Mixed water temperature, level control issues
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I have a question regarding the control of a mixed water process, where hot water and cold water are combined. The parameters that need to be controlled are the liquid level and temperature in the tank (which has no mixing mechanism). Currently, the outlet water temperature fluctuates significantly, with deviations of up to 10 degrees, which causes the level control valve to operate frequently. I would appreciate any suggestions you might have. Also, I wonder if the installation location of the instruments needs to be improved. Thank you in advance
This post was last edited by gxgpww on 2019-1-21 11:48 with the title Process Diagram
It is recommended to check whether a direct water-water mixer would be useful for the project.
Currently, only single-loop PID control is available: Do anyone have any good control methods?
It is recommended to change the control method, as with your current approach the liquid level is controlled by changes in hot water feed, while the temperature is controlled by changes in cold water feed; when both of these change, fluctuations become significant and control becomes ineffective. It is recommended to keep the flow rate of cold or hot water constant, with the temperature controlled via cascade control based on the flow rate of the other type of hot or cold water. Then, the liquid level and the yield from the tank bottom can be controlled in a cascade manner.
There are three sets of adjustments, and there are also requirements regarding the water output. Who did it? Is it that troublesome?
This post was last edited by bfdlwolf on 2019-1-21 at 12:16. It is recommended to use flow control for the hot water entering the tank, temperature control for the cold water entering the tank, and a flow-level cascade control system for the water exiting the tank. Nitrogen is introduced at the bottom of the tank for stirring. Since the flow rate of hot water entering the tank is relatively stable, the flow rate of cold water entering the tank is also stable, and as a result the liquid level in the tank remains stable. This means that the control valves do not need to operate frequently, making it easier to control the temperature
It’s too complicated to use temperature control for both. As mentioned above, it’s simpler to use flow control for the hot water entering the tank, while keeping temperature control for the cold water; in other words, use a temperature sensor to control the valve for the cold water, and keep the flow rate for hot water constant
Under the existing detection system, could we try installing a stirrer to make temperature control easier? (The temperature lag is large). Or switch to the control method suggested by the people upstairs. The traffic is relatively easy to control. This also allows for proportional control of cold and hot water. Alternatively, temperature sensing points can be installed at appropriate locations in the effluent process pipeline, with temperature, level, and flow rate forming a cascade control system.
Since the outlet flow is adjusted according to operating conditions, which involves level control, the hot water flow rate cannot be set at a constant value
It is currently quite difficult to add a mixer; could the cold water inlet pipe be replaced with a spray head, and a thermometer added at the pump outlet for control purposes?