Achieve effective heat exchanger control

This first article in a four-part series explores the nuances of steam supply control

By Cecil L. Smith

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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.

Maximum heat transfer rate
Two constraints can impose the upper limit for exchangers and other heat transfer processes:
Media limited. The process is capable of condensing more steam than can flow through the control valve. In this case, the control valve would be effective up to 100% open.

Heat transfer limited. The maximum steam flow is determined by heat transfer, that is, by UAΔT. Because control valves are commonly oversized, this is the most common situation.

The maximum possible control valve opening is always 100%. This gives the maximum shell pressure, the maximum heat transfer rate and the maximum liquid outlet temperature. Increasing the steam valve opening always increases these three variables. But for large valve openings, the increase will be very small — so small, in fact, that it’s practically zero. Once the shell pressure approaches the steam supply pressure (75 psig), the exchanger approaches the heat transfer limit and is said to be “maxed out.”

For heat transfer processes, the control valve normally is effective up to about 90% of the maximum heat transfer rate. For a constant liquid flow and liquid inlet temperature, the heat transfer rate is proportional to the liquid temperature rise from inlet to outlet. If the shell pressure is equal to the steam supply pressure, the shell steam temperature is 288°F. The maximum possible increase in liquid temperature is from 150°F to 288°F or 138°F. The control valve should be effective for temperature increases of 90% of 138°F or 124°F. This corresponds to a liquid outlet temperature of 274°F.

Figure 3 indicates the valve openings for a liquid outlet temperature of 274°F. For an equal-percentage valve, this is a valve opening of 50% (actually 51%, but let’s round valve openings to the nearest 5%). For a linear valve, this is a valve opening of 15%! This is typical for oversized valves with linear characteristics. If you install a linear valve, it must be properly sized for the process operating conditions.

One of the issues previously raised pertained to flat regions of the process operating line. The slope of the process operating line is the process gain or process sensitivity (again, “process” includes the control valve). A decrease in the process sensitivity has the same effect on loop performance as a decrease in the controller gain, that is, the loop responds more slowly. For an equal-percentage valve, the controller can be effectively tuned and will give consistent performance for valve positions up to 50%. But above 50% the process gain is very small and approaches zero as the valve opening is increased. Even process operators will observe this, saying “Once the valve is half open, it has no effect on anything.”

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