In order to determine the temperature difference in the fire pump bearing housing, Atlantic Pump Industry calculated the conjugate heat transfer (CHT) of the heat transfer process within the housing. The results revealed that volume loss is the primary factor influencing the heating of the bearing housing. As a result, the structural design has been optimized. During operation, the bearing housing often experiences high temperatures, sometimes exceeding 70°C. Excessive heat can degrade lubricant performance. Even within the same product batch, temperature variations are observed, and adjusting bearing type or radial thrust bearing clearance does not effectively control this issue. Therefore, identifying the root cause of temperature variation and implementing effective control measures is essential.
The numerical analysis of CHT identified the main reason for excessive temperatures. CHT problems typically involve two regions: a fluid-filled area and a solid region, with energy exchange occurring through conduction. For heat transfer problems involving fluids, the finite volume method (FVM) is more suitable than the finite element method (FEM). This article uses FVM for its analysis.
The heat transfer calculation results overlooked the impact of lubricating oil on heat transfer and the influence of heat on the system. Since the problem is axisymmetric, a sector model was used for simulation. 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 simulation results, where warm colors represent high temperatures and cool colors indicate low temperatures.
Although computers have strong analytical capabilities, real-world engineering problems can be complex, and some parameters are approximated, which may affect calculation accuracy. It’s crucial to understand these uncertainties when interpreting results. In particular, boundary conditions play a significant role in heat transfer calculations. Water convection heat transfer in the bearing housing is caused by volume loss, and its coefficient depends on the pump's efficiency. Manufacturing deviations can lead to variations in volume loss, causing uncertainty in the heat transfer coefficient. For a fire pump with a specific speed of ns=76, the heat transfer coefficient ranges from 390 to 1240 W/m²·°C when volumetric efficiency is between 90% and 98%.
Air convection, on the other hand, is natural and influenced by environmental factors. Although indoor installation limits airflow changes, wind speed can vary from 0 to 6.4 m/s, affecting the heat transfer coefficient, which typically ranges from 5 to 25 W/m²·°C. Since water convection has a much higher coefficient than air, it is the dominant factor in heat transfer. Designers can control forced convection, as shown in Figure 3, where the effects of water and air convection coefficients on maximum bearing housing temperature are analyzed. The maximum temperature is usually near the bearing away from the impeller. With a single bearing generating 1000W of heat and an ambient temperature of 20°C, the results show that water convection has a greater impact on temperature due to its larger variation range.
To manage temperature effectively, reducing volumetric efficiency slightly may help. If the maximum temperature must stay below 70°C, the water convection heat transfer coefficient should be at least 500 W/m²·°C. The pump's volumetric efficiency and related dimensions should be designed accordingly.
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