Influence of electric heater layout on air conditioning performance

- Apr 29, 2020-

In recent years, China's railway transportation construction has made considerable progress. As of the end of 2017, the railway business mileage reached 127,000 km. , Convenient and comfortable experience. As the main body of rail vehicles carrying comfort requirements, air conditioning units play an indispensable role. The air-conditioning unit can be divided into the following types according to function: a) single cooling type; b) single cooling + electric heating type; c) heat pump type; d) heat pump + electric heating type [1].

The air-conditioning unit of the project involved in this article is a single cooling + electric heating type, the cooling capacity is 30kW, and the air supply and return methods are bottom delivery and bottom return structure. The heating capacity is 22kW, which is provided by two electric heaters. The design power of each electric heater is 11kW. It adopts a U-shaped stainless steel light pipe structure with one end of the outlet and the other end of the U pipe. See Figure 2 and 5. The structural arrangement of the electric heater, blower and return air inlet in the unit, as shown in Figure 1, 1, 7 mixed air filter, 2 right evaporator, 3 electric heater, 4 blower, 5 left electric heater, 6 left evaporator , 8 boxes, 9, 10 return air valve.

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According to TB / T1804-2017 standard, completed the electric heater safety test, electric heating rated heating power test [1]. During the data collation, it was found that the temperature of the measuring points arranged at the two air supply outlets had a large deviation, and the temperature difference between the 4-1 outlet temperature and the 4-2 outlet temperature was a maximum of 14.4 ° C; There is a big difference in temperature, and the temperature difference between 4-2 and 4-1 is up to 14.9 ℃. Test of secondary temperature protector: Simulated blower and primary temperature protector fail simultaneously [1]; the electric heater is in dry burning state, the hot air cannot be taken away by the blower, most of it is gathered inside the unit, so the two blower motors are the highest The temperature values are close, of which the temperature of the 4-1 motor is 136.3 ° C and the temperature of the 4-2 motor is 135.5 ° C. Although the performance of the electric heater of the air conditioning unit meets the procurement specifications and related standards, the air conditioning unit needs to cooperate with the air duct on the vehicle to form a complete ventilation system. The design of the air duct will directly affect the uniformity of the vehicle's ventilation, and Affect the comfort of the vehicle. The EN 14750-1 comfort parameter stipulates: the internal temperature range measured at a distance of 1 m from the ground. This range cannot be greater than 4K on A-type vehicles and 8K on B-type vehicles. The maximum air temperature difference of 14.4 ° C when the air conditioning unit is heating is much larger than the index of 4K and 8K, which undoubtedly brings great challenges to the design of the air duct.

The electric heater has a U-shaped light pipe structure, and the heating area is mainly concentrated in the area of the U pipe and the light pipe, and a part of the pipe at the outlet is used to fix the wire harness, which is not a heating area. In Figure 1, for the convenience of wiring, the electric heater uses a symmetrical structure, which is led from one end, and the U tube is concentrated at the other end. When the electric heater is working, the heat generation on the side of the U-tube is originally higher than that of the outlet end. The two U-tubes with large heat generation are concentrated together and the temperature will be higher, which leads to the air outlet of the U-tube end. The temperature is much higher than the outlet end. In Figure 1, the two blowers are the same model, the motor is arranged on the side of the volute, no matter how adjusted, there will always be a blower motor will appear between the two U-tube ends, which causes the blower The temperature of the motor is much higher than the other end.

After testing and distracting, it was found that the reason for the uneven air temperature of the two air supply ports is that the U-tube end of the electric heater is concentrated on one side, and improving the electric heating layout is undoubtedly the most fundamental measure. Rearrange the electric heater: change the two electric heaters used symmetrically to the same model 5 left electric heater, and arrange the outlet end of the electric heater close to the blower motor to make the environment of the two motors in normal condition The temperature is consistent, and farthest away from the high temperature area of the U-tube. In this way, the heat radiated at each fan is consistent, and the temperature at the motor is lower,  Re-run the air channel simulation test, the outlet temperature of 4-1 is 41.6 ℃, and the outlet temperature of 4-2 is 41.5 ℃, which is basically the same.