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Introduction to Heating Engineering
1. How to control the supply and return water temperature during system operation? What is the temperature difference between supply and return water in China's heating system?
The regulations adopted in China's heating design: the supply water temperature is 95°C, the return water temperature is 70°C, and the temperature difference is 25°C. But in recent years, according to the advanced experience of heating at home and abroad, the temperature and temperature difference of supply and return water have a downward trend, and the design temperature of supply and return water is 80/60 ℃, and the temperature difference is 20 ℃.
2. What types of heat exchange are there? What is the heat transfer coefficient? What is the main form of heat exchange in surface heat exchangers?
There are three forms of heat transfer: conduction, convection, and radiation. For surface heat exchangers, the main forms of heat transfer are convection and conduction. The calculation formula of convective heat transfer is: Q=αA(t2-t1). The calculation formula for heat transfer is: Q=(λ/δ)A(t2-t1). Convective heat transfer occurs on both sides of the heat transfer element in a surface heat exchanger, and heat conduction occurs in the element body.
3. What types of surface heat exchangers are there? What are its principles, advantages and disadvantages?
The main forms of surface heat exchangers are: shell and tube heat exchangers, plate heat exchangers, heat pipe heat exchangers, etc. It can be subdivided into many forms, and its common disadvantages are: large volume, large footprint, large investment, low heat exchange efficiency (compared with the hybrid type), and short life; their advantages are that the condensed water is light in pollution and easy to recycle.
4. Sometimes it is found that some users' radiators are hot but some are not. Why? How to solve?
This is called the hydraulic imbalance of the system, and the causes are more complicated, and there are roughly the following reasons:
(1) The design of the pipe diameter is unreasonable, and the diameter of some parts is too thin;
(2) The resistance of some parts is too large, such as the valve cannot be fully opened;
(3) The system is blocked by sundries
(4) The air in the system cannot be completely removed due to the unequal direction of the pipeline slope;
(5) The system loses a lot of water;
(6) The constant pressure of the system is too low, resulting in running with insufficient water;
(7) The flow rate and head of the circulating water pump are not enough;
To solve a system imbalance problem, first find out the cause and then take corresponding measures
5. What are the pros and cons of gas and water heating?
Although the gas heating system has the advantage of saving investment, it wastes too much energy. According to calculations by authoritative departments, steam heating wastes about 30% more energy than water heating. Therefore, steam heating methods are gradually being eliminated in recent years. The energy wasted by steam heating is mainly manifested in:
(1) The quality of the steam trap is not good enough, the service life is short, the performance is poor, and the steam and water are discharged together;
(2) The heat dissipation of the piping system is large. In addition to the high working temperature, thermal insulation damage and untimely maintenance are also one of the reasons;
(3) The system leaks seriously, the same leakage area, the heat that steam brings out is much larger than that of water. In addition to being uneconomical, steam heating is also unsafe, prone to burns and water hammer accidents. The operation of many systems is accompanied by vibration and water hammer, which affects people's work and rest. In addition, the steam heating room air is dry, which makes people feel uncomfortable. Although the water heating system has appropriately increased the investment, it overcomes the above-mentioned drawbacks.
6. Heating index
The heat supply index is the heat unit consumed per square meter of the building area per square meter maintained at the indoor temperature specified in the design, under the calculated temperature of the local outdoor heating.
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