Phase change and non-phase change issues in heat exchangers
Thread Content
I would like to ask that, during the heat exchange process, if it is necessary to control the outlet temperature of the fluid in the tube side, is it easier to control phase change in the fluid on the shell side or not? (For example, after heat exchange with steam, it becomes condensed water; or with high-temperature heavy oil, it remains in liquid state after heat exchange.) ）
If that is the case, to prevent phase change, the amount of the cooling medium should be increased
Can the outlet temperature on the tube side be adjusted by controlling the flow rate of the fluid on the shell side? I feel like it should be the flow rate on the pipe side that needs to be adjusted! I wonder what everyone thinks
It’s all easy to control; it mainly depends on whether there’s enough heat source. However, phase changes are not good for the equipment
From what the poster means, the process materials are those used in the manufacturing process, and their flow rate is determined by the process itself; it’s not possible to control this flow rate in order to adjust the outlet temperature. It is relatively easy to adjust the shell side, whether using steam or heavy oil; however, the requirements for heat exchange equipment may differ significantly between the two, such as the design pressure ； It is also necessary to consider whether the heat source is easily accessible; for example, if steam is used for heating, it may be required to make additional investments in purchasing boilers and other related equipment. In summary, first, it is necessary to meet the requirements of the process materials (including any special requirements such as tolerance to high temperatures), and second, factors such as the level of investment and the ease of operation and maintenance must be considered.
Let’s start with phase-change and non-phase-change heat transfer: for the same medium, phase-change heat transfer releases significantly more heat (or removes more cold energy) than non-phase-change heat transfer； As for control, those manufacturers have already taken care of the design aspect, so there’s no need to worry; as long as the energy levels of the two heat exchange media are balanced, it’s easy to maintain control. Now, since you are talking about the comparison between water and oil, it comes down to their specific heat capacity values. You also need to consider the pressure of the heat transfer medium, as it is another key factor affecting heat transfer.
It seems strange to me that this issue is presented in such a complicated way. Controlling the outlet temperature of the medium passing through the tubes should primarily involve adjusting the flow rate on the tube side. As for the shell side, whether phase change occurs or not affects the heat transfer coefficient of the heat exchanger, which in turn determines the required heat transfer area or design parameters. However, once the heat exchanger has been selected based on the specific heat transfer conditions, the accuracy of temperature control should remain the same； If we have to determine which is easier in terms of controlling the outlet temperature of the medium in the control pipeline, is it because of phase changes? The heat transfer per unit volume is relatively high, so it’s more difficult to maintain a stable temperature range, right? (But it seems a bit far-fetched.)
However, in many cases it is the flow rate on the pipe side that needs to remain stable, but this can be addressed by adding a reheater line.