Basic knowledge of hot pot stove overview of heat

2022-08-08
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Basic knowledge of boiler - overview of heat transfer knowledge (IV)

the heat transfer process caused by the different temperatures of two objects is called heat transfer

1. Mode of heat transfer

heat is always transferred from high temperature to low temperature. There are three ways of transmission: heat conduction, convection and radiation. The greater the temperature difference, the faster the heat transfer per unit time

(1) heat conduction: heat conduction is the process of transferring heat from a high-temperature object to a low-temperature object through the direct contact of the object, and all parts of the object itself do not move relatively. For example, stirring hot water with a steel spoon will gradually heat up one end of the hand. This is an example of heat conduction. Heat conduction is related to the properties of materials. Different substances have different thermal conductivity, and the same substance has different thermal conductivity at different temperatures. The so-called thermal conductivity refers to the heat passing through a flat wall with a thickness of 1m per hour over an area of 1m2 when the temperature difference is 1 ℃. Its unit is kw/m ·℃, which is indicated by the symbol“ λ” express. According to table 1.2-1, the thermal conductivity of boiler steel plate is 35.6~50.6km/m · ℃, the thermal conductivity of general scale is 1.28~3.14 km/m · ℃, and the average thermal conductivity of scale is about 90% smaller than that of steel plate. If there is scale, the accuracy of the 1-CUT experimental machine on the heating surface will reach level 0.5, which is no problem, and the heat transfer will be greatly reduced. If the scale is too thick, the metal pipe wall will be burned out or the water flow channel in the pipe will be blocked, which will affect the safe operation of the boiler. If soot is deposited on the outer wall of the tube on the heating surface of the boiler, and scale is accumulated on the inner wall of the tube, it will not only affect the normal progress of heat conduction, resulting in the waste of fuel, but also increase the wall temperature of the tube on the heating surface, or even overheat and burn out. Therefore, during the operation of the boiler, soot blowing and sewage discharge must be carried out frequently, and the water treatment must be good to keep the internal and external surfaces of the heating surface clean, so as to ensure the safe and reliable operation of the boiler

table 1.2-1 thermal conductivity of different substances

see table

(2) convective heat transfer: convective heat transfer is a heat transfer process caused by convection of various parts of liquid or gas. In the heat transfer process of the boiler, the water in the heating drum and the boiler circulate, the hot water rises and the cold water falls, so that the heat is transferred to all boiler water; Another example is that the flue gas washes the tube wall of the heating surface of the boiler at a certain velocity, and the heat transfer in the process of contacting the tube wall belongs to the process of convective heat transfer. The heat transfer process of convective heat transfer is related to the properties of fluid, the size of flow velocity, the characteristics of fluid movement, the shape of flow channel and the direction of fluid scouring the pipe. The greater the velocity of flue gas, the more intense the erosion of the pipe wall, and the stronger the convective heat transfer. If the flow velocity is too large, the flow resistance will increase. Because the resistance is proportional to the square of the flow velocity, the power consumption of the induced draft fan will increase sharply, and the pipe wall of the heating surface will be seriously worn. Therefore, when arranging convective heating surfaces, the flue gas flow rate is generally 6~10m/s under normal load. For fire tube boilers, when naturally ventilated, the flue gas flow rate is generally 10m/s; When mechanical ventilation is used, the flue gas flow rate is generally 20m/s. Convective heat transfer is closely related to the structure and arrangement of heating surfaces and the direction of flue gas scouring pipes. The effect of convective heat transfer of small-diameter tubes on the heating surface is better than that of large-diameter tubes; The flue gas scours the pipe laterally, and the convective heat transfer is relatively strong; Among them, the utilization of new materials, new technologies and new energy is very important. When the gas scours the pipe longitudinally, the convective heat transfer is relatively poor. Under the condition of sulfonic scouring, the tubes are staggered, and the convective heat transfer is relatively strong; The tubes are arranged in line, but the convective heat transfer is relatively poor

(3) radiant heat transfer: radiant heat transfer is a process of transmitting energy by means of electromagnetic waves. Part of the heat inside the object becomes radiant energy and radiates to the surrounding. When encountering another object, part of the radiant energy is absorbed and converted into heat energy. The process of radiant heat transfer is going on between the flame and the tube wall in the boiler furnace, and between the sun and the earth. The radiant energy radiated from the unit surface area of any object per unit time and avoids the equipment corrosion caused by the interruption of operation, which is called the radiant heat release intensity. It is proportional to the fourth power of the absolute temperature of the object. Therefore, the radiant heating surface should be arranged in the high-temperature flue gas area, and the water-cooled wall should be arranged in the furnace, which is why. There should not be too many water-cooled walls arranged in the furnace, otherwise the furnace temperature will be reduced too much, the advantage of high-temperature radiation will be lost, and the combustion will be affected and the incomplete combustion loss will be increased. Therefore, a proper proportion should be maintained between the radiant heating surface and the convective heating surface of the boiler. Industrial boilers usually absorb 40% - 50% of the total heat absorbed by the radiant heating surface, and the temperature of flue gas when leaving the furnace and entering the convective heating surface must be kept within the range of 850-1150 ℃

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