This type is to install a two-stage heat exchange device at the smoke outlet of a conventional wall-hung boiler, which is also called "false condensation". In fact, it is also a condensation method, which was developed and used in the late 198s and early 199s, and has been basically eliminated in Europe. Although it does have condensation effect, it is quite different from the fully premixed condensation technology on the market at present.
Advantages:
Low cost and quick enterprise transformation.
Disadvantages:
The content of acidic substances and nitrogen oxides is high, and the corrosion resistance is poor. Mainly because it has not been specially treated in the combustion of the primary main heat exchanger, too much excess air will greatly promote the generation of acidic solution and seriously corrode the parts. Therefore, even if this kind of condensing boiler uses stainless steel to increase anti-corrosion, its low cost also determines that its anti-corrosion ability is very limited, and the life of the secondary condensing heat exchanger is short. Its incomplete premixed air-gas ratio also makes some heat loss.
2. Cast aluminum coil fin type (Lihe Haide heat exchanger, professional equipment selection)
Introduction:
This kind of movement is generally designed and manufactured by boiler manufacturers themselves, with vertical combustion design, cooling in the upper half and condensation in the lower half.
Advantages:
The pipeline of this kind of condensation device has sufficient coarse flow, low requirements for water quality, and it is not easy to scale in the pipeline, so it is not mandatory to use scale inhibitor.
Disadvantages:
A. Due to the design structure, it has a large volume, but the power shortage is not big, and generally it can only be 28KW.
b, it is easy to get stuck and blocked between fins, so it needs to be cleaned regularly with a wire brush. However, due to the use of cast aluminum, you should be very careful when cleaning, so as not to destroy the anticorrosive coating. Once the anticorrosive coating is scratched, it is easy to form corrosion, burning and cracking. Existing domestic manufacturers are developing stainless steel models, but a large number of commercial finished products have not yet been listed.
c, the bottom of the pipeline is immersed in acid condensate, and the pipeline at the liquid level is often the hardest hit area for corrosion, and the serious corrosion burn-through is mainly concentrated here.
3. Form of cast aluminum jacket with ribbed fins (needles)
Introduction:
There is nothing wrong with the finished product itself, mainly because the domestic water quality is too poor, the hardness is high, and there are many particles and impurities, resulting in a large amount of scale accumulation in the jacket, which is difficult to clean. Also, due to the poor quality of domestic gas (high sulfide content) and air (PM off the charts), the ribbed fins (fin pins) in the flue gas heat exchange cavity are easy to gather and adhere to a large number of ash plugging and acidic substances. The biggest problem is that its staggered and criss-crossed ribbed fins (fin pins) design can hardly be cleaned. A manufacturer once designed a special-shaped (serrated) wire brush, but the cleaning effect was very limited, ranging from one year to two years, and the heat exchange efficiency was greatly reduced, so it could only be scrapped.
white matter is a mixture of ash plugging, acidic substances and carbon deposits. Also due to the hardening of impurities, uneven heating and stress changes are caused, and some ribs have been broken.
Introduction of 4, 36 stainless steel main heat exchanger:
Analysis:
This kind of main heat exchanger needs to start with its production process. In the factory, the commercially available 36 stainless steel round tube with a wall thickness of .8mm is coiled into a barrel type by a coiling machine, and the round tube is pressed into a flat round tube by pressing at both ends. Because the long circular tube cannot be completely uniform from beginning to end, it is inevitable that some circular tubes will be pressed too flat and some circular tubes will not be flat enough during the pressing process, resulting in uneven inner diameter, so the method of subsection (unit) pressing is adopted. In this way, the rejection rate is reduced and the yield is improved, and each unit can be designed with a fixed power (8-1KW/ segment). In this way, the number can be increased or decreased at any time according to the design of the wall-hung boiler, and the power number can be adjusted.
However, this has created a new problem. Butt welding cannot be carried out between the two units. On the one hand, there is no welding operation space. On the other hand, the butt welding seam is annular. In the whole main heat exchanger, half of the annular welding seam is at 9 degrees inside the barrel and the other half is at 2 degrees outside the barrel, and the expansion coefficient is the same. At present, it is impossible to find a suitable welding material. Since butt welding in series is not feasible, the manufacturer adopts the parallel connection method, and several units are connected in parallel on a distribution manifold, so that four paths of primary water are heated in the main heat exchanger, and the problem is solved. Just because of adopting the form of multi-pipes, the pipe diameter of each unit must be greatly reduced, and the pipe diameter becomes thinner, which is about 3% of the normal pipe section. In the case of water quality and good after-sales service in Europe, the use is relatively normal.
However, due to domestic water quality problems, such a thin pipe diameter can easily cause scaling or even blockage in the pipe. And because of its parallel connection (for example, if the interfaces are connected in series, there are too many intermediate interfaces, the inner surface is not smooth, and it is easier to scale, and problems such as expansion and water leakage need to be considered), it is particularly easy to cause scaling in one of the units, and then the more scaling this unit is, the more scaling it is, and all impurities will be blocked in this unit after many cycles. And because the other three units can still run normally after the unit is blocked, the scaling phenomenon of all units is very difficult to be found, and the wall-hung boiler will not give an alarm. Users won't know about the unit until it leaks due to long-term dry burning, but it's too late to repair and can only be replaced as a whole.
in order to solve this problem, many manufacturers adopt the method of adding scale inhibitor to the system to delay the generation of scale in the unit. However, the general aging time of scale inhibitor is at most 2 years, which will be shortened according to the water quality. Therefore, it is necessary to test the scale inhibitor concentration as often as possible and add it in time, which is troublesome.
the above picture shows the sectional view of the unit pipeline after being sawed, and you can see the situation after the round pipe is squashed. The thickness of the pipe wall is .8mm, and the gap between pipes is .6mm (according to the requirements of low-temperature corrosion resistance, the thickness of the pipe wall should be at least above .8mm, and the gap between pipes should be at least above .6mm). The wall thickness and gap between tubes of the main heat exchanger have just reached the basic requirements, but far from the good design standards. This is because of the mutual restraint and change among pressurization pressure, constant pressure time, wall thickness, material resilience, gap width and cost, so manufacturers must choose a parameter point that can be balanced in all aspects, and they cannot achieve the best standard in all aspects.
The diameter of the soot and fly ash produced after the combustion of the wall-hung boiler is generally less than 5 microns (.5mm). However, due to the adsorption of acid, many fly ash (the initial fly ash is fluffy and easy to adsorb acidic substances) will stick together, forming large fly ash particles. If the gap between pipes is too small, it is easy to cause ash blockage.
In Europe, the air and gas quality is good, and the fly ash particles are small, so it can be used normally. However, in the case of PM off the charts in China, the ash blockage is very serious. After the high temperature flue gas passes through the gap between pipes, the temperature drops to 5 degrees, which will produce a lot of ash plugging, condensed water and acidic liquid. Because the outer wall of the pipe is circular (non-flattened part), the mixture of ash plugging, condensed water and acidic liquid can't accumulate in this part, so it can only flow back to the gap along the arc surface, and the phenomenon of "topping up" with the later flue gas occurs in the gap, and finally a large amount of mixture of ash plugging and acidic liquid is accumulated at the bell mouth between the two pipes, resulting in poor smoke exhaust and corrosion of the external pipe wall by acidic liquid.
in order to solve this problem, at present, manufacturers mainly adopt the method of cleaning the gaps between pipes with wire brushes every year. However, the design is integrated, and the wire brush cleaning can only solve the cleaning work of the inner wall of the drum, but the outer surface of the drum (between the drum and the shell) still cannot be cleaned.
standard spacing
5. Introduction of 361Ti stainless steel:
Analysis:
The main heat exchanger is made of 316Ti stainless steel, which is added with Ti (titanium element) on the basis of 316L stainless steel, and its scientific name is Mo-Ti-Ti stainless steel.
due to its strong corrosion resistance (enhanced surface corrosion resistance), 316L stainless steel is widely used in military submarines, underwater robots, and other civilian luxuries such as Rolex diving watches and Apple watches. The corrosion resistance of Mo-Ti-Ti stainless steel (316ti) is about 1 times that of 316L stainless steel, especially its super intergranular corrosion resistance. It is an excellent material for making the main heat exchanger of condensing boilers, and is generally only used in the manufacture of large high-end industrial boilers working in special occasions and harsh environments, such as special boilers used in large reservoirs and nuclear power plants.
the design is single tube, with large diameter and sufficient flow, which is not easy to scale. Even if scaling occurs on the inner wall, it is not easy to block. There will be no blocking and failure reporting like parallel connection. If the flow rate decreases obviously, the boiler will also get a cleaning signal due to the over-temperature alarm, which ensures that the fouling and blockage can be cleaned in time, and it is a design that can be maintained. Under the current domestic water quality situation, there is no need to add scale inhibitors.
the manufacturer cuts the 316Ti stainless steel plate, cuts it into long strips, folds it, makes it into a square tube, and puts it on a circular disc machine, which turns it into a spiral barrel. The thickness of the square tube wall is 1.2mm, and the gap between tubes is .8mm, reaching the excellent standard. Thick pipe wall, good corrosion resistance and long service life. The gap width is reasonable, the heat exchange efficiency is high, the flue gas is smooth, and the ash is not blocked and the particles are not stuck.
the flat pipe jacking surface (non-circular) is easy to introduce garbage and impurities, and the phenomenon of "countercurrent" and "pushing cattle" will not occur. The two black indentations on the flat top are the patented diversion groove, which is convenient for the mixture of condensed water, fly ash and acidic liquid to flow into the sump along the diversion groove and be discharged into the sewer. Avoid impurities from flowing back to the gap, and also reduce the residence time of acidic solution on the stainless steel surface.
under the current domestic environmental conditions, internal flushing is not mandatory every year, but it is still recommended to do maintenance every 2-3 years.