Fouling Factors

While revamping a Chemical Process Plant, it is very rare to find a heat exchanger that is a bottleneck to achieve higher throughputs. In other words, heat exchangers more often than not have plenty of in-built design margins. This margin results from providing conservative fouling factors, in addition to the customary practice of designing heat exchangers for 110 % of the heat duty.

Consider the case of a simple condenser condensing organic vapours using cooling water in the tubes. For typical values of ho = 1000, hio = 400 and rd = 0.003 (all in British Units; Thanks to the pioneering text book of Kern, generations of chemical engineers are comfortable in British Units when it comes to heat transfer coefficients), the overall heat transfer coefficient U is 154. With improved treatment of cooling water and annual cleaning, it is possible to specify a less conservative dirt factor. With rd = 0.001, U leaps to 222, an increase of 44 %. Compounded with the 10 % design margin mentioned above, the heat exchanger has a margin of 58 %, good enough for most revamps.

There were times when conservative fouling factors were specified to hide the perceived inadequacies of empirical calculation methods and likely errors in physical properties. But with sophisticated software of HTRI and HTFS, which model the non-ideal flows on the shell side, estimates of clean heat transfer coefficients are more accurate. The case for conservatism in fouling factors no longer exists on this count.

Over sizing of heat exchangers can also be counterproductive. Lower velocities in an oversized exchanger will only promote scaling and accelerate fouling. The research work on fouling is scattered and not much has been done in compiling a meaningful database that can be used to choose economic fouling factors for cost-effective design of heat exchangers. Last year HTRI launched an industry-wide effort to examine the way heat exchangers are designed.