A single pass, countercurrent flow heat exchanger is tested in the clean condition and is found to heat water from 70 to 140 when the flow rate is 100 gallons per minute. The hot side of the heat exchanger uses 230 steam. The tubes inner surface area is 50 ^2 . After being used in the field for several months, the exchanger heats 100 of 70 water to 122. What is the fouling factor in units of ^2 /? Ans: 1.45x10^-4 m^2*K/W.