In order to determine the temperature difference of the fire pump bearing housing, the Atlantic Pump Industry calculated the conjugate heat transfer (CHT) value for the heat transfer process. The results revealed that volume loss is the primary factor influencing the heating of the bearing housing. Based on this analysis, structural dimensions were optimized. During operation, large fire pumps often experience high temperatures in the bearing housing, with temperatures exceeding 70°C. Excessive heat can negatively impact lubricant performance. Even within the same batch of products, the maximum temperature of the housing can vary significantly. Changing the bearing type or adjusting the radial thrust bearing clearance does not effectively control the temperature. Therefore, it is essential to identify the root cause of temperature fluctuations and implement effective control measures.
The main reason for the elevated temperature was identified through numerical simulation of the conjugate heat transfer (CHT) in the bearing housing. CHT problems are typically divided into two computational regions: a fluid-filled area and a solid region. Heat is transferred between these regions via conduction. In terms of numerical methods, the finite element method (FEM) is suitable for pure solid heat transfer problems, while the finite volume method (FVM) is more effective for conjugate heat transfer involving fluids. This study employed the finite volume method.
The heat transfer calculation results ignored the influence of lubricating oil on heat transfer within the tank and the impact of heat transfer on the system. Since the problem is axisymmetric, only a sector needs to be analyzed. Bearings are located at positions A and B, with water inside the frame and the rest consisting of the housing and shaft. The contact between the housing, shaft, and air involves convective heat transfer. Figure 2 shows the calculation results, where warm colors represent high temperatures and cool colors indicate low temperatures.
Although computer simulations are powerful, real-world engineering problems are often complex, and the calculation parameters involve some level of approximation, which can affect accuracy. It is important to understand these uncertainties when interpreting the results. When analyzing the data, the influence of boundary condition variability must be carefully considered. In the heat transfer analysis of the bearing housing, water convection heat transfer is caused by the pump's volume loss. Its heat transfer coefficient depends on the pump’s efficiency and should be controlled during design. However, manufacturing variations can lead to uncertain volume loss, resulting in fluctuating heat transfer coefficients.
For a fire pump with a specific speed of ns=76, when the volumetric efficiency ranges from 90% to 98%, the equivalent heat transfer coefficient varies between 390 W/m²·°C and 1240 W/m²·°C. Air convection, on the other hand, is natural convection, and its coefficient depends on the surrounding environment. Since fire pumps are usually installed indoors, air flow speed remains relatively stable, so the heat transfer coefficient doesn’t change much. If wind speed varies from 0 m/s to 6.4 m/s, the average air heat transfer coefficient can range from 5 W/m²·°C to 25 W/m²·°C. Water convection has a much higher heat transfer coefficient than air, making it the dominant factor in heat transfer. Designers can control forced convection by optimizing pump design.
As shown in Figure 3, the effects of the water convection heat transfer coefficient and the average air convection heat transfer coefficient were analyzed to observe their impact on the results. The temperature displayed in Figure 3 represents the maximum temperature of the bearing housing, typically found near the bearing away from the impeller. With a single bearing generating 1000W of heat and an ambient temperature of 20°C, it is clear that the variation in the water convection heat transfer coefficient has a greater impact on the maximum temperature of the bearing housing compared to the air convection coefficient.
To effectively control the temperature, the volumetric efficiency can be appropriately reduced. If the maximum temperature is kept below 70°C, the water convection heat transfer coefficient should be no less than 500 W/m²·°C. The pump’s volumetric efficiency and related dimensions should be calculated accordingly to ensure optimal thermal performance.
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