Heat exchange efficiency issue

The heat exchange efficiency is first related to the type of heat exchanger you choose; regardless of the type of heat exchanger used, it is possible to determine the required heat exchange area through calculations. It can be said that the flow rate of the fluid directly affects the value of the overall heat transfer coefficient K, which in turn influences the efficiency of heat transfer. Generally speaking, a higher flow rate increases the turbulence in the fluid, leading to more effective collisions and thus facilitating heat transfer. However, an excessively high flow rate can increase the impact of the fluid on the equipment; therefore, a balanced approach is necessary.

This is also related to the fouling thermal resistance of the heat exchanger

To understand the heat exchange effect, one only needs to look at the formula: Q=KF△T. It is related to the heat transfer coefficient, the heat exchange area, and the temperature difference (referring to the logarithmic mean temperature difference, that is, the difference in temperature between the cold and hot fluids). The main task is to determine the heat transfer coefficient, which is the reciprocal of the heat transfer resistance. This resistance consists of the convective resistance outside the tube, the convective resistance inside the tube, the fouling resistance, the contact resistance, and the conductive resistance; generally, it is determined primarily by the first three of these factors, while the contact resistance and conductive resistance are only taken into account under specific circumstances. The flow velocities inside and outside the pipe, as well as the physical properties of the medium (density, viscosity, thermal conductivity, specific heat), are the main parameters that affect the thermal resistance. The higher the flow rate, the greater the Reynolds number, and the higher the degree of turbulence; as a result, the heat transfer coefficient also increases. However, a higher flow rate leads to greater resistance and higher power losses, resulting in high operating costs. Therefore, when designing heat exchangers, it is necessary to find the most economical balance among these factors.

The heat exchange efficiency is certainly related to factors such as the heat exchange area, the material of the heat exchanger, the specific heat of the fluid used in the heat exchanger, the viscosity of that fluid, the turbulence level of the fluid within the heat exchanger, the temperature difference between the inlet and outlet of the fluid, and whether the heat exchanger is covered with dust or scale. As for what was said upstairs that a higher flow rate leads to better heat transfer, I can’t entirely agree with that, because it’s not the flow rate itself that affects heat transfer efficiency; rather, it’s the turbulence level determined by the flow rate (which is also one of the reasons why some heat exchangers are equipped with fins). The flow rate has two effects on heat transfer: on one hand, as mentioned above, a higher flow rate results in a thicker turbulent layer and an increased Reynolds number, which facilitates heat transfer; on the other hand, a high flow rate reduces the time required for heat exchange between the cold and hot media, thereby decreasing the temperature difference between the inlet and outlet, which hinders heat transfer.

I don’t quite understand why, the faster the flow rate, the less effective heat exchange becomes once it reaches a certain level. Indeed, a higher flow rate of the refrigerant means it stays in the heat exchanger for a shorter time. But remember that with a higher flow rate, the total amount of refrigerant that enters and leaves the heat exchanger per unit of time also increases. This leads to higher energy consumption, as the pumps used to transport the refrigerant have to do more work. It also increases the stress on the heat exchanger, affecting its service life. Therefore, it’s not recommended to set the refrigerant flow rate too high. However, from the perspective of heat exchange efficiency, since more refrigerant passes through the heat exchanger per unit of time, this can at least compensate for the heat that isn’t removed due to the short residence time of the refrigerant in the heat exchanger Please explain!

It is related to the thermal resistance caused by heat exchange fouling; it is recommended to use professional cleaning agents. You can contact me for cleaning services – I have environmentally friendly cleaning agents that are non-toxic and non-corrosive. My phone number is 13034516699, and my website is http://www.spqing*ji.com

It seems that what the person upstairs meant is not that it hinders heat exchange, but rather that it makes it difficult to maintain the thermal quality of the refrigerant.

I’ve learned it, I really have; thank you so much

The flow rate cannot be increased indefinitely; in addition to increasing energy consumption, it also raises costs, all of which are factors that need to be taken into consideration.

What is the empirical value of the heat transfer coefficient for forced-circulation heaters? What is the heat transfer coefficient of a steam cooler?