Discussion on the brine heat exchange scheme
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At present, we have an idea to use a molten salt tank to store salt; the salt is heated to 500°C to store heat, which is then used for heating through heat exchange with water. The temperature difference for using molten salt is 500–200°C, while that for water is 20–80°C. The current problem is that with single-tank salt storage for heat exchange with water, it is not possible to ensure the inlet temperature of the molten salt in the hot gas flowing through the salt pipe. In a single-tank setup where the molten salt circulates on its own, the temperature at the inlet of the heat exchanger is 500°C; as circulation continues, the temperature of the molten salt gradually decreases. Since the temperature at the inlet of the high-temperature liquid is dynamic, we control the entire heat exchanger by adjusting the flow rate at that inlet, thereby achieving the purpose of heating. All experts are welcome to join the discussion. Given that the temperature at the high-temperature liquid inlet of such a heat exchanger is dynamic, what is the difficulty level in controlling this heat exchanger? Thank you to all the experts for participating in the discussion. If you are involved in the heat exchanger business, we could work together!
This idea is good, but controlling the flow rate means that the flow will fluctuate constantly. In general, for heat exchangers, the flow rate of the fluid is kept at a specific value or experiences only slight fluctuations, which usually doesn’t cause any problems. However, if the range of flow rate adjustments is too large, it may prevent the heat exchanger from functioning properly. I’d like to ask Teacher Wiseboy, Gu Tongjiu, to take a look and see if it’s possible to achieve this. @wiseboy @Gutongjiu
This is a good idea; it might be able to reduce energy consumption. Due to the temperature changes of the molten salt itself after operation, it is indeed difficult to control the heat exchanger by adjusting the flow rate, as it is hard to maintain this dynamic balance. Because the flow rate is reduced, the residence time of the molten salt in the heat exchanger increases, the depth of heat exchange grows, and the temperature of the molten salt exiting the heat exchanger becomes too low (the extreme case being solidification of the molten salt). It is best to have dual control, primarily through the flow rate and velocity of water, with the flow rate of molten salt serving as a supplementary mechanism. An immature idea, for reference only.
Regarding the original poster’s question, I would like to make two points: 1. In terms of the system – the initial temperature at the inlet of the heat medium is 500 degrees, and it should remain at 200 degrees in the end. There are only two ways to ensure that the water on the cold side does not vaporize: either the flow rate is high enough so that the water reaches outside the heating area before it vaporizes, or the pressure is high enough so that the resulting hot water, when mixed with the cold water from the external circulation, reaches the 80-degree temperature required for heating. This requires the pump and frequency converter to have a fairly high degree of adjustability. 2. Regarding heat exchangers – under the operating conditions here, I recommend using multiple heat exchangers in parallel; as for the type of heat exchanger, only tubular heat exchangers are suitable for this purpose.
Since the ultimate goal is to obtain hot water for heating, the stringent operating conditions that need to be taken into account are the molten salt inlet temperature, a concern mentioned by the building owner. In particular, winter operation conditions must be considered; not only must the molten salt inlet temperature be ensured (factors such as tank insulation also need to be taken into account), but also the insulation of the water delivery system when using softened water for heating in winter, without relying on the existing boiler system. The return water temperature for heating needs to be considered under the most severe conditions, and then a relatively conservative heat exchanger design should be developed through comprehensive analysis. As for the type of heat exchanger, whether to use a parallel configuration depends on your specific water flow rate requirements; for shell-and-tube exchangers, it is advisable to have one unit in operation and another as a backup.
I’m a newcomer to our forum and don’t know how to consult the two teachers. Please give me some advice!
If the flow rate of water is to be adjusted, it becomes impossible to ensure a stable heat output; in that case, the overall water flow rate and the intermediate storage tank must be increased. This might increase equipment investment; are there any other better control methods? Please give me some advice!
I have no knowledge of instrument control and the design of process systems; I lack both design experience and on-site inspection experience. I dare not offer opinions on how this can be implemented in practice. Please ask experts to help resolve this issue.
Thank you for Teacher Gu’s suggestions. The two issues you mentioned do indeed exist, and we are considering ways to address them. As for the heat exchangers, we have chosen shell-and-tube heat exchangers connected in series, and simulations were carried out using HTRI. If it’s convenient for you, I can send you our simulation results so that you can check whether they are reasonable!
Thank you for your comment. We have considered the issue of heat retention, and we have adopted a configuration with multiple shell-and-tube exchangers connected in parallel to carry out the heat exchange task. Thank you
I think it should be easy to achieve. I’ll share my thoughts first; they’re not professional, so feel free to criticize them. On the water side, no changes are needed – it just needs to operate continuously at the desired flow rate； On the molten salt side, a tee is installed; by utilizing the concept of reflux, the molten salt entering the heat exchanger consists of two streams: the \"reflux fluid\" (the relatively low-temperature molten salt that has completed heat exchange) and the \"original fluid\" (the high-temperature molten salt in the tank). The flow rates of these two streams are controlled via frequency conversion by two pumps. Of course, if the amount of \"reflux fluid\" is not fully utilized, it returns to the molten salt tank as well – isn’t that sufficient? The frequency conversion of the two pumps is controlled by a feedback from the temperature of the mixed liquid. Personally, I don’t think it’s difficult