Unsteady heat transfer calculation
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Dear forum members, I have a question regarding unsteady-state heat transfer that I would like to ask you all. Question: There is 20 tons of water in a tank, which is pumped via a pump into a cooler for cooling. The cooling medium is circulating water at 30°C, and it is required that the temperature drop from 60°C to 40°C within 1 hour. How should the flow rate of the pump and the area of the heat exchanger be calculated?
According to the calculations, Q = (60–40) * 4.2 * 20000 / 3600 = 467 KW. The flow rate on the cold side is determined through energy balance to be 80 cubic meters, while the temperature rise on that side is set at 35 degrees. The heat exchange area for the plate heat exchanger is chosen to be 15 square meters, and a pump with a capacity of 100 cubic meters can be used.
How is the heat exchange area determined when cooling needs to be accomplished within a limited time?
This question is quite interesting; various parameters have an impact on the final calculation result. I also had to try various calculations repeatedly before arriving at a relatively reasonable result.
The results for Building 4# are generally reasonable, but it is also necessary to take into account the heat release due to the cooling of equipment, pipelines, and insulation materials (if any). For this case, the flow rate of the circulating water can be reduced over time: 80–100 m3/h at the beginning and around 30 m3/h at the end, in order to keep the temperature difference between the supply and return water of the circulating water relatively small as time goes by. If the circulating water flow does not decrease over time and remains at its initial high level, the temperature difference between the supply and return water at the end will be very small; therefore, some waste is not a major issue. After the design scheme is implemented, it can be considered reliable if the desired cooling target is achieved within 45 to 60 minutes; empirical estimation methods should be sufficient to meet this requirement. Even when using the Batch module in simulation software such as ASPEN for simulation calculations, simplified assumptions are involved, which is not entirely necessary.