Analysis of key technologies in the development of

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Analysis of key technologies in the development of plateau heater 1. Introduction the working principle of plateau heater is that fuel oil is sprayed into the combustion chamber through pressure atomization for combustion, and the high-temperature flue gas produced is heated by the wall heat exchanger, and the outdoor cold air sent into the heat exchange channel by the blower is sent into the indoor (tent) for heating

this heating method provides high air quality, no open fire, good mobility of the unit and high degree of operation automation. It is suitable for military command organs, Yezhan hospital, confidential communication, Yezhan repair and other departments for camping heating in winter. It is also suitable for full speed in cold seasons and field operations in railway, oil and natural gas departments

the General Logistics Construction Engineering Research Institute first developed plain 40kW and 20kW air heaters in China in 1997, and has achieved many successful experiences. With the expansion of application scope and region, the demand for plateau heater is increasing. In order to meet this demand, we began to develop plateau heater in 2000

plateau heater is required to work safely and reliably in an area with an altitude of 5000 meters. At such an altitude, its atmospheric pressure is only 1/2 of that of the plain, which brings a series of problems to the design of the heater. The key technologies are the correct design of furnace and heat exchange area, the improvement of burner and combustion system, the determination of system resistance and the selection of forced draft fan. We have made full use of the successful experience of plain heater and solved the above key technologies by combining experiment and theory. In 2001, we successfully studied 40kW plateau heater and put it into small batch production, which has been well received by users. This article is a brief description of some design principles and key technical solutions we consider in the development process

second, the correct design of furnace and heat exchange area

compared with plain heater, the furnace volume and heat exchange area of plateau heater must be increased. This is because the oxygen content in the air per unit volume on the plateau has greatly decreased, and the contact conditions between oil mist and oxygen in the furnace have become worse. In order to ensure the complete combustion of oil, the flue gas needs to stay in the furnace for a longer time, which requires a larger furnace volume

an important index of combustion chamber volume design is the volumetric heat load QV. Too high or too low QV will increase incomplete combustion loss, affect the economy of the system, and even prone to accidents, making the furnace unable to work. For example, too small QV will make the smoke temperature in the furnace too low, and the flame may be extinguished and the combustion may be unstable. Excessive QV makes the furnace wall temperature too high (the cooling working medium is air), so that it exceeds the allowable temperature of the metal. At the same time, the furnace outlet temperature is too high, resulting in the improper proportion of radiation heat exchange surface and convection heat exchange, and the total metal consumption of the body increases. The correct value of QV should be determined according to long-term operation experience. There should be no obvious eccentricity between the head and counterbore of the plateau heater. During the initial design, there is no operating experience to learn from. We can only make a theoretical analysis and determination on the basis of the plain heater in combination with the characteristics of the plateau. From the above analysis, it can be seen that the plastic film blowing machine industry of the plateau heater is keeping up with the pace of the times, and the QV should be smaller than that of the plain heater

the design of the shape of the combustion chamber must match the performance of the burner. The diameter of the furnace should be greater than the maximum diameter of the flame to avoid wall burning. The length of the furnace should be greater than the total length of the flame

the heat exchange area of the combustion chamber should be designed so that the flue gas temperature at the furnace outlet is controlled at about 1000 ℃

according to the above design principles, combined with the calculation, it is finally determined that the heat load of the 40kW plateau heater is 64% of that of the plain, and the heat exchange area is 1.3 times of that of the plain

the design principle of the heat exchange surface behind the furnace outlet is: from the perspective of heat transfer, the size of the total heat exchange surface should enable the exhaust gas temperature to be about 200 ℃, so that the thermal efficiency of the unit can reach about 88%. In addition, the flow distribution of each parallel air channel should roughly match their heat exchange capacity, so as to reduce the loss caused by the non isothermal air flow mixing at the air outlet of each channel. The heat exchange coefficient on both sides of each heat exchange surface (smoke side and air side) should be as far as possible, but the durability is almost close to that of the ordinary experimental machine, so as to maximize the utilization of the heat exchange surface. Starting from the flow resistance, as the volume flow of flue gas and air on the plateau increases greatly, the resistance loss also increases significantly. Excessive resistance loss not only makes it difficult to select the type of burner and forced draft fan, but also is detrimental to the total cost of the unit (initial investment + regular operation cost). From this point of view, it is necessary to increase the flow area on the smoke side and the wind side

for plateau heaters, the complete combustion of fuel oil often requires a larger air multiplication coefficient, which leads to the decrease of theoretical combustion temperature, the decrease of radiant heat transfer in the furnace and the temperature of furnace wall, which must be compensated by increasing the convective heat exchange surface

the important factor affecting the intensity of convective heat transfer is Reynolds number Re. During plateau operation, if the mass flow rate and the flow area on the smoke and wind sides are unchanged, the re number is unchanged, so that the convective heat transfer coefficient is not affected by atmospheric pressure. However, as mentioned above, in order to reduce the resistance loss of the system, the flow area of the working medium must be increased and the mass flow rate must be reduced, so that the re number and the convective heat transfer coefficient are reduced. This also has high expansibility, which must be compensated by increasing the heat transfer area. In conclusion, considering the above factors and through calculation, the total heat exchange capacity of the plateau heater finally determined is about 1.4 times that of the plain

in order to fundamentally solve the design and calculation problems of this kind of heat exchanger, we have compiled a thermodynamic software, which is verified by the measured results and agrees well. The compilation of thermodynamic calculation software solves the design and calculation problems of this type of series, and the detailed results obtained from the calculation provide a theoretical basis for the further optimization of the structure of the heater

III. improvement of burner and combustion system

burner is the key component of heater, which will directly affect the safety and economy of furnace work

since the volume flow of combustion supporting air required by the unit fuel pipe of plateau burners and the flue gas resistance to be overcome are doubled, such matching types of fuel volume, air volume and furnace internal pressure cannot be found in the conventional burner manual. To this end, we adopted the practice of "big horse pulling small car". For example, the fuel volume of the selected burner ranges from 4kg/h to 10kg/h. In actual operation, its 4kg/h fuel injection volume and combustion supporting air volume close to 8kg/h are used. But even so, during the plateau test, there are still ignition difficulties and combustion instability under high oil pressure. The analysis shows that the main reason is that the excessive volume flow of combustion supporting air causes the excessive wind speed near the nozzle, so that the fire is blown out. The combustion supporting air of the burner is divided into three parts: root air, swirling air formed by the air regulator and peripheral secondary air. It is envisaged to reduce the wind speed of the root wind by increasing the flow area of the secondary air in the outer ring. For this reason, the shape of multiple types of burner and flame barrel is selected, and the flow area of the secondary air in the outer ring is changed by adjusting the position of the air regulator in the flame barrel. Then, the experiments of small oil quantity and large air volume were carried out in the atmospheric environment of plain and plateau to explore the improvement measures of the burner, and the electric heater was used to preheat the combustion supporting air, so as to solve the problems of ignition difficulty and combustion instability during plateau combustion

fourth, the correct calculation of the resistance of the air supply system and the selection method of the air supply fan

the performance of the air supply fan directly affects the heat supply of the unit and the comfort of the air supply temperature. The volume and weight of the forced draft fan affect the overall size and weight of the unit

the premise of correct selection of forced draft fan is the correct calculation of system resistance. Compared with the plain, the pound air volume and resistance on the plateau are doubled, so the correct calculation method of system air resistance is particularly important

the air resistance of the system includes three parts: body, air supply pipe and air return pipe. Due to the complexity of the body structure and the particularity of the structure of the air supply pipe, its equivalent resistance system is nowhere to be checked. In addition, the air inlet of the system is jointly borne by the return air pipe and the shutter. Therefore, when calculating the resistance of the return air pipe, it is also necessary to know the flow in the return air pipe. Therefore, we have carried out a series of resistance tests on the prototype in the cold state, and sorted the test results into the form of the impedance of the cost body and the equivalent resistance coefficient of the air supply pipe, so as to obtain a universal resistance calculation method. This calculation method can be used to predict the wind side resistance loss of the heater of the same structure type when it is running on the plain and plateau

the selection principle of forced draft fan is to first meet the requirements of air volume and air pressure. In addition, the following factors should be considered:

(1) low noise

(2) the P-Q line should be smooth and avoid steep drop and peak

(3) select external rotor fan

(4) the overall size of the fan should be compatible with the size of the heater

when selecting the fan, the known conditions are the air mass flow and the resistance loss of the system under plateau conditions, and the P-Q line given by the fan is characteristic under standard working conditions, so it must be converted when selecting the fan. The conversion method is that if the mass air supply on the plateau is known to be g (kg/h) and the system resistance loss is h (PA), then the air supply volume and air pressure of the fan under standard working conditions are converted as follows:

q= (G/ρ) 3600

P=1.2 ×Δ h/ρ

where ρ Is the local air density on the plateau

considering some unpredictable factors, the safety factors of 1.1 and 1.2 are taken for the air volume and pressure head of the fan respectively

the actual operation of the fan selected according to the above method shows that it meets the requirements of all aspects

in the development of plateau heater, we have solved the above key technologies. After many measurements of plateau operation and long-term operation tests of users, the co component of smoke exhaust is close to zero, the thermal efficiency of the unit is about 85%, and the supply capacity is about 42kw, which fully meets the expected design goal. (end)

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