Heat Exchanger: Forestall Fouling-Factor Foul-ups

Figure 1. Tube in a shell-and-tube exchanger illustrates the five factors that contribute to the overall resistance.

The resultant value comes from subtracting two relatively large numbers from another large number. This concept poses potential problems, as I’ve discussed previously (see February 2015 issue’s “Resist the Temptation”). When using plant data to determine the fouling factor, all errors in the calculation accumulate in the final result.

Plant data are one source of error. We measure the flow rate and the temperatures around the exchanger to calculate duty and provide information for the heat-transfer calculation methods. Are the flow rates and temperatures measured right? We also need the composition of the stream to estimate its physical properties such as viscosity and heat capacity. This becomes especially challenging in exchangers that are partial condensers or partial vaporizers because you really require the rate and composition of each phase.

The correlations we use are another source of error. How good are the correlations for the physical properties needed? In addition, how much scatter appears in the heat-transfer correlations themselves?

A recent analysis I did illustrates how these errors can accumulate. This in- depth study of 60 heat exchangers in multiple units indicated that five had a calculated negative fouling factor after cleaning when their units started up after a shutdown. Another eight had a calculated zero fouling factor.

These 60 exchangers had been in service for more than ten years and had gone through multiple shutdowns. They all received standard commercial handling and cleaning during the shutdown maintenance. It’s highly unlikely they were perfectly cleaned during the shutdown. Also, each one had some amount of corrosion and exchanger wear accumulated over the years. They surely had some fouling resistance present.

The calculated negative-to-zero fouling factors result from data-measurement and physical-property errors as well as correlation uncertainty. In this sample, at least 20% of the exchangers performed better than expected. Due to the physical condition of the exchangers, it’s difficult to say how many performed much worse than expected. Nevertheless, it clearly exceeded 20%.

Fouling factors are useful concepts for monitoring and identifying areas where a plant should take remedial action. However, don’t blindly accept the calculated results — understand the possible errors and how they might affect your operating decisions.