At present, preliminary discussions on several key technical issues encountered in the rapid development of floor heating are proposed, and some methods and suggestions for solving the problems are proposed. These problems include: calorific value calculation on the floor surface, room heat load problem, optimal thickness of the filling layer, heat source sharing problem with the radiator, system of floor decoration layer matching with floor heating, and pressure-maintenance ground layer Structural problems. The paper also carried out a preliminary analysis of the energy-saving problem of floor heating.
I. Introduction
Low-temperature hot water floor radiant heating is an excellent way to heat the room and has many users around the world. As of 1994, about 85% of residential buildings in Korea have installed low-temperature hot water floor radiant heating systems, which is 65% in Canada, 48% in Switzerland, 41% in Germany and 20% in France. Low-temperature hot water floor radiant heating is an improvement measure for people's living quality. It is difficult to sell floor heating for houses without low-temperature hot water floor radiant heating. In the north of China, it has also shown rapid development.
The superiority of floor heating has been extensively discussed in the literature. In terms of research, a lot of work has been done at home and abroad. There are also some local standards and enterprise standards for design, and national standards are being revised. However, in the large number of engineering practices that I have personally engaged in, I feel that the current domestic research on this type of heating is still not deep enough, not systematic enough, and some major technical links are theoretically unclear, and there is no reliable basis in practice. Reflected in some of the current standards, people feel that they are still far from mature as radiator heating. At present, many of the existing local standards and corporate standards are copied from each other, traced back to the source, and there is not enough research basis as a basis. In this article, I will put forward these issues and put forward preliminary views and discuss with the peers.
Second, the surface of the heat problem
The heat generated on the floor surface is one of the most important parameters in the design. A lot of research has been carried out on this issue at home and abroad.
However, these studies still appear to be insufficient for heating heating networks. In some studies, the numerical simulation of the heat transfer process, the convective heat transfer coefficient and the equivalent radiative heat transfer coefficient in the boundary conditions of the anti-floor surface are taken as a fixed value, but in fact they are a function of the surface temperature of the floor and the room temperature. In addition, many studies only perform numerical calculations or experiments on a certain structure, but in fact, various floor structures have different heat transfer performance.
At present, the calculation table of floor calorific value in many local and enterprise standards is mostly from the literature [1]. According to the author, these forms are originated from abroad, and no one in the country checks and corrects the correctness and practicability of these forms. . According to the finite element analysis and calculation of the author, it is considered that the following problems exist in these tables: 1) The thickness of the filling layer in the actual project is controversial, and the value of the person varies within a range, and the cross-linked polyethylene pipe with different pipe diameters can be Select, but the heat is all checked with a table, obviously not accurate; 2) The data in the table shows that the heat is reduced with the increase of the pipe spacing, the trend is correct, but the reduction is not enough, this may Because the filling layer is too thick, the data is too conservative; 3) Only the table data about the amount of ground heat dissipation in the standard, there is no calculation formula, the designer will be at a loss when the local board structure size does not match the data specified in the table.
The calorific value q of the floor surface is related to the pipe diameter d, the cover layer (water content filling, leveling and decoration layer) material and thickness, water temperature ts, room temperature tf and pipe spacing s, etc.
Where: δi - the thickness of each layer in the ground structure, λi - the thermal conductivity of each layer in the ground structure.
The author has started from the above formula, and has carried out a lot of finite element analysis and calculation on the heat conduction, convection and radiation problems in the heating process of the floor. Various combinations of many variables in the above formula are calculated and a lot of new information about the floor is obtained. The calculation table of the floor heat dissipation amount of the board and the simple experimental formulas summarizing the equivalent heat conduction thermal resistance are described in another article.
Third, the floor heating
In general, the design heating load for floor heating should be based on the room heat load, which is calculated based on the heat loss from the enclosure. However, there are still some problems worthy of study and study in the design practice.
1 furniture coverage problem
The surface of the floor covered by the furniture (such as solid bed, solid cabinet, etc.) is nearly infinite, and the area is basically regarded as no heat dissipation.
Where: q'--room heat load,
q--The design of the floor is heating the load.
The problem is that when the residential floor heating design is carried out, it is impossible to predict the coverage rate of each household, and where the coverage is, when the unpredictable furniture coverage has a large difference, it will inevitably cause room temperature inconsistency.
One advantage of floor heating is that it can be easily adjusted for single-family or even single-chamber, while central adjustment is also possible. If the single-family metering is used to charge the heat, then we can design a large heating capacity for the floor. The user can adjust it during the actual operation, but if it is still charged by area, the problem will be great. If the water temperature is strictly controlled centrally, the user who takes care of the large coverage rate can ensure that it can reach 18 °C, which will give the users with small coverage a convenient condition for using the standard. If the situation is small, the coverage rate is small. Control the water supply temperature, the room with large coverage will not reach the specified indoor design temperature. This issue, the author believes that the following measures should be taken. 1) It should be vigorously promoted to charge single households according to heat. When using floor heating, the balance condition of the water system is much better than that of the radiator system. 2) When it is not possible to measure single-family, the coverage of the floor area should be limited in the instructions for the use of floor heating to the user.
2 possible room overheating or super standard heat problem
Unlike radiator heating, the larger floor heating area has a huge heating capacity for the room. According to the calculation of convection and radiation, when the heating unit area heat index is 60w/m2, if the ground is marble, the pipe spacing is s=300mm, the pipe diameter is d=16mm, and the filling layer thickness is δ3=50mm, the supply level is 40°C. The indoor temperature is 18 °C, and the heat index per unit area can reach 84w/m2, which exceeds the heating index per unit area. And in fact, the vast majority of heating sources, there is no difficulty in supplying water with higher water temperatures. Moreover, at the time of the promotion of floor heating, few manufacturers have arranged the pipe spacing to be 300 mm, mostly 200 mm. In this way, the heating capacity of the floor is far greater than the heat load of the room. The water temperature is higher than the design value, and the room is overheated or over-standard heat is often used. In order to solve this problem, the author suggests: 1) try to implement single-family measurement; 2) strengthen the central control link of water temperature during operation to avoid super-standard heat.
Four filling layers and the thickness of the screed
Since the national standards of China have not yet been issued, the local standards for the local standards are not uniform due to the different foreign standards. The literature [1] stipulates that the thickness of the filling layer above the heating pipe should not be less than 30 mm, and the model structure selected in the attached table is the thickness of the filling layer of 60 mm, while the thickness of the filling layer should be 30-40 mm in the literature [2]. From the function of the filling layer, its main purpose is to protect the heating coil, make the surface temperature of the floor uniform, increase the thermal stability, etc. Its thickness is not only related to the heat dissipation of the floor, but also directly affects the height of the building and the design. Load and initial investment. Its value should depend on the selected pipe, pipe diameter and economic analysis, technical analysis and optimization of the problem.
5. Sharing the heat source problem with the radiator heating water system
At present, in the promotion and application of floor heating technology, this is the most frequently encountered problem, because floor heating is an emerging heating method in China, and it is often surrounded by radiator heating. Generally, the radiator water supply temperature needs to be above 80 °C in the coldest day, and the water system resistance loss is small, while the floor heating requires the water supply temperature to be lower than 60 ° C. In some cases, 30 to 40 ° C is enough, and the end resistance can be as high as 3 Rice water column around. Therefore, it is problematic to simply share a single water system with two heating methods.
In practice, however, the requirements for a shared water system are often raised. On the one hand, the housing developers hope to set up one or several buildings to install floor heating in the building where the radiators are heated. What's more, some developers require a certain floor in a building that is heated by a radiator. Even several rooms are equipped with floor heating. This situation is sometimes caused by the reconstruction and expansion of buildings, and sometimes the developer has a process of understanding new things, and it is necessary to see the effects in person to promote it. On the other hand, it is seldom able to arrange low-temperature heat sources separately for floor heating, but only high-temperature (relatively) water heat sources for radiator heating, such as urban heat networks, regional boiler houses, etc. Faced with this situation, as an engineering and technical personnel, simply saying "no" is not conducive to the promotion of new technologies.
A series of reliable methods should be developed to solve the problem of shared heat sources in a variety of situations. The alternative methods of using a single heat source for multi-parameter heating are: 1) the use of heat exchangers, which is the most mature and stable method, but with higher investment; 2) the use of ejector pumps for mixing water, which is in the former Soviet literature. The method introduced requires a large capital indenter, but when the floor heating is connected in parallel with the radiator heating system, this condition is often difficult to meet; 3) using an additional pump to mix water to obtain the required water temperature and water volume; 4) The tandem method, that is, the method of returning water from the radiator system for floor heating and water supply. This is a very attractive idea. Generally, the pressure head of the circulating water pump of the radiator water system is much larger than the circulation resistance. A bypass pipe is added to the return water main pipe to guide the return water to the floor plate through the bypass. The pipe is used for floor heating. The temperature and temperature drop range of the water during the peak and off-peak periods of the heating process saves the power consumption of the pump. This method requires adding a water treatment system at the junction of the two systems to ensure that the water quality of the whole system is free of pollution. 5) Parallel method, when the local surface decoration material allows the water temperature to be higher than 60%, the radiator heating water system can be considered at this time. Simple parallel. Of course, the floor coil should be selected to be larger, and the one-way pipe length should be shorter, so that the water flow resistance of one person's loop is basically the same as that of the radiator heating, and the hydraulic balance is achieved. However, the end resistance is still greater than the radiator. The floor heating system will still be operated with high water temperature, low water volume and large temperature drop. Compared with the normal design parameters, the water supply temperature is much higher, the water volume is much smaller, but the heating function can be basically achieved. Of course, the life of plastic pipes will be affected to some extent. And the water quality requirements for the system are equally high.
Among the above methods, except for the second one, the author has tried to use it in design practice and has already implemented it in engineering. In theory, it should be feasible. In terms of actual effects, some will be verified in this year's heating practice. However, the author believes that floor heating is a new technology, and the above methods are not standardized and mature both theoretically and practically. It is necessary to organize development systems and detailed research.
Development of six pressure-bearing thermal insulation ground structure layer
The floor heating method is especially suitable for large-width rooms such as exhibition halls and workshops. This type of room is usually located on the ground floor and the lower part of the floor coil must be insulated. However, the floor panel including the heat insulating material under such a question must have a large pressure bearing capacity and can withstand the pressure of automobiles and equipment. Such insulation materials have mature products abroad, and domestic production systems for heating and heating networks have not yet been developed or introduced. This requires the experimental development of high-strength inorganic non-metallic cementitious materials, so that its technical performance reaches a small dry shrinkage, no cracking, and has good requirements of compactness and thermal conductivity.
VII. Design and product development of floor decoration layer supporting floor heating
Floor decoration is an important issue that has received great attention from residents.
Whether it is reliable or not, the residents' satisfaction with the floor decoration scheme and products that come with the floor heating will be the key to the large-scale promotion of floor heating technology. It goes without saying that floor heating has certain requirements for floor decoration, that is, it can withstand certain temperature and humidity conditions without deformation for a long time. At present, there are wooden flooring products specially used for floor heating in foreign countries. There are still no domestic fixed-type schemes for floor decoration that are compatible with floor heating, so that residents can choose safely and conveniently. This requires experimental research on deformation and heat aging of floor decoration materials (wood and stone) under dry, wet, rapid heat and cold circulation conditions, and development of ground decoration and matching schemes compatible with floor heating.
Eight energy-saving issues related to floor heating
In the literature, it is said that floor heating can save 30% energy compared to radiator heating. The author believes that this is not accurate. Under the determined indoor and outdoor temperature conditions, the heating energy consumption is mainly determined by the insulation performance of the enclosure structure. The heat transferred to the outside through the enclosure structure, no matter which room heating method is adopted, this part of the energy consumption must be A few additions.
Therefore, it can be said that the heating method of the room has little to do with the heating energy consumption. However, after careful analysis, floor heating can save a small amount of energy compared with radiator heating. The reasons are as follows: 1) The literature [3] points out that when the indoor wind speed is less than 0.05m/s, the average radiation temperature changes by 1 °C and air temperature. The change of 1 °C has the same effect on the human body's thermal sensation. The average radiant temperature generated by floor heating is higher [4], and the indoor temperature is uniform in the height direction, the temperature gradient is small, and the person is in the heating zone, and the radiator The highest temperature zone is above the room, and the temperature gradient is large. When the human feels the same temperature, the average indoor air temperature of the floor radiant heating room is lower than the air temperature of the convection heating room, reducing the room heat load. 2) Floor radiant heating uses low temperature hot water, while lower temperature hot water has less heat loss during transmission than heat sink. 3) Low-level energy sources such as waste heat and solar energy can be utilized to save high energy. 4) The radiator is placed under the window against the wall, and a small amount of heat is short-circuited to the outside, and the floor heating does not have this drawback. 5) When ventilation is performed in winter, energy is saved because the temperature difference between indoor and outdoor is smaller than that of the convective system.
However, it should be noted that the heating capacity of the floor is usually greater than the heat load of the room. If the room temperature is not strictly controlled, some users may use extra heat to generate extra energy.
Whether floor heating is more energy efficient than radiator heating, it is necessary to carefully analyze the above items item by item. This will involve many aspects of the room's thermal process, such as air temperature distribution, wall heat conduction, heat sink cooling process, etc., and research in this area is not perfect.
The above problems are the actual problems that the author encountered in the practice of floor heating technology promotion, and they have not really solved it. The author hopes that this article will lead to further discussion and research by the majority of peers, and promote the healthy development of floor heating technology promotion.
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