6.2. A double pipe exchanger was oversized because no data were available on the rate at which dirt accumulated. The exchanger was originally designed to cool 13,000lb/hr of 100 per cent acetic acid from 250 to 150F by heating 19,000lb/hr of butyl alcohol from 100 to 157F. A design coefficient UD=85 was employed, but during initial operation a hot-liquid outlet temperature of 117F was obtained. It ' rose during operation at the average rate of 3F per month. What dirt factor should have been specified for a 6-month cleaning cycle