Hi,
I would like to know what are the control inputs for the heat exchanger whose model is based on energy balance. If we want to control the temperature of the hot fluid at the outlet, what would be then the control inputs? I would like to know also what the are the possible output that can be controlled and the respective control actions?
Thank you
Typically for heat exchangers, inlet and outlet temperature, inlet and outlet pressure and mass flow rate of the liquid or gas are monitored. Naturally for each side of the heat exchanger. Since it is costly to make all these measurements , sometimes it is limited to a part that is sufficient for monitoring.
It's a very wide question as we have many different types of heat exchangers. But to give a simple and direct answer to your question, first you need to know that in some cases we don't control anything in a heat exchanger. I mean, the heat exchanger will be designed for a working point and either the system parameters (flow, temperature, and pressure) will not change significantly, or it's not that much important for us to have a controlled output.
But looking only into the ones we want and need to control them, mostly the fluid flow rate is the input parameter we control. Let's say you we have a single phase heat exchanger which is designed to cool down a hot fluid (from Th_in to Th_out). In this case, we mostly use the flow rate of the cold fluid to control Th_out. Decreasing the flow rate of the cold fluid will lead to higher temperature of the hot fluid at the exit.
I didn't get your second question. But if I guessed correctly, the outputs you can control in a heat exchanger are definitely hot and cold fluid's exit temperatures. In case the heat exchanger duty is to add/remove a specific amount of heat, then that heat is the controllable output.
In a heat exchanger model based on energy balance, the most common control inputs are:
(1) the mass flow rate of either hot or cold fluid,
(2) the temperature of the cold fluid at the inlet, and sometimes
(3) the rates of bypass or recirculation.
The cold-side mass flow rate is often the best control input for controlling the temperature of the hot fluid outlet because it directly affects how much heat can be transferred. You can also control the cold fluid outlet temperature, the log mean temperature difference (LMTD), or the overall heat duty by changing the flow rates, inlet temperatures, or valve positions. The choice depends on how fast the system can respond and what it can handle, but flow rate modulation is the most useful and common way to control things.
- Badges
- Science topic
- Similar topics
- Engineering
- Materials Engineering
- Powders