A vertical-tube two-pass heater is used for heating gas oil. Saturated steam at 100 lb /in.

A vertical-tube two-pass heater is used for heating gas oil. Saturated steam at 100 lb_ƒ/in.² gauge is used as a heating medium. The tubes are 1 in. OD by BWG 16 and are made of mild steel. The oil enters at 60F and leaves at 150°F. The viscosity-temperature relation is exponential. The viscosity at 60°F is 5.0 cP and at 150°F is 1.8 cP. The oil is 37° API (specific gravity 0.840) at 60°F. The flow of oil is 150 bbl/h (1 bbl 42 gal). Assume the steam condenses in film condensation.The thermal conductivity of the oil is 0.078 Btu/ft·h·°F, and the specific heat is 0.480 Btu/lb·°F. The velocity of the oil in the tubes should be approximately 4 ft/s. Calculate the length of tubes needed for this heater.