A. On-line monitoring of operating heat exchangers

The ability to monitor fouling of operating heat exchangers is especially critical in the heat exchangers that are currently process bottlenecks or have the potential of becoming such, possibly as a result of fouling. However, a significant amount of instrumentation is required. Thus the benefits must be measured against the cost. For complete monitoring of fouling in operating heat exchangers to be achieved, the inlet and outlet temperatures of each fluid must be measured, preferably with Resistance Temperature Device (RTD) technology, and so must the flow rates and pressure drops. Finally, all of the data must be collected and processed in real-time. Data storage and processing may be done on a Distributed Control System (DCS) or on a stand-alone computer. There are a number of very good software programs available for that purpose. One critical aspect of such monitoring is that it must include data trending over a prolonged period.

In some instances, with certain types of processes, there are shortcuts, which may be sufficient. For example, for compressor intercoolers and aftercoolers, one may plot the approach temperature, that is, the temperature of the compressed gas leaving the heat exchanger to the temperature of the entering cooling water. For turbine condensers, a plot of actual vs. ideal vacuum, in mm Hg, may suffice. Ideal vacuum would be, for instance, a plot of mm Hg versus inlet cooling water temperature with zero fouling. In refrigeration machines it may be inlet cooling water temperature versus head pressure.

B. On-line modeling of operating heat exchangers

There are two generally accepted means for modeling process heat exchangers, heat transfer models and pressure drop models. Heat transfer is a more universal technique because it can be used to model either side of a heat exchanger and can be applied to a wider range of heat exchanger designs. On the other hand, pressure drop models can be applied to non-heat transfer applications such as deposition in circulating lines. On-line monitoring usually involves a side stream. The major advantage and disadvantage of on-line modeling is that it must utilize the same circulating fluid as the operating heat exchanger. That is an advantage, because it is the same, including the amount of any anti-foulant/dispersant, which may be in use. If the purpose of modeling is to evaluate another anti-foulant/dispersant technology, that can only be done in addition to what is already in the stream. In order to evaluate an alternate anti-foulant or dispersant as a full substitution, the evaluation must be done either by terminating one treatment and initiating the substitute or use an off-line method such as in the laboratory.