Water at a flow rate of 20,000 kg/h will be heated from 20°C to35°C by hot water at 140°C. A 15°C hot water temperature drop isallowed. A number of 3.5 m hairpins of 3 in. (ID = 0.0779 m) by 2in. (ID = 0.0525 m, OD = 0.0603 m) counter flow double-pipe heatexchangers with annuli and pipes, each connected in series, will beused. Hot water flows through the inner tube. Fouling factors are:Rf i = 0.000176 m2 ? K/W, Rfo = 0.000352 m2 ?K/W. Assume that the pipe is made of carbon steel (k = 54 W/m ? K).The heat exchanger is insulated against heat losses. Assume thatthe allowable pressure drop in the annulus is 10 kPa. The velocityof cold water through the annulus will be around 3 m/s, which wouldrequire a large pressure drop to drive the cold fluid through theinner tube. The pressure drop could be reduced by using severalunits with cold water flowing in four parallel units and hot waterflowing through the inner tube in series. Calculate: a. The overallheat transfer coefficient b. Pressure drop in the annulus c. Theappropriate mean temperature difference.
