What are the heat transfer process and principle application of gas fired heating furnace?

The heat energy generated by fuel combustion is transferred to the heated product, which is the function of industrial furnace. In this process, the two heat transfer processes in the industrial furnace are balanced.

In fact, the two heat transfer processes of gas fired heating furnace are balanced, one is to strengthen the heat transfer process, the other is to weaken the heat transfer process. For example, the former enhances the heat transfer of products in the furnace, and improves the unit productivity and thermal efficiency of the furnace; Strengthen the heat exchange in the heat exchanger, improve the waste heat recovery rate and air preheating temperature; Improve the cooling effect of some water-cooled parts in the furnace and prolong the service life of the equipment. For the latter, such as reducing the heat loss of furnace masonry, heat preservation measures shall be taken for the furnace to improve the heat utilization rate and save energy; Some parts in the furnace shall be prevented from overheating, and necessary insulation measures shall be taken.

The natural gas heating furnace adopts an integral assembled structure. The horizontal large volume cylinder is arranged with heating heating surfaces such as fire tube and smoke tube bundle and cooling heating surfaces such as multi pass convection tube bundle. The cylinder is filled with intermediate heat carrier as the heat transfer medium between the heating and cooling heating surfaces to help the cold and hot fluids achieve the purpose of heat transfer. The intermediate heat carrier can be water, glycol solution and heat transfer oil. Generally, the heating and cooling surfaces are symmetrically arranged along the central axis of the round section of the large cylinder. The fire tube and smoke tube bundle are located below the horizontal axis and symmetrically arranged on the left and right sides of the vertical axis; The multi pass convection tube bundle is located above the horizontal axis, and each return is symmetrically arranged on the left and right sides of the vertical axis.

Heat transfer is a science that studies the laws of heat transfer. As long as there is a temperature difference, the existence of a temperature difference between objects or between two parts of the same object is a necessary condition for heat transfer, and there is always a trend of heat transfer from high temperature to low temperature. Temperature difference generally exists in nature, so heat transfer is a very common natural phenomenon.

Heat transfer is a complex phenomenon. In order to facilitate research, the heat transfer process is divided into three basic modes according to its physical essence: conduction, convection and radiation.

(1) Conduction refers to the heat transfer phenomenon generated by the thermal movement of molecules, atoms, free electrons and other particles. When there is no relative displacement of particles, and physical direct contact exists at different temperatures or different temperatures inside the object. Heat transfer can occur in solids, liquids, and gases. In liquid and solid media, heat transfer mainly depends on the action of elastic waves, the movement of free electrons in metals, and the diffusion and collision of atoms or molecules in gases.

(2) Convection can be divided into two types: one is the heat transfer caused by the relative displacement of each part of the fluid, and the mixing of hot and cold fluids caused by the macro motion of the fluid itself; The second is the heat exchange when the fluid flows through the surface of another object. This heat exchange has a temperature difference, which is called convective heat transfer. The latter includes the former. Convective heat transfer includes the conduction process in the laminar layer near the surface and the convection process outside the laminar layer.

(3) Radiation is a process in which heat energy is transmitted by electromagnetic waves. It has an essential difference between conduction and convection. It not only has the transfer of energy, but also is accompanied by the transformation of energy form, that is, heat energy is transformed into radiant energy, and the radiation is absorbed by the object, and the radiant energy is transformed into heat energy. The transmission of radiant energy does not require the direct contact of heat transfer objects or objects. Radiation propagation in vacuum has no energy loss.

When the natural gas heater works, the industrial natural gas that needs to be heated to meet the process requirements is heated with the heat generated by fuel combustion. The fuel and air are mixed by the burner and injected into the fire tube at the lower side of the large cylinder to produce high-temperature smoke. The smoke is folded into the smoke tube bundle at the other side of the lower part of the large cylinder through the fire tube, and then discharged upward into the atmosphere through the chimney. In this process, the high temperature flue gas transfers heat to the intermediate heat carrier through the fire tube wall and smoke tube bundle wall, and the intermediate heat carrier absorbs heat and heats up; At the same time, the intermediate heat carrier transfers most of the heat to the industrial natural gas to be heated through the wall of the convection tube bundle, and the intermediate heat carrier releases heat to cool down.

In fact, in the process of heat transfer, there are few single heat transfer modes. In most cases, two or three modes occur simultaneously. For example, the burner has both convection heat transfer and radiation heat transfer in the combustion process. For example, in the process of radiating heat outward through the furnace wall, the flame in the furnace transfers heat to the furnace wall in the form of convection and radiation, the furnace wall transfers heat from the internal surface to the external surface in the form of conduction, and the external surface of the furnace wall dissipates heat outward in the form of convection and radiation. Therefore, the heat transfer process in engineering is almost a complex combination of three basic heat transfer modes. This kind of complex process is sometimes regarded as a whole, which is called comprehensive heat exchange.