hours tests were conducted. The results of these tests were summarized as follows. (1) Titanium showed perfect corrosion resistance in the loop test where the brine temperature is 100 and 120°C, DO is less than 10 and 50ppb, the flow rate is 2m/s, and the duration is 1, 000 hours as well as 2, 000 and 5, 000 hour field tests where the brine heater, heat recovery and heat rejection were made of welded tubes 0.3mm thick. (2) The heat transfer rate of titanium tubes decreased to 40% of the initial value after 5, 000 hour operation in the field test. The operation in solution of low pH, where pH of the brine was controlled to 3 for 24 hours, had a remarkable effect on the recovery of heat transfer rate without any corrosion damage of the tubes. (3) Welded titanium tubes of 0.3mm thickness served for the field test plant suffered more or less hydrogen absorption over the total tube length. Laboratory experiments were conducted to investigate the effect of acid pickling, galvanic couples, impressed cathodic current and applied stress on the hydrogen absorption of titanium. (4) Vibration characteristics of welded titanium tubes of 0.3mm thickness were examined using a model condenser, specially devised to simulate the dynamic conditions of MSF condensers. Alternating amplitude stress increased with the dimensionless mean velocity of outside fluid to exceed the fatigue strength of titanium (12kg/mm²) under some conditions. Among the design factors, the position of canopy showed the most remarkable effect on the distribution of the velocity.

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The Object of this study is to determine which aluminum alloys have satisfactory properties as heat transfer tubes for large-scale desalination plants of MSF type and also to find the most suitably condition for the operation of the plant using aluminum alloys. The aluminum alloys studied were selected from practical use viewpoint of corrosion resisting aluminum alloys. The first step test was made on small plate type test pieces in a loop corrosion tester under various conditions of temperature, DO-contents, flow rate and pH value of brine, for about 1, 000hr. The second step tests consisted of a 2, 000hr-field test (24m³/d MSF type) and a 5, 000hr test (in Japan Light Metal Association test plant, 20m³/d MSF type, piping and water box etc. 5052 380kg used) under actual operating conditions. In the first and second tests 3003 and 3004 alloys proved the best, which were followed by 5052. Generally, aluminum alloys are next to titanium and copper alloys in corrosion resistance when used for heat transfer tubes of MSF type desalination plant. But aluminum alloys have some problem on the design and maintenance, especially local corrosion due to contact with other metals. At the high temperature side of brine heater aluminum alloys have a tendency of hard-scale deposition but their general corrosion under the MSF desalting condition is not very sensitive to circumstantial variations and is in the same order as that of copper alloys. On the local corrosion aluminum alloys have a growth of pitting and are feared to develop pitting in long-time use. Finally, this paper concludes that aluminum alloys could not be used for MSF type plant now available, but that there is a possibility of their being used for large scale desalination plant, in future if such plant is designed in due consideration of their properties.

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The application of ferrous materials to heat exchanger tubes in an MSF desalination plant have attracted serious attention, because of their low price and availability. In this national project, the ferrous materials mainly examined were low alloy steels, but stainless steels and coated steels were also tested. The test results of these ferrous materials were as follows. (1) Low alloy steels: In the loop test 17 kinds of low alloy steels were tested, the majority of which were low chromium steels. The corrosion rates of these steels were reduced with increasing content of chromium, but above 5% of chromium the tendency of local corrosion increased. Under the brine condition corresponding to a heat recovery section, 2Cr to 3Cr steels exhibited good corrosion resistance. In the field test, 2Cr to 3Cr steel tubes selected by the loop test were mounted in each stage of the heat recovery section. Contrary to expectation, in the field test the corrosion rates increased more than several times over those in the loop test. From the results of these tests 3Cr-1Al, 3Cr-1Ni and 2Cr-0.1Mo-1.2Mn-0.26Cu steels were selected. The increase of the corrosion rates of these steel tubes in the field test was inferred as follows: deposition of iron rich sludge from circulating brine in the steel tubes (produced by the corrosion of steel shells in contact with the brine) might have an accelerating effect on corrosion of the tubes by the Schikorr reaction. This suggested that continuous removal of iron rich sludge from the steel tubes by sponge-ball cleaning might improve the corrosion resistance of the steel tubes. (2) Stainless steels: In the loop test stainless steels indicated more or less the tendency of local corrosion, especially at the crevice formed by Teflon holders. In the field test, test tubes of stainless steels selected from the loop test were placed in the sections of the brine heater and the heat rejection (where corrosion troubles of copper alloy tubes have been liable to occur). As the results, several stainless steels superior to 316 steel were found. These were three 25 chromium dual phase steels, two high nickel austenite steels and one 25 chromium ferrite steel. However some open type micro pits were observed under sludge in all of these high grade stainless steel tubes. This pitting-like local corrosion may be a form of crevice corrosion caused by the deposition of sludge. Therefore, the continuous removal of sludge by sponge-ball cleaning or the development of more resistant stainless steels is desired. (3) Coated steels: In the loop and the field test some metallic coatings (chromized, aluminized, etc.) and some organic coatings (phenol resin or epoxy resin) were tested. The defect in chromized steel was pitting corrosion originated in pinholes, and that in aluminized steel was the dissolution of aluminum layer which was accelerated by the increased temperature of sea water. Corrosion resistance of the organic coatings was excellent, but because of the extremely low thermal conductivity, coating thickness and the adherence of sludge, the overall heat transfer coefficients were very small. Because of these disadvantages, it seems that the use of coated steel tubes is still not practical.

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The loop test and the field test on 109 kinds of materials at Chigasaki Test Facilities were started in 1969 and finished in 1974, aiming to find out new materials substituted for copper alloys currently used as the heat transfer tubes for Multi-Stage Flash sea water desalination plant (MSF plant). In this report an overall evaluation on a tube bundle was carried out on the basis of the corrosion resistance and the coefficient of overall heat transmission obtained by these tests, adding to the economical factors. Moreover, for the heat transfer tubes of a large scale MSF plant, each material was overall evaluated from the viewpoint of production capacity at present, technique for producing long tube, mineral resources and environment pollution. Finally the optimum materials and tube thickness were discussed. The results are as follows. /Copper alloy: It is confirmed that the current-used materials (BsTF2-4, CNTF 1) have stable characteristics, and an optimum thickness is clarified. /Titanium: The hydrogen absorption phenomenon is observed, though it is confirmed that it has excellent corrosion resistance. For cost reduction it is inevitable to establish the mass production techniques for thin tube. /Aluminum alloy: It is important to establish the operation and chemical control method accompanied with the preventive method against galvanic corrosion and the descaling method. /Low alloy steel: It is promising to put it to practical use as the tubes for the low temperature stage of a heat recovery section in future. However, it is necessary to establish the design of plant for low alloy steel tubes, the descaling method and the corrosion preventive method against oxygen leakage.

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