GB2151744A - Tubes for heat exchangers - Google Patents

Abstract

A heat exchanger tube using sea water or brackish water as a coolant consists of a double tube composed of an inner tube made of copper or copper base alloy and an outer tube made of titanium. The double heat exchanger tube improves significantly both biofouling and corrosion problems due to the adhesion of aquatic living things in salt and brackish water.

Description

SPECIFICATION Tubes for heat exchangers using sea water or brackish water as a coolant This invention relates to tubes for heat exchangers using sea water or brackish water as a coolant.

Heretofore, sea water and other brackish water have been widely employed as a coolant in shell and tube exchangers of condensers or similar equipments. However, in such heat exchangers, aquatic living things (macro and/or micro bio-organisms in salt and brackish water), such as a barnacles, cling to the inside wall of the heat transfer tubes or block them, and, as a result, presents serious problems such as a substantial lowering of the heat transfer ability of the tubes, a significant loss of head of coolant and damages due to corrosion of tubes, etc.As means of preventing these problems, for example, there have been adopted the following methods: by sterilizing aquatic living things in coolants by using chlorine base germicides or temporarily heating a coolant; and by passing sponge balls or brush together with coolants into heat transfer tubes, whereby pushing out or cleaning the aquatic living things adhering to the inside wall of tubes.

Among the known methods above, the first method using chlorine base germicides has become increasingly difficult to be practiced because of natural environment conservation. Therefore, when unchlorinated sea water or brackish water is used as a coolant, only one way is applicable to prevent aquatic living things from adhering to the inside wall of heat transfer tubes in which sponge balls or brushes are passed throughout the tubes together with the coolant during working condensers or the like.

However, this method has the following disadvantages.

(1 ) Since it is impossible to uniformly pass the sponge balls throughout all of the heat exchanger tubes (usually, outside diameter: 20-30 mm, number of tubes: 3,000 to 10,000) to be cleaned, aquatic living things fastening to the tube walls can not be entirely cleaned out of the tubes. Thus, the contamination or blockage problems caused by the adhesion of aquatic living things can not be successfully reduced or eliminated by the prior art method.

(2) When heat transfer tubes made of copper alloys, for example, the copper alloys specified under JIS H 3300 such as C 6870 to C 6872, C 7060, C 7100, C 7150, etc. (hereinafter, the alloy numbers are all represented by aesignations of Japanese Industrial Standard) are employed, erosion-corrosion occurs under the excessive sponge ball passing, developing the leaks of the coolant salt water in service.

(3) When Fe ion is dosed to the coolant water under the controlled use of sponge balls, in view of promoting a corrosion-resistant film formation, the ability of copper to sterilize aquatic living things can not be fully exerted, whereby the aforesaid adhesion of the aquatic living things is resulted.

In tube bundle, the flowing rate is widely distributed and in some tubes the flowing rate is low. The lower flowing rates allow aquatic living things to adhere to the tube walls and, as is clear from Table 1, the tendency of this adhesion greatly depends on the tube material and the flowing rate. When the tube material is, as titanium, highly resistant to corrosion in salt water environment, the unfavorable adhesion set forth above considerably occurs. Test was made under the following conditions to see how the tendency of the adhesion is changed by tube material and coolant flowing rate and the test results are shown in Table 1. The numbers given in Table 1 are the numbers of the striped barnacles with a shell size of 5 mm or larger which adhered to the tube specimens formed from the respective materials.

Size of tube specimens Outside diameter: 25.4 mm Wall thickness of copper or copper base alloy tubes: 1.2 mm Wall thickness of titanium tubes: 0.7 mm Length: 1 m Tested water: Natural sea water Tested period: 6 months (from spring to autumn) Number of Barnaclesim Flowing Rate 0.2 0.5 0.8 1.1 1.5 1.8 21 {miser) Titanium tube 150 70 25 46 20 6 0 Coated Tube* 85 46 60 24 35 9 0 Aluminum Brass Tube 10 6 3 0 0 0 0 10% Cupronickel 12 6 0 0 0 0 0 Pure Copper (Phosphorous- 5 0 0 0 0 0 0 dioxidizedcopper) *Aluminum brass tube whose inside wall was coated with an anticorrosive paint As can be seen from the foregoing table, higher tendency for the adhesion of the aquatic living things was observed in the titanium tube and the coated tube and particularly, in lower flowing rate, the tendency was markedly high. Thus, it is required to pass the coolant at flowing rates not less than 2 m/sec. While, in the cases of using copper and copper base alloy as tube materials, the adhesion does not occur at flowing rates of 1.0 m7sec. or higher.

Further, with respect to the tube specimens used in the test above, the relation between the number of cleanings with sponge balls and the corrosion ofthetube materials were investigated under the following conditions.

Tested coolant: Natura! S-? water (unchlorinated) Flowing rate of the coolant in the inside of the tube: 1.8 miser.

Tested period: 6 months Used sponge balls: 26 mm in diameter, intermediate hardness, and produced by Tasrogge Co.

Number of the sponge bails per one cleaning: 10 TABLE 2 Corrosion Rate lmmlyead Frequencyofclean,ng 0 1 3 5 (Number of cAeaningsl Week) Aluminum Brass Tube 0.05 0.08 0.12 0.21 10% Cupronickel 0.08 0.10 0.14 0.30 Pure Copper (Phosphorous- 0.25 0.39 0.52 0.68 dioxidizedcopper) Coated Tube 0 0 0 0 Titanium Tube 0 0 0 0 As will be seen from Table 2, the copper or copper alloy tubes become increasingly vulnerable to corrosive attack with increase in the number of the cleanings with the sponge balls. Therefore, the number of the cleanings with sponge balls should be restricted.

In practice, plants, in working are periodically, for example, once per week, stopped and cleaned by passing brushes through every tube incorporated therein in order to remove the living things adhering to the tube walls. However, such a periodic cleaning is very expensive and leads to a significantly increased maintenace cost.

It is therefore an object of the present invention to overcome the above disadvantages or problems associated with the prior art, and particularly to provide heat exchanger tubes using sea water or brackish water as a coolant in which the adhesion of aquatic living things can be significantly reduced and, further, leaks of the coolant caused by corrosive attack of salt water can be successfully prevented.

According to the present invention, there is provided tubes for heat exchangers using sea water or brackish water as a coolant, the tube consisting of a double tube composed of an inner tube made of copper or copper base alloy and an outer tube made of titanium.

Brief description of the drawings Figures 1(a) and 7 lbl show a heat exchanger tube of the present invention. Figure 1 (a) is a transverse, sectional view of the tube and Figure 1 (b) is a longitudinal, sectional view of the same.

Figure 2 shows a heat exchanger tube of the present invention jointed to a tube plate to provide a testing sample.

The present invention will now be set forth hereinafter in detail with reference to the accompanying drawings.

Figures 1 (a) and 1 (b) show a heat exchanger tube according to the present invention. Figure 1 (a) is a transverse, sectional view and Figure 1 (b) is a longitudinal sectional view. Referring in detail to these drawings, reference numerals 1 and 2 designate an outer tube made of titanium and an inner tube made of copper or copper alloy. The double tube can be prepared by means of extruding or hydraulic expanding.

Copper or various copper alloys specified in JIS H 3300 may be employed as preferred materials for the inner tube. The copper or copper alloy tubing materials exhibit a relatively high effect in preventing the adhesion of aquatic living things and retain corrosion rate in flowing sea water or other salt water at levels not greater than 0.5 mm/year. On the other hand, titanium as the outer tubing material exhibits a superior resistance to corrosive attack in salt water environments.

Therefore, the heat exchanger tubes of the present invention greatly reduce the adhesion or blockage of aquatic living things by virtue of the sterilizing effect of the copper or copper alloy in the inner tubes. Further, even if the inner tubes are locally etched or pitted by corrosive attack of salt water, the leaks of the salt water flowing within the tubes are prevented by virtue of the high corrosion resistant titanium material used in the outer tubes.Further, even when unchlorinated salt water is employed as a coolant, it is possible to clean the internal walls of the used tubes with sponge balls having a high detergency by using the heat exchanger tubes of the present invention singly or in combination of the titanium tubes or the coated tubes having a high corrosion resistance in salt water environments, and whereby the heat exchangers in service can be kept free from the contamination of aquatic living things and the risk of sea water leakage.

The wall thickness of the inner tubes and the outer tubes is determined according to the required heat transfer ability, mechanical properties, such as resistance to mechanical shock, effective duration of sterization, etc. For example, the thikness of the titanium tube is normally 0.5 mm according to the required mechanical properties.

In practical use of the heat exchanger tubes according to the present invention, the tubes may be arranged over all of the places requiring heat transfer tubes or, in part, only at the particular places of heat exchangers, for example, in cooling regions of non-condensable gases, such as ammonia, and at the portions to which aquatic living things tend to cling.In the latter case of using partially the tubes of the present invention, it is necessary to employ materials having a high resistance to destructive attack of saltwater in the absence of iron ion, for example, titanium tubes or copper alloy tubes having a corrosion resistant coating on the inside surface, in the other portions not suing the tubes of the present invention, since iron ion, of which oxide deposit on tube surface interferes anti-fouling function of copper or copper alloys, is not added to salt water coolant used in the present invention.

Now, one example of double tubes according to the present invention will be described in detail hereinafter.

A double tube was prepared by hydraulic expanding method using an inner tube made of aluminum brass (JIS H 3300, C 6871) and a welded outer tube of titanium (JIS H 4631, TTH 35). The size and the mechanical properties of the inner and outer tubes are given in Table 3.

TABLE 3 Outside Inside Yield Tensile Elongation Diameter Diameter Strength Strength (mum) (mm) Rkgimm2) (kg/mm2) { Aluminum Brass Tube 23.6 22.6 18 46 65 Titanium Tube 25.4 24.4 36 48 30 The double tube thus obtained satisfied the properties (expanding, flattening, season crack, etc.) required for the heat exchanger tubes made of the copper or copper alloys which are specified in JIS H 3300.On the exmination of the over-all coefficient of heat transfer, water vapor and artificial sea water with a flow rate of 2 m/sec. were passed in the outside and the inside of the double tube, respectively, and there was obtained the result of 2,800 kcal/m2hr. C. The value is 90% of that of an aluminum brass tube with a thickness of 1.2 mm.

Thus, such improved properties give the tube of the present invention good utility in heat exchangers using salt water as a coolant.

Further, the double tube prepared in above example was, as shown in Figure 2, joined to the tube plate of a heat exchanger and were tested on its drawing strength and water tightness. In this tests, as shown in Figure 2, the titanium outer tube 4 was partially cut off at the end portion of the double tube 3 and the cut portion was covered with an aluminum brass collar 5 having a length of 20 mm. After expanding the end portion, the expanded portion was jointed to a 30 mm thick tube plate 6 made of Naval brass to provide testing sample.

In Figure 2, reference numeral 7 designates an aluminum brass inner tube 0.5 mm in thickness. As a result of the tests, drawing strength of not less than 600 kg and water tightness of not less thant 60kg/cm2 were obtained and these test results are satisfactory for the intended use.

As previously described, the present invention provides tubes highly suited for use in heat exchangers using sea water or brackish water as a coolant and thereby eliminates the difficulties or problems caused in working heat exchangers due to the adhesion of aquatic living things and leaks of coolant due to the corrosion in salt water environments. Thus, the heat exchangers can exert their ability to the full by using such greatly improved tubes without suffering from the disadvantages encountered in the prior art.

Claims (2)

1. A tube for heat exchangers using sea water or brackish water as a coolant which consists of a double tube composed of an inner tube made of copper or copper base alloy and an outer tube made of titanium.

2. A tube for a heat exchanger substantially as herein described with reference to the accompanying drawings.