Because the low-temperature floor radiant heating project is a concealed project, and the system passes 60 degrees Celsius hot water, it means that when the heating pipe bursts due to quality problems, the ground of the building will be destroyed and the loss will be huge. Therefore, the heating pipe used for the low-temperature floor radiant heating system is required to be more rigid than the plastic pipe for ordinary building: the system has to strictly select the type, quality, manufacturing process and quality of the heating pipe. Only 50 years of life in a low-temperature floor radiant heating system can be achieved.
At present, the pipes used for low-temperature floor radiant heating in the market mainly include PE-X pipe (cross-linked polyethylene pipe), PP-R pipe (three-type random copolymer polypropylene pipe), and PERT pipe (high-temperature resistant polyethylene pipe). , PB pipe (polybutylene pipe), aluminum-plastic pipe, etc., these pipes are used in low-temperature floor radiant heating systems, and each has its own characteristics. Other plastic pipes such as CPVC, PPH, ABS, PE, etc. cannot be used because the performance cannot meet the requirements of the low-temperature floor radiant heating system. Although the aluminum-plastic composite pipe is also included in the usable pipe in the "Technical Regulations for Low Temperature Hot Water Floor Radiant Heating Applications", the anti-aging property of the composite agent-hot melt adhesive in the aluminum-plastic composite pipe is an unknown number, and it is impossible to determine Service life, therefore, it is best not to use.
The performance index of the heated pipe is a complete indicator system that guarantees a 50-year life expectancy. Some companies only do some short-term physical testing of their products, and they claim to be able to use them for 50 years. This is unscientific. It is meaningless for some companies to say how high the temperature of their products can withstand without proving long life. It is scientific and meaningful to comprehensively evaluate the indicators of pressure, temperature and time of use of pipes. In fact, plastic pipes have different working temperatures under the same pressure, and their lifespans are different; at the same temperature, the pressure is different and the life is different. However, a 50-year lifespan cannot be verified by actual experimental methods. Therefore, the verification method of “time-temperature equivalent transformation method†is proposed in the international ISO9080 standard to prove this. The method is to estimate the actual life of the pipe under low temperature use conditions by increasing the experimental temperature and experimental pressure to obtain the life. The extrapolation factor Ke is determined by the temperature difference ⊿T between the test temperature and the actual use temperature. The product of the maximum experimental time Tmax and Ke obtained by the experiment is the longest life that can be achieved by the extrapolation calculation.
According to the principle of performance evaluation of plastic pipes, China Chemical Building Materials Testing Center and some large domestic plastic pipe manufacturers have started 8760 tests on plastic pipes since 2001. This test is an important method for comprehensive evaluation of the service life of plastic pipes. As the heating pipe for low-temperature floor radiant heating system, it should be the first to pass the test.
This test requires the experimental temperature to be chosen to be 110 degrees Celsius. According to the time-temperature equivalent conversion method, the experimental time is set to 8760 hours, because 8760 hours is one year, when the use temperature is 70 degrees Celsius, the temperature difference is 40 degrees Celsius, extrapolation. The factor is 50, which makes it possible to calculate the service life at 70 degrees Celsius: 50 x 8760 = 438000 (hours), which is exactly 50 years.
Related data shows that PE-X tubes have the best heat resistance. In this sense, it is scientific to select a PE-X tube as a heating tube in a low temperature floor heating system. At the same time, PE-X pipes are also relatively inexpensive among the above-mentioned pipes, and therefore, it is also economical to select a PE-X pipe as a heating pipe in a low-temperature floor heating system.
We can also see this from the creep properties of several pipes. The creep characteristic curve is a comprehensive reflection of the long-term stress resistance and high temperature resistance of the pipe, which most realistically reflects the service life of the pipe. From the creep characteristics of the four pipes, it can be concluded that the PE-X tube has no inflection point (ie, a sharp drop in the ring stress) from high temperature to low temperature, from 1 year to 100 years. As time increases, the ring stress of PE-X tubes at the same temperature drops very little (compared to several other tubes); at different temperatures, the degree of ring stress drop is much smaller than other tubes. It can be concluded that the PE-X tube can be well adapted to long-term use under high temperature and high pressure. When several other pipes are above 60 degrees Celsius, the inflection point of the ring stress drops sharply in different time periods (the longest is only ten years, the shortest is less than one year), and the inflection point occurs with the increase of temperature. The time is greatly advanced. PERT has an inflection point in about 14 years under high temperature conditions; although PB has a high stress level, it has an inflection point from high temperature to low temperature, such as 2000 hours (less than three months) at 90 degrees Celsius. Inflection point; the ppr tube also had an inflection point earlier than 70 degrees Celsius. The appearance of the inflection point and the morning and evening are an important reflection of the anti-creep performance of the pipe, and also an important reflection of the life of the pipe. Why is this happening? We know that polymer plastics are divided into thermoplastics and thermosets. In theory, a thermoplastic is a melt that has no fixed melting point and can be melted after molding, and its creep period is short. The thermosetting plastic is a crystal, does not melt after molding, and has a very long creep period, and the performance is almost unchanged in 100 years. Among the commonly used plastic pipes, PP-R, PERT, and PB are thermoplastics, and PE-X (cross-linked polyethylene) is a thermosetting plastic.
PE-X (cross-linked polyethylene) is a high-density polyethylene (PE) added to a crosslinking agent or under radiation, under certain conditions, macromolecular chain formation free radicals are interconnected to form a planar or three-dimensional network structure. Forming. There are three methods, one is silane cross-linking method; the other is peroxide cross-linking method; the third is radiation cross-linking method.
The analysis of the three cross-linking methods is as follows: 1. The silane cross-linking method is to add a silane agent to the polyethylene, and the three-dimensional network cross-linking process is completed in the hydrolysis process after molding, and the cross-linking method requires lower conditions. As long as there is water and a certain temperature, it can be completed, and even in use, cross-linking can be continued until saturated cross-linking is reached, and the formed PE-X is a three-dimensional network structure. The silane cross-linking is divided into one-step and two-step processes from the process. The storage and packaging requirements of the raw materials are high. Otherwise, the raw materials may be pre-crosslinked and scrapped during the storage period, and the storage period of the raw materials is short ( 6 months). The silane cross-linking process is a process of first forming and then cross-linking. Its product features high surface strength and tensile strength, but poor water resistance, product cross-linking will swim with time. The product formed by this method is called PE-Xb. Second, the peroxide cross-linking method is to add a peroxide cross-linking agent to the polyethylene, and gradually complete the planar network cross-linking process at a certain temperature (230 degrees Celsius). The cross-linking conditions required for this cross-linking method are high, and the degree of cross-linking does not change once formed. It is a process of forming after cross-linking. Its product features good surface finish, good flexibility, but poor acid resistance. The product formed by this method is called PE-Xa. Third, the irradiation cross-linking method is a physical cross-linking method. Compared with the former, it is not necessary to participate in the cross-linking process. It is to irradiate the tube product molded from high-density polyethylene by αβγ-ray or electron beam. Thereafter, a new radical is formed in the main chain of the polyethylene macromolecule, and recombination between the radicals forms a crosslink. The cross-linking method requires extremely high conditions, and the cross-linking is very stable. There is a characteristic that the product has a high degree of cross-linking, and as the depth increases, the degree of cross-linking becomes lower and lower. The method is not suitable for the production of products with excessive wall thickness, and the product formed is called PE-Xc.
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