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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If the controller gain is increased to compensate for the reduced process gain, the higher gain will give problems at lower liquid outlet temperatures. This is one case where some people will propose characterization functions, scheduled tuning or other approaches commonly referred to as adaptive control (but, in reality, these are nonlinear controls). A simple approach (scheduled tuning) would be to use one value of the controller gain for valve openings (controller outputs) less than 50% and a higher value for controller outputs above 50%. Unfortunately, the actual cutoff point depends upon operating conditions, especially throughput.

Minimum heat transfer rate
Let’s assume that the condensate from the steam trap flows into a drain. Figure 2a indicates that the shell pressure is below atmospheric when the steam valve position is less than 29% for the equal-percentage valve or 6% for the linear valve. Under these conditions, the condensate won’t flow out of the shell. If the condensate flows into a condensate return system, the minimum shell pressure is approximately atmospheric pressure.

This imposes a minimum heat transfer rate on the exchanger. Atmospheric pressure in the shell corresponds to a shell temperature of 212°F. Because the liquid enters at 150°F, heat will be transferred to the liquid. This heat transfer rate is the minimum that can be continuously sustained by the exchanger in Figure 1. The minimum heat transfer rate gives a liquid outlet temperature of approximately 200°F, regardless of the valve characteristics.

Figure 4 illustrates the effect on the process operating lines. The minimum heat transfer rate corresponds to an opening of 29% for the equal-percentage valve and 6% for the linear valve.

Attempting to operate below this minimum leads to a cycling condition. The following scenario occurs for the equal-percentage valve:

  • The controller positions the valve to less than 29%.
  • The shell pressure drops to atmospheric and the shell begins to fill with condensate. This reduces the effective heat transfer area and the heat transfer rate.
  • The liquid outlet temperature drops below target, causing the controller to increase its output to the control valve and admit more steam to the shell.
  • When the shell pressure exceeds atmospheric pressure, the condensate is forced from the shell, exposing the entire heat transfer area.
  • The heat transfer rate increases, which drives the liquid outlet temperature above its target.
  • The controller decreases its output to the control valve, eventually giving a valve position less than 29%. This causes the cycle to repeat.

How do we avoid operating the process below the minimum heat transfer rate? For normal operating conditions, we could impose a minimum of 200°F for the liquid-outlet-temperature set point. Unfortunately, a variety of variables, including throughput, affect the value for this limit.

Perhaps the surest way to avoid such conditions is to monitor the shell pressure and provide a shell pressure override (Figure 5). For digital implementations, the only additional hardware is the transmitter for the shell pressure. The remainder is implemented in software. For exchangers that discharge the condensate to a drain, the set point for the shell pressure controller would be slightly above atmospheric. For exchangers that discharge into a condensate return system, the set point for the shell pressure controller must be slightly above the pressure required to force the condensate back to the boiler house.

Implications
Figure 6 presents the process operating lines with the limits on operation indicated. Let’s examine each of the aspects previously mentioned:

Limits of process operation. When expressed in terms of the liquid outlet temperature the operating limits are 200°F and 274°F, regardless of the inherent valve characteristics. But when expressed in terms of the control valve position the operating limits definitely depend upon the type of valve:

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