Temperature control methods for heat exchangers: selection between bypass and direct flow regulation approaches

It depends on what you use as the main parameter! If all you need to control is the temperature, then the flow rate has no impact on subsequent processes, so flow control can be used directly! However, under actual operating conditions, flow stability is more important than temperature parameters; that is why bypass flow control is used to regulate temperature!

Controlling the temperature by adjusting the flow rate in the main pipeline can affect the flow rate of the fluid in that pipeline under normal operating conditions. When the temperature changes little, the impact of flow rate adjustments is minimal; however, when the temperature changes significantly, the flow rate experiences substantial variations as a result of such adjustments. On the other hand, adjusting the flow rate through a bypass allows the flow rate in the main pipeline to remain constant, with almost no effect on normal operating conditions!

Okay, doesn’t bypass control offer energy savings by not having direct flow control?

This post was last edited by Hao Liang Zhi Le on 2020-7-17 at 17:11. Is there any phase change during the heat transfer process? If neither the hot nor the cold fluid undergoes phase change, and when the process does not allow changes in the total flow rate, a bypass system is used for control. When bypass control is used, it is required that the heat exchange capacity of the heat exchanger be sufficiently large.

Bypass control is used for mixing cold/hot materials; therefore, it allows for more precise temperature control compared to single-control main circuits, and it also offers a wider range of temperature adjustments. It depends on the actual temperature control requirements of the process. In situations where the total flow rate cannot be changed, if you control the main path, it is necessary to implement control for the bypass as well (dual-valve control). If you only use bypass control (single-valve control), the control effectiveness will depend on the heat exchanger’s resistance drop/total resistance drop. The opening degree of the bypass valve corresponds to the temperature after blending. The ideal situation is that, within the allowable temperature control tolerance, the flow rates in the bypass and the main path are approximately evenly divided (heat exchange design). Control valves are designed based on the operating point; valves with a relatively large size have a smaller operating opening, which is not conducive to control. It is necessary to take into account the disturbances caused by temperature changes on the other side to the temperature control of the main circuit; for example, if the cooling water temperature rises from 28 degrees to 32 degrees, the bypass valve will close, and in extreme cases, automatic operation will not be possible.

For heat exchangers used for cooling purposes, the flow rate of the main fluid cannot be limited. If the temperature of the main fluid stream is relatively high, and it is difficult to maintain a stable temperature for this fluid through water-based heat exchange, then it is necessary to divert some of the fluid that does not need to be cooled via a bypass circuit in order to balance the temperature. This is especially the case when the temperature of the main fluid is low, and reducing the flow rate of the cooling water does not yield quick results.

I feel that, no matter how it is controlled, bypass control is safer; if main flow control is used, a failure of the control valve will result in a shutdown of flow, which can cause pressure buildup

It still depends on the requirements of the process design. The bypass control has several \"weak points\": it is controlled by a single valve, and the process design must allow the main path from not being completely closed. The pressure drop on the equipment side is low, and the control intensity of the bypass is not strong.