How to clean stainless steel plate heat exchanger?

- Aug 17, 2018-

How to clean stainless steel plate heat exchanger


First, the basic principle of plate heat exchanger to remove scale


1) Dissolution: The acid solution easily reacts with calcium, magnesium, and carbonate scale to form a soluble compound, which dissolves the scale.

2) Peeling effect: The acid solution can dissolve the oxide on the metal surface. Destruction and scale bonding. Thereby, the scale adhering to the surface of the metal oxide is peeled off and peeled off.


Second, the plate heat exchanger cleaning scale process requirements


Pickling temperature: increasing the pickling temperature is beneficial to improve the descaling effect. If the temperature is too high, it will aggravate the corrosion of the acid heat treatment plate by the acid washing liquid. It is found through repeated tests that the pickling temperature is controlled at 60~E. Acid pickling concentration: According to repeated tests, the pickling solution should be based on formic acid 81. O%, water 17. O%, buffer 1.2%, surfactant 0.8% concentration, the cleaning effect is excellent.


Third, the specific steps of the plate heat exchanger cleaning scale


1) Washing: Before pickling, first open the heat exchanger, so that there are no impurities such as mud and scale inside the heat exchanger, which can improve the pickling effect and reduce the acid consumption of pickling.

2) Pour the cleaning solution into the cleaning equipment and then inject it into the heat exchanger.


Fourth, the choice of plate heat exchanger cleaning agent


The choice of cleaning agent, currently used for pickling, includes organic and inorganic acids. Organic acids are mainly oxalic acid, formic acid and the like. The main inorganic acids are: hydrochloric acid, nitric acid and the like. According to the heat exchanger fouling and process, material and scale composition analysis:


5. Grinding and polishing in plate heat exchangers


1) Mechanical grinding: Titanium has high chemical reactivity, low thermal conductivity, high viscosity, low mechanical grinding ratio, and easy to react with abrasives. Ordinary abrasives are not suitable for grinding and polishing of titanium. Hard abrasives, such as diamond, cubic boron nitride, etc., the polishing line speed is generally 900~1800m/min. Otherwise, the titanium surface is prone to burns and microcracks.


2) Ultrasonic grinding: By ultrasonic vibration, the abrasive grains between the grinding head and the surface to be polished are moved relative to the surface to be polished to achieve the purpose of grinding and polishing. This has the advantage that the grooves, sockets and stencils that are not ground by conventional rotary tools become easier, but the larger castings are not satisfactory.


3) Electrolytic mechanical composite grinding: Electrolytic grinding tools are used to apply electrolyte and voltage between the grinding tool and the grinding surface. Under the joint action of mechanical and electrochemical polishing, the surface roughness is reduced to improve the surface gloss. The electrolyte is 0.9NaCl, the voltage is 5v, and the rotation speed is 3000rpm/min. This method can only grind the plane, and the grinding of the complex denture bracket is still in the research stage.


4) Barrel grinding: Using the centrifugal force generated by the revolution and rotation of the grinding barrel, the denture in the barrel and the abrasive are relatively frictionally moved to reduce the surface roughness. Grinding automation, high efficiency, but can only reduce the surface roughness and can not improve the surface gloss, the grinding accuracy is poor, can be used with deburring and rough grinding before denture polishing.


5) Chemical polishing: Chemical polishing is achieved by the redox reaction of metal in a chemical medium. The advantage is that the chemical polishing has nothing to do with the hardness, polishing area and structural shape of the metal. Where the contact with the polishing liquid is polished, it does not require special complicated equipment, and is easy to operate, and is suitable for polishing the complex structure titanium denture bracket. However, the chemical polishing process parameters are difficult to control, and it is required to have a good polishing effect on the denture without affecting the accuracy of the denture. The best titanium chemical polishing liquid is prepared by HF and HNO3 in a certain proportion. HF is a reducing agent, which can dissolve titanium metal and play a leveling effect. The concentration is <10%. HNO3 acts as an oxidation to prevent excessive dissolution and hydrogen absorption of titanium. At the same time, it can produce a bright effect. Titanium polishing solution requires high concentration, low temperature and short polishing time (1~2min.).


6) Electropolishing: also known as electrochemical polishing or anodic dissolution polishing. Due to the low conductivity of titanium, the oxidation performance is extremely strong. It is almost impossible to use titanium with water acidic electrolyte such as HF-H3PO4 and HF-H2SO electrolyte. After polishing, after the external voltage is applied, the titanium anode is immediately oxidized, and the dissolution of the anode cannot be performed. However, the use of anhydrous chloride electrolyte at low voltage, has a good polishing effect on titanium, small specimens can be mirror polished, but for complex restorations can not achieve the purpose of complete polishing, perhaps changing the shape of the cathode and additional cathode The method can solve this problem and needs further research.


6. Surface modification of titanium in plate heat exchanger


1) Nitriding: A chemical yellow heat treatment technique such as plasma nitriding, multi-arc ion plating, ion implantation and laser nitriding is used to form a golden yellow TiN coating on the surface of the titanium denture, thereby improving the wear resistance and corrosion resistance of the titanium. And fatigue resistance. However, the technology is complicated and the equipment is expensive. It is difficult to achieve clinical application for surface modification of titanium dentures.

2) Anodizing: Titanium anodizing technology is relatively easy. In some oxidizing media, the titanium anode can form a thick oxide film under the action of applied voltage, thereby improving its corrosion resistance, wear resistance and weather resistance. The anodized electrolyte generally uses H2SO4, H3PO4 and an aqueous solution of an organic acid.

3) Atmospheric oxidation: Titanium can form a thick and strong anhydrous oxide film in high temperature atmosphere, which is effective for comprehensive corrosion and crevice corrosion of titanium. The method is relatively simple.


7. Treatment of casting defects in plate heat exchangers


Internal air holes and shrinkage holes internal defects: can be removed by hot isostatic pressing, but the accuracy of the denture will be affected. After X-ray flaw detection, the surface is removed from the exposed pores and repaired by laser. Surface porosity defects can be directly repaired by laser local welding.


Eight, the removal of the surface reaction layer of the stainless steel plate heat exchanger


The surface reaction layer is the main factor affecting the physical and chemical properties of titanium castings. Before the titanium castings are polished and polished, it is necessary to completely remove the surface contamination layer to achieve a satisfactory polishing effect. The surface reaction layer of titanium can be completely removed by pickling after sandblasting.

1) Sand blasting: The blasting treatment of titanium castings generally uses white corundum coarse spray, and the pressure of sand blasting is smaller than that of non-precious metals, generally controlled below 0.45Mpa. Because, when the injection pressure is too large, the sand particles will strike a strong spark on the titanium surface, and the temperature rise can react with the titanium surface to form secondary pollution, which affects the surface quality. The time is 15 to 30 seconds, and only the sand, the surface sintered layer and the part and the oxide layer on the surface of the casting can be removed. The remaining surface reaction layer structure should be quickly removed by chemical pickling.

2) Pickling: Pickling can quickly and completely remove the surface reaction layer without causing contamination of other elements on the surface. Both HF-HCl and HF-HNO3 acid pickling solutions can be used for pickling of titanium, but HF-HCl acid pickling liquid absorbs a large amount of hydrogen, while HF-HNO3 pickling liquid has a small hydrogen absorption amount, which can control HNO3. The concentration of the hydrogen is reduced, and the surface can be brightly treated. Generally, the concentration of HF is about 3% to 5%, and the concentration of HNO3 is about 15% to 30%.


Nine, stainless steel plate heat exchanger coloring


In order to increase the aesthetics of titanium dentures and prevent the discoloration of titanium dentures under natural conditions, surface nitriding treatment, atmospheric oxidation and anodizing surface treatment can be used to form a pale yellow or golden yellow surface to improve the titanium denture. Aesthetic. The anodizing method utilizes the interference effect of the oxide film oftitanium on light, and naturally develops color, and can form a colorful color on the surface of titanium by changing the cell voltage.


X. Other surface treatment of plate heat exchanger


1) Surface roughening: In order to improve the bonding property between titanium and the facing resin, the surface of the titanium must be roughened to increase the bonding area. Clinically, sandblasting roughening treatment is often used, but sand blasting will cause aluminum oxide contamination on the titanium surface. We use oxalic acid etching method to obtain good roughening effect, and the surface roughness (Ra) can reach 1.50 after etching for 1 hour. ±0.30μm, etching 2h Ra is 2.99±0.57μm, which is more than double the Ra (1.42±0.14μm) of sandblasting alone, and the bond strength is increased by 30%.

2) Surface treatment against high temperature oxidation: In order to prevent the rapid oxidation of titanium at high temperatures, titanium silicon compounds and titanium aluminum compounds are formed on the surface of titanium to prevent oxidation of titanium at temperatures above 700 °C. This surface treatment is very effective for the high temperature oxidation of titanium. Perhaps the coating of such compounds on the surface of titanium is beneficial to the combination of titanium and porcelain, and further research is needed.