Design of the cooling water outlet temperature control system for heat exchangers

It’s possible that the temperature at the cold water outlet is used to control the opening degree of the valve at the cold water inlet

The cold water outlet temperature is controlled by adjusting the cold water inlet flow rate.

Can’t the hot water pump be controlled via an inverter to adjust the hot water flow rate? Can’t a frequency converter be used to control a water pump? Form a cascade system to control it? Also, if it can be controlled by an inverter, what symbol should be used in the control schematic to represent this inverter? Could the expert upstairs give an answer? Thank you~

But it seems that this won’t work, because it’s quite delayed!

The original poster really wants to help you, but after reading your post I’m not quite sure what difficulty you’re facing! The temperature control circuit for a heat exchanger is very simple! I don’t know what you want to do? So, what do you need to control? Is it to control the water temperature at the heat exchanger outlet or the hot water flow rate? An inverter can be used to control the speed of a water pump in order to regulate the flow rate. However, there is a significant delay in this process when using an inverter, as an inverter is, by definition, a device that changes the frequency of the power supply. When it receives a control signal, it first changes the frequency of the power supply to the motor, and only then does the speed of the motor change. Even after the speed changes, the flow rate of the pump does not necessarily change immediately due to inertia! Unlike control valves, it is a shut-off element with fast response! The response time of the temperature control loop is already slower than that of other control loops; therefore, using an inverter as the actuator in this loop can cause oscillations in the control circuit, so its use is not recommended! :L

Thank you very much for your help. What I need to control is the outlet temperature of the cold water in the plate heat exchanger. . . (The device I am using is the A3000 process control experiment system, which has only one electric control valve used to regulate the flow rate of cold water.) The hot water flow rate can only be adjusted through an inverter. . . ) So I thought of two control schemes: 1. Controlling it with a single-loop control system. . The variable to be regulated is the flow rate of cold water entering the heat exchanger, while the control objective is the temperature of the cold water at the outlet. By measuring the water temperature, the controller compares it with the set value and then outputs a control signal to the control valve. . . 2. It is controlled by a cascade control system. . . The hot water flow rate entering the heat exchanger is taken as the secondary objective, while the cold water outlet temperature is the primary objective. . . The output of the main regulator is sent as a setpoint to the secondary regulator, while the output of the secondary regulator controls the actuator. I’m not sure if these two control schemes are good, or do you have any better suggestions? ? ? This post was last edited by Take a look on 2008-2-19 22:12.]

Well, I finally understand what you’re trying to do. My suggestion is not to use a cascade configuration, but one condition must be met: the flow rate of cold water entering the heat exchanger must be sufficient to reduce its temperature to the level you want to control. A cascade control scheme should only be chosen if cold water is insufficient to meet the cooling requirements; the reason is that a good control system should fulfill the control requirements in the simplest, most efficient, and fastest manner possible. The response time for a cascade system to reach control stability is necessarily longer than that of a single-loop system. Since a cascade system has one additional control level compared to a single-loop system, the chances of system disturbances also increase. Therefore, cascade systems are not recommended unless it is necessary to meet certain control requirements.

Also, I didn’t quite understand your choice of cascade control. For the control object you described, I think the cascade loop should work as follows: the ultimate control target should be the temperature at the outlet of the heat exchanger. Therefore, the main controller should have its PV connected to the outlet temperature, while its MV should be connected to the SV used for secondary control. The PV for secondary control should be connected to the hot water flow rate at the inlet of the heat exchanger; however, this parameter isn’t provided in your system. The output MV of the secondary control should be connected to the cold water control valve. The practical purpose of using a cascade configuration here is to eliminate control fluctuations caused by changes in hot water flow rate! :L :L

I also want to ask about this issue: our company uses the temperature of the cooling water exiting the heat exchanger to control the flow rate of the cooling water leaving the system. But some methods involve controlling the inflow rate; could you please explain the advantages and disadvantages of these two control methods?