Design regarding the control of the steam heat exchanger in the waste heat recovery unit and the liquid level control valve in the condensate tank
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In our waste heat recovery system, the heat transfer fluid water/steam heat exchanger is used, with the condensate water exiting from the lower end of the heat exchanger and flowing into the condensate tank. A pipeline runs from the upper part of the same end of the heat exchanger to the upper part of the condensate tank. What is the purpose of this pipeline? Is the flow direction from the heat exchanger to the condensate tank, or is it to recover steam after flash evaporation in the condensate tank? The direction arrows on the drawing are contradictory. Another point is that the control valve at the outlet of the condensate tank is designed to regulate both the liquid level in the condensate tank and the temperature of the hot fluid after heat exchange in the heat exchanger. What is the principle behind this control? The system class talked about cascade control. How to set up automatic control for a control valve? Control the liquid level or control the temperature?
A pipeline runs from the upper part of the same end of the heat exchanger to the upper part of the condensate tank. What is the purpose of this pipeline? --- This is a pressure equalization pipe that connects the condensate tank to the steam side of the heat exchanger, maintaining equal pressures. The outlet control valve of the condensate tank is designed to simultaneously control both the liquid level in the condensate tank and the temperature of the hot fluid after heat exchange in the heat exchanger. — This is indeed cascade control, using temperature to regulate the setpoint for the liquid level in the condensate tank. It can be understood in this way: if the temperature at the outlet of the heater is too high, it causes a rise in the liquid level due to temperature interconnection, which in turn raises the liquid level in the tank. Since the heat exchanger is connected to the condensate tank, the liquid level of the condensate inside the heat exchanger also increases, resulting in a reduction in the heating area. As a consequence, the temperature at the outlet of the heat exchanger drops. It is by changing the heat exchange area of the heat exchanger that the temperature is adjusted.
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