Applying heat exchanger control strategies
Heat exchangers transfer thermal energy between fluids. Although heat transfer is typically efficient, controlling the temperature of the fluid being heated at a specific and stable setpoint can be challenging. However, these challenges can be overcome by understanding heat exchanger control schemes implemented in industry.
Shell-and-tube heat exchanger at a glance
By far, the shell-and-tube is the most common type of heat exchanger used in petrochemical industries because it is suitable for low and high pressure applications (see Figure 1). It consists of an outer shell with a bundle of tubes inside. The tubes are oriented in a straight or in a "U" shape. One fluid runs through the tubes, and another fluid flows through the shell surrounding the tubes to transfer heat between the two fluids (see Figure 2). The set of tubes is known as a "tube bundle."
Heat is transferred from one fluid to the other through the walls of the tubes.
Heat is transferred from the tube fluid to shell fluid to remove heat, or from the shell fluid to the tube fluid to heat the material inside. Fluids can be liquids or gases on either the shell or the tube side. To transfer heat efficiently, many tubes are used, which increases the heat-transfer surface area between the two fluids.
To develop a comprehensive control strategy for any control loop, it’s important to identify the process variable of interest—called the "controlled variable," the manipulated variable, and the different disturbance variables that directly affect the controlled variable.
Consider the heat exchanger shown in Figure 3. The shell side fluid is the process fluid that is required to be heated to a certain temperature setpoint. The resulting temperature is measured at the outlet of the heat exchanger T1Out (controlled variable).
Heating is achieved by passing steam through the tube side. The more steam passing through the tubes, the more heat is transferred to the process fluid, and vice versa. Control of the steam flow F2 (manipulated variable) is achieved by throttling a modulating valve installed on the steam inlet side.
Three major disturbances can affect the process fluid outlet temperature:
- Changes in process fluid flow rate, F1
- Changes in process fluid inlet temperature, T1In
- Changes in steam pressure, causing a change in steam flow rate, F2.
The control objective is to maintain process fluid outlet temperature T1Out at the desired setpoint—regardless of disturbances—by manipulating the steam flow rate F2.
In the feedback control scheme, the process variable, T1Out, is measured and applied to a proportional-integral-derivative (PID)-based feedback temperature controller (fbTC), which compares the process variable with the desired temperature setpoint and in turn calculates and generates the control action required to open or close the steam control valve (see Figure 4).
The most important advantage of the feedback control scheme is that regardless of the disturbance source, the controller will take corrective action. Employing feedback control requires very little knowledge of the process. Therefore, a process model is not necessary to set up and tune the feedback scheme, although it would be an advantage.
The major disadvantage of feedback control is its incapability to respond to disturbances—even major ones—until the controlled variable is already affected. Also, if too many disturbances occur with significant magnitude, they can create unrecoverable process instability.
In the cascade control scheme, instead of feeding the output of the PID temperature controller directly to the control valve, it is fed as a setpoint to a feedback PID-based, steam-flow controller (fbFC). This second loop is responsible for ensuring the flow rate of the steam doesn’t change due to uncontrollable factors, such as steam pressure changes or valve problems.
To understand how this works, consider that the heat exchanger is in steady-state operation, the outlet temperature matches the setpoint, and the controller output of fbTC is constant. A sudden increase in steam pressure will cause steam flow rate F2 to ramp up (see Figure 5). This will cause a gradual change in the controlled variable. Without the flow control loop, fbTC will not take corrective action until the outlet temperature is already affected.
By implementing the cascade strategy, the feedback flow control loop fbFC will adjust the valve position immediately when the steam flow rate has changed to bring the flow back to the value of the previous steady-state condition (because the flow setpoint given by the temperature controller didn’t change as the outlet temperature did not yet change), preventing a change in the outlet temperature before it happens.
Note that the flow control loop must be tuned to run much faster than the temperature control loop, therefore cancelling the effect of flow variance before it affects the process fluid outlet temperature.
Unlike feedback control, feedforward takes a corrective action when a disturbance occurs. Feedforward control doesn’t see the process variable. It sees only the disturbances and responds to them as they occur. This enables a feedforward controller to quickly and directly compensate for the effect of a disturbance (see Figure 6).
To implement feedforward control, an understanding of the process model and the direct relationship between disturbances and the process variables is necessary. For heat exchangers, a derivation from the steady-state model will lead to the following equation, which determines the amount of steam flow required:
F2sp = F1 × (T1OUTsp – T1IN) × (Cp/ΔH)
- F2sp = steam flow rate calculated setpoint to be applied to fbFC
- F1 = process fluid flow rate measured disturbance
- T1OUTsp = process fluid temperature setpoint at the heat exchanger outlet
- T1IN = process fluid inlet temperature measured disturbance
- Cp = process fluid specific heat (known)
- ΔH = latent heat of vaporization for steam (known).
Applying this equation to calculate the required steam flow rate is sufficient to cancel the effects of changes of the process fluid flow rate and temperature. In a perfect world with few enhancements to the process model, this feedforward controller is enough to perfectly control the process. Unfortunately, it’s not a perfect world.
The obvious advantage of using feedforward control is that it takes corrective action before the process is upset. A disadvantage is that it mandates a high initial capital cost because every disturbance must be measured, increasing the number of instruments and the associated engineering costs. In addition, this approach requires deeper knowledge of the process. It’s not always realistic to depend on feedforward control only without taking into account the measured process variable.
An integrated approach that uses feedback, feedforward, and cascade control is shown in Figure 7. This approach is more than capable of accommodating heat exchanger control requirements:
- A feedforward loop will handle major disturbances in the process fluid
- A cascaded-flow control loop will handle issues related to steam pressure and valve problems
- A feedback loop will handle everything else.
Combining the three techniques to optimize heat exchanger temperature control is necessary to minimize process variance, maximize product quality, and ensure energy efficiency in petrochemical industries.
– Shady Yehia is the founder and author of The Control Blog and is the instrumentation, control, and automation proposals and engineering manager in a process technology integration company based in Qatar and operating in the EMEA region. The Control Blog is a CFE Media content partner. Edited by Jack Smith, content manager, CFE Media, Control Engineering, email@example.com.
- Understand the control techniques—feedback, cascade, feedforward, and PID—associated with heat exchanger temperature control.
- Evaluate how process variable upsets and corrections affect control performance.
- Consider the advantages and risks of integrating these control techniques to optimize heat exchanger temperature control.
How effective would these techniques be when using a heat exchanger to remove heat from—instead of supplying heat to—a process fluid?
See related articles below, which offer more information about heat exchangers, temperature control, and PID tuning.