Achieve effective heat exchanger control

Most plants heavily rely on shell-and-tube heat exchangers with liquid flowing through the tubes and steam on the shell. To control the liquid outlet temperature (the controlled variable in control engineers’ terminology) for such a steam heated exchanger, several process equipment configurations are possible, including:

1. control valve on the steam supply
2. control valve on the condensate
3. liquid bypass with one or two control valves.

In this article, we’ll focus on the first configuration. Future articles in the series will look at the others.

The process operating line
In selecting the configuration, designers always consider the process issues, such as the requirement for condensate return, but customarily defer the control issues to those who develop the Process and Instrumentation Diagram (P&ID). This practice needs to change because most modern designs are model-based and the relationships in the model allow generating a graph known as the process operating line that’s the basis for evaluating the control issues.

The process operating line is a plot of the steady state or equilibrium values of the controlled variable (liquid outlet temperature) as a function of the controller output. In a sense the process operating line is one approach to implement the “you have to understand the process” philosophy that’s the essence of process control. Important aspects of this graph are:

• Limits of process operation. The minimum and maximum limits on the control valve position are usually 0% and 100% — however, process problems potentially may arise with operation in certain parts of this range.
• Departure from linearity. Departures less then 20% won’t noticeably affect loop performance. However, departures of 50% or greater will be noticeable and are one source of tuning difficulties.
• Regions where the operating line is flat. The changes in the controller output have no effect on the process in such a region, making it a dead zone. The controller may perform well on both sides of the dead zone but won’t make a smooth transition from one side to the other.
• Regions where the operating line is vertical. The process sensitivity within such a region will be very large, causing the loop to be unstable. This will lead to cycling from one side of this region to the other.
• Effect of process operating variables. Throughput is one such variable, especially in utility processes and batch processes.

Note that the items relate to process engineering issues — not linear systems theory (LaPlace transforms and the like) normally taught in academic courses.

Operating limits
Normal operating conditions clearly should fall between the minimum and maximum limits. If not, either the process design is deficient or the process isn’t running under the conditions for which it was designed. The culprit most likely is the latter; some plants “evolve” from design conditions.

The other mistake is to assume that the process is always operated under its normal operating conditions. We experience a variety of disruptions to production (changes in raw materials, upsets in utility systems, equipment problems, etc.). This is when we are likely to attempt to operate the process beyond the limits. The consequences include cycling conditions (induced by the process, not the controller tuning), windup that isn’t addressed by the windup prevention mechanisms as normally configured, etc. The controls need to recognize the presence of the limits and take appropriate actions instead of attempting to operate beyond the limits.

Consider the exchanger with its control valve on the steam supply. The condensate is discharged through a steam trap into the condensate return system. The liquid is a hydrocarbon fluid that enters at 150°F. The steam supply pressure is 75 psig. Under normal operating conditions, the liquid flow rate is 1,000 lb/min; however, occasionally liquid flow rates up to 4,000 lb/min are experienced. Consequently, the control valve must be oversized for the requirements of normal operating conditions. Of course, further oversizing is the norm. For this example, the control valve is oversized by about a factor of four (relative to what’s required for normal operating conditions).

A common misconception is that every control issue pertains to some aspect of process dynamics. In practice, many if not most of the problems with the controls have their root in the steady state behavior of the process.

The steady state behavior of the exchanger can be understood from the graphs in. Presents the shell pressure as a function of the steam valve position. Figure 2b shows the liquid outlet temperature as a function of the steam valve position — this is the process operating line for the exchanger. The term “process” is somewhat of a misnomer because the valve characteristics (valve size, inherent valve characteristics, etc.) also are incorporated into the operating line. This is why there are two lines, one for a linear valve and one for an equal-percentage valve.