Please see the following repost: Comparison of common heat exchanger types 1. Plate heat exchanger
1. Features: [url= /productpic/200581115FGF5107.jpg][/url]< /p>
(1) Small size, small footprint; compact structure, small size, small footprint, and light weight. Approximately 250m2 of heat transfer area is arranged in each m3 volume, and the floor area is only 1/5 to 1/10 of the tube-type heat exchanger. It is especially suitable for occasions such as technical renovation of old factories that make full use of the original equipment and overcome space limitations.
(2) The heat transfer efficiency is high; generally it can reach 4000~6000W/m2 ℃ (the medium is water-water), which is 2~3 times the heat transfer coefficient of the shell and tube heat exchanger at the same flow rate times.
(3) Flexible assembly; disassembly, cleaning, replacement of rubber pads or replacement of plates are quick and easy.
(4) Low metal consumption, light weight, thin heat transfer plate, small metal consumption, approximately 10kg of metal consumed per square meter of heating area. It is only 1/3 to 1/4 of the tube heater. (5) Small heat loss;
(6) Easy to disassemble, clean and repair; Strong corrosion resistance and long service life
(7) The residence time of the heated material in the heater Short, with few internal dead ends
(8) It has great operational flexibility and wide application range. The number of plates can be increased or reduced as needed to change its heating area or change working conditions.
(9) Plate heat exchangers are also called "self-cleaning heat exchangers" because of the strong turbulence of the internal materials and the smooth surface of the plates, which generates less fouling.
(10) The disadvantage of the plate heat exchanger is that the sealing perimeter is long and easy to leak. The operating temperature can only be lower than 150oC. It can withstand a small pressure difference and has a small processing capacity. Once the plate is found to be fouled, it must be Disassemble and clean
2. Sealing material
★ Nitrile rubber (N) Use temperature ≤110℃
★ EPDM rubber (E) Use Temperature ≤150℃
★
Silicone rubber (C) Operating temperature ≤230℃
3. The corrugation of the plate has two very important functions:
(1) Improve heat transfer performance.
(2) Improve the rigidity and pressure-bearing capacity of the plate
4. Main parameters:
Single-piece heat exchange area (m2), maximum installed area (m2), channel diameter (mm), maximum material volume (t/h), flow rate per channel (kg/s), heat transfer coefficient k value (w/m2.k), and frame size, etc.
5. The following points should be noted when selecting plate heat exchangers:
(1) The heating area cannot be based on the area of ??the original tubular heater. Generally, only the latter is required. 30% to 50%.
(2) Pay attention to the channel diameter of the equipment, that is, the diameter of the corner hole and connecting pipe, which determines the maximum throughput of materials. If a larger channel diameter is selected, the number of plates can be increased to increase material handling capacity if necessary in the future. Insufficient diameter will limit the flow of material. Especially when using low-pressure steam as the heat source, insufficient pipe diameter will result in insufficient steam intake, seriously reducing the performance of the heat exchanger.
(3) The spacing (gap width) between the plates is also very important. This value for most plate heat exchangers is 5~6mm, which can be applied to liquids or slightly positive pressure. water vapor (vapor pressure 0.03~0.1mpa). This value of some products is less than 4mm and can only be used for liquids or steam with higher pressures. If low-pressure steam is used, the insufficient channel area limits the amount of steam inlet and reduces the performance of the heat exchanger.
6. How to use plate heat exchangers
(1) Installation of plate heat exchangers
The connecting pipes of plate heat exchangers must be properly handled . To prevent the weight of the pipeline and the pulling or pushing force of thermal expansion and contraction from acting on its connecting flange, the connecting pipeline of the heat exchanger should be equipped with a 90° elbow. There should be a drain valve before the steam inlet valve to drain the accumulated water and dirt in the pipe before starting the machine to prevent water hammer and dirt from being brought in during startup. The material pipe should be equipped with bypasses and valves, and a bottom drain pipe should be installed at the lowest position. There should be joints for testing water pressure, and the water pressure should be tested on both sides of the channel before use.
A thermometer and pressure gauge should be installed at each connecting pipe opening.
When using steam for heating, the condensate water temperature should be only slightly lower than the steam temperature (calculated as saturation). If it drops too much, it means there is water accumulation in the device, which will significantly reduce its heat transfer performance. The heat exchanger should be equipped with good condensate removal equipment.
The multiple plates of the plate heat exchanger are compressed into a whole by the end covers on both sides and multiple bolts. The end cap on one side is fixed to the frame and used to connect pipelines, and the end cap on the other side can move along the guide rail during assembly and disassembly. When all materials flow in one way, the four inlet and outlet pipes of hot and cold fluids are connected to fixed end caps. This method is the most convenient for management and installation. At this time, large round holes are opened at the four corners of all the plates, running through them from beginning to end.
Each panel has an elastic spacer firmly adhered to the same side. After the bolts are tightened, the thickness of the compressed gasket is equal to the protruding height of the corrugation of the plate. At this time, the protruding ends of the plate and the gasket are on the same plane and are in close contact with each other. Gasket thickness must be accurate. If the gasket is too thick, the corrugations of the plates cannot contact each other and will deform when pressed; if the gasket is not thick enough, when the bolt is tightened, the tops of the corrugations of the plates will be close to each other and then pressed in to form small dents. Easy to perforate and leak. Swedish am-20 type plates are equipped with gaskets with a thickness of 5.4mm after compression.
Before installation, the gasket and the plate must be bonded and fixed together. The plate is pressed with corresponding pits at the position where the gasket is installed, the gasket is placed, and the specified adhesive and process are used according to the material of the gasket for bonding (some gaskets and adhesives need to be heated and hardened). This method is more troublesome when existing equipment needs to replace gaskets, and leakage will occur if it is not handled well. Plate heat exchangers manufactured by foreign manufacturers in recent years can be fixed using a "clip on" method instead of gluing. There is a special structure at the position where the gasket is installed on the plate. A small buckle can be installed to fix it. The gasket is fixed in the groove. This method is simple and easy, and it is also easy to remove and replace the old gasket. This is more suitable for use situations where gaskets are prone to aging.
(2) Assembly and disassembly of plate heat exchanger
The newly purchased heat exchanger has been installed as a whole and can be installed as a whole. It should not be disassembled unless necessary.
The disassembly and reinstallation of plate heat exchangers is very detailed work and needs to be carried out by experienced personnel according to certain rules to ensure that it is well sealed and can be used normally after installation. Improper assembly, disassembly and installation can cause poor sealing and even deformation and damage to the plate, making it difficult to recover.
Before disassembling the heat exchanger, use a steel ruler to measure the original thickness of the plate pack. Measure and record the upper, lower, left, and right corners of the equipment. This thickness should be restored as much as possible during reinstallation. If you increase or decrease the number of plates, you should first calculate the correct total thickness. For example, using 80 pieces of am-20 plates, the nominal thickness is:
80×(5.4+0.8) = 496mm
The difference between the thickness after installation and compression and the nominal value It should be less than 1%. The thickness of the above example should be between 491 and 501mm.
Plate heat exchangers are usually pressed together with 6 to 12 bolts. During assembly and disassembly, these bolts should be tightened or loosened evenly and balancedly, and must not be tightened unevenly.
When disassembling and loosening the bolts, loosen the middle bolts first and then go to the four corners. At first, do 1 to 2 rounds each time, then more times, and repeat multiple times until completely loosened. It is required that during the relaxation process, the total thickness of the plate is measured at the four corners, and the left and right deviations do not exceed 10mm, and the up and down deviations do not exceed 25mm.
When tightening the bolts, the four-corner bolts should be tightened first, and then the middle bolts should be tightened, small by small, and repeated many times. It is required that the asymmetric deviation of the total thickness of the plate group during the tightening process does not exceed the above value. A wrench of a certain length should be used to install and remove bolts so that the applied torque is appropriate. For example, the am-20 type fastening bolt is m39, and the specified wrench length is 550mm.
Plate heat exchangers supplied from abroad often provide a "force-limiting wrench" to limit the tightening torque to a certain limit (the wrench will automatically slip when the applied torque is too large). A wrench that is too long and too much torque is harmful. It may over-compress the gasket and press the corrugated top of the plate into small dents. As a result, a plate heat exchanger in a domestic sugar factory was scrapped due to perforations in many plates.
The fastening bolts of plate heat exchangers are often equipped with small plane bearings, which are installed at the bottom of the compression nuts to reduce the surface friction when tightening the bolts and ensure that the bolts have an appropriate tightening force. When tightening or loosening the bolt, the other end is controlled by the end cap and cannot rotate.
After the heat exchanger is disassembled, the plates should be hung on the body bracket. If they need to be removed and cleaned, they should be placed on a smooth platform. Do not place them on uneven ground, let alone stack multiple plates. To prevent the plate from bending and deforming. Each panel should be hung back into place immediately after cleaning. Do not hit it with a hammer. Hammering the stainless steel sheet will cause changes in the internal structure and reduce its anti-rust performance. After checking that all plates and gaskets are normal, tighten the bolts and reset them.
(3) Cleaning of plate heat exchangers
Plate heat exchangers are also called plate heat exchangers because of the strong turbulence of the internal materials and the smooth surface of the plates, which generates less scale. "Self-cleaning heat exchanger". If the material flow rate is regularly increased or the material flow is reversed during operation, some sediments inside it can be washed away and the working cycle of the heat exchanger can be extended. Before the heat exchanger needs to be opened for cleaning, flushing with a large amount of water can also remove some of the sediments inside it.
The modern chemical industry and food industry are vigorously developing chemical cleaning of various container equipment to simplify cleaning work, and the same is true for plate heat exchangers. In the food industry, "automatic cleaning in-place" (cip) technology has been widely promoted in recent years. The container equipment does not need to be disassembled and opened. It is cleaned with chemicals and water respectively. The operation is automatically controlled by the computer according to the program, which greatly improves the efficiency of cleaning. Efficiency of cleaning work. The formulation and use methods of detergents are also constantly researched and improved.
The material of the plate heat exchanger can withstand the effects of a variety of chemicals, and its internal volume is small, making it easy to clean with chemicals. The chemicals used depend on the type and composition of the scale. For organic deposits, use 2% NaOH or alkaline detergent solution. For oxide or carbonate deposits, use 2% sodium polymetaphosphate or sodium tripolyphosphate. Or 5% ethylenediaminetetraacetic acid, or 0.7% nitric acid (based on the volume of concentrated nitric acid), the use temperature is 50℃, the maximum is 70℃. After chemical cleaning, pump water again and again.
Under good conditions, the plate heat exchanger can not be disassembled all year round. When it is really necessary to disassemble and clean, be careful. Rinse the plates piece by piece with water, wipe them with a soft cloth or brush, or use an appropriate detergent. Do not use steel brushes or other hard tools to avoid scratching the surface of the plate. It should be installed and tightened immediately after cleaning.
2. Shell and tube heat exchanger.
Shell and tube heat exchangers are the most commonly used common structures. They include: fixed tube plate heat exchangers, U-shaped shell and tube heat exchangers, heat exchangers with expansion joints, and floating head heat exchangers. Heater, segmented heat exchanger, jacketed heat exchanger, etc.
The fixed tube plate heat exchanger has the advantages of simple structure, light weight, and low cost; the disadvantage is that the tubes are bent due to thermal expansion.
The U-shaped shell and tube heat exchanger overcomes this shortcoming by making the tube into a "U" shape, with one end fixed and the other movable, so that the heat exchanger is not affected by expansion. It has a simple structure and is light in weight. Its disadvantages are that it cannot be mechanically cleaned, the tube is inconvenient to replace, and the heat transfer per unit capacity and unit mass is low. It is suitable for situations where the temperature difference is large and the fluid medium in the tube is relatively clean.
The heat exchanger with an expansion joint can solve the expansion problem. It uses an expansion joint structure, so it is suitable for fluids with large temperature differences and high-pressure fluids. Because the joints can be removed for cleaning, it can handle easy scaling. Fluid, but not suitable for low-pressure gas, but its disadvantage is that it is complicated to manufacture.
The floating head of the floating head shell-and-tube heat exchanger is not connected to the shell and can be freely retracted. This not only solves the problem of thermal expansion, but also facilitates cleaning. The tube core can be pulled out during maintenance.
For fixed tube sheets, tubes, and sleeve-type heat exchangers, when the shell volume of each shell is 1 m3
, the heat transfer area is about 30 to 40 m3
. For U-shaped shell and tube heat exchangers and floating head heat exchangers, when the shell volume of each shell is 1 m3
, the heat transfer area is about 70 m2
.
3. Spiral plate heat exchanger
The spiral plate heat exchanger is a heat exchanger composed of spiral heat transfer plates. It has better heat transfer performance than the tube-type heat exchanger, has a compact structure, is simple to manufacture, and is easy to install.
The structure of the spiral plate heat exchanger includes spiral heat transfer plates, partitions, cover plates, distance columns, connecting pipes and other components. The structure varies depending on the type. Various types of spiral plate heat exchangers are composed of two steel plates rolled with a thickness of about 2 to 6 mm, forming a pair of mutually separated concentric spiral flow channels. Cold and hot fluids flow alternately with the spiral plate as the heat transfer surface.
According to the different flow patterns and usage conditions of the fluid in the flow channel, spiral plate heat exchangers can be divided into three structural types: I, II, and III, as shown in Figure 5-20.
Type I: Both fluids flow spirally on both sides of the spiral flow channel. Usually, the cold fluid flows from the outer periphery to the center and is discharged, and the hot fluid flows from the center to the outer periphery, which can achieve strict counter-heat transfer and is often used for liquid-liquid heat exchange. Since both sides of the channel are completely welded and sealed, the I-shaped structure is a non-detachable structure.
Type II: In this type, one fluid flows spirally in the spiral channel, while the other fluid flows axially in the spiral channel on the other side. Therefore, both ends of the axial flow channel are open, and both ends of the spiral flow channel are sealed. This type is suitable for situations where the flow rates of the fluids on both sides are very different. It is often used as a condenser, gas cooler, etc.
Type III: In this type, one fluid performs spiral flow in the spiral flow channel, and the other fluid performs a combination of axial flow and spiral flow in the spiral flow channel on the other side. This type is suitable for condensation cooling of steam. The steam first enters the axial flow part to be condensed. After the volume is reduced, it is transferred to the spiral flow channel for further cooling.
It can be seen from the above structure that when the fluid flows between the spiral plates, the centrifugal force forms a secondary circulation and a fixed-distance column disturbance, which makes the fluid flow at a lower Reynolds number (Re=1400~1800). Turbulent flow is formed, the allowable flow rate in the heat exchanger is relatively high (liquid 2m/s, gas 20m/s), and the heat transfer coefficient is relatively high. Since the flow rate of the fluid is high and it flows in the spiral channel, once dirt is deposited somewhere in the flow channel, the flow cross-section there is reduced, and the local flow rate of the fluid there is correspondingly increased, making it easier for the dirt to be washed away , has a certain self-cleaning effect and is suitable for handling suspensions and fluids with high viscosity. Since the flow channel is long and countercurrent heat transfer can be achieved, it helps to precisely control the outlet temperature of the fluid and recover low-temperature heat energy. In the case of pure counterflow, the minimum temperature difference between the outlet ends of the two fluids is only 3°C.
The main disadvantage of the spiral plate heat exchanger is that the operating pressure and temperature cannot be too high. Generally, it can only operate below 2.0MPa and 300~400℃, and the flow resistance is large. In addition, there are problems of difficulty in inspection and maintenance.
4. Chemical cleaning of plate heat exchangers
Plate heat exchangers were first proposed in the 19th century and were used for sterilization in the milk industry in the early 1920s. Because plate heat exchangers have some unique features in manufacturing and use, they have developed rapidly after being successfully used in industry. Most of the early plate heat exchangers were used in the food industry. Nowadays, plate heat exchangers are used in various industries such as food, brewing, chemical industry, machinery, metallurgy, power generation, and transportation.
1. Introduction to plate heat exchangers
Plate heat exchangers are made of thin metal plates (generally about 0.5-1.5mm thick) pressed into corrugated shapes with a certain shape. A heat exchanger formed by stacking heat exchange plates. The working fluid flows through the narrow and tortuous channel formed by the two plates. Hot and cold fluids pass through their respective flow channels in sequence, separated by a layer of plates through which heat is exchanged.
Plate heat exchangers are mainly composed of plates, gaskets, frames, and compression mechanisms.
1) The plates are the main part of the plate heat exchanger, that is, the heat exchange surface. The plate is a whole sheet stamped from a thin metal plate, and is roughly composed of the following parts.
(1) The plate body part, which is the main heat exchange surface part.
(2) Liquid inlet and outlet hole (corner hole), which is the inlet and outlet channel for fluid and also functions as a header.
(3) The inlet and outlet diversion part, which is between the corner hole and the plate body, mainly enables the fluid to flow evenly between the plates. At the same time, it also has the function of heat exchange.
(4) The sealing groove is located around the plate and is used to place the sealing gasket to prevent the working fluid from leaking to the outside and leaking to each other internally.
(5) Positioning holes and hooks are used to ensure mutual positioning and fixation of the plates when assembling them.
Plate heat exchangers can be classified into 4 categories according to the different pattern shapes of the plates: return flow type, wave flow type, mesh type, and spoiler type.
The main materials currently used for plates are austenitic stainless steel, titanium and titanium alloys, nickel and nickel alloys, etc.
2) The sealing gasket is an important auxiliary part in the plate heat exchanger, which mainly ensures the sealing when the fluid flows between the plates; the cross-sectional size of the gasket often affects the working size value of the plate gap, thereby affecting the Its heat release intensity and resistance characteristics.
Gasket material is an extremely critical factor. It has now become an important factor affecting the use range of plate heat exchangers. When selecting gasket materials, we must consider its temperature resistance, pressure resistance, chemical stability, and elasticity. etc properties. Currently commonly used materials include nitrile rubber, EPDM rubber, fluorine rubber, silicone rubber, neoprene rubber, polytetrafluoroethylene, composite gaskets, etc.
3) The function of the rack is to load the plates and also serve as a compression device.
2. Characteristics of plate heat exchanger
2.1 Advantages of plate heat exchanger
1) High heat transfer coefficient. The flow channel of the plate heat exchanger is small, the plates are corrugated, and the cross-section changes are complex, which causes the flow direction and flow rate of the fluid to continuously change, increasing the disturbance of the fluid, so it can achieve turbulent flow at a very small flow rate, and has a high Heat transfer coefficient. It is especially suitable for liquid-liquid heat exchange and heat exchange between fluids with high viscosity.
2) Great adaptability. The required heat transfer area can be achieved by increasing or decreasing plates. A heat exchanger can be divided into several units and can be adapted to heat or cool several fluids at the same time.
3) Compact structure, small size and less consumables. The heat transfer area per cubic meter of volume can reach 250 m2, and only about 15kg of metal is needed per square meter of heat transfer surface.
4) High heat transfer coefficient and low metal consumption make the heat transfer effectiveness reach more than 85%-90%.
5) Easy to disassemble, clean and repair.
6) The dirt coefficient is small. Due to the large flow disturbance, dirt is not easy to deposit; the plate material used is better and there is little corrosion, which makes the dirt coefficient value smaller.
7) Plate heat exchangers mainly use metal plates, so the price of raw materials is lower than that of pipes of the same metal.
2.2 Disadvantages of plate heat exchangers
1) Poor sealing and easy to leak. The gasket needs to be replaced frequently, which is troublesome.
2) The operating pressure is subject to certain restrictions, generally not exceeding 1MPa.
3) The operating temperature is limited by the temperature resistance of the gasket material.
4) The flow channel is small and not suitable for gas-gas heat exchange or steam condensation.
5) Easy to block, not suitable for fluids containing suspended solids.
6) The flow resistance is larger than that of the shell and tube type.
Based on the characteristics of plate heat exchangers and combined with the experience of other cleaning methods, chemical cleaning has a better effect on the cleaning method of plate heat exchangers. Chemical cleaning generally has the following steps.
3. Chemical cleaning of plate heat exchangers
3.1 Preparation before cleaning
The preparation before cleaning mainly includes the following work.
1) Check and understand the equipment’s activation time, usage records, maintenance records, equipment drawings and other information.
2) Understand the materials used in the equipment.
3) Collect and analyze scale samples from the equipment
4) Understand the distribution of scale in the equipment.
5) Conduct scale dissolution experiments and determine the cleaning agent formula and process through experiments.
3.2 Determination of cleaning process
Static cleaning or dynamic cleaning process can be used for undisassembled plate heat exchangers.
1) The static immersion cleaning process is shown in Figure 1.
2) The dynamic cycle cleaning process steps are as follows.
(1) Water flushing and system pressure test. The purpose of water flushing and pressure testing is to remove accumulated dust, sediment, fallen metal oxides and other loose dirt in the system, and to check the leakage of the temporary connection under simulated cleaning conditions.
Figure 1 Block diagram of static immersion cleaning process
(2) Alkali cleaning. Use alkali washing to remove organic compounds and oil stains and soften the dirt to make it easier to remove. The time is 10 to 24 hours, the temperature is required to be 85°C, and the flow rate is below 0.3 m/s.
(3) Rinse with water after alkali washing.
The purpose is to remove the residual alkaline cleaning solution and remove some impurities from the surface and be taken away.
(4) Pickling. The pickling liquid is used to react with impurities such as scale to generate soluble substances.
(5) Rinse with water after pickling. It is to remove the residual acid liquid and falling solid particles for subsequent processes.
(6) Rinsing uses the rinsing liquid to combine with the iron ions remaining in the system and remove the secondary floating rust produced during the water rinsing process to reduce the [Fe2+/Fe3+] content, which is passivation Make preparations.
(7) Neutralization and passivation. It uses passivation agent to form a passivation film on the metal surface to prevent the equipment metal from rusting.
3) The following "circulation + soaking" cleaning process can be used for the disassembled plate heat exchanger.
(1) Manual processing. The purpose is to reduce the amount of scale manually as much as possible, which can not only reduce the processing pressure for the next process, but also reduce the amount of chemical agents, thereby easing the treatment of waste liquid. This process can mainly use nylon brushes, hair brushes, copper brushes, small cleaning machines, etc.
(2) Cycle + soak cleaning. Use cleaning agents to dissolve and peel off dirt, and maintain a certain flow rate through forced circulation to achieve better cleaning results.
(3) Manual processing. After cleaning, the sediments are manually processed.
4. Application examples
The detachable plate heat exchanger used to produce alumina in an aluminum factory has serious fouling, which has seriously affected the normal use of production and needs to be cleaned. Its material is austenitic stainless steel.
The original scale sample is off-white, and the scale layer is divided into a surface layer and a bottom layer. The surface scale is mainly carbonate scale, which can be easily treated; the bottom scale is hard in texture and contains a large amount of acid-insoluble matter, and the general pickling solution reacts extremely slowly with the scale. After analysis, the scale is composed of very dense alumina. Mainly, it also contains some other oxides and a small amount of magnetic iron oxide and carbon. Although the scale layer is thin, it is very hard and dense. Therefore, dealing with the underlying dirt becomes the key to cleaning.
It is difficult to achieve better cleaning results using a single cleaning formula for bottom layer dirt. The scale dissolution test shows that the use of dilute H2S04, cold HNO3, HF and strong alkali cannot dissolve the dirt, and long-term treatment cannot loosen the dirt, and only fine bubbles are generated extremely slowly.
Using concentrated HNO3 under heating conditions can loosen and dissolve the scale.
A12O3+6HNO3 2Al(NO3)3+3H2O
C+4HNO3 4NO2+CO2 +2H2O
Fe2O3+6HNO3 2Fe(NO3)3+3H2O< /p>
FeO+4HNO3 Fe(NO3)3+NO2+2H2O
The cleaning formula was determined through a scale dissolution test. Since the equipment is made of stainless steel, it was determined through repeated experiments that the main cleaning agent should be nitric acid. Loose and dissolve dirt; and add cleaning aids such as accelerators, penetrants and corrosion inhibitors. The cleaning formula is shown in Table 1.
Pharmaceutical name Mass fraction / % Function description Nitric acid 25-30 cleaning main agent X21-3 penetrant has strong corrosiveness, pungent odor, and inorganic weak acid. Has a strong ability to dissolve iron oxide. Lan-8260.3 Corrosion Inhibitor——
Although the higher the temperature, the greater the concentration, and the longer the time, the effect will be better. However, considering the corrosiveness of nitric acid, the temperature, concentration, and The time is limited to a certain extent to reduce the disposal and discharge of waste liquid caused by the use of chemicals.
1) Cleaning method
Clean according to the cleaning process of the disassembled plate heat exchanger.
(1) First use manual brushing and high-pressure water (control the pressure below 10MPa to prevent equipment deformation) to remove the loose scale layer on the plate.
(2) Prepare the cleaning agent in the cleaning tank. Because the concentration of the cleaning fluid is high, the heating temperature should not be too high.
Place the plate into the cleaning tank for circulation + immersion cleaning. During cleaning, dynamic and static should be combined, with static being the main focus. The cleaning time is controlled at about 12 hours. Due to the particularity of the plate heat exchanger, the flow rate of the pickling liquid needs to be strictly controlled during cleaning to prevent excessive flow rate from affecting the corrosion inhibitor efficiency and causing equipment corrosion.
(3) After chemical cleaning, take out the plates and rinse them with a small cleaning machine. The heat exchanger plates will be as bright as new.
(4) Treatment of cleaning waste liquid. The discharge standard after cleaning waste liquid treatment should meet the third-level discharge standard in GB8978-1996 "Integrated Wastewater Discharge Standard". Since the applicable nitric acid concentration is high, the acidity in the waste liquid is strong and cannot be discharged directly. It can be neutralized by adding alkali for treatment. In addition, lime powder (CaO) needs to be added to the waste liquid to deal with other harmful ions.
2) Cleaning results
(1) Corrosion situation. According to the monitoring of the corrosion rate during cleaning, the cleaning corrosion rate is 0.97g/m2. h.
(2) Descaling rate. The descaling rate reaches 100%, and the heat exchanger plates are as bright as new after cleaning. After multiple cleanings of the detachable plate heat exchangers in the aluminum plant and combined with the cleaning results, it was found that disassembly and cleaning of the detachable plate heat exchangers is better, and the cleaning effect can be directly observed, and the cleaning effect can be fully observed. Supplement with other cleaning methods to reduce chemical use. However, dismantling the plate heat exchanger takes a certain amount of time, manpower and material resources.
For all-welded plate heat exchangers, wide-channel welded plate heat exchangers and other types of plate heat exchangers that cannot be disassembled, when it cannot be disassembled, due to the plate heat exchanger The internal fluid flows very quickly, so the circulation speed during cleaning should be strictly controlled below 0.5-0.6 m/s (flow speed within the plate). Prevent excessive flow rate from affecting the corrosion inhibition efficiency of the corrosion inhibitor and causing equipment corrosion. During cleaning, a stabilizer needs to be added to suspend the particulate scale in the cleaning fluid and be brought out of the heat exchanger through the cleaning fluid cycle.
The cleaning of non-detachable plate heat exchangers is more suitable for easily soluble scales, such as carbonate scales; the cleaning of detachable plate heat exchangers is more suitable for difficult-to-dissolve scales, such as materials Dirt etc