Heat exchanger section: [Weekly Topic] What is the impact of fluid flow rate on heat exchange efficiency? (2011.7.4-7.10)
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This post was last edited by w9970165 on 2011-7-4 10:55. Heat exchanger section: [Weekly Topic] 3. What is the impact of fluid flow rate on heat exchange efficiency? (2011.7.4-7.10) The topic for discussion this week is: What is the impact of fluid velocity on heat transfer efficiency? I hope everyone will participate actively. I encourage everyone to share their opinions based on their knowledge and experience, rather than simply copying others’ ideas. Note: Please do not hide your replies; engage in discussions with one another, ask questions, or provide explanations related to the topic. The higher the flow rate of the fluid within the heat exchanger, the greater its heat transfer coefficient. Therefore, increasing the flow rate of the medium within the heat exchanger can **improve the heat exchange efficiency. However, the negative consequence of increasing the flow rate is that it raises the pressure drop across the heat exchanger, thereby increasing the energy consumption of the pumps; hence, there must be an appropriate range for such flow rates.
Effects of excessive flow rate: 1. The outlet temperature fails to meet the design requirements; it cannot drop as much as it should or rise as much as it should, resulting in a reduced heat exchange efficiency. 2. The medium on the other side can achieve excellent heat exchange performance. 3. As the heat transfer driving force increases, the heat transfer coefficient rises, and the amount of heat transferred increases.
Reply 1# w9970165: The higher the flow velocity of the medium within the heat exchanger, the greater its heat transfer coefficient. Therefore, increasing the flow rate of the medium within the heat exchanger can **improve the heat exchange efficiency. However, the negative effect of increasing the flow rate is that it raises the pressure drop across the heat exchanger, thereby increasing the energy consumption of the pumps; hence, there must be a suitable range for such increases. Generally, for media with high viscosity, a lower flow rate is chosen, while for those with low viscosity, a higher flow rate is used.
Reply 1# w9970165: The higher the flow velocity of the fluid within the heat exchanger, the greater its heat transfer coefficient. Therefore, increasing the flow rate of the medium within the heat exchanger can **improve the heat exchange efficiency. However, the negative consequence of increasing the flow rate is that it raises the pressure drop across the heat exchanger, thereby increasing the energy consumption of the pumps; hence, there must be an appropriate range for such flow rates.
Reply 1# w9970165: The higher the flow velocity of the fluid within the heat exchanger, the greater its heat transfer coefficient. Therefore, increasing the flow rate of the medium within the heat exchanger can **improve the heat exchange efficiency. However, the negative consequence of increasing the flow rate is that it raises the pressure drop across the heat exchanger, thereby increasing the energy consumption of the pumps; hence, there must be an appropriate range for such flow rates.
From the heat transfer formula: Q=KAΔT, it can be seen that K is proportional to the amount of heat transferred Q. When the heat exchange area A and the temperature difference ΔT remain constant, the larger the overall heat transfer coefficient K, the greater the amount of heat transferred Q； We all know that the overall heat transfer coefficient K is related to the film coefficients αi and αo inside and outside the tube, and these in turn are related to the Reynolds number of the flow; as the Reynolds number increases, the film coefficients αi and αo also increase (αi and αo are proportional to Re^n, where n > 0) ； Looking at the formula again: Re=（duρ）/μ. The greater the flow velocity u, the higher the Re value. It can be concluded from the above that the greater the flow velocity of the fluid, the better the heat transfer, provided that the requirement for pressure drop is met.
The higher the flow velocity, the greater the heat transfer coefficient; however, I believe the best heat transfer efficiency is achieved when turbulence occurs
I think we also need to consider the heat exchange purpose of the heat exchanger. For media that require heating, the faster the flow rate of the cold fluid, the harder it is to reach the desired outlet temperature per the process requirements. The faster the velocity of the hot fluid, the easier it is to bring the cold fluid to the outlet process temperature. The same principle applies to rings that need to be cooled.
Clarifying what is meant by heat transfer efficiency: if it refers to the heat transfer rate per unit area, then there is only one answer – the faster the rate, the better the efficiency； If what is meant is the performance of a specific heat exchanger under certain operating conditions, it is necessary to analyze different conditions. Take one scenario: if there is a limit on the pressure drop and the fluid has high viscosity, it will not be possible to reach turbulent flow speeds under that pressure drop constraint; in such cases, a suitable low flow speed should be chosen so that the fluid has sufficient time for heat exchange (through natural convection or natural conduction).
:I’m so frustrated – why do some people’s answers exactly match the original poster’s? They should have points deducted; P. Besides, that answer isn’t necessarily correct either; so what’s there to discuss?
:Lol, sometimes I think that if the flow rate is too high, there might not be enough time for heat exchange between the hot and cold streams.