CBEN408Spring2018 ‐1‐ December28,2017

Homework #2 

Problems3.1,3.2(withextrabelow),3.3,&extraproblembelow.Extrafor3.2:Whatistherelativedifferenceinheattransferareaifyouusea1‐2heatexchanger(1shellpass,2tubepasses) instead of pure counter current flow?Consider both thewater on shell side in oneinstance&onthetubesideinanotherinstance.Extraproblem:You’dliketocondense5,000lb/hrofpropaneina4‐passaerialcooler.Considerthefollowing:

Youwanttocondensepropaneatasufficientpressuresothatitwillbesaturatedat120°F. Thepropaneenters asa saturatedgas, exitsasa saturated liquid,& there isnopressure

dropthroughtheexchanger. Ambientairis85°F.Consideritsheatcapacityas0.25Btu/lb·°F. Theexchangershouldhaveaminimum10°Fapproachtemperature.

Answerthefollowing:

Whatmassflowrateofairisneeded(inlb/hr)? If theheat transfercoefficient(basedontheextendedarea) is3.5Btu/hr·ft2·°F thenwhat

surfaceareaisneeded(inft2)?