Seeking advice on the thermal efficiency of heat exchangers
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Friends, I have a question regarding heat exchangers that I’d like to ask you: Given a constant heat exchange area of the heat exchanger, as well as constant flow rates of the hot medium and its initial temperature when it enters the heat exchanger, the requirement is to reduce the temperature of the hot medium from 85 degrees Celsius to 70 degrees Celsius, with the lowest temperature being higher than the highest temperature of the cold medium. Logically, the greater the flow rate of the cooling medium entering the heat exchanger, the faster the cooling speed should be. However, in practice, when the opening degree of the process control valve is adjusted, the cooling speed is the same whether the cooling water valve is fully open (300 t/h) or at 60% of its opening (200 t/h, as determined through process operation experiments). The time it takes for the heat medium to drop from 85 degrees Celsius to 70 degrees Celsius is almost the same. Why is this? (The heat medium is circulated.) I would be very grateful if everyone could help answer this question!
The outlet temperature of the cold medium is different, isn’t it?
Cooling water outlet location, pipe size
Well, I’m a instrumentation technician, so I’ll talk about this from the perspective of valves. 60% opening doesn’t necessarily mean 200t, dude – each type of valve has different flow characteristics... There isn’t necessarily a linear relationship between flow rate and opening degree
1. The flow rate isn’t as high as you said; was it measured by a flow meter? 2. You need to provide the inlet and outlet temperatures of the cooling medium under both high flow rate and low flow rate conditions. 3. You need to provide the flow rates of the hot and cold media. 4. What kind of medium is the hot and cold medium?
I agree with this statement. Like it.
In this case, generally the flow rate of the cold medium is high, the temperature rise at the outlet is not significant, and there is little change in the driving force for heat transfer
Your question is wrong from the very beginning; the premise that \"the greater the flow rate of the cooling medium entering the heat exchanger, the faster the cooling rate\" is incorrect, and as a result, the subsequent questions are also wrong. The efficiency of heat exchange has little to do with the flow rate of the cooling medium; it is rather related to the velocity of that medium. The flow velocities of 300t and 200t within the heat exchanger will definitely be different. If 200t stays in the heat exchanger for a sufficient amount of time, it’s not surprising at all if its heat exchange efficiency is the same as that of 300t. You can determine this simply by checking the temperature of the cooling medium in these two cases.
I don’t know how the instruments determine the relationship between flow rate and velocity; it’s my fault as well – I posted in the wrong place; it should have been posted in the section related to the equipment. I’m sorry about that. Flow rate = velocity × cross-sectional area; the flow rate is directly proportional to the velocity! Is there an error in my proposition? The cross-sectional area of the pipe remains constant, haha!
Please look at what you wrote yourself; you said “the faster the cooling rate”, not the faster the fluid’s speed. There are several factors that affect heat exchange: the area of exchange (which remains constant), the temperature difference between the hot and cold media (also constant), and the flow rate of the hot medium (also constant). The only factor that can change is the flow rate of the cold medium. In other words, the higher the flow rate of the cold medium, the shorter the time it stays in the exchanger. The time spent by the cold medium in the exchanger is an important factor in heat exchange. For example, if 300 tons of fluid stay in the exchanger for only 1 minute, while 200 tons can stay there for 2 minutes, the amount of heat exchanged by them might be the same.
That’s exactly the question I want to ask. Does the heat exchanger have a maximum cooling capacity, such that once this critical value is reached, the cooling effect remains the same regardless of the amount of cooling water used? It’s a matter of residence time, but before reaching the critical value, isn’t it true that the greater the flow rate of the cooling medium, the faster the cooling rate (that is, the speed at which the temperature of the material being cooled drops)? Maybe my statement is a bit confusing; I’m sorry~