Regarding the outlet temperature of the cooling water.
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The original poster works on turbine operation at a power plant. I learned the basics of heat transfer in college. I’ve always noticed a phenomenon that cannot be explained by what I have learned; I hope experts can provide answers, and I’d like to discuss it with friends. In power plants, whether it is a condenser or a cold oil cooler, such simple surface heat exchangers share one characteristic: after throttling the cooling water at its outlet, the temperature of the cooling water at that outlet rises, and this increase is quite noticeable. I learned in school that the heat exchange efficiency is related to factors such as the flow rate of the fluid, the temperature difference between the fluids, and the area involved. Here, after throttling at the cooling water outlet, the flow rate of the cooling water is altered; in other words, the flow rate is changed by, for example, closing the outlet valve. As the flow rate decreases, the heat exchange efficiency will definitely decline. Why does the outlet temperature rise? The heat exchange efficiency decreases, which results in a lower heat transfer rate; at the same time, the flow rate also decreases. There are two variables here – how can we determine the relationship with temperature? I hope an expert can provide an explanation; the simpler it is, the better. It would be great if formulas could be explained as well, as I’m basically a complete beginner in heat transfer. It’s a very simple phenomenon; I just don’t know why from a theoretical perspective. I hope everyone can help me analyze it.
To start with, it should be noted that there are specific requirements for the rise in cycle water temperature, both in the design phase and during operation. Temperature rises of 8 degrees and 10 degrees are common design parameters.
This post was last edited by wiseboy on 2016-1-21 at 19:48. Assuming the specific heat capacity of water is Cp=4.2 kJ/kg and the initial temperature of the water is t1=30°C, then Q = w·Cp·(t2–t1) = w·4.2·(t2–30). ======================================== Initially, w1=10 kg/s and t2=40°C; therefore, Q1 = w1·4.2·(t2–30) = 10·4.2·(40–30) = 420 kJ/s. ======================================== When the flow rate is reduced, assume it becomes w2=5 kg/s and the temperature rises to t2=45°C. Then Q2 = w2·4.2·(t2–30) = 5·4.2·(45–30) = 315 kJ/s. ======================================== Q2
You mean I had thought about this issue of specific heat capacity before, and my own ideas are similar to yours. My question is, why does the flow rate decrease while the outlet temperature rises? What you mean is to work backwards based on the actual conditions: in reality, as the flow rate decreases, the heat transfer amount also decreases, so Q1 > Q2; yet the change in flow rate results in W1 > W2. My question is why does the outlet temperature rise when the inlet temperature remains the same? In other words, it’s about the relationship between △T1 and △T2. According to what you said, that is, although Q1 > Q2 and W1 > W2. But since Q1/W1 < Q2/W2, it follows that △T1 < △T2. The explanation seems reasonable; however, I find it a bit vague. I have thought about this as well – I am aware of both the phenomena and the results – I’m just looking for theoretical support. Your theoretical explanation essentially involves reasoning backwards from the actual observations, and without specific numerical values, analyzing the theory based on those actual results. Pure theory can’t lead to any conclusions without data, right?
Energy conservation! The premise is that the outlet temperature of your cooled material remains constant, as does the flow rate; in other words, Q remains unchanged. Q = w·cp·(t2 – t1), so w1·Δt1 = w2·Δt2. Since w1 > w2, it follows that Δt1 < Δt2 – meaning that a larger temperature difference occurs when the flow rate is low.
This is a normal phenomenon: when the outlet valve is closed or the frequency of the circulation water pump is adjusted (both are minor adjustments), the temperature at the circulation water outlet rises, which enhances the heat exchange effect. You have only considered the decrease in flow rate; such actions not only result in a lower flow rate but also lead to a reduced flow velocity inside the heat exchanger, thereby increasing the heat exchange time. However, this is only the result of making minor adjustments. These are all things that experienced operators tell you about when making slight tweaks
Can it be explained in this way: assuming that the heat exchange requirements of the hot fluid remain unchanged, that is, the amount of heat released stays constant, then a decrease in the flow rate of the cold fluid directly results in an increase in the return water temperature
This is not actually the case, because the heat transfer coefficient is related to the flow velocity. Therefore, as the flow rate changes, the amount of heat removed by the cooling water also changes. As the flow rate decreases, the actual cooling capacity is reduced, and less heat is removed; therefore, the heat transfer amount before and after throttling is different.
It is not conserved here; the cooling effect differs when there is a high flow rate versus when there is a low flow rate. In other words, the amount of heat carried away (absorbed) by the circulating water varies; Q is different in the two cases
Shouldn’t the greater the cooling water volume (the higher the flow rate), the better the cooling effect be? By heat exchange effect, you must be referring to the cooling effect, right? The heat transfer coefficient is related to the flow velocity, within a certain range. The faster the flow rate, the better the heat exchange effect. What I said about low flow rate leading to better heat exchange should be the opposite, right?
What a professional field! I need to read more books!