Look at Liquid Bypass

Such process-side control of tubular exchangers offers advantages.

By Cecil L. Smith, Cecil L. Smith, Inc.

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The general practice today is to implement split-range logic within software (that is, within controls instead of at the control valve). One way to eliminate the dead zone would be to begin closing the bypass valve at a controller output of 40% instead of 50%. Only flow simulations can determine if such changes either reduce or eliminate the flat regions.

Flow and Temperature Control

flow and temperature control
Figure 7. Flow and temperature control: The degree of interaction between temperature and flow
loops depends on the configuration. Click image to enlarge.
In Figure 7(a) a control valve on the steam supply and a control valve on the liquid return regulate liquid outlet temperature and liquid flow. The liquid valve affects both liquid outlet temperature and liquid flow. The steam control valve affects temperature but has at most a very small effect on liquid flow. With this configuration the temperature and flow loops don't interact. The same is true when the control valve is on the condensate instead of the steam supply.

Figure 7(b) illustrates the bypass arrangement. Two control valves are installed, one in the bypass and the other in series with the exchanger. Opening either valve increases the flow. Opening the bypass valve decreases liquid outlet temperature; opening the valve in series with the exchanger raises liquid outlet temperature. This configuration will exhibit at least some degree of interaction.

Figure 8 presents the two possible configurations using two simple feedback loops. In Figure 8(a) the exchanger valve regulates flow and the bypass valve controls temperature. In Figure 8(b) the bypass valve regulates flow and the exchanger valve controls temperature. In multivariable control lingo, the "pairing" is said to be reversed.
liquid bypass configurations
Figure 8. Liquid bypass configurations: Two control configurations can accommodate two simple
feedback loops. Click image to enlarge.

Loop interaction has two aspects:

Steady state. This assesses the degree to which each control valve affects each controlled variable.
Dynamics. If one loop is much faster than the other (at least five times faster), the loops are dynamically separated and the degree of steady-state interaction is irrelevant.

The dynamic separation for most temperature and flow loops is sufficient for either configuration to function. However, for the exchanger the temperature loop is faster than most temperature loops. Consequently, it's advisable to tune the liquid flow controller to respond as rapidly as possible.

From a steady-state interaction perspective, the basis for selecting a configuration can be simply summarized: control liquid flow using the valve with the largest flow. When 70% or more of flow is through the exchanger, the configuration in Figure 8(a) is better. When 70% or more of flow is through the bypass, the configuration in Figure 8(b) is better. But what about the following cases:

1. Flow through the bypass is about 50% of total liquid flow, a point where the degree of steady-state interaction is the greatest.
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